It has long been argued that energetic considerations set a limit to the number of trophic levels that an environment can support. Of the radiant energy that reaches the earth, only a small fraction is fixed by photosynthesis and made available as either live food for herbivores or dead food for detritivores. Indeed, the amount of energy available for consumption is considerably less than that fixed by the plants, because of work done by the plants (in growth and maintenance) and because of losses due to inefficiencies in all energy-conversion processes (see Chapter 17). Thereafter, each feeding link amongst heterotrophs is characterized by the same phenomenon: at most 50%, sometimes as little as 1%, and typically around 10% of energy consumed at one trophic level is available as food to the next. The observed pattern ofjust three or four trophic levels could arise, therefore, simply because a viable population of predators at a further trophic level could not be supported by the available energy.
The most obvious testable predictions stemming from this hypothesis are, first, systems with greater primary productivity (e.g. at lower latitudes) should be able to support a larger number of trophic levels; and second, systems where energy is transferred more efficiently (e.g. based on insects rather than vertebrates) should also have more trophic levels. However, these predictions have received little support from natural systems. For instance, an analysis of 32 published food webs in habitats ranging from desert and woodland to Arctic lakes and tropical seas found no difference in the length of food chains when 22 webs from low-productivity habitats (less than 100 g of carbon m-2 year-') were compared with 10 webs from high-productivity habitats (greater than 1000 g m-2 year-1). The median food chain length was 2.0 in both cases (Briand & Cohen, 1987). Moreover, a survey of 95 insect-dominated webs revealed first that food chains in tropical webs were no longer than those from (presumably) less productive temperate and desert situations, but also that these food chains composed of insects were no longer than those involving vertebrates (Schoenly et al., 1991).
On the other hand, a number of studies on a much smaller scale (e.g. in a group of streams; Townsend et al., 1998) or where resource availability has been manipulated experimentally, have parasites are usually ignored greater primary productivity supports more trophic levels? ...
shown food chain length to decrease with decreased productivity, especially when the decreases take productivity below around 10 g carbon m-2 year-4 (Post, 2002). For example, in an experiment using water-filled containers as analogs of natural tree-holes, a 10-fold or 100-fold reduction from a 'natural' level of energy input (leaf litter) reduced maximal food chain length by one link, because in this simple community of mosquitoes, midges, beetles and mites, the principal predator - a chironomid midge Anatopynia pennipes - was usually absent from the less productive habitats (Jenkins et al., 1992). This suggests that the simple productivity argument may indeed apply in the least productive environments (the most unproductive deserts, the deepest reaches of caves). However, establishing this is likely to prove difficult, since there are other reasons for expecting top predators to be absent from such environments (their size, their isolation, etc.; Post, 2002).
In fact, though, the simple product... or should it ivity argument may have been mis-be total available guided in the first place: what matters energy? in an ecological community is not the energy available per unit area but the total available energy, that is, productivity per unit area multiplied by the space (or volume) occupied by the ecosystem - the 'productive space' hypothesis (Schoener, 1989). A very small and isolated habitat, for example, no matter how productive locally, is unlikely to provide enough energy for viable populations at higher trophic levels. A number of studies appear to support the productive space hypothesis, in that the number of trophic levels is positively correlated with the total available energy -an example is shown in Figure 20.15a. On the other hand, the rare attempts that have been made to determine the separate contributions of ecosystem size and local productivity have detected an effect from size but not from productivity (e.g. Figure 20.15b).
Figure 20.15 (right) (a) The food chain length (FCL) increases with productive space for the food webs of 14 lakes in Ontario and Quebec; productive space (PS) = productivity X lake area; FCL = 2.94PS021, r2 = 0.48. (After Vander Zanden et al., 1999.) (b) Relationships between maximum trophic position and ecosystem size (above) or productivity (below) for 25 lakes in northeastern North America. The maximum trophic position increased with ecosystem size apparently independently of whether productivity was low (2-11 |lg l-1 total phosphorus (TP)), moderate (11-30 |gl-1 TP) or high (30-250 |gl-1 TP). However, when small (3 X 105 to 3 X 107 m3), medium (3 X 107 to 3 X 109 m3) and large lakes (3 X 109 to 3 X 1012 m3) were examined separately, the maximum trophic position did not vary with productivity. The maximum trophic position is the trophic position (FCL + 1) of the species with the highest average trophic position in each of the lake food webs. (After Post et al., 2000.)
Results like these may indicate that total energy is indeed important but is far more dependent on ecosystem size than productivity per unit area. But they may mean, alternatively, that ecosystem size affects food chain length by some other means and available energy has no detectable effect (Post, 2002). One possibility is that ecosystem size affects species richness (it certainly does so - see Chapter 21) and richer webs tend to support longer chains. Unsurprisingly, richness and chain length tend to be associated. Untangling causation from correlation is an important challenge.
0 2.5 5 7.5 10 12.5 15 Productive space In (kg C day-1)
k High productivity □ Moderate productivity ♦ Low productivity k High productivity □ Moderate productivity ♦ Low productivity
107 109 1011 1013 Ecosystem size (volume, m3)
107 109 1011 1013 Ecosystem size (volume, m3)
101 102 Productivity (TP, ig l-1)
If available energy is found ultimately to have no effect on food chain length, it should perhaps be borne in mind that species richness is usually significantly higher in productive regions (see Chapter 21), and that each consumer probably feeds on only a limited range of species at a lower trophic level. Hence, the amount of energy flowing up a single food chain in a productive region (a large amount of energy, but divided amongst many subsystems) may not be very different from that flowing up a single food chain in an unproductive region (having been divided amongst fewer subsystems).
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