Data from: Sunning themselves in heaps, knots, and snarls: the extraordinary abundance and demography of island watersnakes

Snakes represent a sizable fraction of vertebrate biodiversity but until recently, data on their demography has been sparse. Consequently, generalizations regarding patterns of variation are weak and the potential for population projections are limited. We address this information gap through an analysis of spatial and temporal variation in demography (population size, annual survival, realized population growth) of the Lake Erie Watersnake, Nerodia sipedon insularum, and a review of snake survival more generally. Our study spans a period during which the Lake Erie Watersnake was listed as threatened under the U.S. Endangered Species Act, recovered and was delisted. We collected capture-mark-recapture data at 14 study sites over 20 years, accruing 20,000 captures of 13,800 individually marked adults. Lake Erie Watersnakes achieve extraordinary abundance, averaging 520 adults/km of shoreline (ca. 260 adult/ha) at our study sites (range = 160-1600 adults/km; ca. 80-800 adults/ha) and surpassing population recovery and post-delisting monitoring criteria. Annual survival averages 0.68 among adult females and 0.76 among adult males, varies among sites, and is positively correlated with body size among study sites. Temporal process variance in annual survival is low, averaging 0.0011 or less than 4% of total variance; thus, stochasticity in annual survival may be of minor significance to snake extinction risk. Estimates of realized population growth indicate that population size has been stable or increasing over the course of our study. More generally, snake annual survival overlaps broadly across continents, climate zones, families, subfamilies, reproductive modes, body size categories, maturation categories, and parity categories. Differences in survival in relation to size, parity, and maturation are in the directions predicted by life history theory but are of small magnitude with much variation around median values. Overall, annual survival appears to be quite plastic, varying with food availability, habitat quality, and other ecological variables

蛇类是脊椎动物生物多样性的重要组成部分，但直至近年，其种群统计学（demography）相关数据仍较为匮乏。因此，针对种群变异模式的归纳总结尚且薄弱，种群预测的应用潜力亦受到限制。为填补这一信息空白，本研究通过分析伊利湖水蛇（Nerodia sipedon insularum）的种群统计学特征（涵盖种群规模、年存活率（annual survival）、实际种群增长率（realized population growth））的时空变异，并对蛇类存活率的相关研究开展系统性综述。本研究的观测周期覆盖了伊利湖水蛇被列入《美国濒危物种法》列为受威胁物种、实现种群恢复并最终被除名的完整阶段。研究团队在20年间于14个调查点位开展标记重捕（capture-mark-recapture）数据采集工作，累计获取了13800只经个体标记的成蛇的20000次捕获记录。伊利湖水蛇种群密度极高，本研究点位的成蛇平均密度为520只/千米岸线（约合260只/公顷），种群密度区间为160~1600只/千米岸线（约合80~800只/公顷），远超种群恢复及除名后监测的标准阈值。成年雌性个体的年平均存活率为0.68，成年雄性为0.76；存活率因调查点位而异，且在各研究点位中与个体体长呈正相关关系。年存活率的时间过程方差较低，平均值仅为0.0011，不足总方差的4%；因此，年存活率的随机性对蛇类种群灭绝风险的影响或较为有限。实际种群增长率的估算结果显示，本研究周期内伊利湖水蛇种群规模保持稳定或呈增长态势。从更广泛的视角来看，不同大陆、气候带、科、亚科、繁殖方式、体型分级、成熟度分级以及繁殖类型分级的蛇类，其年存活率分布存在广泛重叠。存活率与体型、繁殖类型及成熟度的关联差异符合生活史理论的预测，但效应量较小，且在中位数附近存在较大的变异幅度。总体而言，蛇类年存活率具有较强的表型可塑性，其水平随食物可获得性、栖息地质量及其他生态变量的变化而波动。