bluuebunny/crossref_metadata_2025 huggingface dataset:
Life History and Demography of the Common Mud Turtle Kinosternon Subrubrum in South Carolina, USA
Authors: Frazer Nat B. , Gibbons J. Whitfield , Greene Judith L.
Published: May 2006

This paper presents a life table for the common mud turtle, Kinosternon subrubrum, in a fluctuating aquatic habitat on the Upper Coastal Plain of South Carolina, USA, using data gathered in a 20—yr mark—recapture study. Data on survivorship and fecundity (clutch size, per capita clutch frequency) were assessed and compared to previously published life table statistics for the slider turtle, Trachemys scripta, in the same body of water and for the yellow mud turtle, K. flavescens, in Nebraska. The annual survival rate for adult female Kinosternon (87.6%) is significantly higher than that of adult female Trachemys (77.4%). Similarly, male Kinosternon exhibit an annual survival rate (89.0%) significantly higher than that of male Trachemys (83.4%). The mean annual proportion of female Kinosternon that are reproductively active (50.7%) also is significantly higher than that of Trachemys (37.2%). In addition, survival rate from the time eggs are laid by Kinosternon until the hatchlings enter the aquatic environment (26.1%) is significantly higher than that for Trachemys (10.5%). Comparisons of our findings with those for K. flavescens indicate that these geographically separate populations of congeneric species also differ substantially in age at maturity, mean generation time, and the mean proportion of females that are reproductively active in any given year. Differences were also apparent in mean clutch frequencies and adult survival rates. The differences in life history traits between the two geographically separated populations of congeners seem to be as great as those between the two syntopic populations representing different families (Kinosternidae: K. subruburm and Emydidae: Trachemys scripta). The comparison of life tables for two species from different families having different ecological and evolutionary histories, but living in the same habitat, and of congeneric species in different habitats, is instructive regarding the biological flexibility of species under natural conditions. However, the study suggests that environmental variability has a greater effect on life table statistics than do phylogenetic relationships.
The Effect of Ontogeny on Interspecific Interactions in Larval Amphibians
Authors: Sredl Michael J. , Collins James P.
Published: May 2006

We used 3 x 2 x 2 factorial experiments in a temporary pond to assess the relative importance of predator density (0, 2, or 4 salamander larvae), prey density (20 or 40 tadpoles), and time of prey introduction (early or late) on interactions between predaceous larval salamanders (Ambystoma tigrinum) and their anuran prey (Pseudacris triseriata). We examined how these factors and their interactions affected mass at metamorphosis, length of larval period, growth rate, and survival. These experiments demonstrated the effect of body size on predator—prey interactions by manipulating time of introduction of size—structured prey in a system with a gape—limited predator. Microhabitat differences in primary productivity, sources of predation other than salamanders, and abiotic factors were important determinants of community structure. High levels of salamander predation negatively affected tadpole survival, but affected no other response variable. We detected no density—dependent effects in either taxon. Tadpoles in the early treatment had longer larval periods and lower survival rates than those in the late treatment. Interactions of factors, including time of introduction, were common. Biotic and abiotic factors differentially affecting growth of predator or prey appear to be responsible for these differences. Priority effects in arrival to breeding sites, or climatic events affecting the growth rate of predator, prey, or both, play a role in determining community structure. Interactions of factors complicate the predator—prey relationship. In our experiment, the effects of time of introduction are highly correlated with body size in the two temporal treatments. Our results indicate that more realistic models of predator—prey systems, or population dynamics, should incorporate size structure of predator and prey.
Environmental Causes of Correlations between Age and Size at Metamorphosis in Scaphiopus Multiplicatus
Authors: Pfennig David W. , Mabry Alan , Orange David
Published: May 2006

We examined environmental effects on the correlation between age and size at metamorphosis in spadefoot toad tadpoles (Scaphiopus multiplicatus). These tadpoles occur in ponds of variable duration as two environmentally induced, trophic morphs: a large, rapidly developing carnivore morph and a smaller, more slowly developing omnivore morph. Drawing on data from both natural ponds and artificial pools we examined for both morphs the environmental determinants of, and the intercorrelations among, three life history parameters: age at metamorphosis, size at metamorphosis, and survivorship (both pre— and postmetamorphic). Censuses of four natural ponds revealed that age and size at metamorphosis for both morphs were inversely related, in contrast to many other amphibians and insects with complex life cycles. In artificial pools in which we simultaneously varied two diet components and pond duration, age and size at metamorphosis were negatively correlated in high food treatments and positively correlated in low food, long pond duration treatments. Ephemeral larval habitats should favor rapid differentiation; achieving large body size depends upon the quality and quantity of available food and the time available to develop. A negative relationship between age and size at metamorphosis may occur often S. multiplicatus since this species is exposed to a rapidly diminishing food resource (anostracans). We predict that other organisms with complex life cycles faced with these conditions would possess an inverse relationship between size and age at metamorphosis.
Long‐Path Ftir Measurement of Atmospheric Trace Gas Concentrations
Authors: Gosz James R. , Dahm Clifford N. , Risser Paul G.
Published: May 2006

Extending the ecological knowledge base to larger scales will require new measurement tools. Long—path Fourier—Transform Infrared Spectroscopy (FTIR) is a promising technology because of its ability to measure the concentration of multiple trace gases simultaneously over spatial scales up to 1 km. Detection limits reaching 1μL/KL for numerous gases make this a powerful method with the potential to study spatial patterning of biological processes, fluxes between ecosystems, important processing pathways within terrestrial and aquatic ecosystems, and scale—dependent processing phenomena. Repeated measurements down to the second range add excellent temporal capabilities. This analytical tool, drawing on the latest developments in analytical chemistry, expands the temporal and spatial scales over which ecosystem nutrient cycling energy flow, and biospheric/atmospheric interactions can be studied.
Comparing Fish and Urchin Grazing in Shallow and Deeper Coral Reef Algal Communities
Authors: Morrison Douglas
Published: May 2006

Algal—herbivore (fish and sea urchin) interactions in shallow (<10 m) and deeper (>15 m) coral forereef communities off Discovery Bay, Jamaica, were investigated and compared. Algal functional form groups exhibited different bathymetric distribution patterns. Crustose corallines, the most abundant group in shallow water, decreased in abundance with increasing depth, whereas erect macroalgae increased in abundance with depth, predominating in deeper water. This distribution pattern was correlated with a decline in herbivory over depth. The hypothesis that herbivory affects algal abundance and distribution was tested using grazer exclusions and algal transplants. Five 1—m2 exclusion fences, which excluded the sea urchin Diadema antillarum but not fish, were erected in both shallow (7—8 m) and deeper (17—18 m) water. Removal of Diadema resulted in a pronounced change in the shallow algal community; erect and filamentous algae rapidly overgrew crustose corallines in 2 mo. In contrast, Diadema exclusion had little effect on the deeper algal community. In another experiment, which prevented both fish and urchin grazing, unaltered natural substrates with attached algae were suspended on racks in the water column 30—40 m from the reef at depths corresponding to the fences. Abundances of erect and filamentous algae increased on both shallow and deeper racks. The erect species that increased in abundance in the fences were resistant to fish, whereas only fish—susceptible species increased on the racks. Transplanting the most abundant erect species from the reef crest (Laurencia obtusa, Caulerpa racemosa) and deeper forereef (Lobophora variegata, Dictyota divaricata, Halimeda goreauii) to the shallow forereef revealed that all, except L. obtusa, were readily consumed by Diadema, but all exhibited low susceptibility to fishes. The reef crest and deeper reef are spatial refuges from urchin predation for erect macrophytes. In summer 1983, the Diadema population experienced >95% mortality. On the shallow reef, erect and filamentous algae and fish grazing intensity increased substantially following urchin death, while crustose corallines declined. Conversely, Diadema mortality had little effect on the deeper reef. Algal responses were similar to those observed in the urchin exclusion experiments. The erect species that increased most after mortality are resistant to fishes via chemical defenses. These findings demonstrate that herbivory is largely responsible for the algal distribution pattern on the reef. The results suggest that Diadema plays a major role in structuring the shallow community, but only a minor role relative to fishes in structuring the deeper community. They also indicate that different herbivores can have a differential effect on erect macroalgal composition.
Nutrient Incluence on a Stream Grazer: Orthocladius Microcommunities Respond to Nutrient Input
Authors: Hershey Anne E. , Hiltner Allison L. , Hullar Meredith A. J. , Miller Michael C. , Vestal J. Robie , Lock Maurice A. , Rundle Simon , Peterson Bruce J.
Published: May 2006

A whole—stream enrichment experiment of phosphorus and, further down—stream, of phosphorus and nitrogen, allowed us to examine the growth and density responses of the tube—building larval chironomid Orthocladius rivulorum to nutrient enrichment of the Kuparuk River in arctic Alaska, and to evaluate nutrient effects on the tube microbial community. The larva feeds by grazing a diatom monoculture of Hannaea arcus from the tube exterior, thus direct nutrient effects on the tube microbiota may translate into indirect nutrient effects on the larva. Electron microscopy indicated that tube silk was formed into a sheet, with a filamentous substructure that repeated at 50—nm intervals. Bacterial micro—colonies occurred at the points where the erect diatoms were attached to the silk. Microbial activity of Orthocladius tubes in the P—fertilized section was 2—3 times that of the control section of the river, and total microbial biomass in the P—fertilized section was 3—4 times that of the control section. Chlorophyll a was also higher on Orthocladius tubes downstream of both P and N + P fertilization sites. However, the rate of biomass accumulation on tubes was more rapid downstream of N + P addition, suggesting primary P and secondary N limitation of the rate of primary production in the river. Chlorophyll a was higher on tubes than on rocks or experimental tiles, which indicated that tubes were a more favorable algal habitat for Hannaea. Pupal tubes had less chlorophyll a than larval tubes, suggesting that larval activity may have contributed to the higher algal biomass on tubes. Orthocladius benefitted from the enhanced tube flora; larvae grew larger in the fertilized sections of the stream than in the upstream sections. The results suggest that Orthocladius with its tube and associated biota function as microcommunities that respond directly and indirectly to the surrounding nutrient regime, but have considerable trophic independence from surrounding portions of the epilithon. They may constitute 12—43% of total epilithic algal biomass.
Resource Productivity‐Consumer Species Diversity: Simple Models of Competition in Spatially Heterogeneous Environments
Authors: Abrams Peter A.
Published: May 2006

The relationship between resource productivity and the number of consumer species that can coexist in a patchy environment was examined using simple models of competition. The analysis seeks to determine when diversity should increase with productivity, and when there should be a unimodel relationship between species diversity and resource productivity. The environment is assumed to consist of a number of independent patches in which competition occurs for one or two resources. In one—resource models, productivity—diversity relationships are increasing unless certain types of interspecific competition other than resource depletion occur. In two—resource models, differentiation with respect to use of the resources provides an additional mechanism for coexistence. The effects of productivity on coexistence conditions via this mechanism are examined using four different models. The models differ in whether resources are nutritionally substitutable or nonsubstitutable, and in whether the consumer species adjust the relative values of their two consumption rate constants to maximize fitness. Results for each model suggest that both increasing and unimodal productivity—diversity relationships are possible, depending on the type of resource growth and the definition of productivity. The conditions for coexistence of three species suggest that resource differentiation alone is unlikely to account for high species diversity, especially in the case of nonsubstitutable resources. This analysis makes different predictions than that of Tilman (1982) for several reasons; a major factor is that Tilman (1982) assumes that increasing productivity implies a decreasing coefficient of variation in productivities. Because there are many processes that can produce unimodal and increasing productivity—diversity curves, one cannot infer the mechanism of competition from this relationship.
Does Asynchronous Fruit Ripening Avoid Satiation of Seed Dispersers?: A Field Test
Authors: Gorchov David L.
Published: May 2006

Asynchronous ripening of fruits within individual, temperate, summer—fruiting, bird—dispersed plants has been proposed to be an adaptation to avoid satiation of dispersers when they are not abundant. I tested a prediction of this hypothesis by manipulating ripening synchrony on individual Amelanchier arborea trees and comparing ripe fruit removal rates from these trees to paired controls. Artificially synchronous ripe fruit displays were created by cutting of unripe fruit and replacing them with ripe fruit. Fruit removal was faster for the manipulated, synchronous tree in three pairs, slower in two pairs, and not significantly different in five other pairs, suggesting that synchronous displays did not satiate dispersers. This unexpected result is attributed to violation of the unstated assumption that dispersers are relatively sedentary in their foraging in the summer and do not concentrate at large fruit displays. In this study the major frugivore of A. arborea, the Cedar Waxwing (Bombycilla cedrorum), foraged widely in flocks of 2—6; other reports reveal that some other frugivorous bird species also forage widely in the summer. This nonterritorial foraging implies that the arrival of dispersers is temporally unpredictable from the perspective of each plant, and it is this temporal unpredictability that may favor asynchronous ripening in plants lacking adaptations for ripe fruit persistence.
Fine‐Scale Spatial and Temporal Variation in the Demography of a Perennial Bunchgrass
Authors: Moloney Kirk A.
Published: May 2006

Most life history studies assume that demographic transition rates within a population are temporally and spatially invariant. A violation of these assumptions may have important consequences in the analysis of long—term processes such as population growth and stability. To examine the potential problems, I studied the demography of a size—classified population of Danthonia sericea, a long—lived grass species, using standard matrix analysis techniques. The study was conducted for two June—to—June transition periods at five locations along a well defined soil/vegetation gradient in a field located in Durham, North Carolina. Demographic transition rates, and predictions of long—term population growth, differed markedly between transition periods and among locations, with the biggest differences being attributable to location. Elasticity analysis indicates that, in all cases studied, a large proportion of long—term population growth can be ascribed to demographic processes associated with individuals in the largest size classes. However this aspect of the demography of Danthonia sericea does not differ appreciably among locations of between years and, consequently, does not explain the observed demographic variability. I argue that some of the variability arises from differences in the rate of recruitment of smaller individuals into the large size classes. Although elasticity analysis indicates that the raw rate of recruitment through reproduction is of little importance to population growth, germination and early establishment success may play a significant role in determining long—term population growth rates, particularly if good recruitment years are infrequent. This can be shown if more than one transition period is considered explicitly in the demographic analysis. It is suggested that future demographic studies account for year—to—year variation in transition rates by incorporating stochastic matrix theory in the analysis of long—term population processes.
Ontogenetic Reaction Norms in Lobelia Siphilitica (Lobeliaceae): Response to Shading
Authors: Pigliucci Massimo , Schlichting Carl D.
Published: May 2006

Research on the genetics of development and on reactions to environmental changes (phenotypic plasticity) is important to understand the evolution of natural populations, mostly because of the possible adaptive meaning of some plastic responses, and because of the suggested role of plasticity in maintaining genetic variation. In this paper, we present a study of developmental norms of reaction of two populations of Lobelia siphilitica (Lobeliaceae) to three levels of light availability. We investigated: (1) the plasticity to light of characters determining phenological timing and of end—of—season traits, and (2) the shape of the ontogenetic reaction norms of plant height and number of leaves in the (Time, Environment, Phenotype) space. We found genetic variation for bolting time, fruit production, final leaf number, rate of stem elongation, and rate of leaf production. We detected plasticity for final height, number of leaves, and rates of height growth and leaf production, and genetic variation for plasticity of height growth rate. The ontogenetic reaction norms for leaf number reveal differences among populations and genotypes both in their average (i.e., across environments) ontogenies and in the plastic responses of the ontogenetic trajectories. The ontogenetic reaction norms for plant height, on the other hand, show only a significant Time by Treatment by Genotype effect. The variation among genotypes for both characters can be due to differences in either the rate of growth or its duration. In addition, the same final phenotype may be produced by different ontogenetic trajectories, e.g., rapid early height growth and short duration vs. slower growth and longer duration. The response to selection on plant height or leaf number will depend on the degree of separation in their genetic control and on the amount of genetic variation for these aspects of development. These results suggest that including ontogenetic data in plasticity studies broadens considerably our understanding of how phenotypes may respond to selection and evolve.
A Simulation Model for Shrub Ecosystem in the Semiarid Karoo, South Africa
Authors: Wiegand Thorsten , Milton Suzanne J. , Wissel Christian
Published: May 2006

Plant community dynamics in semiarid regions appear to be "event—driven." The aim of our model is to attain an understanding of the main processes determining the spatial and temporal dynamics of a shrub community in the semiarid Karoo on a large temporal scale and to identify the significant events that drive this shrub community. Vegetation at the study site covers 15—20% of the soil surface and is dominated by five shrubs: Brownanthus ciliatus (Mesembryanthemoideae), Ruschia spinosa (Ruschioideae), Galenia fruticosa (Aizoaceae), Pteronia pallens (Asteraceae), and Osteospermum sinuatum (Asteraceae). Grasses and annuals play little part in the dynamics of this vegetation. The model is based on detailed life history data for the five dominant species and on monthly long—term rainfall data for this region. The method of "dynamic automata" is employed to model individual plants. Growth, death, seed production, germination, and seedling establishment are modelled over long time scales in annual time steps under the influence of the stochastic ad unpredictable rainfall in ungrazed rangeland. In the absence of grazing, survival of seedlings depends on their competitive ability during the seedling stage and their ability to compete with established plants in neighboring cells. The model shows that the dynamics of this shrub community are typified by episodic and discontinuous changes in species composition with intervening quasistable phases lasting some decades. The reason for this episodic behavior is that both recruitment and mortality of plants depend on particular conditions. Using 93 complete data sets with monthly rainfall data taken at the weather station in Prince Albert we show that rainfall is only sufficient for seedling recruitment in 44% of all years for B. ciliatus and in < 30% of all years for the four other species. We identify two types of abrupt and discontinuous changes in species composition: (1) big recruitment events that can only occur if plant density is low and if rainfall conditions are extraordinarily favorable and (2) big mortality events that can only occur if cohorts, originating from big recruitment events, die within a short period of time and if little further recruitment has taken place. However, this behavior is not a property of the biota but is generated by rainfall input to the model. By using a different rainfall input, the model can also display regular cyclic succession. Therefore there appeared to be no contradiction between sudden, discontinuous changes and gradual, continuous and reversible changes in vegetation composition.
Aggregation and the Pearson Chi‐Square Statistic for Homogeneous Proportions and Distributions in Ecology
Authors: Garson Glenn I. , Moser E. Barry
Published: May 2006

The null hypothesis that the proportions or relative abundance among j mutually exclusive categories (e.g., species or sizes) is the same for two or more populations (e.g., locations or sexes, respectively) is often tested with Pearson chi—square goodness—of—fit statistic. The chi—square statistic is derived under the assumption that the objects counted are independently and identically distributed. Count data from samples of cluster of groups of objects, for example, with transects, grabs, trawls, and quadrat usually fail to meet these assumptions. Many articles appear in the ecological literature that fail to realize the implications of violating the assumptions implicit in the use of the chi—square distribution test of homogeneity of proportions. Species aggregation, alone and in combination with increased mean abundance, and a large number of categories are shown to increase Type I error rates. Pooling several samples to get a representative sample does not solve the problem for <14 replicate samples for ecologically plausible levels of aggregation. An easily implemented solution is suggested.
Density Dependence in Two Understory Tree Species in a Neotropical Forest
Authors: Condit Richard , Hubbell Stephen P. , Foster Robin B.
Published: May 2006

Density dependence was examined in two species of neotropical treelets, Faramea occidentalis and Desmopsis panamensis, in a 50—ha plot on Barro Colorado Island in Panama. Survival and growth probability of plants larger than 1 cm dbh (diameter at breast height), and recruitment into the 1—cm class, were assessed as a function of the number of conspecific neighbors in various distance and size classes. Density—dependent effects on survival and growth were strong in Faramea. Performance of 1—8 cm dbh plants declined with increasing numbers of adult neighbors within 1 m, 1—2 m, and 2—4 m, but neighbors at 4—6 had no effect. Performance also declined with increasing numbers of juvenile neighbors <4f cm dbh, but the effect was less pronounced. Saplings of Faramea grew poorly in areas of high juvenile density within 30 m, independent of the effects caused by neighbors within 4 m. In contrast to Faramea, Desmopsis showed no density dependence in survival nor in growth. For recruitment, however, the two species showed similar patterns: recruit density was lower in regions with an adult conspecific within 1 or 2 m distance, but higher in regions with high densities of adult or juvenile conspecifics between 2 and 30—60 m. Desmopsis must have suffered density—dependent effects at stages <1 cm dbh, but the effect vanished by the 1 cm dbh stage. In Faramea, strong neighborhood effects were evident in all size classes <8 cm dbh.
Facilitative and Interfering Effects of Arthrocnemum Subterminale on Winter Annuals
Authors: Callaway Ragan M.
Published: May 2006

The relative importance of interspecific interference and facilitation in natural plant communities is controversial, in part due to the difficulty of manipulating these interactions in the field. I investigated the relative intensity of facilitative and interfering effects of Arthrocnemum subterminale, a subshrub common in the upper zones of California salt marshes, on three species of winter annuals by measuring spatial associations, manipulating Arthrocnemum canopy cover and root distributions, and comparing aspects of the physical microhabitat under Arthrocnemum to that in the open. The annuals Hutchinsia procumbens and Parapholis incurva were positively associated with Arthrocnemum, and survival and aboveground biomass of these species declined when shrub canopies were removed. Spergularia marina was negatively associated with Arthrocnemum, and survival increased when shrub canopies were removed. Combined effects of facilitation and interference were observed for Parapholis survival and biomass, which were reduced by root interference even though the overall effect of Arthrocemum on Parapholis was positive. Arthrocnemum canopies elicited direct effects on the winter annuals, but also altered substrate characteristics. Soil was mounded under the shrubs, averaging 3.14 cm higher in elevation than the surrounding open soil. Soil salinity was 27% lower under Arthrocnemum canopies, soil moisture was 13% higher under shrubs, and shrub understories received less light than open marsh soil. Although Arthrocnemum had species—specific interfering and facilitative effects, and interference and facilitation operated simultaneously, my results suggest that facilitative effects of Arthrocnemum were more significant than interference in the transition zone of the upper marsh. Physical conditions in the open reduced overall annual biomass and survival, but were ameliorated by Arthrocnemum, which altered the species composition and productivity of the upper marsh plant community.
Effects of Canopy Structure on Patch Depression by Grazers
Authors: Laca Emilio A. , Distel Roberto A. , Criggs Thomas C. , Demment Montague W.
Published: May 2006

Patch depression is an essential premise of optimal patch use models. We studied the relationship between patch residence time and intake by Bos taurus. The hypotheses were: (1) grazing changes sward structure and thus, causes patch depression; (2) patches are grazed systematically, by horizon; (3) at equal herbage mass per unit area, tall and sparse swards yield larger bites and thus, exhibit faster and greater depletion than short dense ones. In one experiment, we constructed 0.46—m2 swards of wheat (Triticum aestivum L.) with two contrasting structures but equal herbage mass. In a second experiment we mowed a perennial ryegrass (Lolium perenne L.) field to produce 0.36—m2 patches of three heights (7, 12.5, and 18 cm). The animals were allowed to remove different numbers of bites from each patch type, and we measured bite mass, intake, and residence time. Bite dimensions and location (fresh or gazed area) were also recorded in the first experiment. In the first experiment, bite mass was initially larger and decreased more steeply with residence time in the tall sparse swards than in short dense ones. Because bite dimensions and mass declined but time per bite did not change, instantaneous intake rate declined as patches were grazed. Animals grazed systematically and removed most of the top half of the canopy before biting into the lower horizon. A rectangular hyperbola described intake curves (cumulative intake vs. time) well but ignored the abrupt change in instantaneous intake rate when the top horizon was depleted. A model based on the horizon grazing style accurately depicted this change in slope but disregarded the reduction in bite mass within horizons. Intake curves for the field patches were more variable than for hand—constructed patches, but were well described by a rectangular hyperbola. Taller patches had more herbage mass and allowed greater bite mass than shorter ones. Therefore, intake curves had initially steeper slopes and approached higher asymptotes than in shorter patches. Grazers experienced patch depression at the scale of feeding stations. Plant height and density are the causal factors of this process.
Effects of Nonhost Plant on an Insect Herbivore in Diverse Habitats
Authors: Coll Moshe , Bottrell Dale G.
Published: May 2006

Vegetation texture (i.e., plant density, species diversity, and structural complexity) may influence the abundance of a herbivore by affecting its movement, altering the suitability of host plants, and by changing the herbivore's vulnerability to attack by natural enemies. We investigated these effects on the Mexican bean beetle (Epilachna varivestis) on snap bean (Phaseolus vulgaris) growing in high—and low—density monocultures and intercropped with short (0.5 m) or tall (1.2 m) corn plants (Zea mays). Beetle populations consistently reached higher densities on beans in monocultures than on beans intercropped with tall corn. Neither bean plant density nor plant species diversity alone significantly affected beetle abundance. Instead, the presence of tall corn plants contributed most to lower bean beetle density in the tall corn—bean intercrop, primarily by reducing adult colonization. However, the tall corn also increased the suitability of intercropped bean plants to the beetles because adults preferred to feed on and larvae developed faster on beans in the tall corn—bean habitat. Host plant effects were independent of the influence of microclimate and risk of predation. Abundance of natural enemies of the bean beetle was low and overall, predator density and bean beetle larval survival did not differ significantly among habitats. However, it is difficult to predict the effect or mortality factors, such as natural enemies, on bean beetle populations because larval mortality over time was affected by habitat type (i.e., there was a significant interaction between habitat type and time). Our results were partially consistent with the resource concentration hypothesis. Whereas Mexican bean beetles colonized bean monocultures more readily than they did tall corn—bean diculture, the presence of more suitable host plants in the diculture acted to increase, rather than decrease, bean beetle density in the tall corn—bean habitat. However, higher adult feeding preference and faster larval development on beans in the tall corn—bean habitat did not offset lower adult colonization in this habitat.
Limits to Fruit Production in a Monoecious Fig: Consequences of an Obligate Mutualism
Authors: Bronstein Judith L.
Published: May 2006

Fruit maturation patterns of a Costa Rican fig tree, Ficus pertusa, were studied. Like other figs, F. pertusa is pollinated by a species—specific wasp (Agaonidae) whose female offspring transfer pollen between trees. Pollination success (the proportion of inflorescences pollinated) ranged from 1 to 100% among 21 F. pertusa crops studied over 2 yr and averaged only 65%. Resource availability evidently was limiting to fruit set as well: every crop abscised many inflorescences at a predictable point during growth. This abscission period usually preceded the brief but variably timed period of pollinator arrivals; in most cases every pollinated, undamaged inflorescence set fruit. In contrast, many resource—limited species reduce crop sizes after pollination. Numerous advantages of fruit abortion have been proposed. However, fruit abortion would not be expected in plants that evolved under conditions of pollen limitation; although pollinators have been thought to be overabundant in highly coevolved pollination mutualism, figs' unique flowering phenology may in fact make pollen limitation common. The fact that future pollen carriers as well as seeds mature within fruits may also help explain these unusual fruit maturation patterns.
Latitudinal Variation in Reproductive Timing of a Short‐Lived Monocarp, Daucus Carota (Apiaceae)
Authors: Lacey Elizabeth P.
Published: May 2006

A reciprocal transplant experiment was conducted to examine several aspects of life—history variation in Daucus carota along its latitudinal range in eastern North America. Seeds collected from natural populations at 36°, 42°, and 45° N were sown onto plots at each of the three latitudes. Marked seedlings were followed for 3 yr to examine (1) genetic and environmental components of variation in growth, survival, and year of reproduction; (2) size and age at time of reproduction; and (3) the roles that density— and age—dependent selection may play in producing the variation in age of reproduction. Mean age of reproduction increased clinally from south to north but showed no pattern of variation among different—aged populations within the same latitude. The clinal variation has likely arisen since the introduction of D. carota into North America in the 17th century and has developed independently of any role that growth rate plays in determining age of reproduction. Density influences age of reproduction both directly and indirectly by affecting plant size. Age—specific survival varied strongly over plot latitude but not among populations planted in the same latitude. The ratio of percent germination and survival of 1—yr—olds to percent survival of 2—yr—olds was °3:1, 0.2:1, and 0.15:1 for plots at 36°, 42°, and 45° N, respectively. The experiment suggests that latitudinal variation in age—specific mortality patterns is helping to maintain clinal variation in age of reproduction. Density—independent factors contribute more strongly to age—specific mortality than do density—dependent factors. Age classes are based on times of peak flowering rather than on times of growing seasons. Such intervals allow for better evolutionary interpretation of demographic data.
Assessing Why Two Introduced Conyza Differ in Their Ability to Invade Mediterranean Old Fields
Authors: Thebaud Christophe , Finzi Adrien C. , Affre Laurence , Debussche Max , Escarre Josep
Published: May 2006

Researchers have suggested that species—community interactions determine invasion success. Therefore, it is likely that small biological differences between species interact with habitat characteristics to produce distinct patterns of distribution and abundance throughout a new range. In this study we test the hypothesis that differences in the distribution and abundance of species sharing an identical set of ideal weed characteristics" are explicable in terms of species—specific responses to environmental variation within their new range. Using multifactor experiments, we investigated some of the ecological interactions influencing reproductive success in two very closely related species of annuals having invaded the French Mediterranean region for >150 yr and showing marked differences in their local distribution and abundance patterns. We transplanted seedlings of Conyza canadensis (a species restricted to recently disturbed areas) and C. sumatrensis (a species colonizing early— to mid—successional old fields) at equal densities in three contrasting old fields (6 mo, 4 yr, 17—yr abandonment, respectively) during 1991—1992, a growing season with average rainfall. Individual performances (measured as survivorship, reproductive timing, and reproductive output) were evaluated with respect to: (1) competition with plant neighbors (tested with a weeding treatment), (2) resource availability (tested with nutrient and water addition), and (3) herbivory (tested with chemical limitation). Manipulated factors interacted in a rather complex fashion to influence survivorship and reproduction in both species. However, patterns of relative performance were consistent with relative distribution patterns across Mediterranean landscapes: C. sumatrensis performed better than C. canadensis in all fields, including the youngest one (6 mo old). Herbivory only slightly affected transplant performances. In contrast, competition with plant neighbors had substantial effects on either Conyza species and may be the most important determinant of performance in Mediterranean old fields. The experiment showed unambiguously that the two species differ markedly in their competitive ability, with C. sumatrensis performing better than C. canadensis in the presence of neighboring vegetation. In addition, C. sumatrensis displayed a superior ability to take up and/or to use water and nutrient resources when they become available in competitive environments. We argue that potential physiological or anatomical species differences responsible for this differential susceptibility to local resource reduction by neighbors could involve differences in constructional organization, leaf morphology, and reproductive phenologies.
Second Derivatives of Population Growth Rate: Calculation and Applications
Authors: Caswell Hal
Published: May 2006

The rate of increase of a population depends on the age— or stage—specific vital rates (survival, reproduction, etc.). Sensitivity analysis explores this dependence by calculating the first derivatives of the rate of increase with respect to each of the vital rates, and has become a standard part of demographic analysis. This paper presents a formula for the second derivatives, for the case where the rate of increase is measured by the dominant eigenvalue λ of a population projection matrix. Because the first derivatives of λ are written in terms of the eigenvectors, the second derivatives of λ can be written in terms of the first derivatives of the eigenvectors. I present examples of the second derivatives of λ in age— and size—classified life cycles, and discuss several potential applications of the results: (1) the perturbation analysis of elasticity patterns with applications to conservation biology, (2) the sensitivity analysis of stochastic growth rates, (3) the inclusion of second—order terms in the decomposition analysis of life table response experiments, and (4) the analysis of directional, stabilizing, and correlational selection on life history traits.
Mark‐Resight Population Estimation with Imperfect Observations
Authors: Gardner Shea N. , Mangel Marc
Published: May 2006

Minta and Mangel (1989) developed a Monte Carlo method for estimating population size from mark—resight data in which there is considerable variability in the resighting frequencies among individuals and irregular census surveys. Their method assumed that sightings were perfect, so that all animals present (marked or unmarked) were resighted. We describe an updated version that allows a proportion 1 — p of the individuals present to be overlooked during censusing. We compare confidence interval coverage for the original method and the updated method using simulated data sets, describe a method to estimate p, and evaluate how error in estimating p affects confidence interval coverage. The updated method for estimating a confidence interval performs considerably better than the original estimate. We find that p can be overestimated by 30% or underestimated by 10% and the confidence interval generated still includes the true population size 90% of the time. This technique may improve confidence interval estimates for small and threatened populations.
Evolution and the Consequences of Species Introductions and Deletions
Authors: Abrams Peter A.
Published: May 2006

The addition or deletion of a species from a community is likely to have effects on the trait values of other species and on their population densities. This article argues that current theory is insufficiently developed to provide guidance in predicting what might happen to either traits or population densities. In addition, there has been relatively little empirical work to examine many of the phenomena that have been predicted by the limited available theory. The example of character displacement of competitors is reviewed to reveal some of the gaps in our knowledge about the evolutionary consequences of additions or deletions. The example of evolution of traits related to predation in food webs is used to reveal gaps in our knowledge of the population—level consequences of evolutionary changes initiated by a species addition or deletion. Several approaches to studying combined evolutionary and ecological processes in multispecies communities are discussed. Some previous multispecies models have been too abstract to be easily related to more mechanistic two—species models, but recent methods derived from quantitative genetics may result in significant advances in understanding multispecies systems and their relationship to communities with fewer species.
Paleobiology, Community Ecology, and Scales of Ecological Pattern
Authors: Jablonski David , Sepkoski J. John
Published: May 2006

The fossil record provides a wealth of data on the role of regional processes and historical events in shaping biological communities over a variety of time scales. The Quaternary record with its evidence of repeated climatic change shows that both terrestrial and marine species shifted independently rather than as cohesive assemblages over scales of thousands of years. Larger scale patterns also show a strong individualistic component to taxon dynamics; assemblage stability, when it occurs, is difficult to separate from shared responses to low rates of environmental change. Nevertheless, the fossil record does suggest that some biotic interactions influence large—scale ecological and evolutionary patterns, albeit in more diffuse and protracted fashions than those generally studied by community ecologists. These include: (1) the resistance by incumbents to the establishment of new or invading taxa, with episodes of explosive diversification often appearing contingent on the removal of incumbents at extinction events; (2) steady states of within—habitat and global diversity at longer time scales (107—108 yr), despite enormous turnover of taxa; and (3) morphological and biogeographic responses to increased intensities of predation and substratum disturbance over similarly long time scales. The behavior of species and communities over the array of temporal and spatial scales in the fossil record takes on additional significance for framing conservation strategies, and for understanding recovery of species, lineages, and communities from environmental changes.
Extremes in Ecology: Avoiding the Misleading Effects of Sampling Variation in Summary Analyses
Authors: Link William A. , Sauer John R.
Published: May 2006

Surveys such as the North American Breeding Bird Survey (BBS) produce large collections of parameter estimates. One's natural inclination when confronted with lists of parameter estimates is to look for the extreme values: in the BBS, these correspond to the species that appear to have the greatest changes in population size through time. Unfortunately, extreme estimates are liable to correspond to the most poorly estimated parameters. Consequently, the most extreme parameters may not match up with the most extreme parameter estimates. The ranking of parameter values on the basis of their estimates is a difficult statistical problem. We use data on 401 species from the BBS and simulations to illustrate the potential misleading effects of sampling variation in rankings of parameters. We describe empirical Bayes and constrained empirical Bayes procedures that provide partial solutions to the problem of ranking in the presence of sampling variation.
Population Dynamics and Genetics of Plant Disease: A Case Study of Anther‐ Smut Disease
Authors: Alexander Helen Miller , Thrall Peter H. , Antonovics Janis , Jarosz Andrew M. , Oudemans Peter V.
Published: May 2006

A model by Levin and Udovic (1977) emphasizes the need for integration of studies of the numerical abundances and genetic composition of host and pathogen species. We use their conceptual framework to summarize our collaborative research on the ecological genetics of the anther—smut disease of Silene alba caused by the fungus Ustilago violacea. Our theoretical investigations have revealed the importance of the rate and mode of disease transmission on the likelihood of coexistence between host and pathogen. Our empirical studies have quantified patterns of disease spread on local and regional spatial scales and have shown that host genotypes differ greatly in resistance. Comparable genetic variation in pathogen virulence has not yet been demonstrated. The genetic composition of host populations alters numerical dynamics in experimental populations: disease declines in resistant populations, while host and pathogen appear to coexist in susceptible populations. Ecological outcomes also may be affected by the negative relationship between disease incidence and host flowering time, which may constitute a cost to resistance. We are currently expanding our work on the ecology and genetics of metapopulation dynamics of host and pathogen.
Predator‐‐Induced Life History Changes: Antipredator Behavior Costs or Facultative Life History Shifts?
Authors: Ball Shelley L. , Baker Robert L.
Published: May 2006

We used laboratory experiments to show that the nonlethal presence of pumpkinseed sunfish (Lepomis gibbosus) results in smaller size at emergence, decreased growth and development rates, and lower fecundity of the dipteran Chironomus tentans. Smaller size at metamorphosis is often viewed as a cost of antipredator behavior. However, it may also partly result from a facultative change in life history in which prey increase their development rate to escape their risky larval habitat. To determine the mechanism responsible for this smaller size at emergence, we compared development rates of chironomid larvae raised in the absence and nonlethal presence of pumpkinseed sunfish. When we statistically controlled for effects of predators on larval growth rate, fish presence had no effect on development rate of female chironomids, but significantly slowed male development. These results clearly indicate that C. tentans does not increase its development rate as a means of escaping a risky habitat, and that smaller size at emergence is best viewed as a cost of larvae behaviorally avoiding fish predators. Observed sex differences in life history responses to predators may have important consequences for fitness.