ABSTRACT
Resource partitioning is the consequence of competition for the essential conditions of existence, space and food. An important part plays the respective ecological niche which width depends on physical, chemical and biotic (predators, parasites, competitors, prey) factors. In cases of a surplus of prey off er the strength of competition can decrease whereas, contemporarily, prey partitioning can increase. Competition prevails when resources become short, a consequence for the respective predator is specialisation. However, generalists are able to exploit quantitatively the population alterations of several diff erent prey organisms. Prey partitioning is observed in the SW Baltic Sea in fi ve related gobies. Whereas Pomatoschistus minutus, P. microps, P. pictus and Gobius niger are bottom dwellers, Gobiusculus fl avescens lives suprabenthically and exploits mainly copepods from the plankton (Fig. 3.1.11) (Zander, 1994). G. niger and P. minutus prefer clearly macrobenthos like gammarids, isopods and polychaetes but in P. microps meiofauna like benthic copepods surpass macrofauna and are most abundant. During the seasons, the feeding behaviour can change according to the respective development of plankton which density is high in spring, very low in summer and highest in autumn. Th e consequence is that the plankton feeding G. fl avescens is in summer forced to forage macrofauna on the bottom. However, in autumn the macrofauna feeding P. minutus switches over to the rich supply of plankton. P. pictus presents an intermediate strategy consuming in spring meiofauna, plankton and a smaller part of macrofauna, in the plankton-poor summer meio-and macrofauna, and in autumn mainly plankton (Fig. 3.1.11). Niche overlap is thus high in autumn but low (< 60%) in the other seasons. Similar results are found by Thorman (1982) and Thorman and Wiederholm (1983) in gobies and other small fish at the Swedish west and east coasts. A wide niche overlap between P. microps and P. minutus is found only in early summer, but not between G. niger and these two species, mean values remain very low during a time-span of seven months. Th e Pomatoschistus species present also important overlap values with two stickleback species in summer only; P. microps and Pungitius pungitius are similar by 62% overlap regarding the yearly mean of adults. However, young P. minutus which prefer plankton even compete with Gasterosteus aculeatus by 91% in the mean of the year (Table 3.1.3) (Thorman and Wiederholm, 1983). Generally, it is obvious that ecological segregation of
predators basing on foraging behaviours can be cancelled if supply of food is at its extreme ranges, short as in summer or extreme rich as in autumn (Zander, 1994). Gobies as generalists are able to switch to alternative prey items, as Edlund and Magnhagen (1981) found in laboratory experiments with P. minutus and P. microps. In these, Corophium (Amphipoda) and Chironomus-larvae (Insecta) are offered as prey organisms. When kept separately, at fi rst Chironomus is preferred over Corophium which ratio increases with lower prey density, when kept together, P. minutus prefers Chironomus but P. microps feed almost entirely on Corophium.
