Fleas, tapeworms, Giardia, pinworms: Parasites are all around us. But some animals are more susceptible than others. Take the well-studied chimpanzee, for example: it’s known to host over 100 parasites. In contrast, species like the indri, a lemur only found on Madagascar, are only known to host about 10 parasites. Many other primates are so poorly studied that only one parasite has ever been recorded.
In a new study published in the Journal of Animal Ecology, we examined which traits of both primates and parasites predict the likelihood of their interactions. Using advanced techniques in social network analysis, called the exponential random graph, we were able to simultaneously test the traits of primates and parasites to determine what predisposes primates to infection and what gives some parasites a unique advantage.
For primates, larger species that are found in warmer, wetter climates are more likely to host diverse parasites, compared to smaller species living in drier, cooler climates. Further, species in the same branches of the evolutionary tree and those that live in the same geographic region are more likely to share parasites than more distantly related species found on different continents. Viruses, protozoa, and helminth worms are more likely to infect diverse primates than fungi, arthropods, and bacteria. Parasites that are known to infect non-primate mammals are also more likely to infect diverse primates.
These new results were made possible by the great advances being made in infectious disease ecology. Over the last two decades, Dr. Charles Nunn at Duke University’s Evolutionary Anthropology and Global Health departments has been working with teams of researchers to compile all published records of primate-parasite interactions. Combing through the literature, almost 600 published sources were obtained to glean which parasites are found in over 200 primates species, with over 2,300 interactions recorded. With the analytical tools in social network science mastered by Duke Sociology professor Dr. James Moody, we were able to systematically test how traits of both hosts and parasites affect the likelihood of their interaction for the first time. While many previous studies used subsets of this database and examined either hosts or parasites in isolation, we were able to make new inferences about the critical links in this unique ecological network.
This work builds on a recent study that showed how extinction of primate hosts could lead to the co-extinction of almost 200 parasite species. While at first this might seem like a good thing, in fact it could have negative impacts on biodiversity as a whole. Many parasites don’t actually cause disease or death in the hosts, and some may even have beneficial properties. We simply don’t know enough about these critical and co-evolved relationships to understand what effects host-parasite coextinctions could have in the long-term.
While it might seem strange to worry about parasite extinctions, they are actually an important part of biodiversity and ecosystem functions. Understanding how primates and parasites interact reveals new insights into coevolutionary theory, and could also contribute to the conservation of underappreciated species richness. While from a public health perspective, we’d like to see some parasites disappear, like corona and ebola viruses, from an evolutionary stance, the sheer diversity of parasites and their intimate relationships with their hosts make them fascinating and crucial components of biodiversity.
New research shows that lemurs and their food trees are tightly linked in ecological networks, and that the extinction of lemurs will have cascading effects on ecosystem functions.
Lemurs are the primates found only on Madagascar. They are unique in many ways, and like many organisms, they fit in complex ecological networks. These networks include interactions between lemurs and their food trees. Many interactions are beneficial, or mutualistic; for example, lemurs eat the fruits of trees and disperse their seeds, providing a critical service to the trees. If lemurs go extinct — 98% of species are threatened with extinction due to human activities — the links in the ecological network will be severed, with potentially negative impacts on the trees.
Research published in the Journal of Animal Ecology by Ph.D. student Camille DeSisto, of the Duke University Nicholas School of the Environment, and James Herrera, from the Duke Lemur Center, shows how tightly linked lemurs and trees are in their interaction networks, and the negative impacts of extinction on network resilience. If lemurs do in fact disappear, many trees will be left without a way to disperse their seeds, and may not be able to reproduce effectively.
DeSisto and Herrera used advanced techniques in social network analysis, including exponential random graph models, to test which traits of lemurs and trees predict their probability of interaction. The lemurs with the highest probability of interactions with trees were large, fruit-eating species with a short gestation length, occurring in arid habitats, and with a threat status of Least Concern. Closely related plants were more likely to interact with the same lemur species than distantly related plants, but closely related lemurs were not more likely to interact with the same plant genus.
Simulated lemur extinction tended to increase network structure in some properties, including connectance (% of realized interactions out of all possible interactions) and modularity (how many unique cliques or subcommunities form in the network), but decrease nestedness (the tendency for specialists which feed on only a few trees to be a subset of generalists which feed on many trees) and robustness (tolerance to future extinctions), compared to pre‐extinction networks. Networks were more tolerant to plant than lemur extinctions.
By simulating the loss of lemur and plant species, the authors could predict how network structure will erode over time if threatened lemurs and trees go extinct. The results showed that if the most well-connected lemurs in the network were to disappear, the percentage of trees with interactions would quickly decline, compared to scenarios in which lemurs were removed randomly or if the least-well connected lemurs went extinct. Given the threat status and geographic range size of lemurs, the percentage of trees that would lose their interacting lemurs would be greater than that expected if lemur extinctions were random.
Results also showed that if lemurs go extinct, the resilience of the networks to further disturbance would decrease. This indicates that the current links between lemurs and trees are critical to the stability of these complex ecological networks.
To prevent the loss of key ecosystem functions like seed dispersal, it is critically important to protect lemurs and trees, which depend so crucially on one another for survival. DeSisto is currently conducting field research in Madagascar, studying how well seeds germinate when eaten by lemurs. She created a tree nursery in the forest to grow the seeds obtained from lemur feces, and already has several species germinating. Interestingly, she is also showing how lemurs disperse the seeds of vines, which are an important yet understudied food source when tree fruits are not available. She will continue her research across multiple seasons, to determine how changes in plant phenology affect seed dispersal patterns.
Many conservation programs are currently striving to safeguard Madagascar forests and the diverse species found only in these natural habitats. The Duke Lemur Center has an active conservation program in the northeast, called the DLC-SAVA Conservation Initiative. This program takes a community-based approach to conservation, partnering closely with local stakeholders and actors to develop projects that address the needs of both lemurs and people. By co-creating projects that include alternative and sustainable livelihood strategies, both nature and people benefit from conservation. Natural ecosystems provide important services for people, including locally, such as protecting watersheds and pollinators, as well as globally, such as carbon sequestration. Without the native forests, and the lemurs that call those forests home, people would lose the valuable and irreplaceable services forests provide.
CITATION: “Drivers and Consequences of Structure in Plant–Lemur Ecological Networks,” Camille DeSisto and James Paul Herrera. Journal of Animal Ecology, July 15, 2022. DOI: 10.1111/1365-2656.13776.
A new study investigates why and what they can do about it
Madagascar, famous for its lemurs, is home to almost 26 million people. Despite the cultural and natural riches, Madagascar is one of the poorest countries in the world. Over 70% of Malagasy people are farmers, and food security is a constant challenge. Rice is the most important food crop, but lately an internationally-prized crop has taken center stage: vanilla. Most of the world’s best quality vanilla comes from Madagascar. While most Malagasy farmers live on less than $2 per day, selling vanilla can make some farmers rich beyond their dreams, though these profits come with a price, and a new study illustrates it is not enough to overcome food insecurity.
In a paper published June 25, 2021 in the journal Food Security, a team of scientists collaborating between Duke University and in Madagascar set out to investigate the links between natural resource use, farming practices, socioeconomics, and food security. Their recently published article in the journal Food Security details intricate interactions between household demographics, farming productivity, and the likelihood of experiencing food shortages.
The team interviewed almost 400 people in three remote rural villages in an area known as the SAVA region, an acronym for the four main towns in the region: Sambava, Andapa, Vohemar, and Antalaha. The Duke University Lemur Center has been operating conservation and research activities in the SAVA region for 10 years. By partnering with local scientists, the team was able to fine-tune the way they captured data on farming practices and food security. Both of the Malagasy partners are preparing graduate degrees and expanding their research to lead the next generation of local scientists.
Farmers harvesting the rice fields in Madagascar. Credit: Wikimedia Commons.
The international research team found that a significant proportion of respondents (up to 76%) reported that they experienced times during which did not have adequate access to food during the previous three years. The most common cause that they reported was small land size; most respondents estimated they owned less than 4 hectares of land (<10 acres), and traditional farming practices including the use of fire to clear the land are reducing yields and leading to widespread erosion. The positive side is that the more productive the farm, especially in terms of rice and vanilla harvests, the lower the probability of food insecurity. There was an interaction between rice and vanilla harvests, such that those farmers that produced the most rice had the lowest probability of food insecurity, even when compared to farmers who grew more vanilla but less rice. Though vanilla can bring in a higher price than rice, there are several factors that make vanilla an unpredictable crop.
The vanilla market is subject to extreme volatility, with prices varying by an order of magnitude from year to year. Vanilla is also a labor- and time-intensive crop; it requires specific growing conditions of soil, humidity, and shade, it takes at least 3 years from planting to the first crop. Without the natural pollinators in its home range of Mexico, Malagasy vanilla requires hand pollination by the farmers, and whole crops can be devastated by natural disasters like disease outbreaks and cyclones. Further, the high price of vanilla brings with it ‘hot spending,’ resulting in cycles of boom and bust for impoverished farmers. Because of the high price, vanilla is often stolen, which leads farmers to spend weeks in their fields guarding the vanilla from thieves before harvesting. It also leads to early harvests, before the vanilla beans have completely ripened, which degrades the quality of the final products and can exacerbate price volatility.
In addition to the effects of farming productivity on the probability of food insecurity, the research revealed that household demographics, specifically the number of people living in the household, had an interactive effect with land size. Those farmers that had larger household sizes (up to 10 in this sample) had a higher probability of experiencing food insecurity than smaller households, but only if they had small landholdings. Those larger families that had larger landholdings had the lowest food insecurity. These trends have been documented in many similar settings, in which larger landholdings require more labor, and family labor is crucial to achieving food sovereignty.
The results have important implications for sustainable development in this system. The team found that greater rice and vanilla productivity can significantly reduce food insecurity. Therefore, a greater emphasis on training in sustainable, and regenerative, practices is necessary. There is momentum in this direction, with new national-level initiatives to improve rice production and increase farmers’ resilience to climate change. Further, many international aid organizations and NGOs operating in Madagascar are already training farmers in new, regenerative agriculture techniques. The Duke Lemur Center is partnering with the local university in the SAVA region to develop extension services in regenerative agriculture techniques that can increase food production while also preserving and even increasing biodiversity. With a grant from the General Mills, the Duke Lemur Center is developing training modules and conducting workshops with over 200 farmers to increase the adoption of regenerative agriculture techniques.
Further, at government levels, improved land tenure and infrastructure for securing land rights is needed because farmers perceive that the greatest cause of food insecurity is their small landholdings. Due to the current land tenure infrastructure, securing deeds and titles to land is largely inaccessible to rural farmers. This can lead to conflicts over land rights, feelings of insecurity, and little motivation to invest in more long-term sustainable farming strategies (e.g., agroforestry). By improving the ability of farmers to secure titles to their land, as well as access agricultural extension services, farmers may be able to increase food security and productivity, as well as increased legal recognition and protection.
To move forward as a global society, we must seek to achieve the United Nation (UN) Sustainable Development Goals (SDGs). One of the SDGs is Goal #2, Zero Hunger. There are almost one billion people in the world who do not have adequate access to enough safe and nutritious food. This must change if we expect to develop sustainably in the future. Focusing on some of the hardest cases, Madagascar stands out as a country with high rates of childhood malnutrition, prevalence of anemia, and poverty. This year, more than one million people are negatively impacted by a three-year drought that has resulted in mass famine and a serious need for external aid. Sadly, these tragedies occur in one of the most biodiverse places on earth, where 80-90% of the species are found no where else on earth. This paradox results in a clash between natural resource conservation and human wellbeing.
Achieving the UN’s SDGs will not be easy; in fact, we are falling far short of our targets after the first decade. The next ten years will determine if we meet these goals or not, and our collective actions as a global society will dictate whether we transform our society for a sustainable future or continue with the self-destructive path we have been following. Further research and interventions are still needed to conserve biodiversity and improve human livelihoods.