Analyses of the slopes of size spectra are now widely used to assess the state of marine ecosystems at regional and global scales (Shin et al. 2005
). Observed size spectra typically become steeper (more negative) following exploitation (mainly of fishes); in one survey of fishes the slope of the size spectra became about 1.5 times steeper over the period from 1977 to 1993 (Rice & Gislason 1996
). Demonstrating detectable effects of exploitation on size spectra has been key to their emergence as indicators of marine ecosystems ().
Figure 1. Size spectra describe the relationship between organism size and abundance and can be predicted from the expected joint change in abundance and organismal mass that occurs across one trophic link. The theory behind the scaling of abundance and mass is (more ...)
A rich body of theory exists for predicting the slope of size spectra, and this theory can be used to calculate reference states in fisheries (Jennings & Blanchard 2004
). Jennings and Blanchard found that achieving a slope as steep as that observed in the North Sea requires an unfeasibly low predator
prey mass ratio (of around 10) and/or trophic transfer efficiency (around 0.0025). This suggests that the North Sea is a long way from the theoretical unexploited reference state. This potential for size spectra to provide indicators of ecosystem status, and to allow estimates of distance from reference state, has probably contributed to their use as general indicators of marine ecosystem status (Shin et al. 2005
Are size spectra, and other local allometries, less useful in non-marine ecosystems? Are fishes and the ecosystems they inhabit so different from other species and ecosystems that a universal approach is unsuitable and inapplicable? Is the direct exploitation of larger species, which in part causes the steeper size spectra, so different from other environmental impacts, such as altered nutrient levels, habitat destruction and species invasions?
The answer to these questions appears to be ‘no’: perhaps somewhat surprisingly, size spectra theory even appears to apply in some soil ecosystems. Christian Mulder and his colleagues studied 12 managed grasslands and 10 ex-organic farms abandoned for at least a decade (Mulder & Elser 2009
). Differences in management practices resulted in soil ecosystems differing greatly in soil pH and nutrient ratios. Soils were sampled for the abundance and mass of bacteria, fungi, nematodes, mites, springtails and enchytraeids (e.g. earthworms), and size spectra constructed. Low pH and relatively high ratios of phosphorus to carbon and nitrogen were associated with steeper size spectra, resulting from the relative rarity of larger organisms and abundance of smaller organisms in high phosphorus soils. These links between soil chemistry, farming practices and the characteristics of size spectra indicate the possibility of assessing the status of soil ecosystems, even across large geographical ranges and soil types that are very difficult to compare using more traditional taxonomic indicators. Detailed analysis and modelling of some of the soil biodiversity data collected by Mulder's group, and of one estuarine and two pelagic communities, revealed broad agreement between the theory and observations (Reuman et al. 2008
). It seems that the systematic changes in size spectra that occur in exploited fisheries are occurring in other systems under other types of environmental pressure.