Robert "Rob" K. Swihart

Purdue University
Department of Forestry and Natural Resources
715 W. State Street
West Lafayette, IN 47907-2061
Office Phone: (765) 494-3590
Office Fax: (765) 494-9461
Office Location: Pfendler Hall, Room 125
E-mail Address: rswihart@purdue.edu
Courses and Learning Reprints  Curriculum Vitae

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Research Interests

Effects of Habitat Fragmentation on Populations-For decades most population models in population ecology assumed spatial homogeneity, even though most organisms occupy heterogeneous environments. Environmental heterogeneity is accentuated by human activities. For example, agriculture and residential development create landscapes consisting of remnant patches of forest interspersed among a relatively inhospitable matrix of cropland and subdivisions. This process of habitat loss and subdivision is called fragmentation. A thorough understanding of the consequences of fragmentation for species persistence is especially critical in intensively agricultural areas, because many populations of organisms that once were relatively continuously distributed across a landscape currently exist as distinct subpopulations connected only by dispersal.

We have developed a series of spatially realistic analytical models for these "metapopulations". Our metapopulation models advance theoretical understanding of metapopulation dynamics, because they incorporate age structure, patch dynamics, niche breadth, and phenotypic variation, thus providing considerably more realism than previous models. Empirically, we have characterized the responses of 32 species of vertebrates and several species of acorn weevils to habitat fragmentation in Indiana. I also have compared responses of a wider variety of species to multiple types of agricultural landscapes. We have documented the importance of species-specific ecological profiles, niche breadth, and proximity to geographic range boundaries as determinants of species prevalence and distribution.

Effects of Spatial Structure on Behavior-The dynamics of a population reflects the aggregate fates of its individual members, yet phenomenological population models ignore processes at the individual level. We have addressed the influence of spatial structure on individual behavior, and its link to population dynamics. Our basic research on animal movements has improved the statistical methods used to analyze locational data and led to his discovery of the first allometric relationship describing rates of mammalian movement as a function of body size. We have applied this allometric relationship to a guild of mammalian carnivores in Indiana, demonstrating how differences in rates of movement by individuals can influence species distributions at the landscape level. We also have shown theoretically and experimentally the importance of intra and interspecific differences in behavioral decisions to population dynamics and community structure in agricultural landscapes.

Indirect Effects on Community Structure- Our research has highlighted how direct effects of habitat fragmentation acting on one species can have indirect and sometimes non-intuitive effects on other species. These sorts of effects are important to consider but often are ignored when devising conservation plans. Building on the theory of an "extinction debt" in which a competitively superior species may go extinct following habitat destruction, we used a novel approach that applies the Dirichlet distribution and stochastic simulation models to reveal how habitat fragmentation interacts with species differences to create shifts in species number, species dominance, and community stability. We also demonstrated experimentally the importance of indirect effects of habitat fragmentation on the transmission dynamics of a nematode parasite of raccoons at local, landscape, and regional spatial scales. This finding is significant, as the parasite (Baylisascaris procyonis) is capable of infecting, blinding, and killing humans.  Other studies have documented a human-facilitated range expansion by North American red squirrels into areas formerly dominated by eastern gray squirrels, with concomitant morphological changes and implications for regeneration of nut-bearing trees.

Plant-Herbivore Ecology-I currently am working with Mike Steele and others to understand how mast composition, seed attributes, and fear influence foraging decisions of rodents and the fate of acorns, hickories, chestnuts, and other seeds that they eat and disperse.  I've also studied how environmental factors influence biogeographic variation in interactions of plants and herbivorous mammals. Working with colleagues in plant ecology and in organic chemistry, we have shown that chemical defense by woody plants (1) has a strong ontogenetic component, with highest levels of defense in the juvenile stage; (2) is influenced by interspecific differences in plant life history; (3) varies latitudinally, concordant with variation in discriminatory abilities of herbivores; and (4) appears to have arisen in many areas as an evolutionary response to intense herbivory by mammals. Understanding the nature of such large-scale variation in defense and herbivory has important implications both for forest regeneration and for predicting the effects of global climate change on vegetation. Mathematical models we have developed show how plant toxins can regulate herbivore populations. 

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