Karl Cottenie

Integrating environmental and spatial processes in ecological community dynamics

The processes controlling the abundances of species across multiple sites form the cornerstone of modern ecology. In these metacommunities, the relative importance of local environmental and regional spatial processes is currently hotly debated, especially in terms of the validity of neutral model. I collected 158 published data sets with information on community structure, environmental and spatial variables. I showed that approximately 50% of the variation in community composition is explained by both environmental and spatial variables. The majority of the data sets were structured by species-sorting dynamics (SS), followed by a combination of SS and mass-effect dynamics. While neutral processes were the only structuring process in 8% of the collected natural communities, disregarding neutral dispersal processes would result in missing important patterns in 37% of the studied communities. Moreover, metacommunity characteristics such as dispersal type, habitat type and spatial scale predicted part of the detected variation in metacommunity structure.

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 ...

METACOMMUNITY STRUCTURE: SYNERGY OF BIOTIC INTERACTIONS AS SELECTIVE AGENTS AND DISPERSAL AS FUEL

The relative importance of regional and local processes in determining com- munity structure is a long-standing problem in community ecology. This is especially problematic in communities from highly connected habitats, which undergo two opposing forces: differences in environmental conditions of the habitats lead to divergence of the communities, while the dispersal of individuals leads to convergence of the communities. Using a transplant experiment, we experimentally showed that biotic interactions have a predictable, deterministic impact on zooplankton community structure in a metacommunity of highly interconnected shallow ponds. Fish predation and presence of abundant macro- phyte cover rapidly structured regional communities of zooplankton, both in terms of relative abundances and of cladoceran species richness. Moreover, we could use the relations between the experimentally manipulated environmental conditions and the observed ex- perimental communities to successfully classify the contrasting local communities found in the original neighboring ponds, thus directly relating the experimental results to the actual field situation. Notwithstanding the observed strong impact of local biotic conditions, we also showed that dispersal had an influence on community structure and species richness. Dispersal increased species richness of cladocerans by three species and influenced zoo- plankton community structure. Moreover, dispersal actually made the response to local conditions more deterministic.

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).

Strong Top-Down Control in Southern California Kelp Forest Ecosystems

Global-scale changes in anthropogenic nutrient input into marine ecosystems via terrestrial runoff, coupled with widespread predator removal via fishing, have created greater urgency for understanding the relative role of top-down versus bottom-up control of food web dynamics. Yet recent large-scale studies of community regulation in marine ecosystems have shown dramatically different results that leave this issue largely unresolved. We combined a multiyear, large-scale data set of species abundances for 46 species in kelp forests from the California Channel Islands with satellite-derived primary production and found that top-down control explains 7- to 10-fold more of the variance in abundance of bottom and mid-trophic levels than does bottom-up control. This top-down control was propagated via a variety of species-level direct and indirect responses to predator abundance. Management of top-down influences such as fishing may be more important in coastal marine ecosystems, particularly in kelp forest systems, than is commonly thought.