The ecological effects of exploitation are often not independent of other factors, rather they are strongly interactive. The productivity of an ecosystem, for example, may broadly determine its ability to absorb the effects of exploitation. In highly productive systems the population growth rates of most species will be high, and may compensate moderate mortality inflicted by exploitation. In contrast, unproductive systems like the deep sea often show rapid resource collapse and slow recovery from the impacts of exploitation. The extent of natural disturbance may also be important. If natural mortality from disturbance is high, very little additional mortality may be tolerated. Climatic variability can influence both productivity (e.g., through droughts) and disturbance regime (e.g., through storms and floods). Thus, it can be critical in influencing the effects of exploitation on a population or ecosystem. A population that is very resilient to exploitation in a good year may rapidly collapse in a bad year. Similarly, the effects of human disturbance, such as habitat fragmentation, pollution, or introduction of exotic species, can alter the outcome of exploitation drastically. Accounting for natural (and anthropogenic) variability is thus critical for achieving sustainable exploitation.
often need to be analyzed in a historical context, using a combination of archeological, historical, and recent ecological data. The recent field of historical ecology attempts this. Others have attempted to quantify the effects of exploitation along spatial gradients of exploitation pressure, that often scale with human population density. This space-for-time substitution, however, requires that at least one or more near-pristine sites are included and that factors other than exploitation remain relatively constant across sites. Finally, one can exclude exploiters from replicated protected areas and compare those with adjacent exploited areas. This experimental approach is potentially powerful, but may be influenced by the size, placement, and management of the protected area. Also, the trajectory of recovery in protected areas may not simply reverse the trajectory of exploitation.
Ideally, all three approaches (historical ecology, surveys along exploitation gradients, and exclusion experiments) should be combined in order to understand the full effects of exploitation. Within and among each of these approaches, deep insights are often gained by combining the trajectories of many exploited systems, and searching for common patterns and drivers. This approach can be statistically formalized using recent advances in meta-analysis. Such analyses have proved very important in revealing the general effects of exploitation on populations, communities, and ecosystems.
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