11th ICRS Abstract book - Nova Southeastern University

Oral Mini-Symposium 10: Ecological Processes on Today's Reef Ecosystems
10-37
Fish Demography in The Large
Richard VANCE* 1 , Mark STEELE 2 , Graham FORRESTER 3
1 Ecology & Evolutionary Biology, UCLA, Los Angeles, CA, 2 Biology, California State
University Northridge, Northridge, CA, 3 Natural Resources, University of Rhode Island,
Kingston, RI
Effective management of commercial coral reef fish metapopulations will require
creation of marine protected areas. Informed design of MPA’s must employ
demographic rate functions that drive fish metapopulation dynamics. Unfortunately,
practical constraints usually force measurement of demographic rates on small portions
of individual reefs, a spatial scale far too small to apply directly to metapopulations.
Using these measurements therefore requires some way of “expanding” them to the
spatial scale of entire large coral reefs.
This talk will describe a practical scaling-up procedure. We will show, for the bridled
goby Coryphopterus glaucofraenum (a convenient non-commercially fished species) how
a whole-reef density-dependent death rate function can be constructed from an
empirically measured death rate function that applies at the local scale. The procedure
employs a computer simulation model that follows the fates of individual gobies in all 2m
x 2m cells of a large reef. The reef habitat varies spatially both in its local density of
crevices that provide refuge from predators to goby adults and in the substrate’s
attractiveness to goby settlers. From simulated cell-level demographic data is calculated
a whole-reef per capita death rate, and this rate is then statistically fitted to a function of
whole-reef goby and refuge densities using maximum likelihood methods.
Density dependence at the whole-reef scale is highly statistically significant and can be
described by a function whose form resembles that of the local-scale function. However,
reef spatial heterogeneity forces some adjustment in parameters. Indeed, density
dependence at the whole-reef scale actually proves stronger than at the local scale. We
shall display how scaling up the death rate function is influenced by variability in local
refuge density and in local settler attraction, by the correlation of these reef attributes, and
by the spatial scale on which this habitat heterogeneity occurs.
10-38
Large-Scale Experiment Reveals Effects Of Habitat Structure On Coral Reef Fish
Assemblages
Mark STEELE* 1 , Graham FORRESTER 2 , Jameal SAMHOURI 3 , Clare WORMALD 2
1 Department of Biology, California State University Northridge, Northridge, CA,
2 Department of Natural Resources Science, University of Rhode Island, Kingston, RI,
3 Northwest Fisheries Science Center, NOAA Fisheries, Seattle, WA
The structure of coral reef fish assemblages is usually strongly correlated with attributes
of reef structure. Experimental studies on small patch reefs have improved our
understanding of how certain attributes of reef structure (e.g., shelter hole size) influence
these assemblages, but it is not clear if these results from small-scale experiments
extrapolate to larger spatial and temporal scales. We manipulated the abundance of
rubble habitat to evaluate how this shelter-providing habitat affected the structure of coral
reef fish assemblages at large scales. We added several tons of limestone rubble and
conch shells to one half of each of 5 large (3,000-15,000 m 2 ), isolated reefs near Lee
Stocking Island, Bahamas. We tracked changes in the reef fish assemblage with
underwater visual censuses over 4 years. On rubble addition halves of reefs, several
species of small fishes (damselfishes, wrasses, and gobies) became more abundant,
whereas several similar species in the same families were unaffected. Small-scale habitat
associations and results of previous small-scale habitat manipulations generally did a
good job of predicting which species would be affected by the habitat manipulation and
which were not. Larger species (e.g., snappers, grouper, and squirrelfishes) were
unaffected by the manipulation, presumably because they are not dependent on small
crevices for shelter and their abundance is not influenced by the abundance of smaller
potential prey species that increased as a result of the rubble addition. There was no
strong evidence that any species was negatively affected by the habitat manipulation.
Thus, our results indicate that assemblage structure can by altered by large-scale habitat
manipulations, which increase the abundance of some habitat-dependent species, without
unintended negative effects on other species.
10-39
Extrapolating From Small-Scale Experiments: Predation And Refuge-Shortage in Coral
Reef Fishes
Graham FORRESTER* 1 , Mark STEELE 2 , Jameal SAMHOURI 3 , Richard VANCE 4
1 University of Rhode Island, Kingston, RI, 2 Biology, California State University Northridge,
Northridge, CA, 3 NOAA Fisheries, Seattle, WA, 4 University of California Los Angeles, Los
Angeles, CA
Although field experiments allow rigorous tests of ecological hypotheses, they are usually
limited to small spatial scales. We often want to know if their findings extrapolate to larger
scales, especially when seeking to apply their results to conservation and management.
Experiments on small coral reef fishes (bridled gobies) occupying small habitat patches reveal
that locally density-dependent mortality is inflicted by predators. As prey become crowded,
they suffer a progressively increasing shortage of structural refuges. Consequently, goby
mortality at small-scales is well described by a model in which vulnerability is proportional to
the ratio of gobies to refuges. A manipulation of refuge abundance on entire reefs, which are
the size of small marine reserves and approach the scale at which some reef fisheries operate,
suggests that a similar interaction occurs at this much larger spatial scale. This result is in
accord with a scaling model which indicates that the effects of refuge shortage on entire reefs
can be extrapolated from the model describing goby vulnerability as proportional to the ratio of
fish to refuges. In simple terms, the biological interactions causing density dependence scale
up. Careful extrapolation from small-scale experiments identifying species-interactions may
thus be possible, and so should improve our ability to predict the outcomes of alternate
management strategies for coral reef fishes.
10-40
Musical chairs: competition for unguarded refuges and density-dependent mortality
Jameal SAMHOURI* 1,2 , Richard VANCE 3 , Graham FORRESTER 4 , Mark STEELE 5
1 Northwest Fisheries Science Center, NOAA Fisheries, Seattle, WA, 2 Department of Ecology
and Evolutionary Biology, UC Los Angeles, Los Angeles, 3 Department of Ecology and
Evolutionary Biology, UC Los Angeles, Los Angeles, CA, 4 Department of Natural Resources
Science, University of Rhode Island, Kiingston, RI, 5 Department of Biology, California State
University Northridge, Northridge, CA
Predation is widely recognized as a dominant structuring force in marine populations and
communities. As a result, refuges that reduce predation risk can be an important limiting
resource in prey populations, ultimately mediating the occurrence and strength of densitydependent
mortality. On coral reefs, many fishes retreat to unguarded structural refuges, such as
crevices under coral or other hard substrate, only in response to a predator strike. This behavior
leads to a type of competition for refuges that resembles conceptually the childhood game of
musical chairs: prey scramble to find a safe place to hide when threatened or attacked, and only
when refuges are abundant relative to the number of retreating prey is it likely that individuals
will successfully evade predation. We introduce a new mathematical function to describe the
relationship between structural refuges, population density, and per capita mortality in bridled
gobies (Coryphopterus glaucofraenum) under this form of competition. In this model, we
assume that the proximate cause of all goby deaths is predation, a fixed fraction of individuals
do not respond to predator strikes by retreating to structural refuges, and the remaining
individuals are capable of seeking shelter in a fixed number of unguarded refuges when
threatened or attacked. Therefore, the likelihood of an individual dying during a predation event
is either constant and density-independent or directly proportional to refuge availability, and
these two components of mortality are not additive. In comparison to conventional densitydependent
mortality functions, our musical chairs function provides a much improved fit to
available empirical data for bridled gobies. We suggest that the musical chairs function may
have broad applicability to other coral reef fishes, and more generally, for species in which
refuges from predation are a limiting resource but territorial refuge-guarding does not occur.
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