Since Paine’s classic work in the rocky intertidal describing the effects of Pisaster starfish on the community as a keystone predator species, the concept has become huge to ecology. A keystone species is one with an important role in the community and has a larger effect on the community than would be expected by its abundance. In the rocky intertidal, when starfish were removed from the system, the population size of one of its main prey items, mussels, exploded, crowding out other species and reducing the diversity of the system. The presence of the starfish keeps the populations of other species in check and permits co-existence in the system. We have all also become familiar with other such keystone species like sea otters, wolves, bass, and many other charismatic and less than charismatic species.
If you start to think about a patchwork of habitats connected through dispersal, its easy to imagine that some habitats may be more critical than others. So not only can particular species be important to community properties, but a particular community or ecosystem can be important to the broader metacommunity. Habitats have been shown to help maintain population sizes and diversity, but little has been done to quantify the relative contribution of these habitats to other communities. In the recent Ecology Letters paper, “Extending the concept of keystone species to communities and ecosystems,” Nicolas Mouquet, Dominique Gravel, Francois Mossol, and Vincent Calcagno do exactly what they say in the title. They seek to apply similar criteria for determining keystone species to a network of communities within a metacommunity.
Keystone and burden communities are described as “communities with impacts disproportionately large (positive or negative respectively) relative to their weight in the metacommunity. To study this, they create an approach to assess the influence of a community on metacommunities and metaecosystems by determining the community impact as the change in a community property (e.g. diversity or productivity) as a result of the removal of a community/ ecosystem. They determine the community weight as a result of a measure such as patch area. They can then compare the community impact a particular community has to what would be expected for a community with that community weight. Communities with an impact larger than expected are keystones and those that are smaller are burdens.
Mouquet et al apply this framework to two example systems and find that the particular traits of species, such as the species competitive-colonization ability, species’ ability to invade occupied patches, or competitive ability for resources, in the community help determine the degree of “keystoneness” of the community or ecosystem. In addition, habitat quality (patch fertility) influenced the impact that ecosystem had on the landscape, with more fertile patches being the most important. While environmental conditions of a particular site and the traits of the species found in a site may be important aspects of the keystoneness of a community, the location, connectivity, and rarity of a site may be equally as important and as they mention, worth exploring.
Why this paper is awesome:
- With clearly described metrics and models, they simply and elegantly extend a concept greatly appreciated in ecology, keystone species, to an exciting new area with a wider ecological scope.
- Incorporates existing ideas about the importance of particular habitats and provides a rationale for how we can quantify the impact a habitat may have for a region thus further expanding the concept of metacommunities.
- They more broadly consider the complexity of multispecies interactions in the functioning of landscape as well as discuss that some communities may actually serve as burdens to the rest of the metacommunity.
- With parameterization from field data, it is possible that it could further be developed in natural systems and have applied implications in the preservation and restoration of communities and ecosystems.