Indicators of functional biodiversity are least developed. There are many theories predicting a positive relationship between compositional biodiversity and functional attributes, but the generality and form of the link remain contested. It is unclear whether the link rests on biodiversity in general or on the presence of specific functional groups, or niche complementarity (Loreau et al. 2002). The debate on the functional link between biodiversity and ecosystem net primary productivity is converging on a conclusion that a weak positive link exists, which tends to level off at modest levels of plant diversity (on the order of ten species) (Naeem 2002; Kinzig et al. 2001).
For policy purposes it may be pragmatic to view function as synonymous with the capacity of ecosystems to supply ecosystem services, briefly defined as the benefits people obtain from ecosystems (MA 2003). For example, the capacities to supply water, fish, timber, food, or carbon storage services can be regarded as indicators of ecosystem function. Data on services often exist, but their link to biodiversity remains obscure. In many cases, an equivalent quantum of service could be supplied by a less diverse ecosystem, though perhaps with less resilience.
The Millennium Ecosystem Assessment (MA 2003) makes a useful distinction between biodiversity as an ecosystem service in its own right and biodiversity as an underlying condition necessary for the delivery of many other services. It is easy to confuse natural resources with biodiversity. For example, almost all the food we eat has its origin in a plant or animal, which in turn are components of biodiversity. But it is generally not the diversity of that source that is uppermost in our mind when we think of the nutritional service. Similar nutrition could be, and increasingly is, derived from a very small variety of sources. Nevertheless, there is a key role for biodiversity in food supply: It provides the source of variation needed to adapt the crop to a changing environment and to reduce the risk of catastrophic failure in the event of a widespread stress (e.g., drought or disease). The case for a close connection between biodiversity and human well-being is more direct for the regulating ecosystem services, such as control of pests and diseases.
Although monetary measures of natural capital (Costanza et al. 1997; De Groot 1992), which directly measure biodiversity function in monetary terms, are similarly of limited utility as biodiversity proxies, their ability to link to social and economic drivers and human use is a valuable attribute.
Non-human-centered measures of ecosystem function usually boil down to direct or indirect measures of biogeochemical cycling, such as net primary productivity and evapotranspiration. The greenness of the land or ocean surface and its spatial and temporal variability are proxies for this type of measure that lend themselves to remote sensing.
At the species level, species can be reclassified into functional types or guilds: groups of species that share, to some degree, attributes such as basic physiology, reproductive strategy, trophic position, and response to stress or disturbance (Smith and Schugart 1996). Richness and evenness measures can then be applied to functional types rather than species. The problem is that there is no standard for the definition of functional types, which can be endlessly subdivided until species or even lower basic units are reached.
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