Chapter 6: Principles of ecology
CONTROL OF FIRE DISRUPTS NATURAL PROCESSES
The policy of suppressing forest fires in the western United States, which started around the beginning of the 20th century, illustrates how ecological systems can have unexpected and irreversible responses to human alterations of physical processes. The trees of many western forests are fully adapted to experiencing regular fires in their environment (Barbour et al. 1993). The large older trees have thick bark that makes them resistant to all but the largest fires, and many species cannot even reproduce without fire because the cones will not open unless they are heated to high temperatures. The montane forests of the Sierra Nevada, including the Giant Sequoia forests, are examples of these fire-adapted ecosystems. Under pre-European conditions there were frequent small fires in these forests, and the forests consisted largely of large, mature trees and an open, grassy understory. It was easy to walk through these forests, and both lightning and Native Americans often set small fires that helped keep the forests in this state.
Early European-American forest managers did not understand the role of fire in these ecosystems, and saw it as detrimental to both the forests and the wildlife. They instituted the policy of fire suppression, and as a result the open under-stories began to fill in with smaller trees that would not have been able to survive small fires. This diminished the diversity of wildlife because the open grassy understory was not available to deer and other species that need such habitat. After a few decades of fire suppression there were so many small trees in the forests that there was a high fuel load. In addition, these smaller trees provided a staircase by which fire could get up high enough to reach the crown of the large, mature trees in the forests, which were otherwise immune to smaller fires. In essence, the policy of fire suppression set the stage for enormous raging fires that can destroy every tree in the forest, and surrounding human developments as well. The late 20th and early 21st centuries have seen enormous forest fires throughout the western US that cost extraordinary sums to suppress. It may be quite difficult to return unburned forests to their previous fire regimes of small frequent fires because of the fuel loads that have built up. Indeed, ecological dynamics in these forests have been shifted towards less frequent, high intensity large fires instead of more frequent, low intensity small fires by decades of fires suppression. It addition, the diversity of tree species has declined as the forests shift towards increased dominance by shade-tolerant species such as white fir.
Clearing out the small understory trees might help return these forests to their previous state, but this is expensive because small trees have little economic value. The large, old trees are economically valuable, however, so unfortunately fire risk is increasingly used as an excuse to log large trees in the name of fuel load reduction on both public and private land. Logging the large trees results not only in a major loss of wildlife habitat, but it may actually make the forests less fire resistant in the long run because it is the most fire-resistant trees that are removed. Greater intensity and frequency of forest fires also is predicted to occur with global climate change. Due to changing fire regimes it is unlikely that the forests and wildlife of the western US will ever be what they were before intervention by European-Americans, even in protected areas like national parks.
EMERGENT PROPERTIES AND ECOLOGICAL PROCESSES
The concept of emergent properties leads us to some important insights into the scientific investigation of ecological systems. It is readily apparent that cells are composed of molecules, organisms are composed of cells, populations are composed of organisms, communities are composed of populations of species, ecosystems encompass many interacting communities, and the biosphere is composed of all the ecosystems on Earth. Hence each scale entirely includes the units of the previous scale. But are the properties at each scale predictable from an understanding of dynamics at lower scales? The concept of emergent principles emphasizes that while each higher level may be composed entirely of units from the next lower scale, it possesses distinct properties that are unique to that level, and that cannot be entirely explained by processes at the lower level.
Populations are composed of individual organisms. Part of the dynamics of a population may be explained by understanding the physiology of individual organisms, the next scale down. For example, a long cold winter with lots of snow in the Sierra Nevada can keep lakes covered with ice and snow much longer than usual, preventing light from penetrating and allowing the growth of algae. As a result the trout that live in these lakes may use up all the oxygen in the water and die. However, many of the dynamics commonly observed in populations, such as irruptions, crashes, cycles, and constant densities in a highly variable environment, can only be explained from the interactions between organisms and their larger environment. For example, territorial behavior will often stabilize wildlife populations because it spreads animals, at least in theory, more evenly across the landscape. Many birds and mammals are territorial, particularly carnivorous species. The marten is one such territorial carnivore, an arboreal (tree-climbing) member of the weasel family that lives in the coniferous forests of Canada and the mountainous western United States, where it specializes in hunting squirrels and other small mammals. A marten will aggressively attack another marten that enters its territory, especially when prey densities are low. Individual martens that do not have territories have much lower survival and reproduction rates, because they are much more likely to starve or get attacked by predators. The marten populations of Algonquin Park, Ontario were observed to be surprising constant through time and this stability is probably in large part due to the territoriality of this species (Fryxell el al. 1999). The fact that these population dynamics emerged from population level processes illustrates the idea of emergent properties.
The concept of emergent properties means that many important properties at higher scales of biological organization cannot be entirely explained by understanding lower scales; in other words, "the whole is more than the sum of its parts." Of course, often times much can be explained by looking to the next lower scale of biological organization. The concept of emergent properties may be especially important in ecology because ecological phenomena occur on the scale larger than the predominant scale of natural selection, and as such interactions between organisms and emergent processes may play a larger role. The challenge to biologists is to look for causality at both lower levels and in properties unique to the scale of observation, recognizing emergent properties that occur in the nested hierarchy of biological systems.
REDUCTIONIST VS. HOLISTIC SCIENCE
The concept of emergent properties integrates two different approaches to science. Reductionism seeks to understand phenomena by "reducing" them to their parts, essentially looking for explanation at the lowest scales of organization. This is the traditional approach of Western science, and it has lead to some breathtakingly impressive explanations for numerous phenomena. Physics and chemistry are largely reductionist sciences, and reductionism is generally the main approach in molecular and cell biology. The alternative scientific approach has traditionally been called holistic science, a term which suggests the idea that "the whole is larger than the sum of its parts." Holistic science looks for explanation at the same or larger scale than the phenomenon in question. Unfortunately the term "holistic" is also used by many by non-scientists to indicate all kinds of "fuzzy" thinking and pursuits that have very little to do with the highly technical scientific search for principles of causality from higher organizational scales. Scientific understanding of holistic causality and emergent phenomena is a new approach in contrast to reductionism, and as such is not as well-developed as a method. The cutting edge of theoretical research in this area can be quite complex and quantitative, involving teams of mathematicians and scientists and large computing facilities, very remote from the birds singing in mountain forests.
Holistic scientific explanations are particularly important to include in ecology because ecological systems provide the larger scale context in which many biological processes occur, and hence serve as the basis for the holistic explanations of many phenomena at organismal and lower scales. In addition emergent phenomena, which arise from interactions among organisms, likely are quite important within ecological systems. Thus the structure observed in discrete assemblage of animals is the result of interactions (predation, competition, disease, symbiosis) among them on both long-term (evolutionary) and short-term (ecological) scales. The emergent property often is an apparently stable (persistent through time) community in which each member has a distinct niche with little overlap with other community members (see Chapter 7). In California streams, for example, two small fishes that live in fast water, speckled dace and riffle sculpin, live in different parts of the stream and feed on different invertebrates. When sculpins are removed, however, the dace take over the places where sculpin lived previously and develop a broader diet. They are constrained by the aggressive behavior of sculpin who drive them away from the prime habitat. The pattern of segregation, however, is predominant in most streams.
It is important to emphasize again that both reductionist and holistic explanations are important to biology and ecology because the causes of scientific phenomena can occur at both smaller and larger scales. The degree to which either holism or reductionism is sufficient to explain something may be dependent upon the system in question, and for some questions reductionism may be all that is needed. However, the nature of ecological systems often demand looking for explanation at both larger and smaller scales, as illustrated in the kangaroo rat example on the next page.
Table of Contents
1. Roots of the modern environmental dilemma: A brief history of the relationship between humans and wildlife
2. A history of wildlife in North America
3. Climatic determinants of global patterns of biodiversity
5. Natural selection
6. Principles of ecology
7. Niche and habitat
8. Conservation biology
9. Conservation in the USA: legislative milestones
10. Alien invaders
11. Wildlife and Pollution
12. What you can do to save wildlife
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