Helena Batalha asks how genetically different the three populations of Cape Verde warbler are, and what this means for the species conservation?
Check out the full paper here.
The three populations of the Cape Verde warbler are genetically distinct, and should be managed differently. Unsurprisingly, the smallest population has the highest conservation priority!
Genetics and conservation
When we think about species conservation, what springs to mind is the creation of suitable habitat (where the birds live) or actions to reduce predator numbers. However, it is also important to consider an evolutionary perspective, and assess the genetic make-up of threatened populations when deciding how best to conserve them. This is because genetic factors can play an extremely important role in species conservation. For example, in wild populations, low genetic diversity and inbreeding can lead to increased extinction risk. Genetics can inform us about how related populations are, allowing us to define management units and assess the probability of a species or population becoming extinct.
Lying to the west of West Africa, in the middle of the Atlantic Ocean, is a small group of islands called Cape Verde (pictured below). These rugged, mostly arid islands are home to a bird found nowhere else: the Cape Verde warbler, Acrocephalus brevipennis (that little brown bird below). This species only survives in the humid, densely vegetated areas of these islands and was first described ca. 150 years ago, but not much is known about its biology or conservation threats. Its population size is estimated to be a of maximum 2000 birds and it is confined to three small islands: Santiago, Fogo and S. Nicolau. This makes its conservation status “endangered” (BirdLife International 2016).
Cape Verde islands, off the west coast of Africa (top left) and the Cape Verde archipelago (bottom left). Male Cape Verde warbler Acrocephalus brevipennis, on Fogo (right). These birds are easier to hear than to see. They usually jump around in dense foliage, which makes it hard to get a glimpse of them, but they have a loud and very characteristic song unlike others in Cape Verde. You can imagine how difficult it was to get this photo.
As you can see in the figure above, the distances between these three islands are large, creating barriers to migration. Therefore, it is likely that the populations do not mix and that considerable genetic divergence exists between them. One of the first steps in conserving this species should therefore be to find out more about its evolutionary history and whether it caused them to be genetically different. The aim of our work was to determine the genetic diversity and divergence between the populations on the three islands to understand if they are genetically different. If so, we need to treat each of them carefully to help this diversity survive.
In 2013 and 2014, we searched the densely vegetated areas of three islands for warblers, from coastal sugarcane plantations to forested mountain tops, often only accessible through small dirt roads or narrow trails. As I said, these birds are hard to see, so we attracted them with playbacks of male songs and caught them as they flew into mist-nets. We identified each bird with a unique combination of coloured rings applied on their leg, and collected a small drop of blood.
Back to the lab…
In the lab we ran genetic tests on the blood drops that we collected in the field. We examined two types of DNA fragments: the cytochrome b and some microsatellites. We used two different types of DNA fragments because they tell slightly different parts of the bird’s story: the cytochrome b shows a more ancient part of the history of the birds, and the microsatellites a more recent past. We amplified a few fragments per bird (a bit like zooming in the DNA with a magnifying glass) and recorded which birds had similar fragments and which had different ones (birds with similar fragments are likely to be closer evolutionary relatives than those with very different fragments).
Genetic differences across the islands
The genetic diversity decreases with population size across the Cape Verde warbler populations. This was measured in two different types of DNA fragments (microsatellites = full circles and cytochrome b = empty circles). It is clear from both types of DNA fragment that the genetic diversity is higher for the larger population, Santiago, and decreases as the islands get smaller.
It turns out there is low genetic diversity within the species overall, but considerable divergence between the different islands. This means that each population is genetically unique, and therefore they should all be conserved. The low genetic diversity within this species also means that it could be very vulnerable to environmental threats. For example, if a there was a disease outbreak, there might not be any individuals that could resist the infection and none would survive. In a genetically diverse population, individuals vary in their resistance to the disease and at least some might survive.
As you might expect, the smallest population on S. Nicolau also had the least genetic diversity. This means it is more at risk of extinction, so might need extra help to survive in the long term. In the graph above, we show a gradient of decreasing genetic diversity with population size, with the smallest population of S. Nicolau being the least diverse, and the largest, Santiago, the most diverse. S. Nicolau, which is not only the smallest but also the most geographically isolated, is also more divergent from Santiago and Fogo than these two are from each other. That is because it is harder for the birds to fly between the further away islands, and so they become more isolated genetically too.
This figure shows how much the three populations of Cape Verde warbler mix between islands. Each bar represents an individual with the shading indicating the probability of it belonging to each island population (Santiago: dark grey, Fogo: light grey, S. Nicolau: white). We can see that the populations barely mix, and the individuals from each island have a very distinct genetic composition.
The most closely related species of bird to the Cape Verde warbler is the greater swamp warbler Acrocephalus rufescens, from mainland Africa. Our work suggests that the two species separated sometime within the last half million years. It also suggests that our three Cape Verde warbler populations became isolated from one another 165,000–199,000 years ago. This means that they are genetically different but that this is very recent, with the lack of mixing between the three populations, allowing these differences to appear rapidly. In the figure above, we show that the populations of the three islands do not mix with each other, i.e. there is no migration between the islands.
Our data suggest that each of the three remaining populations of the Cape Verde warbler are genetically distinct and should therefore be treated as different management units, with different conservation priorities. Based on our genetic diversity information, warblers on the small island of S. Nicolau has high conservation priority. Fogo birds need some attention, but Santiago birds are the safest for now. Special attention should thus be given to the small population of S. Nicolau, which currently holds a maximum of about 20–25 breeding pairs and therefore could be considered to be on the verge of extinction. We must point out that our study focused solely on the genetic part of the problem. The relative roles of genetic versus ecological factors in limiting population sizes on any of the islands are not clear and it is important to undertake baseline studies on this species to better inform conservation action.
About the author
Helena is a conservation biologist working at UEA and collaborating with IUCN, BirdLife International and Cape Verde’s Direcção Nacional do Ambiente to help inform conservation efforts for the Cape Verde warbler. She uses genetic and field based techniques in combination to learn about the structure of populations, therefore giving us more insight into how to conserve them. For more information about this work contact Helena at firstname.lastname@example.org.