Migrate, defend, and evolve: Theoretical ecology presented in three different ways

Ecosystems undergo permanent changes. Due to human impact, several of these changes accelerate, like the climate change and the immigration rate of invasive species. Humanity is interested in the enduring stability of the ecosystem in which it lives and of which it is part of. Indeed, several features of species could be identified that can influence the stability of ecosystems. As these features can change due to an evolutionary adaptation to the current conditions, it is important to understand under which conditions a specific feature is beneficial for a species. In this thesis, we investigate three features of species that can influence the stability of ecosystems, namely, prey defense against predator attacks, migration of species between habitats, and intraspecific trait variability in plants. More precisely, we consider reservoir-based defense, which means that the prey has a secretion reservoir that is biosynthetically refilled over time. The prey uses a certain amount of secretion to defend against a predator attack and is inedible as long as it stores at least this amount of secretion. Concerning the migration of species, we will focus on small migration rates for which stochastic effects become relevant, i.e. we analyze the so-called stochastic migration. Finally, we consider that plant individuals within one species or parts of one plant individual differ in their traits and we analyze the impact of this variability on herbivores. Especially, we focus for all three features on the conditions under which the features are beneficial for the species, as representing situations in which these features are evolutionary advantageous. We will show that all three features enhance either species abundance or species diversity in the considered system. Reservoir-based defense can increase both the prey and predator density and is beneficial for the prey when the predator density is not too low and the costs for defense are not too high. Stochastic migration can increase the local and regional diversity in metacommunities; the former even in the limit of very rare migration events. We show that this diversity increase can be ascribed to two effects, namely, the rescue effect and dynamical coexistence. Especially, species of the second trophic level benefit from these effects while species of the third trophic level only benefit from migration when migration rates are high. Finally, we focus on the enormous extent of trait variability in plants. We show that variability in the defense and/or nutrient level of the leaves can per se be beneficial for the plant. We hence present a reason why this large trait variability may have evolved. Furthermore, we show the crucial impact of herbivore preference on these findings. Especially, we find that a plant can per se benefit from large nutrient and defense level variability when it is mainly attacked by specialists that have optimal preference. At the end of this thesis, we discuss, based on these results, how these three features may affect the stability of ecosystems