From population sources to sieves: the matrix alters host-parasitoid source-sink structure.

Field experiments that examine the impact of immigration, emigration, or landscape structure (e.g., the composition of the matrix) on the source sink dynamics of fragmented populations are scarce. Here, planthoppers (Prokelisia crocea) and egg parasitoids (Anagrus columbi) were released among host-plant patches that varied in structural (caged, isolated, or in a network of other patches) and functional (mudflat matrix that impedes dispersal vs. brome-grass matrix that facilitates dispersal) connectivity. Planthoppers and parasitoids on caged patches exhibited density-dependent growth rates, achieved high equilibrium densities, and rarely went extinct. Therefore, experimental cordgrass patches were classified as population sources. Because access to immigrants did not result in elevated population densities, source populations were not also pseudosinks, i.e., patches whose densities occur above carrying capacity due to high immigration. Planthoppers and parasitoids in open patches in mudflat had dynamics similar to those in caged patches, but went extinct in 4-5 generations in open patches in brome. Brome-embedded patches leaked emigrants at a rate that exceeded the gains from reproduction and immigration; populations of this sort are known as population sieves. For species whose suitable patches are becoming smaller and more isolated as a result of increased habitat fragmentation, emigration losses are likely to become paramount, a condition favoring the formation of population sieves. An increase in the proportion of patches that are sieves is predicted to destabilize regional population dynamics.

Related papers:

title: Sources and sinks in population biology.
abstract: The various habitats inhabited by a given species are never of the same quality. When demographic models take into account this habitat heterogeneity, the source-sink concept naturally emerges: a local demographic surplus arises in good quality habitats (source), and a local demographic deficit occurs in habitats of poor quality (sink). Within a landscape, a permanent migration of propagules or individuals from source to sink habitats may lead to a stabilization of the overall demographic system. This simple situation, explored in the recent literature, has surprising properties. In particular, it requires a change in our view of classical concepts such as ecological niche and carrying capacity, it can explain the existence and persistence of local maladaptation and it can improve conservation practice.

title: Sources, Sinks, and Population Regulation
abstract: Animal and plant populations often occupy a variety of local areas and may experience different local birth and death rates in different areas. When this occurs, reproductive surpluses from productive source habitats may maintain populations in sink habitats, where local reproductive success fails to keep pace with local mortality. For animals with active habitat selection, an equilibrium with both source and sink habitats occupied can be both ecologically and evolutionarily stable. If the surplus population of the source is large and the per capita deficit in the sink is small, only a small fraction of the total population will occur in areas where local reproduction is sufficient to compensate for local mortality. In this sense, the realized niche may be larger than the fundamental niche. Consequently, the particular species assemblage occupying any local study site may consist of a mixture of source and sink populations and may be as much or more influenced by the type and proximity of other habitats as by the resources and other conditions at the site.

title: Turnover Rates in Insular Biogeography: Effect of Immigration on Extinction
abstract: Demographic and genetic contributions from nonspecific immigrants tend to reduce ex- tinction rates of insular populations. The MacArthur-Wilson model of island biogeography is modified to provide for this effect of immigration on extinction, which we call the rescue effect. This new model predicts that when immigration rates are high relative to extinction rates, turnover rate is directly related to the distance between an island and the source of colonizing species. A field study of the distribution of arthropods among isolated plants supports the model.

title: Sources, sinks and pseudo-sinks
abstract: It has been suggested that the habitats which a species occupies can be divided into sources and sinks, depending on whether or not local reproduction is sufficient to balance mortality. Source populations are those where reproduction exceeds mortality, surplus individuals dispersing to sink populations where mortality exceeds local reproduction. Sink populations would not be viable in the absence of immigration. A difference equation model is constructed to show that sources and sinks cannot be identified from a simple comparison of the demographic rates between populations, as measured by the numbers of births and deaths. Viable populations may appear to be non-viable simply because the dispersal of individuals into them depresses fecundity or increases mortality as a result of density-dependence. The consequence is that local recruitment appears insufficient to balance local mortality. Viable populations that appear as sinks, as a result of the dispersal of individuals into them, are termed here as 'pseudo-sinks'. They will clearly be difficult to distinguish from genuine sinks on the basis of a simple comparison of the numbers of births and deaths in different populations. Examples of source and genuine sink populations and the data required to establish them are discussed.

title: The spatial structure of populations
abstract: 1. Studies of the spatio-temporal dynamics and structure of populations have identified many categories of population type. However, recognized categories intergrade, making it difficult to assign empirical population systems to single categories. 
 
 
 
2. We suggest that most population categories can be arranged along two axes that combine per capita birth (B), death (D), emigration (E) and immigration (I) rates. The ‘Compensation Axis’ describes the source-sink component of population structure, with source populations exporting individuals (B > D, E > I) and sinks and pseudosinks consuming individuals (B < D, E < I). The ‘Mobility Axis’ describes the involvement of a local population in regional (I + E) rather than local (B + D) processes, running from separate populations, through metapopulations, to patchy populations. 
 
 
 
3. Each sample area within a spatially structured population system can potentially be assigned to a position along each of these axes, with individual sample areas weighted by local population size. The positions of these sample areas and their relative weightings allow the relative importance of different types of process to be judged. A worked exampled is provided, using the butterfly Hesperia comma. This approach shifts the emphasis from pattern (categories that real population systems do not fit) onto process. 
 
 
 
4. In many systems, continuous variation in habitat quality and demographic parameters make clear distinctions between ‘habitat’ and ‘non-habitat’ difficult to sustain. In such cases, we advocate the use of a spatial grid system, with effects of patch size and isolation combined into a single, weighted distance function (neighbourhood). 
 
 
 
5. The relative importance of different processes depends on the spatial scale at which the system is observed. This again emphasizes the value of a process-based approach.

title: Population dynamics in two-patch environments: Some anomalous consequences of an optimal habitat distribution
abstract: The effect of dispersal on population size and stability is explored for a population that disperses passively between two discrete habitat patches. Two basic models are considered. In the first model, a single population experiences density-dependent growth in both patches. A graphical construction is presented which allows one to determine the spatial pattern of abundance at equilibrium for most reasonable growth models and rates of dispersal. It is shown under rather general conditions that this equilibrium is unique and globally stable. In the second model, the dispersing population is a food-limited predator that occurs in both a source habitat (which contains a prey population) and a sink habitat (which does not). Passive dispersal between source and sink habitats can stabilize an otherwise unstable predator-prey interaction. The conditions allowing this are explored in some detail. The theory of optimal habitat selection predicts the evolutionarily stable distribution of a population, given that individuals can freely move among habitats so as to maximize individual fitness. This theory is used to develop a heuristic argument for why passive dispersal should always be selectively disadvantageous (ignoring kin effects) in a spatially heterogeneous but temporally constant environment. For both the models considered here, passive dispersal may lead to a greater number of individuals in both habitats combined than if there were no dispersal. This implies that the evolution of an optimal habitat distribution may lead to a reduction in population size; in the case of the predator-prey model, it may have the additional effect of destabilizing the interaction. The paper concludes with a discussion of the disparate effects habitat selection might have on the geographical range occupied by a species.

title: DISPERSAL AND DYNAMICS
abstract: Mixing and asynchrony of interactions can be expected to stabilize the dynamics of populations. One way such mixing occurs is by dispersal, and Hastings and Gyllenberg et al . have shown that symmetric dispersal between two local populations governed by logistic difference equations can simplify the dynamics. These results are extended here by using a more flexible difference equation and allowing asymmetric dispersal. Although there are some instances where dispersal is destabilizing, its stabilizing effect is enhanced by asymmetry. In addition, very high dispersal rates can induce a stable equilibrium of the metapopulation despite highly chaotic local dynamics. If this equilibrium loses stability, the route to intermittent chaos can be observed. Two new conditions under which dispersal can be stabilizing are discussed. One occurs when the timing of reproduction and dispersal differs in the two patches of the metapopulation. This enlarges the asynchrony of the interactions, and simple dynamics due to dispersal are more likely. The second works by slightly adjusting dispersal rates to control chaotic dynamics. The control can replace chaos by a stable equilibrium. The evolution of dispersal rates is discussed. Since obtaining general criteria for invasion into a population with chaotic dynamics is difficult, no clear conclusions are possible as to whether evolution leads to more stable metapopulations. However, a mutant that controls chaos can invade a resident having the same local dynamics but no control mechanism, so that evolution can lead from chaos to a stable equilibrium.

title: Conservation Lessons from Long-Term Studies of Checkerspot Butterflies
abstract: Long-term studies of checkerspot butterflies (Eu- phydryas spp.) are used to draw conservation lessons with implications for the design and management of reserves for invertebrates. We discuss; 1. The importance of determining the structure of and patterns of mating within demographic units. 2. The lack of congruence between migration and gene flow. 3. The crucial role of habitat diversity in the protection of species which otherwise require only small areas of habitat. 4. The importance of identifying and preserving "reser- voir" populations within metapopulations. 5. The necessity of identifying subtle habitat requirements. 6 The key role of environmental stochasticity in the ex- tinction of small populations.

title: ECOLOGICAL INFLUENCES ON THE DYNAMICS OF A FIELD VOLE METAPOPULATION
abstract: Although metapopulation theory is widely used in basic and applied ecology, there are still few empirical studies that explore the relationships between dispersal, patch size, and the persistence of natural populations. Here, we reanalyzed data from a six-year study of a spatially structured field vole (Microtus agrestis) metapopulation in the Tvar- minne archipelago, Finland. Our goal was to address several issues relevant to metapo- pulation studies: (1) the relationships between within-subpopulation dynamics, dispersal behavior, habitat quality, and metapopulation dynamics; (2) the generality of one of the most common conclusions of metapopulation theory—that smaller and less frequently in- habited islands are less important for metapopulation dynamics; and (3) the comparison of different methods for understanding and predicting dynamics in ''metapopulation-like'' systems. Our results suggest that this vole metapopulation is driven by extinctions and colonizations of island subpopulations. However, contrary to expectation, colonizations by voles from tiny, ephemeral skerry subpopulations were about as important for metapopu- lation persistence as were colonizations from the more persistent subpopulations on large islands. This pattern resulted from less stable vole densities on smaller islands, combined with increased emigration preceding subpopulation extinctions. Either spatially implicit Levins models or incidence function models provided reasonable predictions of the structure and function of this metapopulation, but parameters fitted to incidence functions varied dramatically among years. Our results suggest that models of real metapopulations need not become highly detail oriented or spatially complicated to provide good predictive power. However, applications of metapopulation theory require careful consideration of how un- derlying ecological and behavioral mechanisms will shape metapopulation dynamics of particular species and situations.

title: Effects of an Attractive Sink Leading into Maladaptive Habitat Selection
abstract: Habitat sinks can attract dispersing animals if high mortality or breeding failure are difficult to detect (e.g., when due to human hunting or pollution). Using a simple deterministic model, we explore the dynamics of such source‐sink systems considering three scenarios: an avoided sink, no habitat preference, and an attractive sink. In the second two scenarios, there is a threshold proportion of sink habitat above which the whole population decreases to extinction, but this extinction threshold varies with habitat preference and the relative qualities of the two habitat types. Hence, it would be necessary to know the habitat preferences of any species in a source‐sink system to interpret data on population increases and declines. In the attractive sink scenario, small changes in the proportion of sink habitat may have disproportionate effects on the population persistence. Also, small changes in growth rates at the source and the sink severely affect the threshold and the time of extinction. For some combinations of demographic parameters and proportion of habitat sink, the decline affects the source first; thus, during some time, it will be hidden to population monitoring at the sink, where numbers can even increase. The extinction threshold is also very sensitive to the initial population sizes relative to carrying capacity. Attractive sinks represent a novel aspect of source‐sink dynamics with important conservation and management implications.

title: Pattern and stability in predator-prey communities: How diffusion in spatially variable environments affects the Lotka-Volterra model
abstract: Introducing a spatially heterogeneous environment to the Lotka-Volterra predator-prey diffusion model reveals that spatial pattern is amplified and the dynamics are stabilized by nonlinear environmental heterogeneity and differential species mobility. When predators diffuse and prey are sedentary, heterogeneity in the prey growth rate is amplified in the prey equilibrium distribution. When both species diffuse rapidly, patterns in their distributions are homogenized, but their total abundances are maximized. In contrast to the nonspatial constant Lotka-Volterra model, which features sustained oscillations, perturbations to the heterogeneous environment model decay to equilibrium exponentially fast. When prey are stationary, the rate of decay is proportional to the predator diffusion coefficient. The rate of approach to equilibrium slows as prey mobility increases. These results are interpreted as the creation of a prey refuge by density dependence implicit in the motion of nonuniformly distributed diffusing populations.

title: Environmental heterogeneity and the stability of host-parasitoid interactions
abstract: 1. Spatial heterogeneity has long been recognized as a potentially powerful stabilizing force in host-parasitoid interactions. Most previous work has emphasized heterogeneity in the rate of attacks experienced by individual hosts. In contrast, we highlight the potential importance of spatial heterogeneity in demographic parameters other than the rate of parasitism. 2. Following the model structure explored by Hassell & May (1973) in which hosts and parasitoids are distributed across patches according to fixed distribution rules, we show that, in the absence of heterogeneity in the rate of parasitism, spatial variability in other demographic parameters has no effect on population stability

title: Testing models of source-sink dynamics and balanced dispersal
abstract: I discuss two classes of models that predict patterns of dispersal across heterogeneous environments; the source-sink and balanced dispersal models. Data collected during a long-term study of small mammals in an experimentally fragmented landscape support the predictions of the balanced dispersal model. In addition, a literature review indicates that for 28 species, few empirical studies have enough detailed demographic data to distinguish between assumptions of the two models and to test their predictions. The review indicates both models are poorly supported by empirical studies and many of the studies cited as evidence for source-sink dynamics cannot distinguish a sink habitat from a habitat with a low carrying capacity. I conclude that a continuum should exist between species vagility and the dispersal patterns seen in heterogeneous landscapes. Species with high vagility and the ability to assess habitat quality should tend towards balanced dispersal. In species with low vagility or where dispersal is controlled by abiotic factors (wind, or water currents), source-sink dynamics will be more common. Investigators must collect detailed demographic and movement data if they hope to test these models and further our understanding of the role of spatial dynamics in ecology.

title: SOURCE–SINK DYNAMICS FOR A GENERALIST INSECT PREDATOR IN HABITATS WITH STRONG HIGHER-ORDER PREDATION
abstract: The functional importance of higher-order predators in terrestrial ecosystems is currently an area of active inquiry. In particular, an understanding of the relative influences of prey availability and higher-order predation on predator populations is of immediate relevance to the theory of biological control of herbivorous arthropods. Biological control workers have repeatedly speculated that one cause of failure to establish predators and parasitoids in novel environments is the strong mortality imposed on released agents by higher-order predators. Nevertheless, the ability of higher-order predators to create a habitat where mortality exceeds natality (a “sink” habitat) has never been tested experimentally with a biological control agent in nature. 
 
Although in isolation the predatory lacewing Chrysoperla carnea can consistently produce strong suppression of populations of the aphid Aphis gossypii, the full community of predators when tested together exerts minimal aphid control. The age structure of Chrysoperla spp. populations in cotton fields harboring low to intermediate densities of aphid prey is characterized by a sharp drop in densities from the egg to the first larval instar; this observation is consistent with heavy mortality during either the egg or first larval stage. Egg cohorts followed under unmanipulated field conditions showed relatively high rates of successful hatch, suggesting that the vulnerable developmental stage is the young larva. Larval survival is relatively high in the absence of hemipteran predators, suggesting that prey availability is not the primary limiting factor. Depressed survival is observed in the presence of Geocoris spp., Nabis spp., and Zelus renardii, all common hemipteran predators in cotton. Predation on lacewing larvae appears to disrupt the strong top-down control of aphid populations in cotton. 
 
Chrysoperla spp. densities declined in fields harboring intermediate aphid densities when lacewing subpopulations were experimentally caged to block immigration and emigration. In one year (1993) Chrysoperla spp. densities fell to very low levels, suggesting that the field was either a true sink habitat or a pseudosink with a very low equilibrium density. In a second year (1994), densities declined to what appeared to be a lower but stable density, suggesting that the habitat was a pseudosink. Thus, in both years, declines in Chrysoperla spp. densities were observed following caging, suggesting that Chrysoperla spp. populations are spatially subsidized. Aphid prey availability and higher-order predation interacted strongly in their influence on C. carnea survival: larval survival in the presence of higher-order predators was 5.6% when prey availability was intermediate and 40.5% when prey were superabundant. Spatial heterogeneity in aphid prey densities modulates the intensity of higher-order predation and thereby appears to produce source–sink dynamics of Chrysoperla spp. in cotton fields.

title: The Role of Local Populations within a Landscape Context: Defining and Classifying Sources and Sinks
abstract: The interaction of local populations has been the focus of an increasing number of studies in the past 30 years. The study of source‐sink dynamics has especially generated much interest. Many of the criteria used to distinguish sources and sinks incorporate the process of apparent survival (i.e., the combined probability of true survival and site fidelity) but not emigration. These criteria implicitly treat emigration as mortality, thus biasing the classification of sources and sinks in a manner that could lead to flawed habitat management. Some of the same criteria require rather restrictive assumptions about population equilibrium that, when violated, can also generate misleading inference. Here, we expand on a criterion (denoted “contribution” or 
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title: Catastrophic Extinction of Population Sources in a Butterfly Metapopulation
abstract: Increasing emphasis is being placed on the large-scale and long-term dynamics of populations. A butterfly (Euphydryas editha) metapopulation that was naturally restricted to rocky outcrops in an area of coniferous forest suffered two major perturbations in 30 yr. First, humans clear-cut patches of forest in about 1967. The butterfly colonized the clear-cuts, in which it began to oviposit on a novel host plant, Collinsia torreyi. Breeding success was high, and the clear-cut habitat supported a population source; in the mid-1980s, there was net movement of adults from clear-cut to outcrop, and butterfly densities were elevated on outcrops close to population sources. The second major perturbation came in 1992, when a severe summer frost killed C. torreyi plants (outcrop hosts were not damaged). As a result, E. editha larvae starved in clear-cuts, and the source populations became extinct. This occurrence allowed us to test several predictions of source-sink theory. As predicted, densities declined on outcrops, and particularly on those close to former sources; by 1993, densities on outcrops were no longer correlated with isolation from former sources. True population sinks are predicted to become extinct in the absence of immigration, but this prediction was not observed. Outcrops were not true sinks because E. editha occupied this habitat before the creation of sources, survived the frost on outcrops, and persisted on outcrops in areas where they were isolated from sources. Outcrops were pseudosinks; breeding success was poor at immigration-enhanced densities, but outcrop populations did not rely on immigration to persist. The implications for conservation biology are that populations with high mean density and that act as sources are not necessarily more stable than lower-density or pseudosink populations. This may be especially true of systems subject to recent human disturbance, as in the present case.

title: Confounding of patch quality and matrix effects in herbivore movement studies
abstract: Although the landscape matrix is increasingly incorporated into spatial-ecological population studies, little consideration has been given to the likely possibility that patch quality is confounded with the composition of the matrix surrounding each patch. For example, the nutritional quality of host-plant patches to an herbivore may be highly correlated with matrix composition, consequently obfuscating the importance of the matrix itself to interpatch dispersal. From a literature survey of the effects of the matrix on herbivore movement among host-plant patches, we found that 55% of the studies (6/11) failed to experimentally or statistically isolate the effects of the matrix from potential patch-quality effects on dispersal. Most studies consisted of mark-recapture experiments in natural landscapes where patch equality was not controlled or manipulated. Of the few studies that evaluated the relationship between matrix composition and patch quality, all of them (3/3) found that these two landscape factors covaried. These data suggest that in most matrix studies, effects of the matrix on dispersal may be wholly, or in part, due to underlying differences in patch quality.

title: EMPIRICAL EVIDENCE FOR NONSELECTIVE RECRUITMENT AND A SOURCE–SINK DYNAMIC IN A MAYFLY METAPOPULATION
abstract: Dispersal among spatially subdivided populations and variability in local habitat patch quality can strongly affect local and regional population dynamics. A meta- population of mayflies ( Callibaetis ferrugineus hageni) was studied in which larvae de- veloped in beaver ponds and emerged to the terrestrial habitat as adults; adult females had the potential to disperse among ponds before oviposition. Ponds (patches) differed in the presence or absence of trout. Densities of late instar larvae were more than an order of magnitude greater in fishless ponds. Estimates of adult production, migration, and oviposition rate were used to determine whether the difference in larval abundance between pond types was more consistent with a source-sink or balanced dispersal model. Patch quality among ponds (estimated by the emergence rate of adults) varied from 0 to ;3900 individuals·m 22 ·yr 21 , and was significantly lower in ponds with trout. Survivorship of adult females in the regional terrestrial habitat was low (1.7%). Consistent with the predictions of source-sink theory, local recruitment to the next generation was not related to local emergence. Moreover, comparison of emer- gence and recruitment revealed that some ponds were net exporters of females (sources) and others were net importers (sinks). Ovipositing females did not avoid ovipositing in ponds with trout. Overall, the data were consistent with a source-sink dynamic because patches differed in quality, and because there was net migration of individuals from sources to sinks. The results support the hypothesis that local and regional population dynamics are influenced by spatial variation in patch quality, and by the ability of individuals to both disperse among and assess the quality of habitat patches.

title: Patterns of Dispersal and Dynamics among Habitat Patches Varying in Quality
abstract: Both source‐sink theory and extensions of optimal foraging theory (“balanced dispersal” theory) address dispersal and population dynamics in landscapes where habitat patches vary in quality. However, studying dispersal mechanisms empirically has proven difficult, and dispersal is rarely tied back to long‐term spatial dynamics. We used a manipulable laboratory system consisting of bacteria and protozoa to investigate the ability of source‐sink and optimal foraging theories to explain both dispersal and emergent spatial dynamics. Consistent with source‐sink models and contrary to balanced dispersal models, there was a consistent net flux of protist individuals from high to low resource patches. However, unlike the simplest source‐sink models, intermediate rates of dispersal led to highest abundances in low resource patches. Side experiments found strong density dependence in local population dynamics and differences in average protist body size in high and low resource patches. Parameterization and analysis of a two‐patch model showed that high migration from high to low resource patches could have depressed population density in low resource patches, creating pseudosinks. The movement of individuals and biomass from sources to sinks (a form of ecosystem subsidy) resulted in the convergence of body size and population densities in sources and sinks. Our results indicate a need to carefully consider movement patterns and interaction with local dynamics in potential source‐sink systems.

title: Food Webs in Space: An Island Biogeographic Perspective
abstract: All ecologists are familiar with graphical portrayals of food webs such as that shown in Figure 29.1a—tinkertoy constructions of nodes (e.g., species) connected by lines (feeding relations). This depiction of food webs (or, more formally, its matrix equivalent) has without question helped articulate many important questions in community ecology (e.g., Pimm (1982), Pimm et al. (1991), and Cohen et al (1990)). Yet, as with any powerful conceptual schemata in science, this characterization of community organization both liberates—organizing one’s thoughts in fruitful directions—and enslaves—subtly constraining the questions one tends to ask. In particular, most descriptions of, and models about, food web structure make no explicit reference to space. But all ecological interactions, including trophic relations, are necessarily played out in a spatial arena. For some purposes this observation may well be irrelevant. However, it is becoming increasingly clear that the resolution of many classical problems in community ecology, from the coexistence of competitors (e.g., Hanski (1983)), to the stabilization of predator-prey interactions (e.g. Hassell et al. (1991), to the interpretation of species richness patterns (Cornell and Lawton, 1992), requires a consideration of spatial dynamics. Food web ecology, too, should profit from an explicit incorporation of spatial perspectives.

title: Which traits of species predict population declines in experimental forest fragments
abstract: Theory suggests that species with particular traits are at greater risk of extinction than others. We assumed that a decline in abundance in forest fragments, com- pared to continuous forest, equated to an increase in extinction risk. We then tested the relationships between five traits of species and decline in abundance for 69 beetle species in an experimentally fragmented forest landscape at Mt. Wog Wog in southeastern Australia. The experiment was controlled and replicated. Monitoring ran for two years before forest fragmentation; in this paper, we examine data for five years postfragmentation. We tested five hypotheses: (1) Species that occur naturally at low abundance are more likely to decline as a result of fragmentation than are abundant species. (2) Isolated species are more likely to decline than species that are not isolated. (3) Large species are more likely to decline than small species. (4) Species in trophic groups at the top end of food chains are more likely to decline than species in trophic groups lower in the food chain. (5) Because traits are often shared by related species, populations of more closely related species will respond in the same way. We found that: (1) rare species were more likely to decline than abundant species; (2) isolated species were more likely to decline than species that were not isolated; (3) body size was not correlated with response to fragmentation; (4) among species that declined, predators declined most; and (5) taxonomically related species did not respond in the same way to fragmentation. Thus, our results confirm theories predicting that isolated, rare, or predaceous species will be lost first from fragmented landscapes.

title: Effects of landscape context on herbivory and parasitism at different spatial scales
abstract: Local community structure and interactions have been shown to depend partly on landscape context. In this paper we tested the hypothesis that the spatial scale experienced by an organism depends on its trophic level. We analyzed plant-herbivore and herbivore-parasitoid interactions in 15 agricultural landscapes differing in structural complexity using the rape pollen beetle (Meligethes aeneus), an important pest on oilseed rape (Brassica napus), and its parasitoids. In the very center of each landscape a patch of potted rape plants was placed in a grassy field margin strip for standardized measurement. Percent non-crop area of landscapes was negatively related to plant damage caused by herbivory and positively to the herbivores’ larval mortality resulting from parasitism. In a geographic scale analysis, we quantified the structure of the 15 landscapes for eight circular sectors ranging from 0.5 to 6 km diameter. Correlations between parasitism and non-crop areas as well as between herbivory and non-crop area were strongest at a scale of 1.5 km, thereby not supporting the view that higher trophic levels experience the world at a larger spatial scale. However, the predictive power of non-crop area changed only slightly for herbivory, but greatly with respect to parasitism as scales from 0.5 to 1.5 km and from 1.5 to 6 km diameter increased. Furthermore, the effect of non-crop area tended to be stronger in parasitism than herbivory suggesting a greater effect of changes in landscape context on parasitoids. This is in support of the general idea that higher trophic levels should be more susceptible to disturbance.

title: Host-parasitoid extinction and colonization in a fragmented prairie landscape
abstract: Few field studies of natural populations have examined the factors influencing local extinctions and colonization of empty habitat patches for a prey species and its predator. In this study, I carried out a census of planthopper (Prokelisia crocea; Hemiptera: Delphacidae) and egg parasitoid (Anagrus columbi; Hymenoptera: Mymaridae) incidence and densities in 147 host-plant patches (Spartina pectinata; Poaceae) over seven planthopper generations in a tall-grass prairie landscape. For both species, the likelihood of going extinct in a patch was related to a number of patch-specific variables: density, temporal variability in density, proportion of hosts parasitized (planthopper only), host-plant density, patch size, patch isolation, and composition of the surrounding matrix. Colonization likelihood was related only to the physical attributes of the patch. There was high patch turnover in this prairie landscape. On average, planthoppers went extinct in 23% of the patches and A. columbi went extinct in 51% of the patches in each generation. For the planthopper, extinction likelihood increased with a decrease in patch size and the proportion of the matrix composed of mudflat. Parasitism of eggs had no effect on the extinction likelihood of local P. crocea populations, suggesting that A. columbi may not play a major role in the patch dynamics of its host. The likelihood of extinction for A. columbi was dependent on factors that spanned three trophic levels. An increase in plant density, decrease in host density and decrease in parasitoid density all increased the likelihood of A. columbi extinction within a patch. The dependency on multiple trophic levels may explain the higher extinction risk for the parasitoid than its host. A. columbi extinction was also affected by the matrix habitat surrounding the patch—the effect was the opposite of that for P. crocea. Finally, vacant patches were colonized at rates of 53% and 34% per generation for the planthopper and parasitoid, respectively. For both species, colonization probabilities decreased with an increase in patch isolation. High host densities in a patch also favored high rates of colonization by A. columbi. I discuss how anthropogenic changes to the prairie landscape can affect the metapopulation dynamics and persistence time of this host-parasitoid interaction.

title: MOVEMENT AND SPATIAL POPULATION STRUCTURE OF A PRAIRIE PLANTHOPPER
abstract: The transfer of organisms among patches is a key process influencing the spatial structure and regional dynamics of a population; yet, detailed experimental studies of animal movement among patches are uncommon. I performed a series of mark–recapture studies to quantify the movement of a planthopper, Prokelisia crocea (Hemiptera: Delphacidae), among discrete patches of its host plant, prairie cordgrass (Spartina pectinata). Results from these dispersal studies were used to predict the natural distributions and to characterize the spatial population structure of P. crocea. Planthopper emigration loss per patch increased linearly with the density of female conspecifics and was nonlinearly related to patch size (small > large > intermediate sized patches). Planthopper spatial spread was diffusive and 2.7 times faster among cordgrass patches in a heterogeneous habitat (patches embedded in nonhost vegetation) than within a homogeneous habitat (pure cordgrass). Immigration by planthoppers was an increasing function of patch size but was independent of patch isolation (at the scale of this study). The natural distribution of planthoppers in a prairie fragment, obtained from a survey of 146 cordgrass patches over five generations, was well predicted from the dispersal experiments. Planthopper densities and patch occupancy rates were positively correlated with patch size (cordgrass patches ≥0.8 ha were continually occupied), but uncorrelated with patch isolation. Based on this survey, the rate of patch extinction was 21% per generation, highest in small and moderately isolated patches, and approximately equal to the recolonization rate per generation. Finally, the dynamics of local patch populations were asynchronous, even for patch pairs <10 m apart. I conclude that P. crocea exhibits a population structure most closely resembling a mainland–island metapopulation, but with high patch connectivity. Under these circumstances, processes operating within the few mainland patches are probably more important than regional processes (patch extinctions/recolonizations) in influencing population persistence. 
 
 N. J. Gotelli.

title: MATRIX COMPOSITION AFFECTS THE SPATIAL ECOLOGY OF A PRAIRIE PLANTHOPPER
abstract: To date, there is a lack of well-controlled field experiments that disentangle the effects of the intervening matrix from other landscape variables (e.g., patch geography or quality) that might influence animal dispersal among patches. We performed a field experiment to investigate how the movement of a delphacid planthopper (Prokelisia crocea) among discrete patches of prairie cordgrass (Spartina pectinata) is affected by the composition of the matrix (mudflat, native nonhost grasses, and the introduced grass smooth brome [Bromus inermis]). Within each matrix type, marked planthoppers were released onto experimental cordgrass patches that were made identical in size, isolation, and host plant quality. We found that the emigration rate (planthoppers lost per patch per day) was 1.3 times higher for patches embedded in the two nonhost grass matrix types than for patches in mudflat. The rate of immigration (immigrants per patch per day) into patches isolated by 3 m was 5.4 times higher in the brome than in the mudflat matrix. Patches in the native grass matrix had intermediate immigration rates. In addition, a survey of planthopper distributions in nature revealed that both the within- and among-patch distributions of the planthopper were related to the composition of the matrix. Within patches, individuals accumulated against mudflat edges (relative to patch interiors) but not against nonhost grass edges. Among patches, incidence and density increased with the proportion of the matrix composed of open mud. The matrix was equal to that of patch geography (size and isolation) in its ability to explain the distribution of the planthopper. We suggest that the low permeability of the mudflat relative to a nonhost grass edge may explain these planthopper distributional patterns. Also, because natural cordgrass patches in mudflat were richer in nutrients than those in nonhost grasses, planthoppers may have been more likely to remain and build up densities on the former patches. We predict that the displacement of native matrix types by invasive brome will result in increased connectivity and greater spatial synchrony in densities of planthoppers among cordgrass patches.

title: MATRIX HETEROGENEITY AND HOST–PARASITOID INTERACTIONS IN SPACE
abstract: In this study, I experimentally examined how the landscape matrix influenced the movement, oviposition behavior, and spatial distribution of Anagrus columbi, a common egg parasitoid of the planthopper Prokelisia crocea. Both species exist among discrete patches of prairie cordgrass (Spartina pectinata), the sole host plant of P. crocea. Based on out-planted cordgrass pots bearing host eggs (to assess parasitism) or sticky leaves (traps for adult A. columbi), I found that the distribution of adult female A. columbi and pattern of ovipositions within a cordgrass patch were strongly matrix dependent. Female densities were 59% lower on the edge than interior of patches embedded in a mudflat matrix, but were evenly distributed within patches embedded in a matrix consisting of either native grasses or the exotic grass smooth brome (Bromus inermis). In contrast, parasitism was higher in the interior than edge for patches in all three matrix types. The lack of correspondence between A. columbi density and parasitism was attributed to differences in oviposition behavior: A. columbi parasitized 71% more hosts per capita in the interior than edge for patches embedded in nonhost grasses, but equal numbers on the edge and interior of patches embedded in mudflat. Matrix-dependent differences in the within-patch distribution and oviposition behavior of A. columbi can influence the distribution of parasitism risk and host–parasitoid stability at the patch level. 
 
Matrix composition also affected the pattern of movement through the matrix and the colonization of nearby cordgrass patches. Anagrus columbi females emigrating from a mudflat-embedded patch were captured at very low, but constant, numbers with distance out into the matrix, suggesting that they were reluctant to enter or remain in the mudflat. In contrast, A. columbi females entering a nonhost grass matrix had numbers that were high near the patch border and then declined exponentially with distance. These patterns of movement were likely responsible for the very different colonization rates for experimental patches embedded in different matrix types and located 3 m from a source patch of A. columbi. Patches embedded in brome were colonized at a rate that was 3.0 and 5.7 times higher than for patches in native grass or mudflat, respectively. Finally, based on a census of cordgrass patches spanning five generations, A. columbi densities and proportion of patches occupied generally increased with increasing host density, patch isolation, and the proportion of the surrounding matrix that was mudflat. Patch size had no effect on the distribution of A. columbi. Overall, these data suggest that cordgrass patches in a nonhost grass matrix, particularly smooth brome, have high connectivity relative to patches in a mudflat matrix. Changes in connectivity due to changes in matrix composition can significantly influence host–parasitoid persistence at the metapopulation level. 
 
Corresponding Editor (ad hoc): J. A. Rosenheim.

title: PATCH STRUCTURE, OVIPOSITION BEHAVIOR, AND THE DISTRIBUTION OF PARASITISM RISK
abstract: To date, almost no experimental field studies have attempted to assess the factors that generate heterogeneity in the distribution of parasitism risk, a putative indicator of host–parasitoid stability. In this study, I examined the interaction between a planthopper Prokelisia crocea and its egg parasitoid Anagrus columbi among discrete patches of prairie cordgrass, Spartina pectinata. In particular, I examined how patch geography and host distribution within a patch influenced the distribution of adult parasitoids, parasitoid oviposition behavior, the proportion of hosts parasitized, and the aggregation of parasitism (cv2) and adult parasitoids. 
 
Based on six generations of census data, the distribution of parasitism was strongly aggregated within (cv2 = 3.58), but not among (cv2 = 0.58) cordgrass patches. Parasitism was also spatially and temporally density independent. To determine what influences the distribution of parasitism risk, I selected 26 discrete cordgrass patches, removed all sources of A. columbi, and then quantified the immigration and subsequent oviposition behavior of A. columbi colonists. I found that the number of immigrants significantly increased with patch size and decreased with patch isolation. Patch size had no influence on the per capita hosts parasitized per leaf, but there was a significant twofold increase in per capita attacks from the least to the most isolated patches. The isolation effect was likely due to an optimal oviposition response to dispersal distance by A. columbi. For these experimental patches, substantial within-patch aggregation of parasitism (cv2 = 1.63) did not translate into strong among-patch aggregation (cv2 = 0.13). Searching adult parasitoids were randomly distributed within and among patches and thus did not explain the high cv2 within patches. Interestingly, the aggregation of parasitism risk within a patch was significantly negatively correlated with patch size and positively correlated with patch isolation. The distribution of parasitism risk could be divided into two general components. The within-parasitoid component was attributable to individuals engaging in multiple ovipositions within a leaf and the distance-dependent oviposition response. The latter response was likely the cause for the variation in cv2 with respect to patch size and isolation. Within-parasitoid aggregation has no effect on host–parasitoid stability. The among-parasitoid component of aggregation appears to have been due to heterogeneity in the vulnerability of hosts and an edge effect (parasitism risk is 60% more heterogeneous at the edge than interior of a patch) and is in theory stabilizing. Consequently, a change in landscape structure that leads to an increase in cordgrass edge habitat may promote a more stable host–parasitoid interaction. 
 
 R. F. Denno.

title: The Persistence of Host-Parasitoid Associations in Patchy Environments. I. A General Criterion
abstract: In this article we show that, for host-parasitoid interactions in a heterogeneous environment and with discrete generations, the dynamic effects of any patterns of distribution of searching parasitoids can be assessed within a common, simple framework. The populations are regulated if the distribution of searching parasitoids is sufficiently heterogeneous. Specifically, the square of the coefficient of variation (CV2) of the searching parasitoids per host must exceed unity. This criterion is demonstrated to apply approximately, in general and also in several specific cases. We further show that CV2 may be partitioned into a density-dependent component caused by the response of parasitoids to host density per patch and a density-independent component. Population regulation is enhanced as much by density-independent as by density-dependent heterogeneity.

title: Calopteryx Damselfly Dispersions Arising from Multiscale Responses to Landscape Structure
abstract: Study landscapes Field surveys Simulation models Model evaluation Simulation experiment Results Field surveys Model evaluation Simulation experiment Discussion Empirical distributions Simulations Speculation Responses to this Article Acknowledgments Literature

title: Finding habitat patches and directional connectivity
abstract: For animal species inhabiting patchy environments, the search behavior of individuals and the distance from which they can detect suitable habitat (perceptual range) are key determinants of the functional connectivity of landscapes. We examined the movement behavior and perceptual range of adult cactus bugs (Chelinidea vittiger), which are habitat specialists that feed and reproduce on Opuntia cactus. Movement pathways of walking individuals released into unsuitable matrix habitat (30-3000 m from Opuntia) were highly directional. These results supported predictions of optimal search behavior from published simulation models. A release experiment within natural patch networks indicated that the perceptual range of C. vittiger depended on size of the target patch, matrix structure, and direction of the target patch relative to prevailing winds. A strong effect of wind direction on orientation behavior (and presumed perceptual range) was evident in a release experiment using 'artificial' patches of potted Opuntia. In these two experiments, individuals released 50--100 cm from Opuntia patches were more likely to orientate toward patches located upwind than to those located crosswind or downwind. A reexamination of the pathways of individuals walking in the matrix also revealed a strong bias for movement upwind. Such upwind movement by individuals, both within and outside of patch networks, suggests that C. vittiger uses olfaction to navigate and it complicates our ability to interpret search behavior and to estimate perceptual range. Current techniques for assessing perceptual range have limitations for olfactory-based species. Furthermore, we need to broaden our view of perceptual range and of patch and landscape connectivity. Perceptual range may be anisotropic and connectivity may be directional. An organism-based approach to spatial ecology requires that we consider the dominant senses used by species when navigating around patchy landscapes.

title: Evolution of stepping-stone dispersal rates
abstract: We present a general model of the evolution of dispersal in a population with any distribution of dispersal distance. We use this model to analyse evolutionarily stable (ES) dispersal rates for the classical island model of dispersal and for three different stepping‐stone models. Using general techniques to compute relatedness coefficients in the different dispersal models which we consider, we find that the distribution of dispersal distance may affect the ES dispersal rate when the cost of dispersal is low. In this case the ES dispersal rate increases with the number of demes that can be reached by one dispersal event. However, for increasing cost the ES dispersal rate converges to a value independent of the distribution of dispersal distance. These results are in contrast to previous analyses of similar models. The effects of the size (number of demes) and shape (ratio between the width and the length) of the population on the evolution of dispersal are also studied. We find that larger and more elongated populations lead generally to higher ES dispersal rates. However, both of these effects can only be observed for extreme parameter values (i.e. for very small and very elongated populations). The direct fitness method and the analytical techniques used here to compute relatedness coefficients provide an efficient way to analyse ES strategies in subdivided populations.

title: DENSITY-DEPENDENT COMPENSATION IN BLOWFLY POPULATIONS GIVE INDIRECTLY POSITIVE EFFECTS OF A TOXICANT
abstract: Effects of the toxicant cadmium were studied in blowfly populations, to test whether this abiotic factor could produce indirectly positive effects on demographic rates by interacting with density-dependent processes. A previous study has demonstrated that although cadmium is toxic to blowfly individuals, cadmium exposure of whole blowfly populations nevertheless results in increased mean pupal mass and biomass production. Moreover, statistical modeling of demographic rates has indicated that cadmium-exposed populations have higher larva-to-adult survival. The aim of the present study was to test this model prediction, and to describe underlying density-dependent mechanisms. Blowflies were exposed to cadmium in both low and high densities, in two subsequent generations. In the first generation (low densities) all demographic rates were reduced by cadmium exposure, whereas in the second generation (high densities), survival of old larvae and pupae was higher in cadmium-exposed populations than in control populations. The in- creased survival was due to reduced larval competition, which resulted from lower repro- duction and lower survival of young larvae. Hence, the indirectly positive effects of the toxicant were caused by density-dependent compensatory reactions.

title: The influence of species identity and herbivore feeding mode on top-down and bottom-up effects in a salt marsh system
abstract: In this study we investigated the potential importance of species identity and herbivore feeding mode in determining the strengths of top-down and bottom-up effects on phytophagous insect densities. In 1998, we conducted two factorial field experiments in which we manipulated host plant quality and intensity of parasitoid attack on three salt marsh herbivores, the planthoppers Prokelisia marginata and Pissonotus quadripustulatus (Homoptera: Delphacidae), which feed only on Spartina alterniflora and Borrichia frutescens, respectively, and the gall fly Asphondylia borrichiae (Diptera: Cecidomyiidae), which feeds only on B. frutescens. We increased plant quality through addition of nitrogen fertilizer, and decreased parasitism by trapping hymenopteran parasitoids continuously throughout the study. Herbivore densities were censused biweekly. Increasing plant quality through fertilization increased the density of all three herbivores within 2 weeks of treatment application, and higher densities were maintained for the duration of the study. Reduction of top-down pressure had no effect on either planthopper species, possibly because of compensatory mortality affecting the two species. In contrast, reduction of parasitism significantly increased the density of A. borrichiae galls, perhaps because development within gall tissue reduces the sources of compensatory mortality affecting this species. The results of this study show that the bottom-up effects of plant quality were strong and consistent for all three species, but the strength of top-down effects differed between the two feeding guilds. Thus, even for herbivores feeding on the same host plant, conclusions drawn regarding the relative importance of top-down and bottom-up effects may vary depending upon the feeding mode of the herbivore.

title: The effects of habitat fragmentation on persistence of source-sink metapopulations in systems with predators and prey or apparent competitors.
abstract: We consider systems with one predator and one prey, or a common predator and two prey species (apparent competitors) in source and sink habitats. In both models, the predator species is vulnerable to extinction, if productivity in the source is insufficient to rescue demographically deficient sink populations. Conversely, in the model with two prey species, if the source is too rich, one of the prey species may be driven extinct by apparent competition, since the predator can maintain a large population because of the alternative prey. Increasing the rate of predator movement from the source population has opposite effects on prey and predator persistence. High emigration rate exposes the predator population to danger of extinction, reducing the number of individuals that breed and produce offspring in the source habitat. This may promote coexistence of prey by relaxing predation pressure and apparent competition between the two prey species. The number of sinks and spatial arrangement of patches, or connectivity between patches, also influence persistence of the species. More sinks favor the prey and fewer sinks are advantageous to the predator. A linear pattern with the source at one end is profitable for the predator, and a centrifugal pattern in which the source is surrounded by sinks is advantageous to the prey. When the dispersal rate is low, effects of the spatial structure may exceed those of the number of sinks. In brief, productivity in patches and patterns of connectivity between patches differentially influence persistence of populations in different trophic levels.

title: Biological invasions : a global perspective
abstract: History of biological invasions with special emphasis on the old world invasions of natural ecosystems by plant pathogens patterns, modes and extents of invasion by vertebrates temperate grasslands vulnerable to plant invasions - characteristics and consequences the characteristics of invaded mediterranean-climate regions wildlife conservation and the invasion of nature reserves by introduced species - a global perspective ecosystem-level processes and the consequences of biological invasions theories of predicting success and impact of introduced species the nature and effects of disturbance relative to invasions.

title: A survey and overview of habitat fragmentation experiments.
abstract: Habitat destruction and fragmentation are the root causes of many conservation problems. We conducted a literature survey and canvassed the ecological community to identify experimental studies of terrestrial habitat fragmentation and to determine whether consistent themes were emerging from these studies. Our survey revealed 20 fragmentation experiments worldwide. Most studies focused on effects of fragmentation on species richness or on the abundance(s) ofparticular species. Other important themes were the effect offragmentation in interspecific interactions, the role of corridors and landscape connectivity in in- dividual movements and species richness, and the influences of edge effects on ecosystem services. Our com- parisons showed a remarkable lack of consistency in results across studies, especially with regard to species richness and abundance relative to fragment size. Experiments with arthropods showed the best fit with the- oretical expectations of greater species richness on larger fragments. Highly mobile taxa such as birds and mammals, early-successional plant species, long-lived species, and generalist predators did not respond in the "expected" manner. Reasons for these discrepancies included edge effects, competitive release in the habitat fragments, and the spati.al scale of the experiments. One of the more consistently supported hypotheses was that movement and species richness are positively affected by corridors and connectivity, respectively. Tran- sient effects dominated many systems;,for example, crowding of individuals on fragments commonly was ob- served afterfragmentation, followed by a relaxation toward lower abundance in subsequentyears. The three long-term studies (?14 years) revealed strong patterns that would have been missed in short-term investiga- tions. Our results emphasize the wide range of species-specific responses to fragmentation, the need for eluci- dation of behavioral mechanisms affecting these responses, and the potentialfor changing responses to frag- mentation over time.

title: Finding and Losing Host Plants by Phyllotreta: Patch Size and Surrounding Habitat
abstract: This study examined the effects of size and surrounding habitat of food plant patches on the density, immigration, and emigration of two specialized phytophagous insects. Two flea beetles, Phyllotreta cruciferae and P. striolata, were censused for 3 wk in host plant patches of four sizes, with background vegetation of three types. Densities of both beetle species declined with decreasing patch size. Single isolated host plants (collards) were without flea beetles because of high rates of emigration. By releasing and recapturing marked flea beetles, I found that the emigration rates of both beetle species consistently increased with decreasing patch size. This sensitivity of emigration to host patch size may be common among phytophagous insects because of simple changes in perimeter: area ratios. When this is the case there may be a "critical patch size" below which host islands do not sustain herbivore populations. Immigration rates were also affected by patch size and habitat, but only in P. striolata, not in P. cruciferae. Even in P. striolata, variation in immigration was less consistent and less dramatic than was variation in emigration. Beetles wandering out of patches apparently do not readily return to the host patch. To quantify how far beetles would have to wander before "losing" their host patch, I released marked beetles in two different habitats (a goldenrod field or a cultivated garden) at varying distances (2, 4, 6, or 8 m) from a target patch of collards. For both beetle species, the percentage of individuals discovering the collards after 1 d declined rapidly beyond 2 m and was dramatically reduced by the presence of goldenrod between released beetles and the collard target.

title: Modelling the effect of host patch size on Mexican bean beetle emigration
abstract: The resource concentration hypothesis states that specialized herbivores are more likely to find and remain in pure, large, and/or dense stands of their host plants. This study examined whether in fact herbivores tended to remain longer in large vs. small patches of host plants and attempts to explain the results by modelling herbivore movement. Both movement within a patch and emigration of marked Mexican bean beetles (MBBs) were studied by releasing beetles into small (19 plants) and large (61 plants) patches. Beetles left small patches at a faster rate than large patches, but the difference was not statistically significant. A stochastic model that was developed translates individual patterns of MBB movement into population patterns of beetle distribution within a patch, and predicts how this distribution changes with time. Estimating and testing a model on two separate sets of data provides a more rigorous test than applying the model to the data set on which it was developed. The parameters of the model, therefore, were estimated from data collected in the large patches, and the model was tested by comparing the predicted with the observed spatial distributions of beetles in small patches (i.e., a novel situation). Predicted distributions were very similar to the observed distributions; the predicted proportion of insects remaining in a patch at each census time differed from the observed by < 10% on the average. The model predicts that MBB emigration rate should decrease with patch size, con- firming the observed trend.

title: TOWARD AN INTEGRATION OF LANDSCAPE AND FOOD WEB ECOLOGY : The Dynamics of Spatially Subsidized Food Webs
abstract: We focus on the implications of movement, landscape variables, and spatial heterogeneity for food web dynamics. Movements of nutrients, detritus, prey, and consumers among habitats are ubiquitous in diverse biomes and can strongly influence population, consumer-resource, food web, and community dynamics. Nutrient and detrital subsidies usually increase primary and secondary productivity, both directly and indirectly. Prey subsidies, by movement of either prey or predators, usually enhance predator abundance beyond what local resources can support. Top-down effects occur when spatially subsidized consumers affect local resources by suppressing key resources and occasionally by initiating trophic cascades. Effects on community dynamics vary with the relative amount of input, the trophic roles of the mobile and recipient entities, and the local food web structure. Landscape variables such as the perimeter/area ratio of the focal habitat, permeability of habitat boundaries, and relative productivity of trophically connected habitats affect the degree and importance of spatial subsidization.

title: Empirical evidence for complex source-sink dynamics with alternative states in a butterfly metapopulation
abstract: In heterogeneous landscapes, a species’ habitat may be partitioned into sources and sinks. Conceptually, three kinds of habitat have been described: (1) “sources” are consistent net exporters of organisms; “true sinks” are net importers, and without immigration their populations go extinct; and (3) “pseudosinks” are also net importers, but without immigration they can sustain populations and sometimes can even become net exporters (sources). Previously, I have described sources and pseudosinks in a metapopulation of the butterfly Euphydryas editha and reported the extinction of populations in sources due to an unusual frost. Here, I describe the recolonization of the former sources by migrants from extant populations in the former pseudosinks. For comparison, a series of vacant patches was created in the former pseudosinks. Recolonization was tracked by sampling the population density in the vacant patches. Patches were sampled for 5 years. 
 
Contrary to theoretical expectations, the establishment rate of new populations was 10 times higher in outcrops, the former pseudosinks, than in clearings, the former sources. Initial population density was 150 times higher in outcrops (measured as number of larval webs per square meter). Yet in clearings, only the immigrants had poor reproductive success. On those occasions when a resident population was established, the residents had high reproductive success, and the population grew rapidly. The low recolonization rate of clearings could not be attributed to effects of patch size or spatial barriers. A temporal barrier was hypothesized, in which immigrants arrived too late each year to reproduce successfully on host plants in clearings. The host plant in clearings, Collinsia torreyi, typically senesced during the breeding season, but the host plant in outcrops, Pedicularis semibarbata, did not. The hypothesis proposed that immigrants oviposited later than residents because the immigrants originated in outcrops, which had a different microclimate that delayed adult eclosion. The hypothesis was supported: the newly established resident populations in clearings tended to oviposit 10 d earlier than populations in nearby outcrops. An experiment showed earlier eclosion times in clearings than in outcrops. Another experiment showed much higher larval survival in early clearings than in late clearings. Mortality was correlated with host-plant senescence. 
 
The temporal barrier was strong enough to keep clearings as net importers of butterflies from outcrops, meaning that the net flow of butterflies reversed after the frost destroyed the resident populations in clearings. Thus, “early” clearings had been sources, but “late” clearings were true sinks, giving the system two kinds of source–sink relationships. After the frost, the metapopulation as a whole underwent a demographic source–sink inversion from one locally stable state to the other. The findings suggest that asymmetric constraints on dispersal, due to temporal structure, may be a mechanism for complex source–sink dynamics in herbivorous insects and possibly other animals.

title: Modelling the spatial dynamics and persistence of the leaf beetle Gonioctena olivacea in dynamic habitats
abstract: In dynamic landscapes natural and anthropogenic disturbance as well as succession are responsible for the emergence and subsequent disappearance of suitable habitat patches. Species inhabiting such landscapes are faced with varying number and spatial configuration of patches. A stochastic, spatially explicit simulation model was developed in order to analyse the persistence of the leaf beetle Gonioctena olivacea in a system of dynamic patches of its host plant Cytisus scoparius. The model was parameterized with data from a three-year field study on the spatial configuration, distribution, and turnover of the host plant patches as well as the patch occupancy, extinction, and colonization rates of the beetle. The simulations showed large fluctuations in the occurrence of the beetle in the patches. High levels of occupancy were related to high aggregation of the patches within the landscape. The velocity of patch turnover was found to have a severe effect on the persistence of the beetle metapopulation. Enhancing the turnover rate by only a few patches, the mean time to extinction decreases rapidly. Moreover, the results revealed that not necessarily an effect of connectivity can be detected in the analysis of occupancy patterns in dynamic landscapes, although the colonization of patches is clearly connectivity-dependent. In general, this modelling study demonstrates the importance of detailed information on patch turnover. The amount and spatial distribution of suitable habitat is a major driver of metapopulation dynamics of species in dynamic landscapes.