The extinction risks for many species have been wildly underestimated in a most embarrassing fashion. In this week’s Nature, researchers realized that current extinction risk estimates have failed to account for gender ratio and behavior. In other words, the models assumed that all individuals reproduced in the exact same way, and that’s not true. For example, lots of mammals (gorillas, elephant seals) have a harem social structure in which the number of females determines the reproduction rate, but the total number of males is irrelevant, since only the dominant male gets to reproduce. In species that broadcast spawn, like fish or invertebrates, bigger individuals produce exponentially more (and higher-quality) eggs or sperm than smaller individuals.
I’m not a modeler, so I’m utterly gobsmacked that this problem a) exists at all and b) has just been discovered. Because scientists KNOW that sex ratios and body size play a huge role in reproductive success. I can think of a million examples off the top of my head. For example, many fish are sequential hermaphrodites, changing from female to male (or vice-versa) when they reach a certain body size. So by catching the biggest fish, people remove all the males and the population plummets. Or take the case of the Maine lobster – it’s not the teeny pound-and-a-quarter lobsters keeping the population going, it’s the big monsters that live deep off the continental shelf, so fishing them out is a huge problem. (That’s why Maine has a maximum size limit.) The death of a big male fish or big offshore lobster therefore has much bigger ramifications for the population than the death of a small female fish or wee little lobster.
The authors write:
When we apply our new mathematical model to species extinction rates, it shows that things are worse than we thought,” said Melbourne. “By accounting for random differences between individuals, extinction rates for endangered species can be orders of magnitude higher than conservation biologists have believed.
We’d best fix that, then.
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Uhm, I’m quite amazed as well that current models don’t take this in account.
One would’ve thought the story of, for example, the Passenger Pigeon (colonial bird practising communal roosting and communal breeding, could not survive in flocks of only a couple of thousand) would have it made clear behaviour should be taken in account as a key variable.
Yay Brett! (He was a postdoc in a theoretical lab I used to mooch off of) – and it’s a neat paper. No surprise given Hastings’s involvement in fisheries modeling.
Also, be sure to check out this writeup from Grrrlscientist
Clearly the hermaphroditic species are going extinct because they have invoked the wrath of God with their ambiguous gender identities.
Pretty stunning. Another example off the top of my head that has lead to large scale changes in management of Rockfish species off the west coast is the BOFFF (Big Old Fat Fecund Female Fish) hypothesis. In short, large old females produce significantly more (and often stronger) offspring per year than younger females and can have a disproportionate influence on the structure of the populations. This is also influencing approaches to marine protected areas. Example:
Berkeley, S.A., Chapman, C., and Sogard, S. 2004. “Maternal age as a determinant
of larval growth and survival in a marine fish, Sebastes melanops”. Ecology
85(5):1258-1264. http://californiafish.org/maternal_age.pdf