Effects of productivity on food chain length in tropical insular systems

Background/Question/Methods

Many different drivers have been considered to explain natural variation in the length of food-chains. Productivity, or energy availability, was one of the earliest proposed drivers of food-chain length. It has recently been suggested, however, that productivity may be only secondarily important in determining food chain length – perhaps only of strong importance in extreme systems, or when calibrated by ecosystem size. However, the primary evidence against productivity as a driver of food-chain length, and the support for other factors - particularly ecosystem size - as the alternate explanation derives mostly from studies conducted in freshwater ecosystems. Therefore, the possibility remains that productivity may be important in other systems, particularly terrestrial ones, which operate under very different ecological constraints .In this study we use isotopic data and surveys of community composition and abundance to  test the relative importance of ecosystem size and productivity in influencing food-chain length in a terrestrial setting where experimental replicates (islets) vary naturally in both size and productivity.

Results/Conclusions

We determine that 1) food-chain length increases strongly and linearly with productivity, 2) there is no effect of ecosystem size or productive space on food-chain length, and 3)  the observed changes in food-chain length with productivity are likely achieved by both behavioral and morphological changes of consumers - rather than by changes in consumer identity. Increases in productivity appeared to increase the complexity of the food webs via multiple pathways, and thus to increase the number of trophic exchanges that occurred from the top to bottom of these communities. We suggest several fundamental differences between terrestrial and aquatic systems that may provide insight into different drivers of food chain length identified in this study. These results help to expand our recognition of the range of factors that control food chain length and challenges us to consider how ecological context controls which drivers shape food chain length.