I. WHY MONITOR VITAL RATES?

There are three important reasons why monitoring vital rates (primary demographic parameters such as productivity and survivorship) must be a component of any integrated avian population monitoring scheme (Baillie 1990).

First, environmental stressors and management actions affect vital rates directly and usually without the time lags that so often occur with population size (Temple and Wiens 1989, DeSante and George 1994).

Second, vital rates provide crucial information about the stage of the life cycle at which population change is being effected (DeSante 1992). This information is particularly important for migratory birds that winter in tropical latitudes, because it can determine whether management actions should be directed toward a species - temperate breeding grounds, tropical wintering grounds, or both.

Third, monitoring vital rates provides crucial information about the viability of the population being monitored and about the quality of the habitat or landscape in which the population occurs (DeSante and Rosenberg 1998). Because of the vagility of most bird species, local variations in population size may often be masked or accentuated by recruitment or lack thereof from a wider region (DeSante 1990, George et al. 1992). Thus, density of a species in a given area may not be indicative of population viability due to source-sink dynamics (Van Horne 1983, Pulliam 1988, Donovan et al. 1995).

Estimating primary demographic parameters is critical for understanding population dynamics and is directly applicable to population models that can be used to assess land-management practices by examining the effects of the landscapes they produce on vital rates (Noon and Sauer 1992). Although several studies have investigated relationships between regional landscape patterns and population trends (Sauer et al. 1996, Flather and Sauer 1996), a particular need remains to examine relationships between landscape configuration and vital rates, using standardized methods for collecting vital rate data, at various spatial scales (Villard et al. 1999). To be successful, management actions must be designed to influence the key primary demographic parameter responsible for population decline in a specific target species (DeSante 1995). Such an approach will have a much higher likelihood of success than one based on correlations with presence/absence or relative abundance data (DeSante and Rosenberg 1998, Villard et al. 1999). These considerations necessitate the continued collection of demographic monitoring data, indicate the direction in which analyses of such data should proceed, and emphasize the importance of an integrated approach to monitoring and adaptive management.

MAPS page   II: Overview