A report that hermit crabs are being killed by plastic bottles on the Cocos/Keeling islands. The point being that crabs can climb into the plastic bottles but if they’re lying in a certain way then they can’t get out again. Further, the smell of a dead hermit crab encourages others to come have a look see as that’s how they get to trade up their shells as they grow. So, one getting trapped and dying leads to many doing so.
OK, interesting finding. What we want to know though is, well, is it important?
More than half a million hermit crabs have been killed after becoming trapped in plastic debris on two remote island groups, prompting concern that the deaths could be part of a global species decline.
The pioneering study found that 508,000 crabs died on the Cocos (Keeling) Islands archipelago in the Indian Ocean, along with 61,000 on Henderson Island in the South Pacific. Previous studies have found high levels of plastic pollution at both sites.
That thing we want to know being what neither we nor the scientists do know. The paper is here.
The significant entrapment rate has the potential to negatively impact hermit crab populations. While no population size data exist for any hermit crab species on Henderson or Cocos, and estimates of adult or juvenile survival are not available
We don’t know how many crabs there are. We also don’t know what the death rate is either. We have no clue wither this death rate from plastic is higher, or even lower, than the normal death rate for the species in that place.
No, it isn’t as easy as saying look, they’re dead, the plastic must be increasing the number dying. Because there is a limitation on population size here and it’s not the food supply either. It’s the supply of shells that work as hermitages:
Hermit crabs, including Coenobita perlatus, use the odour of dead conspecifics to locate available shells, increasing shell-acquisition behaviour by up to 10 times (Small and Thacker, 1994; Valdes and Laidre, 2019; Gherardi and Tricarico, 2011), which are a limiting resource
It’s that last there. Shells are a or perhaps the limiting resource on population size.
Which does actually give us a useful method of testing whether this is a problem or not. Are there shells which are suitable to be used which are not being used on those beaches? If so then the plastic bottles are reducing the population below the carrying capacity of that environment. If not then the plastic bottles are a different way for that population to be thinned but not actually a contribution to the thinning of the population.
As we all know what is actually going to happen is that we’ll all be told to stop using plastic because crabs. But we’d really like a bit more of that research before we do.
We around here are willing to undertake that all expenses paid trip to a tropical island in the depths of the European winter in order to count unused crab hermitages. Who is willing to pay for it? Greenpeace mebbe? Or perhaps they’d prefer not to have this question answered?