Understanding how anthropogenic factors influence plant demography and population persistence requires understanding how individual vital rates (e.g., germination, survival, growth, reproduction) respond to global change and how those vital rates combined influence overall population growth and declines. However, few studies have examined demographic responses to global changes across a plant’s entire life cycle. I experimentally manipulated nitrogen and deer herbivory, two drivers of plant population decline and local extinction (defined here as species that have disappeared from a particular area), in the field to test their effects on the population demography of nine confamilial pairs of locally extinct and extant prairie species once found in Michigan prairies. This experiment allows me to investigate: (1) How do vital rates respond to nitrogen addition and deer herbivory? (2) Do these responses differ between locally extinct and extant species? Finally, I use Integral Projection Models (IPMs) to ask how nitrogen and herbivory affect population growth rates (λ) for a subset of species.
Results/Conclusions
Nitrogen decreased survival, especially for locally extinct plants (N x status Χ2=4.16, p=0.04). Extant species experienced higher survival than locally extinct species, but only when small (size x status Χ2=6.06, p=0.01). Extant species also grew more annually than locally extinct species, and this difference was exacerbated under nitrogen addition (N x status Χ2=7.23, p=0.007). Species’ growth was higher when deer were excluded (herbivory Χ2=5.59, p=0.02). Tall plants were more likely to flower, especially extant species in nitrogen-treated plots (height x nitrogen x status Χ2=5.21, p=0.02), and produced more reproductive structures than small plants (size Χ2=7.07, p=0.008). IPM results suggest that locally extinct species demonstrate lower population growth rates, mostly due to lower survival under nitrogen addition. By examining responses across all components of a plant’s life cycle, I find evidence that nitrogen addition negatively impacts plant populations, despite its benefits to growth. This study provides a framework for using recently extinct species to examine contemporary threats to population persistence in changing world and demonstrates the importance of examining the effects of multiple anthropogenic drivers on population dynamics.