Disruptions in Ecosystems

Questions that organize this topic

• How do intensity, duration, frequency, and spatial extent change the effect of a disturbance?
• Why does a changed environment favor some preexisting heritable variants rather than cause organisms to acquire needed traits?
• When does a nonnative species become ecologically invasive?
• How can disease, habitat fragmentation, climate change, and physical events alter several trophic levels?
• Why do primary and secondary succession begin from different biological starting conditions?
• How can before–after data be separated from an unrelated time trend?
• What evidence shows recovery of function, composition, or both?

1. Disturbance is defined by a change and a response

An ecological disturbance is an event or process that changes environmental conditions, resource availability, population sizes, or community structure. A brief flood is a pulse. Repeated mowing is a recurring disturbance. Continuous nutrient loading or long-term warming acts more like a persistent pressure. The label alone does not determine the outcome.

Describe a disturbance with at least four dimensions: intensity, duration, frequency, and spatial extent. A low-intensity fire may leave roots, soil organisms, and seed banks intact; a hotter fire at the same location may remove those legacies. A hurricane can be severe locally yet leave nearby source populations that recolonize the damaged patch. A regional drought can remove that rescue route.

A stressor is a condition that reduces performance or pushes organisms beyond a tolerance range. Stress and disturbance can overlap, but keeping them distinct improves explanations: a storm is an event, while prolonged soil salinity after the storm may be the continuing stressor.

3. Ecological response and evolutionary response use different clocks

Hours after a heat wave, an animal may seek shade and a plant may close stomata. These are behavioral or physiological responses within individuals. Over days or seasons, survival, reproduction, immigration, and emigration alter population size. Across generations, differences in reproductive success can change allele frequencies.

Selection acts on variation that already exists when the environment changes, together with new random mutations that arise without regard to what the population needs. An adaptation is a heritable trait favored by selection because it increases reproductive success in a particular environment. The environment does not instruct organisms to produce the needed allele.

Suppose insects vary heritably in detoxification enzyme activity before a pesticide is applied. Susceptible insects die at higher rates, so resistant survivors contribute a larger share of offspring. The population evolves because variant frequencies change. An individual insect did not evolve resistance during exposure. If one animal reversibly increases enzyme production without a genetic change, that is phenotypic plasticity, not by itself population evolution.

4. Changed conditions create changed selective pressures

A trait is not universally adaptive. Thick fur may improve survival in cold conditions while increasing heat stress after warming. Early flowering may help plants complete reproduction before drought but create a mismatch with pollinators. Selection depends on the relationship between phenotype and reproductive success in the current environment.

To argue that a disturbance caused natural selection, evidence must go beyond a change in abundance. Show that individuals differ in a trait, some of that difference is heritable, and reproductive success differs consistently among trait values under the new condition. A population decline can occur without evolution; evolution can also occur while total population size stays nearly constant.

Heterozygote advantage is a specific case in which the heterozygous genotype has higher relative fitness than either homozygote under the measured conditions. Selection can then maintain both alleles instead of driving one rapidly to fixation. The advantage belongs to the genotype–environment relationship, not to heterozygosity as a universal property. If disease pressure or another environmental condition changes, the relative fitness ranking may change as well.

5. Resistance, resilience, and reorganization are not synonyms

Resistance describes how little a measured variable changes during disturbance. Resilience describes how quickly or how completely it recovers. A system may resist a mild drought but recover slowly after a severe one. Another may lose productivity sharply and regain it quickly.

Recovery of one process does not prove recovery of the original community. Total photosynthesis might return because a few tolerant producers replace sensitive species. If feedbacks maintain a persistently different community and process state, ecologists may describe a regime shift. The evidence must show persistence beyond normal variation, not merely one unusual sampling date.

6. Succession begins with the biological legacies that remain

Primary succession begins where a previous biological community and developed soil are absent, such as newly exposed rock. Weathering, dust accumulation, microbial activity, and early colonists gradually build conditions in which additional organisms can establish. The process is slow because substrate and nutrient reservoirs must develop.

Secondary succession follows disturbance where soil or other biological legacies remain. Roots, seed banks, spores, organic matter, microbes, and nearby survivors can speed recovery. A field after cultivation and a forest after a moderate fire therefore do not begin at the same point as bare volcanic rock.

Succession is not a rigid ladder that every site climbs toward one guaranteed endpoint. Dispersal, climate, soil history, repeated disturbance, priority effects, and species interactions can lead to different pathways. “Later” also does not mean morally better; it identifies a sequence under stated conditions.

7. Disturbance frequency can prevent a stage from establishing

Recovery requires time. If the interval between disturbances is shorter than the time needed for slow-growing species to reproduce or recolonize, the community can remain dominated by early-colonizing species. If disturbance is absent for a long period, shade-tolerant or competitively dominant species may increase. The pattern depends on the organisms and disturbance regime.

Do not assume that an intermediate disturbance level must maximize diversity in every ecosystem. That outcome is a testable hypothesis, not a universal law. A valid investigation samples several intensities or frequencies, includes replication, and identifies the mechanism linking the treatment to survival, colonization, and competition.

8. Introduced is not the same as invasive

An introduced or nonnative species occurs outside its historical range because of human transport, intentional or accidental. Many introduced populations fail to establish; some persist without measurable widespread harm. An invasive species establishes, spreads, and causes ecological, economic, or health impacts in the receiving system.

Successful invasion can involve repeated introductions, rapid reproduction, broad tolerance, release from familiar consumers or pathogens, access to disturbed habitat, or traits poorly matched by resident defenses. None is required in every case. The receiving community and transport pathway matter as much as the species label.

Mechanisms must be named. An invader may compete for light, consume prey with little prior defense, introduce a pathogen, alter fire frequency, modify habitat, or change nutrient cycling. “It outcompetes natives” is incomplete unless the limiting resource and evidence for reduced native performance are identified.

9. Disease can reorganize communities through direct and indirect paths

A novel pathogen may spread rapidly when hosts lack prior resistance, vectors expand their range, or host density and connectivity increase contact. Direct effects include reduced host survival or reproduction. Indirect effects follow when the host supplies food, habitat, shade, or another ecological function.

Imagine a dominant canopy tree declining from disease. More light reaches the understory, soil temperature and moisture change, leaf-litter inputs fall, and organisms depending on cavities lose habitat. Measuring tree mortality alone misses the community pathway. To infer disease causation, compare infected and uninfected hosts or sites, measure pathogen presence, and rule out correlated drought or land-use stress where possible.

10. Human activities alter both habitat and ecological connections

Logging changes canopy cover, microclimate, dead wood, and age structure. Urbanization replaces permeable habitat, fragments populations, redirects water, adds heat and light, and creates transport routes for introduced species. Monocropping concentrates genetically similar hosts and simplifies habitat, which can increase vulnerability to a matching pest or pathogen. These are mechanisms, not judgments based only on land-use names.

Habitat fragmentation has three separable components: less total habitat area, more edge relative to interior, and greater isolation among patches. Edge conditions may be hotter, drier, brighter, or more accessible to predators. Isolation can reduce dispersal, recolonization, and gene flow. A corridor might restore movement, but its effect must be measured because it can also move predators, competitors, fire, or disease.

Some disruptions move through matter rather than habitat area. In eutrophication, excess nitrogen or phosphorus can stimulate producer growth; when the added biomass dies, decomposers consume dissolved oxygen, potentially creating hypoxic conditions that kill organisms unable to escape. The nutrient does not remove oxygen directly—the increase in production and later decomposition supplies the missing causal links. In biomagnification, a persistent contaminant reaches higher concentrations at successive trophic levels because consumers retain material acquired from many prey. This differs from bioaccumulation within one organism over time. A top consumer can therefore receive a high dose even when the contaminant concentration in water is low.

11. Climate change alters means, variation, extremes, and timing

A climate effect is not limited to a higher annual mean temperature. Changes in rainfall distribution, heat-wave frequency, season length, snow cover, ocean chemistry, and extreme events alter resource timing and tolerance limits. Species may respond through plasticity, movement, demographic change, or evolution, but these routes differ in speed and constraints.

Range shifts require suitable habitat and a path to reach it. Evolution requires heritable variation and enough reproduction before extinction. Plasticity must occur within the organism’s reaction range. Species can also shift at different rates, creating phenological mismatches: a consumer may emerge after its short-lived food peak even if both respond to temperature.

12. Geological and meteorological events operate across scales

Volcanic activity can create new substrate locally, bury communities, alter water chemistry, or inject particles and gases into the atmosphere. Storms redistribute sediment and organisms. Ocean–atmosphere cycles can alter temperature and rainfall over large regions. Continental movement changes climate, barriers, and opportunities for dispersal over geological time. A major impact event can change light and climate globally.

Scale controls the biological route. A fallen tree creates a light gap within a forest. A regional hurricane can alter thousands of patches and the source populations that would recolonize them. A global event can change selective conditions across many biomes simultaneously. AP explanations should match the temporal and spatial scale of the proposed response.

14. Calculate a BACI effect before claiming causation

In a before–after–control–impact design, the reference or control sites estimate the change that would have occurred without the focal disturbance. Let the response be native plant cover in percentage points.

The disturbance-associated change is estimated by subtracting the reference change from the impact change:

A simple before–after statement would report a 38-point fall. BACI shows that 2 points of decline also occurred at reference sites, leaving an adjusted effect of 36 points. Strong inference also requires comparable sites, replication, consistent sampling, and no other treatment applied only to the impact group.

15. Measure recovery relative to an explicit target

Suppose restored sites fell from 73% to 35% native cover and reached 61% at year 3. The fraction of the initial loss regained is

This number does not mean the ecosystem is “68.4% healthy.” It measures recovery of one variable toward one reference. Species identities, evenness, soil processes, and food-web structure may follow different trajectories. A management claim should name the response, target, and time horizon.

16. Restoration should repair processes, not only appearances

Restoration begins by identifying and reducing the continuing driver. Replanting without stopping erosion, repeated nutrient input, altered water flow, or reinvasion may produce only a temporary visual change. Process-based actions can include reconnecting habitat, restoring flow or fire regimes, rebuilding soil, reducing propagule sources, and reintroducing missing interactions.

Adaptive management treats management as a testable cycle: state a causal model, apply an intervention, monitor predicted responses, compare with reference or alternative treatments, and revise the model. A successful short-term response should not end monitoring if delayed food-web effects or repeated disturbance remain possible.

17. Design evidence that can distinguish mechanisms

Imagine a wetland where native plant cover falls after an introduced grazer appears. One hypothesis is direct consumption; another is that unusually high water levels reduced plants and also concentrated grazers. Establish replicated enclosures that exclude grazers, procedural enclosures that allow grazer entry, and unenclosed plots across comparable water depths. Measure grazing marks, plant growth, grazer density, light, nutrients, and water level through time.

Native cover recovering only in grazer exclusions would support a consumption effect, especially if damage falls before cover rises. Recovery in every enclosure would suggest a cage artifact. A strong water-level relationship across all treatments would support the abiotic alternative. Multiple mechanisms can operate together, so report effect sizes rather than forcing every result into one cause.

18. Common reasoning errors

• “A disturbance always destroys the ecosystem.” Outcome depends on regime, scale, biological legacies, and the measured response.
• “Organisms evolve because they need to survive.” Selection changes frequencies of heritable variants; mutation is not directed by need.
• “Every nonnative species is invasive.” Invasion requires establishment, spread, and impact, not origin alone.
• “The invader won because it is stronger.” Identify the resource, consumer, pathogen, habitat modification, or other mechanism.
• “Secondary succession starts from nothing.” Soil and biological legacies remain; primary succession lacks those starting conditions.
• “Succession always returns the exact original community.” history, repeated disturbance, dispersal, and feedbacks can redirect the pathway.
• “A before–after decline proves the disturbance caused it.” Background time trends require reference sites or another credible counterfactual.
• “Productivity recovered, so the ecosystem recovered.” Composition and other processes may still differ.
• “Climate change means only a higher average temperature.” Variation, extremes, water, chemistry, and timing also matter.

Check your reasoning