Download Scaling of ecological dominance and resilience in kelp communities

Scaling of ecological dominance and resilience in kelp communities
PAUL K. DAYTON, ED PARNELL, AND JAMES LEICHTER
Scripps Institution of Oceanography, La Jolla, CA 92093-0227
What are the important environmental factors
determining resilience in kelp ecosystems and how do
they exert their influence? Patterns and processes vary in
space and time; often resilience reflects ecological
mediation of the physical and biological processes. The
Macrocystis kelp forest at Point Loma, San Diego,
California exhibited distinct kelp patch structure that
persisted from the 1960s until the late 1990s, resisting
invasion and severe perturbations including massive
storms and El Niños. Under natural conditions important
animals including several species of fish, sea urchins,
abalones as well as sessile invertebrates also persisted.
This resistance stability largely resulted from
interspecific competition, dispersal, recruitment, and
intensity of predation. Eventually a strong La Niña
brought sufficient nutrients to the Macrocystis surface
canopy that the canopy became dense enough to
eliminate all the understory patch structure through
strong shading. As elsewhere, kelps do not resist grazing
by sea urchins in large fronts, the formation of which
reflect available kelp detrital food and reduced predation
by several urchin carnivores. When released from
predation, amphipods can denude Macrocystis, and
heavy settlement of epiphytic organisms can sink or
stress Macrocystis, suggesting stabilizing roles of small
canopy fish predators.
What are the environmental processes that cause these
patterns? Physical processes are important at all scales as
they transport both nutrients and propagules. At smaller
scales the kelps themselves affect the flow rates around
the plants. This is true at the scale of individual
Macrocystis plants such that flows may move sharply
upwards, resulting in an important dispersal process for
the kelp spores. As with Macrocystis, kelps with floating
or stipe-supported canopies such as Pterygophora,
Eisenia, Laminaria spp., Ecklonia, Lessonia, may
interfere with flow, which can result in strong edge
effects. The reduction of flow at the scales of the forest
edge inhibits nutrient transport and dispersal of
propagules into the larger kelp forests. At these scales
the most important forcing functions may be breaking
internal waves and wind. The former may force deep
nutrient rich water and propagules much farther into the
forest whereas the latter breaks down the edge effect at
the canopy and results in surface water flowing across
the top of the forest, thereby potentially advecting
surface larvae into the forest. The availability of light
limits kelp growth and distribution. At small scales it
limits germination and growth, and it is one of the most
important factors limiting small-scale distribution and
abundance.
How general are these patterns and processes over larger
spatial scales? In areas with warmer nutrient poor water
the surface canopy can be thin resulting in welldeveloped understory canopies deriving nutrients from
breaking internal waves. In areas with extreme wave
exposure, nutrients and light are less likely to be
limiting. The understory canopies are more tolerant to
wave exposure and become dominant. Thus resilience of
kelp community structure depends upon shifting patterns
of ecological dominance, which can reflect several
environmental forcing functions. In addition, it also
depends upon cascading patterns of predation on the
kelps, their epiphytes, and sea urchins. Our
understanding of this resilience depends upon synthesis
of both ecological succession and resistance to invasion.
What of the future? The natural distribution and
abundance of most of the associated animals reflects
topographical bottom features and biota other than kelps
themselves. Anticipated global warming effects may
lower the thermocline: but all other things being equal
this will reduce the nutrients in the surface waters and
thin the Macrocystis canopy. Physical processes such as
wind forcing, upwelling and internal waves will continue
although perhaps at somewhat different levels; this
probably implies that the understory kelps and the rest of
the communities may not be much affected by the
projected warming. Unfortunately, the mortality related
to fishing is so severe, that natural patterns are a myth:
most kelp forests have lost most of the functionally
important species including most large predators and
species such as abalones and many other gastropods,
holothurians, and now even sea urchins as a result of
fishing. In many cases other predators, especially
crustacean and small fish acquire community roles that
they did not have before the kelp habitat was so badly
overfished.