Impacts of logging on plantanimal interactions

Logging may inadvertently eliminate individuals of a species upon which pollinators or seed dispersers depend for their various life functions. Pollinators and seed dispersers often depend on a number of specific species for their survival. If only one of these species is eliminated from the range of a pollinator or seed disperser, that mutualist may become locally extinct. If the obligate insect or bird becomes extinct, then the tree species that depends upon it has no chance of reproducing in the wild. However, mutualisms may be less susceptible to extinction when there are spatially differentiated patches that allow a reorganization of the system following a disruption (Bronstein et al. 2003).

Size of openings in the forest, as well as the patchiness of mutualist organization, are important in determining the impact of forest clearing.

When openings in the forest are small, such as in a shifting cultivation plot, re-establishment of mature-forest species can occur as fruits fall into the opening or as seed dispersers traverse it. However, when large areas are cleared, small mammals, birds, and bats hesitate to venture far from the edge of the mature forest (Stouffer and Borges 2001). The loss of seed disperser activity can inhibit the re-establishment of mature-forest species, as has been shown in pastures in Amazonia (Nepstad et al. 1990) and Costa Rica (Holl 1999). In contrast, temperate forest regeneration may be less dependent upon mutualists. Tropical forests tend to be more sensitive to large clear-cuts than temperate zone forests, and logging strategies used in northern forests are therefore not appropriate for the tropics.

Even if the seed-dispersing species is merely reduced in number, reproduction of the tree species may be endangered. The time interval during which seeds of many rain forest species remain viable is often very short (Gómez-Pompa et al. 1972). The fruit or seed must be found soon after ripening. If not, it will germinate or decompose before being transported into the area where reforestation is needed. Other factors, particularly predation, also contribute to low seed germination in disturbed areas. In an Amazon rain forest, seeds of wild species planted in continuous forest were three to seven times more likely to germinate than those in forest fragments (Bruna 1999).

In this section, we have mentioned only a few of the studies that illustrate the importance of mutualisms in maintaining the structure and function of forests through nutrition, protection, pollination, and seed dispersal. There are hundreds if not thousands more. Guides to some of the classical papers in this field are presented by Loiselle and Dirzo (2002) and Marquis and Dir-zo (2002).

0 0

Post a comment