The peninsula effect is a biological diversity pattern found in peninsulas in which the number of species decreases toward the tip of the peninsula. The geometry hypothesis, as one proposed cause of the peninsula effect, attempts to predict this pattern by examining the peculiarities of peninsular geometry. As peninsulas are characterized by their isolated positions, it has been suggested that a decreased immigration-to-extinction rate is the cause of the decrease in species diversity from the base to the tip of a peninsula. We aimed to test the geometry hypothesis on tree species in the Florida peninsula by modeling the latitudinal abundance pattern using sample-based tree inventory data. We postulated that the current abundance distribution of a species is a ramification of past immigration–extinction dynamics in a peninsula, as well as an important indicator of such dynamics in the future. The latitudinal abundance patterns of 113 tree species in Florida in the southeastern United States were simulated with the Huisman–Olff–Fresco (HOF) model using the USDA Forest Service’s Forest Inventory and Analysis (FIA) database. Evidence species for the geometry hypothesis were then selected if the simulated latitudinal abundance pattern was asymmetric with its abundance maxima occurring within the Florida peninsula (i.e., approximately 31.5 degree latitude or lower).
Results/Conclusions
Our HOF model results found only 12 species (13% of 92 species) with an abrupt steep abundance decline along the peninsula as evidence for the geometry hypothesis. This may mean that, first, the geometry hypothesis is not a valid explanation for tree species in Florida or, second, that population dynamics are difficult to trace by the current abundance pattern. Lastly, it could mean that tree species richness gradients in the Florida peninsula are merely driven by environmental factors specific to this region, such as forest patch size or other limiting factors. Yet, those 12-evidence species deserve conservation consideration because, with marked abrupt abundance attrition, these species may be more susceptible to various biotic and abiotic disturbances (e.g., outstanding climate change, hurricane, wildfire, insect infestation, or land fragmentation).