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In a word, yes. Yes it does matter if just one species goes extinct because animals and plants depend upon each other, so the loss of one species affects others within that complex web of relationships
Correction: This article has been updated to reflect proper sourcing.
Islands are special places because they are occupied by many unique animals and plants that are found nowhere else in the world. Tragically, islands are fragile ecosystems that have elevated extinction rates compared to mainlands. This is due to their small size, isolation, and their limited numbers of species. The loss of any of these species destroys the special, often unique, relationships, that evolved on islands, particularly the interdependencies between plants and fruit-eating animals.
Interactions between plants and frugivores are important for maintaining the ability of island ecosystems to respond to threats posed by climate change and human disturbances because frugivores disperse seeds, thereby allowing plants to colonize distant areas. This also serves to redistribute their genes. Most vertebrates living in the tropics and subtropics include fleshy fruits in their diets and up to 90% of all island shrubs and trees depend upon animals to disperse their seeds (i.e.; ref).
Specialized frugivores are strictly tropical or subtropical in distribution (ref). Some groups of birds, such as fruit doves and fruit bats, are critically important for long-distance dispersal of plants from one island to the next, making them important vehicles for colonization of oceanic islands (ref). Meanwhile, large (often flightless) birds, as well as lizards and tortoises are important for moving seeds around the island itself.
But unfortunately for many island species, the first arrival of humans marked the beginning of their descent into extinction. Upon encountering fearless, flightless birds and other strange animals, humans predictably launched a campaign of wholesale slaughter, habitat destruction and introduction of invasive species, particularly rats, cats, pigs and goats, many of which remained on the islands after the explorers departed, and continued the destruction by preying upon ground-nesting and flightless birds and their eggs, and other vulnerable animals.
What are the consequences of island extinctions?
To answer this question, Julia Heinen, a PhD candidate at the Danish Natural History Museum who studies how extinctions influence species interactions within island communities, and her international team of collaborators, focused on fruit-eating island animals.
“We wanted to get the full picture,” Ms. Heinen told ScienceNordic, “so we looked at all of the birds, mammals, and reptiles that eat fruit. We also included animals that have recently become extinct.”
They collected reports of species occurrences of extant and extinct frugivorous birds, mammals and reptiles on 74 tropical and subtropical oceanic islands within 20 archipelagos worldwide (Figure 1).
Their study revealed that almost half of the 74 study islands suffered losses of frugivorous species, and on average, roughly one-third of the original community on these islands disappeared. Further, Ms. Heinen and her collaborators found that animals are not the only living beings affected by extinctions: when one or more species of frugivorous island animals goes extinct, plants are greatly affected too.
“This is because many birds, mammals, and reptiles perform a vital service to the plants by eating their fruits, which contain seeds,” Ms. Heinen told ScienceNordic. “After a while, these seeds will come out again and land somewhere else. This is how many plants move between different areas and make sure their little seeds can grow up in a good spot.”
When frugivorous animals go extinct, plants -- particularly trees -- are deprived of an indispensable mechanism for relocating their seeds to new areas, so they, too, face extinction.
Do some island species have a greater extinction risk than others?
Ms. Heinen and her collaborators wanted to identify whether particular animals are more likely to go extinct, so they compared characteristics possessed by extinct and extant island species to assess what role, if any, they might play in extinctions.
“We saw that large animals that cannot fly go extinct more often than any other,” Ms. Heinen reported. The dodo, Raphus cucullatus, which was a giant frugivorous pigeon that lived on Mauritius, is one well-known example.
But many islands lost large proportions of their original frugivore communities, and some islands, such as many of the Hawaiian Islands, Cook Islands, Tonga Islands, Mascarenes and Seychelles, lost more than 50% of their original frugivore richness. Islands in the Pacific region were especially hard hit: they experienced some of the largest extinction events that occurred during the current geological age. For example, the islands of Oceania lost more than 2,000 bird species after humans arrived (ref).
However, Ms. Heinen and her colleagues found a high proportion of extinctions occurred in other parts of the world, too. For instance, in the Mascarene Archipelago, at least 30 frugivores became extinct (18 birds, 2 mammals, 10 reptiles), including the dodo, the Mauritian giant skink, Leiolopisma mauritiana, and an entire genus of giant tortoises, Cylindraspis spp.
“[W]e saw a knock-on effect of such extinctions,” Ms. Heinen told ScienceNordic. “In fact, the mean weight of all fruit-eating animals on islands has reduced by 37 per cent due to the loss of large animals, such as a giant bird on New Caledonia, several large flying foxes and some of the Galapagos giant tortoises.”
In fact, many of the islands that Ms. Heinen and her collaborators studied had lost their largest frugivore, and some islands lost their second largest frugivore as well, so that only the smallest remained.
“Our data show that the largest animals that can be found on islands today are 51 per cent smaller than the largest animals that used to live there,” Ms. Heinen said.
The loss of so many large fruit-eating animals is a big problem for most island plants because large fruit-eaters have larger beaks and mouths and, thus, can swallow large fruits. Smaller frugivorous animals that still live on these islands simply cannot swallow and disperse large fruits. Thus, many trees that produce large fruits are not redistributed to a suitable location to grow, and as a result, these oceanic islands may end up losing all their trees.
Which types of islands are at greatest risk for extinctions?
To identify which sorts of islands are at greatest risk for extinctions, Ms. Heinen and her collaborators once again compared distinct characteristics. They found that islands that are most likely to suffer disproportionately large extinctions are small, distant from the mainland, and are elevated. Small islands have fewer species of plants and animals to begin with and, because they have fewer resources, humans have a huge impact over the entire island through hunting, habitat destruction and introduction of invasive species. Greater distance from the mainland causes exaggerated evolutionary adaptations in island species, such as flightlessness and gigantism, which increase vulnerability to extinction. The effect of island elevation surprised the researchers, who proposed this might restrict species to small areas of suitable habitat along the elevational gradient, which makes them more vulnerable to extinction when this limited area is lost.
This study provides new insights so hopefully we will learn from our past mistakes, and avoid causing yet more island extinctions in the future.
“We now know what animals and islands are most at risk: large animals that cannot fly, such as the Solomon [Islands] flightless rails and megapode birds, on islands that are small and remote,” Ms. Heinen told ScienceNordic. “And we now need to use this information to help prevent extinctions in the future.”
Already there are a few examples of oceanic islands that have been successfully “rewilded” after the introduction of species that can function as replacements or “ecological analogues” for extinct native species (more here). For example, conservation biologists are starting to replace extinct giant tortoises on the Galapagos, the Mascarenes and the Seychelles with large fruit-eating tortoises (ref).
Granted, rewilding is a controversial conservation method, but it is gaining momentum because this may be the only way we can conserve so many species that are threatened with extinction.
Sources:
Julia H. Heinen, E. Emiel van Loon, Dennis M. Hansen and W. Daniel Kissling (2018). Extinction-driven changes in frugivore communities on oceanic islands, Ecography, 41:1245–1255 | doi:10.1111/ecog.03462
Does it matter if one species goes extinct? ScienceNordic, 6 October 2018.
Does It Really Matter If Just One Species Goes Extinct? | @GrrlScientist
