Differential effects of herbivory and pathogen infestation on plant population dynamics

Plants often interact with antagonists such as herbivores or pathogens. Negative effects on individual plant performance are widely documented, but less is known about whether such effects translate into effects on population viability. In temperate forests, important herbivores include deer. During 2006-2009, I compared vital rates and population growth rates (calculated using integral projection models) between fenced exclosures and grazed control areas, using the perennial herb Phyteuma spicatum as a model species. Deer caused the largest damage to flowering individuals, removing about 24% of all inflorescences and 13% of the above-ground biomass. Only few vital rates seemed to be negatively affected by deer (mainly seed production) and this did not translate into effects on population growth rate. Contrary to expectations, population growth rates tended to be lower in the fenced exclosures in 1 year. This was likely caused by high-pathogen infestation rates, which negatively affected the probability of adult survival and growth. Population growth rate was more sensitive to changes in these vital rates than to changes in seed production. In summary, the results of this demographic study show that grazing effects may be small for long-lived herbs, and that negative effects on vital rates such as seed production may not always translate into effects on population growth rate. The findings also illustrate that other antagonists such as pathogens may be of greater relative importance for differences in population performance than herbivores.