Subsidies primarily affect transient responses, not equlibrium behavior

Background/Question/Methods

The role of allochthonous subsidies in food web dynamics has been studied in empirical systems ranging from the work of Polis and collaborators on exchange from marine environments onto islands to the work of many others since often focusing on aquatic-terrestrial interchanges.  Similarly, there has been substantial interest in the dynamics of resource pulses (in time) from masting and other biological causes as well as from physical environmental forcing. Prior theoretical work on the role of cross-boundary subsidies has focused on the effect on equilibrium dynamics and the number of trophic levels that can persist.Here, we develop a model that integrates resource pulses and spatial subsidies. The model includes three trophic levels and considers both temporally varying and constant subsidies as well as resource pulses input at different trophic levels, ranging from an effect on primary productivity. The model explicitly takes into account different time scales of survivorship of individuals at different trophic levels. This model allows us to determine the effect of subsidies on the response to resource pulses over short time scales as determined by frequency of pulses, dynamics of subsidies, and life histories of species involved.

Results/Conclusions

The model was analyzed using both approximate analytic approaches and numerical solutions.  Viewed one way, the presence of ongoing subsidies has a dramatic effect on the impact of a resource pulse. By raising the base level of resources at lower trophic levels, there is a much larger impact of a resource pulse on population numbers at higher trophic levels. All of the effects can only be understood with an analysis that takes into account transient dynamics, as done here. Viewed another way, there are both substitutabilities and interactions between subsidies across space and subsidies in time in determining average abundances at all trophic levels. All of these results depend strongly on the average lifetime and ongoing resource needs for survivorship of individuals at different trophic levels. The model results are used to explain and understand long standing empirical observations by Polis and colleagues from the Gulf of California.