Understanding white-tailed deer productivity: Population ecology of neonates.

摘要

Despite the documented importance of grasslands to wildlife, few studies have evaluated this habitat for white-tailed deer (Odocoileus virginianus ). In the Northern Great Plains, forested habitat is limited and often fragmented, which could affect fawn ecology. To what extent fawns use Conservation Reserve Program (CRP) grasslands in the Northern Great Plains is unknown. Reductions in CRP enrollments during our study provided a unique opportunity to evaluate neonate ecology during a period of changing availability of cover characteristics in the region. We evaluated behavior, bed site selection, movement, home range use, survival, cause-specific mortality, and resource selection of white-tailed deer fawns from 2007--2009 in north-central South Dakota. Maternal investment plays a critical role in neonate survival, and adults can improve survival of offspring by defending them against predators. During captures of neonates in spring 2007 and 2008 in north-central South Dakota, we documented 24 aggressive encounters by adult female and yearling male and female white-tailed deer defending neonates. Eleven (45.8%) aggressive encounters included yearlings accompanying adult females. Mean ages and weights of neonates that were aggressively defended were greater (P0.0001) than ages and weights of those that were not; adults began protecting neonates at approximately 4 days of age. Male fawns were more likely (P=0.013) to be defended than female fawns. Our data suggested that sex- and age-biased maternal defensive behavior exists in white-tailed deer, and that deer biased maternal investment toward older, male neonates. We captured and radiocollared 81 fawn white-tailed deer from 15 May--15 June 2007--2009 and investigated bed site selection (n=152) in north-central South Dakota. We documented 80 (52.6%) bed sites in tall grass-CRP lands, 35 (23.0%) bed sites were in forested cover, and 37 (24.4%) in other habitats (e.g., pasture, alfalfa, wheat). Bed site selection varied (P0.001) with age and sex of neonate; tree canopy cover (P0.001) and tree basal area (P0.001) decreased with age of neonates with no bed sites observed in forested cover after 18 days of age. Male neonates selected sites with less grass cover (P0.001), vertical height of understory vegetation (P0.001), and density of understory vegetation (P0.001) but greater bare ground (P=0.047), litter (P=0.028), and wheat ( P=0.044) than did females. Probability of bed site selection increased 3.5% (odds ratio=1.035, 95% CI=1.008--1.062) for every 1-cm increase in vertical height of understory vegetation. We recorded 23 mortalities; predation (n=12, 52.2%) was the leading source of mortality followed by hypothermia (n=5, 21.7%). Survival modeling for the summer period using intrinsic variables indicated that survival was influenced by year. Summer survival varied (P=0.002) between years (2007--2009); annual survival rates were 0.94 (SE=0.06, n=22), 0.78 (SE=0.09, n=27), and 0.54 (SE=0.10, n=32), respectively, and corresponded to 41% loss of CRP grasslands in the area over the duration of the study. Survival modeling for the summer period using habitat variables indicated that survival was influenced by CRP patch density, and was greater in home range areas of surviving fawns (X=1.81, SE=0.10, n=63) than in those of fawns that died ( X=0.16, SE=0.04, n=18). Change in CRP influenced (P0.001) home range size; smaller home ranges were indicative of greater quantity of CRP available to fawns. Change in CRP and wheat influenced (P0.001) mean daily movement; smaller movements were characterized by greater availability of CRP and were associated with less acreage of wheat available to fawns. Summer resource selection analyses indicated fawns shifted selection during the summer; mean age at movement ranged from 48.8--58.6 days. During early summer, fawns consistently selected for CRP; selection of wheat progressed temporally from avoided in 2007 to selected in 2009. During late summer, fawns consistently selected for corn habitat and used CRP at least in proportion to its availability. Fawn movement between resource selection clusters was correlated with height of corn (80--85 cm). Reduction in CRP grasslands during our study influenced fawn home range size, daily movements, resource selection, and ultimately survival. However, current legislation mandates continued decrease in CRP enrollments and concomitant increase in the planting of corn for ethanol production. Continued reduction in CRP in the Northern Great Plains may necessitate rapid adjustments in population management (i.e., harvest adjustments, intensified predator control, habitat management) by state agencies.