Bottom-Up Effects on Persistence of a Specialist Predator: Ant Invasions and Horned Lizards

Abstract

Maintaining biodiversity within fragmented landscapes has become a primary focus of ecology. Despite many theoretical and comparative studies on species persistence following fragmentation, experimental studies investigating the mechanisms behind population declines remain rare. In recent years, coastal horned lizards (Phrynosoma coronatum) have disappeared throughout much of their range in coastal southern California. In addition to the direct impact of habitat loss due to urbanization, horned lizard populations continue to decline in the remaining fragmented landscape. One factor that may contribute to their decline is the invasion of exotic species from the surrounding urban matrix. Horned lizards are ant specialists, and they may be particularly vulnerable to changes in the native ant community resulting from the invasion of Argentine ants (Linepithema humile). In this study we examined how Argentine ants influence horned lizard growth rates by raising hatchlings on prey typical of invaded and uninvaded sites. Hatchling horned lizards maintained positive growth rates on a diet of just one native ant species (Crematogaster californica). However, on a diet of Argentine ants or arthropods typical of an invaded community, horned lizard growth rates were either negative or averaged near zero. In addition, when lizards were switched from a diet of Argentine ants to native ants, growth rates increased. One factor contributing to growth was foraging rate; capture success appears partly responsible for why horned lizards specialize on ants vs. other, harder to capture, arthropods. Moreover, a comparison of diets among age classes of coastal horned lizards suggests a diversity of ants is necessary to support lizard populations. While stochastic demographic and environmental processes are often invoked to explain the decline of populations following fragmentation, it appears that horned lizards are disappearing from habitat remnants in southern California at least in part due to the deterministic effects of a biological invasion.