page_title: Ecological extinction
section_title: Invasive species
hierarchical_section_title: Ecological extinction / Invasive species
caption_attribution_description: Linepithema humilis, Argentine ant
context_page_description: Ecological extinction is "the reduction of a species to such low abundance that, although it is still present in the community, it no longer interacts significantly with other species".
  Ecological extinction stands out because it is the interaction ecology of a species that is important for conservation work. They state that "unless the species interacts significantly with other species in the community its loss may result in little to no adjustment to the abundance and population structure of other species".
  This view stems from the neutral model of communities that assumes there is little to no interaction within species unless otherwise proven.
  Estes, Duggins, and Rathburn recognize two other distinct types of extinction:
  Global extinction is defined as "the ubiquitous disappearance of a species".
  Local extinction is characterized by "the disappearance of a species from part of its natural range".
context_section_description: Novaro et al. (2000) assessed the potential ecological extinction of guanacos (Lama guanicoe) 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.
caption: Christian found Argentine ants to disrupt large seed dispersal mutualisms.
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