Success in organisms may be judged by their ability to persist through time, often in the face of harsh and variable environments. An ecological disturbance is an event or circumstance that interrupts the relationship between organism and environment, and the resistance of individuals and populations to disturbance is key to their persistence. Persistence of populations/species over geological timescales involves survival through massive environmental changes (such as extinction events) but in the shorter term and at smaller spatial scale, persistence of populations involves a suite of individual character traits and population-level responses that allow maintenance of the population.
Generally speaking a population's resistance or buffering capacity can be defined as the change to a statevariable internal to the population (abundance for example) relative to the change in some external forcing variable (drought for instance). Where the internal statevariable remains unchanged despite broad changes in the external forcing variable, the population is said to be resistant or strongly buffered. There may be a large variety of external forcing variables and a population may be more or less well buffered to various combinations of them.
Ecological resistance is a concept that asserts that an assemblage of organisms has some ability to repel invading organisms or resist a natural (or anthropogenic) disturbance. There are two components to ecological resistance, biotic and abiotic. Biotic resistance is the ability of the population to resist disease, invasive species, local predators, competitors, and parasites. Also included are anthropogenic forces such as wild harvesting (e.g., fishing). Abiotic resistance is defined as the ability of the population to resist natural and anthropogenic disturbances to the habitat itself.Fire and wind and wave damage are examples of natural disturbances while dredging, land clearing, and pollution would be common anthropogenic ones.
The type and nature of the disturbance is an important factor in determining a population's resistance. Disturbances can be defined generally as changes that occur at smaller ecological timescales and are described based upon three fundamental properties:
1. intensity (i.e., the strength of disturbance, such as the power of a cyclone),
2. frequency (i.e., how often the disturbance occurs, such as the frequency of forest fires), and
3. duration (i.e., how long an individual bout of disturbance lasts, such as the number of hours in a day that extreme temperatures occur).
For example, the disturbance of rain caused by cyclones, which would be intense and short but relatively rare, could be dramatically different than that caused by monsoonal rains which would be moderate but long and relatively common. A population's ability to resist impacts from disturbances and so to persist through time will be a function of the disturbance regime as described by these three factors as well as that population's ability to resist initial impacts and to recover in the interval between disturbances. The speed with which a population can recover is known as resistance. The resilience of a population can have a large impact on long-term persistence and as such is an integral part of resistance and buffering capacity.
Buffering capacity is the scope of a population to offer resistance to environmental perturbations, which may lead to its ability to dominate space, or persist in the face of potentially extinction-causing forces. Clearly an understanding of buffering capacity and resistance is of critical importance in management of both wild and cultivated populations of organisms.
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