Download Abundance and diversity of corals inside and outside the territories

Abundance and diversity of corals inside and outside the territories of
Stegastes nigricans
Jennifer L. Raum
Abstract. In this study, I measured the abundance and diversity of corals inside and outside Stegastes
nigricans territories. Clear patterns of abundance and diversity of coral communities in territories versus
controls show that Stegastes nigricans have a large effect on coral communities. By cultivating their algal
territories, some corals are harmed due to increased competition with algae for space. By defending their
territories, S. nigricans prevent fishes that are harmful to corals from entering their territories, promoting
growth and survival of some corals. I found that diversity of corals is higher where S. nigricans territories
are present compared to control areas with few or no S. nigricans.
Introduction
Stegastes nigricans is a territorial damselfish found in the tropical waters of Moorea,
French Polynesia. Each individual guards an algal mat that is used as food, shelter and a nesting
site. A study on S. nigricans at Reunion Island, France, revealed that dead Acropora is the
preferred substrate on which the damselfish cultivate their territories (Letourneur 2000). It seems
that dead Porites may be their preferred substrate in Opunohu Bay, Moorea. S. nigricans have
been observed moving dead Acropora into their territories at Temae, Moorea in order to increase
the surface area to allow more algal growth (Shima 1992).
S. nigricans live among several genera of corals. The following corals are found at my
chosen study site in Opunohu Bay: Montipora, Fungia, Psammocora, Porites rus, Porites spp.,
Pocillopora damicornis, Pocillopora eydouxi, Cyphastrea, Montastrea, Pavona cactus, Pavona
varians, and Leptastrea. It has been proposed that damselfish may influence the corals that live
among them by preventing fishes that may be harmful to the corals from entering their territories
and by increasing coral-alga competition by cultivating algal mats (Wellington 1982). Physical
factors, such as light, temperature, salinity, sedimentation, and exposure, may play a role in the
distribution of corals (Wellington 1982; Adjeroud 1997). Biotic factors that influence survival of
corals include competition with algae and other corals and predation on corals (negative
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influence) and on algae (positive influence on corals) (Adjeroud, 1997).
This study attempts to find differences in coral abundance and diversity between control
plots and plots containing S. nigricans territories by addressing two questions: (1) Are there
species of corals that are more abundant where S. nigricans colonies are present? (2) Are corals
more diverse where S. nigricans colonies are present?
Methods
Radial Transects
We conducted all sampling at Public Beach on the outer edge of the east side of Opunohu
Bay. I randomly chose 24 patches of live and dead corals, each ranging ~1.8–26.3 m2. Each of 10
patches contained territories of several Stegastes nigricans. Fourteen control patches contained
none to few territories. We used radial transects to sample each patch. Six transects radiated
outward at heading intervals of 60˚, starting at 0˚ and ending at 300˚. Along each transect, we
recorded data that reflected the underlying substrata and/or organisms, using a point contact
method to find percent cover. We sampled an approximate total of 25 m. The total amount (in
centimeters) and percent cover were determined for each component.
Rare Species Area Estimates
We recorded estimates of more rare species by approximating the dimensions (length and
width) of every rare species observed on every plot. (Rare species include: Montipora, Fungia,
Psammocora, Pocillopora damicornis, Pocillopora eydouxi, and “Streas” (Cyphastrea,
Montastrea, and Leptastrea). I divided these two-dimensional area estimates by the estimated
total area of the patch (A = (pi)r2; r = average radius of the 6 measured radii) to determine the
percent cover of each component. This method was carried out in order to supplement the radial
transect method by estimating the percent cover of the smaller and less common species of corals
in another fashion.
Results
Abundance Question
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I plotted a graph to visualize differences for each species between control and territory
plots (Graph 1). Pavona varians, Pocillopora eydouxi, and Psammocora showed the largest
differences between abundance on territories versus controls. (Table 1) Pavona varians and
Pocillopora eydouxi were only found in territory areas. Psammocora was only found in control
plots. “Streas” (Montastrea, Leptastrea, and Cyphastrea), P. rus, Pocillopora damicornis, and
Montipora were all found in both control plots and territories but occurred significantly more in
territory plots. Porites spp. was found in both as well but significantly more in control areas.
I used a Pearson Chi-Square test to determine statistical significance of the data. I found
highly significant differences for all coral types between territory and control areas (p<.0000001,
except Porites rus, p<.03).
I plotted a second graph to determine ecological importance of the differences between
control and territory plots. (Graph 2) I subtracted the total amount of each species (cm) in control
plots from the total territory amount and divided by the total territory amount ((T-C)/T) to
determine a value for each coral type. The obtained value (fraction of relative importance)
relates the difference between amounts of each coral in territories versus controls with the total
amount in the territory. The corals that are favored in territories are more important as the
fraction of relative importance approaches 1. The corals that are favored in controls are more
important as the fraction of relative importance approaches infinity. All coral types show
differences that are ecologically important, except Porites rus. This species was found to be
abundant in both treatments and not extremely favored by either treatment.
We used a second method to add support for the radial transect data. (Graph 3) The corals
represented here are more rare and/or small compared to most Porites spp. colonies. The percent
cover of these corals could have been underestimated by the radial transect method. The percent
cover of each coral type using this method is too small to evaluate statistically. Therefore, this
was simply a tool to strengthen the statistically significant data. Graph 3 shows that many of the
same patterns were demonstrated with this method as with the radial transect method. The values
for each coral are relative to each other and do not reflect abundance of algae or larger, more
common corals. (Table 2) The “Streas,” Psammocora, Pocillopora, and Fungia all showed
consistent patterns with both methods. Pavona varians was detected in only territory plots with
the first method (radial transects) and in both territory and control plots using the second method.
Montipora was found more frequently in territory plots using radial transects. Conversely, it was
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found more frequently in control plots than territory plots using the second method. Differences
may be random.
Diversity Question
Diversity index (H’) was calculated for corals in territory plots and for corals in control
plots using the following formula: H’ = p ln p; p = number of individuals of a species in a
population/total number of individuals in the population. The following results were found:
H’territories = 0.8 and H’controls = 0.4. Diversity of corals in territory plots is twice as high as in
control plots.
Discussion
Pocillopora eydouxi were found exclusively in territory areas (Graphs 1–3). Pocillopora
damicornis were found more commonly in territory areas than in control areas (Graphs 1–3).
This is likely because these are branched corals. If Stegastes nigricans larvae recruit to these
corals, they are able to hide from predators among the branches, increasing growth and survival
(Danilowicz 1996). A similar association was hypothesized to have occurred between another
species of damselfish and branched corals in the Gulf of Panama (Wellington 1982). In this case,
the Pocillopora is positively affecting S. nigricans by providing a site for the larvae to recruit. S.
nigricans may also positively affect Pocillopora by defending territories on which coral
recruitment has been shown to be higher than in other areas (Gleason 1996). By chasing away
fishes that may be harmful to coral recruits, S. nigricans increase the survival of Pocillopora and
possibly other corals.
Porites spp. were found more than twice as often in control areas than in territory areas
(Graphs 1, 2). This is likely due to competition with algae. S. nigricans are found commonly on
Porites colonies. The algal mats of S. nigricans harm Porites via competition for light. The
territories of S. nigricans consist of turf algae that grow thicker than algae in areas that are not
defended. Algal mats are usually uniform in thickness and cover all dead coral in the territory
(Shima 1992). The edge of the algal mat is where competition between algae and coral occurs.
Since the S. nigricans help the algae to grow by preventing herbivores from eating the algae, it
seems that competition pressure on corals by territorial algae is more intense than by algae
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outside of territories.
The data on Montipora may be inconclusive because the two methods showed opposite
results (Graphs 1–3). Radial transect data alone shows that Montipora is more common in
territories than controls. If this method holds true, this may be due to increased recruitment in the
territories that are indirectly protected by S. nigricans by excluding harmful predators. This is
probably the case also for Pavona varians and the “Streas.” These corals were also found
predominantly in territory areas.
Both methods detected Fungia and Psammocora occurring more commonly in control
areas than in territories (Graphs 1–3). The reason for this pattern seems less clear. Algae are
strong competitors against corals. Montipora, Pavona, and the “Streas” seem to successfully
compete with the algae in territories. Porites, however, is killed more so by the algae in
territories. Fungia and Psammocora may suffer the same fate for the same reason. They may be
unable to recruit where algae is present. Another possibility is that more data needs to be
collected for both genera to determine if this pattern truly exists. Fungia had the least amount of
data of all the corals sampled by both methods.
The diversity indices calculated for coral species within and outside S. nigricans
territories show that territories hold a coral composition that is twice as diverse as the control
areas. This is possible due to the following ideas. S. nigricans prevents Porites spp. from
dominating the community by increasing the amount of algae in the community, which in turn
decreases the amount of Porites. S. nigricans also indirectly helps some coral recruits grow and
survive, many of which are more rare species. By increasing the number of rare species and their
colony sizes and by decreasing the dominating Porites, S. nigricans promotes a more diverse
community.
The data found in this experiment showed important differences between the abundance
and diversity of corals inside territories of S. nigricans versus outside of the territories. S.
nigricans have very interesting interactions with the coral communities that exist in a common
habitat. The data show that some corals are more abundant in territory areas, and other corals are
more common in control areas. Calculated diversity indices show a more diverse coral
community within territories of S. nigricans. Competition for algae exists between echinoderms
and S. nigricans (Letourneur 2000). This is an example of other factors that may be impacted by
the presence of S. nigricans. Further studies on coral recruitment, S. nigricans recruitment site
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preferences (Pocillopora?), and further community structure impacts by S. nigricans on
abundance and diversity of other invertebrates and algae will be interesting.
Acknowledgments. Thank you to Nicole Inokuchi for helping with all the sampling and giving valuable input;
Peter Breyfogle and Giacomo Bernardi for helping me catch S. nigricans; Pete Raimondi and Mark Readdie for
helping me with statistics; Yannick and James for providing a great field station; Paola for showing me how to
dissect otoliths; Peter Eldredge for listening; Chad Hanson and Mike Orlando for the late night chats, the music, and
being the BEST roommates; Nora Grant, Teri Sigler, and Peter Eldredge for being the BEST food group; Jenna
Shinen for being such a great “Mom”; everyone for being part of this amazing experience; and the TAs and
professors for sharing your knowledge.
References
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Polynesia. Mar. Ecol. Prog. Ser. 159: 105–119.
Danilowicz BS 1996. Choice of coral species by naïve and field-caught damselfish. Copeia. 3:
735–739.
Gleason MG 1996. Coral recruitment in Moorea, French Polynesia: the importance of patch type
and temporal variation. Jour. Exp. Mar. Biol. Ecol. 207: 79–101.
Letourneur Y 2000. Spatial and temporal variability in territoriality of a tropical benthic
damselfish on a coral reef (Reunion Island). Environ. Biol. Fish. 57: 377–391.
Shima JS 1992. Territory structure and aggregated behavior of damselfish (Stegastes nigricans)
on Moorea, French Polynesia. IB 158 1–16000.
Wellington GM 1982. Depth zonation of corals in the Gulf of Panama: control and facilitation by
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