The peninsula effect may yet be seen as a red herring, but more light is needed

The peninsula effect is a classic biogeographical concept which predicts that the number of species declines from a peninsula’s base to its tip. Two of three hypothesized causal mechanisms, the effects of geological history or habitat on species richness, can be controlled for by study design and/or statistical analysis. The third proposed mechanism (reduced colonization towards the peninsular tip) is attributed to peninsular geometry, and is less easily controlled. Dave Jenkins and Deb Rinne of the University of Central Florida asked two questions: (1) what is revealed by the 4-decade history of research on this concept; and (2) do microcrustaceans in Florida's isolated wetlands reveal a peninsula effect if the effects of history and habitat are controlled for?
Their literature review revealed mixed (49%) support for a peninsula effect, and found that most studies (86%) were not designed to control for alternative hypotheses or to quantitatively compare evidence regarding alternative hypotheses. Also, studies were strongly skewed to vertebrate animals (62% of studies); relatively little is known for other taxa. After controlling for history effects by study design, their own study of microcrustaceans in Florida wetlands revealed that habitat effects dominated (82.5%) the pattern, and virtually no effect of peninsular geometry existed. This result is consistent with effective dispersal of microcrustaceans through geological time.
This study is important because it shows that much illumination is still needed on the long-standing concept of a peninsula effect, and demonstrates that careful study design and statistical analyses can shed needed light. Peninsula effect studies should: broaden in taxonomic focus; control for alternative causative hypotheses (geometry, habitat, or history) in the study design; and quantitatively compare the effects of hypothesized mechanisms on peninsular diversity patterns.

Source paper: Jenkins, D.G. & Rinne, D. (2008) Red herring or low illumination? The peninsula effect revisited. Journal of Biogeography, doi: 10.1111/j.1365-2699-2008-01943.x.

Source of article: Journal of Biogeography highlighted papers