How Human Activities Upset the Balance of Ecosystems

Remember from Grade 7

In Grade 7 you learned about invasive species, biomagnification of persistent toxins, monocultures and overharvesting, and you assessed the environmental impacts of technologies. In Grade 9 you will explain these impacts in terms of dynamic equilibrium: how each one pushes an ecosystem's cycles, energy flow and populations out of balance, and how much pressure an ecosystem can take before it shifts to a new state.

Let's start with a lake that turned green

In August 2014, about 400 000 people in and around Toledo, Ohio, on the shore of Lake Erie, were told not to drink their tap water for about two days (August 2 to 4). A huge bloom of cyanobacteria (blue-green algae) had produced a toxin that got into the city's water supply.

Lake Erie is the shallowest and warmest of the Great Lakes, and it receives runoff from a lot of farmland and cities, in Ontario and in the United States. The bloom was fed by phosphorus: exactly the nutrient the Lake 226 experiment identified decades earlier.

The main ways humans change ecosystems

Scientists often group the main threats to ecosystems with the memory aid HIPPO, plus climate change:

• Habitat loss and fragmentation
• Invasive species
• Pollution
• human Population growth and consumption (which drives the others)
• Overharvesting

Each one disrupts dynamic equilibrium by changing energy flow, matter cycles, or the populations that keep each other in check.

Habitat loss and fragmentation

When forests, wetlands and grasslands are cleared for farms, housing, roads and mines, the species that live there lose food, shelter and breeding sites. In southern Ontario, about 70% of the original wetlands have been drained or filled.

Even when some habitat remains, fragmentation cuts it into small patches:

• Small patches have more edge: more light, wind, predators and invasive plants. Species that need deep interior habitat, such as some songbirds, decline.
• Roads block animals from moving between patches to find food or mates, which reduces genetic diversity.
• Animals are killed crossing roads. Seven of Ontario's eight turtle species are listed as species at risk, and road deaths of nesting females are a major threat.

Solutions include protecting large connected areas, wildlife corridors and eco-passages (tunnels or bridges under or over roads), and restoring wetlands.

Invasive species

Invasive species often have no natural predators or diseases in their new home, so their populations explode and break existing food webs:

• Zebra and quagga mussels arrived in the Great Lakes in ship ballast water in the late 1980s. They filter huge amounts of plankton from the water, which makes the water clearer but takes food from native animals at the bottom of the food web. They also clog water intake pipes.
• Emerald ash borer has killed millions of ash trees in Ontario.
• Invasive Phragmites (European common reed) forms dense stands in wetlands and roadside ditches, crowding out native plants and the animals that depend on them.

Prevention is the most effective tool: clean, drain and dry boats; don't move firewood; don't release pets or aquarium plants.

Pollution

• Nutrient pollution causes eutrophication.
• Persistent toxins (such as mercury, PCBs and some pesticides) bioaccumulate in organisms and biomagnify up food chains, harming top predators.
• Road salt raises chloride in streams and lakes, harming aquatic insects and amphibians.
• Plastics and microplastics are eaten by animals and are now found in water, soil and air.
• Air pollution causes acid deposition (Lesson 7) and adds greenhouse gases (Lesson 10).

Eutrophication is a clear example of a human change to a matter cycle:

Overharvesting

When fishing, hunting, logging or harvesting take organisms faster than they can reproduce, populations collapse. The most famous Canadian example is the northern cod off Newfoundland and Labrador. After decades of heavy fishing with modern trawlers, the population collapsed, and in 1992 the federal government closed the fishery (a moratorium). Tens of thousands of people lost their jobs. Decades later, the cod have still not fully recovered: the ecosystem shifted to a new state in which other species, such as shrimp and crab, became more common for a time.

This shows an important idea: if a disturbance pushes an ecosystem too far, it may not return to its old equilibrium, even when the pressure stops.