Habitat loss is the leading cause of biodiversity loss, and in addition to direct losses, populations may experience transient negative effects due to increased density in remaining habitat. The results of these effects for populations may depend on the nature of intraspecific competition. Scramble competition is likely to have a more negative effect than contest competition, because fewer individuals will be unable to acquire sufficient resources. We were interested in describing the effect of increased caterpillar density on survival and pupation of a migratory butterfly, Vanessa cardui. We reared 80 larvae on an artificial diet in 30 mL containers with ten replicates each of one, two, and three larvae and five replicates with four larvae. We maintained the containers at a temperature of 21° C and checked them approximately every other day.
Results/Conclusions
Overall survival through three weeks was 0.25, but it varied dramatically with the initial number of caterpillars. Containers that started with a single larva had the most success, with a survival rate (95% CI) of 0.70 (0.354, 1.000). Containers with two, three, and four larvae had average survival rates of 0.15 (0.000, 0.322), 0.20 (0.078, 0.322), and 0.20 (0.061, 0.339), respectively. The containers with two or more caterpillars were also slower to pupate, with only 13.1% of surviving caterpillars having entered their chrysalis by the end of week two, compared with 57.1% of surviving caterpillars that were alone in their containers. Overall, these results are more consistent with the scramble model of intraspecific competition, with mortality in two- and three-larvae containers higher than that required to bring the density down to one caterpillar. If these preliminary results hold true for butterflies facing loss of breeding habitat, then the effects of habitat conversion on population size may be larger than expected based on area lost alone.