Interactions, Heterogeneity, and the Determinants of Multispecies Coexistence

Resumen

No living being is an island. Every creature needs resources from its environment to survive and thrive throughout its life. But in a finite world with over 8.7 million species, acquiring such resources is an inherently competitive endeavour. This has resulted in an extraordinary diversity of interactions, with varying strengths that have either negative or positive consequences for species’ performance. Over the last century, ecologists have sought to understand how the balance of these interactions affects a species’ ability to persist in a community. Initially, research focused on studying interactions between pairs of species, finding that coexistence is possible when stabilising niche differences, which occur when intraspecific competition exceeds interspecific competition, outweigh fitness differences that favour one competitor over others. However, species typically engage in interactions that go beyond species pairs, making this pairwise framework unsuitable to explore species persistence in complex communities. Besides, one main characteristic of ecological interactions is that they are not set in stone—on the contrary, they vary through space and time in response to modifications in the surrounding conditions. Therefore, new theoretical developments are providing us with tools to comprehend how interactions shape biodiversity in complex, multispecies communities with environmental heterogeneity, which is of paramount importance in the changing world that we inhabit. In this thesis, I combine the most recent advances in ecological theory with thorough empirical data to explore how different sources of environmental heterogeneity alter biotic interactions between plant species, and in turn how the structure of such interactions shapes the determinants of multispecies coexistence. In the first three chapters, I use a structural stability approach that allows me to investigate the mechanisms of coexistence beyond pairwise combinations of species. In chapter one, I examine how changes in resources (nitrogen availability) and natural enemies (foliar pathogens) modify the mechanisms of plant diversity and composition. To do that, I quantify the intrinsic growth rates and interaction coefficients between eight common Central European perennial plants in an experiment factorially modifying the nitrogen availability and foliar fungal pathogens of a Swiss perennial grassland. I find that both nitrogen addition and pathogen suppression decrease structural fitness differences that drive competitive dominance but, surprisingly, they also promote niche differences that stabilise the dynamics of interacting species. Interestingly, all effects of resources and enemies on the mechanisms of plant coexistence are dependent on the number of interacting species. In chapter two, I use data from 8 years and 150 German grasslands to investigate the effects of land use intensification on the growth rates and interactions of 50 perennial plant species. The data shows that increasing land use causes species loss by reducing structural niche differences in a non-linear way, rather than by increasing differences in fitness. However, I also find that niche differences play a role in maintaining coexistence among the species that persist at high land use intensities. No living being is an island. Every creature needs resources from its environment to survive and thrive throughout its life. But in a finite world with over 8.7 million species, acquiring such resources is an inherently competitive endeavour. This has resulted in an extraordinary diversity of interactions, with varying strengths that have either negative or positive consequences for species’ performance. Over the last century, ecologists have sought to understand how the balance of these interactions affects a species’ ability to persist in a community. Initially, research focused on studying interactions between pairs of species, finding that coexistence is possible when stabilising niche differences, which occur when intraspecific competition exceeds interspecific competition, outweigh fitness differences that favour one competitor over others. However, species typically engage in interactions that go beyond species pairs, making this pairwise framework unsuitable to explore species persistence in complex communities. Besides, one main characteristic of ecological interactions is that they are not set in stone—on the contrary, they vary through space and time in response to modifications in the surrounding conditions. Therefore, new theoretical developments are providing us with tools to comprehend how interactions shape biodiversity in complex, multispecies communities with environmental heterogeneity, which is of paramount importance in the changing world that we inhabit. In this thesis, I combine the most recent advances in ecological theory with thorough empirical data to explore how different sources of environmental heterogeneity alter biotic interactions between plant species, and in turn how the structure of such interactions shapes the determinants of multispecies coexistence. In the first three chapters, I use a structural stability approach that allows me to investigate the mechanisms of coexistence beyond pairwise combinations of species. In chapter one, I examine how changes in resources (nitrogen availability) and natural enemies (foliar pathogens) modify the mechanisms of plant diversity and composition. To do that, I quantify the intrinsic growth rates and interaction coefficients between eight common Central European perennial plants in an experiment factorially modifying the nitrogen availability and foliar fungal pathogens of a Swiss perennial grassland. I find that both nitrogen addition and pathogen suppression decrease structural fitness differences that drive competitive dominance but, surprisingly, they also promote niche differences that stabilise the dynamics of interacting species. Interestingly, all effects of resources and enemies on the mechanisms of plant coexistence are dependent on the number of interacting species. In chapter two, I use data from 8 years and 150 German grasslands to investigate the effects of land use intensification on the growth rates and interactions of 50 perennial plant species. The data shows that increasing land use causes species loss by reducing structural niche differences in a non-linear way, rather than by increasing differences in fitness. However, I also find that niche differences play a role in maintaining coexistence among the species that persist at high land use intensities.