Components of Stability

Holling (1973) originally defined stability as the ability of a community to withstand disturbance with little change in structure, whereas resilience was the capacity of the community to recover following perturbation. Webster et al. (1975) subsequently refined the definition of stability to incorporate both resistance to change and resilience following perturbation. Succession is the expression of resilience. However, the criteria for measuring stability remain elusive. What degree of change can be accommodated before resistance is breached? Does resilience require the recovery of a predisturbance community structure or of ecosystem functions that support a particular community type, and over what scale of space or time?

Webster et al. (1975) developed a functional model to evaluate the relative stability of ecosystems based on the lowest turnover rates (i.e., the longest time constraint) and damping factors (i.e., factors that reduce amplitude of fluctuation) in the system. The system has not fully recovered from displacement until the slowest component of the response has disappeared. They concluded that ecosystems with greater structure and amounts of resource storage were more resistant to disturbance, whereas ecosystems with greater turnover (e.g., via consumption and succession) were more resilient. From a community standpoint, resistance depends on the level of tolerance of the dominant species to characteristic disturbances or other environmental changes (e.g., through protected meristems or propagules) or resource storage; resilience is conferred by species with rapid recolonization and growth rates. Overall, temperate forests, with high biotic and abiotic storage and slow turnover, appear to be most resistant but least resilient to disturbance, and stream systems, with low biotic and abiotic storage and high turnover, appear to be least resistant but most resilient. Resistance and resilience were found to be related inversely, with their relative contributions to stability in a given ecosystem determined by the proportions of K and r specialists (see Chapter 15). Succession appears to represent a trend from more resilient to more resistant communities.

Resistance and resilience are affected by regional species abundance and distribution. Resistance can be compromised by fragmentation, which increases community exposure to external factors. For example, trees in interior forest communities usually are buffered from high temperatures and high wind speeds by surrounding trees and usually have less buttressing than open-grown trees. Fragmentation increases the proportion of trees exposed to high temperatures and wind speeds and thereby vulnerable to moisture stress or toppling (J. Chen et al. 1995, Franklin et al. 1992). Fragmentation also interferes with the adapted abilities of species in the regional pool to recolonize disturbed sites. Species are adapted to levels of dispersal and colonization sufficient to maintain populations within the characteristic habitat matrix of the landscape. If the rate of patch turnover is increased through fragmentation, the colonization rates for many species may be insufficient to provide the necessary level of resilience for community recovery. Such changes in landscape condition may bias evaluation of community stability.

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