Deer browsing and biodiversity: upsetting the balance

"A growing number of Westchester and New York State biologists believe that heavy deer browsing exacerbates the effects of other human-created impacts on the environment such as habitat loss, invasive species proliferation, soil degradation and erosion, acid rain, and pesticides – which together jeopardize the survival of many wildflowers, hinder regeneration of trees and other plants, and potentially threaten important ecological services (e.g. quality of drinking water supplies) provided by healthy forests." 
William S. Greenawalt, Esq., Chairman Citizens’ Task Force on White-tailed Deer and Forest Regeneration

The picture on the top shows a healthy forest floor, with a diversity of plants on the ground, seedlings and small trees.  The second picture shows an over-browsed forest floor, carpeted with the invasive weed garlic mustard and no seedlings or small trees visible.  You can see this for yourself if you go for a walk in the "woods" - check how far you can see(because there are no small trees or shrubs blocking your view!), how much barberry you encounter, and look for small trees - you won't find many!  At the Lasdon Arboretum they've set up an area with a fence where you can look on one side and see what the deer have done, while directly on the other side you see the expected biodiversity.

The deer are nibbling away at the natural diversity of the eastern forest.  Studies show an alarming lack of tree seedlings per acre, and once-dense thickets of rhododendron and mountain laurel are sparse and open.  In some areas of Pennsylvania, where scientists have carefully documented the forest undergrowth, there is simply no viable regeneration of trees and shrubs going on at all.  The white-tailed deer population devours shrubs, saplings, wildflowers and other underbrush - each deer chewing up thousands of pounds of plants and acorns a year.

This phenomenon is referred to as "deer over-browsing".  For many young plants, deer bite off the whole stem of the plant, including its flower, so no seed is formed, thereby preventing reproduction.  Slow-growing trees spend many years in the forest shrub layer before they can grow beyond the reach of deer.  But the biggest threat is not the loss of individual species, but a gradual overall reduction in plant diversity resulting in a “simplified” forest that is less able to bounce back from disturbance, whether natural or manmade.

“In a diverse system, there is always another species that can take up the slack,” said Larry Rhoads, Supervisor of the Forest Pest Suppression Section, PA Dept. of Conservation and Natural Resources.  “If one species is affected by an insect or a disease, there’s another that can move in, shifting the balance. When the forest consists of little more than red maple and hay-scented fern, there’s no backup, no ability to shift and take up the slack when one species is affected, and thus no resilience.” 

Nor are impacts limited to plants.  Some understory birds, most of which are neotropical migrants that journey between Eastern forests and the tropics each year, have been declining 3-5 percent a year. Studies suggest that is largely because of increased nest predation on those species. Deer may be making those species more vulnerable to predation.

 “If you’re trying to nest in the lower six feet of the forest and there is no lower six feet, you’re sticking your nest out in a very obvious place and it’s being hammered relatively soon in your nesting phase,” said William McShea, a research scientist with the Smithsonian Institution’s Conservation and Research Center.

Also, when some species are over-browsed, other plants less desirable to deer take their place. Those new species may be able to outcompete native species that were decimated by the deer population.  So even if the deer population is reduced, and maintained at lower densities over time, diversity may still not be reestablished. 

Too many deer!

One of the most frustrating things that can happen when you install a new mixed shrub border, garden, front foundation planting or annual display is waking up  to find that  some of your new plants have been eaten! Very often, the culprits are whitetail deer.   We've developed so much of the open land and woodlands that their habitat inevitably overlaps with ours.  Yet there's still enough open space for them to find shelter, allowing them to reproduce freely.

What do deer eat in their "natural" habitat?  In most areas, deer eat just about any type of vegetation or mast they can find. They can be picky eaters; if it doesn't taste quite right, a doe will spit it out. Fawns learn which plants, nuts and fruits to eat from their mothers.  A deer's sense of smell tells it whether a plant is good to eat. That's why many strongly scented plants, such as most herbs, are not favored by deer.  That's also why strong scents are used as deer repellants.

Deer forage consists of forbs and mast.  Forbs are herbaceous flowering plants that are not graminoids (grasses, sedges and rushes).  Examples of forbs include clover (which is growing in your lawn), sunflowers (which you're trying to grow for fun) and azaleas (which you were hoping would add curb appeal this spring).

The term "mast" was probably first used to describe a food source for domestic livestock.  Its definition is: "the fruit of oak or beech or other forest trees used as food for hogs and other animals.”  In wildlife biology, mast refers to the fruits of woody plants.  All trees and shrubs produce some type of fruit.  Mast is often characterized as either "hard" or "soft".  Hard mast consists of hard shelled seeds that have a relatively long shelf life and are high in fat, carbohydrate and protein - it's high in energy content, and is a key food source for survival during the winter.  Examples of hard mast include acorns, hazelnuts, hickory nuts, beechnuts and walnuts.

Soft mast is fleshy perishable fruit that is high in sugar, vitamins and carbohydrates.  Soft mast can be an important source of moisture for wildlife in times of drought, as well as a crucial energy source for some species during migration.  Examples of soft mast include cherries, crabapples, apples, persimmons, pawpaws and blackberries.

"The most serious impacts of deer on Westchester County’s flora, fauna, and ecosystems result from browsing of plants for food. Deer consume an average of 4 to 8 pounds of forage per day; one deer can eat one ton of vegetation annually."
William S. Greenawalt, Esq., Chairman Citizens’ Task Force on White-tailed Deer and Forest Regeneration

In winter, deer are often forced to feed on twigs and other woody browse, which are low in nutrients. So if they come upon some really tasty evergreen leaves or buds, they chomp away.  In early spring, when buds start to swell and leaves start to sprout, plants are particularly vulnerable because the deer may be pretty hungry by then.  

The Final Report of the Citizen's Task Force on White-Tailed Deer and Forest Regeneration was issued October 28, 2008, and is available online.  The report explains the history of the problem, cites statistics about deer density in the county and reviews the various methods that could potentially be used to address the deer overpopulation.  There's also analysis of the pros and cons of each idea.  

Soil is Not Dirt!

I'm taking Soil Science at NYBG this semester - its a fascinating combination of chemistry, biology and physics - and I just want to let you know that soil is not dirt.  It's a complex mixture of minerals, organic matter, air and water with well-defined structure that varies with the exact components.  

Our instructor defined soil as: "the natural loose covering over most of the earth's land surface that serves as the medium for plant growth".  

To support plant growth, soil needs to provide

  • both root penetration and anchorage - it's physical consistency needs to be loose enough for root penetration but solid enough to anchor the plant
  • water
  • air
  • needed and essential nutrients