Larval dispersal in reef fishes: biology, ecology, economics Michael - - PowerPoint PPT Presentation

Michael Bode ARC Centre of Excellence for Environmental Decisions, University of Melbourne mbode@unimelb.edu.au

Larval dispersal in reef fishes: biology, ecology, economics

Early research into tropical fish communities

i

Population dynamics on a single patch

Nonlinear dynamics in the absence of dispersal:

Population dynamics on a single patch

Recruitment limitation:

Doherty (1991) Ecology of Fishes on Coral Reefs

Population dynamics on multiple patches

Dispersal is essentially a linear coupling of a multidimensional nonlinear system

• Dispersal is defined by connectivity matrix C
• Matrix elements are the proportion of

larvae from reef i that travel to reef j

1 2 3

Dispersal is essentially a linear coupling of a multidimensional nonlinear system

• Dispersal is defined by connectivity matrix C
• Matrix elements are the proportion of

larvae from reef i that travel to reef j

1 2 3

• P. maculatus. Harrison et al. (2012) Current Biology

Measuring dispersal

• Resource intensive
• Invasive – individual & species level
• Scale limited

– (temporal and spatial)

Part 1: Modelling dispersal

Modelling dispersal

14 16 18 (d) 14 16 18

Source reef Destination reef

Connectivity patterns

Inter-annual variation

Inter-specific variation

Part 2: Dispersal and coexistence

Coexistence needs differences

Reef fish community theory

Metacommunity simulation

• Real distribution of reefs (P = 110)
• Variable dispersal matrices (t = 1, …, 32 years)
• Multiple species (S = 5)

– Identical competitors – Identical niches – Different dispersal behaviour Normally we would expect monodominance

(%)

• Same model
• Two species, identical at a local scale
• Larval dispersal stages of slightly different lengths.
• Three identical patches

Dispersal differences and coexistence

Dispersal differences and coexistence

Dispersal differences and coexistence

Coexistence is possible if each species is a superior disperser over one of the inter-patch distances

Coexistence is possible if each species is a superior disperser over one of the inter-patch distances Mechanism has high predictive power for larger simulations

Dispersal differences support coexistence that:

• Is simple and intuitive
• Driven by common factors
• Can create quite complex patterns
• Creates stable geographic replacement

Mechanisms ¡ are ¡not ¡locally ¡

• bservable. ¡

Part 3: Economic perspectives on dispersal

Measuring dispersal on Manus Island

Plectropomus areolatus. Source: FAO

Timonai Mbunai Pere Tawi Locha

Bioeconomic scales on Manus

Bioeconomic scales on Manus

Community tenure areas Spawning aggregation source areas

Management question

• What is the maximum annual equilibrium

harvest rate from each spawning aggregation?

• How do dispersal externalities affect the

harvesting decision?

Harvested population model

Plectropomus areolatus. Source: FAO

Harvested population model

Harvested population model

Simulation model Population estimates

Independent communities

• Each community chooses:

– a harvest rate on each of their aggregations, – that maximises total equilibrium harvest. – given that other communities act rationally.

Harvests under different coalitions

• Communities are highly heterogeneous
• Describe coalition structures using

partitions

e.g., C0 = {{1} {2} {3} {4} {5}} C1 = {{1} {2},{3} {4,5}} C2 = {{1,2,3} {4,5}} CG = {1,2,3,4,5} – 52 unique coalition structures

Harvest coalition size

• Non-cooperative groups remove 12-25% / FSA /

yr

• Cooperative harvests remove 10-17%
• Grand coalition improve overall catch (by 15%)

and equilibrium population levels (by 70%)

Harvest coalition size

• The current scale of management on Manus

could lead to undesirable outcomes.

• A grand coalition would result in an increase in

catches in every community, for much lower effort

Grand coalition stability

Group 2 leaves coalition

Grand coalition stability

• The grand coalition surplus is insufficient for

a set of side-payments to yield rational cooperation.

Smaller coalitions

• The coalition between Locha and Pere is the only

Nash equilibrium (internal & external stability).

• Almost all the resultant benefits are captured by

the adjacent communities: Tawi and Mbunai

Timonai Mbunai Pere Tawi Locha

Economic impacts of larval dispersal

• The scale of larval dispersal creates

interconnections between communities

• The dissonant scales causes problems.

– Too much dispersal to ignore each other – Too much dispersal to want to cooperate – Not enough dispersal to provide necessary surplus

Collaborators

• Maurice James
• Paul Armsworth
• Glenn Almany
• Lance Bode
• Rick Hamilton
• Luciano Mason
• Geoff Jones
• David Williamson