The role of habitat structure in a freshwater food web

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The Role of Habitat Structure in a Freshwater

Food Web

By

Danielle M.Warfe, B.Sc.(Hons)

A thesis submitted in fulfilment of the requirements for the degree of

Doctor of Philosophy

University of Tasmania

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Dec lcaration

This lbesis contains no material which has been accepted for a degree or diploma by

the U1iversity of Tasmania or any other institution, except by way of background

information which is duly acknowledged in the text. To the best of my knowledge,

this thesis contains no material previously published or written by another person,

exce]Jt where due acknowledgement is made in the text.

Authority of Access

The thesis copy held in the University library may be made available for loan and

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Abstract

Habitat structure refers to the nature ofthe physical structure that provides an

environment for biotic communities. Much of the research in marine and freshwater

systems notes the importance of habitat in community organisation (for example, fish

predators are commonly less effective as habitat structure increases), but few studies

have specifically described the mechanisms by which it influences trophic interactions

and thereby community structure. My research investigated the role of macrophyte

structure in trophic interactions and community structure in the macrophyte beds of a

lowland river.

One of the problems in assessing the role ofhabitat structure is the confusion over the

definition, and therefore the measurement, of habitat structure, particularly in a way

that allows comparison between different habitats and systems. I defined habitat

structure as a combination of the qualitative and quantitative components of structure,

so where macrophytes provide the habitat, this refers to their shape and density. While

macrophyte density is relatively straightforward to quantify, macrophyte shape is

more problematic which has lead to a variety of system-specific measures. I tested

nine different indices of habitat complexity to determine which would best describe

plant shape and best relate to the macroinvertebrate distribution on different

macrophytes. I found a high degree of intercorrelation and redundancy between the

structural indices such that they could be organised into two suites: one describing the

interstitial space and the surface rugosity at coarse scales, the other describing the

"whole plant" attributes of surface area and plant volume and the surface rugosity at

fine scales. In particular, there were two indices which fell into both suites, an index

of refuge space from predation, and the surface rugosity at 5 x magnification. Both

these indices were also the most highly related to macroinvertebrate abundance and

taxon richness, so I suggest they should be incorporated in the development of a

broadly applicable index of macrophyte shape.

As macroinvertebrates responded to the refuge role of macrophytes, I tested if

differences in both macrophyte density and macrophyte shape had any effect on the

prey-capture success of two predators, the southern pygmy perch and a predatory

damselfly. I used two predators to address the impacts of multiple predators; ifhabitat

structure can mediate the outcomes of predator-prey interactions, then it may also

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affect the outcomes of predator-predator interactions. I tested predator success in three

macrophyte shapes at each of five macrophyte densities in a tank experiment.

Surprisingly, there was no effect of plant density, but plant shape was important as

fewer prey were captured, by each predator in isolation and by both predators

combined, in the most structurally complex plant. This indicated that a more

structurally complex plant can negatively affect the prey-capture success of predators,

and also that macrophyte shape can mediate the outcomes of predator interactions.

The implications of this laboratory experiment prompted a field experiment to

determine ifthe influence ofmacrophyte shape on fish predator success translated to

field conditions and affected the macroinvertebrate and periphyton communities in

macrophyte beds. I conducted a two-factorial, repeated measures, randomised

complete block experiment using floating cages in existing macrophyte beds. I tested

the factors ofmacrophyte shape (three types) and the presence or absence of fish

predators using the native southern pygmy perch. I ran the experiment for eight

months, sampling the macroinvertebrate and periphyton communities at 2, 6, 1 0, 26

and 30 weeks. Macrophyte shape had strong, consistent effects on both the

macroinvertebrate and periphyton communities; both were most abundant on the most

structurally complex plant. In contrast, pygmy perch affected only a subset of the

macroinvertebrate community and had minor indirect effects on the periphyton

composition. Contrary to expectations though, pygmy perch had their strongest effects

on vulnerable invertebrate herbivores in the most structurally complex plant.

I concluded that in this system, macrophyte shape has a stronger influence than

macrophyte density on trophic interactions, and constitutes a clear regulating

influence on the macroinvertebrate and periphyton communities such that it precludes

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Acknowledgments

I would like to thank my supervisor Dr. Leon Barmuta for his guidance and advice

throughout this project; his support, patience and assistance with drawing out the best

features ofthis research were greatly appreciated.

I would also like to thank various people for encouraging and thought-provoking

discussions of this research, namely Dr. Peter Davies, Dr. Simon Wotherspoon, Dr.

Thomas Martin, Dr. Barbara Downes, Dr. William Elvey, Reg Majierowski, Jeff

Meggs, John Gooderham, Kath J erie, Paul Reich and the members of the Australian

Society of Limnology (Tasmania).

I am indebted to Henry and Simon Foster for granting access to their property, and

particularly for their support and tolerance in having my cages in the river for a year

and a half.

With regards to the construction of my artificial plants and cages, and the fieldwork

they required, many people kindly donated their time and help at various stages for

which I am extremely grateful: Ash Warfe, Tom Halcombe, Will Elvey, Adam

Uytendaal, Neil Meadows, Stu Meadows (and the rest of the Meadows clan), Mel

Kelly, Pippa Dickson, Jody Bruce, Carl Grosser, Rob Musk, Michelle Gabriel, John

Hanagan, Paula Sheehan, Lisa Meyer, Richard Holmes and Tom Sloane. I thank Brett

Mawbey for generously donating his time for collecting pygmy perch, likewise Simon

Talbot and Hugh Pederson for their underwater photography. I'd further like to thank

Tom Sloane for his help with sample processing.

I am grateful to the technical staff of the School of Zoology for their assistance with

various dilemmas; in particular, Richard Holmes provided invaluable assistance in the

workshop.

I'd like to thank various friends and flatmates over the years for their understanding

and comic relief- too many to name here but you know who you are.

Finally, but by no means least, I cannot thank my family enough for their unremitting

emotional support and encouragement throughout the years I have been doing this.

Thank-you.

I'm just sorry you missed this dad.

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v

1 1 2 3 5 Habitat structure and food-web community dynamics 8

Research strategy and expectations 9

CHAPTER 2: THE QUANTIFICATION OF HABITAT STRUCTURE. 11

Introduction 11

Mcibo& 15

The macrophytes 15

Indices of structural complexity 16

Data Analysis 19

Results 20

Discussion 22

Indices of stmctural complexity 22

Macroinvertebrate distribution and indices of structural complexity 24

Reliability of structural indices 28

Conclusions 29

CHAPTER 3: THE EFFECTS OF HABITAT STRUCTURE ON MULTIPLE

PREDATORS 38

Introduction 38

Mciho& 40

The predators 40

The macrophytes 41

Data analysis 42

Results 43

Behavioural observations 43

Statistical analyses 44

Discussion 44

Macrophyte density 45

Macrophyte shape 46

Multiple Predator Effects 48

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CHAPTER 4: THE EFFECTS OF HABITAT STRUCTURE AND FISH

PREDATION ON MACROINVERTEBRATE COMMUNITY STRUCTURE. 58

Introduction 58

Methods 59

Study site 59

Experimental design 60

Data analyses 65

Results 67

Multivariate analyses 67

Univariate analyses 68

Discussion 71

Habitat Structure 71

Fish predation 73

Fish predation and habitat structure 75

Temporal and seasonal variation 78

Conclusions 79

CHAPTER 5: THE EFFECTS OF HABITAT STRUCTURE AND FISH

PREDATION ON PERIPHYTON COMMUNITY STRUCTURE 90

Introduction 90

Mclho~ 91

Data Analysis 92

Results 94

Periphyton composition 94

Periphyton biomass 96

Discussion 97

Habitat structure 97

Predator effects 99

Temporal and seasonal effects 101

Conclusions 102

CHAPTER 6: GENERAL DISCUSSION 110

Summary of findings 110

Synthesis of findings 112

Effect measurement 112

Prey behaviour 113

Predator behaviour 115

Implications of findings 116

Conclusions 118

REFERENCES 119

APPENDIX A 131

Gut contents oflschnura heterosticta tasmanica from the Macquarie River,

Tasmania. 131

APPENDIX 8: 133

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Identification keys used for macroinvertebrates and periphyton

APPENDIX C:

Macroinvertebrate taxon list

APPENDIX D:

Vulnerability scores for macroinvertebrate taxa sampled during the field experiment.

133

135

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The role of habitat structure in a freshwater food web