Epibenthic Metacommunity Dynamics and Diversity

Understanding the role of biodiversity is a central theme of community ecology. However, how biodiversity is regulated, and how biodiversity impacts ecosystem functioning, is still unclear. This is particularly true for aquatic environments, and even more so for benthic environments, where there has been far less research conducted in comparison to terrestrial environments. It has however, recently become clear that the linked population patches (so-called metapopulations, or at the community level, metacommunities) can play a key role in promoting stability in otherwise extinction-prone populations. The proposed research will use a model microbial system, consisting of benthic algae, ciliates heterotrophic flagellates and bacteria, in order to investigate the interaction of metacommunity dynamics with the effects of predation and disturbance. Working with aquatic microbial systems has not only the advantage that there is much less known about biodiversity controls in these systems as compare to terrestrial habitats, but also that experiments can be run for many more generations than are possible in terrestrial systems. The hypotheses to be tested are: 1) that metacommunity structure allows competitors to co-exist; that this co-existence will depend on the amount of linkage, with highest diversity at an intermediate level of linkage; 2) that predation by generalist predators will reduce diversity, but in combination with metacommunity structure predation will increase diversity; and 3) that disturbance will promote diversity, but only when the disturbance is at a local, rather than regional, scale. Experiments will be conducted in the lab with model communities. Prey will be benthic algae, hetrotrophic flagellates and bacteria. The competing grazers will be epibenthic ciliates. Basins filled with ceramic tiles on which benthic algae have been grown will be the patches, and will be joined with tubing to form a multi-patch environment. Three types of experiments will be carried out: I) Competition-dispersal experiments will be conducted with basins joined to differing degrees and with differing lengths of tubing to produce a range of habitats with many short connections, to those with few long connections. II) Predator-prey experiments will be conducted with ciliates as prey, basins connected at an intermediate level of linkage and the presence of absence of a meiofaunal predator. Experiments will also manipulate the degree to which the predator can disperse, independent of the dispersal rates of the ciliate prey. III) Disturbance experiments will use as the disturbance the replacement of entire basins with in a multi-basin array with new basins with already-established algal communities but no ciliates in order to open patches to colonization. Both the scale and frequency of disturbance will be experimentally manipulated. The experiments outlined in the proposal will increase understanding of how competition, predation and disturbance influence diversity and biomass when the environment consists of linked patches. Moreover, better understanding of the role of spatial heterogeneity is essential to understanding benthic microbial processes.

Understanding the mechanisms that increase biodiversity is not only a central challenge in community ecology, but also of high importance to conservation ecology. Human-induced changes in land-use and fragmentation of formerly connected habitats have resulted in loss of biodiversity. Whether connectivity of such fragments through corridors has a positive effect on biodiversity was analysed in a series of experiments. Benthic (bottom-dwelling) ciliates isolated from ponds and lakes served as test organisms. Due to their small size and high growth rates, these single-celled animals are ideal model organisms. Thousands of animals can be kept in small basins (microcosms) and their short generation times ensure that treatment effects appear within a few weeks. Microcosms were small plexiglass basins filled with water and resource-covered tiles as substrate. Depending on the treatment, several microcosms could be connected with silicon tubing forming a so-called metacommunity. Thus, the local communities were connected by active dispersal of ciliates. Results of the experiments showed that connectivity of communities has a positive effect on the diversity of local communities. When communities were connected, new species could immigrate from neighbouring communities. The level of connectivity, however, played an important role for the effect of connectivity on diversity: When local communities were strongly connected by numerous, short corridors, the superior competitor was able to disperse over the whole metacommunity and eliminated inferior competitors. Thus, diversity of the whole metacommunity decreased with high connectivity. In a predator-prey experiment, a small copepod was used as an efficient predator of the ciliates. When this predator was able to disperse through the corridors, diversity of the prey community strongly declined due to the high predation pressure. In this case, connectivity of communities had a negative effect on diversity. Disturbances are another important factor structuring diversity. In a disturbance experiment, part of the ciliates were removed from the metacommunity at regular intervals. Only when these disturbances completely destroyed selected local communities and thus required reimmigration of species from neighbouring communities did this disturbances lead to species loss. However, disturbances that affected the whole metacommunity but left part of each local community intact, led to a change in the dominance structure of the community but did not result in species extinctions.