Ecology

This page’s sources are US National Park Service (NPS) lesson and teacher materials and research summaries, plus the National Oceanic and Atmospheric Administration (NOAA, the US ocean and weather agency) and the US Geological Survey (USGS), so most examples are North American.

What ecology studies

Ecology is the study of the relationships among living things and between them and their environment.^[1]

An ecosystem is a community of organisms together with its physical environment, interacting as a whole.^{[1, 3]} It includes the non-living, or abiotic, parts of a place as well as the living ones.^[2] Ecosystems are shaped from outside by climate, the rock beneath them and the lie of the land, and from inside by food webs, decomposition and competition.^[4] They are never fixed: fire, floods, pollution and habitat destruction disturb them, and they are often still recovering from past disturbances.^{[2, 4]}

A population is all the members of one species living in the same place, and a community is all the populations that live there and interact.^[2] An organism’s habitat is its address, the place where it lives; its niche is its role there, including its relationships with that place and with other organisms.^{[3, 1]}

Producers, consumers and decomposers

Organisms can generally be sorted into producers, consumers and decomposers.^[2] Green plants are producers: through photosynthesis they turn sunlight into food in the form of sugars and starches.^{[2, 5]} Herbivores such as rabbits eat plants and are called primary consumers, carnivores such as hawks eat other animals and are secondary consumers, and omnivores such as black bears eat both.^[2] Decomposers, including bacteria and fungi, break down dead material and enrich the soil that plants need.^{[2, 5]}

In the sea, plankton are living things carried by tides and currents, unable to swim against them.^[7] Phytoplankton are microscopic plants that photosynthesise near the surface, taking in carbon dioxide and giving off oxygen.^[7] Most zooplankton, the animal plankton, eat phytoplankton and are eaten in turn by larger animals.^[7]

Food chains, food webs and energy

Every animal either eats plants or depends on other species that, in the end, depend on plants; these links form food chains along which energy passes from one organism to the next.^[2] A simple chain runs sun, dandelion, rabbit, hawk.^[5] But most animals have several food sources, so they sit in many chains at once, and linked chains form a food web.^[2] In reality, energy moves through an ecosystem more like a web than a line.^[5]

The sun is the energy source for all food webs.^{[2, 5]} Plants capture its energy, plant-eaters take in part of it, and meat-eaters then eat the plant-eaters.^[5] Each higher feeding level needs more food energy to support it.^[2] Not every link is about eating: insects, birds and bats pollinate flowers, and rodents spread seeds.^[5] Because everything is connected, the loss of one species can reach far, taking away food, shelter or decomposition from others.^[2]

In one study, MacArthur found that five kinds of warblers in the spruce forests of Maine and Vermont ate roughly the same food but found it in different parts of the trees: one habitat, five niches.^[3]

Keystone species: otters, urchins and kelp

A keystone species is one whose place in a food web or nutrient cycle gives it a “disproportionate effect on the persistence of other species”, shaping the makeup of its community.^[1]

Sea otters are a well-known case: they eat sea urchins, which graze on kelp.^[9] As otters bring urchin numbers down, grazing eases and kelp forests flourish.^[9] Urchins left to multiply freely can destroy a kelp forest, often by eating through the holdfasts that anchor the kelp.^[8] Kelp forests in turn give habitat to invertebrates, fish, marine mammals and birds.^[8]

The fur trade had largely wiped out sea otters in southeastern Alaska before 1911.^[9] Since 1993 they have spread rapidly into Glacier Bay National Park; the most recent estimate in the NPS account, from 2012, was more than 8,000 otters.^[9] Scientists also model such predator-prey systems: a USGS model based on prey surveys predicted that two California estuaries could support over 160 sea otters.^[10]

Trophic cascades and the Yellowstone debate

A trophic cascade is when a predator’s effect trickles down one more feeding level, changing the numbers or behaviour of its prey’s own food.^{[11, 12]} “Top-down” means predators reduce prey or change how they behave, so fewer plants are eaten; “bottom-up” means plant growth, powered by sunlight, sets how many prey and then how many predators there can be.^[11]

Wolves were brought back to Yellowstone in 1995, after nearly 70 years’ absence.^[12] For most of the 20th century, elk browsing had kept willow and aspen short; around the time wolves recovered, some of these plants began to grow taller.^[11] Most studies agree on this pattern, and the scientific debate is about why it happened.^[11]

The return of the wolves was not a controlled experiment, so many factors changed at once.^[11] Proposed explanations include a “landscape of fear” that changed where elk browsed, simply fewer elk, weather or climate, and the earlier loss of beavers, which had changed streams.^[11] Apart from the climate explanation, no one argues that top-down effects are unimportant, but the NPS article concludes that the question is not yet settled.^[11]

Bottom-up forces can matter too. In a 29-year study of golden eagles and their main prey, hares and ptarmigan, the authors concluded that plant production, not predation, was the main driver in that system.^[13] Yet across the far north, predation is generally thought to drive the population cycles of many plant-eaters.^[13]

Biodiversity

Biodiversity is the full variety of life, which ecologists tend to consider at three levels: genetic, species and ecosystem diversity.^{[1, 3]} An undated NPS teacher’s guide says we do not know how many species exist, not even to the nearest order of magnitude, and suggests perhaps 10 or 100 million.^[5]

NOAA describes changes in marine biodiversity as a possible early warning of wider change, if they are noticed.^[14] After an unprecedented coral die-off at Flower Garden Banks in 2016, for example, unusually high chlorophyll pointed to heavy runoff from the Mississippi Delta, which may have triggered deadly effects such as bacterial growth; scientists were still working to identify the cause.^[14] In the Arctic, NOAA writes that it is possible the whole social-ecological system of the region has passed a threshold from which it may not easily return.^[14]

Common misconceptions

“Wolves alone brought back Yellowstone’s willows.” Some of the plants did grow taller after wolves returned, but the cause is still debated, with cougars, bears, elk management, climate and beavers all part of the discussion.^[11]

“If predators and prey rise and fall together, predators are in charge.” The eagle study’s authors caution against reading such patterns as strong evidence of top-down control.^[13]

“Sea urchins are simply bad for kelp.” Urchins destroy kelp forests when left to multiply freely, which is what otter predation prevents.^{[8, 9]}

What this page does not cover

The page gives no figures for how much energy passes between feeding levels. It does not cover ecological succession (how communities change over time), the different kinds of close relationships between species, coral reef ecology, nutrient cycles such as the carbon and nitrogen cycles, or ecosystems outside North America.

Going further

See Evolution for how species change, Plants for producers and Oceans for marine life’s setting.