How rat density alters coral reef productivity

In tropical areas, the invasive rats decimate seabird populations. These latter play a key role in the functioning of terrestrial and aquatic ecosystems, such as coral reefs.

Climate change and other human disturbances have important consequences for the structure and functioning of our ecosystems. Profound ecological changes in terrestrial environments – intertidal zones such as estuaries, mangroves or rivers – may ultimately affect and threaten the biodiversity and resilience of other habitats, including marine ecosystems. That said, many animals such as salmon, turtles or hippos play a crucial role as “trophic links” between ecosystems, transferring large amounts of nutrients and organic matter between them, and influencing their productivity and functioning. However, the implication of these natural dynamics is poorly understood, especially in environments where humans have disrupted the connectivity between ecosystems for example by reducing the populations of certain species, or introducing new predators. For instance, seabird species, such as terns, frigatebirds, shearwaters, and boobies, feeding in the open ocean, play a key role in the functioning of certain biotopes. By transferring significant amounts of nutrients (mainly nitrogen and phosphorus) to island environments, through defecation, they enhance the productivity of flora and fauna. In tropical areas, the introduction of rats by human lead to a drastic decrease in seabird populations, ultimately altering productivity and biodiversity in terrestrial and aquatic ecosystems, such as coral reefs.

Invasive rats decrease seabird density, ultimately affecting reef productivity and functions

An international team of researchers studied a “natural experiment” in the Chagos Archipelago in the Indian Ocean, where some islands are infested by black rats (Rattus rattus) and some remain rat-free. The article published in the journal Nature in July 2018 shows that the density of seabirds – and the rate at which they deposited nitrogen – were much higher (760 and 251 times, respectively) on islands without rats, compared to those where the invasive rats were present (i.e.: high predation rates of rats on seabirds). These results were reflected in higher values of nitrogen enrichment in soils and shrubs as well as in algae, sponges and in certain fish species surrounding or present on rat-free islands. In addition, seabirds-derived nutrients might also favor coral reef productivity, increasing growth rate and biomass of herbivorous damselfish (Pomacentridae), and enhancing ecosystem key functions such as bioerosion and predation in reef areas surrounding rat-free islands.

Implications for coral reef resilience to global change

These surprising findings reveal how interconnected different ecosystems can be, and how invasive species (in this case a predator) can influence the functioning and productivity of island and marine ecosystems. The disruption of these linkages can have serious consequences. In 2016, the Chagos Archipelago lost 75% of its coral cover following the El Niño-driven mass coral-bleaching event. The authors of the study hypothesized that coral reefs adjacent to rat-free islands may be more resilient to high temperatures likely because of the large amount of phosphorus that seabirds release, a critical nutrient that would favor the resistance of reef-building corals to thermal stress.

In a time of unprecedented threats to coral reefs, the eradication of rats in tropical areas should be one of the top priorities of conservation programs to offer reef ecosystems an additional chance to further resist and recover from human threats.