Community Interactions & Succession
- Classify symbiotic relationships by their effect on each partner
- Explain how competition, predation, and keystone species shape community structure
- Distinguish primary from secondary succession and predict the sequence of change
Living together: symbiosis
A community is all the interacting populations in an area, and much of that interaction is symbiosis — a close, long-term relationship between two species. The three classic types are named by who benefits. Mutualism (+/+): both partners gain, like a bee getting nectar while pollinating a flower, or gut bacteria helping us digest food. Commensalism (+/0): one benefits and the other is unaffected, like barnacles hitching a ride on a whale. Parasitism (+/−): one benefits at the other’s expense, like a tapeworm or a tick feeding on its host (but usually without killing it outright, unlike a predator).
Competition and the niche
When two species need the same limited resource, they compete (−/−) — both are worse off. The competitive exclusion principle says two species cannot occupy the exact same niche (an organism’s full role: what it eats, where it lives, when it is active) indefinitely; one will outcompete the other. Species often coexist instead by resource partitioning — dividing the resource, like warblers feeding in different parts of the same tree — which shrinks the overlap and lets both persist.
Predation drives adaptation
Predation (+/−) is one organism eating another, and it is a powerful selective pressure. Over time it drives defensive adaptations: camouflage, warning (aposematic) coloration, chemical toxins, and mimicry — a harmless species evolving to resemble a dangerous one for protection. Predators and prey can enter cycles: prey numbers rise, predators feast and multiply, then abundant predators drive prey down, which starves the predators — and the cycle repeats.
Keystone species
A keystone species has an effect on its community far larger than its abundance would suggest — remove it and the whole structure changes. The classic case is the sea star Pisaster: it preys on mussels, and without it the mussels overrun the rocks and crowd out most other species, collapsing diversity. Sea otters (which control sea urchins that would otherwise mow down kelp forests) and beavers (which engineer wetland habitat) are other examples. Keystone species are defined by impact, not by numbers or by sitting at the top of the food chain.
On a rocky shore, a sea star eats mussels. Ecologists remove all the sea stars from one stretch of coast and watch. Predict what happens to the number of other species, and identify the sea star’s role.
- 1.The sea star was the main predator keeping the mussel population in check through predation (+/−).
- 2.With the predator gone, mussels are released from control and multiply rapidly, monopolizing space on the rocks — a limited resource other species also need.
- 3.Through competitive exclusion, the booming mussels crowd out barnacles, algae, and other species, so overall species diversity falls sharply.
A clownfish lives among a sea anemone’s stinging tentacles: the fish gains protection from predators and the anemone gains cleaning and defense from the fish. This relationship is best classified as:
A keystone species is not the same as a top predator or an abundant one. It is defined by disproportionate impact: remove it and the community collapses even though it was never numerous. A common exam distractor calls the most abundant species the keystone — abundance is exactly what a keystone need not have.
Succession: communities rebuild over time
After a disturbance, communities change in a predictable sequence called ecological succession. Primary succession starts where there is no soil at all — bare rock from a retreating glacier or cooled lava. Pioneer species like lichens and mosses arrive first, breaking down rock and building the first thin soil, which slowly makes way for grasses, shrubs, and eventually trees. It is very slow. Secondary succession starts where a disturbance (fire, flood, abandoned farm field) cleared the community but left the soil intact; because soil and often seeds and roots remain, it proceeds much faster.
A forest fire burns an area to the ground, but the soil and many underground roots and seeds survive. The regrowth that follows is:
The whole primary-vs-secondary distinction hinges on one word: soil. No soil to start (bare rock, lava) → primary, slow, begins with pioneer lichens/mosses. Soil already present (after fire or farming) → secondary, faster. State the soil condition explicitly and the classification follows.
Answer the 2 checkpoints as you read.
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