Scarred for life: the other side of the fence debate
Author:
Paul F. Jones
Publication:
Human–Wildlife Interactions 8(1):150–154; 2014
Abstract:
Fence lines crisscross the prairies of North America and are as numerous as the cattle that
they fence in. Fences continue to be erected to define property boundaries, protect drivers from collisions with wildlife, and control the distribution of domestic livestock. In 1879, the
United States produced only 4.5 metric tons of barbed wire; production continued to increase
on a yearly basis. By 1945 the annual production was 210,600 metric tons (Leftwich and Simpson
1978) with a concomitant increase in the erection of fence lines. For example, >51,000 km
of fence lines were constructed on Bureau of Land Management administered lands between
1962 and 1997 in states that supported sagegrouse (Centrocercus urophasianus) populations
(Connelly et al. 2000). In Alberta, Canada, I estimate there to be >67,000 km of fence
lines within 630 townships in the Grassland Natural Region. The construction of new fences
continues today with a growing energy sector, and it will likely match, or perhaps exceed, the
growth in road networks across prairie regions with little regard to their impact on wildlife,
even though much has been learned about their negative effects on wildlife.
The negative effects that fences have on wildlife generally have fallen into one of 3 categories: (1) disruption of movement patterns and associated habitat fragmentation; (2) direct mortality; and (3) indirect mortality (Hayward and Kerley 2009, Somers and Hayward 2012). Fences can disrupt daily and seasonal movement patterns of wildlife (Berger 2004, Suitor 2011). It is now understood that barriers, including fences, to migrating animals is 1 reason for the collapse of a number of migratory
systems worldwide (Berger 2004, Bolger et al. 2008). Fences can be a major source of mortality
for wildlife (Allen and Ramirez 1990 [birds]; Catt et al. 1994; Baines and Andrew 2003; Wolfe
et al. 2007, 2009 [grouse]; Harrington and Conover 2006 [ungulates]). Typically, ungulates
that snare their legs and are restrained for a prolonged period of time, results in the animal’s
death (Scott 1992). Wire fences in Colorado and Utah killed an annual average of 0.25 ungulates
per km of wire fence (Harrington and Conover 2006). These rates of mortality consisted of 0.06
elk (Cervus elaphus) mortalities per km, 0.08 mule deer (Odocoileus hemionus) mortalities per
km, and 0.11 pronghorn (Antilocapra americana) mortalities per km of wire fence (Harrington
and Conover 2006). If these numbers are a reflection of mortalities on ungulates in other
areas, then, the aggregate effect of fences on ungulates is staggering.
The third, and often overlooked, negative effect of fences is indirect mortality. For
pronghorn, and likely other ungulates, such indirect mortality manifests itself as hair
loss. Pronghorn are a grassland species that have adapted to fences differently from other
ungulates. Instead of jumping over an obstacle, as do deer, pronghorns crawl under fences to
cross to the other side. Such behavior is likely due to their evolution on the prairies where
there were limited vertical barriers. If the bottom wire is too low, the fence becomes a barrier to
pronghorn movement (Sheldon 2005, Suitor 2011, Gates et al. 2012). When a pronghorn does
try to jump a fence and becomes entangled in the wires, direct mortality can result (Simpson
and Leftwich 1978, O’Gara and McCabe 2004). Since 2003, fieldwork associated with a
collaborative study on the resource selection, movement patterns, and the effects of fences on
pronghorns have revealed an alarming number Commentary of pronghorns with hair loss. Such loss of hair is due to crawling under barbed fencing where the bottom wire is so low that the pronghorn
rubs hair off on the barbs as it tries to crawl under (Figure 1a–c). A pronghorn typically
drops to its knees to cross under a fence, and with its chest on or near the ground and front
legs bent under its body, it arches its back in a convex position to avoid the bottom wire,
pushes with its hind legs until its head and shoulders are on the opposite side of the fence,
and then slithers forward and begins to stand up (O’Gara 2004a). This approach of crawling
under fences can result in significant hair loss at times, as illustrated by the hair loss on an
animal captured during the telemetry study (Figure 2). Hair loss is typically along the neck,
back, and hind end of the animal (Figures 1 and 3). If the hair loss is significant, the exposed
skin may turn black (Figure 2), which is similar to the exposed skin on moose (Alces alces) that
have lost hair due to infestations by winter ticks (Dermacentor albipictus; McLaughlin and
Addison 1986).
The lack of attention paid to the loss of hair caused by fences to pronghorn is
surprising. Moose suffering from hair loss due to tick infestations were found to have
lower body fat levels and be in poor condition (McLaughlin and Addison 1986). Though
not fully understood, the hair loss in moose presumably results in increased exposure to
thermal stresses, increased metabolic rates, and hypothermia during severe winters
(McLaughlin and Addison 1986, Samuel 1991). Indeed, Glines and Samuel (1989) reported that
moose experimentally exposed to ticks suffered from hypothermia. The effect of hair loss for
pronghorn is not known, but it may be similar to that of moose. The hair of pronghorns is
coarse and provides excellent protection from wind and cold (O’Gara 2004b). Pronghorns are
able to utilize their coat (O’Gara 2004b) and microsites within their environment to cope
with wind and cold temperatures associated with winter (Bruns 1977). Taking away one of
these coping mechanisms can result in negative effects for pronghorn exposed to severe weather.
Whereas hair loss and resulting mortalities in moose from winter ticks predominantly occurs
between February and April (McLaughlin and Addison 1986, Samuel 1991), the loss of
hair in pronghorn occurs year-round, further exacerbating the negative effects. The loss of hair
and its potential negative ramifications is likely more pronounced for pronghorns inhabiting
the northern portion of their range in North America due to the severe winter temperatures
that are common for this geographical area. Further, effort is required to quantify the extent
of hair loss in pronghorn across their range and assess the short- and long-term negative effects.
Most jurisdictions within the range of pronghorns have developed standards for
fences to make them pronghorn and wildlife friendly (Paige 2008, 2012), although it is not
clear how often these guidelines are followed. Continued effort is required to inform and
educate those involved with fence construction about the negative effects of fences on
pronghorns and the need to upgrade to wildlife friendly standards for existing fences and for
all new fence lines. Within these guides there are numerous recommendations for making
fences wildlife friendly. Additional research is required to evaluate the effectiveness of the
suggested fence enhancements to ensure that they do create a more wildlife friendly fence.