So what kind of re-creation will benefit us today and the children of tomorrow—and why? This is at once an intelligent, compound question and a wise one because it’s both present- and future-oriented. Moreover, it raises another interesting question: If what we do is not ecological restoration, what is it?
A simple example of repairing something is mending a hole in a sock, a lesson my mother taught me over fifty years ago. To mend a sock, she had three items: a wooden darning egg (although an old-fashioned light bulb also works), a darning needle, and darning thread. With patience and dexterity, she wove the thread back and forth across the hole. Then, she turned the sock around far enough to weave the thread through the existing strands until the hole was repaired in a neat crosshatch. At this point, the mended portion of the sock was often stronger than the original fabric had been, which meant it took me longer to wear it out a second time. The sock was repaired, but not restored to its original condition. Its physical structure, however, was mended in a way that allowed the sock to continue functioning as a sock.
Another, more complicated illustration is a woman who suffers cardiac arrest and is “brought back to life.” Clearly, she has been physically altered by the episode and psychologically changed by her nearness to death. Therefore, although a medical team can revive her, it cannot restore her to the condition she was in prior to her trauma. Even when a doctor performs a successful triple-bypass surgery, the functionality of the patient’s system is repaired by a surgical creation, although the system itself—and thus the person’s makeup—is different. Nevertheless, the system may function in a nearly normal condition for some years.
It is the same with ecosystems. We repair dislocated or otherwise broken parts by creating an “ecological bypass” in order to maintain the integrity of their processes. In so doing, we generate something other than what existed before. Let’s take the North American prairie as a case in point. For nearly 20 million years, an unbroken swath of grassland 1,000 miles wide stretched from northern Canada, through the midsection of the continent, to Mexico. Then, in less than a century after John Deere invented his steel-bladed plow in 1837, the American prairie all but disappeared. Today, only a fraction is left. The mixed and shortgrass prairie of the Plains states represents about 5% of the ecosystem’s original extent, whereas less than 1% of the lush tallgrass prairie remains to the east. Most of the tallgrass prairie occurs as remnants in pioneer cemeteries and along old railroad rights-of-way.
Now the question becomes one of purpose: Why do you want to repair a patch of prairie? Schlesinger offers sound counsel:
We must remember that we live in an integrated chemical system that spans only a thin “peel” about [12 miles] thick on the surface of planet Earth. How we manage that arena will determine the persistence and quality of life for every one of the species that now inhabit this planet with us. Many species will disappear; others will proliferate globally, bringing huge changes to the ecosystem functions that we have long regarded as “normal.” Like it on not, Homo sapiens will be the supervisor of this arena. We can manage it well, manage it poorly, or through purposeful actions of terrorism and war, we can poison Eden. The chemistry of the arena of life—that is Earth’s biogeochemistry—will be at the center of how well we do.1
A biologically sustainable ecosystem is a prerequisite for a biologically sustainable yield of the broad array of Nature’s interactive services and products on which our way of life depends. In turn, a biologically sustainable yield is a prerequisite for economically sustainable communities, which are a prerequisite for overall social-environmental sustainability. In addition, mended ecosystems could go a long way in counteracting global warming and the extreme weather it fosters.
For most people, however, the purpose of mending an ecosystem is to recover its vegetative beauty (the aesthetics of its biodiversity), which is at once the most notable and visual aspect whereby people connect with it. For them, the composition of species framed in a snapshot is important, particularly those in which they find pleasure, such as prairie flowers.
For people interested in native birds, however, the most important aspect of the mending process lies beyond mere plant-species composition. In this case, the composite structure of the ecosystem is critical because that structure both attracts the birds and serves them as habitat for feeding, reproduction, or both. The same can be said of people interested in butterflies, reptiles, small mammals, or any other group of organisms. In still other circumstances, someone may be concerned about the system’s ability to capture and store water or in making a reinvestment of biological capital as a means of increasing soil fertility.
Each of these approaches is focused on retrieving a selected part of the prairie in an attempt to recapture an aesthetic snapshot, to repair a certain structural condition, or to maintain—and perhaps enhance—a process or function. In this sense, the outcome can be viewed as a commodity or amenity. Whatever the reason, each outcome will be different.
Ecosystems go through what is called “autogenic succession,” which means self-generating or self-induced succession. An example is the characteristic developmental stages a grassland or forest undergoes from bare soil to another grassland or forest—both above ground and below ground.
In considering how autogenic succession functions, a caveat needs to be introduced to counter the notion of discrete stages replacing one another in a predetermined, orderly fashion. Rather than being distinct, the developmental stages form a complex continuum wherein each builds on the dynamics and biophysical nuances of the preceding one. Hence, no two areas ever develop in an identical fashion.
In addition to the relatively orderliness of autogenic succession, all living systems are visited by disturbance regimes, those events that stir the pot, as it were, and keep the systems healthy. A disturbance is any relatively discrete event that disrupts the structure of a population or community of plants and animals, that disrupts an ecosystem as a whole and thereby changes the availability of resources, that restructures the physical environment, or that causes all three kinds of changes.
A simple example is drifting trees in the ocean, which play an ecological role in structuring the biological communities of the outer shores, where rock-dwelling plants and animals compete for space on which to attach themselves. During the high-winter tides, drifting trees batter intertidal communities with sufficient force to smash the plants and animals against the rocks, thereby maintaining species richness through the creation of unoccupied spaces and the consequential flux in developmental stages.
The role driftwood plays in the processes of biotic communities on rocky shores is proportional to its abundance and size. How often an area is battered by large driftwood depends on its location. In a sheltered cove, for example, where such influences are absent, the dominant competitors gradually exclude the subordinate ones, thereby decreasing species diversity, which weakens the community as a whole.2
Ecological disturbances, which can range from a grazing cow or bison to a small grass fire or a major hurricane, are characterized by their distribution in space and the size of disturbance they make, as well as their frequency, duration, intensity, severity, synergism, and predictability. Physical features and patterns of vegetation often control terrestrial disturbances, such as those affecting prairies. The variability of each disturbance, along with the area’s previous history and its particular soil, creates the existing vegetation mosaic.
Human-introduced disturbances, especially fragmentation of habitat, impose stresses with which an ecosystem is ill-adapted to cope. Not surprisingly, biogeographical studies show that connectivity of habitats within a landscape is of prime importance to the persistence of plants and animals in viable numbers in their respective habitats—again, a matter of biodiversity. In this sense, the landscape must be considered a mosaic of interconnected patches of habitats, like vegetated fencerows, which act as corridors or routes of travel between and among patches of suitable habitat.3
Whether populations of plants and animals survive in a particular landscape depends on the rate of local extinctions from an area of habitat and the rate with which an organism can move among existing patches. Species living in habitats isolated as a result of fragmentation are less likely to persist than are those living in connected habitats. Although this is generally true, there are some caveats for both plants and animals.
For example, many species of plants—as opposed only those with small populations—may be vulnerable to genetic erosion, which is the loss of genetic diversity as a result of ongoing fragmentation processes. Because every organism has many unique genes, genetic erosion occurs whenever the gene pool of a species of plant or animal is diminished as individuals die without getting a chance to breed with others in their population. In this way, stored genetic variability is lost, which diminishes the remaining gene pool. This genetic loss is compounded when the plant in question is capable of self-fertilization or mating among related individuals, as opposed to real obligate crossbreeding. Therefore, many fragmented habitats are unable to support plant populations large enough to maintain genetic viability over time.4 Clearly, modifying the existing connectivity among patches of habitat strongly influences the abundance of species and their patterns of movement.
The size, shape, and diversity of patches also influence the patterns of species abundance, and the shape of a patch may determine which species can use it as habitat. Conventional wisdom dictates that habitat fragmentation causes local extinctions of animal populations by decreasing the amount of viable, interior (core) habitat, while simultaneously increasing the effects of the habitat’s edge. It is also widely accepted that interior-dwelling species are better off in bigger fragments because they have a larger amount of suitable habitat. Nevertheless, fragmented habitats in real landscapes have complex, irregular shapes. Irregularly shaped forest remnants in New Zealand, for example, consistently had populations of interior-dwelling species that were reduced by 10 to 100% as a function of the edge effects’ influence on the species. In addition, species within a given habitat fragment tended to exist in small, disjunct populations.5
The interaction between the dispersal of a species and the pattern of a landscape determines the temporal dynamics of its population. Local populations of wide-ranging organisms may not be as strongly affected by the spatial arrangement of habitat patches as are more sedentary species. It is, after all, the relationship of pattern that confers stability on ecosystems—not the relationship of numbers. Stability flows from the relationships that have evolved among the various species. A stable, culturally oriented system, even a diverse one that fails to support these ecologically co-evolved relationships has little chance of being sustainable. Here, it must be understood that plant-species composition is the determiner of structure and function, in that composition is the cause of structure and function rather than the effect.
Until the nineteenth century, when both bison and prairies were all but extirpated, bison ranged across the vast grasslands by the tens of millions. Prior to that time, fire and grazing by bison were the two factors that shaped this great midwestern ecosystem. Fires, started by lightening and indigenous Americans, attracted bison and other herbivores to patches of tender grasses, which they preferred as food, and so they avoided the tender broad-leaved forbs in the same area. In other words, bison grazed heavily on burned areas and avoided other acres altogether. These habitat patches were constantly moving, however, as fires came and went.6
This kind of selective grazing not only kept the grasses in check but also caused the chemical structures in plants to diversify. Because herbivores often fed on chemically similar plants, they imposed selective pressures on the plants to diverge or biased the assemblage of plants within a community toward chemical divergence. As specialization increased and the size of the area in which the plants grew decreased, plant communities tended to be more chemically dissimilar. At fairly local scales and where herbivores had a strong one-to-one interaction with plants, communities had a robust pattern of chemical disparity.
Coexisting herbivores feed selectively in yet another way. The mechanisms employed by coexisting, generalist herbivores can be thought of as complementary diversity. A mainstay of ecological theory is that coexisting species use different resources or, if they use the same resource, they use it differently. Such coexistence is usually attributed to a few species that operate within a fairly narrow latitude.7 But what happens when multiple species of closely related herbivores share critical resources too generally? Species-specific nutritional niches provide a means whereby generalist herbivores might coexist despite broadly overlapping diets.
A study of seven closely related grasshoppers demonstrated that they eat proteins and carbohydrates in different absolute amounts and ratios even from the same species of plants. The grasshoppers’ regulation of their protein-carbohydrate intake elucidates the active nature of dietary selection to achieve balanced diets that provide a buffer in the face of variable food quality.8
Taken all together, the selectivity of indigenous grazers kept the North American prairie habitat in a healthy mix of biodiversity among the various species of grasses, forbs, insects, amphibians, reptiles, birds, and mammals. Some birds, for example, focused on severely disturbed areas, others on largely undisturbed sites, whereas still others, such as prairie chickens, required a mix of habitats.
Ultimately, fire determined the initial landscape-scale pattern and then rearranged its configuration by controlling the grazing behavior of the bison. This dual set of disturbances created a habitat characterized by dynamic vegetation mosaics of various scales and duration over space and time. Allen Knapp, a plant ecologist at Colorado State University in Fort Collins, put it this way: “We have a romantic snapshot view of the prairie when Europeans settled it. But ecological systems are always dynamic, always changing.”9
Therefore, it’s critical that we become thoroughly aware of the silent, often hidden, values of a healthy environment and the huge cost of repairing any of its functions because such repair is all but impossible without losing still other hidden functions. Furthermore, it’s imperative to understand that whenever we view an ecosystem, it’s always in the present moment; this moment—the here and now, which is all we ever have. Patience with Nature’s timetable is thus a critical consideration because healing an ecosystem is a moment-by-moment, day-by-day endeavor in which the visible outcome of one’s labor may be weeks, months, or even years away.
1. William H. Schlesinger. Global Change Ecology. Trends in Ecology & Evolution, 21 (2006):348–351.
2. Chris Maser and James R. Sedell. From the Forest to the Sea: The Ecology of Wood in Streams, Rivers, Estuaries, and Oceans St. Lucie Press, Delray Beach, FL.1994.
3. (1) David J. Rapport, H. A. Regier, and T. C. Hutchinson. Ecosystem Behavior under Stress. American Naturalist, 125 (1985):617–640; (2) David J. Rapport. What Constitutes Ecosystem Health? Perspectives in Biology and Medicine, 33 (1989):120–132; and (3) Monica G. Turner. Landscape Ecology: The Effect of Pattern on Process. Annual Review of Ecological Systems, 20 (1989):171–197.
4. Olivier Honnay and Hans Jacquemyn. Susceptibility of Common and Rare Plant Species to the Genetic Consequences of Habitat Fragmentation. Conservation Biology, 21 (2007):823–831.
5. Robert M. Ewers and Raphael K. Didham. The Effect of Fragment Shape and Species’ Sensitivity to Habitat Edges on Animal Population Size. Conservation Biology, 21 (2007):926–936.
6. Leslie Allen. Prairie Revival: Researchers Put Restoration to the Test. Science News, 172 (2007):376–377.
7. Judith X. Becerra. The Impact of Herbivore-Plant Coevolution on Plant Community Structure. Proceedings of the National Academy of Sciences, 104 (2007):7483–7488.
8. Spencer T. Behmer and Anthony Joern. Coexisting Generalist Herbivores Occupy Unique Nutritional Feeding Niches. Proceedings of the National Academy of Sciences, 105 (2008):1977–1982.
9. Quoted in: Leslie Allen. Prairie Revival: Researchers Put Restoration to the Test. Science News, 172 (2007):376–377.
Text © by Chris Maser 2012. All rights reserved.