Examining species coexistence through the functional trait lens

Abstract

<b>Introduction:</b> I investigate several key knowledge gaps and inconsistencies observed in coexistence literature, specifically gaps associated with spatial heterogeneity, functional trait divergence, and the storage effect. For spatial heterogeneity, empirical research has resulted in all possible heterogeneity-biodiversity patterns, positive, negative, and neutral. The lack of consistency between studies indicates a need to further explore this topic and piece together why various patterns can occur. Although trait divergence has been associated with species coexistence, work is still needed to understand which specific traits play the greatest role in plant communities, how intraspecific trait variation influences coexistence, and to incorporate trait data into community and ecosystem processes. Furthermore, there is a lack of empirical evidence regarding modern coexistence theory and the storage effect, so research is needed to determine if these theories hold true in the natural world. <br>

<b>Methods:</b> In order to address questions surrounding coexistence and plant community assembly, I created the first functional trait database for Nova Scotia, Canada. In total, this database contains 13,458 trait values from 203 species comprising 130 genera and 53 families (Chapter 1). Using this database, I conducted one observational and three experimental studies. The observational study relies on six coastal barren vegetation datasets to determine how spatial heterogeneity and environmental stress influence biodiversity and plant community assembly in turn coexistence (Chapter 3). I examined how soil depth heterogeneity and environmental stress influence plant community assembly over time in an experimental green roof system (Chapter 4). The purpose of the final two experiments was to address questions regarding coexistence and functional trait divergence. I created a greenhouse study examining coexistence under a dynamic watering regime, and a green roof study examining coexistence at low density (Chapter 5). <br>

<b>Main Findings:</b> Environmental stress, rather then spatial heterogeneity, led to an increase in functional trait diversity. Since species with more divergent traits are less likely to compete for limiting resources, this finding indicates the presence of a filter, encouraging coexistence between species that differ in stress tolerant strategies. This finding is further emphasised when specific functional traits are examined. Specifically, coexistence in water-limited environments is possible when species differ in plant height and leaf dry matter content. This dissertation also demonstrates the importance of analysing the functional diversity of individual traits. This analysis allows researchers to understand which specific strategies encourage coexistence and co-occurrence.