ASSESSING THE IMPACT OF EXCESSIVE DEER POPULATIONS
Summary
For many years volunteers of the North Branch Restoration Project (North Branch) have
been observing the increasing impact of browsing deer. The first mention of deer in the
records of the Project was when a doe was observed in Miami Woods in 1980. Populations
increased slowly during the following years until about 10 or 15 years ago when negative
impacts began to be observed by volunteers as some herbaceous species began to disappear
and browse lines appeared on woody vegetation. Since then, the deer population has erupted
and negative impacts have greatly increased.
Extreme adverse impacts currently exist in Miami Woods and St Paul Woods. It appears that
impacts are intensifying in other North Branch sites, but there has been no systematic
assessment.
In an attempt to quantify negative impacts on vegetation, a rapid assessment of impacts on
woody vegetation was made during the summer of 2012 and combined with longer term
observations of impacts on herbaceous vegetation. The pattern of adverse impact on
ecological health with increasing abundance of deer illustrates the need for management of
deer beginning at low densities.
Background
Browsing by white tailed deer is a natural part of healthy prairie and woodland ecosystems.
It is not harmful when populations are of light density and in balance with the vegetative
community. Historically, predators held deer populations in check at a density of about one
per 40 acres of natural area or 16 per square mile. (Wisconsin Department of Natural
Resources, et. al.) Currently within the North Branch watershed densities are far in excess of
that number.
Studies have shown that above a density of 16 per square mile, vegetative quality begins to
deteriorate together with insect and bird populations that rely upon the diminishing plant
species. Currently within the North Branch watershed densities are far in excess of 16 per
square mile, some sites by a multiple of 10 or more.
At a “normal” level of browsing there is no negative impact on the populations of plants
preferred by deer such as lilies and orchids, and those plant populations can be sustained
without special protection from deer. However, as densities increase, a continuum of plant
species decline in abundance and eventually disappear entirely.
The assessment method discussed here is focused on five levels of adverse impact. The “no
affect level” can be thought of as being the zero level at the base of the five point scale.
Woody Vegetation
To assess the impact of the unmanaged herd, a rapid assessment of conditions has been made
by reviewing the condition of woody plants within the sites being restored by the volunteers
of the North Branch Restoration Project.
A checklist was developed and visits were made to all North Branch sites during the period
from July 31 to August 3, 2012. The primary focus of the assessment was on the intensity of
browsing on shrubs and on tree seedlings and saplings. A key indicator was the presence of a
browse line along wooded edges adjacent to lawns, parking areas and paths. A second focus
was on the intensity of browsing on native woody species and on common buckthorn. This
provided a rough assessment based on visual impressions. Observations were made by
briefly walking through the sites and taking notes.
The five level classification of adverse impact on woody vegetation is summarized below.
1. No visible browse
line
No obvious browsing of native woodies or buckthorn.
2. No visible browse
line.
Native woodies lightly browsed, buckthorn not browsed.
3. Partial browse line,
Native woodies heavily browsed, but some still present in
edges. Buckthorn browsed, but foliage extends to the
ground.
4. Browse line continuous
Native woodies and buckthorn heavily browsed, but
buckthorn foliage extends to the ground.
5. Line sharply defined,
Native woodies & buckthorn twigs gone up to five feet.
Forbs
The woody assessment gives a rough picture of the relative severity of impact by deer on
North Branch sites, but it does not address the question of the impact on the remainder of the
vegetative communities. This is particularly the case with broad leaved plants (forbs). Deer
do not eat grasses or sedges except for a brief period in early spring when the new growth of
some sedges and grasses is eaten. This stops when new growth of broad leafed plants
becomes available and has no obvious lasting impact on the browsed plants.
Deer have a hierarchy of preferences among food plants which is generally consistent
although some have individual preferences, probably learned as fawns from their mothers.
The result is that as deer densities increase, preferred plant species are progressively
impacted and then disappear.
At very light densities deer have little obvious impact on woody species, but impact on
preferred forbs begins long before browse lines appear or native woody species are
negatively impacted. Therefore even if a site falls into category one based on assessment of
woody species, negative impacts may be under way in the forb populations. By the time
buckthorn begins to be eaten in stage three, substantial damage will have been done to the
forb community.
Ideally deer browse would be light enough to allow all native plants to survive in numbers
sufficient to sustain their populations, i.e. plants such as orchids and lilies could survive on
their own without being placed in cages. Unfortunately, this zero level of adverse impact
does not exist within the Forest Preserves along the North Branch.
A listing of deer preferences for broad leafed species is shown below. It is based to a large
extent on observations in Miami Woods, but includes information from other sources as well.
As preferred species are eliminated, pressure increases on plants in the next preference
category which are reduced and then eliminated. The resilience of species within a
preference category varies considerably so not all plants in a given category disappear at the
same time.
The five forb categories can be roughly equated to the five levels in the woody assessment.
During woody category 1 the level 1 forbs are being preferentially eaten and premium plants
such as orchids and lilies may disappear. As woodies reach level 2, more level 1 species
will disappear and level 2 forbs will decline. As woodies reach level 3, most level 2 forbs
will be gone and level 3 forbs decline. As woodies reach level 4, most level 3 forbs will be
gone and level 4 forbs will decline. At woody level 5 virtually all lower level forbs will be
gone, leaving only species that deer will not eat.
The forb classification system is still in development and will be refined. For example there
is some question as to whether Indian plantain / cacalia, sneeze weed / helenium, and clear
weed / pilea belong in category 3 or 4.
The plants in level five can tell a story not by their decline, but by their abundance. If deer
pressure is intense, they tend to become the predominant members of the forb community.
Compared to the woody categories, assessment of forb populations can provide a finer scale
interpretation of impacts, but it is somewhat dependent on the availability of baseline
information. If various species were not present when deer densities began to increase, the
deer can not be blamed for their absence. Nonetheless, the absence of preferred species can
provide valuable information.
When Is Deer Management Needed
In some cases evidence of very severe damage has been required before management of deer
populations has been initiated. Unfortunately, this approach allows degradation of
biodiversity and ecological health until low levels are reached. A key question for managers
and for the public is whether it is acceptable to allow high quality habitats to degrade to some
depleted level where brows lines occur and many plant species disappear before deer are
effectively managed?
If diversity and good ecological health are to be protected, especially in high quality areas,
deer management must be initiated during the early stages of deer impact. The 2012
assessment of deer damage within North Branch restoration sites illustrates the damage
occurring at all levels.
A listing of deer preference as observed in North Branch Restoration Project sites follows.
1. Preferred / First to disappear
Aquilegia Canadensis*
Wild columbine
Aster azurureus*
Sky blue aster
Aster Laevis*
Smooth blue aster
Astragalus Canadensis*
Canada milk vetch
Camassia scilloedes*
Wild hyacinth
Cypripedium candidum
White lady’s slipper
Dodecatheon meadia*
Shooting star
Baptisia leucantha*
White wild indigo
Baptisia leucophaea*
Cream wild indigo
Gentiana crinita**
Fringed gentian
Gentiana puberulenta**
Downy gentian
Habenaria leucophaea*
Prairie white fringed orchid
Heuchera richardsonia*
Prairie alum root
Hydrophyllum virginianum* Virginia waterleaf
Lilium philidalphicum*
Prairie lily
Lilium michiganence*
Turk’s cap lily
Tradescantia ohioensis*
Spiderwort
Trillium grandiflorum*
White trillium
Petalostemum candidum*
White prairie clover
Potentilla arguta*
Prairie cinquefoil
Prenanthes racemosa*
Glaucous white lettice
Saxifraga pennsylvanica
Swamp saxifrage
2. Second Preference / Second to disappear
Amorpha canascens**
Lead plant (new growth & flowers)
Arabis laevigata*
Smooth bank cress
Arisaema dracontium*
Green dragon
Asclepias purpurascens*
Purple milkweed
Circium altissima*
Tall thistle
Circium discolor**
Pasture thistle
Corylus Americana*
American hazel
Desmodium illinoensis*
Illinois tick trefoil
Desmodium canadense*
Showy tick trefoil
Eupatorium purpureum
Purple Joe Pye weed
Gentiana flavida*
Yellowish gentian
Geranium maculatum*
Wild geranium
Heracleum maximum*
Cow parsnip
Impatiens capensis*
Orange jewelweed
Iodanthus pinnatifidus*
Violet cress
Lysimachia quadriflora*
Prairie loosestrife
Lobelia siphilitica*
Great blue lobelia
Lonicera prolifera*
Yellow honeysuckle
Petalostimum purpureum** Purple prairie clover
Polygonatum canaliculatum* Smooth solomon seal
Rhus glabra*
Smooth sumac
Solidago riddellii*
Ridell’s goldenrod
Trillium recurvatum**
Prairie trillium
New growth on all shrubs except Tartarian Honeysuckle & Barberry
All seedlings and saplings less than five feet tall
3. Intermediate Preference / About the same as buckthorn
Apocynum cannabinum
Indian hemp dogbane
Arisaema atrorubens*
Jack in the pulpit (seedlings occur, but no flowering plants)
Asclepias sullivantii**
Sullivant’s milkweed
Carex sp
new spring growth eaten on some sedges
Cornus racemosa
Gray dogwood (all new growth eaten in Miami)
Crataegus sp
Hawthorn sp (all new growth and seedlings eaten in Miami)
Helenium autumnale
Sneezeweed
Lespedesia capitata**
Round-headed bush clover
Penstamon calycosus
Smooth beard tongue
Rhamnus cathartica
Common Buckthorn (new growth and seedlings eaten)
Rosa setigera**
Illinois rose
Rosa blanda**
Early wild rose
Rudbeckia subtomentosa
Sweet black-eyed susan
Silphium laciniatum**
Compass Plant
Solidago rigida**
Stiff Goldenrod
Solidago ulmifolia*
Elm-Leafed Goldenrod
Aster novo-angliae**
New England Aster
Aster Sagittifolius Drumundii** Drummond’s Aster
Aster Shortii*
Short’s Aster
4. Next to Last Preference / Seldom Eaten
Asclepias incarnate
Swamp milkweed
Asclepias syriaca
Common milkweed
Asclepias tuberosa
Butterfly weed
Cacalia tuberose**
Indian plantain
Eupatorium maculatum
Spotted joe pye weed
Iris virginica
Blue flag Iris
Mimulus ringens
Monkey flower
Pilea pumila
Clearweed
Parthenocissus quinquefolia** Virginia creeper (all leaves eaten)
Potophyllum peltatum
Rosa multiflora
Solidago altisima
Solidago graminifolia
Solidago nemoralis
Senecio paupericulus
Vernonia fasciculate
May apple
Multiflora rose (new growth eaten)
Tall goldenrod
Grass-leaved goldenrod
Old-Field Goldenrod
Balsam ragwort
Common ironweed
5. Virtually Never Eaten (populations favored by excessive deer browsing)
Berbis thunbergii
Japanese barberry
Bidens frondosa
Beggar’s ticks
Eupatorim rugosum
White snake root
Hackelia virginiana
Stick-seed
Hypericum pyramidatum
Great St Johns wort
Erechtites hieraciflora
Fire weed
Eryngium yuccafolium
Rattlesnake master (browsed early, but not impacted)
Liatris aspera
Prairie blazing star
Liatris pychnostachya
Buttton blazing star
Lithospermum latifolium
Broad-leaved puccoon
Lonicera tatarica
Tartarian honeysuckle (except fresh resprouts)
Monarda fistulosa
Bergamot
Polygonum pensylvanicum Pennsylvania smartweed
Pychnanthemum
Mountain mint
Seymaria macrophylla
Mullen fox glove
Tovara virginiana
Woodland knotweed
Utica procera
Tall nettle
Grasses (except briefly in early spring)
Sedges (except early spring)
*Species lost from Miami Woods since 1981, mostly during the last 10 years
**Species abundance greatly reduced in Miami Woods during the last 10 years
Kent Fuller, Volunteer Steward
Miami Woods and Prairie
North Branch Restoration Project