There’s More to Life Than Predator Eats Prey

By Jade Boyd

It’s not a good idea to put all your eggs in one basket … unless you’re a senita moth.

Found in the parched Sonoran Desert of southern Arizona and northern Mexico, the senita moth depends on a single plant species — the senita cactus — both for its food and for a place to lay eggs. The senita cactus is equally dependent on the moth, the only species that pollinates its flowers.

Senita cacti and senita moths have a rare, mutually dependent relationship, one of only three known dependencies in which an insect actively pollinates flowers for the purpose of assuring a food resource for its offspring.

“Mutualistic relationships like this present a problem for ecological theory,” said Nat Holland, Rice assistant professor of ecology and evolutionary biology. Holland co-discovered the senita moth–senita cactus mutualism in 1995 and has studied it ever since.

The problem is that the moths lay their eggs inside the cacti’s flowers immediately after pollination, and when the eggs hatch, the moth larvae eat the fruit, destroying the plant’s chances to produce seeds. Theory predicts extreme ecological instability for this relationship: As moth populations increase, more fruits are destroyed and fewer new cacti appear, and the spiral continues until both species disappear. But in this case, that hasn’t happened.

Holland, who quipped that his “real” lab is 1,500 miles away, spends several months each year observing the moths and cacti at several locations in the Sonoran Desert, including Organ Pipe Cactus National Monument in southern Arizona. But his primary research site for more than a decade has been a desolate, 30-acre patch of desert straddling three ranches near Bahia de Kino on the Gulf of California. Holland and his students sometimes go weeks without seeing other people at the sites, aside from stray cowboys.

The solitude provides valuable time for Holland to synthesize what he’s learned in the desert, which is important because his ultimate goal is a fundamental rethinking of ecological theory for such mutualistic interactions. “I develop theoretical models that attempt to explain mutualistic relationships like the one between the moth and the cactus,” he said, “and I take those models into the field and examine them empirically to find out how well they predict what really happens.”

Traditional theory of such mutualistic interactions leads to predictions of unbounded population growth or instability and eventual doom due to one species overexploiting another. These predictions clearly don’t square with what Holland and his students see happening in the Sonoran Desert, where both species thrive. His models suggest that one mutualist may exert some control over the other’s population increases, such that neither unbounded growth nor overexploitation ensue.

“I’ve always been interested in the community ecology of mutualism — the larger puzzle — and this moth–cactus relationship is just one piece of that,” Holland said. “When we discovered the relationship, I immediately thought of using it to look at the bigger picture, but I wound up spending a decade working on the population ecology of mutualisms, a prerequisite for understanding the larger puzzle.”

Now Holland is returning to his earlier interests in community ecology. “We want to understand how the structure of mutualistic communities influences the stability and dynamics of individual species and of whole networks of species,” he said. The results suggest that the structures of mutualistic communities complement those of predator–prey food webs, a finding that presents the tantalizing possibility of developing an overarching scheme that incorporates elements of both.

Holland’s research has been funded by the National Science Foundation, the National Geographic Society and the National Park Service.