Optimality-based model of marine zooplankton communities
Zusammenfassung der Projektergebnisse
Marine plankton ecosystem are characterised by high biodiversity, particularly at the lower trophic levels of phytoplankton and zooplankton. Explaining the coexistence of many species in a spatially uniform medium with few known limiting resources (light, macro- and micro-nutrients) is one of the major challenges in biological oceanography. This project aimed to explore the conditions for coexistence at the zooplankton trophic level and elicit a possible mechanism for sympatric speciation, i.e., the evolutionary branching (generation of species) without geographic boundaries. Sympatric speciation has been intensely debated recently, since the introduction of adaptive dynamics has yielded som mechanistic understanding of the processes involved. Previous research on sympatric speciation almost exclusively focussed on mechanisms for reproductive isolation in sexually reproducing populations, such as assortative mating. However, sympatric speciation can only happen if sufficiently different ecological niches exist, in which the newly generated species can coexist. While previous studies always, and usually implicitly, assumed the existence of these niches, the famous paradox of the plankton clearly shows that this assumption is by no means trivial. In the absence of different niches, reproductive isolation would always lead to the displacement of one of the two newly generated species. In this study, we thus went one step back and analysed a possible mechanism for the evolutionary generation of ecological niches within plankton ecosystems. We did not consider the subsequent reproductive isolation, required to finalise the speciation process, since that had be examined already in previous studies. We found that specialisation of predators could generate new ecological niches capable of supporting coexistence of the newly separated species. This mechanism depends critically on the cost of specialisation, i.e., the negative influence of an increased ability to eat the preferred prey on the ability to eat the less preferred prey. For example, a simple behavioural food preference results in a zero-sum game, where the increased ingestion of the preferred prey is completely offset by the reduced ingestion of the less preferred prey. Thus, specialisation can generate new ecological niches only if the cost of specialisation is reduced, e.g., by mutations, resulting in a plus-zero-sum game. We applied this mechanism in several analyses, varying prey traits and total nutrient content of a simple two-predator-two-prey system. Surprisingly, the larger the trait differences between prey species, e.g., maximum growth rate or specific mortality, the lower the likelihood of speciation. Conversely, higher total nutrient content facilitates the generation of new niches and thus can counteract the effect of prey-trait differences. While this result apparently contradicts the observed decline in biodiversity during eutrophication, it is in line with the high biodiversity found in rich ecosystems, such as in oceanic upwelling regions. The reason is likely the difference in time scale: While extinction can happen very quickly, speciation takes very long, owing to the large number of generations required for reproductive isolation. Thus, the decline in biodiversity during eutrophication may be caused by the quick changes in the existing ecological niches, causing rapid extinction. The potential high biodiversity of a rich system can only materialise after sufficiently long periods to allow for several cycles of mutual speciation and niche creation.
