Species Identity, Functional traits, and Resource-Use
Species diversity has two primary components: species richness (the number of species in a local community) and species composition (the identity of the species present in a community). While most research on the relationship between ecosystem diversity and stability has focused on species richness, it is variation in species composition that provides the mechanistic basis to explain the relationship between species richness and ecosystem functioning. Species differ from one another in their resource use, environmental tolerances, and interactions with other species, such that species composition has a major influence on ecosystem functioning and stability.
The traits that characterize the ecological function of a species are termed functional traits, and species that share similar suites of traits are often categorized together into functional groups.
Functionally equivalent species are considered to compete with one another, but this also depends on how finely they are described. In a simple model of a system all herbivores may compete with each other for the plants they eat. A more realistic and finely detailed model will be less likely to have species compete with one another, as herbivorous species tend to specialize on different plant species or even different parts of the same plant.
Species of different functional types don't compete against one another for resources. Carnivores and herbivores, for instance, don't compete with one another. Instead, the carnivores eat the herbivores. These species from different functional groups can exhibit complementary resource-use, meaning that they use different resources or use the same resources at different times. For example, two animal predators may consume different prey items, so they are less likely to compete with one another, allowing higher total biomass of predators in the system. In the case of plants, all species may utilize the same suite of resources (space, light, water, soil nutrients, etc.) but at different times during the growing season — for example, early- and late-season grasses in prairies.
Species have differential importance. So-called 'keystone' species provide critical support to wide arrays of other species with which they interact.
A keystone species is a species whose addition to or loss from an ecosystem leads to major changes in abundance or occurrence of at least one other species
Paine (1969) originally defined a keystone predator as a species that feeds preferentially on the dominant competitor among its prey species.
Thus, the keystone predator’s feeding prevents the dominant prey from excluding other species, and therefore maintains a higher species diversity in the system
For example, in the northeast Pacific, sea otters are a keystone species because, despite their relatively low abundance, they are largely responsible for maintaining the existence of kelp beds and thus have a fundamental impact on coastal ecosystems.
An indicator species is a species whose presence indicates human-created abiotic conditions such as air or water pollution (often called a pollution indicator species). Indicator plant species include common wood sorrel, wild daffodil, wild garlic, common bluebells, etc.
A dominate species that provides much of the biomass or number of individuals in an area