The role of abiotic and biotic factors in limiting Piper abundance and distribution in a lowland Costa Rican wet forest

Niche theory proposes that species are able to coexist because each specializes on a certain portion of the habitats available. Both abiotic resources and the presence or the absence of biotic factors likely influence habitat specialization, although very few previous studies of plant distribution have considered both sets of factors. I conducted an observational study in the primary forest of the La Selva Biological Station in Costa Rica in which I sampled plants of the genus Piper greater than 1 cm at ground level on two different soil types along 26 transects, each 100 m long and 2 m wide. I measured light and herbivory for each individual encountered, as well plant size. I found that herbivory and soil type predict Piper species composition within patches. Moreover, for the most frequent and abundant species of Piper, light is an important predictor of Piper presence and absence. Abundance across the genus was not affected by soil type but mostly by plant size, which could be an indirect effect of competition between seedlings and small juveniles with older and larger plants. Generally, Piper plants suffered more damage by insect herbivores in small patches. However, when these plants are on alluvial soils and at high light levels they suffer less herbivory compared to plants in larger patches but in lower light levels and on volcanic soils. In looking at the distribution of individual species, it does not appear that the most common Piper species differ in their light requirements or in their resistance to herbivores. Thus, although the response to some factors was species-specific, in general the common Piper species seem to be sharing the same niche. I conclude that there is an important role for both abiotic and biotic factors in determining distribution of these understory habitat specialists as a group but not individually. Herbivory, patch size and light levels seem to be determining the habitat in which certain Piper species are able to establish and grow

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