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The threatened Atlantic elkhorn coral, Acropora palmata: Population dynamics and their policy implications

ProQuest Dissertations and Theses, 2011
Dissertation
Author: Tali Vardi
Abstract:
Fossil data from multiple locations indicates that Atlantic elkhorn coral, Acropora palmata, formed shallow reefs throughout the Caribbean Sea since the Pleistocene. Beginning in the 1980s A. palmata has declined to a small fraction of its formerly vast extent throughout the region. In 2006, elkhorn coral was the first coral, along with its sister species, staghorn coral (Acropora cervicornis ), to be included on the U.S. Endangered Species List. We used size-based matrix modeling to parameterize annual A. palmata population dynamics in Florida, over the course of one severe hurricane year (2005) and six calm years (2004, and 2006-2010), incorporating environmental stochasticity as inter-annual variability. We predicted that benthic cover would remain at current levels (4%) for the foreseeable future (until 2030) and beyond (until 2100), suggesting a lack of resilience following the 2005 hurricanes. Standard metrics for the quantification of number and size of individuals are essential to endangered species management. These usually straightforward tasks can be challenging for clonal, colonial organisms. Acropora palmata presents a particular challenge due to its plastic morphology and frequent fission. We quantified three-dimensional colony surface area (CSA), the most ecologically relevant measure of size, for 14 prototypically arborescent A. palmata colonies using three-dimensional digital imaging software. To relate CSA to simple field metrics, we compared loglikelihood values and determined that planar projection was the best predictor. The, tight, linear relationship between planar projection and CSA enables ecological rates, such as reef accretion and gamete production, to be calculated from field data. Finally, we expanded the matrix population model to compare population dynamics in several locations across the Caribbean. The general trend for Acropora palmata is further reductions in population size by 2030. The most striking difference we quantified was between Jamaica, where population size is projected to increase, and all other locations, where population size is projected to remain stable or decline. Density of a key herbivore, the sea urchin Diadema antillarum, was an order of magnitude greater in Jamaica than in any other location. These increases are occurring 30 years after a devastating die-off suggesting that herbivory by urchins may facilitate A. palmata recovery.

TABLE OF CONTENTS

SIGNATURE PAGE

................................ ................................ ................................ ...

iii ! DEDICATION

................................ ................................ ................................ ............

iv ! EPIGRAPH

................................ ................................ ................................ ...................

v ! LIST OF FIGURES

................................ ................................ ................................ ..

viii ! LIST OF TABLES

................................ ................................ ................................ ......

ix ! ACKNOWLEDGEMENTS

................................ ................................ .........................

x ! VITA

................................ ................................ ................................ ..........................

xiii ! ABSTRACT OF THE DISSERTATION

................................ ...............................

xiv ! CHAPTER 1: Introduction

................................ ................................ ..........................

2 ! OVERVIEW OF THE DISSERTATION

................................ ................................ ...

2 ! REFERENCES

................................ ................................ ................................ ...........

7 ! CHAPTER 2: Short - term population predictions of threate ned elkhorn coral in the northern Florida Keys using stochastic matrix modeling

..............................

8 ! ABSTRACT

................................ ................................ ................................ ................

9 ! INTRODUCTION

................................ ................................ ................................ ....

10 ! METHODS

................................ ................................ ................................ ...............

14 ! RESULTS

................................ ................................ ................................ .................

22 ! DISCUSSION

................................ ................................ ................................ ...........

26 ! REFERENCES

................................ ................................ ................................ .........

39 ! CHAPTER 3: Quantifying surface area of a structurally complex, endangered reef - building coral, Acropora palmata

................................ ..................

44 ! ABSTRACT

................................ ................................ ................................ ..............

45 ! INTRODUCTION

................................ ................................ ................................ ....

46 ! METHODS

................................ ................................ ................................ ...............

50 ! RESULTS & DISCUSSION

................................ ................................ ....................

55 !

vii

CONCLUSIO NS

................................ ................................ ................................ ......

62 ! REFERENCES

................................ ................................ ................................ .........

70 ! CHAPTER 4: Regional analysis of Acropora palmata

population dynamics

........

74 ! ABSTRACT

................................ ................................ ................................ ..............

75 ! INTRODUCTION

................................ ................................ ................................ ....

76 ! METHODS

................................ ................................ ................................ ...............

80 ! RESULTS

................................ ................................ ................................ .................

89 ! DISCUSSION

................................ ................................ ................................ ...........

98 ! REFERENCES

................................ ................................ ................................ .......

126 ! CHAPTER 5: Policy considerations for endangered corals

................................ .

129 ! REFERENCES

................................ ................................ ................................ .......

142 !

viii

LIST OF FIGURES

Figure 2 - 1:

Life cycle diagram and corresponding matrice s

................................ .......

33 ! Figure 2 - 2:

Photographs

of Acropora palmata

colonies in size classes 1 to 4.

...........

34 ! Figure 2 - 3:

Percent cover, density, and proporti onal size class distribution of

...........

36 ! Figure 2 - 4:

Ann ual population matrices.

................................ ................................ .....

37 ! Figure 2 - 5:

Elasticity analyses for annual projection matrices.

................................ ...

38 ! Figure 3 - 1:

Morphological variability of Acropora palmata.

................................ .....

63 ! Figure 3 - 2:

Schematic of Acropora palmata colony with length, width, and height

..

64 ! Figure 3 - 3:

Pho to and corresponding wire - frame of an Acropora palmata colony

....

65 ! Figure 3 - 4:

Colony surface area as a linear function of planar projection.

.................

66 ! Figure 4 - 1:

Map of the Caribbean Sea with inset maps.

................................ ............

107 ! Figure 4 - 2:

Relative abundance per size class per survey for each location.

............

108 ! Figure 4 - 3:

Size in cm 2

of individual Acropora palmata

colonies.

...........................

109 ! Figure 4 - 4:

Stable size class distributions (SSDs)

................................ .....................

111 ! Figure 4 - 5:

Stable size class distributions (SSDs) at each location .

..........................

113 ! Figure 4 - 6:

Percent cover of Acropora palmata .

................................ .......................

118 ! Figure 4 - A1: Sampling schedule. .

................................ ................................ .............

123

ix

LIST

OF TABLES

Table 2 - 1:

Summary of size range, mean size, and abundance

................................ ...

35 ! Table 3 - 1:

Fit ted

values for each of the linear models

................................ .................

67 ! Table 3 - 2:

Parameter estimates and standard errors for linear models

........................

68 ! Table 3 - 3:

Raw field data used for fitting log - linear models

................................ .......

69 ! Table 4 - 1:

Triennial matrix population models.

................................ ........................

110 ! Table 4 - 2: Annual m atrix population models

................................ ............................

112 ! Table 4 - 3:

Does site matter?

................................ ................................ ......................

114 ! Table 4 - 4:

Model performance.

................................ ................................ .................

115 ! T able 4 - 5:

Pairwise comparisons of model s with and without location

....................

116 ! Table 4 - 6:

Table of lambda values.

................................ ................................ ............

117 ! Table 4 - 7:

Important ecological factors at each location.

................................ ..........

119 ! Table 4 - A1:

Matrix population models for Curaçao.

................................ .................

120 ! Table 4 - A2:

Matrix population models for

the Florida Keys.

................................ ...

121 ! Table 4 - A3:

Matrix population models for Jamaica.

................................ .................

122 ! Table 4 - A4:

GPS waypoint s

of monitoring plot s .

................................ .....................

1 23 ! Table 4 - A5:

Percent cover as projected in the y ears 2030 and 2100.

........................

125 !

x

ACKNOWLEDGEMENTS

F irst ,

I acknowledge my mother for disallowing me to retract my application to Scripps and my sister for giving me her blessing to flit to California during a difficult time in our family. I’m not sure I would have been as generous if the situation were reversed. I

thank them for encouraging me to do what dad would have wanted, to live life according to my own wants and desires. Second, I thank my perfect husband Sach for encouraging me to complete this dissertation, allowing me the necessary time to do so, formatti ng every single figure, and for making every day feel like I won life’s lottery. George, my best friend since age 16, grounded me when the esoteric side of academia got me down, and provided constant entertainment . Via Daily Phone Calls™ he was

by my side every step. Annie and Kim provided a welcome escape from science friends and kept me firmly grounded in ice cream sundaes. Finally, I will be forever grateful for my

in - laws, the Sokols, for making San Diego a true home for me and for offering boundless su pport. I am truly terrified to live outside driving distance from them .

I’d also like to acknowledge Coast apartments for 6.5 years of reasonable rents, and two and a half years of an ocean view. All with acres of grass and wildlife trails on which to se t my menagerie of creatures free. I can’t imagine a better place to raise Jude. Special thanks to Antonio and Reuben . And to all the wonderful friends and neighbors :

Steve (Lifetime President of the Retirement Community), Rachel, Ty, Sarah, Phil, Ben, Lore n, Megan, Jessica (x2), Jerry, and coast honorary associates –

Melania and Andrea. We slipped, slid, cooked, ate, babysat, catsat, composted, got

xi

engaged, wedded, complained, had babies, studied, and gossip girled together. I will miss this little utopia i mmensely. Overlappingly, the superfriends and breakfast club showed me after two years of stubborn denial that funny smart people existed outside of New York City. Superfriends forever. Extra special recognition to superfriend, Smarthaver, with whom I shar ed a hermetically sealed box in Hubbs Hall for five and a half years, and who served as my interim advisor for several years. How wonderful to share an office with someone so amazingly smart, generous, and goofy. I already

miss her constant transcription o f our cleverness. Dr. Kate Hanson, for forging the path to the dual degrees of mother hood

and doctor dom . Her unpredictable humor combined with her incredible focus were truly an inspiration to me during my last s ix months

at Scripps .

Targeted thank you to

field helpers: Loren, Ayana, Kristen, Katie, and Chris, Brian, A l lan Bright, Abel Valdivia, K. Lindsay Kramer, Discovery Bay Marine Lab, and NOAA SEFSC. This work is a direct collaboration with Margaret Miller and Dana Williams, both of whom have been not hing but helpful, supportive, and encouraging for the past five years. And thank you to NOAA’s Jennifer Moore who so generously funded me and my research for three years. I am grateful to the Smith/Sandin lab group for providing a wonderful support network , especially Rachel for proofreading this thesis . Any errors are wholly her fault.

Finally, thank you to my committee for having a terrifying amount of collective wisdom and agreeing to take me under their tutelage. My gratitude to my advisor, Stuart, exceeds the words “thank you” or “acknowledgement” .

Under no

xii

obligation, he adopted me when I was floundering, built up a lab, and an impressive research program, and managed to make the process of writing the disser t ation satisfying and dare I say, fun.

Chapter 2, in its entirety, was submitted to Endangered Species Research . Vardi, Tali; Williams, D ana;

Sandin ,

S tuart . The dissertation author was the primary investigator and principal author of this manuscript.

Chapter 3 is currently being prepared for

publication. Vardi, Tali ;

Hester ,

J eff;

Sandin ,

S tuart . The dissertation author was the primary investigator and principal author of this manuscript.

xiii

VITA

1999

B.A., Biology

and Environmental Studies, with distinction

1999

M.A., Conservation Biolog y

University of Pennsylvania

2000 - 2005

Grant Writer / Project Manager / Ecologist

Natural Resources Group, New York City Parks Department

2006

Anadromous Fish Population Biologist / Caribbean Reef Ecologist

Oceans Program , Environmental Defense Fund, New York City

2011

Ph . D . , Marine Biology

NSF IGERT Fellow 2005 - 2011

Scripps Institution of Oceanography

University of California, San Diego

PUBLICATIONS

T Vardi ,

DE Williams , SA Sandin . Submitted . Population viability analysis of the endangered Caribbean coral, Acropora palmata, in the northern Florida Keys.

ORAL PRESENTATIONS & PUBLISHED ABSTRACTS

T Vardi

and DE Williams. 2010. Lessons from Jamaica? Initial signs of Acropora palmata recovery using population matrix modeling. Linking Science to Management: A Conference and Workshop on the Florida Keys Marine Ecosystem.

T Vardi , DE Williams, and N Knowlton. 2010. The ESA at ESA: What population models can offer endangered corals in the face of climate change. Ecological Society of America, 95th Annual Meeting.

T Vardi , DE Williams, and KL Kramer. 2008. What is the future of the Threatened Acropora palmata ? Population projections and management recommendations. 11th

International Coral Reef Symp osium.

T Vardi

and JP Kritzer. 2007. Metapopulation dynamics and large - scale restoration planning, Alewives on the south shore of Long Island. 65th Annual Northeast Fish and Wildlife Conference.

T Vardi . 2008. Acropora palmata population viability analys is. Acropora Recovery Team Meeting. (Invited speaker)

xiv

ABSTRACT OF THE DISSERTATION

The threatened Atlantic elkhorn coral, Acropora palmata : population dynamics and their policy implications.

by

Tali Vardi

Doctor of Philosophy in Marine Biology

University

of California, San Diego, 2011

Professor Stuart Sandin , Chair

Fossil data from multiple locations indicates that Atlantic elkhorn coral, Acropora palmata ,

formed shallow reefs throughout the Caribbean Sea since the Pleistocene. Beginning in the 1980s A. palmata

has declined to a small fraction of its formerly vast extent throughout the region. In 2006, elkhorn coral was the first coral, along with its sister species, staghorn coral ( Acropora cervicornis ) ,

to

be

included on the U.S. Endangered Species List. We used size - based matrix modeling to parameterize annual A. palmata

population dynamics in Florida, over the course of one severe hurricane year (2005) and six calm years (2004, and 2006 - 2010), incorporating e nvironmental stochasticity as inter - annual variability. We predicted that benthic cover would remain at current levels (4%) for the foreseeable future (until 2030) and beyond (until 2100), suggesting a lack of resilience following the 2005 hurricanes. Stan dard metrics for the quantification of number and size of individuals are essential to endangered species management. These usually straightforward tasks

xv

can be challenging for clonal, colonial organisms. Acropora palmata

presents a particular challenge du e to its plastic morphology and frequent fission. We quantified three - dimensional colony surface area (CSA), the most ecologically relevant measure of size, for 14 prototypically arborescent A. palmata colonies using three - dimensional digital imaging softw are. To relate CSA to simple field metrics, we compared log - likelihood values and determined that planar projection was the best predictor. The, tight, linear relationship between planar projection and CSA enables ecological rates, such as reef accretion a nd gamete production, to be calculated from field data. Finally, we expanded the matrix population model to compare population dynamics in several locations across the Caribbean. The general trend for Acropora palmata is further reductions in population si ze by 2030.

The most striking difference we quantified was be tween Jamaica, where population size is projected to increase , and all other locations ,

where population size is projected to remain stable or decline. Density of a key herbivore, the sea urchin Diadema antillarum ,

was an order of magnitude greater in Jama ica than in any other location. These increases are occurring 30 years after a devastating die - off suggesting that herbivory by urchins may facilitate A. palmata

recovery.

1

CHAPTER 1: Introduction

2

OVERVIEW OF THE DISSERTATION

This dissertation includes three empirical data chapters. Each of the chapters is intended to stand alone as a publishable unit, and as a result there is some redundancy in the introductory sections and description of research methods. Below I outline the research objectives of each of the data chapters.

Chapter 2: Short - term population predictions of threatened elkhorn coral in the northern Florida Keys using stochastic matrix modeling

Motivation:

Caribbean acroporids, elkhorn and staghorn coral, are the first marine clonal invertebrates to be included on the U.S. Endangered Species List and are listed as critically endangered on the IUCN Red List. A plan for

species recovery is a legal mandate, but the science of coral population and extinction dynamics is not well developed. Although we are less familiar with the population dynamics of clonal as opposed to aclonal organisms, previous studies suggest that non - contiguous colonies vary in vital rates according to their size rather than age ( Hughes 1984, Hughes and Connell 1987) . Size - structured matrix population modeling informs our understanding of the relative importance of these different size classes, or life history stages, on population expansion and enables a prediction of population size structure to assist with recovery

planning.

Clonal organisms evolved to inhabit relatively stable environments ( Coates and Jackson 1985) . However, e lkhorn coral ( Acropora palmata )

inhabits the least stable

3

of reef environments,

the reef flat and upper reef crest, where wave energy is high and

storms frequently inflict severe damage. This

is also the zone of highest resource (food, light) availability and grea test predation intensity ;

t hus ,

A. palmata experiences life in the “fast lane” of coral reefs , where resourc es are plentiful and growth is fast, but also where punctuated disturbance occurs relatively frequently

(Jackson 1991). P opulation dynamics thus vary to some degree interannually ,

and markedly during years with severe storms or hurricanes. A. palmata

matri x population modeling requires parameterization during both storm years and calm years. The Florida Keys, where storms occur frequently, present an ideal study site to parameterize such a model. In 2004, t he National Oceanic and Atmospheric Administration Southeast Fisheries Science Center (SEFSC) initiated

a seven - year A. palmata demographic study. Three hundred individual colonies were identified, tagged, and measured annually. This rich data set allowed a temporally stochastic exploration of Acropora pal mata population dynamics, the first of its kind for a coral.

Objectives:

2.1 To study

the potential importance of large versus small size classes in population persistence using size - structured, temporally (environmentally) stochastic matrix population modeling.

2.2 To predict population density and percent cover of A. palmata on the benthos in the nea r future.

4

2.3

To estimate (a) the density and (b) the size of outplanting that would be necessary for A. palmata

to achieve target levels of benthic cover by 2030.

Chapter 3: Quantifying surface area of a structurally complex, endangered reef - building co ral, Acropora palmata

Motivation:

Three - dimensional colony surface area (CSA) is the mostly ecologically relevant size measurement for any coral, as it rel ates directly to reef accretion

and

is proportional to both sexual and asexual reproductive output (Soong and Lang 1992) , as well as probability of survival (Hughes 1984, Hughes and Tanner 2000) .

Because A cropora

palmata

has a complex branching architecture and plastic morphology, surface area cannot be approximated by a simple shape. As such, A .

palmata

surface area has been approximated by various field metrics , but the relationship between those metrics and actual surface area has never been determined. Three - dimensional digital software was used to create digital representations of 14 A. palmata colonies from which colony surface area was calculated. Using max imum likelihood, linear models relating this colony surface area to simple field metrics (length, width, and height) and planar projection were parameterized and compared.

Objectives:

3.1

To test the assumption that field metrics adequately describe Acro pora palmata

colony surface area (CSA) as estimated by

image analysis software.

5

3.2 T o test whether planar projection (PP, or two - dimensional surface area from a bird’s eye view) correlated more tightly with CSA than linear metrics.

3.3

To explore which

field metrics (or combinations thereof) correlate most closely with CSA.

Chapter 4: Regional analysis of Acropora palmata

population dynamics

Motivation:

Anecdotal accounts of remnant Acropora palmata populations suggest that current abundances and rate s of population depletion or expansion are not consistent throughout the region ( Acropora

Biological Review Team 2005) . We learned from Chapter 2 that A .

palmata

population s

are indeed dynamic over a less than decadal time scale, and that on extrapolating i ts current course , Florida’s A. palmata population is destined for functional extinction. To contextualize this result and to broaden our understanding of this critical ecosystem - building, endangered species, we conducted demographic surveys annually over a minimum of four years in Curaçao (formerly of the Netherlands Antilles) and along the north coast of Jamaica. Curaçao , where hurricanes occur relatively rarely (Bries et al. 2004),

was chosen as a contrast to Florida, where hurricanes

occur frequently an d are a regular source of acute physical destruction and mortality . Disease, bleaching, nutrient concentrations in the water column, and the abundance of herbivorous fish are, of course, also important factors affecting

population dynamics. However, the go al here was to determine if trends in population dynamics associated with hurricanes could be distinguished between the

6

two locations. In Curaçao , we expected rates of population

growth to be stable

( !

approximately equal to 1) and for the stable size dis tribution to comprise primarily large individuals, as the infrequency of hurricane should lead to relatively undisturbed colony growth (assuming ideal background conditions) and relatively little increase in population size due to colony fragmentation . Jam aica was chosen to represent a population in the process of recovery based on the return of a key herbivore, the sea urchin Diadema antillarum , and anecdotal reports on the emergence of young A. palmata colonies. In this location, we anticipated rate of po pulation expansion to exceed rates in Florida and Curaçao. Analyses of population dynamics were contextualized with information from three additional locations: Navassa, Puerto Rico, and Virgin Gorda in the British Virgin Islands. In these supplementary lo cations, demographic data were collected using analogous methods, but only two time points were collected in each location.

Objectives:

4.1 To determine if A. palmata dynamics differ significantly across space, as anecdotal evidence suggests.

4.2 To d etermine the spatial scale of any differences in dynamics, and if any pattern can be discerned.

4.3 To determine if rates of population expansion or depletion ( ! ) correlate with hurricane frequency or urchin density.

7

REFERENCES

Acropora

Biological Review Team. 2005. Atlantic Acropora

Status Review Document. Report to National Marine Fisheries Service, Southeast Regional Office. March 3, 2005. 152 pp + App.

Bries J. M., A. O. Debrot, and D. L. Meyer. 2004. Damage to the leeward reefs of Curaçao and Bonaire, Netherlands Antilles from a rare storm event: Hurricane Lenny, November 1999. Coral Reefs 23:297 – 307.

Hughes T. P.

1984. Population dynamics based on individual size rather than age: a general model with a reef coral example. American Naturalist 123:778 – 795.

Hughes T. P., and

J. H. Connell. 1987. Population dynamics based on size or age? A reef - coral analysis. Am erican Naturalist 129:818 – 829.

Hughes T .P , and J. Tanner . 2000. Recruitment failure, life histories, and long - term decline of Caribbean corals. Ecology 81:2250 – 2263.

Coates, A., and J.B.C. Jackson. 1985. Morphological themes in marine invertebrates in

J.B.C. Jac kson, L. Buss, and R. Cook (Eds.). Population biology and evolution of clonal organisms. Yale Univ ersity

Press, New Haven. 530pp.

Soong, K., and J. Lang. 1992. Reproductive integration in reef corals. Biological Bulletin 183:418 – 431 .

8

CHAPTER 2: Short - term popula tion predictions of threatened elkhorn c oral in the northern Florida Keys using stochastic matrix modeling

9

ABSTRACT

Caribbean e lkhorn coral, Acropora palmata (Lamarck, 1816), was once so widespread and abundant that geologists use its fossils to measure sea level from the Pleistocene through the Holocene. Now it exists a t

a small fraction of its former abundance

and is

listed as t hreatened,

along with its si ster

species , Acropora cervicornis , under the U.S. Endangered Species Act . We conducted annual demographic surveys on the northern Florida Keys population from 2004 - 2010. Percent cover of the benthos, numbers of colonies , and do minance by large individuals declined throughout the study period. We created population matrix models for each annual interval

of the study,

which included a severe hurricane year ( 2005 - 200 6 ) . Hurricane recurrence was simulated stochastically along with m ultiple outplanting scenarios. Further population depletion is predicted given a return time for severe hurricanes of 20 years or fewer. T he largest individuals were shown to have the greatest contribution to rate of change in population size via elasticit y analysis. Active management through o utplanting

can provide

a positive population trajectory over the short

term, especially if larger colonies are transplanted onto the reef. However, the former abundance

of this species suggests that life history traits, specifically rates of growth versus shrinkage measured herein ,

are different from what they must have been in the past. Ultimately, recovery of this species will depend on enacting local short - term manage ment solutions while improving regional and global environmental conditions.

10

INTRODUCTION

Corals are a recent addition to endangered species lists, and the pace of their inclusion is unprecedented. The Atlantic elkhorn coral , A. palmata (Lamarck, 1816),

was classified as threatened on the U.S. Endangered Species List along with i t s congener, Acropora cervicornis

(Lamarck, 1816) , in 2006. Currently, 82

additional corals are considered candidate species (Federal Register 2010). At the international level, the trajectory is similar. In 2008, corals

were included on the International Union for Conservation of Nature's Red List for the first time , and

33% of all reef - building corals with sufficient data were listed as threatened (Vulnerable , Endangered, and Cr itically Endangered) , a percentage that surpasses that of most terrestrial animals ( IUCN 2011, Carpenter et al. 2008).

Like many listed corals, Acropora palmata was

neither historically rare nor tightly restricted in its geographic range

(Gore au 1959, Gei ster 1977, Adey 1978 ) . Though there are unexplained gaps in the fossil record (Hubbard et al .

2008), A. palmata 's persistence, resilience, dominance, and sheer abundance throughout time and space qualified it as the dominant shallow - water

reef builder in t he Caribbean from the late Pleistocene through at least the early Holocene (Adey et al. 1977, Jackson 1992, Pandolfi and Jackson 2001, Hubbard et al. 2005, Pandolfi and Jackson 2006, Hubbard et al. 2008). During this time A. palmata

was dominant on 80% of shallow reefs surveyed throughout the Caribbean and Florida, often forming a monoculture along reef crests and upper reef slopes (Jackson 1992). This percentage

11

dropped to 40% by 1983 and to less than 20% by 1990

(Jackson et al. 2001). Although A. palmata is still present throughout its range (La ng 2003), ecological data reveal

a continuing decline in abundance. As of 2005, most populations were at 2 - 20% of 1970s baselines (Bruckner and Hourigan 2000, Carpenter et al. 2008).

The Atlantic Acropora Status Re view lists the following stressors to A. palmata : disease, temperature anomalies and bleaching, natural and anthropogenic branch breakage, competition, predation, excessive sedimentation and nutrification, boring sponges, toxic compounds in the water column, loss of genetic diversity, and others ( Acropora Biological Review Team 2005). Lists, however, are a deceptively simple presentation of environmental stressors, as feedback loops and synergies lurk between the commas

(Kline et al. 2006) . Further, li sts present a snapshot of a dynamic system in which threats can intensify as population abundance declines. For example, storms can cause direct physical damage and inflict longer - term damage by fragmenting large colonies into smaller colonies that have hi gher rates of mortality (Lirman 2003, Williams et al. 2006). Multiple storms can result in the decrease of asexual recruitment via fragmentation (Williams et al. 2008). Also, density of corallivorous snails, particularly Coralliophila abbreviata , on Atlant ic acroporids can increase dramatically after hurricanes, impeding or preventing population recovery (Knowlton et al. 1990, Baums et al. 2003, del M ó naco et al. 2011).

M atrix population modeling can offer a glimpse of the immediate future of a population of Acropora palmata

colonies. Matrix model s use

demographic data collected over a time frame appropriate to an organism ’ s life history, often annual

12

(Caswell 2001) . Data are converted into a matrix of transition probabilities, delineating the likelihood of

growing from one life stage to another. Each year the number of individuals in each size class is multiplied by the transition matrix, resulting in a projection of the population size structure for the following year. Hughes (1984) developed a size - based matrix model (an adaptation of t he classic age - based matrix model ) for organisms with a clonal life history, where size is more important than age and where individuals can not only grow and die but also shrink

an d

fragment

( Figure 2 - 1 ).

Disturbance is a governing force in coral population structure ,

and during disturbance

events transition probabilities are characteristically different from those during background conditions (Hughes 1984, Fong and Glynn 1998, Edmunds 2010). In the Caribbean, storms and hu rricanes are the major physical disturbance event s

on coral reefs (Gardner

et al.

2005) and are thus a critical component for any coral population model. Though Acropora

palmata is dependent on some level of wave action for asexual reproduction (Highsmith et al. 1980) and sloughing off sediment (Rogers 1983, Acevedo et al. 1989 ), populations exhibit higher rates of fragmentation (Lirman 2003) and can

suffer extreme damage from severe storms (Woodley 1981, Lirman and Fong 1997).

Building on past coral population models, including Lirman's model for Acropora

palmata ( Lirman 2003), we created a stochastic size - based matrix model with disturbance for the A. palmata population of the upper Florida Keys. We collected demographic data f or seven consecutive years, from 2004

to 2010, from

13

which we estimated annual transition rates for six annual intervals. With this rich data set we simulated a stochastic environment by multiplying the population size structure each year by a random draw f rom the six matrices, thereby avoiding certain extinction or certain population expansion ( which arises from deterministic matrix modeling ) . During our study, the population experienced severe hurricane conditions in 2005 ;

thus ,

one snapshot of these distu rbance dynamics was captured. Mild storms occurred in the winter of 2004 and summer of 2008 and were considered background conditions ,

along with 2006, 2007, and 2009. We used our population model to determine critical life history stages, predict future p opulation abundance, explore management actions, and provide a realistic time frame for recovery planning for this population.

Full document contains 161 pages
Abstract: Fossil data from multiple locations indicates that Atlantic elkhorn coral, Acropora palmata, formed shallow reefs throughout the Caribbean Sea since the Pleistocene. Beginning in the 1980s A. palmata has declined to a small fraction of its formerly vast extent throughout the region. In 2006, elkhorn coral was the first coral, along with its sister species, staghorn coral (Acropora cervicornis ), to be included on the U.S. Endangered Species List. We used size-based matrix modeling to parameterize annual A. palmata population dynamics in Florida, over the course of one severe hurricane year (2005) and six calm years (2004, and 2006-2010), incorporating environmental stochasticity as inter-annual variability. We predicted that benthic cover would remain at current levels (4%) for the foreseeable future (until 2030) and beyond (until 2100), suggesting a lack of resilience following the 2005 hurricanes. Standard metrics for the quantification of number and size of individuals are essential to endangered species management. These usually straightforward tasks can be challenging for clonal, colonial organisms. Acropora palmata presents a particular challenge due to its plastic morphology and frequent fission. We quantified three-dimensional colony surface area (CSA), the most ecologically relevant measure of size, for 14 prototypically arborescent A. palmata colonies using three-dimensional digital imaging software. To relate CSA to simple field metrics, we compared loglikelihood values and determined that planar projection was the best predictor. The, tight, linear relationship between planar projection and CSA enables ecological rates, such as reef accretion and gamete production, to be calculated from field data. Finally, we expanded the matrix population model to compare population dynamics in several locations across the Caribbean. The general trend for Acropora palmata is further reductions in population size by 2030. The most striking difference we quantified was between Jamaica, where population size is projected to increase, and all other locations, where population size is projected to remain stable or decline. Density of a key herbivore, the sea urchin Diadema antillarum, was an order of magnitude greater in Jamaica than in any other location. These increases are occurring 30 years after a devastating die-off suggesting that herbivory by urchins may facilitate A. palmata recovery.