When diversity loss leads to a critical transition

Background/Question/Methods

Research has widely revealed that diverse systems are less prone to invasions as well as more constant and reliable in terms of general properties – for instance total biomass, overall structure and functioning – than species-poor systems. However, diverse systems have been known to suddenly collapse, as did Lake Victoria’s native system of cichlids or coral reefs in the Caribbean. Sudden shifts in systems, or critical transitions, are known to occur in the presence of a strong positive feedback. We here investigate, using a simple model, the role of diversity in critical transitions. We follow the idea that diversity might reinforce a positive feedback, thus possibly yielding critical transitions and even alternative states. We draw on the example of Lake Victoria, where it is hypothesised that the native diverse community of cichlids initially prevented the introduced Nile perch from invading by predating on Nile perch eggs. Following diversity loss, this feedback on the introduced predator would have failed, allowing it to invade.

Results/Conclusions

Our model reveals that at high diversity, the predator cannot invade. However, as diversity declines, the system reaches a threshold beyond which remaining diversity suddenly collapses and the invader dominates. Our model yields alternative states, where there is a range of conditions over which the system can exist in either the native or in the invaded state, depending on initial conditions. Higher diversity leads to higher resilience, defined as the diversity decline necessary to tip the system to its alternative state. In our model, resilience conferred by increased productivity of diverse systems and resilience conferred by the higher functional redundancy of diverse systems shape system dynamics differently. While increased productivity allows the system to resist invasion, it also gives rise to a larger range of conditions for which there are alternative states. Increased functional redundancy however cannot give rise to invasion resistance, but in a system that is resistant, it makes each species loss less detrimental. We conclude that diversity loss may uncover strong processes previously masked by the higher resilience of diverse states. Species losses from diverse communities may often be both very sudden and irreversible, especially when these communities exert a feedback on their environment.