Trends are one index of threat to species and can be helpful when assessing whether management intervention is appropriate. Evident trends also are far more useful as an early warning system and preventive measure than they are once recovery is required. Unfortunately, estimating trends for even relatively common species is difficult (see Dunn 2002). Establishing trends for rare species is especially costly because of the difficulty in encountering and sampling the species.
Nonetheless, where they can be credibly estimated, trends of broad groups can reveal habitats most at risk. For example, the proportion of grassland and shrubland birds in North America showing declines is about three times higher than the proportion of forest-dwelling birds. Similarly, the majority of shorebird populations are in decline.
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