Density-dependent effects on physical condition and reproduction in North American elk: an experimental test

Abstract

Density dependence plays a key role in life-history characteristics and population ecology of large, herbivorous mammals. We designed a manipulative experiment to test hypotheses relating effects of density-dependent mechanisms on physical condition and fecundity of North American elk (Cervus elaphus) by creating populations at low and high density. We hypothesized that if density-dependent effects were manifested principally through intraspecific competition, body condition and fecundity of females would be lower in an area of high population density than in a low-density area. Thus, we collected data on physical condition and rates of pregnancy in each experimental population. Our manipulative experiment indicated that density-dependent feedbacks affected physical condition and reproduction of adult female elk. Age-specific pregnancy rates were lower in the high-density area, although there were no differences in pregnancy of yearlings or in age at peak reproduction between areas. Age-specific rates of pregnancy began to diverge at 2 years of age between the two populations and peaked at 6 years old. Pregnancy rates were most affected by body condition and mass, although successful reproduction the previous year also reduced pregnancy rates during the current year. Our results indicated that while holding effects of winter constant, density-dependent mechanisms had a much greater effect on physical condition and fecundity than density-independent factors (e.g., precipitation and temperature). Moreover, our results demonstrated effects of differing nutrition resulting from population density during summer on body condition and reproduction. Thus, summer is a critical period for accumulation of body stores to buffer animals against winter; more emphasis should be placed on the role of spring and summer nutrition on population regulation in large, northern herbivores.

Habitat Interference by Axis Deer on White‐Tailed Deer

“…5). Stewart et al (2005) observed negative feedbacks associated with density dependence in elk in the high-density area. Body condition and pregnancy rates in elk were significantly lower in the highdensity population of elk compared with the lowdensity population.…”

“…Moreover, resources were most available in the low-density area (Stewart et al 2006), where overlap in diet was lowest and palatable species of plants occurred to a greater extent in the diets of both cervid species. We did not have the option of examining resource partitioning during winter when resources are least abundant, because elk were not present on our study areas during that season (Stewart et al 2005(Stewart et al , 2006.…”

“…This northeast area was divided into two study sites with the same 2.4-m high fence, east (842 ha) and west (610 ha), to accommodate experimental comparisons of two populations densities of elk (Stewart et al 2006(Stewart et al , 2009. We divided the northeast area in a manner that resulted in plant communities being near equal in proportions in the east and west study areas (Stewart et al 2005(Stewart et al , 2006. Such study sites are sufficiently large to allow natural movements and other behaviours of large herbivores (Hirth 1977, McCullough 1979, Stewart et al 2002.…”

“…Functional groups of plants included forbs, grasses, sedges, mat-forming shrubs, such as twinflower Linnaea borealis, and upright shrubs, such as serviceberry Amelanchier alnifolia. Plant nomenclature follows Hitchcock & Cronquist (1996) Experimental design During 1997, we began an experiment to examine effects of population density of elk on their physical condition and reproduction (Stewart et al 2005) and on ecosystem processes (Stewart et al 2006(Stewart et al , 2009. We created two populations of elk at high and low density relative to carrying capacity (K), in the northeast east and west study areas on Starkey.…”

The High Cost of Motherhood: End-Lactation Syndrome in Southern Sea Otters (Enhydra Lutris Nereis) on the Central California Coast, Usa

“…Another approach to assessing body condition, one that is less subject to short-term variation in conditions, is to evaluate age-specific length or mass, and/or the rate of body growth in juveniles (Fowler 1990). This can be done by measuring absolute body length or body mass after statistically controlling for age, which is accomplished by fitting some sort of asymptotic growth function (Stewart et al 2005).…”