12.4 Stability, Change and Succession in Ecosystems

An ecosystem is stable when its structure and processes remain within a characteristic range or recover after disturbance. Resistance is the ability to change little during a disturbance; resilience is the ability to return after change. A grassland may burn readily and therefore have low resistance to fire, yet regrow rapidly from protected roots and therefore have high resilience. Stability must be attached to a variable and time scale: species composition, productivity and nutrient retention can respond differently.

Biodiversity can support stability because species may respond differently to stress and partly replace one another's functions. Complex food webs can provide alternate pathways, genetic diversity can supply tolerant variants, and spatial heterogeneity can preserve refuges. Diversity does not guarantee stability under every disturbance, however. Strongly connected networks can transmit effects, and a stress that affects many species through the same mechanism can overwhelm redundancy.

Negative feedback opposes displacement and can stabilize a system: rising prey density supports predators, which then reduce prey. Positive feedback amplifies change. Vegetation loss can expose soil, erosion can reduce plant establishment and further vegetation can be lost. A tipping point is a threshold beyond which feedback drives the system toward a different state, potentially with hysteresis: reversing the original pressure may not be sufficient to restore the previous community.

A mesocosm is a bounded experimental ecosystem that preserves more ecological complexity than a laboratory flask while allowing greater control and replication than a whole landscape. Aquatic tanks, enclosed soil communities and field chambers can test temperature, nutrients, pollutants or species removal. Investigators standardize starting conditions, include controls, replicate independent units and monitor variables such as oxygen, biomass, nutrient concentration and species abundance.

A sealed mesocosm is materially closed but energetically open: light enters and heat leaves. Producers must capture enough energy, organisms must remain within compatible population ranges, and decomposers must return mineral nutrients. Persistence for a short experiment does not prove indefinite sustainability, because slow depletion, genetic change or rare disturbance may not appear. Mesocosm walls also alter dispersal and predator behavior, so results must be transferred to natural ecosystems cautiously.

Agriculture redirects ecological production toward human harvest. Land clearance removes biomass and habitat, fragments populations and can release carbon from vegetation and soil. Monocultures simplify species and genetic diversity; machinery and tillage disturb soil; livestock require feed and produce methane; irrigation changes water movement; and fertilizers add reactive nitrogen and phosphorus. These effects depend on practice and landscape, but they are not captured by yield alone.

Fertilizer manufacture and application accelerate the nitrogen cycle. Crops absorb some added nitrate or ammonium, but surplus nitrate can leach into groundwater or run into surface water. Denitrification can produce nitrous oxide, a greenhouse gas, while ammonia can volatilize and later deposit elsewhere. Harvest removes mineral nutrients from the field, so agriculture often relies on replacement through fertilizer, manure, crop rotation or nitrogen-fixing symbioses.

Sustainable agriculture aims to maintain production while preserving soil, water, biodiversity and future options. Practices include reduced tillage, cover crops, crop rotation, integrated pest management, precise nutrient application, agroforestry, habitat strips and matching livestock density to carrying capacity. Each has trade-offs and must be evaluated using long-term evidence: reducing one pressure can shift another, and local yield, total land demand and off-site impacts all matter.