Species abundance is sometimes used as an indicator of extinction risk, perhaps because rare species can differ from abundant species in terms of traits such as resource use, and otherwise because small populations are more sensitive than large populations to the negative effects of demographic stochasticity. Infrequently examined but equally important is a species’ persistence in a community through time: transient species are more likely to occur as the result of chance dispersal or environmental stochasticity, whereas persistent species may be more closely tied to their local environment. Transient species tend to be rare, and persistent species tend to be abundant; however, there many are ecologically meaningful exceptions to the rule. In this study, we examined whether plant species' responses to experimental nutrient addition depended on their persistence, rarity, or a combination of the two, using data from an 8-year nutrient addition experiment replicated in 27 grasslands on 4 continents. We treated persistence and rarity as complementary axes for predicting species responses to nutrient addition, and also took a categorical approach, classifying species as persistent-abundant, persistent-rare, transient-abundant or transient-rare, and then examining how the effects of nutrient addition differed across these four categories.
Results/Conclusions
Species' persistence and abundance were positively correlated (R2 = 0.52, P<0.001). Consequently, when dividing species categorically, most species observations (a species in a plot observed across 8 years) were either persistent-abundant (28%) or transient-rare (40%). Nonetheless, 18% of species observed were persistent-rare and 14% were transient-abundant. The effects of nutrient addition on cumulative species richness (number of species observed in a plot over time) varied across categories: within sites, nutrient addition reduced persistent-rare species richness, but increased transient-abundant species richness, and had no effect on either persistent-abundant or transient-rare species richness. Nutrient addition increased the increased extinction risk (reduced the persistence) of abundant species only when they were persistent under ambient conditions, but increased extinction risk of rare species regardless of their persistence under ambient conditions. Meanwhile, nutrient addition increased the abundance of rare species, but only when they were persistent under ambient conditions. These treatment effects on abundance and persistence affected species' categorization, with some becoming transient rather than persistent, and others becoming abundant instead of scarce. This framework demonstrates that distinguishing persistent from transient species generates more effective predictions of extinction risk, highlighting the value of long-term data for predicting species responses to perturbations.