11th ICRS Abstract book - Nova Southeastern University

Oral Mini-Symposium 10: Ecological Processes on Today's Reef Ecosystems
10-17
The Influence Of Macroherbivores On The Macoralgal Dynamics Of A Caribbean
Coral Reef
Stephen BOX* 1 , Peter MUMBY 1
1 Marine Spatial Ecology Lab, University of Exeter, Exeter, United Kingdom
The spatiotemporal dynamics of macroalgae remain poorly understood despite being the
spatial dominant on many Caribbean coral reefs. Whilst herbivores are recognised for
their essential role in controlling the abundance and distribution of algae, how they affect
macroalgal dynamics and the availability of potential settlement space for corals is less
clear. Using transition matrices generated from the occupation states of small permanent
quadrats, across successive months, we use generalised linear models to assess the factors
controlling the dynamics of available space and those of two dominant macroalgal
species, Lobophora variegata and Dictyota pulchella, on shallow forereefs in Honduras.
Using exclusion cages the influence of macro-herbivores on these dynamics were then
evaluated. The study found that the rapid occupation and transient dynamics of both
macroalgal species across the surfaces of dead coral restrict available space. Unoccupied
space was at a minimum in the sheltered microhabitats preferred by coral recruits,
resulting in less than 0.06% of the total surface area of dead coral being both in sheltered
habitats and free from algal occupation for six months continuously. This spatiotemporal
limitation may severely restrict successful coral recruitment. When macro-herbivores
were excluded, the availability and persistence of unoccupied space decreased, but the
increase in cover by both macroalgal species investigated here were limited by
competition from other benthic algae. Herbivory influenced algal dynamics across the
surfaces, including the sheltered areas which they could not access. In reducing overall
algal cover, herbivores reduce the reoccupation rate and persistence time of algae,
affecting their dynamics even in the cryptic areas that cannot be grazed directly.
However, the current parrotfish dominated herbivore regime of the study site may
actually be beneficial to these two algal species by restricting competition from other
algal groups.
10-18
Environmental Stress Drives Seasonal Changes in Top-Down And Bottom-Up
Regulation Of Coral Community Structure And Resilience
Peggy FONG* 1 , Matthew WARTIAN 1
1 Ecology and Evolutionary Biology, University of California Los Angeles, Los Angeles,
CA
Theory predicts that the strength and relative importance of processes structuring
communities will vary predictably with changes in environmental stress. In this study, we
used a comparative approach by performing two experiments to assess differences in
bottom-up (nutrients) and top-down (herbivory) forces on changes in tropical eastern
Pacific coral reef community structure and resilience during periods of increasing and
decreasing environmental stress due to upwelling. We found that as upwelling intensity
increased, herbivores were unable to limit increases in algal cover, which expanded into
areas previously dominated by live coral. Additionally, experimental nutrient enrichment
reduced coral and enhanced algal cover both in the presence and absence of herbivores at
the beginning of the upwelling season. Nutrient additions initially shifted the algal
community toward macroalgae, while later it favored turfs. In contrast, in the experiment
conducted when upwelling intensity declined, herbivores effectively reduced algal cover
and increased live coral cover while experimental nutrient enrichment effects were not
significant. However, a combination of herbivory reduction and nutrient enrichment
shifted the algal community to dominance by crustose forms. Based on our findings, we
conclude that both nutrients and herbivory are important factors in determining coral
community structure and resilience, although the relative importance of each was
influenced by environmental stress. Large-scale increases in environmental stress
decreased the relative importance of top-down forces, increased the dominance of
bottom-up forces, and enhanced algal overgrowth of corals. However, once
environmental stress was alleviated, herbivores strongly enhanced coral community
resilience, reversing phase shifts to greater algal dominance.
10-19
Environmental Stress Changes The Relative Importance Of Top-Down (Herbivorous
Fishes) And Bottom-Up (Nutrients) Forces Regulating Community Structure And
Resilience Of Coral Reefs
Matthew WARTIAN* 1,2 , Peggy FONG 3
1 Weston Solutions, Carlsbad, CA, 2 Ecology and Evolutionary Biology, University of California,
Los Angeles, Los Angeles, 3 Ecology and Evolutionary Biology, University of California, Los
Angeles, Los Angeles, CA
Understanding the roles of top-down and bottom-up forces in determining community structure
and resilience has increasingly become the focus of ecological studies as human influences on
ecosystems have intensified, changing the strength of forces, and resulting in phase shifts to
alternate states. Recent phase shifts of marine communities are attributed to human impacts,
including nutrient enrichment, overfishing of predatory and herbivorous fishes, and increases in
environmental stress. For coral reefs, many have hypothesized that herbivores are of primary
importance to reef resilience, limiting the proliferation of macroalgae and enhancing coral
recovery after stress. However, there have been few experimental studies testing this
hypothesis. Here we provide evidence that an intact herbivore community reversed macroalgal
overgrowth of coral following a period of environmental stress. Recovery of coral dominance
after episodic low-temperature stress was dependent upon the presence of herbivores, a topdown
force that removed algal proliferation and prevented further coral mortality. Without
environmental stress, no phase shift was initiated indicating that reductions in herbivory alone
may not cause community shifts. Rather, without environmental stress both herbivory and
bottom-up forcing by nutrients regulated coral/algal dominance, although the magnitude of
community changes was relatively modest. We conclude that the ability of coral communities to
recover from environmental stress is undermined by overfishing, while maintenance of reef
stability is also dependent on nutrients. Our results provide scientific support for the worldwide
call for protection of reef fishes as a key management goal, especially in the face of increasing
nutrification and environmental stress.
10-20
A Biophysical Perspective Of Grazing in Pacific Parrotfish
Sonia BEJARANO* 1 , Victor TICZON 2 , Peter MUMBY 1
1 Marine Spatial Ecology Lab, University of Exeter, Exeter, United Kingdom, 2 Marine Science
Institute, University of the Philippines, Manila, Philippines
Since grazing by parrotfishes plays a crucial role in processes of reef recovery, this study aimed
to understand the way it is affected by important biophysical variables. Data collection took
place in the Palau archipelago during May 2006. Individual bite rate within three contrasting
reef locations was modelled as a function of time of day, tidal direction and height,
species/genus and fish total length (TL). Individual bite area was estimated from jaw
dimensions of parrotfish at the fish market. Bite area for each of the most abundant parrotfish
genera was modelled as a function of TL. Bite rate and bite area models were combined with
local fish census data to predict the grazing impact (m2 hour-1) of individuals of the most
abundant species as well as of the whole assemblage of parrotfish. Most species focused their
grazing on algal turfs but also included a broad range of substrates including live coral. In
general, bite rate was negatively correlated with TL and lower in the morning. In particular, bite
rate differences among species were detected in two sites, a different effect of body size
depending on the genus in one of these sites and a significant effect of tidal height during
midday in another site. Bite rate of initial phase Chlorurus sordidus also seemed to be
moderated as tidal height increased in midday. Bite area of all genera was linearly related to TL,
similar curves for these relationships were obtained for four genera. Output models identify the
relative contribution of some species, species and size classes to overall grazing. We show that
the conversion of fish census data to modelled grazing will potentially give a more insightful
metric of grazing than using biomass as a simple surrogate.
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