Communities: Interdependence, Competition and Abiotic Factors

Every place where living things exist is a mix of two kinds of factors. A biotic factor is a living part of the environment, like a plant, an animal, a fungus, or a bacterium. Biotic factors also include the ways organisms interact with each other. For example, the bacteria living inside an animal's gut help it digest food, and a herd of bison grazing on prairie grass changes the grass population.

An abiotic factor is a non-living part of the environment, like sunlight, temperature, water, soil, or wind. Abiotic factors set the basic conditions of a habitat. A cactus survives in the desert because it can handle the abiotic factors there: strong sun, high heat, and very little rain.

The two kinds of factors work together. The amount of rain (abiotic) decides which plants can grow (biotic), and the plants decide which animals can find food and shelter. When you explain why a population lives where it does, you need to say which influences are living and which are non-living. That is what makes the explanation something you can actually test.

An ecological community is all the populations of all the different species that live together in one area. The species in a community do not live side by side by accident. They affect each other constantly.

Interactions between different species are called interspecific interactions. Each interaction can help one participant (+), hurt it (-), or leave it unaffected (0).

The main types include competition (-/-), where two species both need the same scarce resource, like food, nesting sites, or light, and both end up worse off than if they had the resource to themselves. Predation (+/-) is when one species eats another. Mutualism (+/+) is when both species benefit, like bees and the flowers they pollinate.

Competition matters because resources are limited. If two bird species eat the same seeds and one species disappears, the other usually grows in number because more food is available. Predicting what happens when one competitor is removed is a classic way to test whether two species really were competing.

Life is found almost everywhere on Earth, but it is not spread out evenly. Different species live in different places, and sometimes their ranges overlap.

Each species has a set of environmental conditions within which it can survive and reproduce best. Not surprisingly, those are the conditions it is best adapted to. Many physical, non-living (abiotic) factors influence where a species can live, including temperature, humidity, soil chemistry, and light.

For each factor, a species has a tolerance range: a lower limit and an upper limit beyond which it cannot survive. A fish adapted to cold mountain streams will struggle in warm, slow water because the warmth and low oxygen fall outside its tolerance range.

Adaptations are the features that fit an organism to its factors. Thick fur is an adaptation to cold temperatures. Deep roots are an adaptation to dry soil. When you describe an adaptation, always link it to the specific factor it helps the organism cope with. That link is what turns a description into a real ecological explanation.

Ecologists investigate relationships among organisms and between organisms and their surroundings. Their studies range from individual living things to the entire biosphere.

An environment has biotic and abiotic components. Biotic components include its organisms and their interactions, such as feeding, competition and cooperation. Abiotic components are non-living conditions, including rainfall, temperature, light, soil and water chemistry.

These components affect one another. Physical conditions limit which species can survive somewhere, while organisms alter those conditions. Tree shade cools soil, and earthworms loosen it.

Consider a decline in rabbits. Saying their habitat became worse gives little to investigate. A more specific explanation is that drought reduced the grass available for food. Drought is an abiotic influence; grass is a biotic one. Both can be measured. Identifying the factors and their relationship gives an ecological explanation that can be tested.

Biogeography investigates species' locations across Earth and the reasons for their distribution.

Physical conditions strongly influence which organisms occupy each place. On land, rainfall and temperature help shape tropical forests, grasslands and deserts, which support different communities. Light, soil type, fires and floods also matter.

Aquatic communities depend on water temperature, salt content, oxygen, nutrients and the depth that sunlight reaches. These differences help explain why coral reefs and deep ocean trenches contain such different organisms despite both being ocean habitats.

Abiotic conditions also affect net primary productivity: the rate at which plants store energy as new growth after their own respiration. That growth supplies food and can support more animals and species. Warmth, water and sunlight often favour high productivity. This helps explain rainforest diversity, although productivity alone does not determine how many species live there.