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Diversity, patterns of adaptation, and stability of Nova Scotian kelp beds


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Johnson, CR and Mann, KH 1988 , 'Diversity, patterns of adaptation, and stability of Nova Scotian kelp beds' , Ecological Monographs, vol. 58, no. 2 , pp. 129-154 , doi: 10.2307/1942464.

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There are two alternate community states in the rocky subtidal of the Atlantic
coast of Nova Scotia, an unproductive sea urchin/coralline alga community, and highly
productive kelp beds dominated by Laminaria longicruris. Disease-induced mortality of
the sea urchins (Strongylocentrotus droebachiensis) triggered a switch from the first state
to the second and provided a unique opportunity to study (1) the ability of L. longicruris
to recover its former dominant status, and (2) its stability when competing with other
seaweeds and when perturbed by storms and grazers other than urchins. Rates of recolo-
nization of L. longicruris depended on the proximity of a refugial source of spores. When
reproductive plants were nearby, a closed canopy developed within 18 mo of urchin mortality. When a reproductive population was several kilometres away, there was sparse
recolonization for 3 yr, then a massive recruitment occurred with closure of the canopy in
the 4th yr.
Laminaria is clearly the competitive dominant in the seaweed community. Manipulative experiments showed that the kelp limits the abundance of several understory species,
but there was no evidence that the abundant annual seaweeds limited kelp recruitment.
When sea urchins were rare, the density and growth rates of Laminaria were influenced
mostly by intraspecific competition. When the canopy of adult plants was removed there
was a dramatic increase in kelp recruitment, but the recruits that grew in dense patches in
the clearings were significantly smaller than those of a similar age that grew more sparsely
beneath the canopy. Once the kelp recovered from destructive grazing and formed a mature
forest, it was able to maintain its dominance, even in habitats subject to severe nutrient
stress for 8 mo of the year. For most ofthe year mortality and erosion of laminae outweighed
the effects of recruitment and growth, and the canopy declined, especially during winter
when storms were frequent. Erosion was exacerbated by grazing of the gastropod Lacuna
vincta. However, in late winter and early spring, recruitment and rapid growth restored
the canopy. When severe storm damage was simulated by completely removing Laminaria
in patches, the kelp rapidly recolonized and soon outgrew other seaweeds.
Unlike the competitive dominants in kelp bed systems in the northeast Pacific, L .
longicruris in Nova Scotia manifests multiple patterns of adaptation that enable it to
dominate early and late stages of succession in a range of habitats of different levels of
nutrient stress and of disturbance from storms and grazers. The principal threat to the
stability of the kelp beds is destructive grazing by sea urchins. We suggest that the consid-
erable differences between the dynamios of kelp beds in Nova Scotia and those of the
northeast Pacific, and the high degree of stability of L . longicruris stands in Nova Scotia,
is attributable to the low diversity of kelps and therefore low levels of competition in Nova
Scotia, and to the multiple adaptations of L . longicruris that enable it to tolerate several
stresses and disturbances.
We argue that the dynamics of community organization, and therefore the stability
properties of this system are determined primarily by biological interactions and not by
physical variables. This differs from the kelp communities in the northeast Pacific, in which
both biological and physical factors influence dynamics significantly at a primary level.
We offer a qualitative model of the dynamics of community structure in Nova Scotia that may be viewed as a set of deterministic "subroutines," in which each subroutine describes
the outcome of a particular biological interaction. The subroutine(s) that predominate at
one point in time and space are probably determined mostly by physical hydrographic
variables that have a large stochastic component.

Item Type: Article
Authors/Creators:Johnson, CR and Mann, KH
Keywords: community structure; competition; disturbance; diversity; kelp; Laminaria; multiple adaptations; Nova Scotia; sea urchins; stability; stress; succession.
Journal or Publication Title: Ecological Monographs
DOI / ID Number: 10.2307/1942464
Additional Information:

Copyright by the Ecological Society of America.

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