Transcript 2009, final Lecture 12

Biological Interactions on
Coral Reefs
Mutualsims – Plant-Animal
Mutualisms – Animal-Animal
Habitat Partitioning (Diurnal)
• Nocturnal adaptations
– Bioluminescence
– Day-feeders versus night-feeders
• Fish forage during day or night
• Temporal partitioning of habitat
– Synchronized mass spawning
• Night(s) of full moon, only in certain months
• Corals
• Why do it? Maximize fertilization success
and minimize losses to predators
Predator avoidance - Butterfly Fish
Predator avoidance
Predator Avoidance
Chemical defenses
Lion fish
Cone snail
Warning Coloration
Structural Defense
Coral
Predation
• Huge diversity of organisms
that graze corals
• Remove portions of coral
colony, entire colony not
destroyed. Can abrade tons
of reef per year
• Two groups of fish
– Corallivores = actively feed on
coral polyps (puffers, filefish)
– Omnivores = feed on coral to
remove algae or small
invertebrates (surgeonfish and
parrot fish)
Other Bioeroders
• Tube worms (below)
• Molluscs and Crabs
More Bioeroders
Crown-of-Thorns Seastar
• Acanthaster planci
• Seastar that actively feeds on
corals
• Periodic population
explosions
– Numbers increase from 1 to
1,000s per meter2
– In high numbers they are
capable of completely
destroying entire reefs
Complex Biological Interactions
An interaction that prohibits
Acanthaster from eating
coral
Alpheus (pistol shrimp) and
Trapezia (Crab) live in the coral
branches. When the starfish
approaches the coral both the
shrimp and crab defend the coral
colony by pinching the starfish’s
tube feet
Trapezia pinching tube feet
Trapezia
Evidence of consumer (top-down)
control in coral reef habitats
Trigger fish removal
Elevated urchin density,
reduced algal biomass, and
elevated coral cover
Natural Threats to
Coral Reefs
•
•
•
•
Storms
Natural Predators
Disease
Sea-level rise and
fall
Black band disease
Threats From Humans
•
•
•
•
Overexploitation of reef-associated consumers
Over fishing and poor fishing practices
Sedimentation from development and mining
Water pollution (runoff from above, poor waste
removal, oil spills)
• Careless eco-tourism, divers, boat anchor damage
• Global climate change (global warming, UV radiation)
The Reefs at Risk
Low
Medium
High
Almost two-thirds of Caribbean
reefs are threatened
Low
Medium
High
U.S. Virgin
Islands
Jamaica
Barbados
Overexploitation and coastal
development are the greatest
threats to reefs
40%
35%
30%
25%
20%
15%
10%
5%
0%
O
ita
o
pl
x
re
e
v
n
tio
C
oa
lD
a
st
pm
o
el
v
e
t
n
e
d
n
la
In
Po
n
io
t
llu
M
e
ir n
a
P
n
io
t
lu
ol
Cyanide Fishing
Dynamite Fishing
Poor Land Management
Average erosion rates in
some tropical countries:
30-40 tons/ha/yr
vs. 0.004-0.05 tons/ha/yr
Large scale effects
of sedimentation
Poor waste disposal
Loss of Herbivores
Coral Reef Bleaching
• Episodic coral mortality
• Expulsion of zooxanthellae
• Exact cause not known
– Correlated with increased
sea-surface temperatures
– Excess phosphorus from
runoff and sewage treatment
can inhibit calcification and
lead to bleaching
Coral Bleaching
Coral bleaching is on the rise
Black arrows designate El Nino years
Sea surface temperature (SST) from space
1 °C above “normal” summer maximum
Major Hypotheses to Explain Coral Loss
• Nutrient enrichment
• Loss of herbivores
• Climate change (disease)
Marine Reserves: Are they effective
in restoring consumer density?
• In 76 studies of reserves around the world, densities of fish
increased in 69% of the reserves, average body size
increased in 88% of reserves, and biomass increased in
92% of reserves (Halpern, in press).
• In Looe Key, Florida snapper density nearly doubled and
that of grunts more than quadrupled after fishing was
closed for only two years (Clark et al., 1989).
• Increased size and abundance within these reserves may
lead to a spillover effect, potentially increasing fish
abundance and fishery yield in nearby waters reserve
boundaries (Russ and Alcala, 1999; Crowder et al., 2000).