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Species' range limits can be caused by environmental gradients, and in such cases, abundance is thought to be highest in the center of a species range and decline towards the edge (the abundant-center model). Although in theory decreased abundance is caused by a decline in performance at the edge, it has been shown that performance and abundance are not necessarily related. Few studies have compared abundance and performance in center and edge populations of endemic species, whose ranges may be restricted by the availability of specialized habitat rather than environmental gradients across their range. Additionally, range-wide studies that examine both northern and southern edge populations are rare. We used Roan Mountain rattlesnake-root (Prenanthes roanensis), a perennial plant endemic to the Southern Appalachians (USA), to compare abundance and performance between central populations and populations at the northern and southern edges of the range. To account for multiple fitness components across the life cycle, we measured performance of edge populations as vital-rate contributions to population growth rate compared to the center. Abundance did not decline at the range edge, but some vital-rate contributions were lower in edge populations compared to central populations. However, each edge population differed in which vital-rate contributions were lower compared to the center. Our results do not support the abundant-center model, and it appears that local factors are important in structuring the range of this endemic species. It is important to recognize that when implementing conservation or management plans, populations in close proximity may have substantial variation in demographic rates due to differences in the local environment.

Life-history traits can play important roles in determining the course of ecological species interactions. We explored the consequences of host age on a host-pathogen interaction by quantifying pathogen frequency in an age-structured host population. Our project was motivated by an interest in whether the demographic structure of a host population has consequences for species interactions. In 2 successive years, we planted large cohorts of the perennial herb Plantago lanceolata in its natural environment and observed infection by Fusarium moniliforme, a non-lethal floral fungal pathogen, over 3 years. We documented substantial variation of pathogen frequency across years and between cohorts. Logistic regression revealed that pathogen frequency increased with the number of inflorescences produced and with evidence of prior pathogen presence, whereas it decreased with increasing plant longevity. In addition, interannual variation and an age-year interaction contributed to the observed pathogen frequencies. There was a significant positive effect of age on pathogen frequency overall, but this was not consistent over all ages. Pathogen frequency was higher in 2-year-old plants than in 1-year-olds, suggesting that age-structure can influence the host-pathogen interaction. This pattern did not continue into 3-year-old plants. A possible explanation for this is that selective mortality allows only generally robust plants, and consequently the most resistant plants, to survive to the oldest ages.