Species interactions vary greatly in strength and direction

It should be clear by now that species interactions in a community vary greatly in their strength and direction. Some species have a strong negative or positive effect on the community, while others probably have little or no effect.

ECOLOGICAL TOOLKIT 16.1

Measurements of Interaction Strength

We can measure interaction strength by experimentally manipulating species interactions. The usual procedure involves the removal (or sometimes the addition) of individuals of one species involved in the interaction (the interactor species) and measurement of the response of the individuals of the other species (the target species). There are multiple ways to calculate interaction strength, but one simple way is to calculate the relative interaction intensity (RII) (Armas et al. 2004) using the following equation:

Relative interaction intensity = (C — E) / (C + E) where

C = the number or biomass of target individuals in the presence

of the interactor

E = the number or biomass of target individuals in the absence of the interactor

Interaction strength can vary depending on the environmental context in which the interaction is measured. For example, Menge et al.

How Much Does Predation by Sea Stars Matter? It Depends (Top left) The coastline at Strawberry Hill, Oregon. Plots with (bottom left) and without (middle) cages that excluded sea stars were set up in both wave-exposed and wave-protected areas along the rocky shore. (Bottom right) When mussels were counted and interaction strengths calculated, the results showed that interaction strength was greater in protected than in exposed areas. Error bars show one standard error of the mean. (Graph after B. A. Menge et al. 1996. Food Webs: Integration of Patterns and Dynamics, G. A. Polis and K. O. Winemiller [Eds.], pp. 258-274. Chapman & Hall: New York.) View larger image

The interaction strength “dynamic” (i.e., the relative proportion of strong to weak interactions or positive to negative interactions) is not well understood for any community, because of the numbers of species involved and the many indirect interactions that emerge.

There are some large or abundant species, such as trees, that are likely to have large community-wide effects by virtue of providing habitat or food for other

species. They may also be good competitors for space, nutrients, or light. These species, known as foundation species (Dayton 1971), have large effects on other species, and thus on the species diversity of communities, by virtue of their considerable size and abundance (FIGURE 16.16).

FIGURE 16.16 Foundation versus Keystone Species Species that have large effects on their communities may or may not do so by virtue of their large size and abundance. Some species (lower left-hand corner) have little overall effect relative to their size and abundance, especially if they are redundant in the community. (After M. E. Power et al. 1996b. BioScience 46: 609-620.) View larger image

Some foundation species act by “bioengineering” their environment. These species, known as ecosystem engineers (Jones et al. 1994), are able to create, modify, or maintain physical habitat for themselves and other species. Consider the simple example of the trees mentioned above. Just like any other species, trees provide food for other organisms and compete for resources. However, trees also engineer their environment in subtle but important ways (FIGURE 16.17). The trunk, branches, and leaves of a tree provide habitat for a multitude of species, from birds to insects to lichens. The physical structure of the tree reduces sunlight, wind, and rainfall, influencing temperature and moisture levels in the forest. The roots of the tree can increase weathering and aeration of the soil, and they can stabilize surrounding substrates. The tree's leaves fall to the forest floor, where they add moisture and nutrients to the soil and provide habitat for soil-dwelling invertebrates, seeds, and microorganisms.

FIGURE 16.17 Trees Are Foundation and Ecosystem Engineering Species Treesnot only provide food for and compete with other species, but also act as ecosystem engineers by creating, modifying, or maintaining physical habitat for themselves and other species. (After C. G. Jones et al. 1997. Ecology 78: 1946-1957.) View larger image

Other strong interactors, so-called keystone species, have large effects not because of their abundance, but because of the vital roles they play in their communities. They differ from foundation species in that their effect is large in proportion to their size and abundance (see Figure 16.16). Keystone species usually influence community structure indirectly, via trophic means, as we saw in the case of sea otters (see Figure 16.12A and Chapter 9). Sea otters are considered keystone species because, by preying on sea urchins, they indirectly enhance the presence of kelp, which provides important habitat for many other species. We will consider the role of keystone species in more detail in Chapter

21 and the Case Study Revisited in Chapter 24.

There are also keystone species that act as ecosystem engineers. A great example is the beaver, a species in which just a few individuals can have dramatic effects on the landscape. Beavers dam streams with cut trees and woody debris. Very quickly, flooding ensues and sediment accumulates as the increasing number of woody obstacles slows the water flow. Eventually, the once swiftly flowing stream is replaced by a wetland, containing plants that can deal with flood conditions; plants that cannot do so, such as trees, are lost from the community.

FIGURE 16.18 Beavers Are Keystone Species and Ecosystem Engineers Bydamming streams, beavers created networks of different types of wetlands (shown in red) in a 45-km² watershed on Minnesota’s Kabetogama Peninsula, thus increasing biodiversity within the region.

Why are beavers both keystone species and ecosystem engineers?

(After R. J. Naiman et al. 1988. BioScience 38: 753-762.) View larger image

Finally, it is worth mentioning that there are species that play only a small role in a community's structure and function. Rather than being keystone species or ecosystem engineers, these species are more like bit players: they contribute to the overall diversity of the community, but their presence or absence has little significance for the ultimate regulation of the community (see Figure 16.16). Some of these species may be redundant—that is, they may have the same function in the community as other species within a larger functional group (see Figure 16.4C). Their loss from a community might have little effect as long as other species within the same functional group remain present. We will discuss the role of species in community regulation in more detail in Chapter 19.