Foley Aaron M, Hewitt David G, DeYoung Charles A, DeYoung Randy W, Schnupp Matthew J
Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville, Kingsville, Texas, United States of America.
King Ranch, Inc., Kingsville, Texas, United States of America.
PLoS One. 2016 Oct 6;11(10):e0163592. doi: 10.1371/journal.pone.0163592. eCollection 2016.
White-tailed deer are a culturally and economically important game species in North America, especially in South Texas. The recent discovery of chronic wasting disease (CWD) in captive deer facilities in Texas has increased concern about the potential emergence of CWD in free-ranging deer. The concern is exacerbated because much of the South Texas region is a semi-arid environment with variable rainfall, where precipitation is strongly correlated with fawn recruitment. Further, the marginally productive rangelands, in combination with erratic fawn recruitment, results in populations that are frequently density-independent, and thus sensitive to additive mortality. It is unknown how a deer population in semi-arid regions would respond to the presence of CWD. We used long-term empirical datasets from a lightly harvested (2% annual harvest) population in conjunction with 3 prevalence growth rates from CWD afflicted areas (0.26%, 0.83%, and 2.3% increases per year) via a multi-stage partially deterministic model to simulate a deer population for 25 years under four scenarios: 1) without CWD and without harvest, 2) with CWD and without harvest, 3) with CWD and male harvest only, and 4) with CWD and harvest of both sexes. The modeled populations without CWD and without harvest averaged a 1.43% annual increase over 25 years; incorporation of 2% annual harvest of both sexes resulted in a stable population. The model with slowest CWD prevalence rate growth (0.26% annually) without harvest resulted in stable populations but the addition of 1% harvest resulted in population declines. Further, the male age structure in CWD models became skewed to younger age classes. We incorporated fawn:doe ratios from three CWD afflicted areas in Wisconsin and Wyoming into the model with 0.26% annual increase in prevalence and populations did not begin to decline until ~10%, ~16%, and ~26% of deer were harvested annually. Deer populations in variable environments rely on high adult survivorship to buffer the low and erratic fawn recruitment rates. The increase in additive mortality rates for adults via CWD negatively impacted simulated population trends to the extent that hunter opportunity would be greatly reduced. Our results improve understanding of the potential influences of CWD on deer populations in semi-arid environments with implications for deer managers, disease ecologists, and policy makers.
白尾鹿是北美在文化和经济方面都很重要的狩猎物种，在德克萨斯州南部尤为如此。最近在德克萨斯州的圈养鹿场发现慢性消耗病（CWD），这增加了人们对自由放养鹿群中可能出现CWD的担忧。这种担忧因德克萨斯州南部大部分地区是半干旱环境且降雨多变而加剧，在该地区降水量与幼鹿数量密切相关。此外，生产力低下的牧场，再加上幼鹿数量不稳定，导致鹿群数量常常不受密度制约，因此对额外死亡率很敏感。目前尚不清楚半干旱地区的鹿群对CWD的存在会作何反应。我们使用了来自一个轻度捕猎（年捕猎率2%）鹿群的长期实证数据集，并通过一个多阶段部分确定性模型，结合来自CWD疫区的3个患病率增长率（每年分别增长0.26%、0.83%和2.3%），对鹿群在4种情况下进行了25年的模拟：1）无CWD且无捕猎，2）有CWD且无捕猎，3）有CWD且仅捕猎雄性，4）有CWD且捕猎雌雄两性。模拟结果显示，无CWD且无捕猎的鹿群在25年里平均年增长率为1.43%；若每年对雌雄两性进行2%的捕猎，则鹿群数量稳定。患病率增长率最慢（每年0.26%）且无捕猎的模型中，鹿群数量稳定，但增加1%的捕猎量则导致鹿群数量下降。此外，CWD模型中的雄性年龄结构偏向于更年轻的年龄段。我们将威斯康星州和怀俄明州3个CWD疫区的幼鹿与雌鹿比例纳入患病率每年增长0.26%的模型中，结果显示，直到每年捕猎约10%、16%和26%的鹿时，鹿群数量才开始下降。处于多变环境中的鹿群依靠成年鹿的高存活率来缓冲幼鹿数量低且不稳定的情况。CWD导致成年鹿额外死亡率增加，对模拟的鹿群数量趋势产生了负面影响，以至于猎人的捕猎机会将大幅减少。我们研究结果有助于更好地理解CWD对半干旱环境中鹿群的潜在影响，这对鹿群管理者、疾病生态学家和政策制定者都有重要意义。
