Why Biodiversity Keeps Ecosystems Working

Why not just one kind of tree?

A century ago, American elms arched over many streets in Ontario and Québec. Planting one species everywhere seemed efficient. Then Dutch elm disease arrived, and many of those streets lost most of their elms within a few decades. Today cities plant a mix of species so that one pest cannot wipe out the whole urban forest.

The curriculum asks you to explain why biodiversity is important to maintaining viable ecosystems. In Grade 9 you met resilience and ecosystem services. In this lesson you will look at the mechanisms and the evidence.

1. Resistance and resilience

Every ecosystem faces disturbances: droughts, floods, fires, storms, outbreaks. Two properties keep it viable:

• Resistance: how little it changes during the disturbance.
• Resilience: how quickly it recovers afterwards.

Biodiverse communities usually score higher on both. Different species respond differently to the same stress. In a drought, deep-rooted prairie plants keep growing while shallow-rooted grasses wilt. Afterwards, survivors spread seeds and fill the gaps. In a monoculture every plant responds the same way, so the whole system rises or falls together.

2. Insurance: functional redundancy

In a healthy Ontario meadow, dozens of species of bees, flies, wasps, beetles and butterflies pollinate flowers. If a cold spring hurts one bee species, others still pollinate. This overlap is called functional redundancy. More species means more back-ups for every job: pollination, decomposition, nitrogen fixation, pest control.

Redundancy also explains why species loss can seem harmless at first and then suddenly matter. An ecosystem can lose several pollinators with little visible effect, until the last one in a group disappears and that job stops.

3. Defence against disease, pests and invaders

• Disease and pests spread most easily when hosts are close together and genetically alike. In a mixed forest, a pest that attacks one tree species must travel past many trees it cannot use. In a monoculture, it moves from host to host.
• The American chestnut, once common in southern Ontario's Carolinian forests, was nearly wiped out by chestnut blight, a fungus, in the 1900s.
• Most bananas sold today are one variety, the Cavendish, propagated as clones. A fungal disease (Panama disease, TR4) now threatens it, just as an earlier strain devastated the previous main variety, the Gros Michel, in the mid-1900s.
• Genetic diversity within a species matters too: a population with varied genes is more likely to contain some resistant individuals.
• Invasive species: communities in which most niches are already filled often (though not always) resist invasion better than simplified, disturbed ones.

4. Keystone species

Some species hold a community together. On the Pacific coast of British Columbia, sea otters eat sea urchins. Urchins eat kelp. When the fur trade wiped out sea otters from BC's coast, urchin populations exploded and grazed kelp forests down to bare rock ("urchin barrens"). After otters were reintroduced from Alaska between 1969 and 1972, their population grew, urchins declined where otters returned, and kelp forests grew back, along with the fish, snails and invertebrates that live in them. This chain of effects is a trophic cascade.

5. Ecosystem services and other ways of knowing

Viable, diverse ecosystems give people clean water (wetlands filter it), flood control, pollination of crops, fertile soil, fish, timber, medicines, and places for recreation and spiritual connection. Losing biodiversity weakens these services.

Many First Nations, Métis and Inuit knowledge systems describe people as in relationship with all living things, with responsibilities to them, and emphasize taking only what is needed so that the land stays healthy for future generations. The Three Sisters (corn, beans and squash), grown together by the Haudenosaunee and other nations, is a well-known example of diversity in a food system: beans climb the corn, the bean plants' root bacteria fix nitrogen, and squash leaves shade the soil. Many conservation projects in Canada are now co-led with Indigenous communities, combining their long-term knowledge of the land with Western scientific monitoring.