As human pressure on tropical forests increases, so does the extent of bushmeat hunting — the killing of wild species for food. The adverse effects go beyond the species immediately concerned, and a striking example can be found in a paper by L. F. Pacheco and J. A. Simonetti in Conservation Biology (14, 1766–1775; 2000).
As might be expected, large-bodied animals — including some primates — are usually the preferred bushmeat prey. Many of these animals have fruits and seeds as a major part of their diet; in turn, the dispersal of the plants concerned may depend on the passage of seeds through the animals' guts, and subsequent deposition elsewhere. Most fruit-eaters are generalists, so if one vector is lost there may be others to take its place. But in fruit consumption, size matters, and the selective loss of all large frugivores could seriously impair dispersal in plants with large fruits. Moreover, some plants are relatively specific in their seed-dispersal vectors, and so are most at risk.
Pacheco and Simonetti have studied one such species, Inga ingoides (family Fabaceae), which is one of the common trees in the lowland forests of Bolivia. The seeds of this tree are dispersed almost exclusively by the spider monkey (Ateles paniscus; Fig. 1). Three other primate species feed upon the tree's seeds in the study area, but only spider monkeys ingest them intact and act as effective dispersal agents. The monkeys can travel 1 kilometre in a few minutes and have a gut passage time of about 4 hours. So the seeds can be dispersed some distance from the parent plant, ensuring that the genetic structure of the population of I. ingoides is well mixed.
The research concentrated on the genetic variability of I. ingoides populations, comparing areas where spider monkeys were present with those where they had become locally extinct through bushmeat hunting. After analysing 14 enzyme systems in leaf extracts, the authors found that there was less genetic variation in the seedling populations around parent trees when the monkeys were absent than when they were present. Without the monkeys the seeds simply fall to the ground around their parents. In other words, the monkeys are responsible for maintaining a thorough genetic mix in the population, and in their absence a series of genetically more uniform patches develops.
In general, trees in the tropical rainforest have high levels of genetic diversity. They are usually out-breeding and have efficient gene flow because of their specialized pollination and seed-dispersal mechanisms. Fragmentation of the forest, whether by physical processes (such as clearance) or by interruption to gene flow (as in the case of vector loss), can lead to the local accumulation of potentially detrimental mutations and an overall loss of fitness. As has so often proved to be the case in ecological studies, when one link is removed from a network, the whole system can start to unravel.
