42 results on '"Holt, Robert D."'
Search Results
2. Impacts of Temporal Variation on Apparent Competition and Coexistence in Open Ecosystems
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Holt, Robert D. and Barfield, Michael
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- 2003
3. The Inflationary Effects of Environmental Fluctuations in Source-Sink Systems
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Gonzalez, Andrew and Holt, Robert D.
- Published
- 2002
4. Partial Clonality Expands the Opportunity for Spatial Adaptation.
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Orive, Maria E., Barfield, Michael, and Holt, Robert D.
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ALLEE effect ,GENETIC models ,REPRODUCTION ,PHYSIOLOGICAL adaptation ,LARVAL dispersal ,PHENOTYPES ,HABITATS - Abstract
Reproductive mode may strongly impact adaptation in spatially varying populations linked by dispersal, especially when sexual and clonal offspring differ in dispersal. We determined how spatial structure affects adaptation in populations with mixed clonal and sexual reproduction. In a source-sink quantitative genetic deterministic model (with stabilizing selection around different optima), greater clonal reproduction or parent-offspring association (a measure of the part of the parent's phenotype other than the additive genetic component inherited by clonal offspring) increased the selective difference (difference between phenotypic optima) allowing sink populations to adapt. Given dispersal differences between clonally and sexually produced juveniles, adaptation increased with an increasing fraction of clonal dispersers. When considering migrational meltdown, partially clonal reproduction reduced cases where dispersal caused habitat loss. Stochastic individual-based simulations support these results, although the effect of differential dispersal was reversed, with decreased clonal dispersal allowing greater adaptation. These results parallel earlier findings that for an instantaneous shift in phenotypic optimum, increasing clonality allowed population persistence for a greater shift; here, selective change is spatial rather than temporal. These results may help explain the success of many partially clonal organisms in invading new habitats, complementing traditional explanations based on avoiding Allee effects. [ABSTRACT FROM AUTHOR]
- Published
- 2023
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5. Habitat Selection under Temporal Heterogeneity: Exorcizing the Ghost of Competition past
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Schmidt, Kenneth A., Earnhardt, Joanne M., Brown, Joel S., and Holt, Robert D.
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- 2000
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6. Adaptive Evolution in Source-Sink Environments: Direct and Indirect Effects of Density-Dependence on Niche Evolution
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Holt, Robert D.
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- 1996
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7. Landscape experiments unlock relationships among habitat loss, fragmentation, and patch‐size effects.
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Fletcher, Robert J., Smith, Thomas A. H., Kortessis, Nicholas, Bruna, Emilio M., and Holt, Robert D.
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HABITATS ,FRAGMENTED landscapes ,INSECT communities ,BIOLOGICAL extinction ,FIELD research ,LANDSCAPES - Abstract
Habitat loss is often considered the greatest near‐term threat to biodiversity, while the impact of habitat fragmentation remains intensely debated. A key issue of this debate centers on the problem of scale–landscape or patch–at which to assess the consequences of fragmentation. Yet patterns are often confounded across scales, and experimental designs that could solve this scaling problem remain scarce. We conducted two field experiments in 30 experimental landscapes in which we manipulated habitat loss, fragmentation, and patch size for a community of four insect herbivores that specialize on the cactus Opuntia. In the first experiment, we destroyed 2088 Opuntia patches in either aggregated or random patterns and compared the relative effects of landscape‐scale loss and fragmentation to those of local patch size on species occurrence. This experiment focused on manipulating the relative separation of remaining patches, where we hypothesized that aggregated loss would disrupt dispersal more than random loss, leading to lower occurrence. In the second experiment, we destroyed 759 Opuntia patches to generate landscapes that varied in patch number and size for a given amount of habitat loss and assessed species occurrence. This experiment focused on manipulating the subdivision of remaining habitat, where we hypothesized that an increase in the number of patches for a given amount of loss would lead to negative effects on occurrence. For both, we expected that occurrence would increase with patch size. We find strong evidence for landscape‐scale effects of habitat fragmentation, with aggregated loss and a larger number of patches for a given amount of habitat loss leading to a lower frequency of patches occupied in landscapes. In both experiments, occurrence increased with patch size, yet interactions of patch size and landscape‐scale loss and fragmentation drove species occurrence in patches. Importantly, the direction of effects were consistent across scales and effects of patch size were sufficient to predict the effects of habitat loss and fragmentation across entire landscapes. Our experimental results suggest that changes at both the patch and landscape scales can impact populations, but that a long‐standing pattern—the patch‐size effect—captures much of the key variation shaping patterns of species occurrence. [ABSTRACT FROM AUTHOR]
- Published
- 2023
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8. Theoretical Perspectives on the Statics and Dynamics of Species’ Borders in Patchy Environments
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Holt, Robert D. and Barfield, Michael
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- 2011
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9. Predation and the Evolutionary Dynamics of Species Ranges
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Holt, Robert D., Barfield, Michael, Filin, Ido, and Forde, Samantha
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- 2011
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10. Predator Shadows: Complex Life Histories as Generators of Spatially Patterned Indirect Interactions across Ecosystems
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McCoy, Michael W., Barfield, Michael, and Holt, Robert D.
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- 2009
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11. The Relation of Density Regulation to Habitat Specialization, Evolution of a Species’ Range, and the Dynamics of Biological Invasions
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Filin, Ido, Holt, Robert D., and Barfield, Michael
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- 2008
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12. Canalization Breakdown and Evolution in a Source‐Sink System
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Kimbrell, Tristan and Holt, Robert D.
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- 2007
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13. Temporal Autocorrelation Can Enhance the Persistence and Abundance of Metapopulations Comprised of Coupled Sinks
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Roy, Manojit, Holt, Robert D., and Barfield, Michael
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- 2005
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14. Temporal Variation Can Facilitate Niche Evolution in Harsh Sink Environments
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Holt, Robert D., Barfield, Michael, and Gomulkiewicz, Richard
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- 2004
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15. Allee Effects, Immigration, and the Evolution of Species’ Niches
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Holt, Robert D., Knight, Tiffany M., and Barfield, Michael
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- 2004
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16. Evolutionary Consequences of Asymmetric Dispersal Rates
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Kawecki, Tadeusz J. and Holt, Robert D.
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- 2002
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17. Allee Effects, Invasion Pinning, and Species’ Borders
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Keitt, Timothy H., Lewis, Mark A., and Holt, Robert D.
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- 2001
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18. Spatial Heterogeneity, Indirect Interactions, and the Coexistence of Prey Species
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Holt, Robert D.
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- 1984
19. Vegetation Dynamics in an Experimentally Fragmented Landscape
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Holt, Robert D., Robinson, George R., and Gaines, Michael S.
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- 1995
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20. The factors that favor adaptive habitat construction versus non‐adaptive environmental conditioning.
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Scheiner, Samuel M., Barfield, Michael, and Holt, Robert D.
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EXPECTANCY theories ,GROUP theory ,HABITAT selection ,HABITATS ,CONCEPTUAL models - Abstract
Adaptive habitat construction is a process by which individuals alter their environment so as to increase their (inclusive) fitness. Such alterations are a subset of the myriad ways that individuals condition their environment. We present an individual‐based model of habitat construction to explore what factors might favor selection when the benefits of environmental alterations are shared by individuals of the same species. Our results confirm the predictions of inclusive fitness and group selection theory and expectations based on previous models that construction will be more favored when its benefits are more likely to be directed to self or near kin. We found that temporal variation had no effect on the evolution of construction. For spatial heterogeneity, construction was disfavored when the spatial pattern of movement did not match the spatial pattern of environmental heterogeneity, especially when there was spatial heterogeneity in the optimal amount of construction. Under those conditions, very strong selection was necessary to favor genetic differentiation of construction propensity among demes. We put forth a constitutive theory for the evolution of adaptive habitat construction that unifies our model with previous verbal and quantitative models into a formal conceptual framework. [ABSTRACT FROM AUTHOR]
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- 2022
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21. Do I build or do I move? Adaptation by habitat construction versus habitat choice*.
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Scheiner, Samuel M., Barfield, Michael, and Holt, Robert D.
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PHENOTYPIC plasticity ,HABITATS - Abstract
Trait adaptation to a heterogeneous environment can occur through six modes: genetic differentiation of those traits, a jack‐of‐all‐trades phenotypic uniformity, diversified bet‐hedging, phenotypic plasticity, habitat choice, and habitat construction. A key question is what circumstances favor one mode over another, and how they might interact if a system can express more than one mode at a time. We examined the joint evolution of habitat choice and habitat construction using individual‐based simulations. We manipulated when during the life cycle construction occurred and the fitness value of construction. We found that for our model habitat construction was nearly always favored over habitat choice, especially if construction happened after dispersal. Because of the ways that the various modes of adaptation interact with each other, there is no simple answer as to which will be favored; it depends on details of the biology and ecology of a given system. [ABSTRACT FROM AUTHOR]
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- 2022
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22. The evolution of habitat construction with and without phenotypic plasticity*.
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Scheiner, Samuel M., Barfield, Michael, and Holt, Robert D.
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PHENOTYPES ,PHENOTYPIC plasticity ,LIFE history theory ,HABITAT selection ,HABITATS - Abstract
Habitat construction and phenotypic plasticity are alternative responses to variable environments. We explored evolution along an environmental gradient of habitat construction alone and in combination with phenotypic plasticity using individual‐based simulations that manipulated the fitness benefit of construction and whether construction maintained or eliminated that gradient. Construction was favored when its benefits were more likely to flow to the immediate offspring of the constructing individuals. Habitat construction and phenotypic plasticity traded off against each other or plasticity was selected against, depending on how the optimum environment varied and with the fitness value of construction. When selection favored differences in the amount of construction along the environmental gradient, genetic differentiation for habitat construction increased as the fitness value of construction increased. The degree to which each adaptive response was likely to evolve also depended on the precise ordering of life history events. Adaptive habitat construction does not always occur and may be selected against. [ABSTRACT FROM AUTHOR]
- Published
- 2021
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23. Eco-evolutionary dynamics in fragmented landscapes.
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Legrand, Delphine, Cote, Julien, Fronhofer, Emanuel A., Holt, Robert D., Ronce, Ophélie, Schtickzelle, Nicolas, Travis, Justin M. J., and Clobert, Jean
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ECOSYSTEM dynamics ,FRAGMENTED landscapes ,HABITATS ,BIOLOGICAL adaptation ,POPULATION dynamics - Abstract
It is widely recognized that ecological dynamics influence evolutionary dynamics, and conversely that evolutionary changes alter ecological processes. Because fragmentation impacts all biological levels (from individuals to ecosystems) through isolation and reduced habitat size, it strongly affects the links among evolutionary and ecological processes such as population dynamics, local adaptation, dispersal and speciation. Here, we review our current knowledge of the eco-evolutionary dynamics in fragmented landscapes, focusing on both theory and experimental studies. We then suggest future experimental directions to study eco-evolutionary dynamics and/or feedbacks in fragmented landscapes, especially to bridge the gap between theoretical predictions and experimental validations. [ABSTRACT FROM AUTHOR]
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- 2017
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24. Metapopulations and metacommunities: combining spatial and temporal perspectives in plant ecology.
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Alexander, Helen M., Foster, Bryan L., Ballantyne, Ford, Collins, Cathy D., Antonovics, Janis, and Holt, Robert D.
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PLANT ecology ,SEED dispersal ,METAPOPULATION (Ecology) ,HABITATS ,BIOLOGICAL variation - Abstract
Summary 1. Metapopulation and metacommunity theories occupy a central role in ecology, but can be difficult to apply to plants. Challenges include whether seed dispersal is sufficient for population connectivity, the role of seed banks and problems with studying colonization and extinction in long-lived and clonal plants. Further, populations often do not occupy discrete habitat patches. Despite these difficulties, we present case studies to illustrate explicit integration of spatial and temporal data in plant ecology. 2. First, on the population level, we focused on two early successional species that lack discrete habitat patches. Multi-year data sets taken with a grid approach and simple models permit the analysis of landscape dynamics that reflect regional as well as local processes. Using Silene latifolia, we examined colonization. We found evidence for seed dispersal and connectivity among populations across a large landscape. With Helianthus annuus, a species with seed banks, we determined the degree to which landscape-level patterns of abundance were predicted by local processes (previous year recruitment at a site plus seed banks) vs. seed dispersal. Local processes dominated dynamics. 3. Second, at the community level, we utilized a landscape-level experiment to examine the influence of environmental gradients and spatial processes (dispersal limitation) on community composition during 18 years of succession. Throughout succession, environmental and spatial factors both contributed significantly to spatial variation in species composition (beta diversity). When connectivity was disrupted, space was the dominant factor underlying beta diversity, and this did not change over time. Across more connected communities, spatial effects diminished over succession as the importance of environmental factors increased, consistent with species-sorting metacommunity models. 4. Synthesis. Metapopulation/metacommunity concepts emphasize the interaction between space and time in ecological processes. Spatial processes, such as long-distance dispersal, play a crucial role in creating new populations. Temporal processes, including seed banks, may dominate dynamics at both local and regional scales. The relative importance of spatial vs. temporal processes changes as populations persist and communities assemble over time; these patterns may only emerge after many years. Integrating long-term data with spatial data is thus essential for understanding spatio-temporal patterns inherent in metapopulation and metacommunity theories. [ABSTRACT FROM AUTHOR]
- Published
- 2012
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25. Predicted Impact of Barriers to Migration on the Serengeti Wildebeest Population.
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Holdo, Ricardo M., Fryxell, John M., Sinclair, Anthony R. E., Dobson, Andrew, and Holt, Robert D.
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EMIGRATION & immigration ,HABITATS ,ECOLOGY ,SIMULATION methods & models ,ANALYTICAL mechanics ,BIODIVERSITY ,FRAGMENTED landscapes ,ECOSYSTEM health ,BIOLOGY - Abstract
The Serengeti wildebeest migration is a rare and spectacular example of a once-common biological phenomenon. A proposed road project threatens to bisect the Serengeti ecosystem and its integrity. The precautionary principle dictates that we consider the possible consequences of a road completely disrupting the migration. We used an existing spatially-explicit simulation model of wildebeest movement and population dynamics to explore how placing a barrier to migration across the proposed route (thus creating two disjoint but mobile subpopulations) might affect the long-term size of the wildebeest population. Our simulation results suggest that a barrier to migration—even without causing habitat loss—could cause the wildebeest population to decline by about a third. The driver of this decline is the effect of habitat fragmentation (even without habitat loss) on the ability of wildebeest to effectively track temporal shifts in high-quality forage resources across the landscape. Given the important role of the wildebeest migration for a number of key ecological processes, these findings have potentially important ramifications for ecosystem biodiversity, structure, and function in the Serengeti. [ABSTRACT FROM AUTHOR]
- Published
- 2011
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26. Experimental evidence that source genetic variation drives pathogen emergence.
- Author
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Dennehy, John J., Friedenberg, Nicholas A., McBride, Robert C., Holt, Robert D., and Turner, Paul E.
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PATHOGENIC microorganisms ,HABITATS ,PSEUDOMONAS ,BIOLOGICAL evolution ,BACTERIOPHAGES ,STATISTICAL hypothesis testing - Abstract
A pathogen can readily mutate to infect new host types, but this does not guarantee successful establishment in the new habitat. What factors, then, dictate emergence success? One possibility is that the pathogen population cannot sustain itself on the new host type (i.e. host is a sink), but migration from a source population allows adaptive sustainability and eventual emergence by delivering beneficial mutations sampled from the source's standing genetic variation. This idea is relevant regardless of whether the sink host is truly novel (host shift) or whether the sink is an existing or related, similar host population thriving under conditions unfavourable to pathogen persistence (range expansion). We predicted that sink adaptation should occur faster under range expansion than during a host shift owing to the effects of source genetic variation on pathogen adaptability in the sink. Under range expansion, source migration should benefit emergence in the sink because selection acting on source and sink populations is likely to be congruent. By contrast, during host shifts, source migration is likely to disrupt emergence in the sink owing to uncorrelated selection or performance tradeoffs across host types. We tested this hypothesis by evolving bacteriophage populations on novel host bacteria under sink conditions, while manipulating emergence via host shift versus range expansion. Controls examined sink adaptation when unevolved founding genotypes served as migrants. As predicted, adaptability was fastest under range expansion, and controls did not adapt. Large, similar and similarly timed increases in fitness were observed in the host-shift populations, despite declines in mean fitness of immigrants through time. These results suggest that source populations are the origin of mutations that drive adaptive emergence at the edge of a pathogen's ecological or geographical range. [ABSTRACT FROM AUTHOR]
- Published
- 2010
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27. Niche conservatism as an emerging principle in ecology and conservation biology.
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Wiens, John J., Ackerly, David D., Allen, Andrew P., Anacker, Brian L., Buckley, Lauren B., Cornell, Howard V., Damschen, Ellen I., Jonathan Davies, T., Grytnes, John-Arvid, Harrison, Susan P., Hawkins, Bradford A., Holt, Robert D., McCain, Christy M., and Stephens, Patrick R.
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BIOTIC communities ,ENVIRONMENTAL sciences ,NATURAL selection ,HABITATS ,ENVIRONMENTAL management - Abstract
The diversity of life is ultimately generated by evolution, and much attention has focused on the rapid evolution of ecological traits. Yet, the tendency for many ecological traits to instead remain similar over time [niche conservatism (NC)] has many consequences for the fundamental patterns and processes studied in ecology and conservation biology. Here, we describe the mounting evidence for the importance of NC to major topics in ecology (e.g. species richness, ecosystem function) and conservation (e.g. climate change, invasive species). We also review other areas where it may be important but has generally been overlooked, in both ecology (e.g. food webs, disease ecology, mutualistic interactions) and conservation (e.g. habitat modification). We summarize methods for testing for NC, and suggest that a commonly used and advocated method (involving a test for phylogenetic signal) is potentially problematic, and describe alternative approaches. We suggest that considering NC: (1) focuses attention on the within-species processes that cause traits to be conserved over time, (2) emphasizes connections between questions and research areas that are not obviously related (e.g. invasives, global warming, tropical richness), and (3) suggests new areas for research (e.g. why are some clades largely nocturnal? why do related species share diseases?). [ABSTRACT FROM AUTHOR]
- Published
- 2010
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28. IJEE SOAPBOX: HABITATS AND SEASONS.
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HOLT, ROBERT D.
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HABITAT selection , *SEASONS , *POPULATION dynamics , *HABITATS , *BIOTIC communities , *BIOLOGICAL evolution - Abstract
The article discusses the role of seasonality in habitat selection. According to biologist Steve Fretwell, most populations live in a seasonal environment. He theorizes that the exact temporal pattern of variation in demographic parameters affects the extent at which seasonal cycles affect a population size. Experimental and theoretical studies on the role of seasonality in population dynamics are cited. Also explained are the implications of seasonality to evolutionary process and population mobility.
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- 2008
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29. HABITAT SELECTION AND NICHE CONSERVATISM.
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HOLT, ROBERT D. and BARFIELD, MICHAEL
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HABITAT selection , *HABITATS , *POPULATION biology , *ECOLOGY , *BIOTIC communities , *BIOLOGY - Abstract
Niche conservatism is the phenomenon in which species or other phylogenetic lineages seem to exhibit much the same ecological niche over their geographical ranges or over evolutionary time scales. Previous studies have suggested that optimal habitat selection call constrain niche evolution (and thus may help explain niche conservatism), in effect by preventing populations from experiencing unfavorable sink environments with conditions outside the niche. This paper relaxes a key assumption in prior literature, by allowing habitat selection decisions to be conditional upon an individual's phenotype. We use individual-based simulations of evolution in source-sink landscapes to examine the consequences of phenotype-matching habitat selection. If each individual obeys a "perfect'" habitat selection rule moving to another habitat only if that movement increases the match between its phenotype and the habitat's optimum--then habitat selection can speed up the rate of adaptation to sink environments, and thus facilitate niche evolution. We also note that habitat selection can also indirectly influence the pool of variation available for selection, and thereby potentially alter to some degree this conclusion. [ABSTRACT FROM AUTHOR]
- Published
- 2008
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30. ALTERNATIVE PREY AND THE DYNAMICS OF INTRAGUILD PREDATION: THEORETICAL PERSPECTIVES.
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Holt, Robert D. and Huxel, Gary R.
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PREDATION , *SYMMETRY (Biology) , *OSCILLATIONS , *STANDARDIZATION , *SPECIES diversity , *HABITATS , *POPULATION biology , *FOOD chains , *ECOLOGY education - Abstract
A rich body of theoretical literature now exists focused on the three-species module of intraguild predation (IGP), in which a top predator both attacks and competes with an intermediate predator. Simple models of intraguild predation are often unstable, either because one consumer is excluded, or because sustained oscillations emerge from long feedback loops. Yet, many natural IGP systems robustly persist. Standard models of intraguild predation simplify natural systems in crucial ways that could influence persistence; in particular, many empirical IGP systems are embedded in communities with alternative prey species. We briefly review the key conclusions of standard three-species IGP theory, and then present results of theoretical explorations of how alternative prey can influence the persistence and stability of a focal intraguild predation interaction. [ABSTRACT FROM AUTHOR]
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- 2007
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31. PREDATORS, PARASITOIDS, AND PATHOGENS: A CROSS-CUTTING EXAMINATION OF INTRAGUILD PREDATION THEORY.
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Borer, Elizabeth T., Briggs, Cheryl J., and Holt, Robert D.
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STATISTICAL correlation ,PREDATION ,HABITATS ,HOST-parasite relationships ,SPECIES diversity ,POPULATION biology ,FOOD chains ,MARINE biology ,ECOLOGY education - Abstract
Although the canonical concept of intraguild predation evokes images of predators and prey, several subdisciplines within ecology have developed theory not specifically framed in terms of predation and competition and often using system-specific terminology, yet functionally quite similar. Here, we formulate models combining exploitation and competition in predator-prey, host-parasitoid, and host-pathogen communities to compare dynamics, food web structure, and coexistence criteria for these disparate communities. Although dynamic stability in the coexistence region varies strongly among systems, in all cases coexistence of two consumers on a single resource occurs only if the intraguild prey species is more efficient than the intraguild predator at suppressing the abundance of the basal resource, and if the intraguild predator accrues a sufficient gain from attacking the intraguild prey. In addition, equilibrial abundances of all species in all three formulations respond similarly to increases in productivity of the basal resource. Our understanding of predator-prey and parasitoid-host communities has benefited from explicit examination of intraguild predation (IGP) theory, and we suggest that future research examining pathogen communities, in particular, will benefit substantially from explicit recognition of predictions from IGP theory. [ABSTRACT FROM AUTHOR]
- Published
- 2007
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32. IJEE SOAPBOX: THE UNRAVELING OF NATURE'S INFORMATION WEBS: THE NEXT DEPRESSING FRONTIER IN CONSERVATION?
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HOLT, ROBERT D.
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SCIENTISTS , *PSYCHOLOGY , *ECOLOGY , *HABITATS , *POPULATION biology , *FOOD chains - Abstract
The article reflects on how scientists can use behavioral indicators to monitor the behaviors of individuals of other species to make inferences about the habitat quality they experience. The author shares his personal experience of how he became aware of the behavioral dimension of ecology when he was a graduate student at Harvard University in the 1970s. It discusses the factors that influence predator-prey interactions like heterospecific cuing and the food web. Also noted the direct impacts caused by humans on the information environment which, according to the author, are bound to disrupt preexisting webs of information that organisms historically depended upon in a variety of ways.
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- 2007
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33. The effects of immigration and environmental variability on the persistence of an inferior competitor.
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Long, Zachary T., Petchey, Owen L., and Holt, Robert D.
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SPECIES ,HABITATS ,COLPIDIUM striatum ,ANIMAL migration ,ECOLOGY ,ANIMAL populations - Abstract
We tested the hypothesis that temporally autocorrelated variation should increase the abundance of an inferior competitor sustained by immigration. Temporally autocorrelated variability can increase abundance of the inferior species through effects on demography, the strength of competition, and the mean and variance in the abundance of competing species. We allowed the competitive inferior to immigrate into habitats with constant, variable, or temporally autocorrelated temperature regimes. In the absence of immigration, competitive exclusion occurred, in both constant and variable environments. Immigration permitted persistence of the inferior species, and increased immigration rates led to increased abundance. Temporally autocorrelated variability enhanced this effect of immigration. This ‘inflationary’ effect suggests that the interplay of immigration and environmental variability can jointly influence the outcome of competitive interactions. Our results suggest that an increase in temporal autocorrelation of environmental variability will cause regional processes to increasingly influence local interactions. [ABSTRACT FROM AUTHOR]
- Published
- 2007
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34. Plant productivity and soil nitrogen as a function of grazing, migration and fire in an African savanna.
- Author
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HOLDO, RICARDO M., HOLT, ROBERT D., COUGHENOUR, MICHAEL B., and RITCHIE, MARK E.
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- *
PLANT ecology , *PLANT products , *AGRICULTURAL productivity , *HERBIVORES , *FIRE , *HABITATS , *NITROGEN , *BIOMASS , *PLANT reproduction - Abstract
1 Herbivores can play a key role in affecting ecosystem function, but their direct and indirect effects are often confounded with each other and have rarely been dissected. Predictions for open systems, i.e. those with cross-habitat nutrient fluxes and dispersal, may differ from those expected in closed systems, where no such transfers occur, but these differences have only recently begun to be characterized. 2 We present a theoretical model of plant productivity and soil nitrogen (N) based on the Serengeti ecosystem in order to investigate the interplay among herbivore movement, nutrient transport across habitats (spatial subsidies), the fire regime and the effects of herbivores on N cycling. 3 Model results suggest that the fire regime and herbivore migration are key determinants of primary productivity and fertility, and that the impact of these factors depends on grazing intensity. At low intensity, high grass biomass leads to extensive fires and N volatilization, but this effect is reduced at high grazing intensity. Without migration, the model predicts that primary productivity and fertility initially increase, then decline with increasing grazing intensity. Conversely, seasonal migration decouples the growing and grazing seasons, leading to a monotonic increase in productivity with grazing intensity. Cross-habitat N transport has a relatively modest effect on N dynamics, and the magnitude of the seasonality effect outweighs the spatial subsidy effect. 4 Our model suggests that herbivores and fire may play key interactive roles in regulating producers and determining ecosystem functional properties in grazing systems, both through consumption and via indirect effects on nutrient availability. The direction and magnitude of these effects could potentially vary greatly among ecosystems with resident vs. migratory herbivores, suggesting that spatial coupling through animal movement can be important at both the population and ecosystem levels. 5 This study suggests that the degree to which trophic interactions influence ecosystem function may be strongly modulated by whether systems are open or closed. It also suggests that direct and indirect effects of consumers on their resources (top-down effects) may be far more important than nutrient subsidies in open terrestrial systems. [ABSTRACT FROM AUTHOR]
- Published
- 2007
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35. Trophic cascades across ecosystems.
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Knight, Tiffany M., McCoy, Michael W., Chase, Jonathan M., McCoy, Krista A., and Holt, Robert D.
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PREDATION ,FOOD chains ,ORGANISMS ,HABITATS ,PLANT reproduction ,BIOTIC communities - Abstract
Predation can be intense, creating strong direct and indirect effects throughout food webs. In addition, ecologists increasingly recognize that fluxes of organisms across ecosystem boundaries can have major consequences for community dynamics. Species with complex life histories often shift habitats during their life cycles and provide potent conduits coupling ecosystems. Thus, local interactions that affect predator abundance in one ecosystem (for example a larval habitat) may have reverberating effects in another (for example an adult habitat). Here we show that fish indirectly facilitate terrestrial plant reproduction through cascading trophic interactions across ecosystem boundaries. Fish reduce larval dragonfly abundances in ponds, leading to fewer adult dragonflies nearby. Adult dragonflies consume insect pollinators and alter their foraging behaviour. As a result, plants near ponds with fish receive more pollinator visits and are less pollen limited than plants near fish-free ponds. Our results confirm that strong species interactions can reverberate across ecosystems, and emphasize the importance of landscape-level processes in driving local species interactions. [ABSTRACT FROM AUTHOR]
- Published
- 2005
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36. A Survey and Overview of Habitat Fragmentation Experiments.
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Debinski, Diane M. and Holt, Robert D.
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HABITATS , *NATURE conservation - Abstract
Habitat destruction and fragmentation are the root causes of many conservation problems. We conducted a literature survey and canvassed the ecological community to identify experimental studies of terrestrial habitat fragmentation and to determine whether consistent themes were emerging from these studies. Our survey revealed 20 fragmentation experiments worldwide. Most studies focused on effects of fragmentation on species richness or on the abundance(s) of particular species. Other important themes were the effect of fragmentation in interspecific interactions, the role of corridors and landscape connectivity in individual movements and species richness, and the influences of edge effects on ecosystem services. Our comparisons showed a remarkable lack of consistency in results across studies, especially with regard to species richness and abundance relative to fragment size. Experiments with arthropods showed the best fit with theoretical expectations of greater species richness on larger fragments. Highly mobile taxa such as birds and mammals, early-successional plant species, long-lived species, and generalist predators did not respond in the “expected” manner. Reasons for these discrepancies included edge effects, competitive release in the habitat fragments, and the spatial scale of the experiments. One of the more consistently supported hypotheses was that movement and species richness are positively affected by corridors and connectivity, respectively. Transient effects dominated many systems; for example, crowding of individuals on fragments commonly was observed after fragmentation, followed by a relaxation toward lower abundance in subsequent years. The three long-term studies (>=14 years) revealed strong patterns that would have been missed in short-term investigations. Our results emphasize the wide range of species-specific responses to fragmentation, the need for elucidation of behavioral mechanisms affecting these responses, and the... [ABSTRACT FROM AUTHOR]
- Published
- 2000
- Full Text
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37. Do I build or do I move? Adaptation by habitat construction versus habitat choice*.
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Scheiner, Samuel M., Barfield, Michael, and Holt, Robert D.
- Subjects
- *
PHENOTYPIC plasticity , *HABITATS - Abstract
Trait adaptation to a heterogeneous environment can occur through six modes: genetic differentiation of those traits, a jack‐of‐all‐trades phenotypic uniformity, diversified bet‐hedging, phenotypic plasticity, habitat choice, and habitat construction. A key question is what circumstances favor one mode over another, and how they might interact if a system can express more than one mode at a time. We examined the joint evolution of habitat choice and habitat construction using individual‐based simulations. We manipulated when during the life cycle construction occurred and the fitness value of construction. We found that for our model habitat construction was nearly always favored over habitat choice, especially if construction happened after dispersal. Because of the ways that the various modes of adaptation interact with each other, there is no simple answer as to which will be favored; it depends on details of the biology and ecology of a given system. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
- View/download PDF
38. Use it or lose it.
- Author
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Holt, Robert D.
- Subjects
- *
HABITATS , *ECOLOGY , *GENETIC mutation , *COMPETITION (Biology) - Abstract
Comments on research to determine the origins of ecological specialization. Possible causes of functional degeneration and habitat specialization, including mutations; Confinement of replicate, genetically homogeneous lineages of bacteria to a simple environment; Statement that results may better predict the exclusion of a particular lineage through indirect competition for resources.
- Published
- 2000
- Full Text
- View/download PDF
39. Genetic variation for habitat preference: evidence and explanations
- Author
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Holt, Robert D. and Jaenike, John
- Subjects
ECOLOGY ,BIOLOGICAL variation ,HABITATS - Published
- 1991
40. Spatial heterogeneity, indirect interactions, and the coexistence ofprey species
- Author
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Holt, Robert D.
- Subjects
HABITATS ,COMMUNITIES ,ECOLOGY - Published
- 1984
- Full Text
- View/download PDF
41. Can habitat specialization patterns of Neotropical birds highlight vulnerable areas for conservation in the Atlantic rainforest, southern Brazil?
- Author
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dos Anjos, Luiz, Collins, Cathy D., Holt, Robert D., Volpato, Graziele H., Lopes, Edson V., and Bochio, Gabriela M.
- Subjects
- *
HABITATS , *BIRD populations , *CONSERVATION biology , *RAIN forests - Abstract
Patterns of habitat specialization can be used to infer conservation risks for species whose habitats are being degraded or lost due to anthropogenic activity. We assigned levels of forest type specialization to birds based on their occurrence and local abundance in three basic types of Atlantic rainforest in southern Brazil: seasonal semi-deciduous forest, mixed rainforest, and dense rainforest. We hypothesized that forest types would contain birds characterized by different levels of specialization. Specifically, we predicted that the dense forest would contain more specialists due to its higher plant species richness. Our study focused on a major clade of the suboscine Passeriformes (Thamnophilidae–Furnariidae), the majority of which are understory insectivorous species. We used field data from 16 locations including 35 sites spanning the major forest types in southern Brazil. We found (1) substantial variation in abundance among sampling locations for a single bird species within and between forest types; (2) species-specific associations with forest types; and (3) spatial aggregation among species that share similar levels of specialization. Using our method of qualitative scoring of forest type specialization, three species were considered habitat specialists ( Xenops minutus , Leptasthenura setaria , and Cranioleuca pallida ), twelve species were moderate specialists, sixteen were generalists, five were complete generalists, and twelve species were too rare to be included in the analysis. Contrary to our hypothesis, both mixed and dense forests were equally dominated by specialists. We discuss the implications of our approaches for conservation assessment and planning, both at the single-species level and for aggregate species in assemblages. [ABSTRACT FROM AUTHOR]
- Published
- 2015
- Full Text
- View/download PDF
42. Bird species abundance–occupancy patterns and sensitivity to forest fragmentation: Implications for conservation in the Brazilian Atlantic forest
- Author
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Anjos, Luiz dos, Collins, Cathy D., Holt, Robert D., Volpato, Graziele H., Mendonça, Luciana B., Lopes, Edson V., Boçon, Roberto, Bisheimer, Maria V., Serafini, Patrícia P., and Carvalho, Joema
- Subjects
- *
FOREST birds , *ANIMAL species , *FORESTS & forestry , *FOREST ecology , *HABITATS , *STATISTICAL hypothesis testing , *POPULATION dynamics , *BIRD conservation - Abstract
Abstract: Developing a predictive theory for species responses to habitat fragmentation is a large, complex challenge in conservation biology, and meeting this challenge likely requires tailoring predictions to specific habitats and taxa. We evaluate the effects of fragmentation on forest birds living in three distinct forest ecosystems found in Brazilian Atlantic forest: seasonal semi-deciduous forest (SF), mixed rain forest (MF), and dense rain forest (DF). We test the hypotheses that (1) bird species most prevalent in SF (relative to other habitat types) will be least vulnerable to population declines in fragmented SF, and (2) species with stronger affiliations with DF or MF will be relatively more sensitive to fragmentation in SF. Using an exploratory statistical technique called “Rank Occupancy–Abundance Profiles (ROAPs),” we compared distribution and abundance of birds among large “continuous” areas of each forest type, then compared abundances in continuous SF forests with patterns of abundance in small fragments of SF, where edge effects could play a marked role in population dynamics. Overall, 39 species showed substantially lower local abundance, occupancy, or both in SF fragments versus continuous SF. As predicted, a higher proportion of bird species associated with DF appeared sensitive to fragmentation in SF; by contrast, species most abundant in SF and MF were similarly abundant in fragmented SF. Our study demonstrates how quantifying distribution and abundance in diverse habitats may enhance managers’ ability to incorporate species-specific responses to human disturbances in their conservation plans, and points out ways that even small reserves may have significant conservation value. [Copyright &y& Elsevier]
- Published
- 2011
- Full Text
- View/download PDF
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