[PhD defence] 16/12/2024- Romane BLAYA: "Response of the structure and temporal dynamics of ant communities to spatial characteristics and habitats on small Mediterranean islands" (UMR IMBE)

Romane Blaya will submit her thesis on Monday 16 December 2024 on the subject of "Response of the structure and temporal dynamics of ant communities to spatial characteristics and habitats on small Mediterranean islands".

Laboratory

UMR 7263 IMBE - Mediterranean Institute of Biodiversity and Ecology EECAR

Composition of the jury

Ms ELISE BUISSON	Avignon University	Thesis supervisor
Ms Anna TRAVESET	Instituto Mediterraneo De Estudios Avanzados (CSIC)	Rapporteur
Mr Xim CERDá	Estación Biológica Doñana (CSIC)	Rapporteur
Jean-Philippe LESSARD	Concordia university	Examiner
Ms Ana SANTOS	Universidad Autónoma de Madrid	Examiner
Mr Philippe PONEL	Aix-Marseille University	Thesis co-director
Mr Olivier BLIGHT	Avignon University	Thesis co-supervisor
Ms Jelena BUJAN	Ruđer Bošković Institute	Examiner

Summary

The aim of this thesis is to assess the effects of the spatial characteristics of islands on the structure of ant communities and their temporal dynamics, with a particular interest in the effects of habitat and its changes. Islands are of great interest to researchers because of the conservation issues they raise, but, as nature's laboratories, they are also used to advance fundamental theories. The Equilibrium Theory of Island Biogeography (TEBI) has provided a research framework in island biogeography and ecology for understanding the structure and dynamics of communities, seen as the result of stochastic events of colonisation and extinction, in relation to the size and isolation of islands. However, small islands are considered to be an anomaly in the TEBI due to their lack of area-species relationships: this is the Small Island Effect (SIE). This is known as the Small Island Effect (SIE). Small islands are not only subject to more frequent stochastic extinction events, but are also intrinsically less diverse, particularly in terms of habitats. Although TEBI has been widely studied, understanding the effects of the spatial and ecological characteristics of islands on community structure and dynamics remains a challenge. Using 207 small Mediterranean islands of less than 10 km², our results show that island size is a key predictor of ant species richness. However, using a subset of 36 small Corsican islands, we find that it does not affect temporal turnover. The impact of isolation is less obvious, especially when measured by distance from the mainland, as the islands studied are not very isolated and are often part of a chain of islands. Thus, the proportion of land mass around an island appears to be a better predictor of its species richness. Since low isolation reduces β-diversity, the dynamics of colonisation and extinction would maintain a limited pool of species, which explains why turnover does not respond. The study of the area-species relationship of ants shows the existence of an island size threshold below which species richness increases very slowly with size, which corroborates the EIS. Given that species richness and turnover respond positively to habitat proportion and negatively to habitat diversity respectively, this confirms the importance of considering non-spatial factors for small islands. In line with the habitat heterogeneity hypothesis, the importance of habitats can be explained by the increase in resources and nesting sites. The positive response of ants to habitat change and vegetation diversification following the eradication of witches' claw on a single island also highlights the importance of habitats. It seems that ant communities are relatively more stable on well-diversified sites with undisturbed vegetation. Nevertheless, even if ant community dynamics are low, we observe that, even on relatively well-preserved islands, the composition tends to become more homogenous over time, with the expansion of generalist species, which could be attributed to climate change. Ant diversity on islands appears to be the product of a combination of multiple factors, such as surface area, isolation and habitat availability. The rapid and clear response of ants to habitat change underlines the importance of this factor on small islands, but also the importance of ants as indicator species.

Keywords Formicidae, Biogeography, Small island, Habitat diversity, Ecological restoration, EIS