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Image source: Rachel Gunn snapper (Pomacentridae) is a happy small fish that lives mainly in the tropics and subtropics. Most of them live in the ocean, and a few live in freshwater or brackish waters. These small fish often haunt undersea coral reefs and feed on small crustaceans, plankton or algae.

Amphiprion ocellaris is probably the most famous species of snapper (photo: Ritiks, Wiki) these small fish are very careful to protect their territory from other fish, some of which even spend most of their lives in their own territory (such as Stegastes adustus). Some snappers also raise algae as food rations in their territories, so they are also known as "undersea farmers".

Some algae are rich in fiber, and snappers lack the enzymes to digest the fiber, so they only eat other algae that taste better, such as Polysiphonia urceolata.

"undersea farmers" will remove the untasty algae and spit them out of the "field", leaving only delicious algae and removing sediments from the rocks, leaving room for algae to grow. These snappers take good care of their "fields" and remove intruding sea urchins or starfish.

Scientists have found that a clever species of snapper (Stegastes diencaeus) even tries to protect the floating prawns in the territory for a mouthful of delicious algae, because their droppings are fertilized by algae to make them grow better. Although snappers eat similar small creatures, they seem to be particularly lenient to these shrimps.

Juvenile long-finned Sparus macrocephalus (photo: Laszlo Ilyes, Flickr) in recent years, some Plectroglyphidodon lacrymatus seem to have changed their habits to encircle their territory larger, but less hostile to outsiders within the territory. What on earth happened to make these little fish with a strong sense of territory make concessions?

The bead fixed-toothed Sparus macrocephalus (photo: Wiki) has been investigated by researchers and found that the key has something to do with mice.

Speaking of rats, we naturally think of the dark, damp, garbage-piled corners of the city, and it is hard to imagine how beautiful coral reefs and cute little fish living here are affected by them.

These strange little fish live near the Chagos Islands in the Indian Ocean, which used to be mouse-free. As early as the 18th century, a group of mice came here with European fleets, and because they had no natural predators and ate everything, they became more and more rampant and bred from generation to generation.

This is a typical biological invasion. Rodent invasions are very difficult to eradicate because they are so strong that they eat recipes ranging from grains to fruits, insects to chickens, and even eggshells and human and animal droppings. Rats are now found on more than 80% of the world's islands.

Under natural circumstances, the island has been isolated from the mainland for a long time and developed a very unique ecosystem. Many native species will be hit hard by the emergence of invasive species, and the dodo will suffer.

Dodo skeleton (left) and model (right) (photo source: Bazza Da Rambler, Oxford University Museum of Natural History) the extinction of the dodo is not only related to human hunting, but also related to biological invasion. As humans set foot on the island of Mauritius, animals such as mice, cats, dogs and monkeys came along, and the living space of the dodo became smaller and smaller, making them disappear completely within 200 years after they were discovered by humans.

Rat Island Island is an uninhabited island in the Aleutian Islands, whose name translates literally as "Mouse Island". In 1780, a Japanese seagoing ship ran aground here, and the rats on board ran to the island and became an invasive species. It was the first time that rats appeared on the island. Later, the raging of rats led to the extinction of many seabirds here, and it was not until people launched a large-scale anti-rodent campaign that some seabirds came here to build nests.

Back in the Chagos Archipelago in the Indian Ocean, some islands have been invaded by rats and some have not. Seabirds, especially those that build nests collectively, are very vulnerable. Their eggs and young birds are the source of food for mice. Dr Rachel Gunn, from the School of Biology at the University of Lanchester in the UK, found that seabirds were 720 times denser than those invaded by rats.

When there are fewer seabirds, there is a lot less bird droppings. Bird droppings are rich in nutrients such as nitrogen, which can promote the growth of corals and some undersea plants as well as marine fish, which is essential for the biodiversity of coral reefs. Around the islands where rats live, the nitrogen in the ocean has been reduced by 251 times, resulting in the loss of most of the nutrients of algae on coral reefs.

A free-flying arctic sea bream (photo: Flickr) in the waters around an island that is not invaded by rats, the "farmer" snapper is still diligently guarding its territory, and they will try their best to drive away fish that dare to invade their territory, because the algae in their "fields" are so nutritious that it is worth the effort to fight.

Around the islands invaded by rats, the snappers are less concerned, and they seem to disdain to spend their energy on protecting the barren "farm". Although they are still taking care of their "fields", removing weeds and finding ways to increase the nutrients of algae, these efforts are obviously nothing compared to the nourishment of bird droppings.

Near an island infested by rats, the aggressiveness of the bird bream has become less aggressive (photo source: reference [4]). The ecosystem is in a delicate balance for a long time, and any one of these factors changes. will cause a series of changes to the whole system. The emergence of rats has reduced the number of seabirds, thus affecting the fish in the ocean, which has changed across ecosystems.

It seems that the most effective way to restore the previous ecosystem is to exterminate rats. Although it is difficult to eradicate rats, there are many successful cases. However, the current method of killing rodents is mainly to throw poison bait on the island from the air. I don't know how harmful it is to other creatures on the island.

(photo source: Flickr) in the long run, it will do a lot of good to kill rats. At least, it can expand the population size of Sparus macrocephalus, enrich the community composition of coral reefs and enhance the resilience of the ecosystem.

Reference:

[1] https://en.wikipedia.org/wiki/Damselfish#cite_note-:0-3

[2] https://reefs.com/scientists-take-a-closer-look-at-farming-behavior-of-damselfish/

[3] https://www.popsci.com/environment/invasive-rats-tropical-reef-fish/

[4] Gunn R L, Benkwitt C E, Graham N A J, et al. Terrestrial invasive species alter marine vertebrate behaviour [J]. Nature Ecology & Evolution, 2023: 1-10.

[5] https://zh.wikipedia.org/wiki/%E6%B8%A1%E6%B8%A1%E9%B8%9F

[6] https://en.wikipedia.org/wiki/Rat_Islands

[7] https://www.nhm.ac.uk/discover/news/2023/january/invasive-rats-affecting-behaviour-algae-farming-fish.html

This article is from the official Wechat account: bring Science Home (ID:steamforkids), author: everything.

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