Imagine two farm fields. One is planted with a single crop of corn from fence to fence. The other is an old meadow with dozens of kinds of grasses, wildflowers, insects, birds and soil organisms. A new plant disease arrives that attacks corn. The first field could be wiped out in a single summer. The meadow would hardly notice: if one species is hit, others fill its place. That is the power of biodiversity, and it is one of the main reasons some ecosystems stay healthy for centuries while others collapse. This lesson is about what keeps an ecosystem sustainable: able to keep functioning and supporting life over the long term.
Biodiversity is the variety of life in an area. It includes three levels: genetic diversity (variety within one species, such as the many different-looking wild strawberries or the many varieties of corn), species diversity (how many different species there are and how evenly their numbers are spread), and ecosystem diversity (the variety of habitats, such as forests, wetlands, lakes and grasslands, in a region). The number of different species in an area is called species richness. Ecosystems with high biodiversity are usually more resilient: they resist damage better and recover faster from disturbances such as drought, disease or fire, because many species can do similar jobs.
- Keystone species have a much bigger effect on their ecosystem than their numbers suggest. Beavers are a classic Canadian example: their dams create ponds and wetlands that become homes for fish, frogs, ducks, moose and many insects. Remove the beavers and, as their dams break down, the ponds and wetlands drain away.
- Indicator species are sensitive organisms whose presence, absence or health tells us about the condition of an ecosystem. Frogs absorb chemicals through their thin skin, so declining frog populations can warn of pollution. Many lichens cannot survive in polluted air. Mayfly, stonefly and caddisfly larvae need clean, well-oxygenated water.
- Native species have lived in an area for a long time and are part of its natural food web. Invasive species are introduced from elsewhere and spread so fast that they harm native species (you will meet several in the next lesson). Non-native is not the same as invasive: many introduced species, such as most crops and garden plants, do not spread on their own or cause harm.
Air quality, water quality and soil health
| Ecosystem health measure | What scientists test | Signs of a healthy ecosystem |
|---|---|---|
| Air quality | Ground-level ozone, fine particles (smoke, soot), sulfur dioxide, nitrogen oxides; lichen surveys | Low pollutant levels; many kinds of lichens growing on trees and rocks |
| Water quality | Dissolved oxygen, temperature, pH, nitrates and phosphates, cloudiness (turbidity), bacteria; samples of small stream animals (benthic invertebrates) | High dissolved oxygen, nearly neutral pH, low nutrients, clear water; mayfly, stonefly and caddisfly larvae present |
| Soil health | Organic matter, nutrients (nitrogen, phosphorus, potassium), pH, how well water soaks in, earthworms and microorganisms | Dark, crumbly soil rich in organic matter that holds water and is full of life; little erosion |
Carrying capacity and limiting factors
A population changes in four ways: births and immigration (moving in) add individuals; deaths and emigration (moving out) remove them. With plenty of food and space, a population can grow very fast. But no environment has unlimited resources. A limiting factor is anything that keeps a population from growing larger, such as food, water, space, shelter, disease, predators or harsh weather. The largest population of a species that an ecosystem can support over the long term is its carrying capacity. A population that overshoots its carrying capacity usually falls back down as resources run out, sometimes in a sharp crash. A population near its carrying capacity rises and falls a little from year to year: another example of dynamic equilibrium.
Succession: how ecosystems rebuild
Ecological succession is the gradual, fairly predictable change in the species living in an area over time. Primary succession starts where there is no soil at all, such as bare rock left behind by a retreating glacier or new rock from a lava flow. Pioneer species like lichens and mosses arrive first; they slowly break down rock and add organic matter as they die, building a thin soil. Grasses and other small plants follow, then shrubs, then fast-growing, sun-loving trees, and eventually a long-lasting mature (climax) community such as a mixed forest. Real succession is messier than this neat sequence: the mature community depends on local climate and soil, and fires, storms or insects often reset part of it, so there is not always one fixed end point. Secondary succession happens after a disturbance such as a forest fire, flood or abandoned farm field removes the plants but leaves the soil. Because soil and seeds are already there, secondary succession is much faster than primary succession.
Sustainability means meeting our needs today without damaging the ability of future generations, and of other species, to meet theirs. A sustainable ecosystem has high biodiversity, cycles nutrients efficiently, has clean air, water and healthy soil, and keeps its populations within carrying capacity. In the next two lessons you will see how human activities and climate change push ecosystems out of this balance, and what people are doing about it.