Hungry deer should not be barred from protected native forests, botanists have declared.
A decades-long study of national parks in Ireland has found that grazing deer in oak woodlands is actually good for diversity and helps prevent some plants from taking over the valuable ecosystem.
Researchers found that if red and sika deer and the red-sika hybrid are fenced off, or shot in annual culls as a way of protecting the forest, it becomes significantly less diverse.
But botanists from the School of Natural Sciences in Trinity College Dublin warned that attempts to reintroduce deer should be in moderation as uncontrolled grazing will also have a damaging effect on the woods.
A network of seven experimental deer “exclosures” were surveyed periodically in three national parks in Ireland over 41 years to explain how woods grow and change over time.
The sites inside protected oak woodlands in the Wicklow Mountains, Killarney, Co Kerry, and Glenveagh, Donegal, revealed the surprising results that stopping hungry deer from munching on plants actually decreases floral biodiversity.
Researcher Dr Miles Newman said deer grazing at the correct level is highly important for the conservation of native oak forests.
“Our results certainly have implications for the management of these woodlands as future policy should focus on managing deer – rather than simply excluding them – as part of the overall biodiversity objective,” he said.
“We are now working on the next step to identify what the optimal level of deer grazing may be.”
The results of the study, published in the journal Forests Ecology and Management, show that when deer are blocked from semi-natural oak woodlands the composition and abundance of forest-floor plants is greatly changed.
It said that if the deer and other large herbivores are no longer a threat there is a significantly less diverse collection of plants as some species begin to dominate.
Semi-natural woodlands are a globally important relict ecosystem for biodiversity but it now makes up less than 2% of Ireland’s land cover.
Fencing is increasingly used to protect forests but so too is culling, what the researchers called an emotive issue and beset with practical difficulties.
The botanists said that when appropriate culling is not achievable fencing remains a viable alternative but should only be used for a maximum of 12 years.
The researchers added: “Woodland ecology, it seems, is a little like life – it’s often best to do things in moderation.”