For more than a century, the Hudson's Bay Company bought furs from trappers across northern Canada and kept careful records of how many pelts arrived each year. When ecologists later graphed those records, they found something remarkable: the number of Canada lynx pelts rose and fell in a cycle of roughly 10 years, and the number of snowshoe hare pelts cycled with it, with the hare peaks coming slightly earlier. Neither population was simply growing to a carrying capacity and staying there. Each was being pushed and pulled by the other. No population lives alone. Every species competes, eats, is eaten, or lives in close partnership with others, and a change in one population can ripple through an entire community.
Competition
Competition occurs when organisms need the same limited resource, such as food, water, light, nesting sites or mates. It harms both competitors (), since each gets less than it would alone. Intraspecific competition is between members of the same species; because they have identical needs, it is usually the most intense and is a major density-dependent factor. Interspecific competition is between different species whose niches (their roles, including what they eat, where and when) overlap. In the 1930s the Russian ecologist G. F. Gause grew two protists, Paramecium aurelia and Paramecium caudatum, on the same bacterial food. Grown separately, each species reached a stable carrying capacity. Grown together, P. aurelia kept growing while P. caudatum declined to extinction in the culture. This became the competitive exclusion principle: two species that compete for exactly the same limited resources cannot coexist indefinitely; the better competitor eliminates the other locally. In nature, competitors often avoid exclusion through resource partitioning: they divide the resource so their niches no longer overlap completely. In 1958 Robert MacArthur showed that five species of warblers (Cape May, yellow-rumped, black-throated green, Blackburnian and bay-breasted) all feed on insects in the same spruce trees, but each forages mainly in a different zone of the tree (top, outer branches, inner branches, lower trunk) and uses slightly different feeding behaviour.
Predation and predator–prey cycles
In predation () one organism kills and eats another. Herbivory, where an animal eats a plant, is a related interaction. Predator and prey populations are linked by feedback. When hares are abundant, lynx are well fed, survive and raise more kittens, so the lynx population rises. The growing number of lynx kills more hares, and hares also run short of winter browse, so the hare population falls. With little food, lynx starve and have few young, so the lynx population falls too. With few predators and recovering vegetation, hares rebound and the cycle repeats. The predator peak lags behind the prey peak (typically by one to two years) because predators need time to convert abundant food into new offspring, and they keep dying off for a while after prey have crashed. Field experiments at Kluane in the Yukon, which added food and excluded predators from large plots, showed that the hare cycle is driven by both predation and food supply acting together, not by predators alone.
| Defence | How it works | Examples |
|---|---|---|
| Camouflage (cryptic colouration) | Blends with the background so predators do not detect it. | Snowshoe hare moults from brown to white for winter; ptarmigan; walking-stick insects |
| Warning (aposematic) colouration | Bright, contrasting colours advertise that the animal is dangerous or distasteful; predators learn to avoid it. | Striped skunk; monarch butterfly; yellowjacket wasps |
| Batesian mimicry | A harmless species evolves to look like a harmful one and gains protection without the cost of defences. | Hoverflies (flower flies) with wasp-like yellow and black bands |
| Müllerian mimicry | Two or more harmful species evolve to resemble each other, so predators learn one shared warning faster. | Monarch and viceroy butterflies (both distasteful to birds); bees and wasps sharing black and yellow bands |
| Chemical defence | Toxins, irritants or foul-tasting compounds deter predators or herbivores. | Skunk spray; milkweed's toxic cardiac glycosides, which monarch caterpillars store in their own bodies; poison ivy's urushiol |
| Physical (mechanical) defence | Structures make the organism hard or painful to eat. | Porcupine quills; turtle shells; thorns on raspberry and hawthorn; spines on thistles |
Symbiosis
| Relationship | Effect on each species | Examples |
|---|---|---|
| Mutualism | : both benefit | Lichens (a fungus shelters an alga, the alga supplies sugars); mycorrhizal fungi on tree roots trade minerals for sugars; Rhizobium bacteria fix nitrogen in the root nodules of clover and soybeans; bees pollinating flowers |
| Commensalism | : one benefits, the other is unaffected | Burdock burrs hitching a ride on a fox's fur to disperse seeds; barnacles attached to a whale; birds nesting in a large tree |
| Parasitism | : the parasite benefits, the host is harmed but usually not killed quickly | Winter ticks on moose (heavy loads cause hair loss and "ghost moose"); sea lamprey feeding on lake trout in the Great Lakes; tapeworms in mammals |
Ripples through the community
Because populations are linked through food webs, a change in one population rarely stays local. A keystone species has an effect on its community far larger than its abundance would suggest. Sea otters on the Pacific coast of British Columbia eat sea urchins. The maritime fur trade hunted otters out of B.C. by the early twentieth century; without them, urchins multiplied and grazed kelp forests down to bare rock ("urchin barrens"), removing the shelter and food that kelp gives fish, invertebrates and seals. After otters were reintroduced to the west coast of Vancouver Island between 1969 and 1972, the recovering otter population cut urchin numbers and kelp forests returned. A chain of effects like this, passing down through trophic levels from a predator, is a trophic cascade. Similar reasoning applies in fresh water. Crayfish are a major food of smallmouth bass in Ontario lakes, and they also recycle detritus and graze plants. If crayfish decline, for example because acidification or calcium loss makes it hard for them to build their exoskeletons, bass lose an important prey and may grow more slowly or switch to other food, which in turn changes pressure on small fish and insects. On land, wolves prey on beavers, particularly in summer. Where wolves are removed, beaver numbers can climb, and more dams flood more forest into ponds and wetlands; where wolves return, fewer beavers survive long trips from their ponds, changing where wetlands form. The size of such cascades is still studied and debated, but the principle holds: altering one level of the food web changes the others.
Invasive species show the same ripple effects, often in reverse, because they arrive without the predators, parasites and competitors that controlled them at home. Zebra mussels, native to the Black and Caspian Sea region, reached the Great Lakes in ballast water from ships and were found in Lake St. Clair in 1988. Each mussel can filter up to about a litre of water a day, removing phytoplankton. The water becomes clearer, but the food base for zooplankton and larval fish shrinks, native clams are smothered when mussels attach to their shells, and intake pipes of water treatment and power plants become clogged. The emerald ash borer, a beetle from eastern Asia, was first detected in the Windsor–Detroit area in 2002. Its larvae tunnel through the living layer under the bark, cutting off the tree's transport of water and sugars, and they kill nearly every ash tree they infest. The loss of ash changes light levels on the forest floor, the species that depend on ash for food, and municipal budgets for removing dead trees.