13 February 2015

Their weevil ways: too specialised to survive on the Chathams?


Change has been much on my mind recently. With one son moving into Christchurch to continue with university, another moving to Dunedin to start university and only the third at home for a couple more years (he said hopefully), I have been asked repeatedly about what I will do in the medium term. Down size the house? Maybe look at positions elsewhere? Make a lifestyle change? I'm not sure what motivates these comments as I am happy in my nice, large, old house (it would be great to actually be able to use some of the rooms myself, finally). I enjoy living at Lincoln and working at the University. And I like being an evolutionary biologist. However, on Darwin Day 2015, let me consider my options. If I was to look for another evolutionary biologist position (especially one that focused on biogeography or coevolution) my options would be fairly limited. Within New Zealand there are about seven other similar positions at universities, maybe about the same again in Crown Research Institutes.
Rangitira
In the 20 years that I have been at Lincoln I can remember evolutionary biologist jobs coming up about three times. So it would be very difficult to move within New Zealand. Should I want to go overseas then there are definitely more jobs, but not a whole lot more (and there is a lot of competition!). So I am reasonably tied to Lincoln. Why? Because I am so specialised. To do my job I need a research environment, access to postgraduate students, funding, an organisation that values evolution research and so on. Specialisation, while allowing me to be successful in what I do, has effectively limited my dispersal ability (providing I want to stay being an active evolutionary biologist).

Similar thoughts abound in biology about specialisation. The idea is that specialisation allows a species to do really well as it efficiently utilises a niche but, come a crisis like climate change or a new predator, specialisation will hinder fast adaptation for change for the species and generalists will win out in these situations. So specialists are successful during stable periods but generalists do better in unstable periods, or so theory suggests. It is also thought that species can become so specialised that there is no way back to being a generalist. This has been summarised as Dollo's Law which states that evolution is not reversible. While we know of many examples where evolution does reverse (descendent species becoming more like distant ancestors rather than recent ancestors) specialisation does seem to make such reversals more difficult. For example, parasitism is often portrayed as a dead-end in evolution. Many parasites lose all sorts of traits, such as limbs, complex digestive systems, sense organs and so on, mostly because they live in their food. Once you have lost or substantially changed these traits then it is incredibly difficult to return to a nonparasitic way of life (but not impossible as Rob Cruickshank and I showed for mites a few years ago).
Two coxella weevils getting acquainted. 

For most species this issue, while interesting to evolutionary biologists, is not a real day-to-day concern. However, for threatened species this problem of specialisation can influence the likely fate of the species. If a species is too specialised then it may be too difficult to find a solution that will allow the species to be conserved. If you need to live in a certain type of habitat, say peat wetland, and this habitat is limited or disappearing, then you have a problem. If you need to eat a certain type of food, say a particular type of honey dew, and the insect that produces the dew is limited or disappearing, then you have a problem. Emily Fountain, with Jagoba Malumbres-Olarte, Rob Cruickshank and myself, have investigated this problem in a threatened weevil species, the coxella weevil (Hadramphus spinipennis). This weevil species is found in the Chatham Island group, an isolated archipelago 800km east of mainland New Zealand. The coxella weevil is now found on two small islands, Mangere and Rangatira, which are 15 kms apart. The weevil lives on, and eats, one plant species, Dieffenbach’s speargrass (Aciphylla dieffenbachii). The coxella weevil was once found over the whole Chatham archipelago but now persists on these two small islands in small clumps of speargrass. The speargrass is extremely patchy and this is not helped by the weevils who will graze out patches of a plant before moving to a new patch. Both islands were once over-run with livestock (which found the speargrass tasty) and since their removal there has been much planting of native forest trees and natural regeneration, which removes suitable habitat for the speargrass to grow in. So the coxella fate seems tied to the plant that they have specialised on which must make them susceptable to extinction.
Rangitira with Pitt Island in the distance

Emily and Jagoba surveyed the populations of coxella weevil and speargrass on Mangere and Rangitira. She collected a tarsus off the end of one leg from each weevil that she found and was able to extract DNA from these individuals. Emily, in a paper published in PeerJ, found that there was a small difference between the weevil populations on each island, what you might expect if they had been isolated for a relatively short amount of time (around 1000 years). Despite the differences, this indicates that even these specialised weevils can disperse over water, at least occasionally. There was a lack of genetic diversity which suggests that the weevil populations have been through a bottleneck. There was even some evidence that the coxella weevil is sometimes found on other plant species, although whether they are foraging or not could not be determined. On the upside, populations of weevils and speargrass have not declined as a result of the reforestation efforts at this point. So is specialisation a problem for the survival of the coxella weevil? It's fair to say that specialisation on speargrass will not help with conservation efforts, but so far it does not appear to have consigned this species to extinction just yet. After all, it's still easier for a coxella weevil to find a new speargrass plant than it is for an evolutionary biologist to find an evolutionary biology job!

1 comment:

Mark Anderson said...

Interesting post! I helped monitor these weevils a couple of years ago and saw 3 or 4 on a lancewood tree in the middle of Robin bush - 100s of metres from a speagrass plant - I have a hunch that they must have frequently undergone bottlenecks in historical times with their specialisation, but they must be able somehow to survive - many of these large weevils with their long life spans live on tiny bits of rock with a few plants on!