The is a sample of the information contained in the Reference Section of the Estuarine Ecology Appendix of the Estuary-Net Project.

Ecologists take a number of approaches to their work, but they tend to focus on three levels of organization in the natural world: population, community, and ecosystem,

A population is a group of individuals of the same species living in a given place.

A community comprises populations of a number of different species living together.

Populations and communities do not have rigidly defined boundaries. They are abstractions, or levels of ecological organization rather than actual structures. So, we might have twenty individual egrets making up the egret population in our marsh, and five great blue herons making up the great blue heron population. If these are the only kinds of birds in a marsh, then these two populations comprise the bird community. We can also consider all the animals, plants, etc., living in our marsh and call them the marsh community. This is a way to collectively refer to the populations of every species of living thing in the marsh.

Ecosystem is a term even more inclusive than community, because it encompasses the community and its abiotic environment. Therefore, an ecosystem has components that are living (animals, plants, etc.) and non-living (soil, water, air, nutrients, etc.).

The concept of an ecosystem refers to the flow of energy and nutrients through ecological systems. Whereas, ecologists studying from the population or community perspectives focus on how organisms affect each other and how they are affected by the environment. The word ecosystem is always used by ecologists when they want to refer to the interactions among one or more biotic community(ies) and the associated abiotic features. In this text, the estuary is referred to as an ecosystem, although in other instances it might be considered simply a part of a larger ecosystem. Likewise, we will refer to the river, mudflats, and marsh as habitats within the estuarine ecosystem, in other situations they might be considered ecosystems in and of themselves.

The habitat concept is explained more fully later in the text, but it is important to note here that, although people often define ecosystem as a combination of community and habitat, this is incorrect. Ecosystems are the sum of a community and its physical and chemical environment while habitat of a worm might be the mudflat, but the habitat of a fish that eats the worm might be the river.

HABITAT: Habitat is the place where an organism lives.

Every species has a set of abiotic and biotic conditions and resources that it needs in order to survive, and there are certain places in the world that meet those requirements. These environments are suitable habitats for that species. A habitat is a place where a plant or animal lives. The habitat characteristics that a species requires can be determined by examining the places where it lives.

It is important to keep in mind that the characteristics of a place that are hostile to one species might be benign to another species. Antarctica, for example, is an extremely cold place, but many species find it a perfectly suitable habitat.

The habitat requirements of an organism can be divided into two categories: resources and conditions. Resources are entities (e.g., food, light, water, etc.) that the organism uses or consumes during its lifetime. Conditions on the other hand, are characteristics of the environment that influence the survival of an organism but are not consumed by it (e.g., temperature, salinity, pH). Resources and conditions of a habitat involve the previously discussed biotic and abiotic ecological factors: light, nutrients, oxygen, moisture, temperature, salinity, and space. The factors that organisms use during their lifetime are resources and the factors that simply influence their survival are conditions.

Therefore, if we look at a habitat from the point of view of an individual plant, we should consider the factors the plant needs as resources (light, water, oxygen, nutrients and space) and the conditions that affect its survival (temperature, salinity, and pH). A plant's habitat can be characterized by considering the overall combination of resources and conditions as they occur where the species live. For instance, one plant species might live only in a habitat with medium moist soil and with temperature and sunlight levels varying seasonally within a specific range -- a habitat such as a temperate hardwood forest. Another plant species might live only in a nearby habitat where the temperature and sunlight conditions are the same, but the soil is flooded twice daily with saline water -- in other words, a salt marsh.

Habitat characteristics for animals are somewhat different. Like plants, animals need water as a resource, but they do not solely require light for survival. Rather, animals mostly depend on having appropriate food sources, either plants or other animals, in order to obtain energy.

Animals often depend on plants in another way as well -- for diversifying space. It is no accident that rainforests, with their abundance complexity, and density of foliage, support a large population and diversity of animals. Likewise, it is not coincidence that one finds a greater abundance and diversity of fish in the weedy part of a waterbody than in the areas with little or no submerged vegetation. The more abundant and varied the vegetation the more space available to the animals for shelter and protection.

Environmental conditions, such as temperature and soil type (for burrowing animals), are very important in determining whether a given location is a suitable habitat for an animal species.

Habitats are also defined by their temporal and spatial characteristics. Are the resources and conditions constant, seasonal, unpredictable, or ephemeral? Are they continuous in space, patchy, or isolated? These qualities can make the difference between success and failure for a species.

In sum, a location's suitability for the survival of a species is determined by its resources and conditions. When all of the species' requirements are met, the location is a suitable habitat and the species has the potential to live there. It should be noted that every species can survive in a range of conditions some of which cause it to flourish (i.e., maximum reproduction) and some that are marginal (i.e., minimal reproduction).

The concept of a habitat is different from that of an ecosystem, which refers to the flow of energy and nutrients through an ecological system. A habitat, in contrast, is always considered from the point of view of an individual organism. In practice, an ecologist may use either word, habitat or ecosystem, to refer to the same location, but his/her choice of words reflects the way he/she is thinking about the place. One should say ecosystem when discussing the relationships among all biotic and abiotic components of a place and habitat when viewing a place from the perspective of an individual of a particular species.

The Reference Section contains information on the following:

• Section A - Abiotic Factors
• Section B - Biotic Factors
• Section C - Population//Community/Habitat/Ecosystem      (This Page)
• Section D - Food Webs
• Section E - Disturbance
• Section F - Adaptation
• Section G - Niche
• Section H - Geologic Formation
• Section I - Tides
• Section J - Watersheds
• Section K - Barrier Beaches
• Section L - River
• Section M - Mudflat and Sandflat
• Section N - Salt Marsh
• Section O - Pollution

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