Deer population surge raises concerns for biodiversity in Finland

The white-tailed deer population in Finland has surged significantly since the early 2000s. This increase has raised concerns regarding the damage the deer inflict on crops and gardens, with their impact on both cultivated and ornamental plants being readily observable. Recently, damages caused by deer have also been noted in natural vegetation, particularly in habitats essential for invertebrate species. One such species affected is the rock azure butterfly.

The rapid expansion of the deer population is especially pronounced in the archipelago and coastal areas of southwestern Finland. According to Maija Mussaari, biodiversity coordinator at the Regional State Administrative Agency of Southwest Finland, contributing factors to this growth include milder winters, low snowfall, small predator populations, and winter feeding initiatives aimed at increasing the deer population.

In Turku’s Ruissalo, white-tailed deer and roe deer hinder the regeneration of Finland’s largest oak forests. Young saplings face challenges due to browsing and rubbing by these animals. The swift growth of deer populations threatens rare butterfly species, such as the rock azure and Apollo butterflies, whose larvae rely on specific plant species for survival.

To protect the rock azure, an area has been fenced with landowner approval on Kemiönsaari to prevent deer from consuming its larval food plant, the large-flowered hepatica. Moreover, fencing has shown positive results in conserving the butterfly population where its food plants are safeguarded from deer.

Mussaari emphasizes that while fencing is not a sustainable long-term solution for protecting flora and fauna, it has proven beneficial in allowing host plants to thrive without deer interference. The goal is to halt overgrazing by the white-tailed deer, with hopes for more research on the species’ implications for biodiversity and sustainable population management.