Species Redundancy and Ecosystem Reliability

Naeem’s paper discusses the usage of hypothetical relationships describing ecosystem functions as they provide value to biodiversity. Many of these relationships include species redundancy as a factor. Figure 1 displays charts of these hypothetical relationships, where species redundancy is a factor in any graph where the curve becomes constant for “Ecosystem Function”. The term “redundancy” was originally used to argue that conservation efforts should prioritize species whose contribution to an ecosystem are unique but it has become an argument that conservation of redundant species is unnecessary. Naeem argues that this assessment of redundancy should be reevaluated. “Redundant” species are both necessary and valuable and are a source of reliability in an ecosystem.

Functional groups are used to measure redundancy. A functional group can be defined as a group of species sharing common biogeochemical attributes. Ecosystems can be conceptually divided into compartments: core or peripheral, abiotic or biotic; from which functional groups and redundancy may be determined. Figure 2 provides a representation of these ecosystem compartments. Core biotic compartments include autotrophs and decomposers while core abiotic compartments include nutrients in the form of organic or inorganic matter. Peripheral compartments are either autotroph- or decomposer-derived and may occur at different trophic levels. Links exist between these compartments as shown by arrows in this graphic. Each compartment in this example contains three redundant species.

Naeem further clarifies terms necessary to his argument:

-Local extinction within a functional group is considered a stochastic process.

-Complexity describes the number of functional groups in an ecosystem.

-An ecosystem is said to have failed when it no longer provides the service or goods originally demanded of it. This does not necessitate complete collapse but may be a shift in its function.

-The reliability of an ecosystem (likelihood of failure) is a function of the reliability of its components. As such, more complex ecosystems are more likely to fail.

-While all species are unique, from an ecosystem perspective species are rarely singular in their function. The overall function of an ecosystem will be more significantly altered when changes affect entire functional groups; therefore ecosystems are more sensitive when functional groups are comprised of fewer species.

-Parallel redundancy, or fully compensatory redundancy of a species, can increase the reliability of an ecosystem.

Naeem provides several formulas which model the probability of local extinction of or colonization by a species in a certain functional group, the probability of a functional group providing a service to its ecosystems, and the probability of an ecosystem providing a service given its number of functional groups. Together these show that increasing redundancy increases reliability of an ecosystem, and increasing redundancy is necessary in compensating for the decreased reliability of complex ecosystems. These formulas are however limited in their assumption that extinction and colonization are constant and independent of each other, which is not held in natural processes.

In terms of conservation, it is important to understand that increased rates of global extinction reduces redundancy and diversity of functional groups, and increased fragmentation decreases the recovery effect of colonization. Further research to demonstrate increased compensatory ability with increased redundancy is valuable in showing that a decrease in biodiversity leads to decreased reliability of an ecosystem. The argument can then be made that the maintenance of biodiversity and species redundancy is a requirement for preserving ecosystem services.