Herbivory and sea urchin predation for ecosystem resilience

By Charlotte Johansson, David Bellwood and Martial Depczynski

Introduction

Coral reefs under pressure

Human impacts

Increasing disturbance to reefs

Loss of vital functions (ecological roles)

Gradual erosion of resilience

Introduction

Resilience

“the capacity of a system to absorb disturbance and reorganize while undergoing change so as to still retain essentially the

same function, structure, identity and feedbacks”

(Walker et al 2004)

To understand how ecosystem processes will be affected by disturbances we need to look at the species that contribute to them and what the strength of their combined functional role is

Introduction

Functions are; Processes (feeding) – movement or

storage of materials or energy (Bellwood et al 2004)

Functional groups; A collection of species that perform or

influence the same function, irrespective of their taxonomic affinities (e.g. herbivores, nitrogen fixers etc) (Bellwood et al 2004)

Introduction

4 roving herbivorous groups

Macroalgae

Browsers Grazers Scrapers Excavators

Herbivores

Turfalgae

IntroductionFine scale approach when evaluating resilience:

1. Alternative organism (coral/algae)

2. Herbivorous fish

Functional redundancy (same functional role)

Functional diversity (different functional role)

Responses

3. Non-fish herbivores (sea urchins)

Aim

1. Quantify the distribution of: Macroalgae Herbivorous fish Non-fish herbivores (sea urchins)

2. Evaluate the extent to which herbivorous fish and urchins are likely to control macroalgae

Study location

Ningaloo Reef, Western Australia

1. Alternative organisms

Slope Back Lagoon

-0.2

0.0

0.2

0.4

0.6Coral

Pro

port

ion

± S

E

Slope Back Lagoon

-0.2

0.0

0.2

0.4

0.6Macroalgae

2. Herbivorous fish

Density of herbivores

Slope Back Lagoon0

2

4

6

8

10 BrowsersExcavatorsGrazersScrapers

Den

sity

(10

0m-2

) ±

SE

2. Herbivorous fishA

bund

ance

(%

)

0.0

0.5

1.0 SR = 16FD = 5

SR = 13FD = 5

SR = 8FD = 4

Grazers

C. sordidus C. microrhinos0.0

0.5

1.0SR = 2FD = 1

C. sordidus C. microrhinos

SR = 2FD = 1

C. sordidus C. microrhinos

SR = 2FD = 1

Excavators

LagoonBackSlope

3. Non-fish herbivores

Slope Back Lagoon0

4

8

12

Bioerosion C. microrhinosC. sordidusE. mathaei

Ero

ded

mat

eria

l (kg

.m-2

.yr-

1: ±

SE

)

Slope Back Lagoon0

100

200

300

Abundance

Den

sity

(10

0m2)

± S

E

3. Non-fish herbivores

Important role on Ningaloo Reef

What is driving the pattern of urchins, especially on the slope

Is it a lack of predators?

Urchins = reef degradation

3. Non-fish herbivores

Coris

aygu

la

Choer

odon

rube

scen

s

Leth

rinus

atki

nson

i

Diodo

n hy

strix

Cheilin

us tr

iloba

tus

Aroth

ron

stella

tus

Pseud

obal

istes

fusc

us0

2

4

6

8

10

Suc

cess

ful

3. Non-fish herbivores

Slope Back Lagoon0

0.1

0.2

0.3 BalistidaeLabridaeLethrinidaeTetradontidae

Den

sity

100

m-2

± S

E

0.640 0.537

0.423

Conclusion Macroalgae abundant in lagoon (despite herbivores)

Urchins abundant on slope (despite predators - except specialist balistids)

Urchins and algae often signs of degradation - here they appear to not be

Ningaloo Reef, although relatively intact, appears to be functionally distinct to other reef systems

Acknowledgements

Prof D. Bellwood, Dr. M. Depczynski, Dr. A. Hoey and I. van de Leemput

JCU, ARC Centre of Excellence

AIMS, AIMS@JCU, DEC

Colleagues in the lab and field