The Trophic Levels
Energy cycles through ecosystems through the trophic levels. First, plants will capture the sun’s energy through photosynthesis. Any organism that makes its own food through photosynthesis is called an autotroph or a producer.
Consumers, or heterotrophs, can’t produce their own food. Primary consumers, or herbivores, eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers. These different levels of animals eating one another are called the trophic levels. Because of the laws of thermodynamics, we lose most of the energy so only 10% of energy from the previous trophic level remains as we move up each trophic level.
Since there are several different organisms in ecosystems, the trophic levels aren’t usually represented in a simple chain. Instead, a more realistic model of how energy moves between trophic levels is a food web.
There are also some animals who eat dead organic matter: scavengers, detritivores, and decomposers. These organisms, along with producers, are especially important to ecosystems because they help to regulate and ‘clean up’ the ecosystem. Scavengers, such as racoons and vultures, consume dead animals. Detritivores, such as dung beetles, obtain nutrients by breaking down dead tissues and waste products. Decomposers, such as worms, are fungi and bacteria that convert organic bacteria into small elements and molecules that can be recycled back into the ecosystem.
There are many different ways that organisms interact with each other within an ecosystem. In a predator-prey relationship, one animal will kill and consume another animal. Predators play an important role in regulating prey populations.
In order to avoid being eaten, prey have evolved behavioral, morphological, and/or chemical defenses. For example, prey can run away or hide (behavioral), camouflage or attack predators with spines (morphological), or be poisonous to predators (chemical). Sometimes, prey will simply mimic other organisms’ defense mechanisms. For example, most poisonous frogs are brightly colored. Some frogs are brightly colored, but they aren’t poisonous. However, predators will still avoid eating them.
In a parasitic relationship, one organism (the parasite) will live inside of another organism (the host), and it relies on that organism to survive. Parasitic relationships only benefit the parasite, and they harm the organism in which they live (the host). Pathogens are types of parasites that cause disease in their host.
Both predatory and parasitic relationships will help one animal and harm another animal. However, we also see symbiotic relationships, which help both organisms or they do not affect one organism. In mutualistic relationships, both organisms benefit. Bees pollinating flowers is a common example of mutualism: bees take nectar from the flowers, meanwhile they help the flowers by pollinating them.
Commensalism is another type of symbiotic relationship that benefits one organism, but doesn’t affect the other organism. A bird building a nest on a tree is an example of commensalism. Commensalisms are more common that mutualisms -- almost all organisms will engage in commensalism simply because they live in a community with other organisms.
Competition occurs when organisms each share a limiting resource. Limiting resources can be food, water, shelter -- for plants, it can be sunlight or nutrients from the soil. When two species each compete for the same limiting resource, they can’t coexist. If both lions and hyenas, for example, eat antelope, then only one species can be dominant; the other species will decline.
Species who vie for the same resource, however, can exist together when they divide up the resource based on their behavior or morphology. This is called resource partitioning. If giraffes eat the leaves at the top of the trees, while gazelles eat the leaves closer to the ground, then they can both occupy the same niche without overlapping.
But, if both antelope and gazelles eat leaves closer to the ground, then evolution is going to favor giraffes, because they overlap less with the niche of other shorter animals. And, the taller giraffe will be able to survive and reproduce, while the shorter giraffe will be in the same boat as the antelope and gazelle and they will die off. This is the process of natural selection that occurs both within a species and between several species that live in the same environment.
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