Daphnia metacommunity dynamics : the roles of inbreeding, parasitism, competition, and dispersal

The habitats of many species are not continuous but fragmented, either naturally or increasingly due to human land use. Communities of species inhabiting fragmented habitat patches are connected by dispersal and form a metacommunity, a spatial overlay of the metapopulations of the individual species. The factors enabling coexistence of different species in communities may shed light on the core questions of ecology, which is on the search for explanations for the abundance of species and their interactions with the biotic and abiotic environment. With this thesis, I aimed at contributing to a better understanding of the interplay of potential determinants of species coexistence at the local and regional scales in metacommunities.
I studied a metacommunity of the three generalist filter-feeding planktonic water flea species Daphnia longispina, D. magna, and D. pulex, which form local communities in ephemeral freshwater rock pools although they have largely overlapping niches. To corroborate my observational and experimental findings, I combined them with a long-term demographic data set by V.I. Pajunen on the presence of the three species in over 500 rock pools since 1982.
The largest and most abundant species D. magna is known to harbour a high diversity of ecto- and endoparasites, and the diversity of parasites in local D. magna populations was shown to correlate primarily with population age. I screened rock pool populations of the congeneric species D. longispina over a period of three years and inspected which epidemiological, spatial, temporal, and physicochemical factors explained most of the variance in the number of parasites. Contrary to the situation in D. magna, parasite richness in D. longispina was generally substantially lower, and surprisingly, the best predictor was the presence of D. magna, indicating that the co-occurring congeneric species might serve as a reservoir of parasites from which D. longispina is constantly inflicted. At the same time, the presence of parasites could be the decisive factor mediating coexistence between the two Daphnia species.
Daphnia magna does not only carry the largest parasite burden but is as well the most dynamic species in terms of extinction and colonisation events in the metacommunity. To understand the factors of invasion success of D. magna, I conducted a fully factorial outdoor mesocosm experiment where D. magna clones that were outbred, unparasitised or did not face competition by resident D. longispina had a higher invasion success. The three factors inbreeding, parasitism, and interspecific competition are suggested to affect colonisation dynamics in the metacommunity and to determine whether the sequential arrival of Daphnia species in rock pool habitats can result in local coexistence.
Not only invasion success proved to be diminished by competition, the competitive hierarchy of species is generally considered an important determinant for coexistence. Therefore I studied the interspecific competitive abilities of the three Daphnia rock pool species in two further outdoor mesocosm experiments. The interspecific competitive abilities of forty D. magna clones competing against D. longispina and D. pulex correlated positively and were higher when the clones originated from genetically more diverse populations, which is suggested to be a consequence of reduced inbreeding depression. The competitive abilities of the three Daphnia species competing in pairs against one another followed almost invariably a transitive competition hierarchy, i.e. the largest species D. magna was competitively superior to the intermediately sized D. pulex which in turn was competitively superior to the smallest species D. longispina. Such transitive competitive hierarchies are supportive of the size efficiency hypothesis which states that larger species dominate smaller species in the absence of predation because of their higher feeding efficiency. Conversely, I propose that the rare but naturally possibly more prevalent intransitive competitive relationships are a key factor for the maintenance of clonal and species diversity in Daphnia rock pool communities.
Since rock pools are highly stochastic habitats, a requirement of local coexistence is the sufficient provisioning of immigrants by means of dispersal. Daphnia depend on passive dispersal by resting stages, so-called ephippia. To obtain a direct estimate of the absolute dispersal effort, I trapped ephippia over three years in areas inhabited predominantly by D. longispina and D. magna. The majority of caught ephippia stemmed from D. magna, which supports earlier conclusions that D. magna ephippia are more often exposed to the supposedly dominant dispersal agent wind since the species prefers smaller, more desiccation-prone rock pools. In contrast, I speculate that D. longispina may be dispersed predominantly by means of insects which may be charged by this species' buoyant ephippia.