The role of biotic factors during plant establishment in novel communities assessed with an agent-based simulation model
Background. Establishment success of non-native species can be influenced by interactions with native competitors and novel enemies. Magnitude of interactions between species is mediated by traits, e.g. reflecting competitive strength or defence mechanisms. We combine the concepts to investigate the importance of species traits for successful establishment in a novel community exhibiting biotic resistance in the form of competition and herbivory. Methods. We developed an individual-based
... ion model that tracks survival rate of non-native plants in a native community. Non-native plants are characterized by high or low values of traits relevant for interaction with competitors and herbivores. Results. Traits related to competition had a much greater impact on survival of non-native species than traits related to defence. Survival rates of strong competitors never fell below 50% while survival of weak competitors averaged about 10%. Weak competitors were also much more sensible to competitive pressures including community density, composition and asymmetry while strong competitors responded negatively to changes in competition intensity but hardly to composition or density of native community. High initial numbers of non-native individuals did decrease survival rate of strong competitors but slightly increased survival rate of weak competitors. In the current model configuration we detected only minor advantages in terms of higher survival from high defensive ability as compared to low defensive ability under enemy attack. Surprisingly, herbivory increased survival rate of species classified as weak competitors. Discussion. High survival rates of strong competitors relate to a higher chance of establishment as compared to weak competitors. However, the negative effect of high initial numbers for strong competitive non-natives indicates a self-thinning effect, probably through creation of a highly competitive milieu. For weak competitors, our model predicted increasing survival rates at high initial densities. Positive effects of high propagule pressure have been proposed in field studies and are underlined with this model. However, our model largely overestimated mortality as compared to the preceding experiment. This could be improved by implementing more sophisticated competition modes and traits reflecting tolerance and avoidance of competition by non-native plants. This would, however, be very costly in terms of parameterization. Herbivory had a very low negative effect on survival in general and an advantage of high defence was almost untraceable. However, the positive effect of herbivory on survival of weak competitors indicated that herbivory should not be neglected in estimation of establishment success of species. Although the immediate and expected effect of herbivory might be small, side effects as weakening of resident competitors might play an important role for establishment in a new community.