Good Neighbors: Positive Interaction Can Increase Biodiversity

PULLMAN, Wash.–For a while, in ecology as well as sociology and other studies of interaction, the darker, Malthusian implications of evolution seemed to garner the most attention. Competition was the name of the interactions game. But in ecology, at least, positive interaction has reclaimed respect and close attention.

Positive interactions are simply non-nutritional interactions between organisms that benefit one or the other or both. One well-documented example is the interaction between plants and mycorrhizae, fungi that serve as extensions of the plant’s root system. Another is that between plants and pollinators, which Darwin first detailed.

However, although positive interactions are now known to be so common that many ecologists tend to dismiss them as obvious, surprisingly little is known about their community-wide consequences, particularly how they affect species diversity. As part of her study of salt marshes, Washington State University Vancouver marine ecologist Sally Hacker has long pondered this effect. Her work is the first demonstration of a “facilitator keystone species.”

A keystone species is a species that essentially holds an ecosystem together, as a literal keystone holds an arch together. Many keystone species indirectly increase diversity, by eating or otherwise causing grief to the competitive dominant species, thus allowing other species to invade. But Juncus gerardi, or black rush, displays the first documented positive interaction that directly increases the diversity of its ecosystem.

Salt marshes are ideal for manipulative research into this question because of two factors: there is an enormous amount of variation over very short distances, and positive interactions between plants seem to be most common in physically harsh places.

The New England salt marshes where Hacker did her initial work can be observed as three fairly distinct zones. The high intertidal zone, where tides reach only infrequently, is a relatively benign environment. The low intertidal, on the other hand, with its constant water-logging and high salinity, is harsh for plants. Between these two zones, intriguingly, is where species diversity is highest.

Why was unclear, says Hacker. One clue, however, emerged from some manipulation of the middle intertidal system. Hacker removed the plants from a test plot and found that high salinity, waterlogging, and low oxygen content of the soil resulted. In both the high and low intertidal, however, removing the plants made no difference in the soil conditions.

So, asked Hacker, what is it about the presence of plants in the middle zone that affects the physical conditions? And what is it that affects the species diversity?

There are two key species in New England marshes: Iva frutescans, a tall, broad shrub, and Juncus gerardi. Iva might be considered a good competitor for light, as it forms a canopy. However, in the middle intertidal it occurs in stunted form. It does not occur in the lower intertidal at all. Juncus grows with Iva in the middle intertidal, but not in the low intertidal, and is sparsely distributed in the high intertidal among the tall Iva.

Hacker performed a simple experiment in which she transplanted the various plant species around the three zones. The purpose was to test both the effect of physical stress and how the plants did with and without neighbors.

In the high intertidal, low diversity seems to be due to the fact that the tall Iva shrubs create intense competition. There are no plants in the low intertidal, likely because of environmental stress.

Hacker found that in the middle intertidal, except for one species, all plants without neighbors died–providing a rather straightforward indication that positive interaction did indeed affect diversity.

So obviously, Juncus affects the diversity of species within the middle intertidal. But how it does is not so clear.

Hacker ran a variety of experiments to find out why. What she found was that Juncus ameliorates the harsh conditions in a couple of ways. Juncus grows very densely, shading the soil and decreasing water evaporation, thus decreasing salt accumulation. Also, it has a kind of tissue called “arenchyma,” which allows it to transport oxygen from above-ground parts to the roots. Some of this oxygen leaks out into the soil, which actually helps to oxygenate the soil for other species.

Interestingly, in other work Hacker has found that, rather than help them, Juncus will out-compete plants in other situations. So it is the specific conditions of the middle intertidal zone that allows the interaction to occur.

Not only does Juncus affect plant diversity in the middle intertidal, but it also has an indirect effect on insect diversity.

With simple calculations, Hacker was able to figure what would happen to various insect populations in the marsh if Juncus were to be removed completely. She found that many of the insects would become locally extinct without the positive interaction.

In fact, she predicts that in the middle intertidal alone, 10 of 17 insect species are present because of Juncus. In other words, if Juncus were to disappear, species diversity would decrease by 60 percent.

From a practical conservation standpoint, Hacker’s work will allow more precise decisions in wetland management. For example, to restore or remediate a wetland, managers would be able to determine what species are essential to the system.