bluuebunny/crossref_metadata_2025 huggingface dataset:
Interannual Variation in Greater Flamingo Breeding Success in Relation to Water Levels
Authors: Cezilly Frank , Boy Vincent , Green Rhys E. , Hirons Graham J. M. , Johnson Alan R.
Published: May 2006

In many colonial waterbirds, reproductive success is affected by water levels around the colonies. In a study of the Greater Flamingo (Phoenicopterus ruber roseus) in the Camargue, southern France, we examined annual variation in water levels around the breeding colony site between 1984 and 1991 in relation to (1) the number of breeding pairs, (2) colony productivity (fledglings per breeding pair), and (3) physical condition of fledglings. The number of breeding pairs each year was highly variable and was positively correlated with high water levels during the March—July period. There was no significant relationship between the number of breeding pairs and the number of chicks fledged. Although the productivity of the colony was not affected by water levels during the chick—raising period, fledglings were heavier in years with high water levels. There was no relationship between colony productivity or the absolute number of chicks fledged and the average body condition of chicks in the creche. Differences between the 1984 and 1985 cohorts in the frequency of young birds making breeding attempts (i.e., ages 5 and 6 yr) suggested that body condition at fledging may have important life history consequences. Estimation of reproductive performance in colonial waterbirds, usually limited to the average nest productivity, could be improved by considering average body condition of chicks at fledging as well.
Dynamics in Subdivided Populations of Neotropical Migratory Birds in a Fragmented Temperate Forest
Authors: Villard Marc-Andre , Merriam Gray , Maurer Brian A.
Published: May 2006

To study the hierarchy of variables involved in the year—to—year dynamics in the distribution of Neotropical migratory birds, we compared observed numbers of local extinctions and recolonizations to those expected under six hypothetical mechanisms of settlement by returning spring migrants. These mechanisms were (1) random dispersal among forest fragments; (2) passive sampling of spring migrants according to fragment area; (3) dispersal among fragments between years according to the distance to the nearest fragment occupied the 1st yr; (4) resettlement of spring migrants according to the vertical structure of the vegetation in the fragments; (5) site fidelity of experienced breeders; (6) a combination of mechanisms 2, 3, 4, and 5. We selected four target species: Wood Thrush (Hylocichla mustelina), Black—and—white Warbler (Mniotilta varia), Ovenbird (Seiurus aurocapillus), and Scarlet Tanager (Piranga olivacea). In 50 fragments of mature deciduous forest (3.0—129.8 ha, median = 11.8 ha) within a 10 ° 10 km square, we recorded the presence of any target species and an abundance index in two successive breeding seasons. Absences were validated using song playbacks. Except for the random dispersal model, each hypothetical mechanism was simulated using logistic regressions on empirical data. Frequencies of population turnovers (local extinctions + recolonizations) were 16—28% between the two breeding seasons, varying with species. Observed numbers of population turnovers were best approximated by the combination model (model 6). Expected numbers of population turnovers were consistently higher than observed numbers for models 1—4, suggesting that between—year dynamics were constrained by some process(es). Our simulations suggest that site fidelity might be an important constraint on the distributional dynamics of these species, even when effects of fragment area and habitat on site fidelity are factored out. This finding has important implications for the response of Neotropical migrant birds to the fragmentation of their habitat.
Demography of Birds in a Neotropical Forest: Effects of Allometry, Taxonomy, and Ecology
Authors: Brawn Jeffrey D. , Karr James R. , Nichols James D.
Published: May 2006

Comparative demographic studies of terrestrial vertebrates have included few samples of species from tropical forests. We analyzed 9 yr of mark—recapture data and estimated demographic parameters for 25 species of birds inhabiting lowland forests in central Panama. These species were all songbirds (Order Passeriformes) ranging in mass from 7 to 57 g. Using Jolly—Seber stochastic models for open populations, we estimated annual survival rate, population size, and recruitment between sampling periods for each species. We then explored relationships between these parameters and attributes such as body size, phylogenetic affiliation, foraging guild, and social behavior. Larger birds had comparatively long life—spans and low recruitment, but body size was not associated with population size. After adjusting for effects of body size, we found no association between phylogenetic affiliation and any demographic trait. Ecological attributes, especially foraging guild, were more clearly associated with interspecific variation in all demographic traits. Ant—followers had comparatively long life—spans, but species that participate in flocks did not live longer than solitary species. The allometric associations we observed were consistent with those demonstrated in other studies of vertebrates; thus, these relationships appear to be robust. Our findings that ecological factors were more influential than phylogenetic affiliation contrasts with comparative studies of temperate—zone birds and suggests that the relative importance of environmental vs. historical factors varies geographically.
Constraints on Chemically Mediated Coevolution: Multiple Functions for Seaweed Secondary Metabolites
Authors: Schmitt Tim M. , Hay Mark E. , Lindquist Niels
Published: May 2006

Diterpene alcohols produced by the brown seaweed Dictyota menstrualis deter feeding by numerous species of abundant herbivores. Here we show that these same compound also may prevent fouling organisms from colonizing the surface of this alga. In the field, Dictyota menstrualis plants were less frequently and less heavily fouled than any of the other common seaweed species investigated. In laboratory assays, larvae of the common fouling bryozoan Bugula neritina failed to settle on Dictyota even though they contacted its surface as often as they contacted the surface of a preferred host alga. Rejection occurred only after direct contact with the alga's surface. Rejection of Dictyota was not mediated by water—borne chemical cues or by surface wettability (a physical property of the surface that can affect fouling). The lipid—soluble extract from surface rubbings of Dictyota inhibited larval settlement when placed on other surfaces and contained the diterpene alcohols pachydictyol A and dictyol E. Larvae exposed to these compounds experienced mortality, abnormal development, or reduced rates of development. Although the potential for chemically mediated coevolution between plants and herbivores has been the focus of scores of previous investigations, such coevolution will depend on selection altering the chemical defenses of the plant following the evolution of resistance by herbivores. Such a reciprocal response will be constrained if compounds play multiple roles that are ecologically important. Dictyota produces secondary metabolites that are broadly defensive against a wide variety of consumers and fouling organisms. Although certain consumers may evolve resistance to these metabolites, it is unclear that feeding by these consumers will result in reciprocal responses from the plant. We suggest that coevolved interactions may be uncommon, and that many interactions that appear to be coevolved may result from fortuitous and opportunistic preadaptations.
Competitive Performance and Species Distribution in Shortline Plant Communities: A Comparative Approach
Authors: Gaudet Connie L. , Keddy Paul A.
Published: May 2006

Using a comparative approach, we tested the prediction that relative competitive performance of plant species is correlated with distribution along natural gradients of fertility and standing crop. The mean position of 40 species of herbaceous plants along standing crop, phosphorus, nitrate, magnesium, potassium pH, and percent organic content gradients was calculated based on data from 217 quadrats from lake and river shorelines in Nova Scotia, Quebec, and Ontario. Competitive performance was measured in an outdoor experiments in which species were grown together with a common phytometer, Lythrum salicaria (n = 5 replicated per species). The relative ability of each species to suppress the growth of the phytometer was used as a measure of relative competitive performance. This measure of competitive performance was significantly correlated with the mean position of species on the standing crop (r = 0.081; P < 0.0001), percent organic content (r = 0.71; P < 0.0001); phosphorous (r = 0.70; P < 0.0001), nitrate (r = 0.67; P< 0.0001), magnesium (r = 0.66; P < 0.0001), and potassium gradients (r = 0.61; P < 0.0001). When monocotyledons and dicotyledons were considered separately, the relationship between distribution along the macronutrient gradients and competitive performance was much stronger for monocotyledons. It is suggested that this may reflect a differential influence of disturbance on distribution.
Effects of Flooding On Root and Shoot Production of Bald Cypress in Large Experimental Enclosures
Authors: Megonigal J. Patrick , Day Frank P.
Published: May 2006

Effects of hydroperiod on the root production of bald cypress (Taxodium distichum) saplings were determined in large (8.0 m2 x 1.5 m deep) watertight enclosures over three growing seasons. Our objectives were to determine the effect of continuous and periodic flooding regimes on biomass production, carbon allocation to roots and shoots, and root—system morphology. The effect of the flooding treatments on plant biomass was different for 1—yr—old seedlings and 3—yr—old saplings. After one growing season, root and shoot biomass was highest in the periodically flooded (PF) treatment. After three growing seasons there were no significant differences in total biomass but there were differences in root—to—shoot ratios. Improved growth in the continuously flooded (CF) treatment began in the second growing season and coincided with morphological adaptations to flooding. Such adaptations include the production of water roots, development of intercellular air spaces, and distinctly different root—system morphologies. Periodically flooded cypress allocated more carbon to roots than did continuously flooded cypress and developed deeper root systems. A relatively deep rooting zone may have provided the PF saplings access to water and dissolved nutrients within the water table (50—60 cm deep during summer). Continuously flooded plants had low root—to—shoot ratios and shallow root systems. A relatively shallow rooting zone with ample water and nutrients allowed CF cypress to allocate relatively more biomass to leaves. After 3 yr, total productivity in the two treatments was not significantly different, yet belowground production was greater in periodically flooded saplings (P = .05) and there was a tendency for higher aboveground production in continuously flooded saplings (P = .14). Without the belowground production estimates we might have concluded that CF plants were more productive than PF plants. Most plants can respond to changing resource availabilities by shifting the allocation of carbohydrates to roots or shoots. Because resource availability in freshwater forested wetland ecosystems can be highly variable, studies of production should include estimates of root production.
Effects of a Severe Drought on the Population Biology of a Tropical Hummingbird
Authors: Stiles F. Gary
Published: May 2006

At Finca La Selva, in the Caribbean lowlands of Costa Rica, the Long—tailed Hermit (Phaethornis superciliosus) has a long breeding season (December or January through August or September) during which flowers are abundant, followed by a severe but predictable season of flower scarcity in late October—November, during which most of the annual mortality normally occurs. In 1973, the third of a 4—yr population study of marked lek males, an unusual drought caused a severe flower shortage at the height of the breeding season. During the latter part of the breeding season lek activity of males was reduced, successful breeding by females was drastically curtailed, and masses of both sexes dropped to levels lower than those of the lean season. Interrupted molt, an indicator of energy stress normally seen only in a minority of breeding females, was observed in 25% of adult males, and 50% of females, 1973. Survivorship of lek males through the breeding season dropped from 90% in normal years to °60%, and the total lek population at the start of the 1974 breeding season was one—third lower than before the drought in 1973; the cohort of yearling males was most strongly affected. Recovery of numbers and age structure of the lek male populations to pre—1973 levels required 3—4 yr. Jolly—Seber analyses showed close agreement with survivorship as determined by direct observation (possible because of the lek social system), and permitted estimation of other parameters of interest: populations of marked and unmarked males, probability of recapture of marked males, and recruitment. The average annual survivorship of male P. superciliosus (in normal years) is near the mean value reported for a series of tropical species. A simple model of the reproductive output of female P. superciliosus, assuming no sexual differences in survivorship, suggests that a female breeding only in the prime breeding season (roughly February—July, when over 80% of the fledglings are normally produced) will no more than replace herself in the population; breeding throughout 8—9 mo will permit her to increase her reproductive output by up to 10%, which agrees well with the 3+ yr required by the (male) population to recover from the drought. Possible implications of these findings for tropical bird demography and conservation are discussed.
^(13)C and ^(15)N Enrichment Factors of Feathers of 11 Species of Adult Birds
Authors: Mizutani Hiroshi , Fukuda Michio , Kabaya Yuko
Published: May 2006

The use of stable isotopes of bioelements to analyze diets of animals in nature requires precise knowledge of any changes in the isotope ratios that take place after assimilation. Therefore, the influence of diet on the distribution of carbon and nitrogen isotopes in feathers of 11 species of birds on a constant diet was studied. The standard deviation in °13C and °15N in naturally obtained foods like fish during 6 yr was °1%. Factory—manufactured pellets generally gave lower deviations (0.2% for °13C and °0.6% for °15N). Feathers showed an enrichment relative to food in both 13C and 15N, although its extent varied among different bird species. Isotope ratios for 13C and 15N from diet to feathers converged. The nature of the enrichment factors is discussed in relation to the convergence and temporal window size. Neither enrichment showed an age dependency among adult birds, and neither depended on the nitrogen content in the bird's diet. These results show that the stable isotope approach can be applied not only to these 11 species but to birds whose isotope enrichment factors are so far unknown.
Resistance and Resilience of Lotic Algal Communities: Importance of Disturbance Timing and Current
Authors: Peterson Christopher G. , Stevenson R. Jan
Published: May 2006

We examined effects of disturbance timing on resistance and resilience of epilithic algal communities growing in fast— (29 cm/s) and slow— (12 cm/s) current outdoor experimental stream channels in Kentucky, USA that were either left undisturbed (control) over 33 d following a simulated spate, or were subjected to an additional spate after either 9, 18, 27, 33 d. On day 33, all channels were subjected to a final spate to assess effects of short—term disturbance history on resistance, independently of seasonal influences. Succession proceeded from a sparsely populated community dominated by a small, monoraphid diatom (Achnanthes minutissima) immediately after the initial spate, to dominance by dense floating mats of filamentous green algae (Zygnematales: Mougeotia and Spirogyra) and Synedra spp. by day 21—24. Resistance was generally lower in slow—current communities, both in terms of cell—density reduction and displacement of taxonomic structure, than in fast—current communities. Resistance in slow—current communities varied temporally, with communities least resistant on day 18, when community composition and physiognomy was changing rapidly, and on day 33, when green algal mats began to senesce. On day 33, slow—current communities that had not been recently disturbed (control, D9) exhibited greatest spate—induced loss of algal biomass. Additionally, slow—current communities with high pre—disturbance phaeophytin content (an indicator of algal senescence) also changed most in diatom assemblage structure across the final spate. No such relationship was noted in fast current, suggesting that autogenic factors influenced communities in slow current more than those in fast. Resilience was higher in slow—current communities than in fast current, with disturbed communities reaching biomass and taxonomic structure similar to controls after 3—9 d. High resilience in slow current resulted from enhanced reproduction in some populations following spate—induced biomass reduction and presumed release from nutrient and light limitation, and low shear stress relative to fast—current channels where biomass accrual was limited by current. Interactions between disturbance timing, successional state, and habitat affect the susceptibility of epilithic algal communities to disturbance and likely influence temporal and spatial heterogeneity in stream ecosystems.
Action Principles as Determinants of Ecosystem Structure: The Autonomous Lake as a Reference System
Authors: Vanriel Peter , Johnson Lionel
Published: May 2006

Action in theoretical physics is an abstract quantity describing the overall motion of a system; action has the dimensions of energy x time (in joule—seconds). The Principle of Least Action states that physical systems follow that path for which the action has the least value. By analogy, we hypothesize that, if the principle of least action is universal, ecosystems must be structured on interaction between this principle of least action and an overriding Principle of Most Action. Most action implies energy acquisition, concentration, and conservation. In an autonomous ecosystem a trophic hierarchy is formed in which, for stability, the action increases at each hierarchical level. We predicted that energy density (in joules per gram), being one of the components of most action, must increase at each level along the food chain. This prediction was tested in Keyhole Lake, a small Arctic lake in the Canadian Northwest Territories, and found to be substantiated. The major non—conformity with the hypothesis was found in the zooplankton, which at all times showed the highest energy density. We concluded that the plankton forms an essentially separate compartment of the ecosystem. Total biomass energy in each hierarchal level confirmed the existence of an energy pyramid, in which the relationship between levels exhibited the characteristics of an inverse cooling curve. An autonomous lake, such as Keyhole Lake, may be regarded as the nearest approach in the natural world to a reference system, against which other systems may be evaluated.
Food Supply, Territory Quality, and Reproductive Timing in the Blue Tit (Parus Caeruleus)
Authors: Svensson Erik , Nilsson Jan-Ake
Published: May 2006

We performed supplemental feeding experiments during three breeding seasons of the Blue Tit (Parus caeruleus L.) in order to establish the importance of food in reproductive timing. In particular, we addressed the question of why the response to supplemental food is restricted to 3—6 d in many bird species. Supplemental food advanced the start of egg—laying in all three breeding seasons. The mean difference between fed and control pairs was 4 d in two years and 6 d in another. The amount and quality of the supplemental food used in the experiment were not limiting, since pairs receiving a further increase in amount and quality of food did not start laying earlier than pairs with a standard feeding regime. The amount of food that was consumed increased during the period from 15 d before egg formation until the beginning of egg formation and then remained at a high utilization rate until clutch completion. Although fed pairs, on average, started egg—laying earlier than control pairs, the earliest control females started to lay eggs as early as the first females. Fed pairs were also relatively late in a year when control pairs were late. In 1992, fed pairs did not produce larger clutches or more fledglings, but had nestlings with lower mass at 13 d post—hatching than those of control pairs. Analysis at the territory level revealed that supplemental food affected laying date to a greater extent in low—quality territories, in which laying could be advanced by up to 9—10 d, compared to high—quality territories in which laying was not advanced at all. We conclude that laying date in most Blue Tit females is limited by low food supply early in spring, but some females, occupying high—quality territories, will not advance laying in response to food. Thus, above a certain critical limit, laying date is unrelated to food supply, and birds use other cues in their decision to commence breeding. Such cues are probably also responsible for the between—year variation in laying dates of food—supplemented pairs, since these were provided with the same amount of food during the different years.
Rules and Judgments in Statistics: Three Examples
Authors: Stewart-Oaten Allan
Published: May 2006

Statistical analyses are based on a mixture of mathematical theorems and judgments based on subject matter knowledge, intuition, and the goals of the investigator. Review articles and textbooks, aiming for brevity and simplicity, sometimes blur the difference between mathematics and judgment. A folklore can develop, where judgments based on opinions become laws of what "should" be done. This can intimidate authors and readers, waste their time, and sometimes lead to analyses that obscure the information in the data rather then clarify it. Three familiar examples are discussed: the choice between Normal‐based and non‐parametric methods, the use of multiple‐comparison procedures, and the choice of sums of squares for main effects in unbalanced ANOVA. In each case, commonly obeyed rules are shown to be judgments with which it is reasonable to disagree. A greater stress on model selection, aided by informal methods, such as plots, and by informal use of formal methods, such as tests, is advocated.
Changes in Alaskan Soft‐Bottom Prey Communities Along a Gradient in Sea Otter Predation
Authors: Kvitek R. G. , Oliver J. S. , DeGange A. R. , Anderson B. S.
Published: May 2006

Sea Otter (Enhydra lutris), well documented as "keystone" predators in rocky marine communities, were found to exert a strong influence on infaunal prey communities in soft—sediment habitats. Direct and indirect effects of sea otter predation on subtidal soft—bottom prey communities were evaluated along a temporal gradient of sea otter occupancy around the Kodiak Archipelago. The results indicate that Kodiak otters forage primarily on bivalve prey and dramatically reduce infaunal bivalve and green sea urchin (Strongylocentrotus droebachiensis) prey populations. Bivalve prey abundance, biomass, and size were inversely related to duration of sea otter occupancy. The relative conditions of shells discarded by otters in shallow (<10 m) vs. deep (> 20 m) water at the same sites indicate that otters first exploited Saxidomus in shallow—water feeding areas, and later switched to Macoma spp. in deeper water. Otter—cracked shells of the deep—burrowing clam Tresus capax were rarely found, even at otter foraging sites where the clam accounted for the majority of available prey biomass, suggesting that it has a partial depth refuge from otter predation. The indirect effects of otter predation included substratum disturbance and the facilitation of sea star predation on infaunal prey. Sea stars, Pycnopodia helianthoides, were attracted to experimentally dug excavations as well as natural sea otter foraging pits, where the sea stars foraged on smaller size classes of infaunal bivalves than those eaten by otters. Otters also discard clam shells on the sediment surface and expose old, buried shells during excavation. Surface shells were found to provide attachment sites for large anemones and kelp. Our study shows that sea otters can affect soft—sediment communities, not only through predation, as in rocky habitats, but also through disturbance, and thus retain a high degree of influence in two very different habitat types.
Density‐Dependent Host‐Pathogen Dynamics in Soil Microcosms
Authors: Jaffee B. , Phillips R. , Muldoon A. , Mangel M.
Published: May 2006

Temporal density—dependent parasitism and a host threshold density are important features of disease induced by infectious parasites in populations of aboveground, macroscopic organisms. We determined whether these features also occur in soil microcosms containing a microscopic host (the nematode Heterodera schachtii) and its parasite (the nematophagous fungus Hirsutella rhossiliensis). Soil microcosms are especially interesting because (1) the environment and scale are quite different from conventional host—parasite systems and may result in considerably different disease dynamics, (2) the small size of the soil microcosms, although biologically appropriate, facilitates experimentation and parameter estimation, and (3) some soil—borne, microscopic organisms (such as H. schachtii) are important agricultural pests. Temporal density—dependent parasitism was directly assessed with laboratory experiments in which host density and environment were controlled. A theory, which complements and extends the experiments, was developed to enable direct comparison of observed and predicted dynamics and to provide a stringent test of our understanding of processes underlying the dynamics. The theory was simple, yet explicitly described the essential biology. Parameters for the theory were measured with short—term experiments. We found that the disease dynamics in soil microcosms exhibited both temporal density—dependent parasitism and a host threshold density. However, epidemics were slow to develop. Observed and predicted dynamics were quite similar, indicating that our understanding of the underlying biology was correct.
Self‐Assembling Food Webs: A Global Viewpoint of Coexistence of Species in Lotka‐Volterra Communities
Authors: Law Richard , Blackford Jerry C.
Published: May 2006

It has been suggested that the structure of ecological communities is constrained by dynamical instability of certain configurations of interacting species. Analysis of this has most often been based on asymptotic (local) stability analysis of equilibria at which all species are present, on the grounds that a configuration with an unstable equilibrium is less likely to occur in the real world than one with an asymptotically stable equilibrium. Since species may also coexist on limit cycles or chaotic orbits, we argue that a global condition that species should increase when rare could be more appropriate. Conditions for such "permanence" exist and their use makes it possible to focus on fundamental qualitative questions of coexistence of species, even though the quantitative properties of their dynamics may be very complex. We describe a straightforward and general method for distinguishing between permanent and nonpermanent Lotka—Volterra food webs; this method is due to Jansen (1987). A numerical analysis of four—species food webs with different degrees of omnivory shows that many that would be rejected as unstable in a local analysis are at the same time permanent in the global analysis. Using permanence methods, food webs of high connectance are shown to rebuild themselves more readily than those of low connectance, and therefore to recover more readily from disturbance than those of low connectance. The significance of this result for the relationship between complexity and stability of communities is discussed, together with the merits and drawbacks of permanence for studying the coexistence of species in communities.
The Role of Animals in Dispersing a "Wind‐Dispersed" Pine
Authors: Vander Wall Stephen B.
Published: May 2006

Jeffrey pine (Pinus jeffreyi) produces winged seeds typical of many wind—dispersed pines. However, seedlings often occur in clumps, suggesting that many seeds are scatter—hoarded by animals. An experiment is described that examines how the initially wind—dispersed seeds end up in animal caches. Two arrays of 1064 Jeffrey pine seeds were arranged around two "source" trees in a pattern predicted from models of wind dispersal. Seeds were labeled with radioactive scandium—46 and numbered 1—12 with indelible ink to represent the 12 1—m wide annuli centered on the source tree where they were deployed. Within 2 d, between 95 and 99% of the seeds had been removed by rodents. Fifty—four and 35% of the seeds were found stored at the two sites, most of these in shallow surface caches. Seed caches were located 1.3—54.2 m and 6.2—62.9 m from the "source" trees at sites one and two, respectively. Mean distance of the "wind—dispersed" seeds from the source tree was 4.6 m, and rodents moved seeds a mean of 13.0 m farther at site 1 and 24.7 m farther at site 2. Of 375 minimum dispersal measurements, seeds were carried away from the source tree 362 times. Four rodent species may have participated in caching the seeds, but yellow pine chipmunks (Tamias amoenus) appeared to be the species that cached most of the seeds. In a second experiment where rodents were excluded, seeds placed on the soil surface in fall rarely became buried during the winter, and if not buried, they would not germinate. Jeffrey pine seeds appear to be dispersed in two phases; seeds are initially scattered by the wind, and subsequently carried farther and cached in soil by animals.
Patterns of Gap‐Phase Replacement in a Subalpine, Old‐Growth Forest
Authors: Lertzman Kenneth P.
Published: May 2006

Small—scale natural disturbances involving the death of one to a few trees and creating gaps in the forest canopy are key processes in the population and community ecology of many forests. I examined the patterns of replacement in gaps in a subalpine old—growth forest in southern coastal British Columbia to assess their role in the forest community. I found no evidence that self replacement or reciprocal replacement act to maintain the current community composition. Gapmaker—gapfiller comparisons indicated preferential replacement of all species by Pacific silver fir (Abies amabilis), suggesting that the community is undergoing successional change. Nor did gap size, location within a gap, or local canopy composition appear to exert a strong influence on the species composition of regeneration within the gap. The only circumstance where Pacific silver fir was not overwhelmingly dominant among gapfillers was on stumps, where almost all successful western hemlock (Tsuga heterophylla) gapfillers were located. These patterns suggest that neither species—specific interactions between gapmakers and gapfillers nor variability in gap environments is adequate to maintain the current composition of the forest canopy. However, caution is indicated in projecting a long—term trend from this short—term assessment of community trajectory.
Clutch Predation, Cover, and the Overdispersion of Nests of the Willow Ptarmigan
Authors: Schieck Jim O. , Hannon Susan J.
Published: May 2006

Clutch predation is generally considered a selective force in nest site selection by gound—nesting birds. We examined characteristics of vegetation and cover around nest sites and inter—nest spacing for Willow Ptarmigan (Lagopus lagopus) to determine whether choice of nest location influenced probability of predation. Types of vegetation around successful and depredated nests and random points were similar. However, hens chose sites that had more lateral and overhead cover than that found at random points. Amount of cover was similar for successful and depredated nests. We Suggest that hens choose cover to deter predation on themselves or to reduce energy costs of incubation. Inter—nest distances were larger and nests were located closer to the centroid of the territory than if nest sites had been selected at random. Hens that renested placed their nests farther from their first nests but not farther from neigboring nests than expected if they had located them at random. Probability of a clutch being preyed upon was not related to its distance from a clutch that had previously been depredated. We conclude that, for the breeding densities that we encountered, nests were spaced far enough apart to deter area—restricted search by predators. Predation may have been a factor causing over—dispersion of nests in the past, but current selection may be masked by other factors such as the hen's behavior or stochasticity.
Holocene Variation in Spatial Scales of Vegetation Pattern in the Upper Great Lakes
Authors: Graumlich Lisa J. , Davis Margaret B.
Published: May 2006

While continental—scale patterns of vegetation change during the Holocene clearly record the influence of climatic change, the factors governing change at the landscape scale are less clearly defined. In order to characterize the scales of processes determining vegetation patterns during the Holocene, we analyzed a network of 52 pollen sites in the upper Great Lakes region. Pollen percentage data for three dominant tree genera (pine, Pinus; oak, Quercus; birch, Betula) were interpolated from samples bracketing four target years (500, 2500, 4500, and 6500 yr BP). Smoothed isopoll maps of taxon abundance for each target year show broad trends in pollen abundance that correspond to climatic gradients. Residuals, representing the deviation of each pollen datum from the smoothed valued, indicate the amount of spatial variation in pollen abundance independent of that already modeled as a broad gradient. The three genera differ in the magnitude and pattern of residual variation. Oak residuals are relatively small in magnitude, while pine and birch residuals are relatively large and show greater local variability in sign and magnitude. This indicates local variability in tree abundance, as pollen of all three taxa is readily dispersed by wind. Spatial correlograms, which summarize the strength of spatial autocorrelation as a function of distance between pairs of sites for a given taxon, were calculated separately for each target year and allow the quantification of the dominant scale of variability of each taxon. Oak correlograms corroborate the mapped data in indicating the dominance of region—wide trends. In contrast the birch and pine correlograms indicate that factors operating at scales of 150 to 300 km are as important as region—wide trends in governing pollen abundances. The structure of the correlograms for birch and pine pollen changes through time, with birch showing a more patchy spatial pattern in the mid—Holocene (4500 and 6500 yr BP) as compared to the late—Holocene (500 and 2500 yr BP). Pine, in contrast, shows a more strongly autocorrelated pattern in the mid—Holocene. Our results suggest that substrate, an environmental constraint on vegetation at scales of tens to hundreds of kilometers, has been important in governing the spatial distribution of birch and pine in the upper Great Lakes region. The changing distribution of birch and pine is attributed, in part, to changes in the relative abundance of ecologically dissimilar species within these genera. Further, these observations suggest that spatial scales of tree abundances are dynamic and that constraints imposed by substrate vary in importance in response to long—term climatic variation.
Population Consequences of Larval Crowding in the Dragonfly Aeshna Juncea
Authors: Van Buskirk Josh
Published: May 2006

I studied sources of density dependence in a dragonfly population to reveal how population size is controlled. Larvae of the dragonfly Aeshna juncea, inhabiting rock pools on the Lake Superior shoreline at Isle Royale, Michigan, USA, showed clear evidence of competition when natural densities were high. After 2 yr of larval growth, individuals in crowded pools were developmentally delayed and survived less well than individuals in sparsely populated pools. The stunted larvae caught up with individuals in less crowded pools and reached the final instar in sufficient time to emerge in the summer of their 3rd yr, but even so their final body size remained relatively small. A 5—wk field experiment in natural pools demonstrated the causal connection between density and fitness: feeding activity and growth rates were significantly reduced under crowded conditions, but survival showed no response to density. In unmanipulated rock pools, however, natural increases in density from one year to the next were met with slightly overcompensatory decreases in annual survival. The results illustrate that density dependence operates in a way that could contribute to population regulation in A. juncea, and a 6—yr time series from one of the two study areas showed significant evidence for density—dependent dynamics.
The Influence of Fluctuating Resources on Life History: Patterns of Allocation and Plasticity in Female Guppies
Authors: Reznick David , Yang Anthony P.
Published: May 2006

We investigated how resources are allocated to reproduction and how variations in resource availability influence reproductive allocation, offspring number, and offspring size in guppies (Poecilia reticulata). Our goal was to evaluate how plastic these variables are in response to environmental variation and to characterize the nature of this plasticity. Female guppies which had just given birth (litter 1) were assigned to either high or low levels of food availability until they gave birth to their next litter (litter 2, interval 1). They were then randomly reassigned to either high or low food with the constraint that there be equal numbers of individuals in each of four treatments: high—high, high—low, low—high, and low—low. They were maintained on this level of food availability until they produced their next litter (litter 3, interval 2). We analyzed variables that characterized the female after the birth of the third litter and the offspring in the third litter. These were two—way analyses, with intervals 1 and 2 as the main effects and high vs. low food as the levels of each effect. The qualities of the third litter were influences by both interbrood intervals, indicating that the resources used for producing the litter were derived from both intervals. Specifically, higher food availability during either interval resulted in a significant increase in the number of offspring in litter 3, independent of the size of the mother. This result indicates that the number of offspring produced in a litter will be a function of both the immediate and the past environment. Lower food during either interval resulted in an increase in the number of days between the second and third litters, indicating that, if resource availability is low, the female may delay the initiation of the next litter, allowing her to acquire more resources. Resource availability during both intervals also influences how resources were allocated to individual offspring. Females responded to low food during the first interbrood interval by producing heavier offspring in litter 3. This increase in mass was almost entirely attributable to an increase in fat reserves. Such a result could represent adaptive plasticity, if it can be demonstrated that maternal fitness increases through the production of heavier offspring a low—food environment.
Pattern of Leaf Damage Affects Fitness of the Annual Plant Raphanus Sativus (Brassicaceae)
Authors: Mauricio Rodney , Bowers M. Deane , Bazzaz F. A.
Published: May 2006

We investigated how the pattern of leaf damage influences reproduction, growth, and allocation in the wild radish, Raphanus sativus (Brassicaceae). We removed an equivalent leaf area from plants with four leaves in five treatments ranging from concentrated to dispersed damage: one entire mature leaf removed, on entire new leaf removed, 50% of two mature leaves removed, 50% of two new leaves removed, and 25% of all four leaves removed. Plants in a control group were undamaged. Reproduction, growth, and allocation were not affected by the age of the damaged leaf. However, the pattern of leaf damage significantly affected our three measures of plant fitness: the number of flowers produced, the reproductive biomass, and the total biomass. Plants in the treatment in which the damage was most dispersed had significantly higher flower number, reproductive biomass, and total biomass than an intermediate damage treatment and significantly more reproductive biomass than the concentrated damage treatment. There were no significant differences between the concentrated and intermediate damage treatments and no differences between the dispersed damage treatment and the undamaged control. Our data indicate that more dispersed damage is less detrimental to the plant than more concentrated damage. Therefore, the pattern of leaf damage must be considered in determining the impact of herbivores on plant performance.
How Neighbor Canopy Architecture Affects Target Plant Performance
Authors: Tremmel D. C. , Bazzaz F. A.
Published: May 2006

Plant competition is a process that occurs through the negative effects that individual plants have on resource availability to neighboring individuals. Therefore competition experiments need to examine how different species change resource availability to their neighbors, and how different species respond to these changes–allocationally, architecturally, and physiologically–through time. In a greenhouse study we used a model system of annuals to examine how canopies of species having differing morphologies differed in their architectures and light—interception abilities, and how different species performed when grown in these canopies. Abutilon theophrasti, Datura stramonium, and Polygonum pensylvanicum were grown as "targets," surrounded by neighbors of a single species. Neighbors could be any one of the target species or Setaria faberii. Plants were grown in pots, with one target plant and four neighbor plants. Detailed measurements of neighbor canopy structure and target plant canopy architecture were made at five harvests. Species with different morphologies showed large differences in canopy structure, particularly when grass and forb specie were compared. Setaria, a grass, had a more open canopy than the other species (all forbs), and was a consistently weak competitor. Overall, however, the relative effects of different neighbors on target biomass varied with target species. Target biomass was poorly correlated with neighbor biomass and leaf area, but was highly correlated with a measure of target light—interception ability that took into account both target leaf deployment and neighbor light interception. Despite clear differences among neighbor species in canopy structure and effect on light penetration, the results suggest no broad generalizations about the effects of different species as neighbors. Knowledge of morphological, physiological, and life history characteristics of both the target and neighbor species may be necessary to explain the results of their competition.
A Physiologically Based Tritrophic Perspective on Bottom‐Up‐Top‐Down Regulation of Populations
Authors: Gutierrez A. P. , Mills N. J. , Schreiber S. J. , Ellis C. K.
Published: May 2006

A general tritrophic model of intermediate complexity representing the dynamics of trophic level biomass and numbers is presented. The rudiments of the behavior and physiology of resource acquisition and conversion are incorporated as functional and numerical response models. The tritrophic model is used to examine the effects of trophic position on bottom—up—top—down regulation of populations in theory and in practice. The zero growth isoclines of the interacting populations are used to examine the dynamics of the tritrophic system. The herbivore (M2) and predator (M3) but not the plant (M1) isoclines can be solved explicitly. The plant and herbivore isoclines have two forms that depend on whether the proportion of the trophic level available to its consumer (i.e., its apparency) is greater than or less than its potential per unit biomass population growth rate. Rough estimates of the parameters of these inequalities may be deduced from our knowledge of the search biology of the species and known size to growth rate relationships. The model shows clearly that bottom—up regulation sets the upper limit for trophic—level growth and top—down regulation determines the level of realized growth. The model explains the paradoxes of enrichment and of biological control that arise from the standard Lotka—Volterra models, and its qualitative predictions compare well to the general conclusions of intensive studies on biological control of the cassava mealybug on cassava by an exotic parasitoid. However, discrepancies that were found caution against unconsidered extrapolation of theoretical predictions to specific situations. The model qualitatively defines the dynamics required of a successful weed biological control agent, of a stable fresh water algal—arthropod herbivore—vertebrate predator system, and of a marine phytoplankton—krill—whale system. The utility of the model is its generality and its basis in quantifiable biology.
Inbreeding Depression and Maternal Effects in Aquilegia Caerulea, a Partially Selfing Plant
Authors: Montalvo Arlee M.
Published: May 2006

Little is known about how inbreeding depression and maternal effects change from the seed stage through offspring maturation or how these patterns differ for plants germinated under natural vs. artificial conditions. I measured these effects in perennial Aquilegia caerulea (Ranunculaceae) for both field—sown plants and greenhouse—sown progeny that were subsequently transplanted to the field. Overall, field—sown plants and transplants showed similar cumulative inbreeding depression, averaging 0.54 and 0.52, respectively. Selfed individuals had a lower success than outcrossed individuals in all fitness components measured from the number of seeds produced by the cross of size at year 3, and most effects were significant. Inbreeding depression in survival was worse for field—sown progeny in which mortality was much higher. Furthermore, effects of inbreeding on plant size increased each year for greenhouse—sown transplants, but not for field—sown plants, suggesting inbreeding depression is expressed later when survival is artificially high. The effect of seed mass was investigated in greenhouse—sown individuals. Although selfed seeds weighed significantly less than outcrossed seeds, seed mass was not correlated with emergence or survival. It did, however, correlate with leaf expansion rate, which, in turn, correlated with year 3 size in both transplants and field—sown plants. Maternal effects explained a much larger proportion of variance in seed mass than did level of inbreeding. In the greenhouse, effects of seed parent were strongest on early characters, becoming small and nonsignificant by year 3 when inbreeding effects again become significant. In contrast, maternal effects on juvenile traits of field—sown progeny were negligible.