Using multi-scale spatially explicit frameworks to understand the relationship between functional diversity and species richness

Understanding how ecosystem functioning is impacted by global change drivers is a central topic in ecology and conservation science. We need to assess not only how environmental change affects species richness, but also how the distribution of functional traits (i.e. functional diversity) mediate the relationship between species richness and ecosystem functioning. However, most evidence about the capacity of functional diversity to explain ecosystem functioning has been developed from studies conducted at a single spatial scale. Here, we explore theory, expectations and evidence for why and how species richness and functional diversity relationships vary with spatial scale. Despite the importance of accounting for spatial processes at multiple scales, we show that most studies of the species richness–functional diversity relationship focus on single scale analyses that ignore spatial context. Thus, we discuss the need to establish a spatially explicit, multi-scale framework for understanding the relationship between species richness and functional diversity. As a starting point to developing such a framework, we detail some expected trajectories and mechanisms by which the diversity of species and functional traits may change across increasing spatial scales. We also explore what is known about two important gaps in the literature about this relationship: 1) the influence of spatial autocorrelation on community assembly processes and 2) the variation in the structure of species interactions across spatial extents. We present some key challenges that could be addressed by integrating approaches from community and landscape ecology. This information will help improve our understanding of the relative influence of local and large-scale processes on community structure, while providing a foundation for improving biodiversity monitoring, policy and ecosystem function based conservation

Beware your neighbours: interactions shape population dynamics in natural grassland communities

Interactions between species have long been recognised as being of major importance to patterns of diversity and abundance, yet our knowledge of how interactions operate and vary in natural plant communities remains incomplete. In such diverse system, interactions are both numerous and highly variable, and they depend on both abiotic and biotic context. This makes quantifying interaction strengths between species in natural communities a difficult and complex task, but it is also a necessary one if we are to uncover the driving forces underpinning this variation. Species interactions are an important component of grassland population dynamics, where plants compete for sunlight, space, nutrients and water. Furthermore, the short generation times of most grassland plant species makes them a useful system for examining the effects of interactions. The National Vegetation Survey databank, operated by Landcare Research, hosts time-series of grassland plant abundances from several hundred plots and collected over the span of several decades. I proposed to exploit the potential of this data set in order to examine the effects of interactions on plant population dynamics. My first hypothesis was that including species interactions in models of population dynamics would improve our ability to predict changes in plant abundance in natural grassland communities. To test this hypothesis, I compared the fits of a series of models which included interactions in increasingly complex and biologically meaningful ways. I found that including interactions improved predictions for the overwhelming majority of species. The most successful model included every pairwise interaction, which allowed us to estimate measures of interaction strengths between species. In turn, the distribution of these interaction strengths provided insights into potential community-level differences in stability. The results presented here suggest these differences were driven by elevation, which weakened interactions, and the presence of exotic species, who tended to have stronger interactions than native species. My second study investigated how interactions between species varied along multiple abiotic gradients. I estimated interaction strengths between focal species and four guilds of competitors — forbs, graminoids, woody species and non-woody others — occurring over a range of elevations and latitudes. Both gradients had varying effects on the mean interaction strengths of each competitor guild. On average, increasing elevation and latitude made interactions with forbs facilitative, whereas interactions with graminoids and woody species became less facilitative and more competitive. This variation in interaction effects might be a reflection of the different optima conditions for each group of species. Together, the work that makes up this thesis suggests that interactions between species can be regarded as an important driver of changes in plant abundance in these grassland communities. Plantplant interactions should be included in models of population dynamics in order to improve predictions of changes in abundance. Furthermore, including interactions also uncovers how variable their effects are, to both environmental conditions and identity of the interaction partner. In particular, the relationships between elevation, species functional guild and biological status affected species interactions in complex, and at times unexpected ways. This has important implications for our understanding of how plant interactions shape grassland community dynamics, and thereby how these communities might respond to biotic and abiotic threats

Data from: Accurate predictions of coexistence in natural systems require the inclusion of facilitative interactions and environmental dependency

1. Coexistence between plant species is well known to depend on the outcomes of species interactions within an environmental context. The incorporation of environmental variation into empirical studies of coexistence are rare, however, due to the complex experiments needed to do so and the lack of feasible modelling approaches for determining how environmental factors alter specific coexistence mechanisms. 2. In this paper, we present a simple modelling framework for assessing how variation in species interactions across environmental gradients impact on niche overlap and fitness differences, two core determinants of coexistence. We use a novel formulation of an annual plant population dynamics model that allows for competitive and facilitative species interactions, and for variation in the strength and direction of these interactions across environmental gradients. Using this framework, we examine outcomes of plant-plant interactions between four commonly co-occurring annual plant species from Western Australian woodlands. We then determine how niche overlap and fitness differences between these species vary across three environmental gradients previously identified as important for structuring diversity patterns in this system: soil phosphorus, shade and water. 3. We found facilitation to be a wide-spread phenomenon and that interactions between most species pairs shift between competitive and facilitative across multiple environmental gradients. Environmental conditions also altered the strength, direction and relative variation of both niche overlap and fitness differences in non-linear and unpredictable ways. Synthesis We provide a simple framework for incorporating environmental heterogeneity into explorations of coexistence mechanisms. Our findings highlight the importance of the environment in determining the outcome of species interactions and the potential for pairwise coexistence between species. The prevalence of facilitation in our system indicates a need to improve current theoretical frameworks of coexistence to include non-competitive interactions, and ways of translating these effects into explicit predictions of coexistence. Our study also suggests a need for further research into determining which factors result in consistent responses of niche overlap and fitness differences to environmental variation. Such information will improve our ability to predict outcomes of coexistence, invasion events and responses of whole communities to future environmental change

Estimating interaction strengths for diverse horizontal systems using performance data

Abstract Network theory allows us to understand complex systems by evaluating how their constituent elements interact with one another. Such networks are built from matrices which describe the effect of each element on all others. Quantifying the strength of these interactions from empirical data can be difficult, however, because the number of potential interactions increases nonlinearly as more elements are included in the system, and not all interactions may be empirically observable when some elements are rare. We present a novel modelling framework which uses measures of species performance in the presence of varying densities of their potential interaction partners to estimate the strength of pairwise interactions in diverse horizontal systems. Our method allows us to directly estimate pairwise effects when they are statistically identifiable and to approximate pairwise effects when they would otherwise be statistically unidentifiable. The resulting interaction matrices can include positive and negative effects, the effect of a species on itself, and allows for non‐symmetrical interactions. We show how to link the parameters inferred by our framework to a population dynamics model to make inferences about the effect of interactions on community dynamics and diversity. The advantages of these features are illustrated with a case study on an annual wildflower community of 22 focal and 52 neighbouring species, and a discussion of potential applications of this framework extending well beyond plant community ecology

Accurate predictions of coexistence in natural systems require the inclusion of facilitative interactions and environmental dependency

Coexistence between plant species is well known to depend on the outcomes of species interactions within an environmental context. The incorporation of environmental variation into empirical studies of coexistence are rare, however, due to the complex experiments needed to do so and the lack of feasible modelling approaches for determining how environmental factors alter specific coexistence mechanisms. In this article, we present a simple modelling framework for assessing how variation in species interactions across environmental gradients impact on niche overlap and fitness differences, two core determinants of coexistence. We use a novel formulation of an annual plant population dynamics model that allows for competitive and facilitative species interactions and for variation in the strength and direction of these interactions across environmental gradients. Using this framework, we examine outcomes of plant–plant interactions between four commonly co-occurring annual plant species from Western Australian woodlands. We then determine how niche overlap and fitness differences between these species vary across three environmental gradients previously identified as important for structuring diversity patterns in this system: soil phosphorus, shade and water. We found facilitation to be a widespread phenomenon and that interactions between most species pairs shift between competitive and facilitative across multiple environmental gradients. Environmental conditions also altered the strength, direction and relative variation of both niche overlap and fitness differences in nonlinear and unpredictable ways. Synthesis. We provide a simple framework for incorporating environmental heterogeneity into explorations of coexistence mechanisms. Our findings highlight the importance of the environment in determining the outcome of species interactions and the potential for pairwise coexistence between species. The prevalence of facilitation in our system indicates a need to improve current theoretical frameworks of coexistence to include noncompetitive interactions and ways of translating these effects into explicit predictions of coexistence. Our study also suggests a need for further research into determining which factors result in consistent responses of niche overlap and fitness differences to environmental variation. Such information will improve our ability to predict outcomes of coexistence, invasion events and responses of whole communities to future environmental change

Data from: Accurate predictions of coexistence in natural systems require the inclusion of facilitative interactions and environmental dependency

1. Coexistence between plant species is well known to depend on the outcomes of species interactions within an environmental context. The incorporation of environmental variation into empirical studies of coexistence are rare, however, due to the complex experiments needed to do so and the lack of feasible modelling approaches for determining how environmental factors alter specific coexistence mechanisms. 2. In this paper, we present a simple modelling framework for assessing how variation in species interactions across environmental gradients impact on niche overlap and fitness differences, two core determinants of coexistence. We use a novel formulation of an annual plant population dynamics model that allows for competitive and facilitative species interactions, and for variation in the strength and direction of these interactions across environmental gradients. Using this framework, we examine outcomes of plant-plant interactions between four commonly co-occurring annual plant species from Western Australian woodlands. We then determine how niche overlap and fitness differences between these species vary across three environmental gradients previously identified as important for structuring diversity patterns in this system: soil phosphorus, shade and water. 3. We found facilitation to be a wide-spread phenomenon and that interactions between most species pairs shift between competitive and facilitative across multiple environmental gradients. Environmental conditions also altered the strength, direction and relative variation of both niche overlap and fitness differences in non-linear and unpredictable ways. Synthesis We provide a simple framework for incorporating environmental heterogeneity into explorations of coexistence mechanisms. Our findings highlight the importance of the environment in determining the outcome of species interactions and the potential for pairwise coexistence between species. The prevalence of facilitation in our system indicates a need to improve current theoretical frameworks of coexistence to include non-competitive interactions, and ways of translating these effects into explicit predictions of coexistence. Our study also suggests a need for further research into determining which factors result in consistent responses of niche overlap and fitness differences to environmental variation. Such information will improve our ability to predict outcomes of coexistence, invasion events and responses of whole communities to future environmental change

The evolution of niche overlap and competitive differences

Competition can result in evolutionary changes to coexistence between competitors but there are no theoretical models that predict how the components of coexistence change during this eco-evolutionary process. Here we study the evolution of the coexistence components, niche overlap and competitive differences, in a two-species eco-evolutionary model based on consumer-resource interactions and quantitative genetic inheritance. Species evolve along a one-dimensional trait axis that allows for changes in both niche position and species intrinsic growth rates. There are three main results. First, the breadth of the environment has a strong effect on the dynamics, with broader environments leading to reduced niche overlap and enhanced coexistence. Second, coexistence often involves a reduction in niche overlap while competitive differences stay relatively constant or vice versa; in general changes in competitive differences maintain coexistence only when niche overlap remains constant. Large simultaneous changes in niche overlap and competitive difference often result in one of the species being excluded. Third, provided that the species evolve to a state where they coexist, the final niche overlap and competitive difference values are independent of the systems initial state, although they do depend on the models parameters. The model suggests that evolution is often a destructive force for coexistence due to evolutionary changes in competitive differences, a finding that expands the paradox of diversity maintenance. A two-species eco-evolutionary model based on consumer-resource interactions and quantitative genetic inheritance shows how evolution among competitors changes the components of stable coexistence.Funding Agencies|Australian Research CouncilAustralian Research Council [DP170100837]; [VR 2017-05245]</p