Population decline assessment, historical baselines, and conservation

Scientific and historical knowledge of worldwide animal-population decline is fragmentary at best. However, understanding historical population trends is essential for informing best efforts to preserve species. We reviewed the literature of long-term studies of population declines across a set of animal taxa and found that only 15% of the studies used data older than 100 years, and 58% of the studies lacked continuous data. Based on our review, we describe five general approaches to studying population declines: counting, correlative, evolutionary, geochemical, and historical. The most common method of population assessment was a census/counting approach (75% of studies) followed by a range mapping/correlative approach (17% of studies). Evolutionary, geochemical, and historical approaches are used less often but, in combination with traditional counting and correlative methods, they hold great potential as tools for conservation. The multidisciplinary approaches we identify and advocate here will be useful for understanding and potentially reversing population declines and ultimately stemming the tide of extinctions currently underway.