Bertarelli-Annual-Report-2024-LR - Flipbook - Page 39
Our Impacts in 2023
Marine Science 2023
37
Jewel damselfish, Chagos Archipelago © Rachel Gunn
Terrestrial Invasive Species
alter Marine Vertebrate
Behaviour
Gunn, R., Benkwitt, C., Graham, N., Hartley, I., Algar,
A., Keith, S. (2023) ‘Terrestrial invasive species alter marine
vertebrate behaviour’ - Nature Ecology & Evolution https://
doi.org/10.1038/s41559-022-01931-8
For the first time, the presence of invasive rats on
tropical islands has been shown to affect the territorial
behaviour of fish on surrounding coral reefs.
Nutritional resources are key drivers of territorial
behaviour, with territory size primarily determined by the
nutritional requirements that allow organisms to meet
their short-term energetic needs. Seabirds are globally
important contributors to nutrient transfer, responsible
for a cascade of nutrients through terrestrial and marine
ecosystems by depositing guano on islands after feeding
in the open ocean. Invasive species, including black rats
(Rattus rattus), disrupt this nutrient pathway by driving
declines in seabird densities via predation.
The study used a natural experimental system in the
Chagos Archipelago of five rat-infested and five ratfree islands. The focal species was the jewel damselfish
(Plectroglyphidodon lacrymatus), a herbivorous tropical
reef fish that “farms” algae in the branches of corals and
aggressively defends small solitary territories.
A combination of behavioural observations, territory
mapping, benthic surveys and isotope sampling were used
to link territorial behaviour to nutritional resources.
Around islands with intact seabird populations, the jewel
damselfish defend their patch, typically less than half a
square metre of the reef, to protect their turf algae food
source. The higher enriched nutrient content of the turf
algae makes it worth the energy cost needed to defend.
However, jewel damselfish on reefs adjacent to rat-infested
islands were much more likely to have larger territories,
and five times more likely to behave less aggressively than
those who lived on reefs adjacent to islands without rats.
Larger territories are needed because the algal turf was
less nutrient-rich due to the presence of rats and missing
seabird-derived nutrients, to the extent that it is almost not
worth fighting for.
The algal farming of damselfish affects the balance of
corals and algae on the reef. Their aggression towards
other fish can influence the way those fish move around
and use the reef. While the consequences of these
behavioural changes are currently unknown, ecosystems
evolve a delicate balance over long time-scales and
behavioural changes are often the first response to
environmental changes.
This study adds to the evidence base behind the need to
eradicate invasive rat populations from tropical islands.
The removal of invasive rats could restore the territorial
behaviour of farming damselfish, which could scale up to
benefit coral reef community composition and resilience.
