Spatio-Temporal Variation in Age Structure and Abundance of the Endangered Snail Kite: Pooling across Regions Masks a Declining and Aging Population

Abstract

While variation in age structure over time and space has long been considered important for population dynamics and conservation, reliable estimates of such spatio-temporal variation in age structure have been elusive for wild vertebrate populations. This limitation has arisen because of problems of imperfect detection, the potential for temporary emigration impacting assessments of age structure, and limited information on age. However, identifying patterns in age structure is important for making reliable predictions of both short- and long-term dynamics of populations of conservation concern. Using a multistate superpopulation estimator, we estimated region-specific abundance and age structure (the proportion of individuals within each age class) of a highly endangered population of snail kites for two separate regions in Florida over 17 years (1997-2013). We find that in the southern region of the snail kite-a region known to be critical for the long-term persistence of the species-the population has declined significantly since 1997, and during this time, it has increasingly become dominated by older snail kites (> 12 years old). In contrast, in the northern region-a region historically thought to serve primarily as drought refugia-the population has increased significantly since 2007 and age structure is more evenly distributed among age classes. Given that snail kites show senescence at approximately 13 years of age, where individuals suffer higher mortality rates and lower breeding rates, these results reveal an alarming trend for the southern region. Our work illustrates the importance of accounting for spatial structure when assessing changes in abundance and age distribution and the need for monitoring of age structure in imperiled species.

Fig 1. Age-class specific apparent annual range-wide survival of snail kites in Florida, modified from Reichert et al. [16].

Results from previous analyses reveal that adult survival declines significantly in snail kites beginning at age 13. Estimates are based on modeling encounter histories of 2084 known age individuals from 1992–2008 using an extension of the Cormack-Jolly-Seber model for open populations accounting for age-class specific variation in survival [16]. Error bars represent 95% confidence intervals.

Fig 2. The primary wetlands located within the breeding range of the snail kite in Florida that were surveyed from 1997–2013.

Wetlands that comprise the snail kite’s breeding range in Florida were previously classified into two regions (north and south) based on patterns of observed annual dispersal between wetlands [20]. All wetlands were systematically surveyed for banded and unbanded snail kites multiple (4–6) times during the peak of the snail kite breeding season (March 1^st–June 30^th) from 1997–2013.

Fig 3. Superpopulation size of adult snail kites at the regional and range-wide scales.

Derived point estimates of superpopulation size (⦁) with 95% CI’s of snail kites during the breeding season (March1-June 30^th) in northern region, southern region, and over the entire breeding range. Estimates were derived based on model-averaged parameter values using extensions of the open robust design multistate model. Solid and dashed grey lines represent predicted mean and 95% CI’s from GAM used to assess trends in estimates of superpopulation size. Blue lines indicate periods of significant increase and red lines indicate periods of significant decrease in population size, both identified using methods of ‘finite differences’.

Fig 4. Age distribution of snail kites at the regional and range-wide scales.

Estimated proportions of adult snail kites across three biologically-relevant age classes (0–1 years, 1–12 years, and 13+ years) in the northern region, southern region, and over the entire breeding range. Proportions of individuals in each age class were calculated using model-averaged estimates for the superpopulation size of banded individuals in each age class.

Fig 5. Comparison of estimates with 95% CIs of range-wide superpopulation size of snail kites during the breeding season in Florida using single-state and open robust design multistate models.