In the sections that follow, our goal is to present key ecological concepts in a variety of ways that we believe will allow for use in a multiplicity of situations and applications. The first is a one-paragraph statement that defines ecology and relates it to other disciplines. The next section then presents a rather comprehensive set of concepts that generally follows the “traditionally-organized” (even classical) hierarchical presentation so often used in ecology textbooks from Odum’s Fundamentals of Ecology (1953) to the present. Many of the concepts themselves would be familiar to readers of textbooks from the past 60 years. However, some are relatively new, appearing within the past decade – especially at recent Ecological Society of America meetings.
We recognize that many faculty and program directors have already implemented these ideas on some level. As we move toward a multidimensional approach to ecology education, the 4DEE Task Force welcomes your ideas and contributions that illustrate how to teach and assess and/or develop and implement programs using the 4DEE framework.
The Task Force envisions that the framework will evolve over time and welcomes your input. Participate in our survey and let us know what you think about the 4DEE framework.
Ecology is the part of biology that examines the interrelationships between organisms and their environment. Like all sciences, ecology is based on objectively understanding nature, typically through careful observation, hypothesis testing, data-collection, experimentation, and modeling. While ecology and natural history both seek to understand nature, the former is explicitly more scientific in its approach, while the latter is more descriptive. Ecology draws heavily upon, and contributes to, other areas of biology, other sciences like chemistry, physics, meteorology, and earth science, as well as to other fields like mathematics, economics, medicine and sociology. Ecology is often studied to understand how nature operates, aside from any direct benefit to, or impact from, humans – an approach often called basic ecology. Conversely, ecology is often studied with the goal of utilizing the gathered information toward understanding human impacts on nature or preserving the ability of the earth to sustain all forms of life – including humans – an approach called applied ecology. Ecology is related to – but distinct from – environmentalism, which is the philosophy and social movement that seeks to limit human impacts on natural systems. Like those of other sciences, ecological concepts can change based on new findings and on new interpretations of old data.
I. Core Concepts Generally Organized According to the Ecological Hierarchy
Many factors in the environment affect the way that component organisms survive, reproduce, disperse, and distribute themselves in nature. Some of those factors are physical or abiotic features, including temperature, moisture, light, and soils. Other factors, called biotic, involve the presence of other organisms that may act as competitors, pollinators, dispersers, predators, prey, disease-causing agents, or mates.
Among environmental factors, some are resources that organisms consume and incorporate into their bodies. Others are regulators that control life processes. Those resources and regulators determine where a particular species is able to survive. Physiological ecology seeks to explain the relationship between survival, growth, and reproduction in relation to the various physical and biotic features of the environment.
Nature offers an almost infinite number of combinations of environmental conditions for organisms to live – collectively defined as habitat: A habitat can be defined rather precisely as the “place” where a species lives (a cavity within a tree). Conversely, a habitat can be defined more broadly as terrestrial, aquatic, marine, etc. A given area can offer excellent habitat for one species (e.g., deciduous forest for black bear), or poor habitat for another (e.g., deciduous forest for polar bear).
Among broadly defined habitats, terrestrial refers to land-based, marine to those in oceans, and aquatic to freshwater types like streams and lakes. Wetlands are transitional between terrestrial and aquatic or marine habitats. In terrestrial environments, soils comprise an important habitat for many taxa and rooting medium for plants.
In nature, environmental conditions often change continuously from one place to another or over time. For example, temperatures typically decline as one ascends a mountain. Plant and animal assemblages often track those changes, forming patterns that are visible to the naked eye – or by satellite images. The shift from one assemblage type to another caused by continuous environmental changes is called a gradient.
The combination of a species’ habitat and its role in nature is referred to as its niche. For example a woodpecker and a squirrel living in the same forest would have different niches because they eat different foods. Individuals of different species are able to coexist in a given location because they have different niches, which reduces the chance that one outcompetes the other. An important distinction exists between a species’ fundamental niche (where it could persist without other organisms) and its realized niche (where it does persist when other organisms are present).
Organisms of a given species that occur together comprise a population. Populations can grow by adding more individuals, but various environmental factors prevent populations from growing indefinitely. Populations can also decline to the point of extinction. Populations are often studied by demographers, who follow cohorts of individuals born at the same time to determine whether mortality is uniform throughout the life-span of the species or is concentrated among younger or older individuals.
Each species has life history features that include typical life span, the number of times that females reproduce during their lives, and the number of offspring that they produce. Life history features develop through evolution, and reflect adaptation to physical and biological conditions offered by the species’ habitat.
Each species is distributed over the earth’s surface based on a combination of its dispersal capabilities and its physiological ability to tolerate site conditions; the area occupied being called its range. Biogeography is the branch of ecology that examines species’ distributions – typically over the earth’s surface. Some species have wide (cosmopolitan) distributions, whereas others have narrow (endemic) distributions. Many animals are broadly distributed due to migration. Ranges can change over time due to climatic shifts and dispersal / extinction events.
Species that live in proximity to their place of evolutionary origin are termed natives. Alien species evolved elsewhere, and have been introduced on purpose or accidentally. Some alien species, termed invasives, undergo population expansions in their new habitat, often outcompeting natives.
In most areas, multiple species live together within a defined spatial area, and may interact in a variety of ways, forming an ecological community. Communities have certain properties not found in populations. Species diversity encompasses the number of species and their relative proportions in a community. Dominance expresses the degree to which a species has more individuals or biomass than others in a community. Stability expresses the degree a community resists change or returns to an original state after disturbance. Each community is unique, but those occurring in similar environments are generally similar to one another. Adjacent communities often blend into each other.
The biological variation found in a defined spatial area can occur at different levels, including genetic, phenotypic, species, community, and ecosystem. This variability is often called biodiversity. While the term biodiversity is most often used to describe species richness or diversity, this common usage should not restrict its correct wider definition.
Individuals and species that share a community often engage in competition for resources, particularly when their availability is limited. Exploitation competition involves gathering of the resource more quickly or efficiently than the other individual or species, while interference competition involves preventing other species access to resources. The result of a competitive interaction may be that both species harm each other, or that one species wins and the other loses. In some cases, species evolve ways to reduce competition, thus allowing them to coexist.
All of the organisms in a defined area, along with its physical environment, comprise an ecosystem. The ecosystem is considered to be the fundamental functional unit of ecology, much as a species is the fundamental unit for taxonomy. Ecosystems are composed of producers (e.g. plants) that incorporate energy from the sun into living tissue, consumers (e.g., animals) that eat producers or other consumers, and decomposers (e.g. fungi and bacteria) that break down organic matter. An important process in ecosystems is predation, in which one organism (the predator) eats another (the prey). Predators and prey often regulate each other’s populations. Predators that eat plants are termed herbivores, while those that eat other animals are carnivores. Some organisms like bacteria, fungi, and protozoans cause disease in other organisms and in so doing negatively affect survival and reproduction of the afflicted individual.
The ordering of organisms in an ecosystem according to predation patterns (producers -> herbivores -> carnivores) is called a food chain. In most ecosystems, predation patterns are complex, and thus the system of ordering is best called a food web.
Grazing food chains are based on herbivores that eat living plants (e.g. rabbits eating grass). Detritus-based food chains are based on herbivores (millepedes, earthworms, snails, caddisflies) that eat dead plants.
Ecosystems have certain emergent properties including energy flow and nutrient cycling. Through those processes, ecosystems are viewed as being important transformers of materials and energy.
All organisms and ecosystems depend upon energy in order to function. In any given ecosystem, energy is passed from source to producers to consumers in a process called energy flow. Most of the energy is lost back to the environment during each stage of passage, with only a small part (10% average) being retained among the next feeding level. Energy cannot be recycled in an ecosystem.
All ecosystems process nutrients, which are elements that comprise the building blocks of all organisms. Key nutrients include carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, calcium, and potassium. Plants typically absorb nutrients from the soil or air, and incorporate them into their bodies. Nutrients are in turn passed to herbivores that eat the plants, and to those that eat the herbivores. Decomposers return nutrients to the environment, and thus be reused by the plants – in a process called nutrient cycling. Therefore, unlike energy, nutrients can be recycled in an ecosystem.
Ecosystem productivity is a measure of the amount of energy that is incorporated within the ecosystem and passed from one level to the next during a specific period of time. Ecosystems vary in their rates of productivity. Factors that limit productivity include low moisture or nutrient availability and extreme temperatures.
All living things in an ecosystem are interconnected through networks of relationships. Those relationships can result from behavioral interactions between many animal species, or they can be the result of patterns of predation, reproduction or competition.
Within an ecosystem, various trophic levels can control the numbers of individuals in other trophic levels. Control from below occurs when a trophic level is limited by insufficient food. Control from above occurs when a level is limited by predation. In a trophic cascade, predators in a food web affect the abundance or behavior of their prey, thus releasing the next trophic level down from predation.
Ecosystems have interactions besides predator-prey relationships. Parasitism is a common interaction in ecosystems in which one species (the parasite) feeds upon another species (the host). Parasites and their hosts coevolve over time. [Note that this can be broadened to include disease]
Mutualism is a common interaction between two species in which both partners benefit from the interaction – often by providing nutrients or protection to the other. Examples include mycorrhizae (tree roots and fungi) and lichens (fungi and certain algae).
Some of the functions of ecosystems (e.g., filtration of water or air, preventing downstream flooding, providing food) can be considered ‘useful’ in some way to human standard of living, and are called ecosystem services. Many ecologists and economists are placing monetary values on ecosystem services, thus providing an argument for the benefits of conserving biodiversity.
Landscapes are ecological units that comprise a few to many square miles. Within a landscape, it is possible to recognize patches of ecosystem types that are distinct from other patches. Patches are often linked by corridors, which allow organisms to move from one patch to another. Conversely, landscapes also have barriers, which are linear arrays of unsuitable habitat that prevent individuals from moving from one area to another.
Large regions of the world with similar climates (rainfall and temperature) typically form recognizable groupings of plants and animals that are called biomes. Examples of biomes include the arctic tundra, the eastern deciduous forest, grassland, and desert.
The largest level of ecological interest is the biosphere, which includes all life on earth – including that on the land, in the oceans, and in the atmosphere. Many important processes occur at the global scale including: large-scale patterns of species distributions and migrations, atmospheric circulation patterns, and ocean currents.
All life — from individual organisms to species to ecosystems — changes over time. Those changes can be due to outside physical or biological factors (disturbance) that impose new conditions that may be temporary or long lasting. Or the change may be due to the organisms progressing through their life cycles, to altered relationships between the species, and to the species changing by evolution. Ecosystems that remain unchanged in the face of disturbance are said to be resistant. Those that recover quickly after disturbance are said to be resilient. In some biological systems, change occurs slowly, and we may perceive the system to be in a steady state. Other systems appear to fluctuate between two or more states in a predictable manner.
Ecosystems, including those recovering from disturbance or growing on previously uninhabited sites, go through a process called succession, in which certain species colonize the site early and are replaced by species characteristic of mature ecosystems. Colonizers tend to have good dispersal ability and often grow quickly, while mature-site species are excellent competitors.
II. Cross-Cutting themes
Our scale of observation (small / large; short-term / long-term) greatly influences how we interpret ecological processes. Spatial scale can range from microscopic to global. Temporal scale can range from milliseconds to billions of years. At one time, ecology was based on our powers of observation, including what we could see with our own eyes during timescales ranging from seconds to years. The development of various technologies allows us to extend our observations to larger and smaller spatial and temporal scales.
Ecological entities comprise systems – which consist of connected parts that form a more complex whole. While individual ecological systems are often perceived as being independent entities, they are also part of larger systems. Similarly, systems also contain smaller systems. To that end, systems are said to be nested. Changes within a system can affect the sustainability of the systems that are nested within it as well as the larger system in which it exists.
Individuals within a species often have different responses to the environment due to their genetic composition. The properties of species can change over the course of generations thanks to the process of evolution – in which the relative frequencies of heritable genetic information changes over time. A primary cause of evolution is natural selection, in which members of a species that have a certain genetic composition enjoy higher rates of survival and reproduction in a given set of environmental conditions than members having a different genetic makeup. As a result, the genetic composition of the offspring more closely resembles that of the successful parents. When environments are changing, populations can track those changes genetically so that the offspring are better able to survive and reproduce in the new environment through adaptation. Evolution depends on mutation to create new genetic variability on which natural selection can act. In some cases, small populations can evolve due to chance events associated with survival and reproduction. Biologists distinguish between microevolution, which is change in heritable characteristics within species over short evolutionary timescales, and macroevolution, which is the larger-scale formation of new species, often over longer periods of time. Coevolution involves the evolution of two groups that are in close association with each other. Examples include plants and pollinators, as well as parasites and hosts.
Species vary in their distribution over the earth, with some (endemics) being restricted to a limited geographic range, while others have broader (cosmopolitan) ranges. The study of the causes and implications of such patterns called biogeography. Species found near where they evolved are termed natives. Aliens are species brought to a new area, often by humans. Invasives are alien species that prove to be superior competitors in their new settings, often driving natives to local extinction. Some invasives can cause physical harm to structures, clog water intake systems, and reduce the value of pastureland. Only a fraction of non-native species that colonize a new area become established, and of that only a fraction of these established invaders cause harm to humans or to biodiversity. But yet, they can have a large impact on natural diversity of many ecosystems. Many governments have programs that seek to eradicate invasives. An important challenge is to predict invasiveness and therefore prevent introduction of harmful invasive organisms.
III. Human Dimensions
More than any other species, humans have changed the earth’s ecosystems (anthropogenic impact). At present, ecologists are particularly concerned about preservation of biological diversity, the effects of global climate change, and the ability of ecosystems to sustain life.
Many regions of the earth are experiencing changes in temperature, rainfall, and storm intensity due to global climate change. While such changes are known to occur over the earth’s history, changes in the past several decades are attributed to human activities – especially burning fossil fuels (coal, oil and natural gas). Many scientists and others are concerned that continued use of fossil fuels will make climate change worse over the coming century, potentially leading to stresses on human standard of living and threatening the integrity of natural systems.
Some materials in the ecosystem are harmful to one or more species – and are called toxins. Toxins can occur naturally or be introduced by humans. As materials are passed from one trophic level to the next, key toxins may be built up or concentrated, in a process called biomagnification or bioconcentration).
Natural systems often provide a benefit to humans, which are broadly termed ecosystem services. Examples include the ability of plants and algae to produce oxygen, wetlands to filter polluted streamflow, and marine ecosystems to produce shellfish for human consumption. Ecological economics seeks to monetize the beneficial functions that natural ecosystems serve.
The nature of ecosystems and the ecological interactions that occur in urbanized areas tend to be different than those in more rural or natural ecosystems. Urban areas tend to have a higher proportion of alien species, and food webs are thought to be simpler. The transition between natural systems and urban areas are often gradual, resulting in a rural-urban gradient that is recognized by many ecologists.
Farms, orchards, and pasturelands often have different species composition and ecological interactions than natural ecosystems. For example, farms that grow a single crop (monocultures) have much reduced species diversity than typically found in nature and are not ecologically sustainable. Agroecology studies the ecological interactions of farmed and grazed areas, seeking to understand how crops can be grown in a way that enhances ecosystem stability and biodiversity.
Many ecologists recognize that there are ethical dimensions to our relationship to nature. Stewardship is a philosophy and practice that embodies the responsible planning and management of resources. Environmental justice seeks to reduce the environmental costs of human development on human communities that are at a socioeconomic disadvantage.
Several areas of ecological study seek to create or preserve natural ecosystems of higher value than those that are degraded by humans. Conservation involves the preservation or protection of natural ecosystems. Natural resource management represents a set of practices that preserve and enhance desired features of natural ecosystems for future generations. An example would involve regulating fish catch in the ocean so that populations of desired species will be available in the future. Restoration seeks to create productive natural ecosystems from degraded ecosystems such as sites impacted by mining.
IV. Science Practices
Ecologists are often called upon to perform habitat assessments. In so doing, they analyze the physical and biological features of a site in relation to its suitability for a species or a group of species to live there.
Wetlands are areas that are transitional between terrestrial and aquatic or marine habitats. Because they often clean polluted water, prevent downstream flooding, and provide habitat to plants and animals, wetlands are viewed as valuable ecosystems that are protected by law from destruction. Ecologists often perform wetland delineations that identify the boundary between a wetland and its adjoining upland. They also provide valuation assessments of wetlands to determine the importance of ecological systems to the listed functions that they provide.
Stream macroinvertebrates are organisms like insects, worms, and mullosks that live in bodies of flowing water. Some macroinvertebrates are indicative of clean water, whereas others are more tolerant of pollution. Ecologists are often called upon to conduct macroinvertebrate assessments in streams to indicate the quality of the water. Such ecologists must understand sampling protocols and identification features of macroinvertebrates.
A critical part of ecological work involves the ability to identify and often preserve organisms belonging to a particular group such as mammals, vascular plants, birds, algae, insects, or reptiles and amphibians. To that end ecologists typically have the ability to recognize and identify certain groups by sight or sound and be able to identify unknowns using a taxonomic key or field guide. Ecologists should also be able to collect and preserve voucher specimens that can be placed in taxonomic collections for verification.
Ecologists typically collect and analyze data as part of their work. Collecting data is often facilitated by using sensors form parameters of interest connected to dataloggers. Complex data must be analyzed, often by using appropriate statistical techniques. Data are also summarized using visualization tools that prepare graphs and charts. Ecologists increasingly depend on informatics approaches in which data are exchanged between computers connected online. Data mining involves seeking data from often remote sources online. Those data can then be combined through a meta-analysis to derive a larger scale picture than would be otherwise possible.