The Stability of Boundary Regions Between Kelp Beds and Deforested Areas

Abstract

Two distinct organizational states of kelp forest communities, foliose algal assemblages and deforested barren areas, typically display sharp discontinuities. Mechanisms responsible for maintaining these state differences were studied by manipulating various features of their boundary regions. Urchins in the barren areas had significantly smaller gonads than those in adjacent kelp stands, implying that food was a limiting resource for urchins in the barrens. The abundance of drift algae and living foliose algae varied abruptly across the boundary between kelp beds and barren areas. These observations raise the question of why urchins from barrens do not invade kelp stands to improve their fitness. By manipulating kelp and urchin densities at boundary regions and within kelp beds, we tested the hypothesis that kelp stands inhibit invasion of urchins. Urchins that were experimentally added to kelp beds persisted and reduced kelp abundance until winter storms either swept the urchins away or caused them to seek refuge within crevices. Urchins invaded kelp bed margins when foliose algae were removed but were prevented from doing so when kelps were replaced with physical models. The sweeping motion of kelps over the seafloor apparently inhibits urchins from crossing the boundary between kelp stands and barren areas, thus maintaining these alternate stable states. Our findings suggest that kelp stands are able to defend themselves from their most important herbivores by combining their flexible morphology with the energy of wave‐generated surge. The inhibitory influence of this interaction may be an important mechanism maintaining the patchwork mosaics of barren areas and kelp beds that characterize many kelp forest ecosystems. Corresponding Editor: J. D. Witman.

Latitudinal, seasonal, and small-scale spatial differences of the giant kelp, Macrocystis pyrifera, and an herbivore at their southern range limit in the northern hemisphere

“…The increasing probability of finding sea urchins outside of the kelp bed detected in this study in all populations in spring has been shown for other kelp forests (Konar and Estes 2003). Sea urchin distribution may spread out and away from the kelp forest during increased nitrate conditions in spring when macroalgal recruitment may be greater outside of the kelp canopy due to increased light there.…”

Multiple stable states and relationship between thresholds in processes and states

“…Paine & Trimble (2004) stated that the shift is evidence for multiple stable states, but it appears that the existence of hysteresis is inferred from the sudden shifts over time. Konar & Estes (2003) addressed abrupt spatial shifts in the boundary between kelps and barrens. They manipulated both kelps and urchins and provided good evidence that the presence of kelps keeps out urchins and thus sets the boundary.…”

“…abrupt regime shifts from corals to macroalgae in tropical systems, and from macroalgae to barrens or mussel beds in temperate systems, which result in changes in abundance of exploited fish stocks (Done 1992, Hare & Mantua 2000, Konar & Estes 2003, deYoung et al 2004, Paine & Trimble 2004, Mumby 2009, Petraitis et al 2009). Ecologists are especially interested in these sudden shifts because they are common in a variety of situations, often occur unexpectedly, and are difficult to reverse.…”

Sea-urchin feeding fronts

“…It has also been shown, by using physical models of large algae, that the movement of foliose algae by the water may restrict urchin movement (Konar and Estes, 2003). In this paper, the conse-50 quences of differential motility in different habitats will be explored, whatever its cause.…”