Population dynamics in central and peripheral populations of a narrowly endemic plant
As climate change begins to shift species’ ranges and cause population extinctions, it is critical to understand how species’ ranges are structured. The abundant-center model postulates that an environmental gradient in habitat quality from the center to the periphery of the range results in reduced abundance and performance at the range periphery. A demographic approach is an ideal method for testing the performance predictions of the abundant-center model. Estimates of population growth rates, which account for both the positive and negative contributions of multiple fitness components across an individual’s life cycle, can be compared between central and peripheral populations, and the demographic vital rates that reduce population growth rate in peripheral populations, and potentially limit the range of the species, can be identified. Using four years of field data, we parameterized integral projection models for Roan Mountain rattlesnake-root (Prenanthes roanensis), a perennial plant of the Southern Appalachians, to test whether population growth rates are lower in peripheral populations compared to center populations. Additionally, we examined vital rate contributions to the variation in population growth rate between center and peripheral populations to determine which vital rates may be involved in limiting the northern and southern edges of the range.
Population growth rates were high in the center of the range, but peripheral population growth rates were not necessarily lower. Because the two lowest population growth rates occurred in peripheral populations, without a larger sample size, we cannot rule out the possibility that habitat quality is more likely to be lower at the range periphery. The vital rates responsible for the differences between central and peripheral population growth rates varied among populations, suggesting no single vital rate limits the northern or southern edges of the range. Overall, our results suggest that local site factors, rather than latitudinal gradients, are more important in population persistence, and populations will sustain themselves through different life-history contributions. Therefore, each population may respond differently to climate change. Furthermore, it is important to recognize that when implementing conservation or management plans, populations in close proximity may have substantial variation in demographic rates.