David Deane | ALES Graduate Seminar

A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES PhD Final Exam Seminar by David Deane. This seminar is open to the general public to attend.

Thesis Topic: Species diversity in discrete habitat networks

PhD with Dr. Fangliang He.

Seminar Abstract:

Human activities are increasingly fragmenting the natural world. A unifying theme in this thesis is understanding the expected patterns of species richness within artificially sub-divided or naturally discrete habitat networks. I start by testing a common expectation regarding the species composition of small habitat patches, which are usually assumed to support only common, widespread species. If that were true, then the complete loss of the smallest habitat patches would not reduce the number of species in that network. In a meta-study of 175 published studies, I found that loss of only the smallest patches, comprising less than 20% of total habitat area, would reduce species richness by an average of 12.7% species, more than twice the 5.8% predicted from species-area relationships. This argues strongly that groups of small patches should not be assumed to comprise only common species. I then explored a second commonly held, but little tested, theory: that rapid accumulation of species when combining groups of small patches is due to their high beta diversity. Using 38 published abundance datasets, I test competing explanations for the observed difference in the species richness of groups of small patches, relative to the largest patch using confirmatory path analysis. I found that beta diversity explained no more varation than evenness of species abundance distributions and patch-size-dependency in the completeness of sampling. Both increased and decreased evenness contributed to differences in species richness suggesting multiple mechanisms are involved. I next used sampling theory to develop and validate a suite of models allowing the effects of sub-division on species richness to be distinguished from the effects of habitat loss. I used these models to explore the effects of sub-division on species richness for different spatial and abundance distributions. I simulated the destruction of 20-90% of original habitat area, comparing the number of species that would be present if the remaining habitat was divided into 1-32 patches. I show that only when individuals are randomly positioned will the number of species be unaffected by sub-division; any amount of intraspecific aggregation results in an increase in the expected species richness of sub-divided, relative to contiguous, habitat of equal total area. In other words, several small patches should contain more species than a single large patch as this is a probable consequence of intraspecific spatial patterns and uneven distribution of abundance. However, as the amount of sub-division of a given area increases (i.e., as the remaining habitat is broken into more, but smaller, pieces), the number of additional species reaches asymptote and thus sub-division produces a ‘diminishing return’ on total observed species for a given habitat area. This thesis makes applied and theoretical contributions to conservation biology and has clear implications for the SLOSS and fragmentation per se debates. From an applied perspective, it refutes two widely held, but largely untested, assumptions relating to rapid species accumulation in small patches. Its main theoretical contribution is the application of sampling theory to predict the expected effects of sub-division on species richness.