7.6-7.7 AUTHENTIC SCIENTIFIC TEXT

An ecosystem is the combination of abiotic and biotic components present in a particular location. Changes in either the abiotic or biotic components can alter the populations of many different organisms. These changes are called disturbances and can be either natural (volcanic eruption, drought) or manmade (increase in nutrient input by adding fertilizers, extinction of a species by habitat removal). Several examples illustrate the effect of disturbances and ecosystem dynamics.

Environmental changes, such as deforestation, disease, human activities, floods, droughts, fires, bring changes in resources that will cause some organisms or populations too perish or move, while permitting other organisms or populations to thrive.

Sometimes rapid environmental changes from abiotic (nonliving), biotic (living), or intrinsic factors can change an ecosystem dramatically. When a hurricane crashes ashore and floods the area with a huge amount of salty ocean water, the freshwater ecosystem plants and animals cannot survive and will perish (die), while other organisms that adapted to the saltier water will continue. When an ecosystem changes rapidly, with a forest fire for example, an existing organism is faced with one of two fates. When human activity such as deforestation destroys an ecosystem or invasive species begin to compete for resources, if the organism has the ability to move, it may migrate, seeking a more compatible ecosystem. If it cannot change locations, however, it may perish (die). In the areas where trees and grasses burned, seedlings begin to sprout, animals move in, and life begins anew. Nature often finds a balance when environments change; some organisms may die, while others find an opportunity to survive.

In areas of severe drought, the ecosystem that was dependent upon a plentiful supply of water will disappear, and plants and animals that can exist on far less water will move in and thrive (do well). While drought obviously affects aquatic ecosystems, it also impacts forests. Trees that are stressed by drought become less resistant to diseases such as insect infestations. The lack of water can result in decreasing reproductive success of small prey animals, which in turn affects predators such as coyotes and bears. Trees killed by insects provide fuel for wildfires, removing shelter and killing animals. Depending on the length of the drought and the lifespan of the organisms, effects can be short term or long term.

Coyotes are large natural predators present in many habitats. In urban habitat fragments, humans remove coyotes as they can present a danger to domestic animals. In fragments where coyotes are absent, the number of smaller predators such as striped skunk and raccoon increases. This effect is called mesopredator release. These smaller predators hunt smaller prey animals, including birds.

When coyotes are absent, bird species diversity and abundance decreases. Removal of large predators in other ecosystems due to hunting or habitat loss has similar complex effects. Biologists and policy makers can examine food webs and predict the effects of the local extinction of species as well as how to mitigate the effects.

Biotic and abiotic components change ecosystems through ecological succession.

Ecosystems are not built in a day. The dynamic, interdependent assemblages of species and environments that make up an ecosystem develop over many years through a process called ecological succession. Ecosystems have living (biotic) and nonliving (abiotic) components. The biotic and abiotic components of an ecosystem are interdependent; each component changes the nature of the other interacting components. Ecological succession begins when an empty area containing abiotic resources (space, nutrients, water) becomes available for organisms to inhabit. Events such as floods, volcanoes, wildfires, plate tectonic movement, climate change, and human activity create empty habitats where ecological succession may begin.

Resource availability impacts individual organisms as well as populations of organisms within an ecosystem.

Ecosystems are complex, interactive systems that include both biological (biotic) communities and physical (abiotic) components of the environment. Organisms grow, reproduce, and perpetuate their species by obtaining necessary resources throughout interdependent relationships with other organisms and the physical environment.

Organisms rely on physical factors, such as light, temperature, water, solid, and space for shelter and reproduction. Earth’s varied combinations of these factors provide the physical environments in which its ecosystems (e.g., deserts, grasslands, rain forests, marine, and freshwater) develop and in which the diverse species of the planet live.

Different environmental factors lead to different ecosystems.

Competition is an interaction between organisms by which one organism gains access to a limited resource at the expense of the other organism not gaining the same access to the same resource.

Competition between species is called interspecific competition, while competition within species is called intraspecific competition. Competition becomes more important as the number of individuals increases or the availability of resources decreases. It is important in all organisms. In plants, competition can be over nutrients, such as nitrogen, water, light, space, and other abiotic factors.

These whitetail bucks are demonstrating competition within populations.

When stable conditions change, ecosystems change.

In other words, a complex set of interactions within an ecosystem can keep its numbers and typesof organisms relatively constant over long periods of time under stable conditions. If a modest biological or physical disturbance to an ecosystem occurs, it may return to its more or less original status (i.e., the ecosystem is resilient), as opposed to becoming a very different ecosystem. Extreme fluctuations in conditions or the size of any population, however, can challenge the functioning of ecosystems in terms of resources and habitat availability.