Development of the Decomposer Food-Web, Trophic Relationships, and Ecosystem Properties during a Three-Year Primary Succession in Sawdust

There has been considerable interest in characterising the general properties of food-webs and trophic interactions but little is known as to how these develop along successional gradients. From a field study we determined how the decomposer food-web develops during a three-year primary succession of sawdust. Microfloral and microfaunal groups increased over the initial 1-2 years of the study and then reached a state of «dynamic equilibrium». During this period of longer-term stability there was considerable short-term instability, with severe oscillations most likely attributable to predator-prey cycles involving nematodes, bacteria and fungi. Populations of top predatory nematodes closely mirrored those of the bacterial-feeding cephalobid nematodes three months earlier, indicative of a strong trophic association. Other relatively close linkages were also detected, but the majority of links involving nematodes appeared weak, suggesting that the distribution of food web interaction strengths is strongly skewed. Food chain length and food-web complexity increased over the fist three months of the study but remained invariant thereafter, demonstrating that connectedness approaches to food-webs are extremely insensitive to primary succession. While some of our findings were in broad agreement with Odum's theory of ecosystem succession (e.g. initial biomass buildup, development of connectedness food-web properties), others were not. These included: colonisation early in the succession by the largest organisms in the food-web (mites and Collembola); little difference in early colonising ability between nematodes with r-selected and K-selected traits: a short-term «instability» of microflora and microfauna later in the succession indicative of high nutrient exchange rates; and a detectable rise in the microbial respiration:biomass ratio over the final year probably due to nutrient limitation. Consideration of trophic dynamics aided the understanding of successional trends, and an appreciation of the complex patterns of (trophic) species interactions appears necessary for a more complete interpretation of the patterns and mechanisms of ecosystem succession