Habitat

A habitat is the environment in which an organism, species, or community lives. Habitats can be classified in a number of ways in order to compare them at different times, across different geographic areas, and in terms of different life history strategies.

Within nearly every type of habitat there exists a species that has adapted to that habitat, from the deep ocean floor to the polar ice caps. These specializations mean that the organism can survive in one type of habitat but not necessarily in another, a concept termed habitat requirement. Organisms that are free to move about can choose which habitat they will live in, and these choices are made based on the costs and benefits of each place.

In order to make comparisons between the many that exist, habitats need to be classified in some manner. First, it is important to understand that what is a good environment to one organism, such as the middle of the ocean for a shark, may not be good for another, such as a desert-dwelling lizard. Thus, classification of habitat types is usually done in reference to a certain species or group of species.

One method of classifying habitats is temporally. Over time a habitat may be constant, with little change from the viewpoint of a particular organism, or it may be seasonal, where there is a predictable pattern of favorable and unfavorable periods for that organism. An example of a constant environment is a cave. A cave's temperature stays at a constant temperature, within a few degrees of the mean annual temperature of the area; therefore, a bat can find refuge from the extreme high and low daily temperatures. The cave is also a consistent shelter for the bat because there is no rain in a cave.

Habitats can also be unpredictable, alternating between favorable and unfavorable periods for variable amounts of time, or ephemeral, meaning there are periods that are predictably short followed by unfavorable periods of variable, frequently extensive duration. To a water-loving amphibian, ephemeral habitats are intermittent streams that only run after heavy rainfalls and dry up to an uninhabitable state between rains.

If classifying habitats spatially, they can either be constant, with a fairly uniform distribution of resources like food and refuge, or they can be patchy, with resources occurring in small, dense locations that are scattered around an area that is otherwise without any resources. It is easy to see how these different habitats could select for different behaviors, including types of locomotion, searching, and communication among individuals to relay locations of food patches or defend territories.

Habitats can also be classified by their effects on the growth and life history of the species in question. On the one hand, in size-beneficial habitats large individuals have a greater chance to successfully compete and reproduce within their own species. For example, a large male deer that successfully defends a harem of females is more likely to mate. Also, the risk of predation may be greater for smaller adult deer, which are subject to attacks by wolves.

On the other hand, certain habitats are size-neutral or size-detrimental. In these habitats mortality may occur equally to all individuals, such as when a seasonal spring dries up. Or in such a habitat larger individuals may have a disadvantage; for example, an aerial predator may find it easier to see larger individuals or find it more worthwhile to chase them because of the greater food reward. Additionally, in a resource-rich environment there may be no within-species competition that would favor larger individuals.

The type of growth that is preferred in a given habitat will often correlate to the reproductive strategy of a species. In a size-beneficial habitat it is advantageous to reproduce less frequently and have fewer, larger off-spring. This is also called a k-selecting habitat. A reproductive adult may have to delay reproduction for a long time in order to store the energy necessary to produce a large offspring. That offspring will then have a much better chance at survival. In a size-neutral or detrimental habitat, producing smaller, greater numbers of offspring is often a good strategy. This is also known as an r-selecting habitat.

Habitats are as varied as the animals that live in them and each could be infinitely described, but another general way to think of them is according to their measurable characteristics, or parameters. Examples of habitat parameters, or characteristics, include temperature, moisture, substrate type, nutrient availability, altitude (or depth in water), and amount of light and wind (or current in water). Each of these parameters shapes the organisms that live there or imposes certain habitat requirements that limit the types of organisms that can move in.

Consider the cave example again. This habitat has a constant temperature, high moisture, low nutrient availability, and no light. Because of these characteristics, organisms that evolved to live in caves lose the ability to withstand temperature extremes and low moisture but in exchange gain the ability to withstand long periods with little food, partially by lowering their metabolism relative to their surface-dwelling relatives.

Other lost features of cave-dwelling organisms are sight, which takes considerable energy, and pigment, two things totally unnecessary in a habitat with constant darkness. These habitat parameters also restrict the kinds of animals that can successfully move into the cave environment. Raccoons, even though they are not cave adapted, can use the cave because they are nocturnal and accustomed to using their keen sense of smell to find their way through the dark and hunt for food.

These examples make it easier to understand the complexity of habitats and their specialized requirements. Because most organisms have adapted to their habitats and may not necessarily survive in another, it is extremely important to maintain habitats in their natural state in order to ensure the survival of the species that live there.