Stability and Change

Ecosystems are open systems: energy flows IN (sunlight) and OUT (heat), but the same finite pool of nutrients is recycled internally between organisms and the abiotic environment. Long-term stability depends on several factors working together.

Requirements for stability:

• A continuous supply of energy — usually sunlight captured by producers; without it, food chains collapse.
• Recycling of nutrients — carbon, nitrogen and phosphorus must cycle through decomposers back to producers (a one-way leak of nutrients would exhaust the system).
• Genetic diversity within populations — variation provides the raw material for adaptation to disturbance and disease, buffering against extinction.
• Climatic stability — relatively constant temperature and rainfall keep the community within tolerable limits.

Two related properties describe how a stable ecosystem responds to disturbance:

• Resistance — the ability of a community to remain unchanged when subjected to a disturbance (e.g. a mature forest barely altered by a dry season).
• Resilience — the ability of a community to recover and return to its original state after a disturbance (e.g. a grassland regrowing after fire).

A diverse ecosystem with many interconnected feeding relationships tends to be both more resistant and more resilient, because loss of one species can be compensated by others.

Keystone species have an effect on community structure that is disproportionately large relative to their abundance or biomass. Removing one can cause a cascade of changes.

Classic examples (trophic cascades):

• Sea otters eat sea urchins. Remove otters → urchin numbers explode → urchins overgraze kelp forests → the whole kelp-forest community collapses.
• Grey wolves at Yellowstone. Reintroducing wolves (1995) reduced elk numbers and changed elk grazing behaviour → willow and aspen recovered along rivers → beaver and bird populations rose → even river-bank erosion changed. A predator at the top reshaped the whole ecosystem — a top-down trophic cascade.

Feedback governs whether a disturbance is damped or amplified:

• Negative feedback counteracts a change and restores the original state — it is stabilising. Example: if a prey population grows, predators have more food and increase, which then reduces prey numbers back down.
• Positive feedback amplifies a change, driving the system further from its original state — it is destabilising. Example: melting Arctic ice exposes dark ocean, which absorbs more heat, causing more melting.

Tipping points. A tipping point is a threshold beyond which a small additional change causes a large, often irreversible, shift to a new stable state. Positive feedback can push a system past a tipping point — e.g. a coral reef bleaching repeatedly until it flips to an algae-dominated state that resists returning to coral.

Ecological succession is the gradual, directional change in a community over time.

• Primary succession begins on newly exposed bare substrate with NO soil and no prior life — e.g. cooled lava, bare rock, a retreating glacier. Pioneer species (lichens, mosses) colonise first; they tolerate harsh conditions and begin to build soil.
• Secondary succession occurs where a disturbance (fire, abandoned farmland) has cleared a community but soil and seeds remain, so it proceeds much faster.

Through successive seral stages, conditions change (soil deepens, shade increases), allowing new species to outcompete the pioneers, until a relatively stable climax community is reached. During succession, species diversity and total biomass generally increase, and food webs become more complex (raising stability).

Human disturbance reduces stability and diversity:

• Deforestation removes producers, breaks nutrient cycles (nutrients wash away), reduces biodiversity and can release stored carbon — lowering resistance and resilience.
• Biomagnification — fat-soluble, non-biodegradable pollutants become increasingly concentrated at successive trophic levels. DDT (a pesticide) magnified up food chains, thinning the eggshells of top predators like birds of prey. Mercury magnifies through aquatic food chains into large predatory fish. Because top consumers eat many contaminated prey, they accumulate the highest, often toxic, concentrations.

Mesocosms are small, enclosed experimental ecosystems (e.g. a sealed glass tank or a fenced field plot) used to study sustainability and stability under controlled conditions. A closed mesocosm must self-sustain: producers, consumers and decomposers must balance so that energy input (light) and internal nutrient recycling keep the system going. They let researchers manipulate one variable while replicating treatments — but they are simplified and small, so results may not scale to real ecosystems.