dc.description.abstract | 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. | es |