Competition workshop

Question 1

Where did it take place

Answer 1

Moorea – French Polynesia

Question 2

Who are the predators and prey?

Answer 2

Prey- damselfishes (Dacyllus)
Big Predators- Honeycomb Grouper, Jacks
Small Predators- Sandperch, Morray eels, Squirrelfish

Question 3

Methods

Answer 3

• Predator exclusion experiments
• Observations
• Observations with infrared video

Question 4

what were the mains questions posed by this work?

Answer 4

• 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 ?

Question 5

What are the four main findings of the predator exclusion experiment?

Answer 5

• 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

Question 6

What are two main findings of the size-selective predator exclusion?

Answer 6

• Losses also density dependent – regardless of corals exposed to all or just small bodied predators
• Suggests small-bodied predators responsible for most losses

Question 7

What do the authors determine in terms of the relative timing and identity of the attackers at the large scale?

Answer 7

• 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

Question 8

What is critical about the behaviour of damselfishes, and why is this relevant to both the larger and more fine scale timing of attacks?

Answer 8

• 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)

Question 9

What do the following figures show

Answer 9

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

Question 10

Where are the fish that are attacked?

Answer 10

• 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

Question 11

How do fish positions vary with density?

Answer 11

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

Question 12

How do aggression and density work together

Answer 12

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

Question 13

In three sentences, what are the main take-away messages from this study.

Answer 13

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

Question 14

what is Interference competition

Answer 14

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)

Question 15

Were are the authors able to successfully answer their main overall questions, and if so, how?

Answer 15

• 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