How do alien species replace native biota? This question has fuelled several well-established hypotheses referring to three levels of biological organization: 1) species level e.g. hypothesis of competitive exclusion, 2) community level e.g. mutualism disruption, invasional meltdown, 3) ecosystem level e.g. concept of ecosystem engineers or transformers, which alter disturbance regime, succession and productivity. Most of these hypotheses have however seldom been evaluated in natura using threatened native species, which makes it difficult to identify which ecological mechanisms prevail in causing biotic homogenization. Our project aims to fill this knowledge gap by asking two questions: Q1) through which processes do invasive alien species predominantly act? Q2) Do these processes change in magnitude along the species extinction trajectory i.e. in the course of a native species' decline?

The last survivor of a population of Pyrostria commersonii, a critically endangered tree endemic to Réunion and Mauritius (dark bark) in a dense thicket of the alien shrub Ligustrum robustrum (light bark), Cirque de Cilaos, Réunion. ©Robin POUTEAU

These questions will be addressed by combining: 1) plot inventories in populations of threatened trees endemic to the Mascarenes islands (Indian Ocean), 2) cutting-edge community models supplemented by pot experiments, and 3) remote sensing analyses of major ecosystem shifts and exogenous disturbances that occurred in the last > 70 years over inventoried populations. Hopefully, the complementary nature of biological measurements and their nested scales will reveal what type of pressure invasive alien plants exert on LC, NT, VU, EN and CR endemic species, thus allowing targeting the ultimate mechanisms of biotic homogenization in local conservation efforts and upcoming international agendas.

The ecology of threatened plant species with different levels of extinction risk will be studied at different spatial and time scales. Q1 will be jointly addressed by three WPs targeting nested levels of biological organization (Fig. 1). WP1 will investigate pairwise interactions between threatened endemic species and invasive alien species over more than 10 years . WP2 will focus on the disassembly of native communities by invasive alien species using a ‘snapshot’ approach. WP3 will deal with historical ecosystem shifts driven by invasive alien species. The different mechanisms tested in WPs1-3 are mutually non-exclusive and complementary over time on the extinction trajectory. WP4 will address Q2 by revealing the prime mechanisms at different steps of the extinction trajectory.

Figure 1. Conceptual diagram showing the complementarity in considered biological (vertical axis) and time scales (horizontal axis) between tasks (WPs) 1, 2 and 3. The different levels of biological organization will be approached using complementary scientific disciplines.

Our focus will be on trees because they are perennial and play a critical role in structuring most terrestrial habitats. As 86% of recent extinctions concerned island endemic species, this project will be conducted on trees endemic to the Mascarene islands, which comprise (from west to east) Réunion, Mauritius and Rodrigues. Although this archipelago has been among the last places on Earth to have been reached by humans, it is among the most ecologically devastated. The emblematic case of the Mauritian dodo has become the very symbol of recent extinctions but many less well-known plant examples also occurred  (or are currently occurring) in the Mascarenes. Emphasis will be placed on Mascarene endemics found on Réunion, including single-island endemics but further data on Mascarene endemics extirpated from Réunion might also be collected on Mauritius and Rodrigues.

An isolated individual of the endangered Mascarene endemic tree Zanthoxylum heterophyllum found in a dense monospecific thicket of Leucaena leucocephala. The tree died between 2013 and 2023. ©Arnaud RHUMEUR (2013) and Robin POUTEAU (2023).