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Ecosystems work with each other no matter how far or disconnected they are. Animals and birds have impacts on each other’s lives and deaths. According to a new study, invasive rats on tropical islands are creating winners and losers among tiny underwater animals near the bottom of the food chain that inhabit surrounding coral reefs. Now, scientists are saying the resultant changes can rewire food chains on the reefs.Tiny fish and invertebrates that live at the bottom of the sea on coral reefs are known collectively as ‘cryptofauna’. These creatures are often overlooked and little studied, but they play a vital ecological role and represent a cornerstone of energy transfer on coral reefs.Led by researchers at Lancaster University and the University of Texas, a team of marine scientists studying these tiny critters on reefs around remote islands in the Indian Ocean have discovered that their populations are much different around islands infested by invasive rats compared to reefs around islands without rats.
Seabirds, rats and small invertebrates
Invasive rats, which arrived as stowaways on ships hundreds of years ago, have decimated the seabird populations on the islands, eating their eggs, chicks and at times even adult seabirds.
Reefs surrounding rat-free islands are dominated by tiny ‘cryptobenthic’ fish species, such as gobies and triplefins, whose biomass is more than five times greater than that of small invertebrates. But around rat-infested islands, the population of small invertebrates such as coral crabs, porcelain crabs and snapping shrimps is higher. Moreover, the biomass of the cryptobenthic fish and invertebrates is nearly equal.The secret to how rats are able to impact these marine creatures lies in how they are affecting the seabird populations on the islands they have infested. Invasive rats, which arrived as stowaways on ships hundreds of years ago, have decimated the seabird populations on the islands, eating their eggs, chicks and at times even adult seabirds. On the islands without rats, the populations of seabirds are a whopping 760 times greater.Seabirds have a vital role in recirculating nutrients from the open ocean where they feed, to the islands where they roost, breed and nest. Their droppings, known as guano, are rich in nutrients that, when washed off the islands into the sea, act as a natural fertiliser for the surrounding coral reef ecosystem.The study area in the Chagos Archipelago in the remote Indian Ocean provides a living laboratory to compare marine ecosystems around islands with and without rats. Researchers from Lancaster University have conducted years of studies looking into the effects of invasive rats on marine ecosystems, from how changes in nutrient flows affect the growth of corals to the territorial behaviour of fish species.
Cryptobenthic fish versus invertebrates
This latest study, published in Ecology, is one of the first to reveal the changes to benthic cryptofauna and the species that prey on them.
This latest study, published by Ecology, is one of the first to reveal the changes to benthic cryptofauna and the species that prey on them, offering surprising new insights for the study team.“Contrary to our expectations, the low-nutrient conditions that rats create do not negatively affect all reef organisms,” said Laura-Li Jeannot, lead author of the study and PhD researcher at Lancaster University. “Nutrient-loss impacts are not uniform. There are winners and losers.“The discovery of the role of cryptic invertebrates is interesting, as really very little is known about these critters, even less so than for cryptobenthic fishes. How they respond to disturbances or transfer energy through systems was almost entirely undocumented,” she said.The researchers believe that fish have a number of competitive advantages when the seabird nutrient flow remains intact. They have extraordinarily high rates of growth and reproduction. “As a result, they have high nutrient and energy requirements, and those can be matched in nutrient-rich environments. This allows these fish populations to flourish and expand in favourable conditions – such as seabird-fertilised reefs,” said Jeannot. With expanding populations, they colonise habitats that would be otherwise occupied by cryptic invertebrates. “Beyond chasing invertebrates away from their homes, they also outcompete them for food: our results show that cryptobenthic fish, when more abundant, likely drive invertebrates to seek alternate, less preferred resources,” she added.On the flip side, the researchers believe that the main reason invertebrates have relative success near rat-infested islands is due to the loss of competitive pressure from cryptobenthic fishes, which are much less abundant in nutrient-poor reefs. Invertebrates’ lower metabolic rates could also be an advantage in lower nutrient environments as they don’t require as many nutrients to sustain a certain level of biomass.
A changing food web
The researchers found that mixed carnivorous fish rely more on cryptobenthic fish in seabird environments, and that the productivity of fish-eating predators soars while invertivore productivity stagnates near seabirds.
Given the position of cryptobenthic fish and invertebrates near the bottom of the coral reef food chain, researchers expect big impacts on the rest of the food web. “Cryptofauna are the main prey source on reefs, and will often represent the first step in transferring nutrients from primary producers to larger predatory fishes. Any change that affects them is likely to have cascading effects on their predators,” said Laura-Li Jeannot.While much less is known about the role of cryptic invertebrates in wider reef productivity, cryptobenthic fish have been estimated to contribute up to 60% of total consumed biomass on coral reefs, so they are highly important for nutrient and energy cycling on reefs. The researchers found that mixed carnivorous fish rely more on cryptobenthic fish in seabird environments, and that the productivity of fish-eating predators soars while invertivore productivity stagnates near seabirds.“Cryptobenthic fish are a nutrient-dense, protein-packed, highly digestible resource for predators; on the other hand, much of invertebrates’ mass is represented by a tough outer shell. In nutrient-rich environments, fish are also way more abundant than invertebrates, meaning they likely require less foraging. As a result, they represent a more energetically optimal prey near seabird islands for predators looking for a quick snack,” said Assistant Professor Dr Simon Brandl from the University of Texas and co-principal investigator of the study.“These findings suggest a shift in food chains from fish to invertebrate-mediated pathways near seabird-poor islands. Seabird nutrient flows do not just enhance reef productivity and health; they are also influencing trophic structure. Therefore, invasive species like rats that cut off these nutrients can drive significant changes to the food webs of the reefs,” added Professor Nick Graham of Lancaster University and co-principal investigator of the study.These results point out important conservation implications: safeguarding seabirds and their nutrient subsidies is also a way to protect the integrity of reef food webs.
