Forestry and Natural Resources

Office Phone  (765) 494-3590 
Office Fax:    (765) 494-9461
Office Location:   PFEN 125 
E-mail Address:  rswihart@purdue.edu

Robert K. "Rob" Swihart
Head, Dept. of Forestry and Natural Resources

Purdue University
715 W State Street
West Lafayette, IN 47907-2033

Research Interests -

• Effects of Habitat Fragmentation on Populations
• Effects of Spatial Structure on Behavior
• Indirect Effects on Community Structure
• Plant-Herbivore Ecology

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 hostile 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.

Rob and colleagues in mathematics have developed a series of spatially realistic analytical models for these "metapopulations". Their metapopulation models represent a major advance in our 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, Rob and his students have characterized the responses of 32 species of vertebrates to habitat fragmentation in Indiana. Rob also has collaborated with ecologists in the Netherlands to compare responses of a wider variety of species to multiple types of agricultural landscapes. They have documented the importance of species-specific ecological profiles, niche breadth, and proximity to geographic range boundaries as determinants of species prevalence and distribution. The theoretical models and empirical studies have resulted in the development of a series of metrics to unite spatial structure and species profiles into "ecologically scaled indices" for measuring the suitability of a landscape for persistence by species with differing ecological requirements.

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. Rob's research team has addressed the influence of spatial structure on individual behavior, and its link to population dynamics. His basic research on animal movements has served as a cornerstone for improving 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. He and a former student subsequently 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. By combining metapopulation theory with graph theory, he and a current student are working on an improved method for estimating the connectivity of a landscape from an individual's perspective. He and his students 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- Rob's research also 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, Rob and colleagues have 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. Working with colleagues in Veterinary Pathobiology, the importance of indirect effects of habitat fragmentation on the transmission dynamics of a nematode parasite of raccoons was shown experimentally at local, landscape, and regional spatial scales. This finding is significant, as the parasite (Baylisascaris procyonis) is capable of infecting, blinding, and killing humans.

Plant-Herbivore Ecology-Environmental heterogeneity also occurs at large spatial scales and can be influenced tremendously by human activities. Rob has studied how environmental factors influence biogeographic variation in interactions of plants and herbivorous mammals. Working with colleagues in plant ecology and in organic chemistry, he has 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. Currently, Rob is developing models to explore the ecological and evolutionary dynamics of plant-herbivore interactions that are shaped by toxin-mediated functional responses.