Effects of biotic neighborhoods on the assembly of annual plant communities in semi-arid gypsum systems: An approach based on common garden and soil translocation experiments

Abstract

One of the main topics of community ecology has been historically the study of mechanisms and processes influencing the formation and structure of organism assemblages. In the early 20th century, two opposing hypotheses summarized the theories developed at that time. Gleason (1926) argued that communities were formed from stochastic processes while Clements (1916) proposed the existence of certain ecological rules in a deterministic version. Currently, it is proposed that communities are structured based on theories such as ecological neutrality and niche theory, along with environmental filtering processes. It is widely accepted that community formation is governed by the combined participation of both stochastic and deterministic processes. Additionally, the resulting diversity in an assemblage can be explained by complementary processes, grouped in terms of speciation, dispersal, drift, and selection, which interact with each other. However, there is still much to investigate regarding the relationship between these four processes and the patterns observed in nature. In recent decades, assembly rules have been widely studied, proposing that these determine the distribution and diversity of plant communities hierarchically, with dispersal filters first, followed by ecological filters, first abiotic and then biotic. Within these, plant-plant interactions play a crucial role, influencing functional and phylogenetic diversity, especially at small spatial scales. The coexistence of species within an assemblage will be conditioned by establishment limits derived from the environment and by biotic interactions. The latter may limit species coexistence, as in the case of competition, or may promote it through facilitation interactions or niche complementarity. Competition, widely studied throughout the history of ecology, can cause the displacement of less competitive species. Facilitation improves physical conditions or increases resource availability, and along with diffuse interactions, contributes to maintaining diversity. Niche complementarity allows species occupying different ecological niches to coexist in the same space at the same time. In this thesis, we assess how the initial degree of evolutionary relatedness within the community (phylogenetic diversity) may determine the response of plant communities to a key filtering factor in Iberian gypsum systems: water availability. The effects on the assemblages resulting from the addition of a potentially key native species are also analyzed, and whether abiotic filtering defined by water and light availability modulates these effects. The importance of this thesis lies in its experimental approach, which allows for an understanding of how communities are structured and inferring processes through the manipulation of mentioned factors. Experimental work has been conducted with both designed and manually created communities, as well as with natural communities in Mediterranean gypsum ecosystems. The use of phylogenetic diversity as a manipulated variable, rather than as a response variable (which is the traditional approach in ecological studies), constitutes a key point in the thesis, allowing for the unraveling of cause-and-effect relationships. In this way, the mechanisms involved in community and species responses to environmental filtering factors can be identified, and ultimately, the mechanisms involved in species coexistence and assembly processes can be inferred. The addition of a potentially key species to its native community while also experimentally manipulating abiotic conditions (light and water availability) is also interesting as it allows inferring mechanisms and processes behind community assembly and trying to deduce the possible plant-plant interactions that may be resulting from them, depending on environmental conditions. In this thesis, taxonomic, functional, and phylogenetic diversity are key tools to understand the underlying mechanisms of plant community assembly. These tools offer an integrated and in-depth view by considering both classic diversity, ecosystem functionality of species, and their evolutionary relationships. Functional diversity reveals the specific adaptations and roles of each species in the environment, while the phylogenetic perspective provides information on evolutionary relationships, interactions, and environmental filtering processes.