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