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Functional groups are non-phylogenetic, aggregated units of species sharing an important ecological characteristic and playing an equivalent role in the community (Cummins, 1974; Smith et al., 1997; Steneck, 2001; Blondel, 2003). In other words, Functional groups are defined as sets of species showing either similar responses to the environment or similar effects on major ecosystem processes (Gitay and Noble, 1997). In addition, FGs can be identified as clusters in trait space through multivariate statistics, without a priori of classifications regarding to particular responses to environment or influences on ecosystem processes (Hooper et al.,2002 ) Functional diversity refers to the range and value of organismal traits that influence ecosystem properties (Tilman 2001). This can be expressed in a variety of ways, including the number and relative abundance of functional groups (e.g. Tilman et al., 1997; Hooper, 1998), the variety of interactions with ecological processes (Martinez, 1996), or the average difference among species in functionally related traits (Walker et al., 1999).
A loss of biodiversity may, both directly and indirectly, affect ecosystem function and service (Chapin et al., 2000). As FGs provide a link between species diversity and ecosystem function (Grimm, 1995; Bengtsson, 1998; McCann, 2000) the FG approach can be applied to investigate and predict global environmental change impacts and feedbacks on ecosystem structure and function (Steffen et al., 1996; Diaz and Cabido, 1997; Woodward et al., 1997; Grime et al., 2000).
To a first approximation, FGs respond as a unified whole to their environment. As FGs are independent of any specific species composition they allow comparison to be made between different communities or communities at different stages of their development (Simberloff and Dayan, 1991; Tilman, 2001).
FG traits do not necessarily need to be taxonomic characteristics; although if connected to a particular morphological trait all members of a certain taxon tend to hold the same ecological function. FGs based on different form-of-feeding traits are differentially sensitive to environmental change and are potentially good indicator species.
The co-existence of species within a FG is only possible because of spatially and temporally different exploitation of food and environmental resources (Ritchie & Olff, 1999; Wilson, 1999; van der Putten et al., 2004); species within FGs exhibit interchange ability redundancy) (Blondel, 2003). If environmental conditions change, while species composition within FGs may alter the FG itself may persist. The more extreme the environmental change the fewer the number of common species present before and after the perturbation (-advantages of FGs, Voigt et al., 2007).
Improving our understanding of diversity–function relationships across ecosystems will require a categorization of species, or of species attributes, that can be related to function (Walker et al., 1999).
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