Erik Michels

ZOOPLANKTON METACOMMUNITY STRUCTURE: REGIONAL VS. LOCAL PROCESSES IN HIGHLY INTERCONNECTED PONDS

Local communities can be structured by both local interactions (competition, predation, environmental variables, etc.) and by regional interactions (dispersal of individuals between habitats). Using data from a three-year study on 34 neighboring ponds in an interconnected pond system, we tested whether zooplankton communities show a metacommunity structure, how much variation in zooplankton community structure is related to local environmental factors vs. spatial configuration (taking into account the dispersal pathways), and what environmental variables are the locally structuring forces. In three different years, we found evidence for a metacommunity structure. We also found that spatial and environmental components act independently of each other due to the small geographic area and the high dispersal rates in this system. Despite these homogenizing forces, local environmental variables (associated with alternative equilibria in shallow lakes) were strongly related with zooplankton community structure ...

Geographical and genetic distances among zooplankton populations in a set of interconnected ponds: a plea for using GIS modelling of the effective geographical distance

In systems of interconnected ponds or lakes, the dispersal of zooplankton may be mediated by the active population component, with rivulets and overflows functioning as dispersal pathways. Using a landscape‐based approach, we modelled the effective geographical distance among a set of interconnected ponds (De Maten, Genk, Belgium) in a Geographic Information System (GIS) environment. The first model (the Landscape Model; LM) corrects for the presence of direct connections among ponds and was based on the existing landscape structure (i.e. network of connecting elements among ponds, travelling distance and direction of the current). A second model (the Flow Rate Model; FRM) also incorporated field data on flow rates in the connecting elements as the driving force for the passive dispersal of the active zooplankton population component. Finally, the third model (the Dispersal Rate Model; DRM) incorporated field data on zooplankton dispersal rates. An analysis of the pattern of genetic differentiation among Daphnia ambigua populations inhabiting 10 ponds in the pond complex reveals that the effective geographical distance as modelled by the flow rate and the dispersal rate model provide a better approximation of the true rates of genetic exchange among populations than mere Euclidean geographical distances or the landscape model that takes into account solely the presence of physical connections.

Zooplankton on the move: first results on the quantification of dispersal of zooplankton in a set of interconnected ponds

In systems of interconnected ponds or lakes, the dispersal of zooplankton may be mediated by the active population component, with rivulets and overflows functioning as dispersal pathways and the dispersal being unidirectional. Such systems offer the possibility to study the impact of dispersal rates on local population dynamics and community structure, and provide opportunities to quantify dispersal in the field in a straightforward manner. In this study, dispersal of active zooplankton populations among interconnected ponds was quantified directly in the field by sampling the small waterways connecting the ponds. The number of dispersing zooplankton sampled in connecting elements was on average high (almost 7000 ind h−1). However, the contribution of dispersing individuals to total population size in the target ponds was very limited (< 1% 24 h−1.). Only a weak diel pattern in dispersal rates was observed.

Zooplankton community structure and environmental conditions in a set of interconnected ponds

We studied the zooplankton community structure in a set of 33 interconnected shallow ponds that are restricted to a relatively small area (‘De Maten’, Genk, Belgium, 200 ha). As the ponds share the same water source, geology and history, and as the ponds are interconnected (reducing chance effects of dispersal with colonisation), differences in zooplankton community structure can be attributed to local biotic and abiotic interactions. We studied zooplankton community, biotic (phytoplankton, macrophyte cover, fish densities, macroinvertebrate densities), abiotic (turbidity, nutrient concentrations, pH, conductivity, iron concentration) and morphometric (depth, area, perimeter) characteristics of the different ponds. Our results indicate that the ponds differ substantially in their zooplankton community structure, and that these differences are strongly related to differences in trophic structure and biotic interactions, in concordance with the theory of alternative equilibria. Ponds in the clear-water state are characterised by large Daphnia species and species associated with the littoral zone, low chlorophyll-a concentrations, low fish densities and high macroinvertebrate densities. Ponds in the turbid-water state are characterised by high abundances of rotifers, cyclopoid copepods and the opposite environmental conditions. Some ponds show an intermediate pattern, with a dominance of small Daphnia species. Our results show that interconnected ponds may differ strongly in zooplankton community composition, and that these differences are related to differences in predation intensity (top-down) and habitat diversity (macrophyte cover).

Temporary pools are not `enemy-free'

Temporary pools are traditionally considered as refuges where the conspicuous anostracans are protected from predation. While this is true for the size-selective predation by fish, there is compelling evidence that invertebrate predation is an important biotic stress regulating temporary pool communities. In rock pools in southeastern Botswana, we studied the impact of some suspected invertebrate predators on populations of the freshwater anostracan Branchipodopsis wolfi by means of observations and manipulative experiments. In a survey of 45 pools, the relationship between B. wolfi natural population sizes and the abundance of suspected predators were never negative for turbellarians and mosquito larvae. When dragonfly larvae, notonectids or tadpoles were present, the anostracan populations were generally non-existent or very small. In enclosure experiments with turbellarians, there was a significant effect of predation within one hour of the start; the average daily predation rate was about 1/4 anostracan per turbellarian. Anostracans from a pool with few turbellarians were slightly less vulnerable than those from a turbellarian-rich pool. Furthermore, there was an indication of males being predated on more than females. With dragonfly larvae and notonectids, the predation effect was marked with all six anostracans in an experiment eaten in less than one day by a single predator (predation rate: about one anostracan every 2 h per predator). In a behavioral study, both sexes of B. wolfi avoided swimming above sediment that held more turbellarians than the open patches; there was no evidence for chemical communication with respect to this behavior.

Genetic composition of resident populations influences establishment success of immigrant species

We conducted an outdoor container experiment to test the hypothesis that the genetic composition of resident populations influences the establishment success of immigrant species. We manipulated the genetic compositions (source populations) of populations of the water flea Daphnia magna, a strong competitor in pond and shallow lake zooplankton communities, and monitored the establishment success of immigrant cladoceran species of a regional species pool. We show that establishment success is affected by the source population of the resident D. magna as well as by the presence/absence of macrophytes and the presence/absence of fish in the containers. Our results provide evidence that the genetic composition of resident populations can impact community assembly and metacommunity dynamics, and that community genetics can influence ecosystem functioning.

The influence of food quality on the phototactic behaviour of Daphnia magna Straus

We examined the influence of food quality on the phototactic behaviour of Daphnia magna. Cohorts of a positively phototactic D. magna clone were fed nine diets differing in quality. Diets were obtained by substitution of a fraction of unicellular algae ( Scenedesmus acutus) by a biomass-equivalent of fresh yeast ( Saccharomyces cerevisiae). Some of the diets were enriched with an inoculum of ciliates and organic compounds added as a hay infusion filtrate. Animals fed with a diet containing at least 25% algae showed a similar phototactic behaviour as animals fed with a diet that contained 100% algae. Addition of ciliates to the yeast diets resulted in a lower mortality and a higher reproductive rate compared to diets without a ciliate supplement. The presence of ciliates did not influence phototactic behaviour. Experiments testing for a maternal effect showed that the phototactic behaviour of the animals was strongly influenced by diet during the early developmental stages, but was not influenced by the diet of the mother.