noids during the end-Permian mass extinction, disparity can rebound to even greater levels [e.g., McGowan (2004) for ammonoids]. Critically, these studies suggest that at least within broad body plans, the developmental process does not become so increasingly constrained with time as to limit the exploration of morphospace.
Holling (1973) defined resilience as the magnitude of disturbance that a system can absorb before shifting to an alternative state. Ecological studies have demonstrated that the loss of biodiversity can imperil ecosystem services and functions (Loreau et al., 2001, 2002; Folke et al., 2004; Balvanera et al., 2006; Cardinale et al., 2006; Worm et al., 2006), potentially leading to a negative feedback loop further reducing diversity. An assessment of experiments on grassland biodiversity (Hector and Bagchi, 2007) demonstrated a positive relationship between the number of species considered and the overall functioning of multifunctional ecosystems. These results contradict claims of ecological redundancy in ecosystem function (McCann, 2000) and suggest that many, if not most, species do play important roles in ecosystems.
The challenge in analyzing functional diversity is to establish appropriate metrics. For ecological studies Petchey and Gaston (2006) conclude that tabulating the number of functional groups or types is not reliable. Paleontologists thus face significant, although not unsurmountable, problems in identifying the ecological services and functions because the most straightforward paleontological approach is to categorize taxa of interest into different functional groups, such as carnivores, herbivores, suspension feeders, etc. Such categories can often readily be identified in fossils and can be consistent across larger taxonomic groups. Paleontologists have long discussed the selective impact of mass extinctions on trophic groups, such as the pervasive extinction of epifaunal, suspension-feeding marine taxa during the end-Permian mass extinction (Erwin, 1993).
Macroecological guilds were developed as an extension to ecological guilds, encompassing a suite of species (not necessarily related) competing for a similar resource (Bambach, 1983). The concept has primarily been applied to large-scale paleoecological trends, rather than more intensive studies of extinction episodes. One limitation of the guild approach, however, lies in identifying the critical limiting resources that define the members of a guild. A more operational concept is ecospace, which focuses on general modes of life and can be defined independently of species. For marine animals these modes of life are defined in terms of motility, or ability to respond to disturbance; tiering or relationship to the substrate (burrowers versus swimmers), and feeding strategy, or means