Conservation efforts, by definition, are aimed at keeping things as they were. When this is impossible, ecologists and policy makers try to at least minimize anthropogenic damage to the Earth's species, without attempting so many simultaneous tasks that each effort becomes ineffectual. The species that receive this sort of triage tend to be taxonomically unique, such as the tuatara, or charismatic megavertebrates, such as the giant panda. Such a strategy prioritizes species over ecosystems, and can only be undertaken with the understanding that eventually all the tuataras and pandas on Earth may only be able to live in zoos. However, triage remains a potentially useful tool if it is applied based strictly on ecological importance. If conservationists focus on keystone species and ecosystem engineers, they will be most able to protect diversity and maintain structure within threatened ecosystems.

Keystone species are usually understood to act trophically and to exert an effect disproportionate to their relative biomass within an ecosystem. They often occur at or near the highest trophic levels of a food web, as carvinores with high per capita effects and low collective biomass. However, keystones can act non-trophically as well, pollinating or dispersing other species. The textbook case of a keystone species is the starfish Pisaster, which plays a major role in supporting diversity. In one computer simulation, when it is removed, the Simpson Diversity Score1 of its ecosystem fell by 4 points, as the Pisaster’s prey were allowed to competitively exclude each other. Other species in the same simulation exerted far less of an effect on their biotic environments (Power 1996).

A species can also be of particular importance to the physical structure of an environment. Ecosystem engineers change the quality, quantity, and distribution of resources other than their own biomass. These organisms allogenically construct or autogenically compose entire habitats such as coral reefs, forests, and beaver ponds; in doing so they affect key environmental attributes such as nutrient mixing, temperature, and drainage. Excepting ecosystem engineers which are also keystone species, such as beavers, leaf-cutter ants, and badgers, ecosystem engineers commonly live at high densities and, like Antarctic algae or reef building coral, may be near the lowest trophic level of a food web (Lawton 1994). Their effects are drastic, but can be taken for granted, potentially causing them to be overlooked in conservation efforts.

There are, however, some ecosystem functions to which all species contribute equally. Low-diversity communities constructed experimentally so as to resemble depauperate descendants of high-diversity communities show decreased rates of carbon dioxide uptake and decreased plant productivity, with a constant decline in each function confirming Lawton’s “ratchet” hypothesis (1994). Another caveat lies in the fact that many studies that appear to attribute a major role to a given species in fact attribute that role to a given guild. A hypothetical experiment which labels an ecosystem’s decomposer as an ecosystem engineer does not determine whether ecosystem function is further dependent on a diverse guild of decomposers, or whether one species is sufficient. And even seemingly redundant species can contribute to an ecosystem’s resistance and resilience in the face of disturbances that may never be experimentally observed (Walker 1992). Conclusive decisions as to the allocation of conservation resources can only be made for ecosystems that are thoroughly studied and well understood, and even then minor-interactors must never be written off entirely.

Works Cited:
  • Lawton, H. What do species do in ecosystems? OIKOS 71(3), 1994.
  • Power, M.E. and others. Challenges in the Quest for Keystones. BioScience 46 (8): 609-620, 1996.
  • Walker, B. H. Biodiversity and ecological redundancy. Conservation Biology 6 (1): 18-23, 1992.

1This is a unitless ranking of an ecosystem's diversity that takes both its richness (the number of different species it contains) and eveness (the degree to which species are spatially interspersed with each other) into account.