Persistence of forest tree species in human-modified landscapes of central Panama
Throughout the tropics old-growth forests have been cleared for timber extraction and agricultural expansion, and degraded by selective logging, fires and overhunting. The human-modified landscapes that result are a mosaic of old-growth forest fragments, degraded forest, regenerating forest and agricultural land, and these landscapes now cover large areas of the tropics. Only 9.8% of the tropical forest biome is strictly protected, and therefore the long-term conservation of tropical forest biodiversity is dependent on the capacity of human modified landscapes to maintain viable populations of tropical forest species. However, studies that investigate species diversity and composition across diverse tropical landscapes are lacking, and the factors that determine the presence of old-growth forest species in these landscapes remain poorly understood. To investigate the species richness and composition of woody species across a human-modified agricultural landscape in central Panama, we conducted inventory surveys of all woody shrubs and trees at 26 sites. These sites included living fences, gallery forest and regenerating forest patches. Specifically we investigate 1) how species composition compares with that of neighboring old-growth forest within the Barro Colorado Nature Monument, and 2) whether species characteristics or life-history traits can explain abundance across the landscape.
A total of 454 species were found across the human-modified landscape, with 258, 365 and 318 species present in living fences, gallery forest and regenerating forest patches respectively. Of the 303 species present in Barro Colorado 50-ha old-growth forest plot, 244 (81%) were present in the landscape. Species overlap with old-growth forest was greatest in gallery forest (72%) and lowest in living fences (47%). The most abundant species across the landscape (i.e. those present in the greatest number of sites) tended to have low shade-tolerance and/or human-uses (such as, fruit trees, or trees suitable for animal fodder or fencing). In contrast, species present in old-growth forest but absent from the agricultural landscape tended to have large, animal dispersed seeds. These species will be most threatened by agricultural expansion and loss of old-growth forest. Our data indicate that human-modified landscapes may provide suitable habitat for many forest species, but detailed studies are required to determine the mechanisms that maintain species populations across these landscapes.