Where did it take place
Moorea – French Polynesia
Who are the predators and prey?
Prey- damselfishes (Dacyllus)
Big Predators- Honeycomb Grouper, Jacks
Small Predators- Sandperch, Morray eels, Squirrelfish
Methods
- Predator exclusion experiments
- Observations
- Observations with infrared video
what were the mains questions posed by this work?
- What are the predator types in this system?
- Is there a period during diel cycles when losses are more important?
- What mechanisms underly this density dependence in per-capita mortality rate?
- Is this mortality after larvae settle from plankton due to predation?
Are predators responsible for the previously observed density dependent loss of young Dascyllus ?
What are the four main findings of the predator exclusion experiment?
- On corals excluding predators, no increased loss
- On corals exposed to predators, proportion lost increases
with density - Density-dependent pattern of mortality related to predation
- Losses of juveniles not due to movement
What are two main findings of the size-selective predator exclusion?
- Losses also density dependent – regardless of corals exposed to all or just small bodied predators
- Suggests small-bodied predators responsible for most losses
What do the authors determine in terms of the relative timing and identity of the attackers at the large scale?
- Diel pattern of density-dependent predator induced mortality
- Explored whether strength of density-dependence
differed between light and dark periods - Attacks mostly as dark period began – while fish sheltered in coral/anemone
- Groupers/squirrelfish dominate predation events
What is critical about the behaviour of damselfishes, and why is this relevant to both the larger and more fine scale timing of attacks?
- First and most attacks occur during night times by nocturnally active predators as fish begin sheltering
- Substantial drop in predation 1–1.5 hour post-sheltering
- Fish harass better protected fish to displace
them and get better protected positions - Chases alert predators whose sight detects movements
- Chased fish more likely to be attacked
- More chases/predator alerts in more densely packed corals (density-dependent)
What do the following figures show
In both species, higher proportions lost at night during sheltering
* In both cases, losses increase with increasing fish density
especially at night
* Density has less of an effect on damselfish lost during day
* Night predators created the strongest density dependence whereas daytime predators created a almost no density dependence
Where are the fish that are attacked?
- Most attacks occur on the edges and in the water column
- Depending on habitat, different micro-areas more
susceptible to predation than others - Anemones seem safer than corals – but fish also show intraspecific aggression which increases with density
How do fish positions vary with density?
As density increased, the proportion of aggressive acts/5min also increased
* Leads to increases in proportion of individuals in more vulnerable micro-habitat with increasing density
How do aggression and density work together
Position after chase is better or the same: Chaser: 90% of events
Chasee: 18% of events
* Density-dependent aggression interactions occur while damselfish are sheltered
* This leads to density-dependent vulnerability to crepuscular and nocturnal attacks by predators in the area
In three sentences, what are the main take-away messages from this study.
One of few studies precisely showing how density results in variation in per capita mortality rates, regulated by predation-based mortality by small local reef predators
* Supports hypothesis that density-dependent competition can be mechanistically linked to other factors such as aggression and predation to structure reef fish populations
* Interference competition for predator-free space was the main cause of density- dependent mortality
what is Interference competition
Competition in which individuals behave in a way that reduces exploitative efficiency of another competitor (e.g. damselfishes competing for enemy-free space though aggression)
Were are the authors able to successfully answer their main overall questions, and if so, how?
- Increases density of damselfish = increasing proportion of individuals in vulnerable (edge, water column) areas
- Density-dependent aggression displaced individuals to risky locationsmaking them vulnerable to predation by smaller-bodied predators
- Thus, authors able to address all of their main objectives successfully
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Ichthyology21
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sampling methods4
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Aquatic habitats32
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oxygen and metabolism23
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Energetics18
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morphology of locomotion24
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Feeding morphology34
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Fish Genetics30
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Geometric morphometrics19
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Individuals and life history traits24
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Populations and Production26
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Conservation28
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Competition workshop15
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Shape analysis4
