REGIONAL VERSUS LOCAL HELMINTH PARASITE RICHNESS IN BRITISH FRESH-WATER FISH - SATURATED OR UNSATURATED PARASITE COMMUNITIES

The relationships between regional species richness and local species richness were examined in respect of helminth parasite communities in 32 species of freshwater fish in the British Isles. Fish were divided into 5 categories, for each of which the goodness of fit of the relati onship to linear, exponential and power function models was tested. Fo r all categories of fish combined, there was a significant, positive c urvilinear relationship. Nested within this were two other patterns. F or introduced fish, a linear model provided the best fit; for euryhali ne and relict species it was impossible to determine the best model, b ut for the other categories the relationship was curvilinear and was b est fitted by a power function model. The linear relationship found fo r introduced fish was interpreted as a temporary situation, reflecting the shortage of time for the communities to become saturated. It corr esponded to the linear part of the curvilinear relationship of the oth er categories, which is believed to represent the fundamental form of the relationship for parasite communities. The communities reached a s aturation level of richness, corresponding to the asymptote of the cur ve, which fell well below regional species richness. Explanations for local saturation are discussed, but neither community structure nor su pply-side ecology can yet be preferred. It is concluded that local pat terns in helminth community richness, in contrast to those in fish ass emblages, are not significantly influenced by patterns on a larger, re gional spatial scale and so regional species richness is not a key det erminant of local species richness, nor does a knowledge of regional p atterns improve predictability of local patterns.