ESTIMATING MORPHOLOGICALLY DETERMINED CONNECTANCE AND STRUCTURE FOR FOOD WEBS OF FRESH-WATER INVERTEBRATES

1. Connectance is a parameter of central importance in determining foo d-web structure, but the processes determining its value remain unclea r. In evaluating possible explanations it is useful to know what patte rns, and values, of connectance occur in food webs assembled at random from a set of species in a regional species pool; i.e. where the numb er of links is determined by the morphological features of the species present, not by the immediate effects of energetics or stability on t he particular web. 2. This study examines, by means of laboratory expe riments, the occurrence of potential feeding interactions among a set of freshwater invertebrate species randomly selected from different fr eshwater sites in a geographical region. The results from pairwise fee ding trials are used to construct two 'theoretical' food webs, in whic h the patterns and values of connectance are examined. 3. Analyses of these webs indicate that their structure is consistent with the observ ed values in previously documented 'real' webs. Directed connectance v alues of 0.12-0.16 (or less) suggest that the assembled webs are no mo re connected than many freshwater webs from natural systems. The numbe r of links per species increases curvilinearly with the number of spec ies, during web assembly, consistent with recent hypotheses. 4. These results also indicate that quantifying, and understanding the determin ants of, trophic generalism or specialism does have implications for u nderstanding how connectance is constrained in real webs. Freshwater i nvertebrates seem to be relatively generalist, and freshwater food web s perhaps correspondingly highly connected. Such arguments have implic ations for interpreting other aspects of food-web structure in these s ystems, and for parameterizing models that are based on connectance.