Sunday, December 21, 2008

Population Bottleneck


I came across this piece about Cheetahs and had to think of the Sharks.

Dunno if anybody has focused on this yet - but with 90% to 98% of the oceanic Shark populations already gone, we may well be facing the exact same outcome even if we did manage to pull them back from the brink of extinction. Deprived of Genetic Diversity, the population will eventually crash - regardless of numbers and of whether they are still being hunted.

Not that I really had any high hopes anyway - but alas, it just shows how incredibly complex the issues are, and how difficult it is to restore a viable balance once the damage has progressed beyond its tipping point. And let there be no doubt that when it comes to pelagic Apex Predators, it probably has already.

It's a complicated topic but if you're interested, you may want to start with this and then take it from there. There is also this about the (controversial) concept of Minimum Viable Population Size. And if you like formulas, this (sort of) explains Effective Population Size.

It will be interesting to see the results of the ongoing genetic studies that are presently being conducted on various Shark species, among them one about Bull Sharks to which we have and are continuing to contribute.

Luckily, when it comes to coastal Sharks, the damage to stocks looks less irrevocable - so far. Shark Conservation has gained considerable traction and I believe that we really do have a fighting chance - but we need to do something right now.

Fingers crossed that we're not too late.

1 comment:

Jeff - ScienceSays.net said...

Thanks for posting that, I'm really glad you enjoyed it! What a surprise to see one of my own articles pop up on my favorite shark blog!

I'm a huge shark fan and have actually thought a lot about how sharks could be going through a bottleneck.

There's no question that any species that goes through a population reduction like the one we've caused in sharks will suffer from a loss of genetic diversity. Long-term, this will limit the number of healthy individuals that are even possible for the population - longer, longer-term, it will change the rate at which sharks evolve.

However, there are two reasons to hope for sharks:

1) Species are capable of recovering from small populations, and a single individual can increase the diversity of a population exponentially. The trick with population science is that there is a tipping point beyond which a population should never go, and it's a moving target due to random environmental factors. I suspect that many shark species, with the exception of the critically endangered, despite being small fractions of their former population, still have enough individuals left to preserve a large part of their available genetic material. I imagine those species would recover easily in the absence of fishing pressure.

2) Sharks are so old that genetic diversity might not be as critical as an issue as it is for cheetahs. The cheetah emerged about 18 million years ago, while sharks have been around as early as 400 million years ago and most modern species have been predominantly unchanged for 100 million years.

It stands to reason that each individual shark would have more genes in common with another than the average cheetah. I'd have to look into this more, but I imagine that shark genetics are fairly stable, save the odd mutation here and there, and that they would be more resilient as a result.

Thanks again, and I'm glad you liked the post.