Olden Julian D
Centre for Limnology, University of Wisconsin-Madison, Madison, WI 53706, USA.
Philos Trans R Soc Lond B Biol Sci. 2007 Mar 29;362(1479):461-72. doi: 10.1098/rstb.2006.1974.
In landscape ecology, substantial theoretical progress has been made in understanding how critical threshold levels of habitat loss may result in sudden changes in landscape connectivity to animal movement. Empirical evidence for such thresholds in real systems, however, remains scarce. Streambed landscapes provide a strong testing ground for studying critical thresholds because organisms are faced with substantial environmental heterogeneity while attempting to overcome the physical force of water. In this study, I report on the results from a series of experiments investigating the influence of habitat abundance and current velocity on the movement dynamics of two stream herbivores (caddisfly larva Agapetus boulderensis and snail Physa sp.) that differ substantially in how they perceive landscape structure. Specifically, I ask whether critical thresholds to herbivore movement exist in streambed landscapes. By exploiting the pattern recognition capabilities of artificial neural networks, I found that the rate, sinuosity and directionality of movement by Agapetus and Physa varied nonlinearly according to the abundance of habitat patches, current velocity and habitat-current interaction. Both the study organisms exhibited threshold responses to habitat abundance, yet the location and slope of these thresholds differed between species and with respect to different current velocities. These results suggest that a critical threshold in functional connectivity (i.e. the connection of habitat patches by dispersal) is not an inherent property of the landscape, but in fact emerges from the interplay of species' interactions with landscape structure. Moreover, current velocity interacted with habitat abundance to elicit strong upstream-oriented movement for both the species. This suggests that dispersing individuals may be polarized in the upstream direction and therefore functional connectivity is not equal in all directions. Such results highlight the need for future research addressing the sources of variability of critical threshold effects in ecological phenomena.
在景观生态学领域，对于理解栖息地丧失的临界阈值水平如何导致景观连通性对动物移动的突然变化，已经取得了显著的理论进展。然而，实际系统中此类阈值的实证证据仍然稀缺。河床景观为研究临界阈值提供了一个有力的试验场，因为生物体在试图克服水流的物理力量时面临着巨大的环境异质性。在本研究中，我报告了一系列实验的结果，这些实验调查了栖息地丰度和水流速度对两种河床食草动物（石蛾幼虫Agapetus boulderensis和蜗牛Physa sp.）移动动态的影响，这两种动物在感知景观结构的方式上有很大差异。具体而言，我探究了河床景观中是否存在食草动物移动的临界阈值。通过利用人工神经网络的模式识别能力，我发现Agapetus和Physa的移动速率、弯曲度和方向性根据栖息地斑块的丰度、水流速度以及栖息地与水流的相互作用呈非线性变化。这两种研究生物对栖息地丰度均表现出阈值响应，但这些阈值的位置和斜率在物种之间以及不同水流速度下有所不同。这些结果表明，功能连通性（即通过扩散连接栖息地斑块）的临界阈值并非景观的固有属性，而是实际上源于物种与景观结构相互作用的结果。此外，水流速度与栖息地丰度相互作用，引发了两种物种强烈的向上游移动。这表明扩散个体可能在上游方向上表现出极化，因此功能连通性在所有方向上并不相等。这些结果凸显了未来研究解决生态现象中临界阈值效应变异性来源的必要性。
