11th ICRS Abstract book - Nova Southeastern University

Oral Mini-Symposium 10: Ecological Processes on Today's Reef Ecosystems
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Interactions Among Inhabitants Of Branching Coral: Multiple Predator Or
Competitor Effects?
Russell SCHMITT* 1 , Sally HOLBROOK 1 , Andrew BROOKS 2 , Jennifer LAPE 1
1 Ecology, Evolution & Marine Biology, University of California Santa Barbara, Santa
Barbara, CA, 2 Marine Science Institute, University of California Santa Barbara, Santa
Barbara, CA
The ability to forecast change in coral reef communities requires sufficient understanding
of how multiple factors operate together, yet the complexity of real systems often is
simplified by lumping species into such functional groups as predators or competitors.
Oversimplification can obscure important influences on the strength of interactions, such
as non-additive effects of multiple predators (MPEs) on shared prey that can either
reduce or enhance their mortality risk. In French Polynesia, we explored interactions
involving two putative predators (arc-eye hawkfish Paracirrhites arcatus, red-spotted
coral crab Trapezia rufopunctata) and several damselfishes (Dascyllus flavicaudus, D.
aruanus, Chromis viridis). Field and laboratory observations revealed that both hawkfish
and coral crabs were capable of capturing and consuming recently settled damselfish.
Because these species all use the branches of the coral Pocillopora eydouxi as shelter
from ‘external’ predators, the hawkfish, coral crabs and damselfishes also may be
competitors for enemy-free space. A survey suggested that the density of damselfish on
P. eydouxi was reduced in the presence of either an arc-eye hawkfish or red-spotted coral
crab. A field experiment confirmed that both hawkfish and coral crabs reduced
survivorship of recently settled damselfish, and that their combined per capita effects
were additive. Additivity occurred because the strongest effect of these species on
damselfish was from competition for enemy-free space that increased their vulnerability
to external predators; only 25% of damselfish mortality attributable to hawkfish resulted
from consumption by hawkfish, while none of the mortality caused by crabs resulted
from predation by crabs. Hence, in this case, there were no emergent effects of multiple
predators. These results underscore the need to understand the particular mechanisms by
which species exert their influence on others.
10-46
Indirect Effects On Coral Dynamics From Interactions Between Resident Fishes
Sally HOLBROOK* 1 , Russell SCHMITT 1 , Andrew BROOKS 2
1 Ecology, Evolution and Marine Biology, UCSB, Santa Barbara, CA, 2 Marine Science
Institute, UCSB, Santa Barbara, CA
Mortality rates of coral often scale inversely with colony size, and hence factors that
enhance the growth of smaller, more vulnerable size classes can be important to the
dynamics of coral populations. In a field experiment, we found that branching coral in
the genus Pocillopora grew a third faster when a colony was occupied by a group of
damselfishes; the amount of growth enhancement varied positively with the number
(biomass) of resident damselfish. We undertook field studies to determine what
influences the occurrence of damselfish groups on Pocillopora in Moorea, French
Polynesia. Field surveys revealed three distinct fish occupancy patterns that were related
to coral size: (1) no coral 80% of large corals
(>75 cm circumference) hosted a group of damselfish, more than half of which also had
co-occurring hawkfish. Field experiments and observations revealed that arc-eye
hawkfish could prevent the establishment of a damselfish group on intermediate-sized
corals by suppressing recruitment of young damselfish (reducing settlement rates,
increasing early mortality rates). By contrast, hawkfish could not stop groups of
damselfish from colonizing larger corals because they could not prevent immigration of
older (larger) damselfish and they appeared less efficient at reducing larval recruitment in
large corals. Thus the growth - and perhaps survival - rates of intermediate-sized
Pocillopora can be influenced substantially by a small piscivore. These results indicate
that biotic interactions among species of fish that use corals as habitat can have
substantial but non-obvious indirect effects on the dynamics of corals.
10-47
Recruitment And Population Dynamics Of A Coral-Reef Fish: A Large-Scale Long-Term
Field Experiment
Mark HIXON* 1 , Todd ANDERSON 2 , Kevin BUCH 3 , Darren JOHNSON 4 , Christopher
STALLINGS 5
1 Department of Zoology, Oregon State University, Corvallis, OR, 2 San Diego State University,
San Diego, CA, 3 Texas A&M University at Galveston, Galveston, TX, 4 Oregon State
University, Corvallis, OR, 5 Florida State University, St Teresa, FL
The relationship between the input of newly settled recruits and post-settlement demographic
rates in populations of coral-reef fishes is complex, and has thus been controversial. To
examine this relationship at the spatial scale of entire local populations and the temporal scale
of entire generations, we conducted an 8-year field experiment in the Bahamas. First, from
1998 to 2002, we monitored demographic rates (recruitment, survival, growth, and egg
production) in four populations of the bicolor damselfish (Stegastes partitus). During this 4year
baseline period, two "near sites" adjacent to the Exuma Sound source of larvae exhibited
relatively high recruitment rates and largely density-dependent dynamics. In contrast, two "far
sites" located some 20 km from the Sound on the Great Bahamas Bank exhibited low
recruitment and density-independent dynamics, in accordance with the "recruitment limitation"
hypothesis. Second, during four consecutive recruitment seasons (2002-2005), we reduced
recruitment 10-fold at one near site by removing new recruits as they settled, and bolstered
recruitment 10-fold at one far site by transplanting new recruits. We continued demographic
monitoring at all four sites into 2006. Examining all 8 years, population dynamics remained
mostly density-dependent (compensatory) at the near sites, yet became inversely densitydependent
(depensatory) at the far sites. The cause of depensation appears to have been a
combination of relatively low habitat complexity and high physical (including hurricane)
disturbance at the far sites, such that new recruits suffered higher mortality rates at lower
densities. We conclude that a combination of factors, rather than larval supply alone,
determines the level and kind of dependence on population density exhibited by demographic
rates.
10-48
Habitat Competes For Fish Larvae: Increased Fish Production Despite Settlement
Redirection
Adrian STIER* 1 , Craig OSENBERG 2
1 Zoology, University of Florida, Gainesville, FL, 2 University of Florida, Gainesville, FL
Declines in the quality and quantify of coral reefs can decrease the abundance and diversity of
the organisms that dependent on the reef habitat. As a result, coral reef restoration (e.g., via
deployment of artificial reefs or coral fragments) has been proposed to reverse long-term
degradation. The effect of new habitat remains unclear, primarily because added habitat may
simply redistribute individuals, rather than increasing the regional size of a population. This
uncertainty has led to the “attraction-production” controversy. Redirection alone does not
increase production; it just redistributes it. However, if fish compete, then redistribution of
larvae can enhance regional populations by lowering densities on existing reefs and thereby
reducing the effect of density on post-settlement survival. Settlement redirection has never been
previously tested in a field experiment. Thus, we designed a field experiment to quantify the
degree to which new habitat increases settlement (or redirects it). Addition of new habitat
increased total settlement, but decreased density. However the effect on settlement was much
less than expected based on the increase in reef habitat: habitat increased 6-fold, but settlement
increased less then two-fold. Thus, settlers are redirected to new habitat, but total settlement
increases, and local density decreases. Based on the observed strength of density-dependence in
these systems, we demonstrate how habitat addition should lead to a reduction in density, an
increase in regional density, and a significant increase in fish production by ameliorating
negative effects of density.
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