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DISSERTATION

Presented in Partial Fulfillment of the Requirements for

the Degree Doctor of Philosophy in the Graduate

School of the Ohio State University

By

Earl William Campbell III, B.A., M.S.

*****

The Ohio State University 1996

Dissertation Committee: Approved by

Jonathon B. Bart, Adviser

Thomas C. Grubb Adviser Thomas E. Hetherington Department of Zoology

William M. Masters UMI Number: 9630865

Copyright 1996 by Campbell, Earl William All rights reserved.

UMI Microform 9630865 Copyright 1996, by UMI Company. All rights reserved.

This microform edition is protected against unauthorized copying under Title 17, United States Code.

UMI 300 North Zeeb Road Ann Arbor, MI 48103 ABSTRACT

The effect of Brown Tree Snake (Boiga irregularis) predation on the

lizards of Guam was quantified by contrasting lizard abundance, morphology,

size, and behavior in two 1-ha plots of secondary forest from which snakes had

been removed and excluded, versus two 1-ha control plots in which snakes

were monitored but not removed or excluded. Resident snakes were removed

with snake traps, and immigration was precluded by electrified snake barriers

surrounding the treatment plots. I sampled lizards in all plots quarterly for a year following snake elimination in the treatment plots. Although the density of lizards in all plots was very high (mean of 13,210 lizards/ha in unmanipulated plots), lizard numbers significantly increased in the treatment plots (to approx. 19,650 lizards/ha). Results of systematic lizard population monitoring before and after snake removal suggest that the abundance of an introduced skink, Carlia fusca, increased significantly and the abundance of two species of gekkonids, Lepidodactylus lugubris (native) and Hemidactylus frenatus (introduced) increased substantially on snake free plots. No changes were observed in the abundance of Emoia caeruleocauda, a native skink.

Additionally, the mean snout vent length of all lizard species increased in the treatment plots, but not in the control plots following snake removal. No behavioral changes were observed in lizard species. A second goal of this experiment was to determine the practicality of snake exclosures for endangered species repatriation and snake quarantine facilities for cargo and vehicles leaving island. These results suggest that large scale snake exclosures will be feasible after refinements are made to the barrier to reduce rat and typhoon damage. Dedicated to my family and Meg

iv ACKNOWLEDGMENTS

I would like to thank Thomas Fritts and Gordon Rodda of the

Department of the Interior's National Biological Service (NBS) for their guidance and support throughout my dissertation. Each of these individuals had a strong personal commitment to the completion of a project which was a logistical, mental, physical, and emotional challenge. Without their efforts, as well as the efforts of those who directly aided in the construction of the snake exclosures and data collection, this project would not have been completed. I firmly believe that the vision and commitment that Thomas Fritts has had guiding the scope of brown tree snake research within the NBS sets a standard which others trying to deal with ecological catastrophes should follow. Tom's staunch commitment to the importance of the scientific method to gain insight into a politically sensitive topic where many would prefer a quick fix of questionable practicality is an example of his strengths. Gordon Rodda is one of the most insightful researchers I have ever encountered. Much of the strength of the data I report today is due to months of planning with Gordon prior to barrier construction, in addition to daily advice at a distance. Gordon is clearly one of today's eminent herpetologists whom people will refer to tomorrow. I am thankful that I had a chance to work with both Tom and Gordon.

I am indebted to the aid and support of my advisor, Jonathan Bart at the

Ohio Cooperative Fish and Wildlife Research Unit (OCFWRU). Jon provided me with a situation where I could follow my dreams and visions. Like many prospective graduate students, I interviewed at several programs. At one institution I interviewed with one of the "fathers" of a modern field in conservation science who told me that there was no future or reason to conduct research on introduced species problems on oceanic islands. I didn't take that advice and luckily was accepted at the Ohio State University (OSU). Jon's strengths in sampling design and biostatistics have aided in the clarification of trends in this data. His support and advice throughout this research has been of great value and I thank him for it.

I wish to thank Jon Bart, Tom Fritts, and Gordon Rodda for reviewing many documents I have written over the last seven years. This has been a thankless task but has improved my writing abilities. Tom Hetherington should be recognized for his advice and thoughts during my tenure at OSU. He has aided with many details of this work. I would like to thank my dissertation committee members; Tom Fritts, Tom Grubb, Jr., Tom Hetherington, and Mitch

Masters, for their advice and comments on the planning of research and reviewing this document. Craig Clark was the primary individual who aided me with my field research on a daily basis during my tenure on Guam. Craig's technical skills and high standards for fieldwork were a major factor in the success of this project. Craig not only worked with me on a daily basis but was my primary housemate while living on Guam. Muchos gracias.

Heidi Hirsh, Environmental Officer, at the Andersen Air Force Base,

Guam (AAFB) acted as a liaison for this research on Base. Heidi's coordination made the logistics of this research much easier. Her efforts made this work much easier. The Department of Defense should be thanked for allowing much ofthis research to be conducted on their property.

I would like to thank Craig Clark, Rick Blakely, Tom Fritts, and Gordon

Rodda for assistance with initial barrier testing. Logistical assistance for this aspect of my research was provided by Mike McCoid and the staff of the

Division of Aquatic and Wildlife Resources (GDAWR) and Heidi Hirsh (AAFB).

The construction of the snake barrier was done with the aid of several individuals. I would like to thank Craig Clark, Jay Graham, Matt Reid, Kerensa

King, Gordon Rodda, Brendan McCarthy, and volunteers from AAFB for aid in construction of the barrier. Staff of the 633rd Civil Engineering Squadron and

Explosive Ordnance Disposal Unit at AAFB aided in the clearing of plot perimeters.

I would like to thank the following individuals who aided in the monitoring of snake and lizard populations on the snake free and snake present plots and maintenance of the snake barrier: Craig Clark, Jay Graham, Thomas

Sharp, Grant Beauprez, Todd Mabee, Stan Kot, Greg Campbell, Marie

Timmerman, Jo Ann White, Dave Worthington, Tom Fritts, Gordon Rodda, Pat

Fritts, Ren6e Rondeau, and Kerensa King.

Kelly Wollcott, John Sanchez, Leslie Morton and Rosemarie Concepcion of the United States Fish and Wildlife Service's Guam National Wildlife Refuge

(USFWS-GNWR) should be thanked for their support during this research. The refuge provided office, laboratory, and storage space which was used during latter phases of this research.

During my tenure on Guam logistical assistance in Ohio was provided by Diane Rano, Roxanne Shull and Jonathan Bart at the OCFWRU. Linda Wolfe and Michelle Hampton of the NBS, Washington D.C. provided logistical assistance through Tom Fritts's office.

On Guam, I wish to thank friends who provided support in numerous ways to this project. Tim Sherwood (GDAWR) provided me with free housing for several months. Mike and Rebecca McCoid (GDAWR) were extremely supportive of my efforts during the first year of this work and are missed on

Guam. Gary Wiles, Bob Beck, Bob Anderson, Tino Aguon, Kelly Brock, Mike

Ritter, Marvin Aguilar, and Lillian Mariano of the GDAWR all provided advice and support during this project. Ernie Kosaka (USFWS-Honolulu) should be thanked for his continual support of this and other NBS brown tree snake research efforts. The following individuals in Guam and the Commonwealth of the

Mariana Islands should be thanked for their friendship and advice while I lived

on Guam: Eva Beyer, Rick Blakely, Kelly Clark, Dan Grout (USFWS-Rota),

Doug Gomez, Annie Marshall, John Morton (USFWS-Guam), Jorge Phocas

(USFWS-Guam), Kelvin and Thea Osbourne, and Dave Worthington. It would

be a gross error not to thank members of Agana Hash House Harriers for

weekly reality breaks and sanity checks (Honor and ON ON! to all).

The following individuals should be thanked for friendship and advice

while I was in Columbus: Craig Clark, Brad Coupe, Randy Dettmers, Nadeem

Ghani, Barry Muller, and Joe Robb. Carol and Dean Boccetti provided me with a housing and a sense of place during a time of need and a room when I visited

Columbus periodically (thank you). My parent's, Earl and Thurid Campbell and brother, Greg Campbell, should be thanked for encouragement and support through the years. Both Megan Laut and Greg Campbell should be thanked for making my return to Columbus an extremely enjoyable experience.

Lastly, the United States Departments of Interior and Defense provided funding for this work through the Ohio Cooperative Fish and Wildlife

Research Unit at the Ohio State University.

ix VITA

November 24,1964 ...... Born - Evereux, France

198 5...... Career Trainee Fellowship, Jersey Wildlife Preservation Trust Training Centre, Jersey, Channel Islands

198 6 ...... B.A. Zoology, Ohio Wesleyan University

1989...... M.S. Biology, Bowling Green State University

1989 -1990...... Teaching Assistant, Department of Zoology, The Ohio State University

1990 - present...... Research Assistant, Ohio Coop. Fish & Wildlife Research Unit, Department of Zoology, The Ohio State University

PUBLICATIONS

Research Publication

Tolson, P. J. and E. W. Campbell. 1989. Typhlops richardi (Richard's Blind Snake) Arboreal Activity. Herpetol. Rev. 20: 75.

Campbell, E. W. 1991. The effect of introduced roof rats on the bird diversity of Antillean Cays. J. Field Ornithol. 63(3): 343 - 348.

Campbell, E. W. and M. J. McCoid. 1993. Geographic Distribution. Pelodiscus sinesis. Herpetol. Rev. 24: 63.

McCoid, M. J., E. W. Campbell, and B. W. Alokoa. 1993. Efficacy of a chemical repellent for the brown tree snake (Boiga irregularis). The Snake. 25:115 -119.

McCoid, M. J., T. H. Fritts, and E. W. Campbell. 1994. A brown tree snake (Colubridae: Boiga irregularis) sighting in Texas. Texas J. of Sci. 46(4): 365 - 368. Campbell, E. W. in press. Barriers to movements of the brown tree snake (Boiga irregularis). In G. H. Rodda, Y. Sawai, D. Chiszar, and H. Tanaka, (eds.), Snakes, Biodiversity, and Human Health: Case Studies in the Management of Problem Snakes.

Campbell, E. W., T. IT. Fritts, G. H. Rodda, and R. L. Bruggers. in press. An integrated management plan for the brown tree snake on Pacific Islands. In G. H. Rodda, Y. Sawai, D. Chiszar, and H. Tanaka, (eds.), Snakes, Biodiversity, and Human Health: Case Studies in the Management of Problem Snakes.

Fritts, T. H., M. J. McCoid, G. H. Rodda, and E. W. Campbell, in press. The biology of the brown tree snake, Boiga irregularis, super predator or successful opportunist. In G. H. Rodda, Y. Sawai, D. Chiszar, and H. Tanaka, (eds.), Snakes, Biodiversity, and Human Health: Case Studies in the Management of Problem Snakes.

Rodda, G. H., T. H. Fritts, E. W. Campbell, in press. The Feasibility of Controlling the Brown Tree Snake in Small Plots.JnG. H. Rodda, Y. Sawai, D. Chiszar, and H. Tanaka, (eds.), Snakes, Biodiversity, and Human Health: Case Studies in the Management of Problem Snakes.

Rodda, G. H., M. J. McCoid, T. H. Fritts, and E. W. Campbell III. in press. Population Trends and Limiting Factors for Boiga irregularis. In G. H. Rodda, Y. Sawai, D. Chiszar, and H. Tanaka (eds.), Snakes, Biodiversity, and Human Health: Case Studies in the Management of Problem Snakes.

FIELDS OF STUDY

Major Field: Zoology

xi Table of Contents

Dedication...... iv

Acknowledgments ...... v

Vita...... x

List of Tables...... xiii

List of Figures...... xv

Introduction...... 1

Purpose...... 4 Organization...... 4

Chapter

I. Barriers to Dispersal of the Brown Tree Snake (Boiga irregularis)...... 6

Materials and Methods ...... 8 Results and Discussion ...... 12

II. The Effect of Brown Tree Snake Predation on the Island of Guam's Extant Lizard Assemblages...... 18

Methods...... 22 Results ...... 34 Discussion...... 67

Literature Cited 82 LIST OF TABLES

Table Page

1.1 Snake barrier designs, sample sizes and duration of exclosure experim ent...... 9

1.2 Wiring configuration of four electrical barrier prototypes...... 11

2.1 Mean snake abundance and ratios of snake abundance on snake present and snake free plots ...... 36

2.2 Evidence that snakes declined in abundance between pre­ treatment and transitional monitoring p e rio d s...... 37

2.3 Brown tree snake population estimates for 1 ha. plots during transition phase and number of snakes removed from exclosures...... 39

2.4 Carlia fusca adult abundance during pre-, transitional, and post­ treatment monitoring periods...... 43

2.5 Estimates of change in lizard population size due to snake rem oval...... 44

2.6 Lepidodactylus lugubris adult abundance during pre-, transitional, and post-treatment monitoring p e rio d s...... 46

2.7 Hemidactylus frenatus adult abundance during pre-, transitional, and post-treatment monitoring periods...... 49

2.8 Estimates of change in Hemidactylus frenatus in individual snake exclosures due to snake removal...... 50

2.9 Carlia fusca juvenile abundance during pre-, transitional, and post-treatment monitoring periods...... 53

2.10 Emoia caeruleocauda abundance during pre-, transitional, and post-treatment monitoring periods...... 55

xin 2.11 Mean perch height, perch diameter, and ratio of perch height/diameter for geckos observed before and after snake removal...... 57

2.12 Estimate of change in gecko perch height and perch diameter due to snake removal...... 60

2.13 Percentage and sample size of gecko observed with broken tails...... 64

2.14 Percentage of geckos that did not move during escape tests ...... 65

2.15 Mean number of taps needed to stimulate geckos to move during escape behavior tests ...... 66

2.16 Density of lizards captured during removal sampling ...... 68

2.17 Mean snout vent length of lizards captured during removal sampling ...... 69

xiv LIST OF FIGURES

Figure Page

1.1 Fate of snakes in first experiment...... 13

1.2 Fate of snakes in second experiment...... 14

2.1 Arrangement of 1 ha. snake exclosure plots and snake present p lo ts...... 23

2.2 Brown tree snake density estimates for snake free and snake present plots...... 29

2.3 A theoretical removal for schedule for determining how many days of snake free trapping must occur to ensure that there was a >95% chance that no snakes remained on p lo ts...... 38

2.4 Number of snakes removed from two one hectare snake exclosures between 31 August 1995 and 31 December 1993...... 40

2.5 Carlia fusca adult abundance for snake present and snake free plots...... 42

2.6 Lepidodactylus lugubris adult abundance for snake present and snake free plots...... 45

2.7 Hemidactylus frenatus adult abundance for snake present and snake free plots...... 48

2.8 Carlia fusca juvenile abundance for snake present and snake free plots...... 52

2.9 Emoia caeruleocauda abundance for snake present and snake free plots...... 54

2.10 Mean Hemidactylus frenatus perch height for snake present and snake free plots ...... 58

xv 2.11 Mean Lepidodactylus lugubris perch height for snake present and snake free plots ...... 59

2.12 Mean Hemidactylus frenatus perch diameter for snake present and snake free plots ...... 61

2.13 Mean Lepidodactylus lugubris perch diameter for snake present and snake free plots ...... 62

xvi INTRODUCTION

The Brown Tree Snake, Boiga irregularis, was accidentally introduced to the Island of Guam during the late 1940's (Savidge, 1987 and Rodda et al.,

1992a). Over the past forty years this nocturnal, rear-fanged colubrid snake with an average length of a little over a meter, has reached high population densities (Rodda et al., 1992a). The snake's irruption has coincided with the extirpation of 17 species of vertebrates, including most native forest birds and several lizard species (Engbring and Fritts, 1988 and Rodda and Fritts, 1992). In addition to its ecological impact, this species also has had an economic impact on the island through frequent power outages caused by snakes shorting out powerlines while climbing them (Fritts et al., 1987 and Fritts and Chiszar, in press) and through the decimation of local poultry production (Fritts and

McCoid, 1991). The snake also is a potential health risk to hum ans (Fritts et al.,

1990, and Fritts et al., 1994). Of particular interest is the high frequency of infants being bitten by the snake, though no bites are known to have been fatal

(Fritts et al., 1994). Lastly, there is grave concern that the snake will be transported off Guam through civilian or military transport and colonize other

Pacific Islands (Fritts, 1987; Fritts, 1988 and Fritts et al., in press).

Since the Brown Tree Snake's impact on Guam's native forest birds was documented in the early 1980's (Savidge, 1987) several researchers have investigated the biology and control of this species intensively (Rodda et al., in press - c). The ultimate goal of this research has been to reduce the impact of the snake on Guam and to control its spread to other Pacific Islands (Fritts, 1988 and Campbell et al., in press).

During the past ten years, staff of the Department of Interior's National

Biological Service have conducted research to develop traps which would efficiently capture Brown Tree Snakes (Rodda and Fritts, 1992; Rodda et al.,

1992b; and Rodda et al., in press - b). For the past three years, these traps have been used on a limited geographic scale to reduce snake abundance near commercial and military ports and airports on Guam. An understanding of the interaction between trap effectiveness and Brown Tree Snake biology is clearly needed in order to use trapping as a control technique to eliminate snakes from critical sites on Guam.

Brown Tree Snakes have high nightly movement rates (Rodda et al., in press - a). In 1991, Gordon Rodda attempted to eliminate snakes from a 1.5 ha plot on the Orote Peninsula, Guam, and observed no appreciable decline in snake captures over a 15 day period. This was due to the movement of new snakes into the trapping grid, replacing snakes that had been removed by traps.

Using the average trap effectiveness obtained from this removal experiment,

Gordon Rodda calculated it would take 228 days to reach a theoretical criterion of 0.1 snakes remaining in a 1 ha plot. Reproductive recruitment and

2 immigration during this time span could further prolong snake removal. This high movement rate of snakes on Guam is further supported by the results of mark-recapture population estimates. A five to seven percent turnover in the snake populations per night in 1 ha trapping grids has consistently been found over a time span of several years (Rodda et al., in press - b).

The high movement rate of snakes on Guam makes the development of an effective snake barrier crucial to the creation of snake-free areas. An effective snake barrier, in association with an active trapping program, could be used to create exclosures and snake-free plots. If practical, these exclosures could be used on Guam to create snake-free wildlife habitat and sanitized zones in the vicinity of ports and airports (Rodda et al., in press - a).

Researchers in Okinawa and the Ryuku Archipelago, Japan, working with a venomous pest snake, the Habu (Trimeresurus flavoviridis), have had little success eradicating snakes from any areas (Hayashi et al., 1983,1984, in press; Tanaka et al., 1987; Shiroma and Akamine, in press); including small islands (Katsuren et al., in press). In all cases, sites were surrounded by barriers which were supposed to reduce or eliminate snake dispersal into a removal site, and snake removal was attempted with trapping.

Differences in Brown Tree Snake and Habu biology and trap success suggest that the creation of snake exclosures may be more practical for Brown

Tree Snake control than has been shown for the Habu. Brown Tree Snakes move more often and greater distances than the Habu, a sedentary viper

3 (Tanaka et al., in press and Rodda et al., in press - c). This higher movement increases the chance that a snake will encounter a trap within a snake exclosure and be removed from a control site. Brown Tree Snakes are also more likely to disperse out of a snake exclosure and not return, if the barrier surrounding the exclosure is designed to encourage one-way movement of snakes over the barrier. Additionally, the capture success of Brown Tree Snake traps are 10 -100 fold higher than Habu traps (Rodda et al., in press - b).

Purpose

This study had three primary objectives: 1. to design a barrier to Brown

Tree Snake dispeisal, and determine the feasibility of controlling Boiga movement with barriers; 2. to create two one-hectare Brown Tree Snake exclosures and determine the long-term practicality of large-scale Boiga exclosures; and 3. to determine the predatory effect of Brown Tree Snakes on

Guam's extant lizard assemblages through a predator removal experiment using the two one-hectare Brown Tree Snake exclosures. I report the results of the first and third objectives of this study in this dissertation, and will address the second objective of this study in future publications.

Organization

This dissertation is divided into 2 major sections: I) a description of the design and testing of a barrier to Brown Tree Snake dispersal and II) an

4 investigation of the predatory impact of the Brown Tree Snake on Guam's extant lizard fauna. Chapter I addresses the applied issue of snake barrier design and is a manuscript already accepted for publication (Campbell, in press). Chapter II is a basic research question which was answered using the applied techniques of snake control I and others developed.

5 CHAPTER I

Barriers to Dispersal of the Brown Tree Snake (Boiga irregularis)

While it is presently not practical to eliminate established populations of the Brown Tree Snake, Boiga irregularis, it may be possible to control this species on a smaller scale. Snake densities in an area can be lowered if resident snakes are removed and dispersal into the site is eliminated (Rodda et al., in press - a). One method to reduce movement into an area is to build a barrier.

In the case of the more sedentary Habu (Trimeresurus flavoviridis), barriers have been constructed to maintain snake reduced or snake-free zones (Shiroma and Akamine, this volume). Barriers can also be used to help prevent movements of snake into cargo or vehicles that might spread them to new islands. Given the high movement rate of the Brown Tree Snake (Rodda et al., in press - a), an effective barrier to dispersal may be particularly beneficial.

The design of any practical barrier to deter snake disperal must incorporate features which recognize the unique morphological and behavioral adaptations of these legless vertebrates. Because snakes have flexible long and slender bodies, they are able to climb certain surfaces and enter spaces unavaliable to most quadrapeds of similar mass. Snakes can displace their body weight to more than four points (in the case of a quadraped) during movement and thus are more likely to avoid direct contact with point source stimuli.

6 Electrical fencing is commonly used to control the movements of domestic and wild animals (McKillop and Sibley, 1988). However, the use of such barriers to control snake dispersal has been limited to efforts of Japanese herpetologists to control the movements of Habu (Hayashi et al., 1983,1984).

These barriers are presently being used to stop Habu from entering villages on

Tokunoshima and Amamioshima in the Kagoshima Prefecture of Japan (Tanaka et al., 1985,1987). Electrical barriers are also being used to prevent movement of Habu into electrical substations in the Okinawa Prefecture of Japan (Y.

Miyagi, pers. comm.) and on selected power poles on Guam. Stimulated by the

Japanese successes, the staff of Guam's Division of Aquatic and Wildlife

Resources is placing electrical barriers on trunks of trees used by nesting pairs of endangered Mariana Crows (Corvus kubaryi; Aguon et al., in press) to reduce the risk of Brown Tree Snake predation on eggs and chicks.

Non-electrified barriers are also used to control snake movements.

Nylon mesh, metal mesh, and concrete block fences have been designed to reduce Habu dispersal, with varying degrees of success (Shiroma, 1981;

Nishimura, 1983,1984b, 1984c). The most cost effective design is a 70 cm high

7.0 mm square mesh nylon fence placed at a 60° angle to the ground and secured to the substrate to prevent Habu from crawling underneath it

(Nishimura, 1984c). Traps have also been designed to be placed against the net fence near openings (e.g. roads) to reduce the chance that snakes moving along the fence will enter a protected area (Nishimura, 1984a). The Okinawa net fence

7 is presently being used in association with Habu traps in an experiment to reduce Habu densities in Kitanakagusuku Village, Okinawa (Shiroma and

Akamine, 1992). This paper describes the testing of electrical barriers which may ultimately be used to exclude the Brown Tree Snake from endangered species habitats, power stations, and other areas.

Methods and Materials

I tested the efficacies of various snake barrier designs in two experiments

(Table 1.1). Results of the first experiment were used to refine the design of the fences in the second experiment. For both, I put individually marked snakes in

3.5 m2 enclosures for several days to quantify escape and mortality rates (Table

1.1). I checked the fate of all snakes in every enclosure each morning. I believe that both escape and mortality rates of snakes on fences in the wild will be much lower than in test enclosures. In contrast to experimental enclosures, a snake repelled from a fence in nature is not as likely to encounter another fence.

In the first experiment, two fencing materials were tested with four different wiring configurations (Table 1.1). In the second experiment one fencing material was substituted, and one wiring configuration was abandoned.

During the second experiment I also recorded whether snakes died on the fence or on the ground. Further, tests were run with a larger snake sample size for a shorter period of time (Table 1.1). In addition to experiments 1 and 2 ,1 watched

8 Exp.l Exp. 2

Number of designs tested 8 5

fencing materials tested Tensar ®, polyethylene nylon net, polyethylene

wiring configurations 4 3

Snakes tested/design 10 >50

Duration in enclosure (d) 7 3

Table 1.1. Snake barrier designs, sample sizes and duration of enclosure experiments. 10 snakes individually at night in each of the enclosures to see the behaviors involved in moving over, under, or through the fences.

I used a Speedrite HB12 electrical fence energizer (3700v @ 500 ohms, 1.2

J maximum output) to charge 16 ga. aluminum wires. Enclosures were constructed out of three different fencing materials: 1) Tensar® Polygrid tm WB

(The Tensar Corporation, Morrow, Ga.), a UV treated high density polyethylene fencing material with 24.5 X 5.5 mm oval holes; 2) UV treated high density polyethylene netting (Memphis Net and Twine Co., Inc.,

Memphis, Tenn.) with 6.5 mm X 6.0 mm parallelogram holes, and 3) hexagonal knotless nylon netting with 7.5 mm holes (Memphis Net and Twine

Co., Inc., Memphis, Tenn.).

The fences were approximately 110 cm tall, with a variety of charged wiring configurations (Table 1.2). During all trials, the negative wires were grounded. The bottom wire was positively charged and higher wires were alternated as positive and negative. This ensured that a snake contacting oppositely charged wires while attempting to climb up the fence would be shocked and repelled. To construct enclsoures, I attached wires to the inner surface of the polyethylene fencing materials using cable ties. Prior to initiating this study I observed large snakes forcing their heads underneath fencing material and escaping. I consequently buried fencing materials 10 cm into the ground.

10 # of wires Distance (mm) of wires from the ground and polarity in parentheses

5 10(+) 30(-) 50(+) 70(-) 90(+)

4 30(+) 50(-) 70(+) 90(-)

3 50(+) 70(-) 90(+)

4a 10(+) 50(-) 70(+) 90(-)

Table 1.2. Wiring configurations of four electrical barrier prototypes. In all cases negative conductors were grounded. Results and Discussion

During the first enclosure experiment snake escape rates ranged from 0 -

40 % and mortality ranged from 30 - 90 % (Figure 1.1). Although overall escape rates were low for the Tensar® fencing materials, behavioral observations suggested that small snakes could not be prevented from going through the large mesh; testing on this material ended. It was observed that dead snakes hanging across positively charged wires and negatively charged wires (or the ground) on an electrical barrier can drop voltage subsequently passing through wiring by increasing resistance. Feasibly, Several dead snakes hanging on the wiring of an electrified barrier could the lower voltage produced by the charger to a level that does not deter snake dispersal, thus rendering a barrier useless. Starting with the second experiment, I collected data on the site of snake mortality.

In the second enclosure experiment, testing continued on the five- and four-wire polyethylene netting fences. A new fencing material, nylon netting, was also tested in three different configurations. Escape rates in this second experiment ranged from 0 - 37% for the various fencing designs (Figure 1.2).

There were no escapes from the five-wire nylon and polyethylene netting enclosures, and the four-wire nylon netting enclosure had only a 2 % escape rate. Mortality of snakes ranged from 20 - 78% (Figure 1.2). Results bearing on barrier design can be grouped into three general topics: conductor/ fencing

12 ■ Escape ESDead □ Repelled Alive 100%

5-PI 4-PI 3-PI 4a-Pl 5-T 4-T 3-T 4a-T Wiring configuration / Fence material

Figure 1.1. Fate of snakes in the first experiment. Two fence materials were tested; polyethylene netting with 6.5 mm X 6.0 mm parallelogram holes (PI) and Tensar® Polygrid™ WB with 24.5 X 5.5 mm oval holes (T). Four different wiring configuration were tested (Table 1.2). with 7.5 mm wide holes (N) and polyethylene netting with 6.5 mm X 6.0 mm parallelogram holes (PI). Three Three (PI). holes 1.2). (Table parallelogram tested Xmm 6.0 mm 6.5 were with netting configurations wiring polyethylene different and (N) holes wide mm 7.5 with Figure 1.2. Fate of snakes in the second experiment. Two materials were tested; hexagonal knotless nylon netting netting nylon knotless hexagonal tested; were materials Two experiment. second the in snakes of 1.2.Fate Figure Percent of snake sample % 0 0 1 50% 0%-L 5-N ■ Escape E3 Dead on Fence S Dead on Ground □ Repelled Alive Repelled □ Ground on Dead S Fence E3on Dead Escape ■ H Wiring configuration / Fence material Fence / configuration Wiring 4-N - 5P 4-PI 5-PI 3-N material compatibility, effective wiring configurations, and difficulties associated with excluding a broad range of snake sizes.

The stiffness of the polyethylene fencing material made it very difficult to attach conductors so they would remain close to the plastic mesh. Over the passage of time, even minor irregularities in the supporting terrain would cause deep furrows in the fencing material between attachment points. Many snakes died during the first experiment hanging near or adhering to cable ties used to attach charged wires to the polyethylene fencing materials.

Nylon netting in the second experiment did not need cable ties, thereby reducing furrows and the number of dead snakes hanging on the fence. I compared the site of snake death, either on the ground or on the fence, to evaluate if fences constructed out of nylon netting had a lower chance of being rendered inoperable by dead snakes hanging on wires (Figure 1.2). Fewer dead snakes were found on wiring of the tieless nylon netting designs (Gi = 6.388, P

< 0.05). Of the two fencing designs that had no escapes during the second experiment, the five wire nylon configuration had the lowest percentage of snakes that died on the conductors. The nylon four-wire fence had a significantly lower escape rate than the polyethylene four-wire fence (Gi =

34.087, P < 0.001). With the exception of Tensar® fences, all fence designs had an inverse relationship between snake escape rate and mortality (r6 = -0.841, P <

0.002).

15 Differences in wiring configuration effectiveness were also observed.

Snakes had a higher chance of escape as the number of wires on polyethylene or nylon netting enclosures decreased (Figures 1 and 2; X7 =0 .887, P < 0.005).

This is because of the increased chance that a snake will encounter an electrified wire and be repelled, before it becomes entwined in the wiring of the fence.

Finding an effective barrier to exclude all size classes of snakes proved to be problematic. During the first experiment the Tensar® fencing material had low escape rates for all wiring configurations. However, I observed a 420 mm snout-vent length Brown Tree Snake passing unimpeded through the material during behavioral observations. Further testing ceased on this material since it was not available in a smaller mesh size.

Behavioral observations on a large Brown Tree Snake (snout vent length

> 1350 mm) were made for each enclosure in the second experiment. This individual was placed in each enclosure and could escape all fencing due to it's size. Fortunately, snakes this large are presently very rare at most sites on

Guam and most individuals of this size are male (Rodda et al., in press - b).

In conclusion, the five-wire nylon netting fence was the most successful barrier design for stopping Brown Tree Snake dispersal. This fencing design had no escapes, except for the large snake during behavioral observations.

These results suggest that this design merits testing on a larger scale such as a snake exclosure (Shiroma and Akamine, this volume; Rodda et al., in press - a).

16 Eventually, barriers such as this one may be used to keep Brown Tree Snakes out of electrical facilities, ports, airports, and wildlife habitat.

17 Chapter II

The Effect of Brown Tree Snake Predation

on the Island of Guam's Extant Lizard Assemblage.

During the past 30 years, many field experiments have been conducted in terrestrial and aquatic communities to evaluate the influence of predation and competition on community structure (Schoener 1983, Sih et al., 1985; Pimm

1991; Crawley 1992). Though the importance of inter- and intraspecific competition in structuring communities is well supported, the evidence is far more equivocal as to wheter predation affects prey abundance. Roughgarden and Diamond (1986) feel that three lines of evidence support the importance of predation on prey abundance: 1. a wealth of experimental field evidence from many terrestrial and marine systems (Holmes and Price 1980, Menge and

Lubchenco 1981, Pacala and Roughgarden 1984); 2. high abundances of hare, reindeer, elk, and rats on predator-free islands (Scheffer 1951, Troyer 1960,

Schnell 1968, Klein 1968, W indberg and Keith 1976); and 3. the reductions in prey populations often achieved by introduced predators (Diamond and Case

1986). Crawley (1992) reviewed terrestrial predator exclosure experiments and felt that few, if any, had conclusively demonstrated the ability of predators to regulate prey communities. Crawley thought most experiments had led to

18 increases in prey breeding success. Crawley discounted these increases, stating that they were almost certainly transitional and that subsequent prey population declines were not detected due to the short term nature of these studies. Crawley also felt that many studies alleging a major impact of predator removal were anecdotal, often referring to alien herbivores and/or predators, and that some of the experimental studies were poorly replicated.

Unlike terrestrial communities, there are numerous studies in aquatic communities which have clearly demonstrated the direct ana indirect effects of predation on prey community density (Kerfoot and Sih, 1987; and Hairston

1989). This difference between terrestrial and aquatic systems may in part be due to the greater practicality of conducting both large and small scale experimental manipulations of predator densities in aquatic systems.

Given the contradictory views associated with the debate over the importance of predation in terrestrial communities, there is a need to evaluate this issue through well designed predator removal experiments on appropriate spatial scales. One group of organisms that has frequently been used in observational and manipulative studies of competition are lizards in island and arid ecosystems (Roughgarden 1995, Losos 1994, Pianka 1986, and Dunham

1980). No manipulative studies have clearly demonstrated an effect of predation on lizard community dynamics. Hairston (1989), when reviewing studies evaluating the importance of competition on arid lizard communities, felt there was a need to understand the effect of lizard predators on lizard

19 population dynamics. Experimental demonstration of the role predators play in structuring Anolis lizard communities is also needed in the Caribbean

(Losos 1994).

Observations of present biogeographic patterns of island lizards suggest that the historical introduction of predators and competitors affects subsequent lizard abundance and distribution (Case et al., 1992). There is clear evidence that predation by introduced species has historically caused the extinction of endemic island lizards (Pimm 1991; Case et al., 1992, Rodda et al.,

1991; and Rodda and Fritts 1992).

Competition between endemic and introduced lizards on islands has not been shown to cause the extinction of endemic lizards (Case and Bolger 1991), though competition from introduced lizards has been observed to negatively affect the abundance and/or behavior of endemic species (McKeown 1978;

McCoy 1980; Cogger et al., 1983; Gibbons 1985; Jarecki and Lazell 1987; Rodda et al., 1991; Rodda and Fritts 1992; Zug 1991; Losos et al., 1993). On oceanic islands, introduced lizards have been responsible for dramatic declines in abundance and/or localized extinction of other introduced lizard species through competition (Case and Bolger 1991), though the exact mechanisms are unknown. In the Pacific, research has recently begun to explore the role that competition plays in shaping gecko assemblages (Case et al., 1994). A series of experimental studies have documented a decline in the abundance of a native parthenogenic gecko, Lepidodactylus lugubris, through exploitive competition

20 by an introduced sexual gecko, Hemidactylus frenatus (Petran et al., 1993; and

Petran and Case 1996). In anoline lizard communities in the Caribbean, which have been the subject of intensive study, interspecific competition, not predation, has been suggested as the major force structuring those communities in most sites (Losos 1994).

Experimental manipulations have been suggested as the only way to demonstrate conclusively the role predators play in structuring anole communities (Losos 1994). Up to this date, no experimental manipulations have been conducted to determine the effect of predation on extant lizards assemblages (terrestrial or arboreal; introduced or native) on tropical islands.

Observational studies have suggested that predation, in addition to interspecific and intraspecific competition, may affect the abundance and behavior of anoline lizards in sites of high predator densities (McLaughlin and

Roughgarden 1989; Campbell 1989). By conducting a predator exclusion study on the island of Guam, I was able to address this issue directly by assessing the effect that an introduced predator, the Brown Tree Snake (Boiga irregularis), has on the abundance and behavior of Guam's extant lizards.

Four species of lizards (two gecko and two skink) were found on the site where I conducted the exclosure experiment. The two geckos were

Lepidodactylus lugubris, a native parthenogenic species, and Hemidactylus frentus, an introduced species sexual species. The two skinks were Emoia

21 caeruleocauda, a native species, and Carlia fusca, an introduced klepto- cannabalistic species.

Methods

Two one-hectare Brown Tree Snake exclosures and two one-hectare control plots (Figure 2.1) were constructed in early, second-growth tangen- tangen (Leucaena leucocephala) forest on Andersen Air Force Base - Guam. The snake exclosures were surrounded by an electrified barrier designed to stop snake dispersal (Chapter 1). Snakes were removed from the exclosure plots using traps and hand capture during night-time visual snake surveys. Control plots had no barrier surrounding them. Each plot was transected by three trails which were used for lizard and snake surveys (Figure 2.1).

Snake and lizard populations were monitored in snake exclosures and control plots twice prior to the removal of snakes from snake exclosures and once each quarter for one year following removal. Pre-treatment monitoring was conducted during February and March 1993, prior to construction of the snake barrier. A transitional monitoring was conducted during July and

August 1993, immediately following construction of the snake barrier but prior to its electrification. Transitional monitoring allowed me to detect any effect of barrier construction on snake and prey abundance in snake exclosures.

Quarterly monitoring on all four plots was initiated in late January 1994, three months after the mean date of snake removal on the exclosure plots. Following

22 Transect Lines Snake Exclosure Perimeter Mag. N. Snake-Present Plot Perimeter

100 m

Figure 21. Arrangement of the 1 ha. snake exclosure plots and snake-present plots (from Rodda et al, in press - a). completion of the experiment, a complete census of lizards was conducted in a

10 m2 area within each of the four plots to further validate the results of the exclosure study.

Mark-recapture and index techniques were used to determine snake densities and show that snakes continued to be absent from the exclosure plots.

Abundance of terrestrial lizards was monitored using catch-release, adhesive trapping (Campbell et al., 1993). Abundance of arboreal lizards was measured using night-time visual surveys.

In statistical analyses to estimate treatment effects, results from the two snake-free plots were combined and the results from the two snake-present plots were combined. This step recognized that the plots were adjacent to each other, rather than being selected independently from a larger population, and that movements between plots, especially the control plots, by the species we studied or other species affecting them, may have occured.

Changes in abundance were evaluated in two steps. First, we determined whether temporal changes in treatment effect were evident. This analysis was carried out by regressing the ratio of species abundance (index for snake-free plots/index for snake-present plots) against time (i.e., the four post-treatment periods). If the treatment effect, measured as a ratio of population sizes, changed through time in a consistent manner, then the regression coefficient should be significant. When this test was non-significant, we calculated single index values for the snake-present and snake-free plots during pre-treatment,

24 transitional, and post-treatment periods. I use the symbols ybP, ybf,yap, and y af where y is the mean index value, b = before treatment, a = after treatment, p = snake-present plots, f = snake-free plots. "Before" refers to either the pre­ treatment or to the transitional monitoring periods. Standard errors (SE) were also calculated for each estimate (details below).

I carried out two tests to determine whether a treatment effect was detectable. I describe the tests below and then discuss their relative strengths and weaknesses. The first test assumed that in the absence of a treatment effect, the index values would be equal (apart from sampling error) in the post­ treatment period. Under this assumption, the point estimate of percent change due to snake removal was (ra -1) * 100 where ra = y af / y ap. The SE(ra) was estimated using the standard formula for the ratios of correlated means (e.g.,

Cochran 1977, eq. 6.12). A one sample t test was used to determine whether the point estimate was significantly different from the null value of 0.0. For example, if the post-treatment mean was 4 on the snake-free plots and 3 on the snake-present plot, then the estimate of the treatment effect was (1.33 -1 ) * 100

= 33%, a 33% increase in abundance.

The second test assumed that in the absence of a treatment effect, the ratios of the index values would be equal (apart from sampling error) before and after treatment. Under this assumption, the point estimate of percent change due to snake removal was ((ra/ n,) -1) * 100 where ra/rb = ( y af /

Yap)/(ybf / ybP )• The SE(ra/rb) was estimated using the standard formula for

25 the ratio of two ratios (e.g., Cochran 1977, eq. 6.95). As in the first test, I used a one sample t test to determine whether the point estimate was significantly different from the null value of 0.0. For example, if the post-treatment ratio of index values (ra or yaf / yap) was 3 and the pre-treatment ratio of index values

(rb or ybf / ybP )was 2, then the estimate of the treatment effect was (1.5 -1) *

100 = 50 %, a 50 % increase in abundance.

The t test on the post-treatment ratio of species abundance assumes that in the absence of a treatment effect, species abundance levels will be the same on snake-free and snake-present plots. Though it is realistic to assume that the abundance of a particular species on my four plots fluctuated at the same level on all four study plots, it is unlikely that this fluctuation would be exactly the same on each of the plots. The second test, a comparison of the pre-treatment

(or transitional) and post-treatment ratios of species abundance on snake-free and snake-present plots, takes into account that the ratio of species abundance on each of the snake-free and snake-present plots may vary over time. Though it is hard to choose which test is more realistic, I have chosen to be conservative and assume that the treatment affect is significant only if both t tests indicate significance.

Snake abundance

To obtain estimates of Brown Tree Snake population size on all plots prior to snake removal and on snake-present plots following snake removal, a

5X5 grid of 25 traps (25 m spacing between traps) was constructed in each plot

26 and monitored for 16 to 60 days. During the snake eradication, a 7X 7 grid of

49 traps (14.3 m spacing between traps) was used for snake removal. After snake removal, snake trapping was conducted on a quarterly basis on snake- free and snake-present plots. A 5 X 5 grid of 25 traps was used on the snake- present plots to obtain mark-recapture estimates of snake populations and a 7 X

7 grid of 49 traps was used on the snake eradication plots to detect snake incursions and insure low or no snake presence on snake exclosures.

Snake trapping to determine snake densities was conducted for 16-60 consecutive days on all pre-treatment and control plots. Records were kept on individual snake capture histories through all trapping sessions. Each new snake was marked with a uniquely numbered passive integrated transponder

(PIT) tag for identification (Lang, 1992) prior to release.

Brown Tree Snakes were also captured by hand during standardized nighttime visual surveys for arboreal lizards. Methods of marking and data collection were the same for snakes captured by hand and in traps, and data from both capture methods were used in the mark-recapture estimates.

Snake traps were constructed out of commercially available crayfish

(funnel) traps (Rodda et al., in press - b). Each trap was baited with a live mouse, housed in a separate cage inside the trap. A hinged flap was placed in the funnel's opening to prevent snakes from leaving the trap, and a shelter, made from a plastic pipe, was placed inside the trap. A plastic roof was placed over the trap to prevent snakes and mice from overheating.

27 Snake capture histories were analyzed using the open population modeling program SURGE (Lebereton et al., 1992, Rodda et al., in press - c) for

16 to 20 day capture periods during each trapping session. During the pre­ treatment monitoring period, the coefficient of variation (CV) of snake mark- recapture population estimates for all four plots was extremely high due to low recapture success (Figure 2.2). Though mark-recapture estimates from subsequent monitoring periods had acceptable CV's, an index of snake abundance, the mean number of snakes captured per night per plot, was used to determine whether barrier construction significantly affected snake abundance in exclosure plots between pre-treatment and transitional monitoring periods.

The indices of snake abundance calculated for all four plots had substantially lower CV's compared to mark-recapture density estimates during the pre­ treatment monitoring period. T-tests, similar to those described earlier in these methods, were used to see if there was: 1. a significant post-construction (or transitional monitoring period) difference in the ratio of snakes captured on snake-free and snake-present plots; and 2. a significant difference between the ratios of snake-free and snake-present plots snake population indices before and after barrier construction.

Snake Removal

Following barrier construction and the transitional monitoring period, snake removal was continued on each removal plot until several days had elapsed without any captures. The required number of days without a capture,

28 Snake-Pres. 1 Snake-Pres. 2 — O— Snake-Free 1 — A— Snake-Free 2

100.0 0 --

80.00--

u 01 ? 60.00 - - 01 a A 0J 2 40.00-- c CD

20.00 - -

0.00 PRE TRANS POST 1 POST 2 POST 3 POST 4 Sampling period

Figure 2.2. Brown Tree Snake density estimates and 95% confidence intervals for snake-free and snake-present plots during pre-treatment, transitional, and quarterly monitoring periods. Upper estimates of each of the pre­ treatment confidence intervals was greater than 110 snakes per hectare. n, was determined in the following way. If a single snake was present on the plot during n days of trapping, then the probability it escaped capture was (1 - p)n where p = capture probability per day (assumed constant). I estimated p using SURGE and calculated n so that (1 - p)n < 0.05. At the end of n days of trapping (with no captures) it could be stated "If a snake was present on the plot n days ago, then the probability that we would have captured it is >0.95.

Therefore, we assume that in fact no snakes are present on the plot."

Lizard Abundance

Night time surveys for arboreal lizards were conducted at the start of each monitoring period. The survey protocol on each plot was as follows; on each of ten nights (1900-2400), one and a half of the transects (150 m) on each of the four plots was covered by two surveyors working separately on exterior transects and as a pair surveying opposite sides of interior transects (Figure

2.1). Thus, each transect was surveyed five times by each observer. During each survey period, the sequence in which transects were surveyed was determined systematically, assuring that each transect within each plot was surveyed equally by both observers. The same two surveyors made all observations except during the transitional monitoring period when one of the surveyors was replaced. Surveyors used headlamps to find arboreal lizards and recorded the following for each individual sighted: lizard species, perch height, perch diameter, perch plant species, location, and time. During the last sampling period, observers also noted whether gecko tails were broken,

30 regrown, or normal as a method to evaluate intra- and inter- specific aggression and to compare gecko species level effects of snake predation following the removal of snakes.

At the start of each monitoring period, following the completion of arboreal lizard surveys and snake trapping, terrestrial lizards were monitored using adhesive trapping. Twelve adhesive traps (Bauer and Sadlier 1992, and

Rodda et al., 1993) were placed along the three trails each plot (Figure 2.1) to monitor terrestrial lizard abundance (four traps per transect). Trapping lasted seven days during each sampling period, and traps were moved 3 m forward along each transect for each subsequent trapping session. Trapping was conducted in the morning (0730 -1100) during the period of peak skink activity and before the risk of mortality due to overheating became too great. Traps were checked each half hour, and all lizards captured were released. Lizard age-class or size (snout vent length), time, and trap locality, were recorded for each individual. Carlia fusca were considered adults if they had a snout vent length over 49 mm (M. J. McCoid, pers. comm.).

In the pre-removal and transitional periods, the sample size was the number of days and the response variables were number of geckos/100 m of transect, skinks/trap or snakes/trap. Standard errors were calculated using the customary formula for simple random samples. Statistical inferences apply solely to the period of the monitoring, not to earlier or subsequent periods. In the post-removal periods, I viewed the four sampling periods as a random

31 sample from the 12 month interval. Statistical inferences thus applied to the entire period. The sampling plan was multiple stage, with 4 periods as primary units. For each index, a single response variable was calculated for each period

(i.e., average numbers of geckos /100 m, skinks/trap, or snakes/trap, and the four quarterly numbers were used in the customary formula for simple random sampling to calculate standard errors for each measurement). This reduction of the data (to a sample size of 4) avoided pseudoreplication during the post­ removal period. Changes in arboreal and terrestrial lizard abundance, perch height, and perch diameter on snake-free and snake-present plots before and after snake removal were analyzed using ratio testing as described earlier in these methods. Differences in gecko tail breakage rates were compared using

Fisher's exact test (Zar, 1996).

Test of Gecko Escape Behavior

While conducting standardized night-time surveys for geckos, it was noted that Hemidactylus frenatus and Lepidodactylus lugubris differed in their escape behavior in response to hand capture. This difference may have been an important factor which contributed to the response of each gecko species to predator removal from snake exclosures. Following the completion of the exclosure experiment, a test on gecko escape behavior was conducted between

Hemidactylus and Lepidodactylus on the exclosure experiment study site.

Using five observers , escape behavior of geckos was recorded for each gecko sighted within an arm's reach of the observer. The observer tapped the branch

32 the gecko was on, 120 mm below the gecko (i.e. proximal to the trunk of the tree), five times at a rate of one tap/second and recorded the number of taps until the gecko flushed (or if it did not flush). Each gecko was then captured when possible and later preserved.

Fisher's exact test was used to evaluate the difference in proportion of geckos flushing, between the two gecko species. A t test was used to evaluate the difference, between the two species, in average number of taps before flushing (for animals that did flush). Only one trial per animal was conducted, and no evidence of temporal or spatial effects could be detected. Trials were therefore regarded as a simple random sample, and the sample size was the number of trials for Fisher's exact test and the number of trials in which animals flushed for t tests.

Removal Census of Lizards

Following the completion of the exclosure experiment and the gecko escape behavior test, removal (or quadrat) sampling was used to census lizards on a 100 m2 sub-sample of each plot used for the exclosure experiment (Jaeger and Inger, 1994). This sampling was accomplished in association with removal of all vegetation from 100 m2 areas near the center of each of the one hectare plots. Any terrestrial movement of lizards in or out of the 100 m2 areas was stopped by a 0.5 m aluminum barrier coated with spray-on Lithium grease; arboreal movements were proscribed by a 1 m wide gap seperating thequadrat from all contiguous vegetation. In the day after the creation of the lizard fence,

33 all vegetation within the 100 m2 areas was reduced to small pieces suited for close inspection and all lizards were captured by hand or adhesive trap capture.

All lizards captured during the removal experiment were identified, measured

(snout vent length and weight), and preserved. In February 1995, we conducted removal sampling in a 100 m2 sub-sample of both snake exclosures and one of the snake-present plots. During this time period we conducted removal sampling on a 25 m2 sub-sample of the second snake-present plots. To get a census of a 100 m2 sub-sample of this area, three more 25 m2 removal plots were constructed and sampled in this plot in late April and early May 1995.

Results

Snake population monitoring and snake removal

During the transitional and post treatment monitoring periods of the snake removal experiment there were an average of 34 snakes per hectare (SE =

4.35, n = 10) on snake-present plots (Figure 2.2). Forty seven snakes were captured on snake-free plots during the 365 days of trapping following the completion of snake removal (0.06 incursions detected per day per plot). Thirty eight (81%) of these incursions occurred during the last three months of the exclosure experiment a period of high rainfall and when the fence was increasingly fragile. During this time period, snake trapping was conducted continuously to assure low snake densities for the predator removal experiment.

34 A significant decline in the mean number of snakes captured on snake

exclosure plots compared to snake-present plots was detected between the pre­

treatment and transitional monitoring periods (Tables 2.1 and 2.2).

During the transitional (pre-electrification) monitoring period, the mean

probabilities of capturing a snake in each snake exclosure (p) were 0.125 and

0.175. Using the mean value for p for both plots (0.15), we estimated that 20

snake capture-free days were needed in each snake exclosure plot in order to

assume that there was >95% chance that no snakes remained (Figure 2.3). In the

first snake exclosure plot, 15 snakes were removed in 18 days. The second plot

took longer; 16 snakes were removed in 78 days (Table 2.3 and Figure 2.4). The

difference in duration of the snake removals between the exclosures may have

been due to leakage of snakes through holes rats chewed in the fence. On 26

October 1993, a marked snake previously captured outside the exclosures

during the removal phase of the experiment was captured in the second

exclosure.

Assessment of Lizard Abundance

Significant increases in adult Carlia fusca abundance were observed

following Brown Tree Snake removal. Substantial increases in abundance were also observed for Lepidodactylus lugubris, Hemidactylus frenatus, and juvenile

Carlia fusca. Post snake removal increases in abundance were significant for

Lepidodactylus lugubris and juvenile Carlia fusca for two of three tests, though overall significance could not be claimed. There is not strong evidence

35 Period Snake-free Snake-present Ratio of plots plots capture rates'

Pre-treatment 0.94 0.86 0.91

Transitional 2 3.46 1.72

'(mean number of snakes captured on snake-free plots/mean number of snakes captured on snake-present plots) per sampling period.

Table 2.1. Mean snake capture rates (mean snake captures/plot/ day) and ratio of snake captures on snake-present and snake-free plots during the pre-treatment and transitional monitoring periods. A perfect match between control and treatment plots, perfectly sampled, would give a 1.0 ratio of capture rates. In this case, snake densities appeared slightly greater on snake-free plots during the pre-treatment monitoring period and greater on the snake-present plots immediately following barrier construction and prior to snake removal.

36 H0: Ra= l d H0: Ra/R b = l c

Ratio of Capture Ratesc 1.72 1.90

Estimate of % change 72% 90%

SE 7% 17%

95% C.I. 62 - 82 % 64 -116 %

P - value 0.001 0.006

1 (mean number of snakes captured on snake-free plots/mean number of snakes captured on snake-present plots) per sampling period. d Test assumes that, in the absence of construction, snake populations would be the same size after treatment. The point estimate of % change in population size due to the treatment was (r„ -1) * 100. ‘Test assumes that, in the absence of construction, the ratios of snake population sizes would be the same before and after treatment. The point estimate of % change in population size due to the treatment was (r4/r b -1) * 100.

Table 2.2. Evidence that snakes declined in abundance between pre-treatment and transitional monitoring periods (index values and ratio of capture rates from Table 2.1). "R" represents parameter values and "r" represents sample estimates. Comparisons (% change) that indicated increases in snake abundance between snake-free and snake-present plots following barrier construction or over time are shown as positive values.

37 c 0.75 •m

1 0.25 X u

0 5 10 1520 25 30 Days since last snake captured on plot

Figure 2.3. A theoretical removal schedule showing the probability of a snake being present and uncaught and the number of days without a capture. In this case (assuming a daily capture probability of 0.15 for snakes present in a plot), the probability of a snake being present and undetected falls below P = 0.05 after about 19 days without a capture. Plot population estimate Number (95% C.I.) Removed

Snake Free 1 12 (8 -18) 16 Snake Free 2 13 (8 - 25) 15 Snake Present 1 26 (17 - 42) Snake Present 2 30 (21 - 45)

Table 2.3. Brown Tree Snake population estimates for 1 ha. plots during transition phase (pre-removal) and number of snakes removed from exclosures.

39 o Figure 2.4. Cumulative removal of snakes from two one hectare snake exclosures between 31 August and 31 and August 31 between exclosures snake hectare one two from snakes of removal 2.4. Cumulative Figure December 1993. December Snakes 0 1 2 1 -- 14 6 1 0 + 4 2 6 8 - - - - t U -- -- 0 AM / /

I u I? 1 H • • A AAA / 10 ---- A'A 1 I ---- • • 20 1 ---- 1 ---- Snake-Free 1 — 040 30 1 ---- 1 ---- 1 ---- 1 ---- Day — 50 1 ---- a —

Snake-Free 2 Snake-Free 1 ---- 07 090 80 70 60 1 ---- 1 ---- 1 ---- 1 ---- 1 ---- 1 ---- 1 ---- 1 ---- 100 1 supporting an increase in Emoia caeruleocauda abundance following Brown

Tree Snake removal, though one of three tests was significant.

Adult Carlia fusca abundance increased significantly following snake removal. Mean adult Carlia fusca abundance was similar on snake-free and snake-present plots during the pre-treatment and transitional monitoring periods, but increased 113% on snake-free plots following snake removal

(Figure 2.5 and Table 2.4). Increases were significant (Table 2.5) in comparisons of: 1. post-treatment lizard abundance on snake-free and snake-present plots

(H 0: Ra = 1), and 2. pre-treatment/post-treatment and transitional/post­ treatment ratios of lizard abundance on snake-free and snake-present plots (H„:

R a/R b = l).

Between the pre-treatment monitoring period, and barrier construction and snake removal, mean Lepidodactylus lugubris abundance increased 110% and 350% respectively on snake-free plots (Figure 2.6 and Table 2.6). Following snake removal, mean quarterly Lepidodactylus abundance was 153% greater on snake removal plots compared to snake-free plots. This post-treatment increase in Lepidodactylus abundance between snake-free and snake-present plots was significant for two of the tests(H 0: Ra = 1 and H 0: Ra/R b = 1 for the pre-/post­ treatment monitoring periods). Conversely, the transitional/post transitional ratio (H 0: Ra/Rb = 1) did not differ significantly from 1. Thus, removal of Brown

Tree Snakes did not have an overall significant effect on Lepidodactylus lugubris abundance.

41 Figure 2.5. Adult Carlia fusca abundance and standard error for snake-present and snake-free plots during periods. during plots monitoring snake-free and post-treatment snake-present for quarterly and error transitional, standard and pre-treatment, abundance fusca Carlia Adult 2.5. Figure Average number of Carlia captured per trap day 1.50 -- 1.50 2.50 -- 2.50 3.50 j 3.50 0.00 1.00 2.00 -- 3.00 0.50 - - 0.50 ------

PRE TRANS POST 1 POST 2 POST 1 POST TRANS PRE + ■Snake-Pres. 1 ■Snake-Pres. + 1 ■Snake-Pres. 2 “ -O - -Snake-Free 1 “ -A - -Snake-Free 2 -Snake-Free - -A “ 1 -Snake-Free - -O “ 2 ■Snake-Pres. Sampling period Sampling ------POST 3 POST 4 POST 3 POST + Period

Plot Pre-Treatment Transition Post-Treatment

Snake-Free -1 1.51 1.13 2.40 Snake-Free - 2 0.85 1.25 2.68 Snake-Free - mean 1.18 1.19 2.54

Snake-Present -1 1.68 1.21 1.44 Snake-Present - 2 1.10 1.14 1.40 Snake-Present - mean 1.39 1.18 1.42

mean s.f/m ean s.p. 0.85 1.01 1.79 SE 0.01 0.02 0.13

Table 2.4. The abundance of adult Carlia fusca during pre-, transitional, and post-treatment monitoring periods. Carlia adundance was measured as the mean number of skinks captured per trap day per monitoring period. The post-treatment data displayed here is the grand mean for all four post-treatment monitoring periods. Carlia Lepidodactylus Hemidactvlus Carlia Emoia Variable adult juv.

Ho: R.: = V Estimate of % change 79% 153% 72% 29% 2% Standard error 14% 2 1 % 23% 15% 19% P-value 0.010 0.005 0.052 0.148 0.935

Ho: R./Rb = l a Pre-treatment Estimate of % change 113% 140% 27% 54% 7 L /O Standard error 11% 22% 16% 15% 26% P-value 0.020 0.008 0.189 0.033 0.036

Transition Estimate of % change 77% 83% - 12% 51% - 12% Standard error 10% 32% 14% 14% 17% P-value 0.005 0.081 0.455 0.038 0.526

‘ Test assumes that in the absence of snake removal lizard populations would be the same size after treatment. The point estimate of % change in population size due to the treatment was (r, -1) * 100. d Test assumes that in the absence of snake removal the ratios of lizard population sizes would be the same before and after treatment. The point estimate of % change in population size due to the treatment was (r,/rb -1) * 100.

Table 2.5. Estimates of change in lizard population size due to snake removal and tests for treatment effect (index values from Tables 2.4, 2.6,2.7, 2.9, and 2.10). "R" represents parameter values and "r" represent sample estimates. Population comparisons (% change) that indicated increases in lizard abundance between snake-free and snake-present plots following snake removal or over time are shown as positive values. Contrary comparisons are shown as negative values. Snake-Pres. 1 Snake-Pres. 2 “ ~0“ ■ Snake-Free 1 “ * Snake-Free 2 £ 2.00 -r o u 1.80 - - a» & •d 1.60 + 1.40 - - cn CD 3 1.20 - -

1.00 - -

o 0.40 - -

Figure 2.6. Mean Lepidodactylus lugubris abundance and standard error bars for snake-present and snake- free plots during pre-treatment, transitional, and quarterly post-treatment monitoring periods. Period

Plot Pre-Treatment Transition Post-Treatment

Snake-Free -1 0.24 0.33 0.91 Snake-Free - 2 0.16 0.50 0.89 Snake-Free - mean 0.20 0.42 0.90

Snake-Present -1 0.19 0.30 0.33 Snake-Present - 2 0.19 0.30 0.38 Snake-Present - mean 0.19 0.30 0.36

m ean s.f/m ean s.p. 1.05 1.38 2.53 SE 0.01 0.03 0.28

Table 2.6. The abundance of Lepidodactylus lugubris during pre-, transitional, and post­ treatment monitoring periods. Lepidodactylus adundance was measured as the mean number of geckos observed per 100 m per monitoring period. The post-treatment data displayed here are the grand means for all four post-treatment monitoring periods. Hemidactylus frenatus abundance increased on snake-free plots following the removal of Brown Tree Snakes (Figure 2.7), though this trend was not statistically significant. Mean Hemidactylus abundance on both snake-free and snake-present plots increased 62 -163 % between pre- and transitional, and post-treatment monitoring periods (Table 2.7). The ratio of Hemidactylus abundance on snake-free plots compared to snake-present plots was greatest during the transitional monitoring period. During the post-treatment monitoring period this ratio (H0: Ra = 1) was significant (Table 2.5). Neither of the comparisons (pre-treatment/post-treatment or transitional/post-treatment) of Hemidactylus abundance ratios (H0: Ra/Rb = 1) were significant.

Because the Hemidactylus H0: Ra = 1 test was near the threshold of significance, I decided to examine these data more closely. I compared changes in Hemidactylus abundance for individual snake-free plots with the mean lizard abundance of both snake-present plots (Table 2.8). Both the post treatment comparison of Hemidactylus abundance (H0: Ra = 1) and the pre­ treatment/ post-treatment comparison of Hemidactylus abundance ratios (H():

Ra/Rb = 1) were significant for snake-free plot 1, though transitional/post­ treatment comparison of Hemidactylus abundance ratios (H0: Ra/Rb = 1) for plot 1 was not significant. This result caused me to reject the conclusion that

Hemidactylus abundance increased significantly following snake removal on plot 1. On snake-free plot 2, no significant differences (Table 2.8) were

47 plots following the transitional monitoring period. monitoring snake- transitional both the and snake-free following both plots on bars error standard and abundance frenatus Hemidactylus Mean 2.7. Figure present plots during pre-, transitional, and post treatment monitoring periods. Snakes were removed from snake- from removed were Snakes periods. monitoring treatment post and transitional, pre-, during plots present Avg. number of Hemidactylus sighted per 100 m 0.00 8.00 j 9.00 1 4.00 -- 4.00 - ■ 5.00 2.00 6.00 7.00-- 3.00 -- 3.00 . 0 0 ------

PRE TRANS POST 1 POST 2 POST 3 POST 4 POST 3 POST 2 POST 1 POST TRANS PRE + ■Snake-Pres. 1 ■ Snake-Pres. 2 ~ -O - - Snake-Free 1 “ "A” * Snake-Free 2 * Snake-Free "A” “ 1 - Snake-Free - -O ~ 2 ■ Snake-Pres. 1 ■Snake-Pres. + Sampling period Sampling + + + 5 , Period

Plot Pre-Treatment Transition Post-Treatment

Snake Free -1 1.49 1.63 4.02 Snake Free - 2 2.66 2.07 4.64 Snake Free - mean 2.08 1.85 4.33

Snake Present -1 1.44 1.13 2.57 Snake Present - 2 1.66 0.77 2.47 Snake Present - mean 1.54 0.95 2.50

mean s.f/mean s.p. 1.35 1.95 1.72 SE 0.02 0.04 0.18

Table 2.7. The abundance of Hemidactylus frenatus during pre-, transitional, and post­ treatment monitoring periods. Hemidactylus adundance was measured as the mean number of geckos observed per 100 m per monitoring period. The post-treatment data displayed here are grand means for all four post-treatment monitoring periods. Variable Snake-free Snake-free plot 1 plot 2

H„: Ra = l c Estimate of % change 60% 84% Standard error 17% 29% P-value 0.040 0.062

Hn: Ra/R b = l d Pre-Treatment 64% 6% Estimate of % change 17% 16% Standard error 0.033 0.725 P-value

Transition Estimate of % change -7% -15% Standard error 14% 16% P-value 0.640 0.399

c Test assumes that in the absence of snake removal lizard populations would be the same size after treatment. The point estimate of % change in population size due to the treatment was (r» -1) * 100. dTest assumes that in the absence of snake removal the ratios of lizard population sizes would be the same before and after treatment. The point estimate of % change in population size due to the treatment was (ra/r b -1) * 100.

Table 2.8. Estimates of change in Hemidactylus frenatus population size in individual snake exclosures due to snake removal and tests for treatment effect (index values in Table 2.7). "R" represents parameter values and "r" represents sample estimates.

50 observed in lizard abundance following snake removal for either test (H0: Ra = l o r H 0: R a/R b = 1).

Like adult Carlia fusca abundance (Table 2.5), mean juvenile Carlia fusca abundance was similar on snake-free and snake-present plots during the pre­ treatment and transitional monitoring periods and increased (40%) on snake- free plots following snake removal (Figure 2.8 and Table 2.9). This juvenile population increase was not as strong as that seen in adult Carlia fusca and was not significant in the comparison between post-treatment abundance levels on snake-free and snake-present plots (H0: Ra = 1). Both comparisons of the ratios of juvenile Carlia fusca abundance on snake-free and snake-present plots before

(pre-treatment and transitional) and after snake removal (H0: R a/R b = 1) were significant. Because the results of the H0: Ra = 1 test were not significant, I do not feel confident that abundance of juvenile Carlia fusca increased significantly on snake-free plots following snake removal.

Overall, my results suggest that Emoia caeruleocauda abundance did not increase following snake removal. No apparent trends in Emoia caeruleocauda abundance were observed between snake-free and snake-present plots following snake removal (Figure 2.9 and Table 2.10). During the pre-treatment monitoring period, Emoia abundance was half as large on snake-free plots compared to snake-present plots. Between the pre-treatment and transitional monitoring periods, Emoia abundance declined 47% on snake-present plots while Emoia abundance stayed relatively constant on snake-free plots. Thus

51 during pre-, transitional, and post-treatment monitoring periods. Snakes were removed from snake-free from removed were Snakes periods. monitoring post-treatment and transitional, pre-, during Figure 2.8. Mean juvenile Carlia fusca abundance and standard error for snake-free and snake-present plots snake-present and snake-free for error standard and abundance fusca Carlia juvenile Mean 2.8. Figure plots following the transitional monitoring period. monitoring transitional the following plots Avg. number of Carlia captured per trap day 1.50 2.50 3.50 1.00 2.00 3.00 0.00 0.50 PRE TRANS POST 1 POST 2 POST 3 POST 4 POST 3 POST 2 POST 1 POST TRANS PRE m Sampling period Sampling Snake-Pres. 2 “ "0 “ - Snake-Free 1 “ "A” * Snake-Free 2Snake-Pres. * Snake-Free "A” “ 1 - Snake-Free “ "0 “ 2 Snake-Pres. 1 Period

Plot Pre-Treatment Transition Post-Treatment

Snake-Free -1 1.19 1.01 1.35 Snake-Free - 2 1.06 1.04 1.66 Snake-Free - mean 1.13 1.03 1.51

Snake-Present -1 1.21 1.27 1.09 Snake-Present - 2 1.48 1.13 1.24 Snake-Present - mean 1.35 1.20 1.16

mean s.f/mean s.p. 0.84 0.85 1.29 SE 0.01 0.02 0.25

Table 2.9. The abundance of juvenile Carlia fusca during pre-, transitional, and post­ treatment monitoring periods. Carlia adundance was measured as the mean number of skinks captured per trap day per monitoring period. The post-treatment data displayed here are the grand means for all four post-treatment monitoring periods. snake-present plots during pre-, transitional, and post-treatment monitoring periods. Snakes were removed were period. Snakes montoring periods. transitional the monitoring following both post-treatment plots and and snake-free snake-free transitional, from both pre-, for bars error during standard plots and snake-present abundance caeruleocauda Emoia Mean 2.9. Figure Average number of Emoia captured per trap day .0 j 1.20 1.00 - - - 1.00 0.80 -- 0.00 0.20 0.40- - 0.60- -

PRE TRANS POST 1 POST 2 POST 1 POST TRANS PRE ■ Snake-Pres. 1 ~ “ O—- Snake-Pres. 2 “ -O - * Snake-Free 1 ~ • Snake-Free 2 • Snake-Free ~ 1 * Snake-Free - -O “ 2 Snake-Pres. O—- “ ~ 1 ■ Snake-Pres. Sampling period Sampling + POST 3 POST 4 POST 3 POST + Period

Plot Pre-Treatment Transition Post-Treatment

Snake-Free -1 0.43 0.33 0.53 Snake-Free - 2 0.29 0.40 0.56 Snake-Free - mean 0.36 0.37 0.54

Snake-Present -1 0.88 0.39 0.58 Snake-Present - 2 0.48 0.24 0.48 Snake-Present - mean 0.68 0.32 0.53

mean s.f/mean s.p. 0.53 1.16 1.02 SE 0.005 0.01 0.20

Table 2.10. The abundance of Emoia caeruleocauda during pre-, transitional, and post­ treatment monitoring periods. Emoia adundance was measured as the mean number of skinks captured per trap day per monitoring period. The post-treatment data displayed here are the grand means for all four post-treatment monitoring periods. 16% more Emoia were detected on snake-free plots compared to snake-present

plots during the transitional monitoring period. Statistical results (Table 2.5)

support these observations, detecting no significant difference in the post­

treatment ratio of lizard abundance (H 0: Ra = 1) and the transitional/post­

treatment monitoring period comparison of lizard abundance ratios(H 0: R a/R b

= 1). A significant difference was observed between the pre-treatment/post­

treatment monitoring period comparisons of lizard abundance ratios(H 0: Ra/R b

= 1).

Changes in Gecko Perch Usage Following Snake Removal

Overall, gecko perch diameter and height did not increase significantly

for either Hemidactylus frenatus or Lepidodactylus lugubris. No apparent

trends in perch height were observed for either species (Table 2.11, Figures 2.10

and 2.11) and statistical testing supported these conclusions (Table 2.12).

Following snake removal, Hemidactylus perch diameter appeared to be

proportionally greater on snake-present plots compared to snake-free plots

(Figure 2.12). Though Hemidactylus perch diameter declined between the pre-,

transitional, and post-treatment monitoring periods for both treatments, the

greatest proportional difference between snake-free and snake-present plots

was observed during the post-treatment monitoring periods (Table 2.11). This

ratio (H0: Ra = 1) was statistically significant (Table 2.12), though pre­

treatment/post-treatment and transitional/post-treatment comparisons of

Hemidactylus perch diameter ratios (H0: Ra/R b = 1) were not significant. No

56 Ratio of perch Snake-free plots Snake-present plots height/diameter*

Perch Height Hemidactylus Pre-Treatment 1.82 1.87 0.97 Transition 2.48 2.27 1.09 After snake removal 2.35 2.43 0.97

Lepidodactylus Pre-Treatment 1.78 1.83 0.97 Transition 1,92 1.93 0.99 After snake removal 2.27 2.29 0.99

Perch Diameter Hemidactylus Pre-Treatment 23.98 23.12 1.04 Transition 24.93 27.49 0.91 After snake removal 21.31 18.78 1.13

Lepidodacylus Pre-Treatment 21.72 18.29 1.19 Transition 11.54 10.59 1.09 After snake removal 15.48 12.45 1.24

* (mean perch height or diameter on snake-free plots/mean perch height or diameter on snake-present plots) for each species per sampling period

Table 2.11. Mean perch height (m), perch diameter (mm), and ratio of perch height/ diameter for geckos observed before and after snake removal on snake-free and snake-present plots. present plots during pre-treatment, transitional, and post- treatment quarterly monitoring periods. monitoring snake- both quarterly and post- treatment snake-free and both for error transitional, standard and pre-treatment, height during perch plots frenatus present Hemidactylus Mean 2.10. Figure Perch Height (m) 0.00 1.00 2.00 4.00-- 3.00------PRE TRANS POST 1 POST 2 POST 3 POST 4 POST 3 POST 2 POST 1 POST TRANS PRE Snake-Pres. 1 Snake-Pres. Sampling period Sampling Snake-Pres. 2 — O— Snake-Free 1 — A— Snake-Free 2 Snake-Free A— — 1 Snake-Free O— — 2 Snake-Pres. snake-free plots following the transitional monitoring period. monitoring transitional the snake- following and plots snake-free for bars snake-free error standard and height perch lugubris Lepidodactvlus Mean 2.11. Figure present plots during pre-, transitional, and post-treatment monitoring periods. Snakes were removed from removed were Snakes periods. monitoring post-treatment and transitional, pre-, during plots present Perch Height (m) 0.00 1.00 2.00 .0 - - 4.00 .0 - - 3.00

A ------PRE TRANS POST 1 POST 2 POST 3 POST 4 POST 3 POST 2 POST 1 POST TRANS PRE 1 ------■ Snake-Pres. 1 “ “ 0 — Snake-Pres. 2 — O— Snake-Free 1 — A— Snake-Free 2 Snake-Free A— — 1 Snake-Free O— — 2 Snake-Pres. — 0 “ “ 1 ■ Snake-Pres. 1 ------Sampling period Sampling 1 ------1 ------1 ------Perch Height Perch Diameter

Variable Hemidactvlus Lepidodactvlus Hemidactvlus Lepidodactvlus

Estimate of % change -3% -1% 13% 24% Standard error 3% 3% 2% 11% P-value 0.378 0.799 0.003 0.111

Ho-.R./Rh = l d Pre-Treatment Estimate of % change -1% 2% 9% 5% Standard error 7% 9% 14% 21% P-value 0.892 0.863 0.554 0.839

Transition Estimate of % change -12% 0% 25% 14% Standard error 6% 9% 15% 22% P-value 0.163 0.975 0.196 0.570

1 Test assumes that in the absence of snake removal gecko perch height / perch diameter would be the same after treatment. The point estimate of % change in population size due to the treatment was (r, -1) * 100. ‘‘Test assumes that in the absence of snake removal the ratios of gecko perch height / perch diameter would be the same before and after treatment. The point estimate of % change in population size due to the treatment was (ra/r b -1) * 100.

Table 2.12. Estimates of change in perch height and perch diameter due to snake removal and test for treatment effect (index values from Table 2.11). "R" represents parameter values and "r" represents sample estimates. Figure 2.12. Mean Hemidactvlus frenatus perch diameter and standard error following Brown Tree Snake Tree Brown following error standard and diameter perch frenatus Hemidactvlus Mean 2.12. Figure periods. Snakes were removed from snake-free plots following the transitional monitoring period. monitoring transitional the following plots monitoring treatment snake-free from post and removed transitional, were pre-, Snakes during plots periods. snake-present and snake-free for removal Perch Diameter (mm) 15.00 - - 15.00 10.00 20 25.00-- 30.00-- 35.00-r 0.00 -- 5.00 . 0 0 -- -

-- - -

1 PRE TRANS POST 1 POST 2 POST 1 POST TRANS PRE Snake-Pres. 1 Snake-Pres. ------+ Snake-Pres. 2 - - -o- - - Snake-Free 1 - - -A- - - Snake-Free 2 - - -A- 1 - -- -- Snake-Free --o- 2 Snake-Free Snake-Pres. Sampling period Sampling + POST 3 POST POST 4 POST Figure 2.13. Mean Lepidodactvlus lugubris perch diameter and standard error for snake-free and snake-present and snake-free for error standard and diameter perch lugubris Lepidodactvlus Mean 2.13. Figure plots during pre-, transitional, and post treatment monitoring periods. Snakes were removed from snake-free from removed were period. Snakes monitoring periods. transitional the monitoring treatment following post plots and transitional, pre-, during plots

Perch Diameter (mm) 20.00 20.00 10.00 10.00 + 30.00 0.00 + + PRE TRANS POST 1 POST 2 POST 3 POST 4 POST 3 POST 2 POST 1 POST TRANS PRE + Snake-Pres. 1 Snake-Pres. 2 — O— Snake-Free 1 — A— Snake-Free 2 Snake-Free A— — 1 Snake-Free O— — 2 Snake-Pres. 1 Snake-Pres. + Sampling period Sampling + + + clear trends were apparent in Lepidodactvlus perch diameter during the experiment (Figure 2.13), though the proportional difference in perch diameter was greatest between snake-free and snake-present plots following snake removal (Table 2.11). As expected, neither statistical test (H0: Ra = 1 or H0: Ra/Rb

= 1) detected significant changes in Lepidodactvlus perch diameter following snake removal (Table 2.12).

Gecko Species Tail Breakage Rates with and without Snake Predation

There was no significant difference between the proportion of

Hemidactvlus frenatus or Lepidodactvlus lugubris with broken tails on snake- free or snake-present plots during the fourth post-treatment monitoring period

(Table 2.13). A significant difference in tail breakage was detected between

Hemidactvlus and Lepidodactvlus on snake-present plots. No significant difference in tail breakage was detected between these species on snake-free plots.

Test of Gecko Escape Behavior

A statistically significant difference in escape behavior was observed between Hemidactvlus frenatus and Lepidodactvlus lugubris (Table 2.14).

Significantly more Lepidodactvlus did not flush (i.e. stayed in place) during behavioral trials compared to Hemidactvlus. If a gecko flushed during behavioral trials, more taps were needed to flush Hemidactvlus compared to

Lepidodactvlus, though this difference was not statistically significant (Table

2.15).

63 % of animals with broken tails Sample

Species Snake Present Snake Free Snake Present Snake Free P-value

Hemidactylus 39 37 198 475 0.73

Lepidodactvlus 23 31 60 16 0.53

P-value 0.03 0.79

Table 2.13. The percentage and sample size of Hemidactvlus frenatus and Lepidodactvlus lugubris observed with broken tails during the last post-treatment monitoring period on snake-free and snake- present plots. Fisher's exact test (two-tailed) was used to test for differences in gecko tail breakage rates. Species % stay n P-value

Hemidactylus 24% 234 < 0.001

Lepidodactylus 63% 46 -

Table 2.14. The percentage of Hemidactvlus frenatus and Lepidodactvlus lugubris that did not move during escape behavior tests. Fisher's exact test (two-tailed) was used to test for differences in gecko escape rates.

65 Species mean taps SE n P-value

Hemidactvlus 1.78 0.11 179 0.40

Lepidodactvlus 2.12 0.38 17

Table 2.15. The mean number of taps on a perch branch needed to stimulate Hemidactvlus frenatus and Lepidodactvlus lugubris individuals to move during escape behavior tests. These results do not include individuals that did not move during testing. A two tailed t test using seperate variances was used to detect differences in tap frequency between treatments.

66 Removal Census of Lizards

Using removal sampling following the completion of the last quarterly monitoring period, we determined that there were an average of 19,650 and

13,210 lizards per hectare in the snake removal and snake-present plots respectively (Table 2.16). For all four lizard species, density per hectare was higher on snake-free plots compared to snake-present plots. In both snake-free and snake-present plots skinks, Carlia fusca and Emoia caeruleocauda, were more abundant than geckos, Hemidactvlus frenatus and Lepidodactvlus lugubris. The mean snout vent length of all four lizard species captured during removal sampling was greater on snake-free plots compared to snake-present plots (Table 2.17).

Discussion

Snake Population Monitoring and Removal

The exclosure project validated our snake population monitoring techniques and verified that snake exclusion is possible on a landscape scale.

The number of snakes removed from each of the snake exclosures during the snake removals fell within the 95% confidence intervals of the transitional phase SURGE mark-recapture population estimates. Following snake removal, far fewer snakes were detected in snake exclosure plots compared to snake- present plots. Low levels of snake incursions were detected in exclosures, particularly during the latter three months of this experiment. Several factors

67 Variable Carlia Emoia Hemidactvlus Lepidodactvlus Total

Plot

Snake-Free 9,150 4,100 3,050 3,350 19,650 Snake-Present 5,020 3,790 1,900 2,500 13,210

% diff.a 82% 8% 61% 34% 67%

*The point estimate of % difference in lizard density is (snake-free/snake-present -1) * 100.

Table 2.16. The mean density of lizards per hectare of four species of lizards captured during removal sampling conducted on snake free and snake present plots following the final post-treatment monitoring period. Snake-Free Snake-Present

mean SVL SE n mean SVL SE n P-value

Carlia fusca 51.80 0.69 183 46.98 1.19 100 < 0.001 Emoia caeruleocauda 46.99 0.80 82 42.01 1.29 74 < 0.001 Hemidactvlus frenatus 49.41 0.74 61 45.07 1.32 30 < 0.001 Lepidodactylus lugubris 37.80 0.78 67 34.67 1.54 33 < 0.001

Table 2.17. The mean snout vent length (SVL), measured in millimeters, of four species of lizards captured during removal sampling conducted on snake-free and snake-present plots following the final post­ treatment monitoring period . A two tailed t test using pooled variances was used to detect differences mean SVL between treatments. could have lead to snake incursions: 1 rodent or ungulate damage to the barrier

material; 2. photodegradation of the barrier material causing holes and tears; 3.

battery failure causing the electrical fence charger not to function; and 4.

typhoon damage to the barrier. When snake barriers around either exclosure

were found to be damaged, repairs were made within 24 h and snakes removed

when caught. Snake exclosures, with refinement to the barrier, can be used to

reduce snake densities or eliminate snake populations on a landscape scale.

Several factors could have influenced the rate at which snakes were

initially removed from exclosures. First and most problematic, Brown Tree

Snakes were able to breach the barrier at a low frequency. Second, snakes

quiescent early in the removal due to digestion of large prey items might not

have been captured until later in the removal when they began moving again.

During the removals, we were aware of one incident where a marked Brown

Tree Snake breached the barrier and we believe the snake may have entered the

plot through one of the holes that rats chewed in the barrier mesh.

The significant decline observed in snake abundance in the exclosure

plots compared to snake-present plots between the pre-treatment and

transitional monitoring periods suggests that snake predation pressure on lizards may have been reduced partially in snake exclosures and to a lesser extent in snake-present plots prior to snake removal. This decline may have been due to human activity on the exclosure plot perimeters during barrier construction or the presence of the barrier prior to electrification.

70 Impact of Brown Tree Snake Removal on Lizards

The Brown Tree Snake exclosure experiment supports the hypothesis that

snake predation significantly impacts Guam's extant lizard fauna. Following

snake removal, the abundance of Carlia fusca, an introduced skink, increased

significantly. Substantial, though not uniformly significant increases, were also

observed following snake removal in both geckos, Lepidodactvlus lugubris, a

native species, and Hemidactvlus frenatus, an introduced species. Snake

removal did not appear to affect the abundance of the native skink, Emoia

caeruleocauda. In addition to changes in abundance of three of four lizard

species following snake removal, the snout vent length of all four lizard species

was significantly greater in snake exclosure plots compared to snake-present

plots. Removal of Brown Tree Snakes did not appear to affect gecko perch behavior.

Though I attempted to control factors that increased variation in lizard sampling techniques, it is clear that sampling variation was a factor affected my ability to detect significant changes in lizard abundance following snake removal. Species delectability, observer variation, and habitat heterogeneity were factors that affected the sensitivity of gecko abundance indices using night-time visual censuses. Weather, time of day, temperature, and species specific behavior clearly influenced the sensitivity of both gecko and skink abundance indices using, respectively, night-time visual censuses and day-time adhesive trapping. Further reductions in sampling variation, when practical,

71 may increase the sensitivity of conclusions of future studies using indices of lizard abundance for comparison.

The adult class of Carlia fusca was the only group of lizards whose abundance increased uniformly in the, pre-/ post-treatment, and transitional/post-treatment comparisons. The response of juvenile Carlia fusca to snake removal was not as strong as that observed in adults. Comparisons of pre-/post-treatment and transitional/post-treatment ratios of juvenile Carlia fusca abundance were significant but post-treatment comparisons of lizard abundance on snake-free and snake-present plots were not. The age/size difference in Carlia response to snake removal may be due to three factors.

First, juvenile abundance on both plots appeared to fluctuate seasonally in both treatments, while adult abundance did not (Figure 2.5 and 2.8). These fluctuations may reflect seasonal patterns in juvenile recruitment between wet and dry seasons. Second, adult Carlia immigration through the snake barrier may have been a source of Carlia recruitment into the snake-free plots, particularly when Carlia juvenile recruitment was seasonally low on both snake-free and snake-present plots. Third, cannabalism by increasing adult

Carlia populations on snake-free plots may have had similar predatory effects to Brown Tree Snake presence on juvenile Carlia populations on snake present plots.

The strong response of Carlia fusca to Brown Tree Snake removal is supported by two recent studies of Brown Tree Snake stomach contents.

72 McCoid (1990) found that Carlia fusca constituted 21 - 52 % of ingested prey in four sites on Guam (N = 149 snakes). Campbell (1991) found similar results for

Brown Tree Snakes (N = 60) captured in forest habitats on Guam during the summer of 1990, with skinks compromising 60% of snake diets. Sixty two percent of the skinks (N = 15) found in my study were identified as Carlia fusca.

Rodda et al., (in press) state that terrestrial skinks are an important component of the diet of adult snakes which forage more frequently on the ground.

Juvenile snakes, which are smaller, forage more frequently in trees and eat approximately equal numbers of geckos and skinks. In my study, Carlia fusca constituted 58% of all prey items (92% of all skinks; N = 11) found in snakes foraging on the ground and 19% of all prey items (33% of all skinks; N = 4) found in snakes foraging in trees. The high proportion of Carlia in snake diets may be due to two factors: 1. ease of capture and 2. higher relative abundance of

Carlia compared to other lizard species (Table 2.16).

On numerous occasions, during night-time surveys, I and other researchers have observed Brown Tree Snakes actively foraging in mowed grass. The primary lizards found in this habitat during night-time surveys is

Carlia fusca, which often sleeps in the grass at night. This behavior may make

Carlia fusca accessible and vulnerable to predation by the Brown Tree Snake than are nocturnally active lizards, such as geckos. This idea is supported by several studies. In Australia, where the Brown Tree Snake is native, agamid lizard species which sleep exposed in foliage, compose a disproportionately

73 large component of known snake diets (Shine, 1991). Similarly in the forests on the northern end of Guam in 1985, the introduced anole, Anolis carolinensis, was the primary prey item found in snake stomachs ( >50%) though it was uncommon during night-time visual surveys (< 5% of lizards species observed were Anolis; Smith and Fritts, unpub. data; Rodda and Fritts, 1992). This species, like certain Australian agamids, sleeps at night in exposed sites on foliage. Presently, anoles are rare in the forests of Guam and are common only in localized sites in urban habitat (Rodda et. al., 1992). Rodda (1992) suggests that unvigilant lizards which sleep in exposed sites at night are particularly vulnerable to the Brown Tree Snake's systematic technique of foraging.

Carlia fusca was introduced to the island of Guam sometime between the mid-1950's and the mid-1960's (Rodda and Fritts 1992; and McCoid 1993). By

1977, this species was common in all areas of Guam (Moore 1977 cited in

McCoid 1995). Carlia fusca is an extremely aggressive lizard, readily taking food from other small terrestrial lizards (Rodda et al., 1991). Rodda et al. (1991) suggested that Carlia fusca may adversely affect the abundance of a previously common native skink, Emoia caeruleocauda. In urban and disturbed habitats, it appears that Carlia fusca has replaced Emoia caeruleocauda (Rodda and Fritts,

1992). The expansion of Carlia fusca across the island of Guam clearly has provided a highly fecund alternative prey for Brown Tree Snakes in sites where less fecund ectothermic and endothermic prey became locally extinct due to snake predation. High Carlia fusca abundance, in addition to the presence of

74 introduced Anolis, may have supported substantial snake populations during the late 1970's to early 1980's in Northern Guam. These high denisities of snakes caused the decline and subsequent extirpation of most native forest birds on Northern end of the island during this time period.

The results of the snake exclosure experiment suggest that Brown Tree

Snake predation has minimal, if any, effect on Emoia caeruleocauda abundance.

This supports the statement by Rodda and Fritts (1992) suggesting that the scarcity of Emoia on Guam is not due to snake predation but ecological displacement by Carlia, even though Emoia are a component of snake diets.

The continued persistence of Emoia in secondary forest habitats on Guam may be due to predation on Carlia by Brown Tree Snakes. The removal of Brown

Tree Snakes may have released Emoia populations from snake predation in exclosures and this population limiting factor may have been replaced by higher levels of direct competition with increasing Carlia populations.

Presently, the dietary overlap of and competition between these two skinks is being evaluated (McCoid 1995).

Though the overall effect of Brown Tree Snake removal was not statistically significant for Lepidodactvlus lugubris, this species had the greatest proportional post-treatment increase in abundance (153%, sig., P = 0.005) of all species studied in this experiment. Since Lepidodactvlus is parthenogenetic, it may have a demographic advantage compared to a sexual lizard species such as

Hemidactvlus frenatus due to a higher reproductive capacity (Petren and Case,

75 1996). This advantage may be the reason for the initial surge in Lepidodactvlus lugubris abundance following predator removal. Of the two gecko species present during this study, Lepidodactvlus populations may have been the more vulnerable to snake predation.

Three lines of evidence support the hypothesis that Lepidodactvlus lugubris are easier arboreal prey for Brown Tree Snakes compared to

Hemidactvlus frenatus. Lepidodactvlus fled from novel stimuli significantly fewer times than Hemidactvlus in escape behavior tests conducted as part of this dissertation (Table 2.12). Rodda and Fritts (1992) noted that Hemidactvlus was unusually adept at avoiding hand capture compared to Lepidodactvlus. Of a sample of 99 geckos of both species that the authors attempted to capture, the escape rate for 77 Lepidodactvlus was 4%, whereas 36% of 22 Hemidactvlus escaped. Fritts (pers. comm.) has also noted that Hemidactvlus frequently drop from trees when threatened; a behavior uncommon in Lepidodactvlus in similar circumstances. Though it is difficult to determine if lizard tail breakage is due to intra- or inter- specific aggression or predation (Schoener 1979, Pianka

1986, and McCoid, in press), the comparison of gecko tail breakage rates conducted on snake-free and snake-present plots as a part of this study suggests that Lepidodactvlus have significantly higher tail breakage rates compared to

Hemidactvlus where Brown Tree Snakes are present. It should be noted that this result doesn't preclude the possibility that gecko tail breakage may be due in part to inter- and intra- specific interactions between gekkonids on Guam; it

76 merely suggests that predation can be a factor causing tail breakage.

It is difficult to determine the many indirect effects of snake predation on

Guam's extant gecko populations from the results of this exclosure experiment.

Petren and Case (1996) have shown that exploitive competition may be the mechanism for the displacement of Lepidodactvlus lugubris by Hemidactvlus frenatus in urban habitats in the Pacific. On other Pacific Islands, where

Hemidactvlus frenatus has been introduced, it is commonly found in urban habitats, but is uncommon in forest habitats (Case et. al, 1994). In contrast,

Hemidactvlus frenatus is common in the forests of Guam (Rodda et. al, 1991).

One factor that may have aided the spread of Hemidactvlus frenatus into forest habitats on Guam is the presence of the Brown Tree Snake (T. Fritts and G.

Rodda, pers. comm.), which appears to have reduced or eliminated potentially competing geckos such as: Gehyra oceanica, a large carnivorous gecko;

Perochirus a teles, a formerly abundant arboreal species; Gehyra mutilata, a generalist gecko; and Nactus pelagicus, a terrestrial species (Rodda and Fritts,

1992). Thus the Brown Tree Snake has aided Hemidactvlus spread into Guam's forests by reducing interspecific competition, while preying on expanding

Hemidactvlus populations in these habitats.

Other studies have suggested that Brown Tree Snake abundance appears to be limited by food availability in Guam and in it's native range (Rodda et al., in press - d). There is some evidence of long term fluctuations in snake and prey abundance in forest habitats near my study site (Rodda, pers. comm.). At

77 the time of this experiment, prey populations may have been high and Brown

Tree Snake populations were in an expansion phase. Thus at the time of this

work, I believe food availability may not have been a limiting factor for snake

populations on Guam. During my experiment, the mean density of snakes on

snake-present plots was 34 snakes/ha. and lizard censuses conducted during

removal sampling on snake-present plots suggest there was a standing crop of

about 259 skinks and 129 geckos as potential prey per snake. This is a

substantial prey base for an individual Brown Tree Snake in the size classes

which feed on lizards and underscores the significance of any numerical

response to predator removal by lizards.

Through a series of large scale predator removal experiments and

reintroduction experiments over a five year period, Newsome et. al (1989) and

Pech et. al (1992) showed that introduced mammalian carnivores, foxes (Vulpes

vulpes) and feral cats (Felis catus), could suppress irruptive introduced rabbit

(Oryctolagus cuniculus) populations in semi-arid Australia, where the rabbit

population is below a certain threshold. Above this threshold, predation by

foxes and cats was unable to suppress rabbit populations. These results suggest

that predation, in addition to other environmental factors, can suppress

irruptive prey populations.

In temperate habitats, where mammalian prey population growth is characterized as cyclical rather than irruptive (Newsome 1990), several

exclosure experiments have attempted to clarify the importance of predation.

78 Krebs et. al (1995) conducted a large scale (1 km2 plots) long term (8 year) predator removal experiment in boreal forests of the Yukon to study the effect of predation and food abundance on snowshoe hare (Lepus americanus) population cycles. The results of this study suggest that both factors have an additive effect which generates hare cycles. Norrdahl and Korpimaki (1995) manipulated the densities of birds of prey in 3 km2 agricultural habitats in western Finland to see the impact of kestrel (Falco tinnunculus) and

Tengmalm's owl (Aegolius funerus) predation on small mammals. The authors manipulated bird of prey abundance by removing all nest sites in five plots and increasing nest sites in five other plots. Following bird of prey population manipulation, short term increases in the sibling vole (Microtus rossiaemeridionalis) and the common shrew (Sorex araneus) were observed.

Since there were mammalian predators on the study site, it is unclear whether vole and shrew abundance would have remained elevated.

From the studies I have reviewed, it appears that predation affects mammalian prey abundance additively in association with other environmental factors such as food availability. My results suggest that Brown

Tree Snakes affect abundance of certain species of lizards on Guam but these results don't clarify the interaction of predation with other environmental factors, interspecific competition, and food availability

Brown Tree Snake predation affected the size of all species of lizards found on my study site. Following Brown Tree Snake removal, the significant

79 increase in lizard size on snake-free plots compared to snake-present plots suggests that more lizards were likely to reach larger size classes due to decreased snake predation. This change in lizard size distribution due to predation on larger individuals is similar to that seen in a population of

American water snake (Nerodia sipedon sipedon) inhabiting a fish hatchery where employees were paid a bounty per snake killed (Bauman and Metter,

1975). Employees removed larger snakes due to ease of capture. Eventually this culling caused a younger age distribution of snakes at the hatchery, with little or no reduction in the total population on the site. In the case of the

Nerodia culling, and of Emoia careluleocauda on my study site, predation on adult reptiles decreases the mean snout vent length in the population, but doesn't affect prey abundance. These data suggest that the populations are limited by other factors than predation. Perhaps food availability in the case of

Neroida in the fish hatcheries and competition with Carlia fusca and juvenile survivorship in the case of Emoia. The strong increase in Carlia fusca,

Lepidodactvlus lugubris, and Hemidactvlus frenatus abundance following snake removal, in association with significant increases in snout vent length, supports the hypothesis that Brown Tree Snake abundance, at the levels present during the removal experiment, is an important factor regulating their population levels, age structure, and recruitment.

No significant changes in gecko perch behavior were observed following Brown Tree Snake removal. This may be due to several factors.

80 First, inter- and intra- specific competition between geckos for food, refuge, or egg laying sites may be a more important factor than snake predation in determining perch height. Second, the duration of the experiment may have been to short for shifts in prey behavior to occur following snake removal.

Third, the ease with which geckos moved in and out of the exclosures suggests that some geckos within the exclosures were exposed to Brown Tree Snake predation. If changes in gecko perch behavior following predator removal were to occur, it is more likely that it would happen in isolated snake-free sites where geckos are protected from snake predation for several generations.

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