Individual variation in trophic interactions shape natural communities
Trophic interactions vary with body size in most animals, and therefore also among individuals within populations. Together with food-dependent body growth, this diversity of interactions and interaction strengths results in feedbacks that govern animal community dynamics. Size-dependent interactions and variation in individual body size (due to energy-dependent body growth) have proven to be fundamental drivers of fish community dynamics. However, their importance in natural food-webs has primarily been demonstrated in closed, often small-scale systems such as lakes, subject to experimental or accidental manipulations.
In this talk I will explain how theory on size-structured community dynamics can guide analyses of observation data to identify (i) size-dependent trophic interactions, (ii) size- and food-dependent individual performance, (iii) size- and food-dependent community dynamics, and (iv) mechanisms that may uphold alternative stable states in food-webs, all in large open systems. The analyzed examples address the interactions between predatory fish in the Baltic Sea, their main fish prey at intermediate trophic levels, and partly shared invertebrate prey lower in the food-web. Combined analyses of dynamic models of these core food-webs, of data on the performance of predator individuals (such as growth and fecundity) and information on prey availability, as well as of time series of population sizes reveal that individual variation in trophic interactions, arising through size- and energy-dependence, are highly important for fish community dynamics also in open, exploited, food-webs, such as the Baltic Sea. Addressing variation in individual performance and in interactions, as well as population dynamics across trophic levels, through dynamic food-web modelling combined with natural observational data can resolve mechanisms underlying food-web dynamics in natural systems, impossible to manipulate experimentally.