However, contrary to expectation, the researchers have found that the \u2018low-nutrient conditions\u2019 that rats create do not negatively affect all reef organisms.<\/p>\n
Laura-Li Jeannot, lead author of the study and PhD researcher at Lancaster University, said: \u201cNutrient-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.\u201d<\/p>\n
The researchers believe that cryptobenthic fish thrive in nutrient-rich, seabird-fertilised environments because their biology demands it.\u00a0<\/p>\n
\u201cCryptobenthic reef fishes are characterised by extraordinarily high rates of growth and reproduction,\u201d said Jeannot. \u201cAs 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 \u2013 such as seabird-fertilised reefs.\u201d\u00a0<\/p>\n
That expansion, the team found, actively displaces invertebrates. \u201cBeyond 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.\u201d<\/p>\n
Invertebrates, conversely, benefit from the absence of that competitive pressure in nutrient-poor reef environments. Their lower metabolic rates give them an advantage when nutrients are scarce as they simply require less to sustain themselves.<\/p>\n
The consequences for the wider food web are significant. Assistant Professor Dr Simon Brandl, from the University of Texas and co-principal investigator of the study, explained why the shift in cryptofaunal communities matters so much for the predators above them.\u00a0<\/p>\n
\u201cCryptobenthic fish are a nutrient-dense, protein-packed, highly digestible resource for predators; on the other hand, much of invertebrates\u2019 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.\u201d<\/p>\n
The study\u2019s result show that the main mode of seabird nutrients moving up the food web is through fish, which feed both specialised fish-eating predators as well as more generalise carnivores.\u00a0<\/p>\n
\u201cAs such, when seabird nutrients are added or taken away from ecosystems, there is a rewiring of how energy travels up food webs, and a reconfiguration of larger fish communities,\u201d said Jeannot.<\/p>\n
Professor Nick Graham of Lancaster University, co-principal investigator of the study, said the findings suggest that seabird nutrient flows do not just enhance reef productivity and health but influence the trophic structure, too.\u00a0<\/p>\n
\u201cTherefore, invasive species like rats that cut off these nutrients can drive significant changes to the food webs of the reefs,\u201d he concluded.<\/p>\n
The study was conducted in the Chagos Archipelago in the remote Indian Ocean \u2013 a site that has become a living laboratory for understanding the relationship between island ecosystems and surrounding coral reefs, and where Lancaster University researchers have spent years investigating the cascading effects of invasive rats on marine environments.<\/p>\n