Chapter 6: Principles of ecology
PRINCIPLES OF ECOLOGY
Conservation of wildlife requires an understanding of ecology, the science devoted to study of the interactions among organisms and their environment. Ecology is defined as "the study of the abundance and distribution of organisms" (Begon, Harper and Townsend 1996). Understanding why animals thrive where they do requires an intimate knowledge of both the organisms and they environment in which they live, including other organisms. Ecology is a subdiscipline of biology, the scientific study of life. Biology is a broad discipline that spans many levels of organization, from molecules to ecosystems. In this chapter we present the broad principles that are incorporated into ecological thinking, or, as Aldo Leopold so gracefully stated, into "thinking like a mountain."
THE IMPORTANCE OF SCALE IN BIOLOGY
Traditionally, ecology focuses on the larger scales in biology, from the individual organism through populations, communities, ecosystems, and the biosphere (Box 6.1).
- Populations are interbreeding groups of individuals of the same species, generally living in the same contiguous habitat.
- Communities are interacting populations of different species.
- Ecosystems are comprised of both the biotic (living) and abiotic (non-living) factors in a given area; they contain both the broad biological community and all the physical processes (such as weather, soil, hydrology, nutrients, energy flow etc.) that influence that community.
- The biosphere is global in scale, and includes all the biological and physical processes that allow for and influence life on Earth.
- Higher scales of organization contain smaller scales (e.g., a given community contains populations of various species), yet all scales also possess properties that are unique to that scale and that cannot be deduced from properties of included scales. For example, the nature and strength of interactions among two species of meadow mice cannot be fully explained even with the most sophisticated models of their population dynamics. Thus, many interactions at the community level are emergent properties of communities.
In contrast, molecular and cell biology studies scales smaller than the individual organism. The great difference in the scale of the subject matter of these sub-disciplines of biology has resulted in large differences in the study methods, ways of thinking, and disciplinary culture of these two sub-disciplines. In fact, these sub-disciplines are now essentially treated as different fields at most major universities, with different departments, undergraduate majors, and faculty who do not read the same literature or go to the same conferences. As a result a dichotomy has been growing within biology. There are also biologists who work at the individual organism scale, studying topics like anatomy, physiology, and behavior, but even many of these "organismal" biologists will tend toward either the broader-scale ecological way of thinking or the smaller-scale molecular/cellular approach.
This leads us to ask the following questions. Why does a divide seem to occur in biology at the scale of the individual organism? Why are the biological processes, and the ways of studying them, so radically different at larger, ecological scales vs. smaller, molecular and cellular scales? What happens at the scale of the individual organism that is so crucial to biology, and how might it explain the seemingly dichotomous nature of modern biology?
Natural selection is the driving force of adaptation and evolution, and it is such an important topic in biology that we have devoted an entire chapter to it (Chapter 5). Biologists generally agree that natural selection occurs predominantly on the scale of the individual organism (Williams 1966), although some argue that it can theoretically occur at other scales as well (Wilson 1980). Each individual organism acts to maximize its own survival and ability to produce offspring that are in turn able to survive and reproduce (called fitness by biologists), even at the expense of other organisms of the same species. Thus, natural selection means that those heritable traits that increase the fitness of an individual organism have a greater probability of being present in future generations within the population. The fact that selection is nearly always strongest on the scale of the individual organism has important ramifications for understanding ecology (Levin 2002). At scales smaller than the individual organisms, the units (cells, genes) are inextricably dependent upon each other for survival. As a result, cooperation among these units is high, and overall they work together for the "good" of the whole organism. However, it is not necessarily easy to keep these units working together, and just how the conflicts among genes and cell lineages were resolved so that multi-cellular organisms could evolve is a fascinating question in evolutionary biology. In fact, cancer is one instance where cells increase their own rate of replication at the expense of the other cells, even though that results in the demise of the entire organism. Our immune systems keep this from happening most of the time, removing those cells that are not working for the good of the whole, yet still 24% of Americans get cancer at some point in their lives (Masters 1996).
In contrast, at scales larger than that of the individual organism, the units generally do not work together for the good of the whole. Each individual organism is out to maximize its own individual survival and reproduction, even if it does not benefit the population, community, or ecosystem. The amazing thing is that in spite of the fact that essentially all the sub-units of ecological systems are behaving in a selfish "cancerous" way, populations and communities continue to thrive and ecosystem processes continue to exist. The conditions for life are not regulated for the good of the whole ecosystem, but rather the persistence of life emerges from the interactions between organisms and the environment. This may sound brutal, and in many respects it is. In natural systems the vast majority of organisms die before reproducing, usually being eaten by other organisms or starving to death. Despite this, organisms do things to benefit each other, although they do so mainly if it also benefits them as individuals or does not harm them in any way. This is in direct contrast to many popular versions of ecological systems, which often describe communities and ecosystems as a "balanced" assemblage of organisms working together to maintain life. The old "balance of nature" paradigm basically assumed each ecosystem had some ideal state that it could maintain indefinitely, at least in the absence of humans. We now realize that even in the absence of human influence, ecosystems are constantly changing in response to changes in the environment (e.g., climate change caused by volcanic eruptions, big floods on rivers, fires in forests) and the organisms that live there (evolution). In fact, if there is a new paradigm for ecology it is that "the only constant is change." Certainly, the fossil history of life provides good evidence of this!
Despite the ever-changing nature of ecological systems, all organisms and species within an ecosystem are dependent upon other life forms for their existence. It is clear that predators, such as wolves, are dependent upon their herbivore prey, such as elk and moose, for existence. It is also readily apparent that elk and moose are dependent upon the plant species they eat. Those plants are in turn dependent upon the microorganisms that form soil, and often on symbiotic fungi in their roots, called mycorrhizae, to obtain water and nutrients. Without the mycorrhizae there quite likely would be no wolves! The wolves themselves also have indirect effects that benefit other species. For instance, ecologists have observed that where wolves are present in the Greater Yellowstone Ecosystem of the northern Rocky Mountains, there is greater abundance and diversity of songbirds (Berger et al. 2001). This is because when wolves are present there are fewer moose present in the tree-lined riparian zones of streams. Moose are voracious herbivores, and they can lower plant diversity in the riparian zone by over-grazing preferred species of trees and shrubs. When wolves are present the moose densities go down, plant diversity goes up, and as a result the diversity of rare songbirds that are dependent upon the riparian habitat also increases. The interdependence of species in ecological communities is also illustrated by the example of beaver creating habitat for dozens of other species and increasing ecosystem productivity as described in Chapter 2. Even the oxygen in the atmosphere that all animals require accumulated from the photosynthetic action of algae in the oceans over the millennia.
All of these ecological interdependencies are crucial to the persistence of wildlife, and all other forms of multicellular life on earth. However, the fact that life is dependent upon interactions in ecological systems in which the organisms are not acting for the benefit of the whole gives ecological systems some distinct, and disturbing (to us), characteristics. Thus, alteration of ecological systems is often irreversible (at least within the time frame of human lives), and in general the exact response of ecological systems to perturbations is difficult to predict. Humans accelerate the processes of ecosystem change and push them in unanticipated directions, particularly through alteration of physical processes. This change often has undesirable consequences for humans, which is why we need to take such care to protect ecosystems and biodiversity. Scientific investigation of ecosystem properties can be challenging, but ecologists are making significant progress in better understanding life at larger scales of organization.
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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