Faculty of Environmental Design, University of Calgary, Calgary, Alberta, Canada.
PLoS One. 2013 May 8;8(5):e64311. doi: 10.1371/journal.pone.0064311. Print 2013.
Ongoing debate about whether food webs are primarily regulated by predators or by primary plant productivity, cast as top-down and bottom-up effects, respectively, may becoming superfluous. Given that most of the world's ecosystems are human dominated we broadened this dichotomy by considering human effects in a terrestrial food-web. We studied a multiple human-use landscape in southwest Alberta, Canada, as opposed to protected areas where previous terrestrial food-web studies have been conducted. We used structural equation models (SEMs) to assess the strength and direction of relationships between the density and distribution of: (1) humans, measured using a density index; (2) wolves (Canis lupus), elk (Cervus elapahus) and domestic cattle (Bos taurus), measured using resource selection functions, and; (3) forage quality, quantity and utilization (measured at vegetation sampling plots). Relationships were evaluated by taking advantage of temporal and spatial variation in human density, including day versus night, and two landscapes with the highest and lowest human density in the study area. Here we show that forage-mediated effects of humans had primacy over predator-mediated effects in the food web. In our parsimonious SEM, occurrence of humans was most correlated with occurrence of forage (β = 0.637, p<0.0001). Elk and cattle distribution were correlated with forage (elk day: β = 0.400, p<0.0001; elk night: β = 0.369, p<0.0001; cattle day: β = 0.403, p<0.0001; cattle, night: β = 0.436, p<0.0001), and the distribution of elk or cattle and wolves were positively correlated during daytime (elk: β = 0.293, p <0.0001, cattle: β = 0.303, p<0.0001) and nighttime (elk: β = 0.460, p<0.0001, cattle: β = 0.482, p<0.0001). Our results contrast with research conducted in protected areas that suggested human effects in the food web are primarily predator-mediated. Instead, human influence on vegetation may strengthen bottom-up predominance and weaken top-down trophic cascades in ecosystems. We suggest that human influences on ecosystems may usurp top-down and bottom-up effects.
关于食物网主要受捕食者还是初级植物生产力调节的持续争论，分别被视为自上而下和自下而上的影响，这可能变得多余了。鉴于世界上大多数生态系统都受到人类的主导，我们通过考虑人类在陆地食物网中的影响，拓宽了这一二元对立。我们研究了加拿大艾伯塔省西南部的一个多种人类用途的景观，而不是之前进行陆地食物网研究的保护区。我们使用结构方程模型（SEM）来评估以下因素之间的关系的强度和方向：（1）人类密度，使用密度指数来衡量；（2）狼（Canis lupus）、麋鹿（Cervus elapahus）和家牛（Bos taurus），使用资源选择函数来衡量；（3）饲草质量、数量和利用率（在植被采样点测量）。通过利用人类密度的时间和空间变化来评估关系，包括白天与黑夜，以及研究区域内人类密度最高和最低的两个景观。在这里，我们表明，食物网中人类介导的饲草效应优先于捕食者介导的效应。在我们简约的 SEM 中，人类的出现与饲草的出现最相关（β=0.637，p<0.0001）。麋鹿和牛的分布与饲草相关（麋鹿白天：β=0.400，p<0.0001；麋鹿晚上：β=0.369，p<0.0001；牛白天：β=0.403，p<0.0001；牛晚上：β=0.436，p<0.0001），而且白天（麋鹿：β=0.293，p <0.0001，牛：β=0.303，p<0.0001）和晚上（麋鹿：β=0.460，p<0.0001，牛：β=0.482，p<0.0001），狼与麋鹿或牛的分布呈正相关。我们的研究结果与在保护区进行的研究形成对比，后者表明食物网中的人类影响主要是通过捕食者介导的。相反，人类对植被的影响可能会加强自下而上的主导地位，并削弱生态系统中的自上而下的营养级联。我们认为，人类对生态系统的影响可能会篡夺自上而下和自下而上的影响。
