Species Interactions: The Root of Success or Failure?

Each week on the blog we pull back the curtain on our Wildlife Ecology & Conservation graduate students’ seminar class. This post is by master’s student Allison Cauvin. 

Dr. John Terborgh wants to answer an increasingly pressing question for biologists all over the world: Why do some species go extinct? It’s a complex issue tied to several other fundamental questions. How, for instance, does a species’ interactions play a role in their potential extinction or survival? What role does habitat play? What can we do differently to preserve already endangered species?

Dr. John Terborgh is a distinguished member in the field of tropical ecology. He received his BS, MS, and PhD at Harvard, specializing in plant physiology for his doctoral degree. Dr. Terborgh has served  on the faculty at the University of Maryland, Princeton University, and Duke University, where he founded and directed the Duke University Center for Tropical Conservation. Dr. Terborgh has numerous publications to his name, including articles published in Nature and Ecology. He has also written several books that focus on themes of biodiversity and conservation. The breadth of his work has merited notable accolades including a MacArthur Fellowship and the Daniel Giraud Elliott Medal. In addition, Dr. Terborgh served for 18 years as the operator of the Cocha Cashu Biological Field Station in Peru, which gave him firsthand experience in learning how tropical organisms interact with one another.

A toucan species found in Venezuela.

During his time in Venezuela, Dr. Terborgh studied how the recently formed Lago Guri—a lake created by a new dam—was changing local habitat and species abundances. He found small, newly formed islands where bird abundance was between two and twenty times higher than on the mainland. And on the smaller islands, where there was a high density of birds, the howler monkey density was roughly thirty times higher than on the mainland. Given this unexpected finding, Dr. Terborgh wondered if there was an interaction between the howler monkeys and the birds that allowed for elevated populations.

After conducting several surveys, Dr. Terborgh concluded that the increase in howler monkey density resulted in increases in howler monkey dung – this, in turn, allowed nutrients to cycle through the environment at a much higher rate than on the mainland. Tree and plant density increased from the higher nutrient availability, which provided food for the bugs that the birds feed on. With more bugs, these small islands could support greater densities of tropical birds.

His study provides an interesting insight into how tropical species may react to habitat fragmentation, which is occurring at an increasing rate globally. Understanding how human activity impacts the biological landscape is crucial to land and species conservation, and this study takes a wide perspective by analyzing effects across several levels of the food chain.

Dr. Terborgh is also interested in Australia as a case study for how humans affect the biological landscape. He points out that Australia is the only continent that failed to retain a top predator. Roughly 40,000 years ago, humans came to Australia and wiped out the large animals known as megafauna. Before the aboriginal humans came to Australia, there were marsupials as big as rhinos, and lizard predators that dwarfed the komodo dragon. When these large animals disappeared from Australia, smaller herbivore species grew more abundant and diverse.

Today, however, we are riding the wave of Australia’s next big extinction event. Foxes and feral cats, both introduced within about the last 150 years, are decimating small mammal populations including endangered species. The problem is that in the absence of large apex predators, small predators such as foxes and feral cats have no population control. Also, as novel predator species, their prey has no inherent fear response, making the foxes and cats particularly effective predators. Without a means to control populations of these introduced predators, Australia could very well witness the extinction of several of its small native mammal species.

A dingo on the prowl.

A dingo on the prowl.

Dr. Terborgh thinks there is a solution, however. The dingo, introduced to Australia roughly 4,000 years ago, is considered a nuisance and a potential predator to livestock. Significant resources have been allocated to mitigate dingo populations. Control efforts include hunting, poisoning, and building large fences. And yet native small mammal populations have been coexisting with the dingo for at least 4,000 years, and studies have shown decreased fox abundance in areas where the dingo is present. While there is not much information on how dingoes affect cat populations, Dr. Terborgh argues for halting the systemic persecution of the dingo. He believes the dingo will serve as a predator for the introduced foxes and cats, and may slow their population growth enough to sustain communities of native small mammals. It’s a long road to changing attitudes in Australia, but hopefully this can be done in time to save some of its more endangered small mammal populations.

All in all, the success or failure of a species is a complex issue. Many factors, including those not described here, must be taken into account. Dr. Terborgh believes one of the keys to understanding potential extinction lies in a species’ interactions with others around it. In the case of the tropical birds of Venezuela, they benefit from an indirect interaction with the howler monkeys. While in Australia, small native mammals are being decimated by introduced predators. Understanding how these interactions can affect species’ survival is essential to saving our increasingly threatened global biodiversity.