Scale dependency in seagrass dynamics: how does the neighboring effect vary with grain of observation?

Although the importance of spatial scale in ecology has been increasingly recognized, the effects on ecological processes of changing the grain size of the observation have rarely been tested for empirical populations. A seagrass bed is an ideal system to study scale-dependency because it occurs in two-dimensional shallow soft-bottoms and can be monitored on a broader scale by using remote-sensing techniques. To investigate the grain dependency of seagrass spatial dynamics, we analyzed the effect of neighboring vegetation on the annual transition between vegetated and unvegetated states in a seagrass meadow in Futtsu, Tokyo Bay. The presence or absence of seagrass vegetation was observed at different grains from aerial photographs taken annually over 17 years. We detected the presence of a neighboring effect both in the increasing process (transition from the unvegetated to the vegetated state) and the decreasing process (vice versa) of vegetation. In the increasing process, the intensity of the neighboring effect was positive with the small grain, but the effect decreased to 0 with grain of ca. 20 m. In the decreasing process, the neighboring effect was negative with the small grain and increased to 0 with grain of ca. 30 m. The observed grain dependency in the neighboring effects also varied among different positions of the bed and among different years. The grain dependency in the increasing process cannot solely be explained by shoot elongation of the seagrass, which can cause positive neighboring effects only at small grain (≤6 m). The neighboring effect at the greater grain can be regarded as an emergent property.