The following essay appeared in the April 2016 newsletter of the Columbia Audubon Society (The Chat), but we thought we’d repost it here as it deals directly with our struggles against abundant deer on this farm.
While deer are a natural part of many North American ecosystems, there is concern that some populations have grown beyond the carrying capacity of their environment. Studies using exclosure fences have documented more biodiversity and lusher growth in areas from which deer are restricted, and the reverse in areas where deer are abundant. While this has direct consequences on botanical diversity, it also has disturbing implications for birds which share this disturbed habitat. Al Cambronne wrote about this in his fascinating 2013 book Deerland:
Deer reduce the total density of plants in the understory, but they also alter species composition and diversity. Scientists don’t understand (the) indirect effects of overabundant deer as clearly as they do the more simple, direct ones . . . If the forest understory is gone completely, it stands to reason that ground-nesting birds will be more exposed to predators and the elements . . . As plants in the midstory die or graduate into the canopy, birds that nest and forage there will be homeless too.
Dramatic deer-exclosure study in Wisconsin; image courtesy of Dr. Thomas Rooney,
Wright State University.
Cambronne cites a study by biologist David deCalesta in which deer enclosures (rather than exclosures) were used to study the effects of different population densities on forest health. These enclosures simulated deer densities of 10, 20, 40, and 65 deer per square mile; for reference, Cambronne states that many suburban areas are over 100. After ten years, the researcher counted birds in each area.
“The pens with the most deer had 37% fewer birds and 27% less species diversity. Some species disappeared as soon as deer reached densities of twenty per square mile. Even phoebes and robins had gone missing at sixty-five deer per square mile.”
A followup study on the same landscape by Dr. Timothy Nuttle, twenty years later, suggested that these effects could linger for many years. As Cambronne quotes Nuttle,
. . . we found that one simple relationship seems key. Both deer and caterpillars like to eat leaves from the same trees, and for the same reasons. They’re more delicious and digestible than the leaves on other trees. If deer eat those leaves first, before the seedling or saplings can even turn into trees, then caterpillars don’t get them. Fewer caterpillars in the canopy, fewer birds . . . this browsing legacy persists long after . . . trees have escaped browsing by growing into the canopy.
All this reminded me of a talk Frank Thompson gave at the January 2015 CAS meeting, discussing how sufficient shrubby cover improved the fledgling survival rates of songbirds, even species that nested high in the forest canopy (see the March 2015 Chat). Could abundant deer affect this need as well?
Joanna and I started thinking about trying an exclosure experiment on our own diversified farm, where abundant deer have long been a source of damage and frustration in our fields, pastures, and woods. Hunting never seems to make a dent in the overall population, and a whole-farm deer fence has seemed impractical and unaffordable. So we decided to use the birding field trip and work day we’d scheduled for early March to help implement our own experiment.
After enjoying two hours of birding with 28 species recorded, attendees helped us work on our low-tech, low-cost deer exclosure: a fence made of stacked brush, the natural result of our winter forest-improvement work. Combined with a higher electric line, we hope to exclude deer entirely from this ~1/2 acre test area, which lies directly across known deer trails. We’re interested to see how the forest responds this summer and in future years.
Trialing a brush & electric line fence as a low-cost deer barrier; photo by Eric Reuter.