Rare — and doing just fine

We often think species that exist only in small numbers are also in danger of going extinct. But it seems being rare is not always a bad thing.

There are fewer than 50 Maui's dolphins left in the world. Their relatives, the vaquitas, are even less "common." Both are on the brink of extinction. But do small numbers automatically mean a species is likely to disappear? Not according to a team of researchers from the University of California, Davis.

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In a perspective paper published in the journal "Ecology Letters," they considered which factors make a species likely to thrive, even if its population is small. They also looked at what factors are likely to contribute to a species' demise.

For starters, it helps if the adult animals or plants are highly mobile. That seems obvious since, if there are few of them and they are spread out, they may have to travel far to find a suitable mate. Alternatively they need to have a good intermediary who can take their gametes (seeds, sperm or eggs) to a mate. It also helps if they can signal and/or attract a mate over a long distance. 

Where fertilization takes place is important as well. If it happens inside or near an adult, that's a plus. To us and most other mammals out there that would seem obvious. But many species simply release their gametes into the air or water in the hope they will either find a mate or even just combine with their drifting counterparts.

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A famous and spectacular example of this is the broadcast spawning of many types of coral, which can turn otherwise clear stretches of ocean into a cloudy soup of gametes. Such a strategy works well if there are many others like you nearby but not if you are few and far between.  

Speaking of sperm and eggs, for a real edge in terms of survival, it helps if you're not too set on being male or female but open to changing your sex if the occasion calls for it. Many types of snail and worm fit the bill, as do some fish, among other animals. The advantage is obvious. If there are very few of you around and you're all male and have to stay that way, you're not going to have any offspring anytime soon.

When does rare mean thriving?

Some species are very successful at being rare. They can survive in small numbers, scattered over large distances. Sadly, that doesn't hold true for the Maui's dolphins and vaquitas.

Both species live in a single small habitat — the west coast of New Zealand's North Island and the northern part of the Gulf of California, respectively.

Furthermore, both didn't initially exist in such small numbers but human activity — specifically fishing — has pushed them to the brink of extinction.

"By learning how a species can be rare, we can also learn how to protect species that cannot be rare," said UC Davis professor Geerat Vermeij.

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The paleontologist is one of the paper's authors. Since rare species are also often not as extensively investigated as common ones, however, the authors say more research is needed to test their theories.

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