Species assembly rules and the biodiversity-ecosystem functioning relationship / Regras entre assemblÃias de espÃcies: relaÃÃo entre biodiversidade e funcionamento do ecossistema

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

The biological diversity of ecosystems has important implications for their functioning. However, in order for reliable predictions to be made on the effects of species extinctions and/or additions on particular ecosystem functions, accurate knowledge is required on the ways in which species interact. These interactions in turn are important drivers of community structure and diversity. Direct interspecific interactions involve predator-prey interactions and competition for resources, whereas facilitation and inhibition are examples of indirect interactions between species. The latter type of interactions is crucial to an adequate understanding of the degree of redundancy among species within a given trophic guild. By now the concomitant contribution of bottom-up (i.e. resource-controlled) and top-down (i.e. predation-controlled) forces on community structure and dynamics has been widely accepted. But due to the great variability in experimental circumstances and test organisms, the results and conclusions are far from consistent. Nematodes are the most abundant and speciose metazoans in almost all marine soft sediments. Their communities are typically characterised by a high local diversity, and many species belonging to single functional guilds tend to co-occur. They are potentially important consumers of primary producers as well as of primary decomposers (bacteria), and are prey to a variety of predators, including other nematodes. It is hitherto unclear whether nematode community structure and dynamics tend to be controlled predominantly by resource availability or by predation. The importance and role of horizontal interactions, i.e. within a trophic level, such as competition and the indirect interactions (mentioned above), are even less well understood. The rules underlying species assembly in complex and speciose communities are a sum of direct and indirect interactions, occurring simultaneously in any direction: top-down, bottom-up and horizontal. Life history theory emphasizes that the impact of a stressor (for instance resource limitation, presence of predators,â) on populations of organisms is determined by its effect on the fitness of a given population. More specifically, the concept of maximizing fitness is the key to gaining insight into the demographic effects of such stressors at the population level. We have performed microcosm experiments addressing bottom-up, top-down and horizontal effects on the fitness of populations of bacterivorous nematodes. A first experiment investigated whether and how nematodes may affect colonization of substrata by bacteria through their production of mucus trails. We were able to demonstrate that nematode mucus secretions consistently become colonized by a very low-diverse subset of the bacterial community present in the environment, and that this effect is nematode-species-specific. This microbial âgardeningâ strategy provides novel insight into recently reported species-specific impacts of bacterivorous nematodes on bacterial communities, and may well have important implications for interspecific interactions among bacterial-feeders. Subsequently, we investigated the effect of resource availability on the population dynamics of three species of bacterial-feeding nematodes which typically occur in sympatry in their natural habitat. Resource availability was defined as bacterial (E. coli) density, and the response of each nematode species was studied in monospecific culture. Each nematode species showed a differential response to food availability, differences among species becoming manifest in parameters like reproduction rate, development time, maximal densities and individual as well as population biomass. One nematode species had an optimal fitness at the highest food availability, whereas the other two preferred intermediate food densities. Based upon these results, we constructed a hypothetical curve of how a community simultaneously comprising these three species would be expected to respond to food availability, and tested this in a parallel microcosm experiment in which the three species were now inoculated together. Our results did not follow the pattern expected from the individual speciesâ responses, and this as a result of three different horizontal interactions acting simultaneously within this artificial nematode community: facilitation, inhibition and competition for food. In a following set of experiments, we elaborated upon the inhibitory and competitive interactions between the two species showing the most similar food-density dependence. In addition to population development, we now also studied food-finding (through chemotaxis) and food assimilation (following the fate of a 13C tracer). We confirmed the inhibitory effect between both species in a more complex feeding environment comprising a multitude of bacterial strains, but found it to become asymmetric (only one species negatively affecting the other, not vice versa), in contrast to the results from the previous experiment where the effect was largely symmetric. Surprisingly, food-finding and food assimilation of both species showed clear evidence of interspecific facilitation, the opposite of inhibition. Evidently, behavioural parameters studied at the short-term may yield different conclusions than population parameters studied over a longer term. However, there was concordance in the fact that food (i.e. bacterial) diversity weakened the observed interspecific interactions. Finally, we studied top-down effects, resulting from the introduction of a predaceous nematode, on the population development of these same two species in monocultures. The predaceous nematodes exerted a significant effect over total densities, age and size distribution and even sex ratio of the prey species. This effect was strongly dependent on, but not linearly proportional to, predator density. As such, we have shown that all three types of interactions (bottom-up, top-down and horizontal) are important drivers of population dynamics and community structure within the trophic guild of bacterivorous nematodes. We conclude that the indirect horizontal interactions are of particular relevance to our understanding of how species diversity within this trophic guild may affect ecosystem functions related to microbial activity, but that these indirect interactions in turn are bottom-up and top-down affected.

ASSUNTO(S)

ecosystem functioning estuÃrios predaÃÃo sucessÃo ecolÃgica detritus food webs nematodes regras entre assemblÃias de espÃcies predation species assembly rules bactÃria succession funcionamento dos ecossistemas top-down control facilitaÃÃo cadeia trÃfica de detritos controle ecolÃgico descendente chemotaxis controle ecolÃgico ascendente nematÃdeos quimiotaxia relaÃÃes ecolÃgicas bottom-up control estuarine bacteria trophic interactions facilitation biodiversidade ciencias biologicas

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