Functional Diversity or Ecospace

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

178 / Douglas H. Erwin of acquiring energy (Bush et al., 2007). The six possibilities along each of these axes define a 3D grid of 216 possibilities, of which only 92 appear to be occupied (Bambach et al., 2007). As with marine guilds, many different taxa can occupy each of these different modes of life, so identifying which modes are occupied across a mass extinction may not be particularly informative. One could imagine a more intensive study in which this framework was used to chronicle across mass extinction both how many modes of life were exhibited by various clades and the changing density of occupation by various clades of particular modes of life. Such a study would be particularly informative if it revealed differences in extinction intensity between different functional groups.

In some cases it may be possible to apply more rigorous analyses to the problem, such as the comparison of food web structures. Ecologists have developed a rich toolkit for studying the network properties of food webs (Martinez and Dunne, 2000), and with a working group at the Santa Fe Institute we have recently shown that such methods can be applied to Cambrian fossil communities. Although ecologists have access to direct feeding observations and gut contents in practice they often rely on morphology and other data also available to the paleoecologist. Our results demonstrate that ancient food webs can be reliably reconstructed, opening up the potential to study changes in the network properties of ecosystems across mass extinctions (Dunne et al., 2008).

Modeling changes in functional diversity, trophic complexity, and food web structure in the search for patterns that can be observed in the fossil record is another approach. We developed a simple model in which extinction was simulated by the collapse of primary productivity, triggering reductions in the diversity of higher trophic levels (Solé et al., 2002). The results imply that the trophic structure of extinction may influence the tempo and pattern of recovery. More detailed computer simulations of the effects of both productivity loss and resulting secondary extinctions through a food web further emphasize the importance of the network structure in the pattern of extinction (Roopnarine, 2006; Roopnarine et al., 2007). Although the significance of these results is limited because of the lack of empirical input into the food web structure, it suggests something of the insights that may eventually result.

An additional area that could prove important in understanding the loss of functional diversity is the correlation between scaling relationships and ecological networks, particularly as biodiversity collapses. For example, metabolic scaling theory posits linkages across metabolic activity, form, population size, species diversity, and other variables (West et al., 1997; Enquist et al., 2003, 2007). The apparent relationship between metabolic activity and some mass extinctions (Bambach et al., 2002; Knoll et al., 2007), suggests that the relationship between scaling theory and extinction is worth exploring, as are species-energy relationships (Evans et al., 2005; Hawkins et al., 2007a).

Ecological networks also provide a host of services to the community, ranging from clean water to fine-scale modification of climate (microhabitats). These ecological services have been a subject of considerable interest among conservation biologists, but have not been addressed in deep time. For example, what was the impact on the water quality in shallow marine ecosystems as a consequence of the loss of so many articulate brachiopods, crinoids, bryozoans, and other filter feeders during the end-Permian mass extinction? This issue is probably best investigated through stable isotope studies of nutrient flow or geochemical cycling (West et al., 2006) or where the services leave a tangible fossil record.

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