Lawrence Kendra L, Wise David H
KL2 Consulting, LLC, Silver Spring, MD, United States of America.
Department of Biological Sciences, and Institute for Environmental Science and Policy, University of Illinois at Chicago, Chicago, IL, United States of America.
PeerJ. 2017 Jul 21;5:e3572. doi: 10.7717/peerj.3572. eCollection 2017.
Theory predicts strong bottom-up control in detritus-based food webs, yet field experiments with detritus-based terrestrial systems have uncovered contradictory evidence regarding the strength and pervasiveness of bottom-up control processes. Two factors likely leading to contradictory results are experiment duration, which influences exposure to temporal variation in abiotic factors such as rainfall and affects the likelihood of detecting approach to a new equilibrium; and openness of the experimental units to immigration and emigration. To investigate the contribution of these two factors, we conducted a long-term experiment with open and fenced plots in the forest that was the site of an earlier, short-term experiment (3.5 months) with open plots (Chen & Wise, 1999) that produced evidence of strong bottom-up control for 14 taxonomic groupings of primary consumers of fungi and detritus (microbi-detritivores) and their predators.
We added artificial high-quality detritus to ten 2 × 2-m forest-floor plots at bi-weekly intervals from April through September in three consecutive years (Supplemented treatment). Ten comparable Ambient plots were controls. Half of the Supplemented and Ambient plots were enclosed by metal fencing.
Arthropod community structure (based upon 18 response variables) diverged over time between Supplemented and Ambient treatments, with no effect of Fencing on the multivariate response pattern. Fencing possibly influenced only ca. 30% of the subsequent univariate analyses. Multi- and univariate analyses revealed bottom-up control during Year 1 of some, but not all, microbi-detritivores and predators. During the following two years the pattern of responses became more complex than that observed by Chen & Wise (1999). Some taxa showed consistent bottom-up control whereas others did not. Variation across years could not be explained completely by differences in rainfall because some taxa exhibited negative, not positive, responses to detrital supplementation.
Our 3-year experiment did not confirm the conclusion of strong, pervasive bottom-up control of both microbi-detritivores and predators reported by Chen & Wise (1999). Our longer-term experiment revealed a more complex pattern of responses, a pattern much closer to the range of outcomes reported in the literature for many short-term experiments. Much of the variation in responses across studies likely reflects variation in abiotic and biotic factors and the quality of added detritus. Nevertheless, it is also possible that long-term resource enhancement can drive a community towards a new equilibrium state that differs from what would have been predicted from the initial short-term responses exhibited by primary and secondary consumers.
理论预测在以碎屑为基础的食物网中存在强大的自下而上的控制作用，但对以碎屑为基础的陆地系统进行的实地实验却发现了有关自下而上控制过程的强度和普遍性的相互矛盾的证据。可能导致相互矛盾结果的两个因素是实验持续时间，它会影响对降雨等非生物因素的时间变化的暴露程度，并影响检测到接近新平衡的可能性；以及实验单元对迁入和迁出的开放性。为了研究这两个因素的作用，我们在一片森林中对开放地块和围栏地块进行了一项长期实验，这片森林曾是早期一项短期实验（3.5个月）的地点，该短期实验使用的是开放地块（Chen和Wise，1999年），实验结果表明，对于真菌和碎屑的14个分类群的初级消费者（微生物碎屑食性动物）及其捕食者，存在强大的自下而上的控制作用。
在连续三年的4月至9月期间，我们每隔两周向十个2×2米的森林地面地块添加人工高质量碎屑（补充处理）。十个类似的对照地块作为对照。补充地块和对照地块中有一半被金属围栏围住。
节肢动物群落结构（基于18个响应变量）在补充处理和对照处理之间随时间发生了分化，围栏对多变量响应模式没有影响。围栏可能仅对随后约30%的单变量分析产生影响。多变量和单变量分析显示，在第一年，一些但不是所有的微生物碎屑食性动物和捕食者存在自下而上的控制作用。在接下来的两年里，响应模式比Chen和Wise（1999年）观察到的更为复杂。一些分类群表现出一致的自下而上的控制作用，而其他分类群则没有。多年间的变化不能完全用降雨差异来解释，因为一些分类群对碎屑补充表现出负响应而非正响应。
我们的三年实验没有证实Chen和Wise（1999年）所报道的对微生物碎屑食性动物和捕食者都存在强大且普遍的自下而上控制作用的结论。我们的长期实验揭示了一种更为复杂的响应模式，这种模式更接近许多短期实验在文献中报道的结果范围。不同研究中响应的许多变化可能反映了非生物和生物因素以及添加碎屑质量的变化。然而，长期的资源增加也有可能使一个群落朝着一个新的平衡状态发展，这个状态不同于根据初级和次级消费者最初的短期响应所预测的状态。
