What are the indirect effects of resource-pulses, short term increases in resource availability, and how do top down effects impact the stability of lower trophic levels? For instance, should the effects of resource pulses become weaker as they move up trophic levels due, to energy loss, to metabolism and the inefficiency of ingestion and digestion by consumers? Contrary to this the trophic level that is directly impacted by a resource pulse has been found to have a smaller relative response compared to species indirectly impacted by the initial resource pulse. However, what is the mechanism that causes this amplification? Using a theoretical approach, we explore whether top down effects can amplify the impacts of resource-pulses. To do this, we modeled the biomass of a tri-trophic food chain using differential equations and represented resource-pulses by allowing potential primary productivity (PPP) to fluctuate at high or low frequencies. Using Fourier analysis we were able to develop analytical solutions for what each trophic levels amplitude (stability) would be when experiencing fast or slow fluctuations in PPP. We then compared the stability of food chains with 1, 2, & 3 - trophic levels in order to quantify the impact of the removal/addition of a new trophic level to ecosystem stability.
Results/Conclusions
We found that top down effects can cause the consequences of resource pulses to be amplified, as they are transferred from one trophic level to another. When PPP fluctuated rapidly primary producers’ fluctuated the most and the effect of the resource pulses was dampened as it was transferred from one trophic level to another. However, as PPP began to fluctuate more rapidly the effects of the resource-pulses became more amplified in the primary and secondary consumers. This resulted in primary producers exhibiting low amplitude fluctuations while the higher trophic levels exhibited high large amplitude fluctuations. Finally, when PPP fluctuated slowly the effects of the resource pulse would appear to bypass the primary consumer and only the primary producer and secondary consumer would appear to show large amplitude fluctuations. Furthermore, when we compared the effect of removing the secondary consumer we found primary producer’s began to show greater fluctuations in their biomass but as PPP began to fluctuate more rapidly predators played an important role in stabilizing primary producers from resource pulses. In summary, we have found that predators play an important role in determining the stability of resource pulses.