Natural ecosystems can absorb a lot: a hard winter, a fire, a bad year for berries. They bounce back because the changes are usually limited, and because species have had thousands of years to adjust to each other. Human activities are different. In a few decades we can drain a wetland that took 10 000 years to form, carry a mussel across an ocean in a ship's ballast tank, or spray a chemical that builds up in eagles hundreds of kilometres away. When change is this fast or this large, an ecosystem can be pushed out of its dynamic equilibrium and may not return to the way it was. Knowing how these impacts work is the first step to preventing them.
Habitat loss happens when natural areas are cleared or drained for farms, housing, roads, mines or logging. It is the biggest single threat to biodiversity worldwide. Southern Ontario has lost roughly 70% (about 72%) of the wetlands that existed before European settlement, mostly drained for farming and building. Even when some habitat is left, roads and developments can cut it into small, isolated patches. This is habitat fragmentation. Small patches support fewer species, animals that need large territories (like wolves and black bears) cannot find enough space, and roads kill animals trying to cross between patches. Turtles crossing roads to lay eggs are a well-known Ontario example. Solutions include protecting large connected areas, building wildlife corridors and road underpasses, and restoring wetlands.
Invasive species
An invasive species is a species introduced, usually by people, into an area outside its natural range, where it spreads quickly and harms native species, ecosystems or the economy. Invasive species often succeed because they left their natural predators, parasites and diseases behind, they reproduce fast, and native species have no defences against them. Ontario has many; three well-known examples are below.
| Invasive species | How it arrived | Harm it causes | Control efforts |
|---|---|---|---|
| Zebra mussel (small freshwater mussel from Eastern Europe) | Carried in ships' ballast water; found in Lake St. Clair in 1988 and had reached all five Great Lakes by 1990 | Filters plankton out of the water, robbing native animals of food; smothers native mussels; clogs water-intake pipes of towns and power plants. Its close relative the quagga mussel has since spread even more widely, especially in deeper water | Ballast-water rules for ships; cleaning, draining and drying boats before moving between lakes |
| Emerald ash borer (metallic green beetle from eastern Asia) | Likely carried in wooden packing material; first identified in 2002 around Detroit, Michigan, and across the river in Windsor, Ontario | Its larvae tunnel under ash bark and cut off the flow of water and nutrients, killing nearly every untreated ash tree they infest; millions of ash trees have died in Ontario | Not moving firewood, removing infested trees, injecting valuable trees with insecticide, releasing tiny wasps that attack the beetle |
| Purple loosestrife (tall wetland plant with purple flower spikes from Europe and Asia) | Brought in as a garden plant and in ships' ballast in the 1800s | Forms dense stands that crowd out cattails and other native plants, reducing food and shelter for wetland animals; a single plant can make millions of seeds | Since 1992, release of leaf-eating Galerucella beetles that feed mainly on loosestrife; this biological control has greatly reduced loosestrife at many Ontario release sites |
Pollution and eutrophication
Pollution is any harmful substance or energy added to the environment: smoke and exhaust in the air, chemicals and plastics in water, pesticides and road salt in soil. One very important kind of water pollution in Ontario is extra nutrients. Eutrophication is what happens when a lake or river receives too many nutrients, especially phosphates and nitrates from farm fertilizer, manure, lawn fertilizer and sewage. The nutrients feed a huge, fast growth of algae called an algal bloom, which turns water green and blocks sunlight from reaching plants below. When the algae die, decomposers break them down, and their cellular respiration uses up the dissolved oxygen in the water. Fish and other animals can suffocate, leaving low-oxygen dead zones. Some blooms are caused by cyanobacteria that release toxins dangerous to people and pets.
Bioaccumulation and biomagnification
Some pollutants, such as mercury, DDT and PCBs, are persistent: they break down very slowly and are stored in fatty tissue instead of being excreted. Bioaccumulation is the buildup of such a substance in one organism over its lifetime, because it takes the toxin in faster than it can get rid of it. Biomagnification is the increase in concentration of the toxin at each higher trophic level. A small fish eats thousands of plankton and keeps most of their toxin; a large fish eats many small fish; a fish-eating bird eats many large fish. Because only about 10% of energy passes up each level, each predator must eat a large mass of food, and the toxin from all of it piles up. Top predators end up with concentrations thousands or even millions of times higher than in the water.
- DDT and birds of prey: the insecticide DDT biomagnified in birds at the top of food chains, such as ospreys and bald eagles (which eat fish) and peregrine falcons (which eat other birds), making their eggshells so thin they broke during incubation. Populations crashed. After most uses of DDT were restricted in Canada and the United States in the 1970s, these birds slowly recovered.
- Mercury at Grassy Narrows: from 1962 to 1970, a chemical plant at the pulp and paper mill in Dryden, Ontario, released an estimated 9000 kg or more of mercury into the English–Wabigoon River system. Mercury biomagnified in fish, a central food for the Anishinaabe people of Grassy Narrows (Asubpeechoseewagong) and Wabaseemoong First Nations, and many community members have suffered long-term health effects. It is a lasting example of how ecosystem damage harms people too.
Sustainable agriculture: innovations that help
- Cover crops: plants such as clover, rye or radish grown between main crops. Their roots hold soil against erosion, they soak up leftover nutrients so less washes into streams, and legume cover crops add nitrogen.
- Crop rotation: changing which crop grows in a field each year (for example corn, then soybeans, then wheat). It breaks pest and disease cycles, and legumes in the rotation restore soil nitrogen.
- No-till (or reduced-till) farming: planting seeds directly into the leftover stalks of last year's crop instead of ploughing. This protects soil from erosion, keeps more carbon and moisture in the ground, and supports earthworms and soil fungi.
- Precision agriculture: using GPS-guided equipment, drones, soil sensors and satellite images to apply water, fertilizer and pesticide only where and when they are needed, saving money and reducing runoff.
- Integrated pest management (IPM): monitoring pest numbers and combining methods such as natural predators, resistant crop varieties, crop rotation and traps, using pesticides only when pests pass a set threshold.
- Vertical farming: growing crops indoors in stacked layers under LED lights, often in water-based (hydroponic) systems. It uses far less land and water and needs few pesticides, but it uses a lot of electricity and so far suits mainly leafy greens and herbs.