A snapshot of extinction in action
Although headline-grabbing research has been undertaken by Kew staff to look back and identify the likely date of extinction for well-documented species such as the dodo (estimated at 1690), it is more difficult to look to the future and predict with certainty an exact time at which a species will meet its end. However, this was exactly the question we were facing in relation to the 21 remaining individuals of the Madagascan endemic plant Eligmocarpus (a member of the legume family).
Introducing Eligmocarpus cynometroides
Eligmocarpus cynometroides is the sole member of a genus of flowering plants currently restricted to the south-east of Madagascar. Its dense wood makes it attractive to local people for use as structural support in homes and to be burnt to produce charcoal and its range has therefore been much reduced in the last 30 years.
It is now known only from a single highly-threatened locality in Petriky, a littoral forest remnant in southern Madagascar which, despite well-intentioned attempts at conservation and re-introduction, is now threatened by future mining activities.
The forest sits on a substrate of the titanium-bearing mineral ilmenite, which is economically valued for its use as a pigment in paint, paper and plastics. Although assumed to have once formed a continuous band along much of the 1,600 km east coast of the island, the littoral forest now exists only as isolated remnants. This humid forest, thought to have originated 8-10 million years ago on the coast before spreading to sandy substrates within several kilometres of the shoreline, remains one of the island’s least known and insufficiently documented vegetation types.
Threats and challenges
Leading up to this project, the most time-critical threat to the survival of Eligmocarpus was posed by mining activities. However, the challenges facing the species are more wide-ranging.
Because the remaining patches of littoral forest are situated close to human settlements, and have very limited formal protection, we have already observed a decline from 27 to 21 individuals since 2007. Additionally, of those 21 individuals 12 are saplings, indicating that they are unlikely to have yet reached reproductive maturity. Eligmocarpus is sadly listed as ‘Critically Endangered’ on the IUCN Red List of Threatened Species.
Levels of both seed production and germination in the remaining wild individuals are very poor, giving little chance for natural regeneration. This, then, was the situation prior to our investigation and it did not seem a happy one.
With the vast collections of plant specimens in Kew’s herbarium at our fingertips we were able to examine collections made during a period when Eligmocarpus cynometroides was more widespread. We could effectively look back in time, as well as collecting and analysing plants from present-day populations. Our aim was to look at the past and present distributions and to investigate the genetic structure of the surviving and extinct populations. This would give us information on how the populations are related and also enable us to assess what diversity has been lost as a result of the decline.
We started by extracting DNA from all 21 extant individuals and from herbarium material from populations that had been destroyed (see map below for the location of study sites), then using DNA sequencing and population genetics techniques and analysis of past and present bioclimatic data to investigate them.
What we found was that, although there is a core of individuals in the Petriky area that are genetically similar, the population is still genetically diverse and that this diversity is not correlated to geography (see map below). Our results showed that the groups are intermixed, making conservation practices even more challenging. This was a somewhat puzzling situation, as a species that had colonised the coast and spread inland would be expected to have a genetically uniform population.
The breakthrough, when it came, was twofold. Firstly, we stopped thinking of the species as a potential early coloniser which had made it to the coast but was struggling to gain a foothold, and instead considered it as a relic, struggling to hold on.
Secondly, based on its morphology, we concluded that the fruit would float and act as a diaspore (the reproductive portion of a plant that is dispersed), distributing the seeds down the river network. Our colleagues in Madagascar were able to confirm this for us.
Based on this, we can now hypothesise that the river network has played an important role in dispersing Eligmocarpus. Under this hypothesis, the origin would have been close to the populations currently located further inland, in the Andohahela area, (see map below) and the individuals used the river network to colonise the area and finally reach Petriky. Interestingly, Ranopiso is at the intersection between two river basins and we might expect that Eligmocarpus previously occurred in the whole area prior to human deforestation (ca. 1,150 years ago).
The hypothesis of river dispersal would also explain the large ecological niche of Eligmocarpus, which might have had the ability to shift between biomes by using the river network to establish new populations on riverbanks and seacoasts. However, as the individuals in Petriky are currently restricted to the dune habitat (10 m above sea level) and do not have direct contact with either rivers or the sea, further investigation into previous climatic conditions was necessary to validate the aquatic-dispersal hypothesis.
During the past 10,000 years (i.e. since the start of the current interglacial - the Holocene), there have been significant variations in sea level in this region. It is therefore probable that in the past individuals of Eligmocarpus would have been in closer proximity to water than they are today, thereby allowing the fruit to float and disperse, and to subsequently reach the riverbanks and shore. The relatively high genetic richness of the population in Petriky suggests that it results from several independent dispersal events to this area, consistent with the river dispersal hypothesis.
So, while the human impact on Madagascar (including both historic land use/clearance and mining activities) seems unlikely to have initiated the reproductive and germination issues facing Eligmocarpus, it will almost certainly be the main contributing factor to the extinction of the genus, particularly given the climatic challenges facing Madagascar in the future. This study has, however, also provided local stake-holders and interested parties with a fantastic opportunity to perform some potentially genus-saving actions prior to the commencement of mining activities.
We therefore hope that Eligmocarpus could also be used as a flagship species for conservation of the littoral forest. One of the take-home messages from this investigation is that conservation efforts, including those undertaken by mining companies, should not only target the expansion of the protected offsets around actual mining sites, but also take into account the biology and ecology of the plants, as these are clearly linked to historical distribution and related dispersion through geological time. We should view this as protecting the process rather than just the pattern.
- Dion and Sven -
- Devey, D.S.*, Forest, F.*, Rakotonasolo, F., Ma, P., Dentinger, B.T.M.*, Buerki, S.* (2013). A snapshot of extinction in action: The decline and imminent demise of the endemic Eligmocarpus Capuron (Caesalpinioideae, Leguminosae) serves as an example of the fragility of Madagascan ecosystems. South African Journal of Botany (Special Issue: Towards a new classification system for Legumes), 89: 273-280. Access this article online.