A trait-based approach to deciphering diversity change and its consequences on ecosystem functioning

Abstract

Biodiversity and ecosystem functioning represents a prominent field in community ecology. Despite decades of research, the relevance of biodiversity effects in natural communities remain uncertain. Amidst rapid shifts in biodiversity, ensuring generalizability and predictability is essential if we aim at mitigating the decline of ecosystem functions. Functional traits serve as a versatile tool across disciplines, offering a shared currency for addressing critical ecological questions. Functional trait-based ecology presents a valuable framework, as it allows for a more mechanistic and coherent examination of the Biodiversity and ecosystem function relationship (BEF) by facilitating the integration of co-existence and assembly processes. The broad and successful implementation of a trait-based approach in plants has prompted recent calls to extend its application to terrestrial arthropods. This is particularly important in the subtropics and tropics, which harbor the greatest diversity, much of which remains unknown. In this thesis, I contribute to the growing body of trait-based ecology by describing a new experimental approach (chapter 2) that allows for the segregation of the ground foraging ant assemblage in function of their body size. Through the filtering of a subset of ant species, I was able to demonstrate that ants of different sizes present unequal contributions to scavenging, with larger species contributing significantly more than smaller ones. By adopting this approach, we can evaluate the consequences of non-random functional trait change without determent to the current community. Next, I investigate the consequences of functional changes using invaded systems as a natural experiment. Along an invasion density gradient, I applied the response-effect trait framework (Chapter 3). By correlating traits to invasion density, I identified key response traits while simultaneously determining effect traits by estimating their impact on ecosystem function efficiency. I conclude that Solenopsis invicta does alter the resident assemblage, however, scavenging and predation increase with invasion density. From there, I examine the importance of intraspecific variation in effect traits (Chapter 4). Using recent advancements in trait-based analysis, I demonstrated that polymorphic species had higher functional richness, and this led to a more even contribution across scavenging, carbohydrate removal, and predation. Finally, I assessed the behavioral mechanisms behind the observed functional differences between invaded and resident ant assemblages (Chapter 5). I find that pairs of resident species with similar body size, are associated with higher competitive ability, and were more aggressive. Aggression between similarly sized ants could lead to reduced S. invicta abundance. Overall, my work aims to expand trait-based ecology of arthropods to include biodiversity and ecosystem functioning. I accomplish this by providing methods to empirically test in the field, identify and estimate the importance of effect traits, and their variation, as well as, shed light on some of the potential behavioral mechanisms driving the observed patterns.