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articles


An exchange of views on Marine Reserves and Trophic Cascades
25/01/08
source : Various UK and USA  Vol 311 - Science (www.sciencemag.org)


Effect of Nassau grouper on parrotfishes

To investigate the feasibility of grouper consuming parrotfishes of various sizes, we
established two allometric scaling relationships. Firstly, the relationship between gape
width (G) and total length (TL) was established for the dominant predator, E. striatus, using
seven anaesthetised individuals within the reserve.

Gape width, G = -0.5998+(0.167×TL) with a coefficient of determination, r2, of 0.96.

Secondly, at least three individuals of Scarus iserti, Sc. vetula, Sparisoma viride and Sp.
aurofrenatum were photographed in both their terminal- (TP) and intermediate- phases (IP)
to determine the relationship between fork length (FL) and maximum dorso-ventral height
(H). Dimensions of the fish in each photograph were digitally analysed and the resulting
scaling relationships used to estimate the maximum height of each parrotfish surveyed in
situ.




Grazing behaviour of parrotfishes

We adapted the statistical model of Bruggemann (S4) on grazing by two species of
parrotfishes in Bonaire (Sparisoma viride and Scarus vetula) by extending it to multiple
scarid species. The model assumes that allometric relationships between fork length and
both bite rate and mouth size can be extrapolated within genera.

Bite rate, r, of Sparisoma spp. = CSc ((1088 – (17.12 FL)) – Species offset)
(1a)

Bite rate, r, of Scarus spp. = CSp ((3329 – (33.00 FL)) – Species offset) (1b)

Bite rate, r, (h-1) is calculated as a function of species (s), length (l), and life phase (p) using
(1a or 1b) where FL is the fork length (cm), C is a weighting factor for life phase such that
values for the genus Scarus (CSc) are 0.85 for TP and 1 for IP and juveniles and those for
Sparisoma (CSp) are 0.80 for TP, 1 for IP and 0.84 for juveniles. Species-level offsets in the
genus Scarus are 0 for vetula, 1196 for taeniopterus; 1714 for iserti. Offsets in the genus
Sparisoma are 260 for aurofrenatum, 142 for rubripinne, 264 for chrysopterum, and 56 for
viride. Offsets were based on 20 minute observations of grazing intensity in Belize (n=18
per species).

Total grazing intensity, expressed as area of a reef grazed per unit time, is calculated as a
function of size- and species-specific bite rates and bite sizes (S4).

Effect of Nassau grouper on parrotfishes

To investigate the feasibility of grouper consuming parrotfishes of various sizes, we
established two allometric scaling relationships. Firstly, the relationship between gape
width (G) and total length (TL) was established for the dominant predator, E. striatus, using
seven anaesthetised individuals within the reserve.

Gape width, G = -0.5998+(0.167×TL) with a coefficient of determination, r2, of 0.96.

Secondly, at least three individuals of Scarus iserti, Sc. vetula, Sparisoma viride and Sp.
aurofrenatum were photographed in both their terminal- (TP) and intermediate- phases (IP)
to determine the relationship between fork length (FL) and maximum dorso-ventral height
(H). Dimensions of the fish in each photograph were digitally analysed and the resulting
scaling relationships used to estimate the maximum height of each parrotfish surveyed in
situ.




Grazing behaviour of parrotfishes

We adapted the statistical model of Bruggemann (S4) on grazing by two species of
parrotfishes in Bonaire (Sparisoma viride and Scarus vetula) by extending it to multiple
scarid species. The model assumes that allometric relationships between fork length and
both bite rate and mouth size can be extrapolated within genera.

Bite rate, r, of Sparisoma spp. = CSc ((1088 – (17.12 FL)) – Species offset)
(1a)

Bite rate, r, of Scarus spp. = CSp ((3329 – (33.00 FL)) – Species offset) (1b)

Bite rate, r, (h-1) is calculated as a function of species (s), length (l), and life phase (p) using
(1a or 1b) where FL is the fork length (cm), C is a weighting factor for life phase such that
values for the genus Scarus (CSc) are 0.85 for TP and 1 for IP and juveniles and those for
Sparisoma (CSp) are 0.80 for TP, 1 for IP and 0.84 for juveniles. Species-level offsets in the
genus Scarus are 0 for vetula, 1196 for taeniopterus; 1714 for iserti. Offsets in the genus
Sparisoma are 260 for aurofrenatum, 142 for rubripinne, 264 for chrysopterum, and 56 for
viride. Offsets were based on 20 minute observations of grazing intensity in Belize (n=18
per species).

Total grazing intensity, expressed as area of a reef grazed per unit time, is calculated as a
function of size- and species-specific bite rates and bite sizes (S4).

WE DISAGREE WITH CHAPMAN ET AL.’S ASSER


tion that potentially high levels of shark abundance were an important contributor to the
observed increase in parrotfish grazing within
the Exuma Cays Land and Sea Park (ECLSP).

First, a key finding of our study was that
larger-bodied parrotfish species achieve an
escape from grouper predation and therefore the
interaction between groupers and parrotfish is
surprisingly weak, reducing parrotfish grazing
by only 4 to 8%. Although we cannot discount
the possibility that even greater biomasses of
grouper could eventually constrain grazing, current biomasses are already at least double that of
other reported levels in the Caribbean.

Second, there is no empirical evidence that
sharks regulate grouper biomass. In Bascompte
et al. (1), the modeling of shark-grouper interactions ignores active prey selection and prey size
refugia and may prove to be simplistic once studied empirically. Thus, we agree that the ecosystem functioning of sharks warrants further study.

Third, it is difficult to imagine that sharks
would have a strong impact on groupers but
not on parrotfishes. Indeed, Bascompte et al.

(1) found that only four of 200 tritrophic relationships containing sharks, groupers, and
parrotfish contained two or three strong inter-
CREDIT: CORBIS

www.sciencemag.org SCIENCE VOL 312 28 APRIL 2006 527

Published by AAAS


LETTERS


actions. Of those four, two included direct
negative impacts of sharks on parrotfish that
would tend to counteract the positive indirect
impact postulated by Chapman et al. Moreover, none of these tritrophic food chains
involved the large-bodied species of parrot-
fishes, which undertake most of the grazing.

Even if sharks are unusually abundant in
the Bahamas, their direct impacts are unlikely
to differ across reserve boundaries because the
shark species that most likely feed on groupers
(and large parrotfish) range over large areas
(2). Therefore, our results show that despite
background shark predation, both groupers
and parrotfish can achieve high biomasses
when fishing levels are vastly reduced.

The key process driving these patterns is
fishing mortality. The impact of a reserve
depends on both the complexities of trophic
cascades within the reserve and the differential
in fishing mortality across its boundaries [our
Report, (3)]. Our paper and that of Bascompte
et al. (1) show that trophic cascades can be
highly complex and occasionally surprisingly
weak. We also show that even minor reductions
in fishing mortality can overwhelm the negative influences of weak trophic cascades. Therefore, reductions in fishing mortality within
Caribbean reserves will almost always lead to a
net increase in parrotfish grazing.

PETER J. MUMBY,1 FIORENZA MICHELI,2
CRAIG P. DAHLGREN,3 STEVEN Y. LITVIN,2
ANDREW B. GILL,4 DANIEL R. BRUMBAUGH,5,6
KENNETH BROAD,7 JAMES N. SANCHIRICO,8
CARRIE V. KAPPEL,2 ALASTAIR R. HARBORNE,1
KATHERINE E. HOLMES5

1Marine Spatial Ecology Lab, School of BioSciences, University
of Exeter, Prince of Wales Road, Exeter EX4 4PS, UK.
2Hopkins Marine Station, Stanford University, Oceanview
Boulevard, Pacific Grove, CA 93950–3094, USA. 3Perry
Institute for Marine Science, 100 North U.S. Highway 1,
Suite 202, Jupiter, FL 33477, USA. 4Institute of Water and
Environment, Cranfield University, Silsoe, Bedfordshire
MK45 4DT, UK. 5Center for Biodiversity and Conservation,
American Museum of Natural History, Central Park West at
79th Street, New York, NY 10024–5192, USA. 6Marine
Protected Areas Science Institute, National MPA Center, 110
Shaffer Road, Santa Cruz, CA 95060, USA. 7Rosenstiel
School of Marine and Atmospheric Science, Division of

Marine Affairs and Policy, University of Miami, 4600
Rickenbacker Causeway, Miami, FL 33149, USA. 8Resources
for the Future, 1616 P Street, NW, Washington, DC 20036,
USA.

References

1. J. Bascompte, C. J. Melián, E. Sala, Proc. Natl. Acad. Sci.
U.S.A. 102, 5443 (2005).
2. D. D. Chapman, E. K. Pikitch, E. Babcock, M. S. Shivji,
Mar. Technol. Soc. J. 39, 42 (2005).
3. F. Micheli, P. Amarasekare, J. Bascompte, L. R. Gerber,
Bull. Mar. Sci. 74, 653 (2004).
A Not-So-Abrupt Departure

GIVEN R. A. KERR’S PAST RECORD OF OUTSTAND


ing science journalism, we were surprised by
the unfortunate errors and omissions in his
ScienceScope piece “Knock hockey,” surrounding issues of how scientists gauge temperature
over the last two millennia (10 Mar., p. 1359).The
assertion that “[Michael] Mann made himself
scarce throughout the proceedings, even abruptly
departing as [Stephen] McIntyre stood to make a
final comment” is incorrect. Mann’s early departure had to do with his teaching obligations and a
limited flight schedule. Mann was unaware of
who may have been making comments as he
departed following the official close of the presentation period, and he has been responsive to all
requests made of him by the committee.

Big News

AAAS Science Journalism Awards
Call for Entries

The AAAS Science Journalism Awards honor distinguished reporting
on science by professional journalists. The awards are an internationally recognized measure of excellence in science reporting for
a general audience. They go to individuals (rather than institutions,
publishers or employers) for coverage of the sciences, engineering
and mathematics.

U.S. CATEGORIES
Awards will be presented for U.S. submissions in the following
categories:

• Large Newspaper • Magazine • Television
• Small Newspaper • Online • Radio
INTERNATIONAL CATEGORY

Open to journalists worldwide, across all news media.

• Children’s Science News
Deadline: August 1, 2006 SPONSORED BY

www.aaas.org/SJAwards

528 28 APRIL 2006 VOL 312 SCIENCE www.sciencemag.org

Published by AAAS



VWE DISAGREE WITH CHAPMAN ET AL.’S ASSER


tion that potentially high levels of shark abundance were an important contributor to the
observed increase in parrotfish grazing within
the Exuma Cays Land and Sea Park (ECLSP).

First, a key finding of our study was that
larger-bodied parrotfish species achieve an
escape from grouper predation and therefore the
interaction between groupers and parrotfish is
surprisingly weak, reducing parrotfish grazing
by only 4 to 8%. Although we cannot discount
the possibility that even greater biomasses of
grouper could eventually constrain grazing, current biomasses are already at least double that of
other reported levels in the Caribbean.

Second, there is no empirical evidence that
sharks regulate grouper biomass. In Bascompte
et al. (1), the modeling of shark-grouper interactions ignores active prey selection and prey size
refugia and may prove to be simplistic once studied empirically. Thus, we agree that the ecosystem functioning of sharks warrants further study.

Third, it is difficult to imagine that sharks
would have a strong impact on groupers but
not on parrotfishes. Indeed, Bascompte et al.

(1) found that only four of 200 tritrophic relationships containing sharks, groupers, and
parrotfish contained two or three strong inter-
CREDIT: CORBIS

www.sciencemag.org SCIENCE VOL 312 28 APRIL 2006 527

Published by AAAS


LETTERS


actions. Of those four, two included direct
negative impacts of sharks on parrotfish that
would tend to counteract the positive indirect
impact postulated by Chapman et al. Moreover, none of these tritrophic food chains
involved the large-bodied species of parrot-
fishes, which undertake most of the grazing.

Even if sharks are unusually abundant in
the Bahamas, their direct impacts are unlikely
to differ across reserve boundaries because the
shark species that most likely feed on groupers
(and large parrotfish) range over large areas
(2). Therefore, our results show that despite
background shark predation, both groupers
and parrotfish can achieve high biomasses
when fishing levels are vastly reduced.

The key process driving these patterns is

Letters to the Editor
Letters ( 300 words) discuss material published
in Science in the previous 6 months or issues of
general interest. They can be submitted through
the Web (www.submit2science.org) or by regular
mail (1200 New York Ave., NW, Washington, DC
20005, USA). Letters are not acknowledged upon
receipt, nor are authors generally consulted before
publication. Whether published in full or in part,
letters are subject to editing for clarity and space.
fishing mortality. The impact of a reserve
depends on both the complexities of trophic
cascades within the reserve and the differential
in fishing mortality across its boundaries [our
Report, (3)]. Our paper and that of Bascompte
et al. (1) show that trophic cascades can be
highly complex and occasionally surprisingly
weak. We also show that even minor reductions
in fishing mortality can overwhelm the negative influences of weak trophic cascades. Therefore, reductions in fishing mortality within
Caribbean reserves will almost always lead to a
net increase in parrotfish grazing.

PETER J. MUMBY,1 FIORENZA MICHELI,2
CRAIG P. DAHLGREN,3 STEVEN Y. LITVIN,2
ANDREW B. GILL,4 DANIEL R. BRUMBAUGH,5,6
KENNETH BROAD,7 JAMES N. SANCHIRICO,8
CARRIE V. KAPPEL,2 ALASTAIR R. HARBORNE,1
KATHERINE E. HOLMES5

1Marine Spatial Ecology Lab, School of BioSciences, University
of Exeter, Prince of Wales Road, Exeter EX4 4PS, UK.
2Hopkins Marine Station, Stanford University, Oceanview
Boulevard, Pacific Grove, CA 93950–3094, USA. 3Perry
Institute for Marine Science, 100 North U.S. Highway 1,
Suite 202, Jupiter, FL 33477, USA. 4Institute of Water and
Environment, Cranfield University, Silsoe, Bedfordshire
MK45 4DT, UK. 5Center for Biodiversity and Conservation,
American Museum of Natural History, Central Park West at
79th Street, New York, NY 10024–5192, USA. 6Marine
Protected Areas Science Institute, National MPA Center, 110
Shaffer Road, Santa Cruz, CA 95060, USA. 7Rosenstiel
School of Marine and Atmospheric Science, Division of

Marine Affairs and Policy, University of Miami, 4600
Rickenbacker Causeway, Miami, FL 33149, USA. 8Resources
for the Future, 1616 P Street, NW, Washington, DC 20036,
USA.

References

1. J. Bascompte, C. J. Melián, E. Sala, Proc. Natl. Acad. Sci.
U.S.A. 102, 5443 (2005).
2. D. D. Chapman, E. K. Pikitch, E. Babcock, M. S. Shivji,
Mar. Technol. Soc. J. 39, 42 (2005).
3. F. Micheli, P. Amarasekare, J. Bascompte, L. R. Gerber,
Bull. Mar. Sci. 74, 653 (2004).
A Not-So-Abrupt Departure

GIVEN R. A. KERR’S PAST RECORD OF OUTSTAND


ing science journalism, we were surprised by
the unfortunate errors and omissions in his
ScienceScope piece “Knock hockey,” surrounding issues of how scientists gauge temperature
over the last two millennia (10 Mar., p. 1359).The
assertion that “[Michael] Mann made himself
scarce throughout the proceedings, even abruptly
departing as [Stephen] McIntyre stood to make a
final comment” is incorrect. Mann’s early departure had to do with his teaching obligations and a
limited flight schedule. Mann was unaware of
who may have been making comments as he
departed following the official close of the presentation period, and he has been responsive to all
requests made of him by the committee.

Big News

AAAS Science Journalism Awards
Call for Entries

The AAAS Science Journalism Awards honor distinguished reporting
on science by professional journalists. The awards are an internationally recognized measure of excellence in science reporting for
a general audience. They go to individuals (rather than institutions,
publishers or employers) for coverage of the sciences, engineering
and mathematics.

U.S. CATEGORIES
Awards will be presented for U.S. submissions in the following
categories:

• Large Newspaper • Magazine • Television
• Small Newspaper • Online • Radio
INTERNATIONAL CATEGORY

Open to journalists worldwide, across all news media.

• Children’s Science News
Deadline: August 1, 2006 SPONSORED BY

www.aaas.org/SJAwards

528 28 APRIL 2006 VOL 312 SCIENCE www.sciencemag.org

Published by AAAS

Editorial Note:

This is a discussion and exchange of views on the efficacy of marine reserves and looks at the existence and level of trophic cascades. Clearly, a lot of research is needed to arrive at robust, unequivocal scientific conclusions !!



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