The latest issue of the Ibis, the journal of the British Ornithologists’ Union, had several excellent papers. One was an overview of the impacts that deer browse has on habitat quality and subsequently bird life. I’d like to highlight some salient points, because they illustrate the interconnectedness of ecosystems, and can serve to help the average person understand that too many deer don’t just mean fewer wildflowers in the woods.
We’ll start with that simple point. Deer browsing changes under- and mid-story vegetation not only by changing the species composition (skewing it towards unpalatable species) but by reducing the abundance and density of trees, shrubs, and vines. For birds, this can mean a loss in available nest sites and roost sites, and an increased vulnerability to nest predation. This seems fairly intuitive. The impacts of deer browse on the food supply of birds is often less direct.
Deer prefer to eat growing shoots of plants, which not only affects the growth of the plant, but may also delay or prevent flowering. This can reduce the number of pollen-seeking insects on which birds may feed during the breeding season. No flowers means no fruits or seeds, which birds consume later in the season, often fueling migratory flights or providing winter forage. Of course, deer also eat fruit and seeds, putting them in direct competition with birds.
Many insects, such as lepidoptera larvae which are so important to birds feeding nestlings, also prefer to feed on actively growing plant tissue just as the deer do. This paper notes a number of studies have found that these invertebrates can be reduced by deer browse in the vegetation layers that can be reached by deer.
Leaf litter thickness often also changes in forests with high densities of deer. Their browsing can alter the amount of sunlight that reaches the forest floor, and decreased density of trees and shrubs, as well as direct grazing on herbaceous plants, often coincides with more grasses or bare ground. This can all result in reduced number of some types of leaf litter invertebrates that are important to ground foraging bird species.
The New York Times just had a short article on the stupendous increase in White-tailed Deer in the United States — doubling in population the last twenty years to an estimated 32 million animals. That’s 12 million more than were here prior to European settlement, in far less space. The Times graphic shows that here in Michigan, I have a 1 in 86 chance of hitting a deer with my car in the next year, the second-highest odds in the nation.
The Ibis paper (and indeed the entire issue) is available for free at the journal home page.
Gill, R. M. A. and R. J. Fuller. The effects of deer browsing on woodland structure and songbirds in lowland Britain. Ibis 149:119-127. doi:10.1111/j.1474-919X.2007.00731.x
Here are some links to previous posts I’ve done that included the impact of deer overpopulation: