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closeDate of human arrival in Hawaii
Posted by Rosalind on 20 May 2011 at 17:33 GMT
I looked in vain for a clear presentation of the extinction dates of Geograpsus directly compared with archaeological radiocarbon dates that unambiguously indicate human arrival in the Hawaiian Islands coincided with the extinction of this crab. The paper also is missing a likely human behavioral mechanism that would account for the proposed extinction occurring immediately upon human arrival. For example, it is hard to imagine how a small human founding population, living only on the coasts of the islands, whereas the crabs apparently preferred inland habitat, could have focused on this animal to the point of extirpating it forever. Rattus exulans is a highly unlikely predator, and pigs were likely to have been managed locally, not roaming the inland areas where early settlers were absent.
I suggest the authors re-think their attribution of human causation in this case, and to also familiarize themselves with recent archaeological findings, which show that human arrival in Hawaii was no earlier than c. 750 years ago. Therefore to improve this paper, the relatively late dates for human arrival in Hawaii need to be directly compared with the established date for Geograpsus extinction, and the precise method for how this extinction date was determined also needs to be made explicit. Without these considerations, the paper's proposal that human activity caused Geograpsus to go extinct is in serious doubt.
RE: Date of human arrival in Hawaii
paulay replied to Rosalind on 21 May 2011 at 12:13 GMT
The relevant question for understanding the potential influence of humans in the extinction of G. severnsi is not the absolute dates of crab extinction and human arrival, but the relative dates of the two. From stratigraphic context it is clear that these crabs survived into the Polynesian, but not European era, as described in our paper:
“Abundant Geograpsus fossils have been taken from two excavations with well-dated stratigraphic context, which were studied to document changes in the Hawaiian biota and landscape following human colonization: the Pu'u Naio lava tube on Maui [69] and the Māhā'ulepū Caves and Sinkhole on Kaua'i [40]. Deposits at Pu'u Naio extend back to >7,750+/−500 BP across four major stratigraphic units. Unit I is youngest, corresponding roughly to historic times, Unit II dates from the Polynesian expansion, with humans arriving late in Unit III. Most of Units III and IV predate human colonization. The majority of the crab remains we studied are from Unit III, with a few from Unit IV. Cory Pittman (in litt. 11.IV.2004) quantified the abundance of crab fragments at Pu'u Naio and found the last fragments in the basal-most layer of Unit II. Thus at this site G. severnsi survived to, but disappeared rapidly after, human arrival. Burney et al. [40], [41] came to a similar conclusion for Māhā'ulepū, where crabs are present in all pre-human layers, show a decrease in size during the early human period, and are absent from more recent strata.”
As to the exact mechanism of extinction, this remains speculative. We know that land crabs do not do well on islands where mammals are present (see paper). Geograpsus undertake mass migrations to the sea for larval release, traversing the coastal belt, and are highly vulnerable at these times. Pacific cultures are very much aware of the timing of these migrations and take advantage of them to capture crabs for consumption. Introduced rodents have been implicated in numerous animal extinctions on islands. St. Clair (2011) (in a paper that was brought to my attention post publication) provides an overview of the impact of rats (including R. exulans) on the decline and extinction of island invertebrates. He concludes that large bodied invertebrates are among the most vulnerable. As we note in the paper, the eminent crab biologist, Peter Ng said: “I believe in many cases, it is the young which are very vulnerable to things like rats, civets, squirrels, monitors etc. and the pressure of these predators on the juveniles forces them to small islands where such predators are absent.” Thus while knowing what exactly finished off G. severnsi must remain speculative, correlation of its extinction with human arrival, and vulnerability of land crabs to humans and other mammals strongly support the hypothesis that there is a causal link between the two.
St Clair, JHJ. 2011. The impacts of invasive rodents on island invertebrates. Biological Conservation 144:68-81.