Posted by: chrismaser | October 6, 2009

COMPOSITION, STRUCTURE, AND FUNCTION

It is critically important for people to both understand and accept multiple scales of diversity across an array of ecological conditions if the heterogeneity of habitats—and species—is to be maintained. They need to understand that diversity is mediated by such events as a falling leaf, a blown-over tree, a fire, a hurricane, a volcano, or an El Niño weather pattern. Each scale of disturbance alters—both destroys and creates—a habitat, or collective of habitats, by renegotiating the composition, structure, and function of plant communities that, in turn, create a time-space array of still different scales, dynamics, and dimensions of diversity that can be used by animals that alter plant communities, that become still different communities and habitats, and so on. Therefore, to caretake an ecosystem on a biologically sustainable basis, it is necessary to have the best possible understanding of diversity itself, because biological, genetic, and functional diversity are in many ways the cumulative effect of Nature’s diversity in all its various dimensions.

Small-scale diversity in a stream canyon in eastern British Columbia, Canada, and landscape-scale diversity of the basalt plateau on the Washington side of the Columbia River Gorge, which is the state boundary between Oregon and Washingon.

To maintain ecological function means that the characteristics of the ecosystem must be maintained in such a way that its processes are sustainable. In other words, if you want large woodpeckers to live in your forest, you must have species of trees that not only grow large enough to accommodate them but also are allowed to grow large enough to accommodate them. Consequently, the characteristics of concern are: (1) composition, (2) structure, (3) function, and (4) Nature’s disturbance regimes, which periodically alter an ecosystem’s composition, structure, and function in some dramatic way. Such alterations, however, are seldom readily apparent in the short term because they advance as cumulative effects, which pass through a “lag period” before crossing a threshold manifestation that is clearly visible.

To understand what I mean, think of each of these kinds of diversity as an individual leg of an old-fashioned, three-legged milking stool. When so considered, it soon becomes apparent that if one leg (one kind of diversity) is lost to some sort of “disturbance,” the stool will fall over. Fortunately, a considerable number of functional back-ups are built into an ecosystem in that more than one species (biological diversity passed forward through genetic diversity) can usually perform a similar function (functional diversity) and thereby maintain the functional resilience of the system.

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.

A Joshua tree in the desert of southern Nevada, and the jungle of southern Malaysia.

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’s 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.

The roots of a Malaysian tree, which lives in low areas that flood, and a Malaysian tree with a similar root structure that lives on higher ground. Which of these two micro-habitats could support the greatest variety of insects?

As primary (old) Amazonian forests are cleared, pastures and secondary (young) 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 seventy-six to thirty species per plot immediately after the forest was cleared, and the resultant composition of the new community differed from that of the old. 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 sixty-six 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 forty-seven 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 six or seven 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

malaya33

Ants (top) and a well-worn ant “highway” (bottom) on the jungle floor of southern Malaysia.

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, which 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 its 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, the root systems of the surviving indigenous plants—especially those of the native grasses—radically decreased the amount of soil erosion in those areas.

In this case, a simple physical alteration in the orientation of the rows of exotic grass plants 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 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, 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 to be.

An immediate, ecological difficulty lies in the propensity of 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 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 wheatgrass affects another component of the ecosystem, the sequestration of atmospheric carbon. When the prairie was converted to wheatgrass, a much more effective sink for carbon was lost—one that would tie up 480 million tons (435 meteric tons) beneath rich native prairie, thereby removing it from the atmosphere. Therefore, the effects of introducing 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. But, understanding these kinds of dynamics requires patience because much of the ongoing change is initially unnoticed and thus unrecognized as cumulative effects—unless, of course, the ecosystem of interest experiences a disturbance, such as a fire, drought, landslide, hurricane, or flood.


 

Related Posts:

• The Link Between Nature’s Commons And Our Cultural Commons

• The Commons Usufruct Law

• Planet Earth As A Biological Living Trust

• The Key Of Choice

• Sunlight Is The Earth’s Only True Investment Of Energy

BIODIVERSITY–THE VARIETY OF LIFE

• Biodiversity–The Variety Of Life

2. Disturbance Regimes

3. Cumulative Effects, Lag Periods, And Thresholds

4. Biological Diversity

5. Genetic Diversity

6. Functional Diversity

7. Nature’s Services–Ecological Wealth Across Generations


BIODIVERSITY—OUR ENVIRONMENTAL INSURANCE POLICY

• Earth Before Oxygen

• The Advent of Oxygen

• The Long, Slow Path To Life As We Know It

• From Whence Comes Today’s Biodiversity?

• What—Exactly—Is Biodiversity?

• Biodiversity—Our Social-Environmental Insurance Policy

• Endangering Our Environmental Insurance Policy


• Soil–The Great Placenta

• Air–The Breath Of Life

• Water–A Captive Of Gravity

 


ENDNOTES

  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 C. M. D’Antonio and (2) 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.


Sunrise—a magnificent symbol of the commons and a new today.


Text and Photos © by Chris Maser, 2009. All rights reserved.

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If you want to contact me, you can visit my website. If you wish, you can also read an article about what is important to me and/or you can listen to me give a presentation.



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