Ecological Extinction - Invasive Species

Invasive Species

Novaro et al. (2000) assessed the potential ecological extinction of guanacos (Lama guanocoe) and lesser rheas (Pterocnemia pennata) as a prey source for native omnivores and predators in the Argentine Patagonia. These native species are being replaced by introduced species such as the European rabbit, red deer, and domestic cattle; the cumulative damage from the increased herbivory by introduced species has also served to accelerate destruction of the already dwindling Argentine pampas and steppe habitats. This was the first study to take into account a large number of diverse predators, ranging from skunks to pumas, as well as conduct their survey in non-protected areas that represent the majority of southern South America. Novaro and his colleagues found that the entire assemblage of native carnivores relied primarily on introduced species as a prey base. They also suggested that the lesser rhea and guanaco had already passed their ecological effective density as a prey species, and thus were ecological extinct. It is possible that the niches of introduced species as herbivores too closely mirrored those of the natives, and thus competition was the primary cause of ecological extinction. The effect of introduction of new competitors, such as the red deer and rabbit, also served to alter the vegetation in the habitat, which could have further pronounced the intensity of competition. Guanacos and rheas have been classified as a low risk for global extinction, but this simplistic view of their demography doesn’t take into account that they have already become functionally extinct in the Argentine Patagonia. Novaro and his colleagues suggest "this loss could have strong effects on plant-animal interactions, nutrient dynamics, and disturbance regimes ..." This is a prime example of how current conservation policy has already failed to protect the intended species because of its lack of a functionally sound definition for extinction.

Seed dispersal mechanisms play a fundamental role in the regeneration and continuation of community structure, and a recent study by Christian (2001) demonstrated a shift in the composition of the plant community in the South African shrublands following an invasion by the Argentine ant (Linepithema humile). Ants disperse up to 30% of the flora in the shrublands and are vital to the survival of fynbos plants because they bury the large seeds away from the dangers of predation and fire damage. It is also crucial for seeds to be buried, because nearly all seed germination takes place in the first season after a fire. Argentine ants, a recent invader, do not disperse even small seeds. Christian tested whether the invasion of the Argentine ant differentially effected small and large-seeded fauna. He found that post-fire recruitment of large-seeded flora was reduced disproportionately for large seeds in sites already invaded by Argentine ants. These initial low large-seed density recruitments will eventually lead the domination of small-seeded fauna in invaded habitats. The consequences of this change in community structure highlight the struggle for dispersal of large-seeded flora that have potential reverberations around the world because ants are major ecological seed dispersers throughout the globe.