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Habitat Size Mediates the Effects of Environmental Factors on Species Richness
Date of Award
Doctor of Philosophy (PhD)
The Species Area Relationship (SAR) is one of the oldest and most fundamental patterns in ecology. Researchers have long known that a larger habitat will support more species than a smaller habitat. In the early 20th century, a predictive mathematical model was developed giving researchers an opportunity to understand the consequences of habitat loss for patterns of species richness and diversity. Species richness refers to the number of species observed within a given area. Related to species richness, species diversity is any statistical metric that allows researchers to compare species richness values measured across habitats that have varying probabilities of sampling individuals of the species present. Researchers have used the SAR model to understand the number of species that could be supported by a given amount of habitat, but have also used the SAR to compare the number of species supported by different habitats, to into infer important environmental factors that lead to differences in species richness among habitats. Ecologists have given great attention to studying the roles that these environmental factors play in determining how many species a given habitat will support. Yet, very little consideration has been given to the way in which habitat size alters the effects of those environmental factors on species richness patterns. This dissertation explores the influences that habitat size has on environmental factors that have been proposed as mechanisms that increase or maintain species richness in local habitats. I present three chapters each describing the findings of experiments that explored the interplay between habitat size and three commonly invoked mechanisms that affect local species richness: energy, species dispersal, and habitat heterogeneity.
In Chapter 1, I present an experimental test of the species-energy-area relationship, where energy and area are predicted to interact in a positive way to increase the total number of species in a habitat. While much attention has been given to the interaction between habitat size and energy, researchers have failed to recognize that energy input may not be directly acting as a mechanism increasing species richness, but instead increasing the density of individuals, which increases the likelihood of detecting species in high-energy habitats. The 2nd chapter addresses the way in which habitat area alters the importance of dispersal for maintaining species richness in habitats, where small habitats are predicted to benefit more from dispersal than large habitats. Despite classical models in ecology predicting that species dispersal should be most important in small habitats, no conclusive experimental evidence exists showing that habitat size alters the importance of dispersal on species richness in local communities. Finally, in the 3rd chapter I present data from an experiment showing how the importance of habitat heterogeneity depends on habitat size and the aggregation of species among habitats. Habitat heterogeneity has been implicated as one of the most important mechanisms required to increase and maintain species richness in local communities. However, a meta-analysis of heterogeneity experiments revealed that experimental evidence is inconclusive, because many researchers have found positive and negative relationships between habitat heterogeneity and species richness. Additionally, the Area-Heterogeneity-Trade-Off (AHTO) model, developed to predict the relationship between habitat heterogeneity and species richness, has proven to be difficult to test in natural environments, due to the correlation of alternative environmental parameters with habitat heterogeneity. The results of this experiment support the main predictions of the AHTO, but show that the patterns predicted by the model can arise from alternative mechanisms than what the model predicts.
The results from the experiments presented within this dissertation advance ecologists' understanding of how environmental variables that are thought to drive patterns of species richness are mediated by the size of the habitat being studied. Furthermore, the results of these experiments add to a longstanding debate in ecology over the importance of habitat size in maintaining species richness. According to the SAR model, the reduction in size of a contiguous habitat will result in an immediate reduction in species richness. Some researchers have suggested, however, that the SAR model always under-predicts the loss of species richness as habitat size decreases. The research presented in this dissertation supports the conclusion that as habitat size decreases, species will be lost at a rate different from the prediction of the SAR model, due to the complex and non-linear relationships between habitat size and environmental factors like energy and heterogeneity that affect the number of species a local habitat could support.
Chair and Committee
Tiffany M Knight
Allan Larson, David Fike, Garland Allen, Jonathan Myers, Kevin Smith
Schuler, Matthew Scott, "Habitat Size Mediates the Effects of Environmental Factors on Species Richness" (2015). Arts & Sciences Electronic Theses and Dissertations. 434.