Recruitment-driven, spatially discontinuous communities: a null model for transferred patterns in target communities of intestinal helminths.

ABSTRACT

Populations and therefore communities of intestinal helminths of vertebrates are fueled by recruitment of new individuals from outside the host. The source of new individuals is often an intermediate host that harbors several infective propagules of 1 or more species. Hence these source communities are transmitted in packets of infective propagules to target communities in definitive hosts. Packets not only provide recruits to target communities, but, because a packet of propagules possesses its own structure, it may also transmit structure to the target community. We use this system to examine the contribution that structure in the source pool of propagules makes to the structure of recruitment-driven target populations and communities. By treating the dynamics of such target populations and communities as immigration-death processes, we conclude (1) Unlike a birth-driven population a recruitment-driven target population will grow to an asymptotic limit even in the absence of density-dependent processes or reaching carrying capacity; (2) the frequency distribution of the number of recruits entering target populations will determine the frequency distribution of adults in target populations; (3) interspecific associations among species in the source community will be transmitted to target communities, but the magnitude of the transmitted associations will depend upon the relative survival rates of the species; and (4) for associations of equal magnitude in a source community, the magnitude of a transferred negative association will be less than the magnitude of a positive association in a target community. Two examples of source communities in salt marsh crabs reveal that source infracommunities exist with the hypothesized structure. Further, the source helminth communities display a greater number of positive than negative interspecific associations. The inequity in transfer and the existence of a greater proportion of positive associations in source communities may explain the widespread occurrence of excess positive associations that has been noted in recruitment-driven communities.