15.15: THIS IS HOW WE DO IT: Investigating ants, plants, and the unintended consequences of environmental intervention.

Mutualisms between plants and animals are hugely important to the functioning of ecosystems and to maintaining biodiversity. But the complexity of the webs of interactions within ecosystems makes it difficult to predict the impact of a change in one part of the system. This has important implications as we try to protect valuable environments and natural resources.

Consider a situation that many natural resource managers encounter. Suppose that a population of organisms with great value appears to be threatened by the predatory behavior of another species. To protect the valuable population, the natural resource manager might be tempted to intervene, blocking the predator from harming the valuable population. This seems like a reasonable plan. But how can we be sure?

In fact, our plan, with all its good intentions, may not work. Worse, it may have exactly the opposite effect of what we intend. Let’s look at a situation that, surprisingly (but unambiguously), revealed a scary truth: seemingly helpful, straightforward manipulations may have significant, unintended, and negative consequences.

It started when a biologist visiting Africa observed an intervention that was in place to help a plant species—acacia trees. The intervention: large groups of these trees were fenced off to protect them from the destructive herbivory of elephants and giraffes.

The natural resource managers assumed that the enclosures, by protecting the trees from herbivores, should benefit the plants. The acacia trees ought to grow faster and live longer. But the visiting biologist noticed that the several hundred plants within the enclosures—distributed across six 10-acre plots of land that had been fenced off for more than 10 years—weren’t doing very well. Compared with unprotected acacia plants in plots adjacent to the enclosures, most of the protected plants looked unhealthy, and many were dying. A group of researches decided to find out why.

The researchers began by observing acacia trees closely to figure out what normally occurs (that is, outside enclosures) in these plants. Here’s what they found out:

• 1. Acacia plants and certain types of ants have a mutualistic relationship. The plant makes hollow thorns, in which the ants live, and produces sugary nectar, which the ants consume.
• 2. When something touches the tree—such as an elephant eating leaves—the ants swarm out and defend the tree. This reduces herbivory on the plant.

The biologists’ next step was to make careful observations of the acacia trees within the enclosures to figure out what was happening as a result of the intervention to keep out elephants and giraffes. Each piece of information they collected was like one piece of a puzzle. Here’s what they found for the enclosed trees:

• 1. Reduced predation → reduced investment in defense against predators. When protected from elephants and giraffes, the acacias somehow detected the reduced herbivory and reduced their investment in supporting the symbiotic ants: they decreased by one-third the number of nectar-producing sites, and they produced fewer swollen thorns in which the ants could live.
• 2. Reduced support of ants → reduced defense behavior by ants. When the nectar reward dwindled, the ants played a lesser role in plant defense: there was a 30% reduction in the number of ants remaining on the plants, and among the remaining ants, 50% fewer individuals responded to disturbances of the plant—and those that did were significantly less aggressive in their response.
  

• 3. Reduced number of ants → opportunity for competitors to get in. When the number of mutualistic ants dwindled, this created an opportunity for competing (non-mutualistic) species of ants to take up residence in the acacias. The proportion of trees occupied by these competing, non-mutualistic ants doubled.
• 4. Increased number of competitor ants → increase in stem-boring beetles and plant damage. The competitor, non-mutualistic ants encouraged beetles to infect the plants: the beetles bored cavities in the acacia stems, where the competing ants could lay eggs. And when beetles bore cavities, plants suffer. The researchers documented that acacia growth was much slower, and mortality was doubled.

These two changes, on their own, should not have been bad for the enclosed plants. Why?

In just a few short steps, the researchers were able to piece together what was happening:

• 1. Big herbivores eat acacia plants.
• 2. Restricting herbivores causes acacia plants to invest less in mutualistic ants.
• 3. Reduced support of the ants helps competing ants and stem-boring beetles to invade.
• 4. Formation of bore holes by the beetles slows plant growth and increases plant mortality.

In other words, the researchers learned why restricting big herbivores by placing enclosures ultimately harmed the acacia trees.

Beyond this specific message, the researchers noted that their results illustrate the extreme complexity of community interactions.

The disappointing but unavoidable implication is that, within an ecosystem, seemingly helpful, straightforward manipulations may have significant, negative consequences. Or, in the words of poet Robert Burns, “The best-laid schemes o’ mice an’ men / Gang aft agley [often go awry].”