Biodiversity and Human Impact

• Genetic diversity — the variety of alleles (versions of genes) within a species or population. It is why individuals of one species differ, and it gives a population the raw material to adapt when conditions change.
• Species diversity — the variety of species in a community. It has two parts: species richness, the number of different species, and species evenness, how equally individuals are spread among those species.
• Ecosystem diversity — the variety of ecosystems in a region, such as wetlands, forests, lakes and grasslands. Ontario alone stretches from tundra near Hudson Bay, through the boreal forest and the mixed Great Lakes–St. Lawrence forest, to the Carolinian forest of the southwest.
• Structural diversity — the variety of physical structures and habitats within an ecosystem, such as the canopy, understorey, shrub and ground layers of a forest, or fallen logs and rocks. More structure means more places for different species to live and feed.

Two communities can have the same richness and still differ greatly. Picture two ponds with the same four species. In one pond, the individuals are split evenly; in the other, one species makes up almost all of them. The even pond is more diverse, because a randomly chosen organism is much less predictable. Simpson's index of diversity captures both richness and evenness in one number between 0 and 1. Here $n$ is the number of individuals of each species and $N$ is the total number of individuals of all species. The closer the index is to 1, the more diverse the community. Watch the convention: some sources call the fraction $\frac{\sum n(n-1)}{N(N-1)}$ itself Simpson's index, $D$, and write the diversity index as $1-D$. This course uses the "1 minus" form throughout, so a higher value always means more diversity.

A resilient ecosystem can resist a disturbance, such as a drought, disease or new predator, and recover afterwards. Diversity helps in several ways. With many species, several often do similar jobs, such as pollinating flowers or decomposing leaves, so if one species declines another can fill its role. This backup is called functional redundancy. A food web with many links gives predators other prey to switch to. Genetic diversity means that some individuals are likely to survive a new disease or a hotter summer and pass on their resistance. Diverse ecosystems are also harder for invading species to take over, because fewer resources and niches sit unused. People depend on biodiversity too. Ecosystems provide ecosystem services: pollination of crops, water filtration by wetlands, soil formation, flood control, and oxygen production. Many medicines come from living things: the cancer drug paclitaxel was first isolated from the bark of the Pacific yew, and aspirin is related to salicin, a compound found in willow bark. Every species lost could be a medicine or crop gene never discovered.

• Warmer water holds less dissolved oxygen, which stresses cold-water fish such as lake trout and brook trout and shrinks the habitat they can use.
• Ranges shift toward the poles and uphill as species follow the temperatures they are adapted to. Species that cannot move fast enough, or have nowhere left to go, such as those on mountaintops, decline.
• Timing mismatches occur when warming changes when plants flower or insects emerge, but migrating animals that depend on them arrive on their usual schedule.
• Drought and flooding from changing rainfall can dry out or drown wetland habitats.
• Some species gain: warmer conditions let some organisms, including certain pests, expand into new areas, which can further stress native species.

• Protected areas — national and provincial parks and conservation areas keep whole ecosystems intact, protecting all three levels of diversity at once.
• Habitat restoration and corridors — replanting native species, restoring wetlands, and connecting fragments with strips of habitat so populations are not isolated.
• Laws — Canada's Species at Risk Act and, in Ontario, the Species Conservation Act, 2025 (which replaced the Endangered Species Act, 2007, in March 2026) list species at risk and set rules for activities that could harm them. Supporters say the newer Ontario law speeds up approvals for projects; critics say it protects fewer species and habitats.
• Controlling invasive species — preventing their spread (e.g. cleaning boats between lakes) and biological control, such as releasing leaf-eating beetles that feed on purple loosestrife.
• Captive breeding and seed banks — breeding endangered animals for release into the wild, and storing seeds to preserve the genetic diversity of plants and crops.
• Sustainable harvesting — fishing quotas, crop rotation and mixed planting reduce pressure on wild populations and soil.