1.1 - Introduction to Ecosystems
The main objective for this topic is explaining how resource availability impacts species interactions. We will explore relationships such as predator/prey dynamics, competition, and symbiosis. The accompanying skill is describing an environmental concept.
Ecosystem Basics
Before analyzing how organisms interact, you need the foundational components of an ecosystem.
Term | Definition | Key Characteristics |
Individual | A single organism. | One living thing. |
Population | A group of organisms. | All members must belong to the same species. |
Community | All the living organisms within a specific area. | Includes all living things, like trees, animals, bacteria, and fungi in the soil. |
Ecosystem | Includes all components, both living and nonliving. | Living (biotic) components interact with nonliving (abiotic) components, such as water, soil, and rocks. |
Biome | A large geographical area. | Defined by a similar climate, which ultimately determines the types of plants and animals that can survive there. For example, the tropical rainforest is characterized by high rainfall and warm temperatures. |
Species Interactions and Outcomes
This chart summarizes the outcomes of different types of organism interactions.
Interaction | Outcome (Species 1 / Species 2) | Summary |
Competition | Lose / Lose () | Organisms fight for a shared resource like food or shelter, which limits their population size. |
Predation | Win / Lose () | One organism consumes the other for energy. The consumed species is harmed or killed. |
Parasitism | Win / Lose () | One organism (the parasite) lives on or in another organism (the host) and feeds on it, usually without killing it right away. Ticks on deer and tapeworms in mammals are examples. |
Mutualism | Win / Win () | Both species benefit from the relationship. |
Commensalism | Win / Unaffected () | One species benefits while the other is neither helped nor harmed. This is sometimes called a “free rider” relationship. Birds making nests in trees is a classic example. |
A Closer Look at Predation
Predation happens when one organism uses another as its energy source.
- Herbivores: Animals that eat plants are technically practicing predation because they use the plant for energy. A giraffe eating tree leaves may not fit the traditional view of a predator, but the action counts as predation in the strict sense.
- True Predators (Carnivores): Animals like lions or leopards that consume other animals. They are distinct from herbivores.
- Parasites: Organisms that use a larger host organism for energy. Parasites often live attached to the outside of the host or inside it. They typically draw energy without killing the host. Think mosquitoes, tapeworms, or the sea lamprey, which drains blood from fish.
- Parasitoids: A specific type of parasite that lays its eggs inside the host. When the eggs hatch, the larvae eat their way out, which generally kills the host. A parasitic wasp injecting eggs into a caterpillar is the classic example.
How resource availability shapes predator and prey numbers: prey are the predator's food resource. When prey are plentiful, more predators survive and raise young. As predators increase, they eat prey faster than the prey can reproduce, so the prey population drops. Then the predators run short of food and their numbers drop too, which lets the prey recover. The Canada lynx and the snowshoe hare repeat this cycle about every 10 years, and the hares' own food supply (twigs and shrubs in winter) also plays a part (see 3.4 for the step by step cycle).
Symbiotic Relationships
Symbiosis is a long term, close association between two organisms of different species. The word roots mean “a state of living together” (sym = together, bio = living, osis = state of being). Symbiosis by itself is just the condition of close living, and it isn’t necessarily beneficial or harmful. Symbiotic relationships can include mutualism, commensalism, and even parasite-host relationships.
Mutualism (A Form of Symbiosis)
Mutualism is a symbiotic relationship that benefits both species, leaving both better off than they’d be alone. In some cases the species are so interdependent that they can’t survive apart.
- Example 1, the coral reef ecosystem: Coral are tiny animals (polyps) that build the reef structure, which provides a home for algae. The coral also provides carbon dioxide () that the algae need for photosynthesis. In exchange, the algae produce sugars through photosynthesis, which the coral uses for energy.
- Example 2, lichen: This relationship between a fungus and algae is so close that they’re considered a single, composite organism. The algae provide energy in the form of sugars, while the fungi supply the nutrients the algae need to photosynthesize and grow.
- Example 3, mycorrhizal fungi: these fungi grow on and into plant roots and spread thin threads (hyphae) far into the soil. They bring the plant water and phosphorus, a nutrient that's often scarce, and the plant feeds the fungi sugars from photosynthesis. Most land plants have these partners, and they matter most where soil nutrients are limited.
Commensalism (A Form of Symbiosis)
Commensalism is a symbiotic relationship where one species benefits and the other is neither helped nor harmed ().
- Example 1, cattle egrets: these birds follow grazing cattle and eat the insects the cattle stir up from the grass. The egrets get an easy meal, and the cattle aren't affected.
- Example 2, epiphytes: plants like orchids and Spanish moss grow on the branches of trees to reach sunlight. They get water and nutrients from rain and air, not from the tree, so the tree isn't harmed.
Parasitism (A Form of Symbiosis)
Parasitism is a symbiotic relationship where one species (the parasite) benefits by feeding on another species (the host), which is harmed (). Unlike a typical predator, a parasite lives on or in its host for a long time and usually doesn't kill it quickly, since a dead host stops being a food source.
- Example, blacklegged (deer) ticks: they attach to deer, mice, and people and feed on their blood, and they can pass on Lyme disease. Tapeworms, mosquitoes, and sea lampreys (above) are parasites too.
Competition and Resource Partitioning
Competition is a lose-lose situation. Species competing for resources have fewer survivors, which means smaller overall populations.
- Intraspecific competition: competition between members of the same species, like oak seedlings competing for the same patch of sunlight. Members of one species need exactly the same resources, so this competition is often intense.
- Interspecific competition: competition between different species that need the same limited resource, like lions and spotted hyenas competing for the same prey on the African savanna (each also steals the other's kills).
- Resource availability sets how strong competition is: competition only happens when a resource is limited. In a wet year with plenty of food and water, competition is weak. In a drought, the same species compete hard and fewer individuals survive. If two species need the exact same limited resource in the same place at the same time, one usually outcompetes the other and pushes it out (this is called competitive exclusion), which is why resource partitioning matters.
Resource partitioning (also called resource sharing) is a mechanism that reduces competition by letting different species use the same resource in slightly different ways. This differentiation is a product of evolution, which favors traits that allow species to share a resource without direct conflict. It enables larger population sizes because species don’t have to compete for the exact same resource in the same place at the same time.
There are three main types:
- Temporal partitioning: Competing species use the resource at different times of day. For instance, wolves and coyotes may hunt small mammals at different times.
- Classic example: hawks and owls both hunt mice and voles. Most hawks hunt during the day and most owls hunt at night, so they share the same prey without meeting head on.
- Spatial partitioning: Species use different areas of a shared resource. Examples include:
- Different grass species extending roots to different depths, shallow versus deep, to access water and nutrients.
- Warbler species occupying separate portions of the same tree for nesting and feeding.
- Morphological partitioning: Species evolve slightly different body features to access different parts of the resource. For example, a ferret and an ermine may evolve different tooth patterns and jaw sizes, which lets them hunt different sized prey and avoid direct competition.
- Real example: on the Galápagos Islands, several species of ground finches live together but have different beak sizes. Big, deep beaks crack large, hard seeds, and small beaks handle small, soft seeds, so the species split up the seed supply instead of all competing for the same seeds.