Population Variation in the Life History of a Land Fish, Alticus arnoldorum, and the Effects of Predation and Density

Abstract

Life history variation can often reflect differences in age-specific mortality within populations, with the general expectation that reproduction should be shifted away from ages experiencing increased mortality. Investigators of life history in vertebrates frequently focus on the impact of predation, but there is increasing evidence that predation may have unexpected impacts on population density that in turn prompt unexpected changes in life history. There are also other reasons why density might impact life history independently of predation or mortality more generally. We investigated the consequences of predation and density on life history variation among populations of the Pacific leaping blenny, Alticus arnoldorum. This fish from the island of Guam spends its adult life out of the water on rocks in the splash zone, where it is vulnerable to predation and can be expected to be sensitive to changes in population density that impact resource availability. We found populations invested more in reproduction as predation decreased, while growth rate varied primarily in response to population density. These differences in life history among populations are likely plastic given the extensive gene flow among populations revealed by a previous study. The influence of predation and density on life history was unlikely to have operated independently of each other, with predation rate tending to be associated with reduced population densities. Taken together, our results suggest predation and density can have complex influences on life history, and that plastic life history traits could allow populations to persist in new or rapidly changing environments.

Fig 1. Predicted changes in female reproduction and growth as a function of potential changes in mortality that are biased towards older females (Scenario A), younger females (Scenario B), or equal across age classes (Scenario C).

Fig 2. The relationship between predation and (A) adult density and (B) juvenile density.

Symbols are shaded according to the level of predation, from white (low predation) to black (high predation).

Fig 3

Variation among populations in reproductive investment as a function of predation (A). Also shown (B) are population coefficients computed using the best-supported model for predation reported in Table 1. The dashed lines around coefficients are the upper and lower 95% confidence interval of the computed trend between reproduction and predation. Symbols are shaded according to the level of predation, from white (low predation) to black (high predation).

Fig 4

Variation among populations in growth rate as a function of adult density (A). Also shown (B) are population coefficients computed using the best-supported model for adult density reported in Table 2. Symbols are proportionally filled according to adult density, from white (low adult density) to black (high adult density). See Fig 3 legend for other details.

Fig 5. Variation in ovarian weight and growth rate among populations.

Plots show population coefficients computed from the best-supported model reported in Tables 1 and 2. Symbols are shaded according to the level of predation, from white (low predation) to black (high predation).