G. it. Ent., 12 (54): 1-30
December 2007
The Tárcoles Line:
biogeographic effects of the Talamanca Range
in lower Central America
by
BERT KOHLMANN & M. JUSTIN WILKINSON
Abstract - This paper documents the unusually clear biogeographic frontier formed by the border between the Pacific dry forest (Mexican Pacific Coast Province) and the Pacific rain forest (Western Panamanian Isthmus Province) at the Grande de Tárcoles River and its coincidence with the Costa Rica - Panamá microplate boundary. Our approach begins with analyses based on the study of the position of the biogeographic border with several environmental variables. We investigated the possible respective effects of the coincidence of several environmental barriers, represented by plate tectonic borders, topography, winds, climate and vegetation parameters, on the formation of the frontier between two biogeographic provinces along the Costa Rican Pacific-facing slope, by using space photography and environmental gradient models. A detailed case analysis of the distributional limits of Scarabaeinae dung beetles at the biogeographic border is also presented and analyzed. All groups of organisms studied so far (dung beetles, butterflies, amphibians, reptiles, fishes, birds and vegetation) show a sharp distributional boundary at the Grande de Tárcoles River. Environmental modeling showed the existence of regional gradients for solar radiation and evapotranspiration as well as the existence of steep local gradients for temperature, rainfall and soil moisture coincident with the biogeographic boundary. Dung beetle population dynamics show marked differences in seasonality on either side of the boundary. The clear biogeographical frontier between the Mexican Pacific Coast and the Western Panamanian Isthmus Provinces correlates well with the fault system that separates the Caribbean plate from the Costa Rican - Panamá microplate. The origin of the fault system is directly attributable to the effect of the submarine Cocos ridge. This feature has raised land surface altitudes far above the average values of mountain ranges in the southern Central America region, in the form of the Talamanca mountain range. Topographic barriers created by this uplift generated dependent climatic zones. Rotor wind circulations associated with the Talamanca massif enhance precipitation only in the Talamanca sector, that is, on the central and south Pacific - facing slopes of Costa Rica. We propose a casual chain that leads from tectonic activity to modern topography to climate and hence to biogeographic distributions including a biogeographic node. This study portrays one of the few known plate boundaries in Latin America that coincides with clear-cut distributional limits separating major biogeographic provinces.
