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Biodiversity:
Ecological Effects of Species Introductions and Extinctions
by Waterose et. al.
Changes in stability occur when biotic interactions are altered and changed. Human activities produce changes that affect patterns of interaction and linkages of species to one another and with other species in the community.
Deletion is the removal of species from a community. Clear cutting is a good example of deletion. This kind of practice can cause a profound effect on the biotic linkage. With deforestation, comes the deletion of lower plants and animals that depend on this ecosystem for survival. Within a web, species deletion may vary considerably, depending, among other things, on the species richness of the system, the trophic level from which the species are removed, and the degree of inter-connectedness of the species removed.
Although a species rich system might be expected to be more resistant to the loss of plant species, the occurrence of keystone plant species such as Ficus spp in highly diverse tropical forest suggest that removal of certain plant species can indeed have a profound effect on species composition and linkages in the rest of the web.
Addition of species to biotic linkages can also have an adverse impact on the ecosystem. On average, communities seem to be highly resistant to species introduction. In some cases introduced species overpower the native species and take over. Scotch broom, zebra mussel, the Argentine ant, and the rabbits introduced in Australia are some examples.
Alteration of ecosystems can also cause extinction. Farming is a good example of this. Fifty years ago, there were approximately 50,000 different varieties of rice in India. Today there are only 3,000 varieties left, of which only 30 are extensively cultivated.
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