Robert Swihart | Purdue University (original) (raw)

Papers by Robert Swihart

Environmental Modelling and Software, 2007

Landowners need a simple-to-use, readily available tool to contribute to sustainable management o... more Landowners need a simple-to-use, readily available tool to contribute to sustainable management of family forests. We developed an Internetbased decision-support system, 4S Tool (Forest Stand Software Support System), which is designed to encourage informed forest management and to bridge the gap between a new generation of family forest owners (e.g. exurbanites) and natural resources professionals. We describe the technical and educational components of the 4S Tool and present several considerations for application to other geographic regions. The tool requires users to provide information on the species group, typical tree size, and density of trees in their forest. Users also select a forest management technique to apply to their ''virtual forest.'' The tool then estimates forest dynamics and wildlife habitat availability for up to 40 years into the future. The output provides projected changes in commercial timber, as well as changes in the diversity of berries, ferns, herbs, flowers, birds, and mammals as a consequence of the selected management option. Projected changes are based on primary and secondary data sources and modeled via the Forest Vegetation Simulator. 4S Tool provides a means of enhancing landowner awareness of options associated with management of forest property and provides users with access to additional resources and natural resource professionals to aid in future forest planning efforts.

Presence-absence data are used widely in analysis of wildlife-habitat relationships. Failure to d... more Presence-absence data are used widely in analysis of wildlife-habitat relationships. Failure to detect a species' presence in an occupied habitat patch is a common sampling problem when the population size is small, individuals are difficult to sample, or sampling effort is limited. In this paper, the influence of non-detection of occurrence on parameter estimates of logistic regression models of wildlife-habitat relationships was assessed using analytical analysis and simulations. Two patterns of non-detection were investigated: (1) a random distribution of non-detection among occupied patches; and (2) a non-random distribution of nondetection in which the probability of detecting a species in an occupied patch covaried with measurable habitat variables. Our results showed that logistic regression models of wildlife-habitat relationships were sensitive to even low levels of non-detection in occupancy data. Both analytic and simulation studies show that non-detection yields bias in parameter estimation of logistic regression models. More importantly, the direction of bias was affected by the underlying pattern of non-detection and whether the habitat variable was positively or negatively related to occupancy. For a positive habitat coefficient, a random distribution of nondetection yielded negative bias in estimation, whereas linkage of the probability of non-detection to habitat covariates produced positive bias. For a negative habitat coefficient, the pattern was reversed, with a random distribution of non-detection leading to positive bias in estimation. A release-recapture livetrapping study of small mammals in central Indiana, USA, was used to illustrate the magnitude of non-detection in a typical field sampling protocol with varying levels of sampling intensity. Estimates of nondetection error ranged from 0 to 23% for seven species after 5 days of sampling. We suggest that for many sampling situations, relationships between probability of detection and habitat covariates need to be established to correctly interpret results of wildlifehabitat models. #

Ultrasonic detectors are widely used to survey bats in ecological studies. To evaluate efficacy o... more Ultrasonic detectors are widely used to survey bats in ecological studies. To evaluate efficacy of acoustic identification, we compiled a library of search phase calls from across the eastern United States using the Anabat system. The call library included 1,846 call sequences of 12 species recorded from 14 states. We determined accuracy rates using 3 parametric and 4 nonparametric classification functions for acoustic identification. The 2 most flexible classification functions also were the most accurate: neural networks (overall classification accuracy ¼ 0.94) and mixture discriminant analysis incorporating an adaptive regression model (overall classification accuracy ¼ 0.93). Flexible nonparametric methods offer substantial benefits when discriminating among closely related species and may preclude the need to group species with similar calls. We demonstrate that quantitative methods provide an effective technique to acoustically identify bats in the eastern United States with known accuracy rates. ß 2011 The Wildlife Society.

Forests dominated by oak, beech, hornbeam, and boreal conifers are characterized by resource puls... more Forests dominated by oak, beech, hornbeam, and boreal conifers are characterized by resource pulses. Contemporary eastern U.S. oak forests may exhibit dramatic resource pulses in part because of the loss of American chestnut, which comprised 25-50% of the canopy throughout its range. Chestnut loss may have reduced mast resources for wildlife. A newly developed, blight-resistant hybrid makes reintroduction feasible within several years. We use historical data to model the effects of American chestnut mast on consumer populations, illuminating the potential ecological effects of a successful restoration. We parameterized a stochastic population growth model with mast production data to compare consumer population dynamics both pre-and post-blight. We explored the effect of maximum intrinsic population growth rate, degree of density dependence, and reliance upon mast on consumer response to chestnut loss. We parameterized the models for white-footed mouse, eastern chipmunk, gray squirrel, and white-tailed deer. At a northern site 14 years post-blight, simulated annual mast production decreased 80% and the coefficient of variation (CV) increased 60%. At a southern site 35 years post-blight, annual mast production decreased by 35% and the CV increased by 76%. Smaller, more variable mast crops translated to reduced abundance and increased variability in simulated consumer populations. White-footed mice were the most responsive, exhibiting a 48% decrease in population size and 57% increase in interannual variation post-blight. The reintroduction of blightresistant chestnut may fundamentally alter predator-prey interactions, gypsy moth outbreaks, and Lyme disease hot spots through its effect on the character of resource pulses that drive consumer dynamics.

The ability to make a priori assessments of a species' response to fragmentation, based on its di... more The ability to make a priori assessments of a species' response to fragmentation, based on its distribution in the landscape, would serve as a valuable conservation and management tool. During 1997-1999, we monitored 717 scent stations to examine seasonal use of forest patches, corridors, and crop fields by coyotes (Canis latrans), domestic cats (Felis catus), foxes (Vulpes vulpes and Urocyon cinereoargenteus), raccoons (Procyon lotor), striped skunks (Mephitis mephitis), opossums (Didelphis virginiana), and long-tailed weasels (Mustela frenata). For each species we developed landscape-based ecologically scaled landscape indices (ELSI), and we modeled species spatial distribution across three spatial scales (landscape-level, element-level, and local habitat-level). Our results suggest that these predators view landscape fragmentation at different spatial scales and demonstrate strong interspecific differences in their response to elements of the landscape. All species except coyotes and domestic cats avoided agricultural fields. In general, predator species that were more mobile (i.e. high ESLI for landscape connectivity; coyotes) were characterized by landscape-and element-based logistic models. In contrast, models including local habitat features generally were most appropriate for less mobile or more stenophagous predators (e.g. long-tailed weasels). Our analysis extends the application of the ESLI concept to species assemblages that do not appear to function as metapopulations, and it highlights the importance of examining spatial scale and species-specific responses to habitat fragmentation. We discuss the relevance of these findings for defining ecological landscapes, understanding predator-prey interactions at multiple spatial scales, and conserving predator and prey populations in fragmented landscapes. #

A community is ''nested'' when species assemblages in less rich sites form nonrandom subsets of t... more A community is ''nested'' when species assemblages in less rich sites form nonrandom subsets of those at richer sites. Conventional null models used to test for statistically nonrandom nestedness are under-or over-restrictive because they do not sufficiently isolate ecological processes of interest, which hinders ecological inference. We propose a class of null models that are ecologically explicit and interpretable. Expected values of species richness and incidence, rather than observed values, are used to create random presence-absence matrices for hypothesis testing. In our examples, based on six datasets, expected values were derived either by using an individually based random placement model or by fitting empirical models to richness data as a function of environmental covariates. We describe an algorithm for constructing unbiased null matrices, which permitted valid testing of our null models. Our approach avoids the problem of building too much structure into the null model, and enabled us to explicitly test whether observed communities were more nested than would be expected for a system structured solely by species-abundance and species-area or similar relationships. We argue that this test or similar tests are better determinants of whether a system is truly nested; a nested system should contain unique pattern not already predicted by more fundamental ecological principles such as species-area relationships. Most species assemblages we studied were not nested under these null models. Our results suggest that nestedness, beyond that which is explained by passive sampling processes, may not be as widespread as currently believed. These findings may help to improve the utility of nestedness as an ecological concept and conservation tool.

Many patch-based metapopulation models assume that the local population within each patch is at i... more Many patch-based metapopulation models assume that the local population within each patch is at its equilibrium and independent of changes in patch occupancy. We studied a metapopulation model that explicitly incorporates the local population dynamics of two competing species. The singular perturbation method is used to separate the fast dynamics of the local competition and the slow process of patch colonization and extinction. Our results show that the coupled system leads to more complex outcomes than simple patch models which do not include explicit local dynamics. We also discuss implications of the model for ecological systems in fragmented landscapes.

2001. Changes in transmission of Baylisascaris procyonis to intermediate hosts as a function of s... more 2001. Changes in transmission of Baylisascaris procyonis to intermediate hosts as a function of spatial scale. -Oikos 93: 213-220.

Environmental Modelling and Software, 2007

Landowners need a simple-to-use, readily available tool to contribute to sustainable management o... more Landowners need a simple-to-use, readily available tool to contribute to sustainable management of family forests. We developed an Internetbased decision-support system, 4S Tool (Forest Stand Software Support System), which is designed to encourage informed forest management and to bridge the gap between a new generation of family forest owners (e.g. exurbanites) and natural resources professionals. We describe the technical and educational components of the 4S Tool and present several considerations for application to other geographic regions. The tool requires users to provide information on the species group, typical tree size, and density of trees in their forest. Users also select a forest management technique to apply to their ''virtual forest.'' The tool then estimates forest dynamics and wildlife habitat availability for up to 40 years into the future. The output provides projected changes in commercial timber, as well as changes in the diversity of berries, ferns, herbs, flowers, birds, and mammals as a consequence of the selected management option. Projected changes are based on primary and secondary data sources and modeled via the Forest Vegetation Simulator. 4S Tool provides a means of enhancing landowner awareness of options associated with management of forest property and provides users with access to additional resources and natural resource professionals to aid in future forest planning efforts.

Presence-absence data are used widely in analysis of wildlife-habitat relationships. Failure to d... more Presence-absence data are used widely in analysis of wildlife-habitat relationships. Failure to detect a species' presence in an occupied habitat patch is a common sampling problem when the population size is small, individuals are difficult to sample, or sampling effort is limited. In this paper, the influence of non-detection of occurrence on parameter estimates of logistic regression models of wildlife-habitat relationships was assessed using analytical analysis and simulations. Two patterns of non-detection were investigated: (1) a random distribution of non-detection among occupied patches; and (2) a non-random distribution of nondetection in which the probability of detecting a species in an occupied patch covaried with measurable habitat variables. Our results showed that logistic regression models of wildlife-habitat relationships were sensitive to even low levels of non-detection in occupancy data. Both analytic and simulation studies show that non-detection yields bias in parameter estimation of logistic regression models. More importantly, the direction of bias was affected by the underlying pattern of non-detection and whether the habitat variable was positively or negatively related to occupancy. For a positive habitat coefficient, a random distribution of nondetection yielded negative bias in estimation, whereas linkage of the probability of non-detection to habitat covariates produced positive bias. For a negative habitat coefficient, the pattern was reversed, with a random distribution of non-detection leading to positive bias in estimation. A release-recapture livetrapping study of small mammals in central Indiana, USA, was used to illustrate the magnitude of non-detection in a typical field sampling protocol with varying levels of sampling intensity. Estimates of nondetection error ranged from 0 to 23% for seven species after 5 days of sampling. We suggest that for many sampling situations, relationships between probability of detection and habitat covariates need to be established to correctly interpret results of wildlifehabitat models. #

Ultrasonic detectors are widely used to survey bats in ecological studies. To evaluate efficacy o... more Ultrasonic detectors are widely used to survey bats in ecological studies. To evaluate efficacy of acoustic identification, we compiled a library of search phase calls from across the eastern United States using the Anabat system. The call library included 1,846 call sequences of 12 species recorded from 14 states. We determined accuracy rates using 3 parametric and 4 nonparametric classification functions for acoustic identification. The 2 most flexible classification functions also were the most accurate: neural networks (overall classification accuracy ¼ 0.94) and mixture discriminant analysis incorporating an adaptive regression model (overall classification accuracy ¼ 0.93). Flexible nonparametric methods offer substantial benefits when discriminating among closely related species and may preclude the need to group species with similar calls. We demonstrate that quantitative methods provide an effective technique to acoustically identify bats in the eastern United States with known accuracy rates. ß 2011 The Wildlife Society.

Forests dominated by oak, beech, hornbeam, and boreal conifers are characterized by resource puls... more Forests dominated by oak, beech, hornbeam, and boreal conifers are characterized by resource pulses. Contemporary eastern U.S. oak forests may exhibit dramatic resource pulses in part because of the loss of American chestnut, which comprised 25-50% of the canopy throughout its range. Chestnut loss may have reduced mast resources for wildlife. A newly developed, blight-resistant hybrid makes reintroduction feasible within several years. We use historical data to model the effects of American chestnut mast on consumer populations, illuminating the potential ecological effects of a successful restoration. We parameterized a stochastic population growth model with mast production data to compare consumer population dynamics both pre-and post-blight. We explored the effect of maximum intrinsic population growth rate, degree of density dependence, and reliance upon mast on consumer response to chestnut loss. We parameterized the models for white-footed mouse, eastern chipmunk, gray squirrel, and white-tailed deer. At a northern site 14 years post-blight, simulated annual mast production decreased 80% and the coefficient of variation (CV) increased 60%. At a southern site 35 years post-blight, annual mast production decreased by 35% and the CV increased by 76%. Smaller, more variable mast crops translated to reduced abundance and increased variability in simulated consumer populations. White-footed mice were the most responsive, exhibiting a 48% decrease in population size and 57% increase in interannual variation post-blight. The reintroduction of blightresistant chestnut may fundamentally alter predator-prey interactions, gypsy moth outbreaks, and Lyme disease hot spots through its effect on the character of resource pulses that drive consumer dynamics.

The ability to make a priori assessments of a species' response to fragmentation, based on its di... more The ability to make a priori assessments of a species' response to fragmentation, based on its distribution in the landscape, would serve as a valuable conservation and management tool. During 1997-1999, we monitored 717 scent stations to examine seasonal use of forest patches, corridors, and crop fields by coyotes (Canis latrans), domestic cats (Felis catus), foxes (Vulpes vulpes and Urocyon cinereoargenteus), raccoons (Procyon lotor), striped skunks (Mephitis mephitis), opossums (Didelphis virginiana), and long-tailed weasels (Mustela frenata). For each species we developed landscape-based ecologically scaled landscape indices (ELSI), and we modeled species spatial distribution across three spatial scales (landscape-level, element-level, and local habitat-level). Our results suggest that these predators view landscape fragmentation at different spatial scales and demonstrate strong interspecific differences in their response to elements of the landscape. All species except coyotes and domestic cats avoided agricultural fields. In general, predator species that were more mobile (i.e. high ESLI for landscape connectivity; coyotes) were characterized by landscape-and element-based logistic models. In contrast, models including local habitat features generally were most appropriate for less mobile or more stenophagous predators (e.g. long-tailed weasels). Our analysis extends the application of the ESLI concept to species assemblages that do not appear to function as metapopulations, and it highlights the importance of examining spatial scale and species-specific responses to habitat fragmentation. We discuss the relevance of these findings for defining ecological landscapes, understanding predator-prey interactions at multiple spatial scales, and conserving predator and prey populations in fragmented landscapes. #

A community is ''nested'' when species assemblages in less rich sites form nonrandom subsets of t... more A community is ''nested'' when species assemblages in less rich sites form nonrandom subsets of those at richer sites. Conventional null models used to test for statistically nonrandom nestedness are under-or over-restrictive because they do not sufficiently isolate ecological processes of interest, which hinders ecological inference. We propose a class of null models that are ecologically explicit and interpretable. Expected values of species richness and incidence, rather than observed values, are used to create random presence-absence matrices for hypothesis testing. In our examples, based on six datasets, expected values were derived either by using an individually based random placement model or by fitting empirical models to richness data as a function of environmental covariates. We describe an algorithm for constructing unbiased null matrices, which permitted valid testing of our null models. Our approach avoids the problem of building too much structure into the null model, and enabled us to explicitly test whether observed communities were more nested than would be expected for a system structured solely by species-abundance and species-area or similar relationships. We argue that this test or similar tests are better determinants of whether a system is truly nested; a nested system should contain unique pattern not already predicted by more fundamental ecological principles such as species-area relationships. Most species assemblages we studied were not nested under these null models. Our results suggest that nestedness, beyond that which is explained by passive sampling processes, may not be as widespread as currently believed. These findings may help to improve the utility of nestedness as an ecological concept and conservation tool.

Many patch-based metapopulation models assume that the local population within each patch is at i... more Many patch-based metapopulation models assume that the local population within each patch is at its equilibrium and independent of changes in patch occupancy. We studied a metapopulation model that explicitly incorporates the local population dynamics of two competing species. The singular perturbation method is used to separate the fast dynamics of the local competition and the slow process of patch colonization and extinction. Our results show that the coupled system leads to more complex outcomes than simple patch models which do not include explicit local dynamics. We also discuss implications of the model for ecological systems in fragmented landscapes.

2001. Changes in transmission of Baylisascaris procyonis to intermediate hosts as a function of s... more 2001. Changes in transmission of Baylisascaris procyonis to intermediate hosts as a function of spatial scale. -Oikos 93: 213-220.