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tomtits and robins

Tomtits and robins were the focus of the first question in the 2016 Schol Bio paper. Specifically, Chatham Island tomtits and robins, which are found only on the Chathams. While at one point they were common and widespread on the islands, the tomtit is classified as nationally endangered, while the black robin is nationally critical & at very high risk of extinction.

The question paper provides two pages of resource materials (maps, photos, and text), and asks students to

Analyse the information provided in the resource materialA and integrate it with your biological knowledge to discuss

  • the reasons why the black robin has a higher risk of extinction than the Chatham Island tomtitB
  • the impact of human intervention on the survival AND evolution of the black robin population.

Compared to the more generalist tomtits, black robins are quite specialised in their habitat and diet. The robins prefer mature forest, with a closed canopy and open understory, while tomits live in mature forest but also venture into shrublands and tussock. This means that the tomtits' options in terms of food and nest sites are less limited. The fact that the robins' poweres of dispersal are limited, so they can't move to suitable habitat on other islands, doesn't help.

One of the fun things about encountering robins in bush on the mainland is that they are ground feeders - you can scuff up some of the leaf litter, step back, and watch them come down to peck through it in search of the invertebrates that they prey on. Tomtits feed at multiple levels in the forest, taking fruit & leaves as well as invertebrate animals. Their more specialised diet means the robins are more at risk following loss of habitat - or a dry year that makes their prey harder to come by. Their ground-feeding habit also means that they're more exposed to predation, something that is also the case for their nesting habits: robins prefer cavities in trees, while tomtit nests are generally quite well concealed (not that this would stop a hungry rat, possum, or mustelid from seeking them out). 

It also takes longer for black robins to replace any losses to predation, let alone grow their population. This is because, compared to tomtits, they have a lower reproductive rate: normally the robins produce one (sometimes two) clutches of 1-4 eggs a year, while tomtits may rear up to three lots of offspring a year, with 3-4 eggs per clutch. (The fact that robins can produce that extra clutch, if the first doesn't survive, was crucial to the efforts to save them when their effective population size was down to a single breeding pair.) The result is that the robins are at greater risk of extinction. 

The fact that the robin population got so low (down to 5 birds in total, with that single breeding pair) means that they went through a severe bottleneck event. As a result of this, and of the subsequent unavoidable inbreeding, there is very little genetic diversity in their population, even though there are now around 250 birds on two islands. This means that the population may not have sufficient variation to allow at least some individuals to survive any significant environmental change. The discovery of birds with deformed beaks, poor bone development, or a distinct lack of feathers has been attributed to that high level of inbreeding.

As the resource information (& a couple of the links above) makes clear, human intervention was the only thing that brought the robins back from the brink and ensured their survival to date. Thus, inducing double clutching, by taking the first clutch and placing the eggs with surrogate parents (first warblers & then, when that wasn't successful because the warblers couldn't provide the right food, tomtits) saw a marked increase in population size. (However, this did come with the risk that the robin chicks would imprint on the wrong parents, something that did actually happen.) Translocating the robins to other islands not only provided suitable habitat and food for the growing population, it also meant that their eggs weren't 'all in one basket': if a predator or disease knocked out the birds on one island, the other could still survive. 

However, conservation workers were pretty much developing their techniques with the robins as they went along, and their interventions did have an impact on the birds' gene pool. I've already mentioned the impact of inbreeding, which can result in increased odds of harmful alleles being expressed. Back in 1984, when someone noticed that a robin had laid her egg on the rim of the nest rather than in the bowl, nudging the egg back into the nest seemed the right thing to do. Unfortunately, by 1989 over 50% of the females were laying rim eggs - the DoC team had inadvertently selected for a dominant, harmful, allele (you can read the original paper here). That is, human action had countered natural selection: normally the egg would have fallen from the nest, or at the least would not have been incubated. Once researchers identified the problem, egg nudging stopped, with the result that natural selection kicked in and the frequency of the allele dropped markedly: now only 9% of females lay eggs on the nest's rim. 

Translocation and fostering could also affect the population's gene pool, and thus its evolution. If there are different selection pressures on the different islands, this could change allele frequencies in the gene pool. And, as previously mentioned, using another species as surrogate parents - while essential at the time - can lead to robins imprinting on the wrong parents and hybridising with them, something that's been confirmed by analysis of microsatellite DNA.

But if it weren't for the dedication and hard work of scientists and conservation workers, the black robin (and many other NZ species) would already have gone the way of the dodo.

 

A I wrote about this in my previous post

B Remember, this question asked students to compare the two species. So a good answer would make that comparison explicit; you shouldn't focus on the robin alone.

 

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