AP Environmental Science

Ecosystems and their parts

An ecosystem is a community of living organisms together with the nonliving environment they interact with. The living parts are biotic factors (plants, animals, fungi, bacteria) and the nonliving parts are abiotic factors (sunlight, temperature, water, soil chemistry, salinity, oxygen). The two sets act on each other all the time: plants change the soil they grow in, and the soil's nutrients and moisture decide which plants can grow there at all.

A population is all the individuals of one species in an area, and a community is all the populations living and interacting in that area. The interactions between species depend mainly on how much of each resource is available, since food, water, light, space and shelter are always limited somewhere, and how species deal with that shortage shapes who survives.

Predator and prey

In a predator-prey relationship, the predator is the organism that eats another organism, the prey. The two populations often rise and fall together in a pattern: when prey are abundant, predators have plenty of food and their numbers grow; more predators then reduce the prey population; the shortage of prey makes predator numbers fall; and with fewer predators the prey recover. Real populations are also affected by weather, disease and alternative food, so the cycles are rarely as tidy as this.

Predation acts as a selective pressure on both sides. Prey that are faster, better camouflaged or better defended are more likely to survive and reproduce, and predators that are better at finding and catching prey do the same, so each side drives adaptations in the other over many generations.

Competition

Competition happens when organisms need the same limited resource. It can be within one species (intraspecific competition) or between different species (interspecific competition). Intraspecific competition is usually the stronger of the two, because members of one species need exactly the same resources.

When two species compete for the same resource in the same place, the species that uses it more efficiently tends to do better. If the overlap is complete, one species can be pushed out of that area altogether, which is called competitive exclusion.

Resource partitioning

Species can avoid excluding each other by using the resource in different ways, in different places or at different times, which is called resource partitioning. This reduces the negative effect of competition on survival, so more species can share a habitat.

• Different ways: warblers feeding on the same spruce tree can each take insects from different parts of the tree, or of different sizes.
• Different places: one lizard species hunts on the ground while a related species hunts in the canopy.
• Different times: some predators hunt by day and others by night, so the same prey is shared without direct conflict.

Symbiosis

Symbiosis is a long-lasting, close association between two species that live together. The three types are defined by what each partner gains or loses.

A parasite usually weakens its host without killing it quickly, since a host that dies early ends the parasite's food supply. A parasitoid or a predator, by contrast, kills what it feeds on.

Mutualism matters for ecosystems because it links the nutrient supply of one species to the survival of another. Corals house photosynthetic algae in their tissues, and the coral gets much of its energy from the sugars that the algae produce while the algae receive shelter and nutrients. If the algae are lost, as in coral bleaching, the coral is left short of energy.

How resource availability shapes these interactions

When resources are abundant, competition is weak, populations of consumers grow and more species can coexist. When a resource becomes scarce, for example in a drought, competition intensifies: individuals with small advantages in finding or using the resource survive more often, species that partition poorly are excluded, and predators may switch to other prey. Scarcity also pushes species toward closer symbiotic relationships in some cases, since partners that supply a missing nutrient become more valuable.

The same idea explains why a change in one resource can move through a whole community. If a drought reduces seed production, seed-eating rodents decline, and so do the predators that depend on the rodents; species that compete with the rodents for seeds may increase.

Worked interpretation of a population graph

Suppose a graph shows hare numbers peaking in one year and lynx numbers peaking about two years later, with each peak followed by a fall. The lag exists because lynx reproduction depends on food that has already become abundant: hare numbers rise first, the lynx have a good food supply and their numbers follow, and the larger lynx population then cuts the hare population back. The fall in hares is followed, again after a delay, by a fall in lynx. The description of a relationship between two variables (predator numbers lag prey numbers) is enough to identify predation as the interaction; the lag shows that neither population is controlled by the other alone.