The key factors driving species distribution within fluvial island systems worldwide are
poorly known. Previous studies have shown that herbivore abundance and proportion
in the mammal assemblage can be indirectly influenced by soil fertility via production
of nutritious fruits and leaves and higher ecosystem dynamics linked to vegetation
growth on more productive soils.The goal of this study was to investigate which
environmental (soil fertility, tree basal area, forest cover and distance from sampling
point to nearest mainland) and anthropogenic factors (logging, distance from the
sampling point to nearest human community) were most closely-linked to observed
patterns of mammalian abundance and proportions of omnivorous and herbivorous
mammals on fluvial islands of the Rio Negro, Amazonas State, Brazil. Several of these
islands are located on the left bank of the river, and so receive nutrient-rich sediments
from the Rio Branco. We expected that our chosen environmental and anthoropogenic
factors could predict island mammalian abundance patterns. We used camera traps to
examine this, surveying 49 sampling sites across 60 days when water levels were
seasonally low.We calculated the proportion of herbivorous and omnivorous mammals
per site, and assayed their relationships to predictor variables with a series of
generalized linear models (GLM). As expected, the number of records was low on all
studied fluvial islands, compared with upland terra-firme studies. In 70,560 sampling
hours we obtained 126 independent camera trap records, for a total of 11 mammal
species. Total mammal abundance and proportion of the generalist omnivore Didelphis
marsupialis was not affected by our chosen environmental and antropic variables.
However, soil fertility played a central role in determining the proportion of herbivorous
species.Thus, gradients of soil fertility can help to explain herbivore distributions on
riverine islands, and define crucial areas for conservation and future
restoration.Additionally, logging, distance to the nearest human community and to the
mainland had a positive effect, and forest cover a negative effect, on the proportion of
herbivorous mammals, but did not affect generalist omnivorous species. Therefore,
results suggest that island soil fertility can differentialy impact proportions of mammals
from the same feeding guild.
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