Predators and Mutualists

August already, and I am moving into late summer collections at Jerry Smith Park.

This season I don’t have students working with me in the field. Avila is a small college, and the timing of biology majors moving through our programs isn’t always conducive to having research assistants on my summer team.

Even when you work as part of a field crew, ecology is a solitary endeavor. There comes a moment every day when collective experience is distilled into one-on-one interaction with the creatures that make your study site their home. Individual silence opens the way for a broad exchange based on a range of sensory impressions, a revealing dialogue without words.

I’ve found authors and poets who manage to capture this experience – if fleetingly – in some of their stories. The most recent example is Richard Powers’ The Overstory, which I may come back to in a later post. For the moment, let me just say: You must add The Overstory to your list of recommended reading.

This past week at Jerry Smith Park, a jumping spider caught my eye. She sat still long enough on a compass plant (Silphium laciunatum) for me to capture her in a somewhat blurred photo.

She reminded me of another spider, Dipoena banksii (schmidtii), that caught my attention in the lowland rain forests of Costa Rica many years ago.

A really tiny spider – about the size of the capital “D” in its name – D. banksii inhabit plants of the peppercorn genus, Piper.  Piper plants, in turn, are engaged in a special relationship with small ants of the genus Pheidole.

Spiders, ants, and plants: these interconnected organisms eventually gave me my doctoral thesis, and many years of research beyond.

Pheidole ants protect Piper plants from herbivores (insects that eat plants) in exchange for food and shelter. This kind of relationship, where two species help each other out, is called a mutualism.

Another example of mutualism is pollination, where bees or butterflies transfer pollen from one flower to another in exchange for food (pollen and nectar).

Mutualisms are very common. In fact, they may be the most important type of ecological interaction. Yet historically, they seem to have received less attention than other interactions such as competition and predation. This has changed since I began my career. Even so, we have yet to fully understand the profound role mutualistic interactions have in organizing natural communities and determining the course of evolution.

As a graduate student, I was generally interested in mutualisms and specifically in the question of how predators might affect the costs and benefits of mutualistic interactions. Dipoena spiders, along with their plant host and ant prey, gave me the opportunity to explore this question in a pristine tropical ecosystem.

Today, while the focus of my research has changed, I still think about the dynamics of predation and mutualism as I study bee communities of prairie remnants in temperate environments.

Predators abound at Jerry Smith Park – not just spiders, but also dragonflies and robber flies, among others. These winged arthropods cluster around patches of flowers. They snatch bees and butterflies in flight as would-be pollinators seek nectar from prairie plants.

When I began working on Dipoena, I wondered whether the impact of predators was powerful enough to break down mutualisms. Piper ant-plants were a good system for this question, because not all Piper plants support ant colonies. This indicates plants may have a choice whether to participate in the mutualism. If the costs of supporting an ant colony become too high, plants opt out.

In the end, my research showed Dipoena spiders reduced the ants’ ability to protect Piper plants from herbivores. However, the measurable impacts were small. I never found evidence the damage to the plant was sufficient to override the benefits of the mutualism.

A glance across the broad network of mutualisms that characterizes any ecosystem indicates that these are very robust interactions. As data accumulates, I believe we will find predators most often “skim off the top” of abundance created by mutualisms. Predators may do some damage while exacting benefits for themselves. Rarely, however, can they topple the real advantages of interspecific cooperation.

By now, I know my regular readers are reaching for the metaphor – because there’s always a metaphor, isn’t there?

I’ll leave the metaphors to you this week, but with a cautionary note.

Sometimes the stories of nature are just that: stories of nature. They have aren’t about us, but about the organisms we study. Good things can happen when we use the power of metaphor to translate these stories into our every day, human reality, but bad things can happen too.

For example, with this story, who you interpret as “mutualist” and “predator” might be very different from who someone else interprets as “mutualist” and “predator.”

From an ecologist’s point of view, nature doesn’t tell moral tales. There’s no right or wrong in the role organisms play, whether in the rain forests of Central America or in the tall grass prairies of the Great Plains.

I do believe, however, there’s a point to consider in the fact that mutualisms are so ubiquitous and important in natural systems. Yet, across the history of ecology, they have received less attention relative to more exploitative relationships such as predation and competition.

This observation, however, is not a reflection on nature. It’s a reflection on ourselves as scientists and as a society.