October 2006
Isalo National Park, Madagascar. Photo: Bernard Gagnon.
“It’s a dream come true to me to have this kind of group assembled,” said Anne Yoder, professor of biology at Duke University and director of the Duke Lemur Center, speaking to 26 scientists who had come from all over the United States as well as England, France, Finland, Germany, and Madagascar to meet for three days in June 2006 in Durham, North Carolina.1 The group included botanists, zoologists, ecologists, paleontologists, molecular biologists, statisticians, computer modelers, and taxonomists. What drew this diverse group together? A fascination with Madagascar and with evolution.
Madagascar: Evolutionary hotspot
In Yoder’s words, “Madagascar has often been described as one of the world’s greatest natural laboratories for the study of evolution.” An island 400 kilometers off the eastern coast of South Africa, Madagascar is only about the size of Texas, yet it contains an astonishing collection of plants and animals that are characterized by
- diversity: an estimated 200,000 animal species are present2
- endemism: most Madagascar species occur only in Madagascar; 7 of at least 160 plant families on the island live nowhere else3
- imbalance: some globally widespread taxonomic groups are absent, while others are unusually diverse
All of the island’s mammals, not counting bats, are endemic. Madagascar supports at least 50 different species of lemurs, which are primates that live nowhere else on Earth, but no monkeys or apes live there. More than 95 percent of Madagascar’s reptile and amphibian species are endemic. Out of 28 frog families worldwide, only three live on the island. But within these three families, there are more than 300 species. Some amphibians and reptiles are conspicuously absent: No salamanders or vipers live in Madagascar.4
There are at least 12,000 different plant species and more are identified every year. The endemic Madagascar periwinkle plant has become well known because it is used in medicine to treat childhood leukemia. According to George Schatz of the Missouri Botanical Gardens, “We really don’t have any idea how many plant species are in Madagascar. It may go to 14,000 or 16,000.” Texas, by comparison, contains about 5,500 plant species.
Threats to Madagascar’s flora and fauna
Madagascar is one of the poorest countries in the world. More than 18 million people live there, and the population is growing rapidly. The vast majority of Malagasy people (as inhabitants of Madagascar are called) eke out an existence supported by slash-and-burn agriculture. Farmers clear a plot of land, cultivate it for a few years until the soil is depleted, then move on to clear another patch of forest. Forest is also burned to provide grazing areas for cattle and logged for construction materials, firewood, and charcoal production.5
Humans arrived on the island only about 2,000 years ago but have already left a heavy mark:
- It is estimated that only 10 to 15 percent of the island’s habitat remains undisturbed.
- Deforestation has caused massive erosion, as the island’s soils wash into the ocean.
- To add to the ecological disaster, many animals are illegally hunted for meat or for the international pet trade.
- On top of it all, global climate change looms as a serious threat.
Michelle Zjhra, of Georgia Southern University, who studies Madagascar’s trees, said, “Since I’ve been collecting, the number of these species has gone way up, which means we’re only just finding the tip of the iceberg. We’re racing against the clock to document the diversity.”
To date about 2.7 percent of Madagascar’s land area (16,131 km2) is officially protected.6 In response to the ecological crisis, in 2003 the president of Madagascar, Marc Ravalomanana, announced he would triple the amount of land under protection in his country before 2008.
Scientists convene
Ravalomanana’s announcement has generated a lot of interest in the scientific community; scientists hope their knowledge of the evolutionary history of the island’s biota will help identify the best areas to protect.
Yoder and Claire Kremen, assistant professor in the department of environmental science, policy, and management at the University of California-Berkeley, called together the international group in June at the National Evolutionary Synthesis Center (NESCent) in Durham, North Carolina, with these goals:
- to share and compare their research on the evolutionary history of Madagascar’s plants and animals
- to organize small collaborative working groups
- to apply their knowledge to help identify conservation priorities in Madagascar
Like birds of a feather, scientists tend to spend most of their professional meeting time with their own kind, gathering at conferences sponsored by groups such as the Geological Society of America and the American Society for Microbiology. The NESCent meeting, in contrast, brought together a variety of different specialists. Joel Kingsolver, NESCent’s associate director, explained, “We’re interested in getting people together who may not have met before and fostering new collaborations.”
Studying the unusual
Yoder and Kremen hope the NESCent meeting will jumpstart data-sharing among scientists to help figure out in more detail what geological events, climatic conditions, and evolutionary processes led to Madagascar’s present-day assemblage of plants and animals.
There’s no easy answer to how evolutionary processes led to such an unusual biota, but several factors clearly play a role:
- geographic isolation
- size—Madagascar is the fourth largest island in the world
- variety of habitats, including desert, rain forest, mountainous regions, and seashore
- tropical location
Because of the island’s isolation, Madagascar’s organisms follow their own evolutionary paths with little or no competition or genetic exchange with the world’s other plants and animals. The size of the island and wide variety of habitats provide many different niches for different species to fill. The tropical location may allow evolution to proceed faster: Scientists in New Zealand announced this spring that among the 45 plant species they studied, molecular changes in DNA (which drive evolution) occur at a faster rate in tropical climates compared to temperate ones.7
The geological story of Madagascar’s isolation
Millions of years ago, Madagascar was part of a large continent called Gondwana. What would later become Madagascar was nestled between parts of what would later become South America, Africa, India, Antarctica, and Australia. About 165 million years ago, Gondwana began to break up. After separating from Africa and the other continents, Madagascar and India remained joined until 88 million years ago, when India split away and headed on a collision course to Asia. Since then, Madagascar has been on its own.
This is currently the most widely accepted theory, but new fossil evidence has led some scientists to speculate that land bridges connected Antarctica to the southern tip of South America as well as to the southern tip of India-Madagascar for 40 million years or more after Madagascar and Africa parted ways.
The Mozambique Channel, which separates Madagascar and Africa, is so deep that even during times of low sea level there was no land bridge between the two. That means that plants and animals living in Madagascar today have either evolved from what was there when it first became isolated or evolved from individuals that arrived on Madagascar’s shores after floating, swimming, rafting, or flying across the Mozambique Channel—a mechanism called “waif dispersal.”
“Entire trees may have been dislodged from a west African mangrove [forest], rafted across, and germinated on the Madagascar shore,” said Kobinah Abdul-Salim, a botanist at Ohio State University. Other plants and animals could have hitched rides on rafting trees. Animals might have flown or swum. While waif dispersal seems highly unlikely, it only has to happen once or twice for a particular organism to establish a new colony. Once established, the new colony has virtually no contact with the parent colony in Africa, allowing the daughter colony to follow a different evolutionary path.
Given any particular plant or animal in Madagascar, how do scientists figure out whether it evolved from an organism that was present on the island when it first became isolated, or from an organism that arrived by means of waif dispersal? Here are the first steps:
- construct an evolutionary tree of the organism and related groups
- add dates to different branches of the tree, if possible
- pinpoint the geographic locations of relatives, called “sister groups,” living in other parts of the world today
Constructing evolutionary trees using genetic analysis
New genetic techniques have given scientists new ways to construct evolutionary trees. In the past, scientists primarily compared physical characteristics of organisms. Those that shared similar physical characteristics were assumed to be more closely related than those that didn’t. Fossils were placed on the tree the same way.
Today, scientists compare DNA samples from living species. The molecular structure of DNA from closely related species is more alike than the DNA of distantly related species.
Genetic studies uncover surprising relationships. Bart Buyck, of the National Museum of Natural History in Paris, who studies fungi, said, “For two centuries, the whole systematics [of fungi] has been based on what they look like. With the arrival of molecular techniques, we know now this system is worthless. For example, what were supposed to be different families before are now a single genus.” Even for animals such as frogs and ants, genetic analysis is showing that some look-alike species are not as closely related as previously believed, whereas some species have one or more very different forms (males and females, for example).
Genetic analysis is quickly being adopted by more and more evolutionary scientists. Miguel Vences, an expert on Malagasy frogs who works at the Technical University of Braunschweig in Germany, said, “It’s amazing to see how much has been done in the past five years. In the next couple of years, if we work on it, we could get a lot more [data].”
Assigning dates to evolutionary trees
The two most common ways of putting ages on the branches of an evolutionary tree are to use fossil ages or the more recent method using DNA evidence.
Unfortunately, according to paleontologist David Krause, from Stony Brook University, “The fossil record for Madagascar stinks.” He should know: He’s been going on fossil hunting expeditions in Madagascar for about a dozen years. Many of the rocks and sediments in Madagascar are not suitable for preserving fossils; there are virtually no fossils between 26,000 years old and 65 million years old. Krause digs up fossils about 70 million years old. What he’s found indicates that the faunal community living in Madagascar then was not as endemic nor as imbalanced as today. Many are similar to fossils of the same age found in South America and India. Most do not appear to be ancestors of the animals in Madagascar today. Some people speculate that many of Madagascar’s animals became extinct 65 million years ago as a result of the same meteor impact that doomed the dinosaurs, but there’s currently no solid evidence for this.
In the absence of fossils, scientists sometimes use “molecular clocks” to put dates on evolutionary trees. The assumption is that mutations in DNA occur at more or less regular rates through time, so the number of differences in the DNA between two species can be used to infer the amount of time that has passed since the two species diverged from one another. In recent years, it’s become obvious that most mutations do not occur at regular rates, so this method is not infallible. However, modern statistical methods have allowed scientists to use molecular clock data in conjunction with fossil data to produce ever more precise age estimates.
Comparing sister groups
Even when fossil or DNA dates are not available, scientists can look at sister groups to figure out where a Malagasy species arose. Sister groups are different branches of an evolutionary tree that share a common ancestor. (All life forms share a common ancestor, but evolutionary scientists are usually looking at the most recent one for two or more particular groups.)
If a sister group of a particular Malagasy species lives in South America, for example, scientists speculate that both groups evolved from an ancestor that lived on Gondwana before it split apart. If the only sister group is in Africa, and the divergence between the two groups appears to have occurred fairly recently, then the species likely evolved from individuals that arrived in Madagascar by means of waif dispersal.
The results so far
Scientists think there are only a handful of terrestrial groups in Madagascar today that evolved from Gondwanan animals—some turtles, boas, and iguanas. These animals are most closely related to animals that live in South America. Most other Malagasy animals appear to have descended from animals that arrived on the island from Africa in the more recent past. For example, there are four main groups of nonflying mammals in Madagascar:
Black-and-white Ruffed Lemur (Varecia variegata) at Dudley Zoo, England. Photo: Creative Commons, flickr member jo-h’s.
- tenrecs (insect-eating shrewlike mammals)
- lemurs
- carnivorans (such as the fossa, a catlike relative of the mongoose)
- a subset of rodents
Each of these groups has been shown, through DNA analysis, to be the direct descendents of groups of animals in Africa. Dating techniques, including fossil work and molecular clock data, indicate that the species in all four groups most likely arose after Madagascar became isolated.
If waif dispersal explains the presence of most organisms present on the island today, then it is not surprising that the biota is so imbalanced. It would be more surprising if representatives from all common tropical taxonomic groups had managed to make the unlikely trip, survive, and flourish.
Working together to fill in the blanks
Evolutionary scientists would like to know more about the sequence of arrival of various groups in Madagascar, which would help explain the unusual present-day collection and evolutionary processes in general. What if a plant washes up on shore, but its natural pollinator is not present? What if a newly arrived animal finds its preferred niche already filled or is faced with a new predator? As scientists continue to refine the evolutionary trees of different organisms, a sequence of arrival will start to emerge, and some of these questions can be addressed.
Sometimes the evolutionary tree of one organism can illuminate something about another organism. Plants and their specialist herbivores are a good example. David Lees, a butterfly specialist from the Natural History Museum in London, said, “We don’t know when butterflies originated. Recently people have made claims for early origins of butterflies—84 million years ago, or between 82.5 million and 95 million years ago. This seems to me to ignore that the fossil record of the host plants is far better. The plants that some of these butterflies feed on are not that old.”
Lees and other meeting attendees plan to form a working group to compare the evolution of plants and their pollinators in Madagascar. Another member of the working group, Michelle Zjhra, asked, “Do the [evolutionary] patterns in the trees correspond to the patterns of the lemurs that disperse the fruit?” Lemurs, who eat the fleshy fruit of many trees, are significant pollinators and dispersers of trees in Madagascar.
By synthesizing the evolutionary data available so far on Malagasy plants and their pollinators or dispersers, be they butterflies, hawkmoths, or lemurs, the researchers hope to address questions about how the evolution of one group affects or responds to the evolution of other groups. Lees and Zjhra also hope their work will inspire others to do research in Madagascar.
Sharing data leads to discoveries
One of the most exciting moments in the meeting came when David Vieites, of the University of California-Berkeley, presented his brand-new reconstruction of past habitats in Madagascar. (“This is all new stuff,” he said. “I just finished working on it last night.”) Using information about worldwide climate inferred from dated pollen samples from all over the world, Vieites assembled maps of Madagascar in different geologic time periods, showing the changing locations and extents of habitats.
When Vieites showed the group the maps, someone shouted out, “There’s Brian’s lost rain forest!” Indeed, the Pleistocene map showed a tiny patch of rain forest at the southern tip of Madagascar. Just the day before, ant expert Brian Fisher had told the group about some rain forest ants that live deep in rock crevasses in southern Madagascar, an area that’s currently arid. He had speculated a rain forest must have been there in the past. Vieites said of his maps: “It’s very preliminary. We need to calculate the error. But it is very promising. The ant people, the plant people, the butterfly people were happy with it.”
Improving conservation plans
Claire Kremen studies Malagasy butterflies and computer habitat modeling at the University of California-Berkeley. She is particularly interested in using computer models to help prioritize areas for preservation. It’s an endeavor fraught with unknowns. Scientists are trying to work out which attributes are most important when evaluating the value of a piece of land as a preserve:
- size
- shape
- connections to other preserves
- number of species that live there
- number of individual organisms that live there
- ratio of rare to common species
- diversity of habitats
- stability or fluctuation of habitat type over geologic time
- presence or absence of habitat degradation by humans
- presence or absence of features that encourage the development of new species
There are other questions as well. Does protecting land for the benefit of one species protect other species too? It has long been assumed that protecting large “charismatic” species, such as tigers, would automatically help smaller species. Recent computer modeling by Craig Moritz of the University of California-Berkeley indicates that preserving parcels of land to best protect large vertebrates would not necessarily do a good job protecting invertebrates, whereas conserving habitat for invertebrates might have a positive impact on larger organisms.
And what about global climate change, which will likely disrupt many habitats whether they are in national parks or not? Craig Moritz asked, “Should we prepare for recovery from climate change? Can we locate geographical areas that might drive diversification of species?”
No one knows the answers to these questions. But as these scientists and others continue to piece together the evolutionary history of Madagascar, what they learn will help predict the conditions that are necessary to protect Madagascar’s plants and animals—and the evolutionary processes that produced them—in the future.
Anne Yoder summed up her hopes this way: “How are we going to take this magnificent biota and save it? That’s my dream for what’s going to come out of this meeting.”
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