Disturbance and thresholds

Ecosystems have a history of disturbance that influences the current composition of the ecosystem. Self-organization capacity in concert with natural selection and evolution results in an ecosystem that reflects past conditions and disturbances. Some organisms are more successful in relative stable conditions; some can only compete successfully in dynamic and ever-changing conditions. Disturbances (e.g., fires, floods, tides, seasons, and plate tectonics) periodically introduce forces that reset ecosystem structure at varying temporal and spatial scales. If a disturbance occurs that creates a large change in the abiotic or biotic environment, an ecosystem can cross a threshold which makes it unable to recover to its previous state. Examples include loss of soil fertility that cannot be restored, changes in water table levels that lead to desertification or wetland development, loss of a keystone species, and introduction of new diseases or species.

Human-managed systems typically lose structural and functional diversity (fewer species providing fewer goods and services) and become more spatially uniform over time, for example, agricultural fields, managed forests, and urbanization. The alteration of natural disturbance cycles (e.g., fire suppression and flood control) disrupts the patch dynamics of ecosystems and generally reduces habitat complexity and thus biodiversity. As uniformity increases, resistance to disturbances decreases. For example, spatially extensive monocrops, whether mountain forests, urban trees, or agricultural crops, can be destroyed by a single virulent disease vector. Monocrops tend to be genetically similar, do not have the variability to resist diseases, and diseases spread rapidly to adjacent identical, susceptible plants.

This homogenization of human-managed systems highlights the differences between engineering resilience and ecological resilience. Designing for traditional engineering resilience seeks stability and permanence; systems with high engineering resilience return to a stable equilibrium point quickly after a disturbance. Ecosystems do not exist around a stationary equilibrium point. Ecological resilience is a measure of how large a disturbance an ecosystem can absorb and maintain its original structure and function. Major disturbances, whether chronic or instantaneous, such as volcanic eruptions, glacial advances or retreats, overfishing, or sea-level rises and falls, may cause an ecosystem to cross a threshold. The ecological resilience of ecosystems can be exceeded by a major disturbance. This typically results in a dramatic change in species composition and the start of a new self-organization process perhaps with different abiotic conditions.

Ecological engineering recognizes the stochastic nature (unpredictability) of disturbances in space and time and seeks designs that tolerate multiple states while still meeting the design purpose, thus benefiting humans and protecting the environment. Ecological engineers recognize the hubris in the thinking that ecosystems need to be managed constantly or extensively to provide goods and services. Ecological engineering designs take into account the variability in time and space of processes and species composition across the landscape.

Traditional engineering design incorporates factors of safety in design parameters. Risk analysis estimates the probability of failure of the design, and energy and material are used to enable the structure to resist failure that can result in harm to humans or infrastructure. This is called fail-safe design. The risks are known and are more or less predictable. Efficiency, constancy, and predictability are guiding principles for traditional engineering. Ecological engineers recognize that over time the forces of nature can overcome any affordable design. Persistence, change, and unpredictability are hallmarks of ecological theory. Ecosystems are complex systems with many variables and risk may come more from unknown (or unrecognized) sources than from known sources; that is, the probability of occurrence of the risk is unknown or in some cases, the risk itself is unknown. For this reason, ecological engineering strives for safe-fail design, that is, when the design fails, the failure takes place such that extensive harm to humans, infrastructure, and the ecosystem is minimized. When considering design alternatives, ecological engineers choose the one that has the best worst-case outcome.

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