Volume 68, Issue 4, July 2015, Pages 349–353
Habitat Selection by Free-Ranging Bison in a Mixed Grazing System on Public Land ☆
- Under a Creative Commons license
Open Access
Abstract
Domestic livestock have replaced bison (Bison bison) on almost all the remaining rangelands of North America. One of the few places where bison and cattle (Bos taurus)
comingle on shared rangelands is in the Henry Mountains (HM) of
southern Utah. Ranchers there are concerned, however, that bison are
selecting the same grazing areas that are needed by cattle. We used
global positioning system telemetry on bison across the entire HM
rangeland to determine which habitats are most important for bison
throughout the seasonal cycle. Sexual segregation was also measured
(using the segregation coefficient, SC) to determine if bison
bulls exert localized impacts by congregating in certain habitats
separate from cow/calf groups. The HM bison exhibited low levels of
sexual segregation for both the breeding (SC = 0.048) and nonbreeding seasons (SC
= 0.112). We found bison habitat use to be diverse and dynamic, with
bison grazing effects distributed widely across habitats throughout the
seasonal cycle. Patches of grassland, whether naturally occurring or
created through burning or mechanical treatments, were favored
regardless of their distance to water. Our findings should assist
ranchers and agency personnel in moving forward with the integrated
management of free-ranging bison and cattle on the HM rangeland, with
implications for bison conservation on public lands elsewhere in the
United States.
Keywords
- GPS telemetry;
- habitat selection;
- human-wildlife conflict;
- Resource Selection Function;
- sexual segregation
Introduction
With
commercial ranching and subsistence pastoralism being practiced on 40%
of the earth’s land surface, resolving human–wildlife conflicts on
rangelands is a major challenge in global biodiversity conservation (Wrobel & Redford, 2010).
Rangelands constitute much of the matrix of land within which protected
areas are embedded, and this matrix is especially important for
sustaining viable populations of large ungulates (Redford et al., 2011), of which the American bison (Bison bison)
is a case in point. Once numbering in the millions, the entire North
American plains bison population declined to < 100 wild animals by
the late 1800s (Hedrick, 2009).
Bison numbers have rebounded to ~ 500 000 thanks to conservation
efforts, but only ~ 20 000 of these bison are found in conservation
herds, with the remaining ~ 480 000 being found in commercial livestock
production herds (Freese et al., 2007). Of those, most are intensively managed on fragmented landscapes and are introgressed with cattle genes (Halbert & Derr, 2007).
In addition to concerns of disease transmission, perceived competition
with livestock is one of the main factors prohibiting large-scale bison
restoration on a continental scale (Freese et al., 2007).
One of the only places where free-ranging plains bison comingle with
cattle on open rangeland is in the Henry Mountains (HM) of southern
Utah.
Established in the early 1940s with bison from Yellowstone National Park (Nelson, 1965 and Popov and Low, 1950),
the HM bison herd now numbers ~ 325 adults (posthunt) and is controlled
primarily by sport hunting. The presence of bison on public allotments
leased for cattle grazing has become a source of contention between
local cattle ranchers and the state and federal management agencies (UDWR, 2007). A search for mentions of the HM bison in a major Utah daily newspaper (Deseret News)
and the Utah Legislature archives revealed an increase in the conflict,
with no mentions before 1991, eight mentions between 1991 and 1995, and
13 mentions in between 2007 and 2012 ( Ranglack & du Toit, 2015a).
The main concern expressed by the ranchers was that bison were reducing
the standing crop of grass in summer on allotments that were designated
for cattle in winter.
To
complicate the issue, the HM bison herd is a public resource managed by
a state agency (Utah Division of Wildlife Resources), but the HM
rangeland is mainly a checkerboard of federal and state land with a
federal agency (Bureau of Land Management) responsible for regulating
cattle grazing. The cattle are owned by individual ranchers and
corporations with permits to graze about 4 200 cows (with calves) in
winter and 800 in summer, whereas the bison herd comprises < 400
adults year-round.
Most studies of bison and cattle interaction have focused on the ecological comparability of the two grazers (Allred et al., 2011 and Kohl et al., 2013),
which is important considering that cattle have replaced bison across
the majority of the historic bison range. However, for the restoration
of bison at an ecologically meaningful scale, bison and cattle will
likely graze on shared rangelands. This leads to many concerns from the
livestock producer community, primarily concerning disease transmission,
property damage (especially to crops and fences), and competition for
grazing resources (Gates et al., 2010). With adequate surveillance and management, disease concerns can be controlled (Nishi et al., 2002), and the movements of cattle across the landscape can be controlled through spatial management of water and mineral licks (Bailey, 2004 and Porath et al., 2002),
reducing the need for fencing. Competition, however, is difficult to
manage, and so it is important to quantify bison habitat use before
implementing management actions aimed at addressing perceived conflicts
with cattle habitat needs. Early work in the HM discovered that bison
and cattle have 91% dietary similarity (van Vuren & Bray, 1983),
indicating a high potential for competition and leading to a local
perception that bison are strong competitors with cattle for grazing
resources. Dietary overlap alone might not, however, be an indicator of
competition if habitat use by bison and cattle is differentiated in time
and space. A previous study in one part of the HM identified only a 29%
overlap in space use, with bison ranging farther—in both distance and
elevation—from water than cattle (van Vuren, 2001).
In the Great Plains, too, cattle stay close to water and prefer wooded
areas, whereas bison movements are less influenced by distance to water
and they display no preference for wooded areas (Allred et al., 2011).
Identifying
overlaps in habitat use throughout the year is important for
understanding the overall dynamics of a mixed-species grazing system,
but competition is most likely to occur during the season in which
grazing resources are most limiting (Odadi et al., 2011).
On the HM rangeland, winter is the most limiting season, when annual
grasses have died and perennial grasses have reallocated nutrients to
their roots. We thus used global positioning system (GPS) telemetry on
bison to determine their patterns of habitat use through each phase of
the seasonal cycle. Our main objective was to provide rangeland managers
and ranchers with accurate information regarding where and when bison
use habitats of particular importance to cattle. Also, because sexual
segregation is common in bison elsewhere, we investigated the
possibility that bison bulls, although small in number, could degrade
certain habitats if they “camped” there in bachelor groups whereas mixed
cow–calf groups roamed more widely.
