Ecology

Ecology is the interdisciplinary scientific study of the distributions, abundance and relations of organisms and their interactions with the environment. Ecology is also the study of ecosystems. Ecosystems describe the web or network of relations among organisms at different scales of organization. Since ecology refers to any form of biodiversity, ecologists research everything from tiny bacteria's role in nutrient recycling to the effects of tropical rain forest on the Earth's atmosphere. The discipline of ecology emerged from the natural sciences in the late 19th century. Ecology is not synonymous with environment, environmentalism, or environmental science. Ecology is closely related to the disciplines of physiology, evolution, genetics and behavior.

Like many of the natural sciences, a conceptual understanding of ecology is found in the broader details of study, including:

• life processes explaining adaptations
• distribution and abundance of organisms
• the movement of materials and energy through living communities
• the successional development of ecosystems, and
• the abundance and distribution of biodiversity in context of the environment.

Ecology is distinguished from natural history, which deals primarily with the descriptive study of organisms. It is a sub-discipline of biology, which is the study of life.

There are many practical applications of ecology in conservation biology, wetland management, natural resource management (agriculture, forestry, fisheries), city planning (urban ecology), community health, economics, basic & applied science and it provides a conceptual framework for understanding and researching human social interaction (human ecology).

[edit] Levels of organization and study

Ecosystems regenerate after a disturbance such as fire, forming mosaics of different age groups structured across a landscape. Pictured are different seral stages in forested ecosystems starting from pioneers colonizing a disturbed site and maturing in successional stages leading to old-growth forests.

Because ecology deals with ever-changing ecosystems, both time and space must be considered when describing ecological phenomena. In regards to time, it can take thousands of years for ecological processes to mature. The life-span of a tree, for example, can include different successional or seral stages leading to mature old-growth forests. The ecological process is extended even further through time as trees topple over and decay.

Ecosystems are also classified at different spatial scales: the area of an ecosystem can vary greatly from tiny to vast. For instance, several generations of an aphid population and their predators might exist on a single leaf. Inside each of those aphids exist diverse communities of bacteria. The scale of study must at times be quite large, when studying the life of the tree in the forest where bacteria and aphids live. To understand tree growth, for example, soil type, moisture content, slope of the land, forest canopy closure, and other local site variables must all be examined; to understand the ecology of the forest, complex global factors such as climate must be considered.

Long-term ecological studies provide important track records to better understand ecosystems over space and time. The International Long Term Ecological Network manages and exchanges scientific information among research sites. The longest experiment in existence is the Park Grass Experiment that was initiated in 1856. Another example includes the Hubbard Brook study in operation since 1960. Ecology is also complicated by the fact that small scale patterns do not necessarily explain large scale phenomena, otherwise captured in the expression 'the sum is greater than the parts'. These emergent phenomena operate at different environmental scales of influence, ranging from molecular to planetary scales, and require different sets of scientific explanation.

To structure the study of ecology into a manageable framework of understanding, the biological world is conceptually organized as a nested hierarchy of organization, ranging in scale from genes, to cells, to tissues, to organs, to organisms, to species and up to the level of the biosphere. Ecosystems are primarily researched at (but not restricted to) three key levels of organization, including organisms, populations, and communities. Ecologists study ecosystems by sampling a certain number of individuals that are representative of a population. Ecosystems consist of communities interacting with each other and the environment. In ecology, communities are created by the interaction of the populations of different species in an area.

Biodiversity (portmanteau of the words biological diversity) describes all varieties of life from genes to ecosystems and spans every level of biological organization. There are many ways to index, measure, and represent biodiversity. Biodiversity includes species diversity, ecosystem diversity, genetic diversity and the complex processes operating at and among these respective levels. Biodiversity plays an important role in ecological health as much as it does for human health. Preventing or prioritizing species extinctions is one way to preserve biodiversity, but populations, the genetic diversity within them and ecological processes, such as migration, are being threatened on global scales and disappearing rapidly as well. Conservation priorities and management techniques require different approaches and considerations to address the full ecological scope of biodiversity. Populations and species migration, for example, are more sensitive indicators of ecosystem services that sustain and contribute natural capital toward the well-being of humanity. An understanding of biodiversity has practical application for ecosystem-based conservation planners as they make ecologically responsible decisions in management recommendations to consultant firms, governments and industry.

[edit] Ecological niche

Termite mounds with varied heights of chimneys regulate gas exchange, temperature and other environmental parameters that are needed to sustain the internal physiology of the entire colony.

The ecological niche is a central concept in the ecology of organisms. There are many definitions of the niche dating back to 1917, but George Evelyn Hutchinson made conceptual advances in 1957 and introduced the most widely accepted definition: "The niche is the set of biotic and abiotic conditions in which a species is able to persist and maintain stable population sizes." The ecological niche is divided into the fundamental and the realized niche. The fundamental niche is the set of environmental conditions under which a species is able to persist. The realized niche is the set of environmental plus ecological conditions under which a species is able to persist. Organisms have functional traits that are uniquely adapted to the ecological niche. A trait is a measurable property of an organism that strongly influences its performance. Biogeographical patterns and range distributions are explained or predicted through knowledge and understanding of a species niche requirements. For example, the uniquely adapted nature of each species to their ecological niche means that they are able to competitively exclude other similarly adapted species from having an overlapping geographic range. This is called the competitive exclusion principle. Important to the concept of niche is habitat. The habitat describes the environment over which a species is known to occur and the type of community that is formed as a result. For example, habitat might refer to an aquatic or terrestrial environment that can be further categorized as montane or alpine ecosystems.

Biodiversity of a coral reef. Corals adapt and modify their environment by forming calcium carbonate skeletons that provide growing conditions for future generations and form habitat for many other species.

Organisms are subject to environmental pressures, but they are also modifiers of their habitats. The regulatory feedback between organisms and their environment can modify conditions from local (e.g., a pond) to global scales (e.g., Gaia) and over time and even after death, such as decaying logs or silica skeleton deposits from marine organisms. This process of ecosystem engineering has also been called niche construction. Ecosystem engineers are defined as: "...organisms that directly or indirectly modulate the availability of resources to other species, by causing physical state changes in biotic or abiotic materials. In so doing they modify, maintain and create habitats."

The ecological engineering concept has stimulated a new appreciation for the degree of influence that organisms have on the ecosystem and evolutionary process. The niche construction concept highlights a previously underappreciated feedback mechanism of natural selection imparting forces on the abiotic niche. An example of natural selection through ecosystem engineering occurs in the nests of social insects, including ants, bees, wasps, and termites. There is an emergent homeostasis in the structure of the nest that regulates, maintains and defends the physiology of the entire colony. Termite mounds, for example, maintain a constant internal temperature through the design of air-conditioning chimneys. The structure of the nests themselves are subject to the forces of natural selection. Moreover, the nest can survive over successive generations, which means that ancestors inherit both genetic material and a legacy niche that was constructed before their time.

[edit] Population ecology

The population is the unit of analysis in population ecology. A population consists of individuals of the same species that live, interact and migrate through the same niche and habitat. A primary law of population ecology is the Malthusian growth model. This law states that:

"...a population will grow (or decline) exponentially as long as the environment experienced by all individuals in the population remains constant."

This Malthusian premise provides the basis for formulating predictive theories and tests that follow. Simplified population models usually start with four variables including death, birth, immigration, and emigration. Mathematical models are used to calculate changes in population demographics using a null model. A null model is used as a null hypothesis for statistical testing. The null hypothesis states that random processes create observed patterns. Alternatively the patterns differ significantly from the random model and require further explanation. Models can be mathematically complex where "...several competing hypotheses are simultaneously confronted with the data." An example of an introductory population model describes a closed population, such as on an island, where immigration and emigration does not take place. In these island models the per capita rates of change are described as:

dN / dT = B − D = bN − dN = (b − d)N = rN,

where N is the total number of individuals in the population, B is the number of births, D is the number of deaths, b and d are the per capita rates of birth and death respectively, and r is the per capita rate of population change. This formula can be read out as the rate of change in the population (dN/dT) is equal to births minus deaths (B – D).

Using these modelling techniques, Malthus' population principle of growth was later transformed into a model known as the logistic equation:

dN / dT = aN(1 − N / K),

where N is the number of individuals measured as biomass density, a is the maximum per-capita rate of change, and K is the carrying capacity of the population. The formula can be read as follows: the rate of change in the population (dN/dT) is equal to growth (aN) that is limited by carrying capacity (1 – N/K). The discipline of population ecology builds upon these introductory models to further understand demographic processes in real study populations and conduct statistical tests. The field of population ecology often uses data on life history and matrix algebra to develop projection matrices on fecundity and survivorship. This information is used for managing wildlife stocks and setting harvest quotas.

[edit] r/K-Selection theory

An important concept in population ecology is r/K-selection theory. It was introduced in 1967 in a book entitled The Theory of Island Biogeography and was one of the first predictive models to explain life-history evolution. The premise behind this model is that forces of natural selection change according to the density of the population. When an island is first colonized, the density of individuals is low and the population size increases with reduced levels of competition and an abundance of available resources. Under such circumstances a population experiences density independent forces of natural selection, which is called r-selection. When the population becomes crowded, it reaches the island's carrying capacity, and individuals compete more heavily for limited resources. Under crowded conditions the population experiences density-dependent forces of natural selection, called K-selection.

In the r/K-selection model, the first variable r is the intrinsic rate of natural increase in population size and the second variable K is the carrying capacity of a population. Different species evolve different life-history strategies spanning a continuum between these two selective forces. An r-selected species is one that has high birth rates, low levels of parental investment, and high rates of mortality before individuals reach maturity. Evolution favors high rates of fecundity in r-selected species. Many kinds of insects and invasive species exhibit r-selected characteristics. In contrast, a K-selected species has low rates of fecundity, high levels of parental investment in the young, and low rates of mortality as individuals mature. Humans and elephants are examples of species exhibiting K-selected characteristics, including longevity and efficiency in the conversion of more resources into fewer offspring.

[edit] Metapopulation ecology

Populations are also studied and conceptualized through the metapopulation concept. The metapopulation concept was introduced in 1969 "as a population of populations which go extinct locally and recolonize." Metapopulation ecology is another statistical approach that is often used in conservation research. Metapopulation research simplifies the landscape into patches of varying levels of quality. Like the r/K-selection model, metapopulation models have also been used to explain life-history evolution, such as the ecological stability of amphibian metamorphosis shifting life stages out of aquatic patches and into terrestrial patches. In metapopulation terminology there are emigrants (individuals that leave a patch), immigrants (individuals that move into a patch) and sites are classed either as sources or sinks. A site is a generic term that refers to places where ecologists sample populations, such as ponds or defined sampling areas in a forest. Source patches are productive sites that generate a seasonal supply of juveniles that migrate to other patch locations. Sink patches are unproductive sites that only receive migrants and will go extinct unless rescued by an adjacent source patch or environmental conditions become more favorable. Metapopulation models examine patch dynamics over time to answer questions about spatial and demographic ecology. The ecology of metapopulations is a dynamic process of extinction and colonization. Small patches of lower quality (i.e., sinks) are maintained or rescued by a seasonal influx of new immigrants. A dynamic metapopulation structure evolves from year to year, where some patches are sinks in dry years and become sources when conditions are more favorable. Ecologists use a mixture of computer models and field studies to explain metapopulation structure.

[edit] Community ecology

Community ecology is a subdiscipline of ecology which studies the distribution, abundance, demography, and interactions between coexisting populations. An example of a study in community ecology might measure primary production in a wetland in relation to decomposition and consumption rates. This requires an understanding of the community connections between plants (i.e., primary producers) and the decomposers (e.g., fungi and bacteria). or the analysis of predator-prey dynamics affecting amphibian biomass. Food webs and trophic levels are two widely employed conceptual models used to explain the linkages among species.

[edit] Food webs