Identifying drivers of pumpkinseed invasiveness using population models

Pumpkinseed (Lepomis gibbosus) is listed among the most invasive fish species and has been demonstrated to have adverse impact on native species when present in high numbers. However, not all introductions result in high density populations. There are several possible underlying mechanisms behind this variation in population density (e.g. water quality, availability of nesting substratum and biotic interactions), but their relative importance remains poorly known. With this in mind, we studied vital rates (i.e. growth, maturation and reproduction) of pumpkinseed in 19 isolated standing waters of different pumpkinseed density. The fish were collected in early summer to determine their density, sex, age, growth and reproductive effort as well as environmental variables (i.e. availability of nesting substratum, acidity, nutrient concentrations, fish assemblage structure). To construct a population projection model with which to assess the relative importance of each vital rate for the growth of the populations, a stable population structure was assumed. Most environmental variables that affected vital rates (e.g. pH effect on individual growth) had little effect on population growth, or the associations were spurious (e.g. the negative effect of nesting substratum availability on gonad production). The environmental effects were dictated by a strong density dependent feedback of pumpkinseed density on the growth of age 2 fish, and gonad size and maturation state. This finding has important repercussions for management of pumpkinseed invasions: if only part of the population is removed or if complete eradication is followed by a re-introduction, then the population will rapidly recover to its former size. It was not possible to identify environmental drivers of pumpkinseed survival because the data had to be pooled across populations in order to estimate survival rates. However, a negative correlation was found between pumpkinseed and predator density, indicating that predator-induced mortality may be key in determining pumpkinseed invasiveness. Although the measure still needs thorough evaluation, introduction of native piscivores, especially northern pike (Esox lucius), may be a suitable way to prevent pumpkinseed from becoming the dominant species and reduce damage to local biodiversity.