In our new article, my colleagues and I found strong relationships between the distribution and abundance of primates from Madagascar and the trees from which they eat fruits, leaves, and flowers. We then used those relationships to make estimates of the total population sizes for endangered species to better understand their conservation status. The paper, “Estimating the population size of lemurs based on their mutualist food trees”, was just published in the Journal of Biogeography.
Lemurs are the endemic primates of Madagascar, found nowhere else on earth. Because of their long isolation and unique ecologies, more than 100 lemur species have evolved. This amazing diversity of species is threatened by the loss of habitat in Madagascar, as well as hunting. Recent efforts have shown that over 90% of lemurs are threatened with extinction. It is critically important to estimate the total population size of threatened species to monitor their potential decline in the future.
Most lemurs are herbivores, and some rely heavily on a few tree species for food. Many lemurs eat fruits and spread their seeds, making them important for healthy ecosystems. Previous research on many groups of plants and animals has illustrated the importance of interactions among species in determining the geographic distributions and abundances of species. For example, some butterfly species need specific host plants, and so the butterfly cannot exist in a place if the host plant does not exist, even if the climate is appropriate. In our study, we use the distribution of 14 lemur food trees to map the distribution of 19 lemur species and estimate their abundance. Using new data from field observations of endangered lemur species and statistical methods, we created estimates of the total range and population sizes of lemurs.
We found that the geographic range of all 19 lemurs and the abundance of most lemurs is correlated with the distribution of the food trees. For all lemurs, the lemur range was strongly correlated with the range of their food trees, more so than with climate alone. Our estimates of abundance also show that for most species, the population sizes are greater than 10,000 individuals, which is the size below which the species is considered threatened with extinction. For example, the ring-tailed lemur (Lemur catta) may have a total population size of over 500,000 individuals! This is great news because recent press by BBC and IFLScience have suggested lemurs may be 'doomed' with extinction.
Ring-tailed lemur in Isalo National Park. Ring-tailed lemurs may be abundant, but collecting animals for meat and the pet trade still threaten fragmented populations.
In contrast, the sportive lemurs in the southeast (genus Lepilemur) are relatively rare, with a high estimate of 7,000 individuals. The black-and-white ruffed lemur (genus Varecia) is particularly vulnerable, with the lowest estimates of approximately 1,400 individuals. Sibree's dwarf lemur (Cheirogaleus sibreei) may have less than 2,000 individuals in the southeast, though the species's range includes sites in central and northeastern Madagascar as well. Some lemurs had population sizes greater than 10,000 individuals, but are still at a high risk of extinction due to unsustainable hunting and deforestation. In another paper recently published in Conservation Biology, my colleague Dr. Cara Brook showed that one species, the indri, may go extinct because of hunting in some areas.
The sportive and black-and-white ruffed lemurs have fewer than 10,000 individuals in the wild
Sibree's dwarf lemur is only found on high mountains
The Indri is threatened by hunting pressure in the northeast
For several species, we were able to compare our estimates of recent population size to past estimates from the year 2000. At least two species seem to have declined in population size dramatically. While approximately 100,000 red-fronted brown lemur (Eulemur rufifrons) were estimated to exist in the southeast around the year 2000, we estimated that only 15,000-20,000 individuals occurred in 2014. Another species, the gentle bamboo lemur (Hapalemur griseus) from the southeast was estimated to have less than 20,000 individuals in the year 2000 while we estimate there to be approximately 10,000 in 2014. These trends are related to habitat loss as well as the new methods we used to estimate the population sizes.
The red-fronted brown and gentle bamboo lemurs have much smaller population sizes than estimated in the year 2000
These results are significant because many lemur species are important for spreading seeds and as food for predators, and they are a unique branch on the primate family tree. If they go extinct, we will lose those ecosystem functions and evolutionary history. The Malagasy government has taken action to protect the most valuable and diverse habitats remaining in Madagascar. Malagasy and international conservationists and researchers work closely with the government, educational system, and nongovernment organizations to turn research results into conservation action. More effort is needed because Madagascar is an extremely impoverished country with a high population growth rate and a high demand for natural resources.