Communities and interactions
Species in a community compete, eat each other, and cooperate. These interactions shape which species live where.
Niches and habitats
A habitat is where an organism lives; its niche is its role there: what it eats, when it is active, and what conditions it needs. No two species can occupy exactly the same niche in the same place for long, a principle called competitive exclusion.
Competition
Competition happens when organisms need the same limited resource. It can be within a species or between species. Species often reduce competition by resource partitioning: several warbler species feed in different parts of the same tree.
Predation
Predators kill and eat prey. Predator and prey populations often cycle: more hares feed more lynx, which then reduce the hares, after which the lynx decline. Prey defend themselves with camouflage, warning colours, spines, and toxins; predators evolve better hunting traits in return.
Herbivory and defences
Herbivores eat plants, and plants fight back with thorns, tough leaves, and chemical defences such as caffeine and nicotine, which evolved to deter insects. Some plants release chemicals that attract predators of the insects eating them.
Symbiosis
Symbiosis is a close, long-term relationship. In mutualism both benefit, as with bees and flowers, or the bacteria in your gut. In commensalism one benefits and the other is unaffected, as with barnacles on a whale. In parasitism one benefits at the other's expense, as with tapeworms and ticks.
Keystone species
A keystone species has an effect far larger than its numbers suggest. When sea otters were hunted out, sea urchins exploded and destroyed kelp forests; when otters returned, the forests recovered. Wolves reintroduced to Yellowstone changed elk behaviour and allowed streamside trees to regrow.
Ecological succession
Succession is the gradual change in a community over time. Primary succession starts on bare rock, as after a volcanic eruption, with lichens and mosses building soil. Secondary succession follows a disturbance, such as a fire, that leaves soil intact, and is faster. Both can lead to a relatively stable community.
Biodiversity
Biodiversity includes genetic diversity, species diversity, and ecosystem diversity. Diverse communities tend to be more stable and productive and provide services humans rely on, such as pollination and clean water. Tropical rainforests and coral reefs are among the most diverse ecosystems.
Key takeaways
A niche is an organism's role; competitive exclusion limits overlap. Competition, predation, and herbivory drive coevolution. Symbiosis includes mutualism, commensalism, and parasitism. Keystone species shape whole communities. Succession rebuilds communities after disturbance. Biodiversity supports stability and ecosystem services.
Flashcard
Where an organism lives
Check your understanding
Bees get nectar from flowers and the flowers get pollinated. This relationship is: