Nt. Hobbs et al., UNGULATE GRAZING IN SAGEBRUSH GRASSLAND - EFFECTS OF RESOURCE COMPETITION ON SECONDARY PRODUCTION, Ecological applications, 6(1), 1996, pp. 218-227
In many areas of western North America, populations of elk are believe
d to harm production of cattle by competing with them for limited supp
lies of native forage. We examined effects of variation in the populat
ion density of elk (Cen,ns elaphus canadensis) during winter on growth
and reproduction of cattle during spring, using a randomized complete
block experiment conducted in sagebrush grassland during four years.
We manipulated elk numbers to achieve four levels of population densit
y (0, 9, 15, and 31 elk/km(2)), replicated each level three times, and
observed responses of cattle to these manipulations. Mean birth dates
of calves born to cows in the intermediate (9, 15 elk/km(2)) treatmen
t levels were delayed by 5 d relative to birth dates of calves born to
cows in controls (0 elk/km(2)), but these trends were not statistical
ly significant (quadratic effect F-1,F-6 = 3.2, P = 0.13). Calf body m
ass at birth was not significantly influenced by treatment, but calf b
ody mass at the end of spring declined linearly (F-1,F-6 = 7.9, P = 0.
03) with increasing elk population density, from a mean of 80.2 kg in
the control to 73.0 kg in the 31 elk/km(2) treatment. Calf body mass a
t weaning was weakly depressed by treatment, with the largest treatmen
t effects occurring at the 9 elk/km(2) level (F-1,F-6 = 8.8, P = 0.02)
. Body mass of cows at the end of the spring grazing season tended to
decline linearly with treatment, but these tendencies were not statist
ically significant (linear contrast F-1,F-6 = 1.9, P = 0.22). We did n
ot find significant effects of treatment on cow body mass at time of w
eaning (control vs, others contrast F-1,F-6 = 3.3, P = 0.12) or on nat
ality rates (control vs. other contrast F-1,F-6 = 1.2, P = 0.31), alth
ough, in both cases, values for the control tended to exceed the treat
ments. Body mass of cows (t(1) = -3.9, P = 0.0003) and calves (t(1) =
-3.9, P < 0.0001) at the end of the spring grazing season were quadrat
ically related to the biomass of available herbaceous forage during sp
ring. We observed a threshold in effects of forage supply on cattle pr
oduction at about 45 g/m(2) of live and dead herbaceous biomass. Cattl
e production declined with declining forage biomass when forage supply
fell below this threshold. Total cattle production (kilograms per cow
per year), was quadratically related to elk population density (F-1,F
-6 = 5.8, P = 0.05), Average cattle production in the control ((X) ove
r bar = 248 kg . cow(-1). yr(-1)) exceeded the mean of the other treat
ment levels ((X) over bar = 224 kg . cow(-1). yr(-)1, F-1,F-6 = 6.7, P
= 0.04). Quadratic responses in cattle performance were apparently ca
used by compensatory growth after the spring grazing season: growth ra
tes of cows during spring were inversely related to their subsequent g
rowth rates during summer and fall (F,,, = 33.5, P < 0.0001, r(2) = 0.
50). We conclude that elk grazing caused reductions in cattle producti
on, but the magnitude of the effects were not proportionate to elk pop
ulation density. Our studies revealed that competition between cattle
and elk operates in a threshold fashion. If sufficient forage is avail
able to cattle following elk grazing, elk populations will not harm ca
ttle performance, In the system we studied, this threshold occurred at
approximate to 45 g/m(2) of live and dead herbaceous biomass.