Top‐down effects on population dynamics of Eriocrania miners (Lepidoptera) under pollution impact: does an enemy‐free space exist?

Abstract

In areas disturbed by pollution, populations of herbivorous insects may reach high densities. This study was conducted to test one of the hypotheses attempting to explain this phenomenon – that pollution creates an enemy‐free space for herbivores. We monitored the population densities of Eriocrania leaf‐mining moths on mountain birch, Betula pubescens subsp. czerepanovii, in the vicinity of the nickel–copper smelter in Monchegorsk (Kola Peninsula, NW Russia) over twelve years (1994–2005) and assessed larval mortality from parasitoids, ants and birds. The mean density (mines/birch leaf area) of Eriocrania populations in severely disturbed habitats (industrial barrens) was about 2.7 times higher, and peak densities 2–4 times higher, than in pristine forests. Temporal population variability (measured as the coefficient of variation of log‐transformed densities) increased with an increase in pollution load. The proportion of infested trees was not affected by pollution, but mine distribution among trees was more clumped in the polluted sites. Eriocrania populations in disturbed sites fluctuated independently of each other, whereas populations in forest sites fluctuated in synchrony. Larval mortality caused by parasitoids was lower in disturbed sites only during those years when populations of Eriocrania reached high densities; mortality from ants and birds did not differ between disturbed and undisturbed habitats in either high or low density years. In undisturbed forests the rate of population change correlated negatively with previous‐year parasitism, suggesting that parasitoids are the key demographic factor in Eriocrania population dynamics. In the habitats heavily disturbed by pollution no such correlation was found, which means that negative feedback with parasitoids is disrupted: parasitoids fail to follow host population growth, thus creating an enemy‐free space for Eriocrania leafminers.