The composition of an ecosystem consists of the number and kinds of organisms that grow in a particular area, as well as the length of time they live and then persist after death. The length of a particular organism’s life, plus the length of time its body persists after death, is critical—particularly with long-lived and large-sized plants, such as trees. For example, a coast redwood tree may influence its habitat for more than two millennia, whereas a passing black bear may affect the habitat for only half an hour. The bear is a transient component while it’s in the habitat, and even if it dies there, it is still a transient when compared with a redwood.
Structure, in turn, is an outcome of the composition of plants that grow in a particular locale because each individual and each kind of plant grows differently. The cumulative effect of how they grow creates the vegetative structure we see above ground as well as the structure below ground, which is unseen. The combined features of composition and structure allow certain functions to take place within a given area of a particular landscape, whether it is a desert, a prairie, or a tropical forest. As a simple illustration, let’s examine the clearing of tropical forest and its effect on soil macrofauna in southeastern Amazonia.
As primary Amazonian forests are cleared, pastures and secondary forest occupy an increasing amount of space in the landscape. Their presence has a variable effect on the soil macrofauna, particularly invertebrates, such as beetles, ants, termites, spiders, and earthworms. In one study, the richness of the soil macrofauna fell from 76 to 30 species per plot immediately after the forest was cleared, and the composition of the new community was different. Ants, termites, and spiders were most affected by the disturbance.
In plots situated where deforestation had taken place several years earlier, the effect was dependent on the type of land use—a pasture in which the grasses and forbs were continuously grazed, for example, or one where the land was allowed to lie fallow and thus could begin to recover toward a forest. The richness per plot in old clearings left fallow rose to 66 species, and the composition was closer to that in the primary forests than to that on land used in other ways. Although macrofaunal communities showed richness close to that in the primary forest in all fallow areas, the species richness of earthworms and beetles recovered only as areas next to the forest regained their forest cover. In contrast, species richness per plot remained low in pastures, just 47 species. The data show that clear-cutting the forest is a major disturbance to the soil macrofauna, whose recovery potential is much higher in fallow areas than in pastures, even 6 or 7 years after logging. In Amazonia, therefore, areas that lie fallow after they are clear-cut may play a critical role in the conservation of soil macrofauna.1
Returning to North America, a simple management decision in southeastern Oregon provides an excellent example of what can happen to the composition, structure, and function of a remnant grassland when how to repair it is not understood. The only way to “rehabilitate” areas in southeastern Oregon following rangeland fires in the 1970s, while I was working in the area, was to plant crested wheatgrass (an exotic species from northern Asia) and call doing so “fire rehab.” In fact, the habitual response of the federal-government range managers was to increase forage for cattle by planting rows of crested wheatgrass with a mechanical range drill, but this response was not based on any understanding of the ecosystem. Moreover, the seedings were touted as a way to control soil erosion, which was nowhere evident, as rill erosion was legion in the seedings.
The challenge began when the U.S. Department of the Interior, Bureau of Land Management, hired me to conduct a wildlife survey on 5.2 million acres. One of the first plants I encountered was hundreds of acres of crested wheatgrass. This tussock-forming grass (which grows in small, thick, coarse clumps) was planted in neat, well-spaced rows that were all but devoid of life other than the grass itself.
These uninhabitable areas had experienced the extirpation of virtually every indigenous species within their borders. In comparison, however, the areas surrounding the seedings were brimming with native life, and I wondered why. The answer occurred to me on a hot summer’s afternoon, as I examined the seemingly endless seedings, and how simple—how very simple—the answer was. The grass had been planted in north-to-south rows, which allowed the blazing sun to perpetually scorch the bare soil between the rows as it traveled in a southerly arch from morning till evening.
Conversely, in a few areas, where the rows had inadvertently been planted east to west, the grass plants themselves created a physical barrier that shaded the ground from one row to the next. Here, plants and animals could survive, although less abundantly in both numbers and species than in the areas surrounding the seedings. Nevertheless, their survival radically decreased the amount of soil erosion in those areas because of the soil-holding root systems of the indigenous plants, especially the native grasses.
In this case, a simple physical alteration in the orientation of the rows of exotic grass helped mend two functional problems—those of wildlife habitat and soil erosion, both of which shared the same self-reinforcing feedback loop of helping to stabilize the ecosystem of which they were a part. These are the most obvious problems inherent in the alien plantations, but the multitudinous acres of crested wheatgrass harbor subtle, long-term problems, some of which require decades to become apparent, by which time they’re out of control.
Crested wheatgrass was imported during the drought-stricken Dust Bowl years of the Great Depression to augment livestock forage because local prairie grasses were failing to provide sufficient fodder for the cattle. The invasion of wheatgrass was sealed when farmers discovered that it withstood drought and overgrazing, had a long growing season, and made good hay. Today, through the unflagging efforts of ranchers and government agencies, such as the Bureau of Land Management, crested wheatgrass covers twenty-five million acres of prairie and shrub-steppe in North America north of Mexico. It is not, however, the problem-free panacea for the livestock industry that it is trumpeted as being.
An immediate, ecological difficulty lies in the propensity of crested wheatgrass to devote most of its energy above ground to the production of shoots, while maintaining only a meager root system. Indigenous grasses, in contrast, do not grow as tall, but they form prodigious networks of roots, which anchor soil in place and enrich it with nutrients and organic matter. As a result, soil in crested wheatgrass plantations contains significantly fewer nutrients and less organic matter than does soil in native grasslands.
The fact that native grasses put more nutrients into the soil and plant tissue than crested wheatgrass affects another component of the ecosystem, the sequestration of atmospheric carbon. When the prairie was converted to crested wheatgrass, a much more effective sink for carbon was lost—one that would tie up 480 million tons beneath rich native prairie, thereby removing it from the atmosphere. It seems, however, that the effects of introducing crested wheatgrass to increase short-term livestock feed extend well beyond the displacement of indigenous species and the reduction of diversity, to include the alteration of pools of energy and nutrients and how they flow within the prairie ecosystem.2
In these two scenarios (logging in Amazonian forests and crested wheatgrass seedings), the species composition of the vegetation was clearly the determining factor in how a given area functioned as habitat. Understanding these kinds of dynamics, however, requires patience because much of the ongoing change is initially unnoticed and thus unrecognized as cumulative effects.
1. The preceding two paragraphs are based on J. Mathieu, J.-P. Rossi, P. Mora, and others. Recovery of Soil Macrofauna Communities after Forest Clearance in Eastern Amazonia, Brazil. Conservation Biology, 19 (2005):1598–1605.
2. The preceding discussion of crested wheatgrass is based in part on: (1) Janice M. Christian and Scott D. Wilson. Long-Term Ecosystem Impacts of an Introduced Grass in the Northern Great Plains. Ecology, 80 (1999):2397–2407, and (2) C. M. D’Antonio and P. M. Vitousek. Biological Invasions by Exotic Grasses, the Grass/Fire Cycle, and Global Change. Annual Review of Ecology and Systematics, 23 (1992):63–87.
Text © by Chris Maser 2012. All rights reserved.