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Stable isotope analyses reveal impacts of resource availability and interspecies competition on body sizes of California Channel Islands deer mice Joy Zhang

Under the supervision of Dr. V. Louise Roth and Dr. John Mercer, Department of Biology, Duke University

May 2018

______Research Supervisors

______Faculty Reader

______Director of Undergraduate Studies

Honors thesis submitted in partial fulfillment of the requirements for graduation with Distinction in Biology in Trinity College of Duke University

Abstract

Island rodent populations have challenged Foster’s rule for insular mammal body size with inconsistent size patterns when compared to mainland populations. Many factors have been implicated in models of island rodent size changes including island area, climate, predation, and competition with other species. Connecting these factors is their influence on resource availability and how rodents preferentially consume different amounts of macromolecules such as carbohydrates and proteins. I studied rodent diet using stable isotope analysis, in the absence of sampling stomach contents or surveying the flora and fauna in the area where the rodents were trapped. Utilizing stable isotope analysis, I examined carbon (13C) and nitrogen (15N) stable isotopes in deer mice (Peromyscus maniculatus) and black rats’ (Rattus rattus) hair collected from recent and historical samples captured on the California Channel Islands. I hypothesized that larger deer mice body size would correlate with greater protein consumption and higher 13C and 15N concentrations. Additionally, I hypothesized that significant variation in deer mice body size and 15N values between islands would be explained by differences in resource availability on islands with or without nesting seabirds and the presence or absence of other rodent species that compete with deer mice for resources. While 13C did not reliably predict the origin of rodent diet components, body mass, body length, and 15N concentration appeared to correlate with availability of protein from seabird materials (eggs and hatchlings) and the absence of competing rodent species. In interpreting the significant differences in body mass and 15N concentration for deer mice on islands with and without seabirds, I considered El Niño Southern Oscillation (ENSO) weather effects on seabird reproductive behavior and species distribution since the deer mice were collected in different years. In the comparison of Anacapa Island deer mice before and after rat eradication, it is possible that artificial selection of larger Anacapa Island deer mice occurred due to the trapping and re-release of a small population of deer mice on Anacapa Island during black rat eradication.

Introduction

Animal Body Size Variation

As children, we often first learn about the diversity of body size among animals with a comparison of how small mice are compared to elephants. While body size variation is easily observable among small mice and large elephants, why exactly body size varies among species and within subpopulations of one species is far more difficult to explain. Furthermore, body size variation has long been correlated with trends in many animal characteristics such as metabolism

(Kleiber, 1932), resource use (Calder, 2001) and lifespan (Speakman, 2005). Despite the well- studied relationships between body size and physiological and ecological characteristics of various species (Kaustuv, 2008), patterns of change in body size have proven more difficult to describe. Many biological rules have been proposed to describe trends in body size temporally, such as Cope’s Rule, which postulates that species tend to increase in size over evolutionary time

(Stanley, 1973), or geographically, such as Bergmann’s Rule, which proposes that animal body size increases with increasing latitude (Huston & Wolverton, 2011).

Like Bergmann’s Rule, Foster’s Rule also identifies a broad geographical trend in animal body size. Foster’s Rule states that island mammal populations show size differences compared to mainland ones. J. Bristol Foster concluded that smaller mammals tend to evolve larger body size on islands while larger mammals tend to evolve smaller body size on islands. Foster originally established these trends within a few taxa. Insular rodents displayed gigantism when compared to mainland populations and insular lagomorphs, carnivores, and artiodactyls displayed dwarfism (Foster, 1964). Researchers have hypothesized that factors such as relaxed predation threat on islands and island area influence the direction of size change for different taxa (Meiri et al., 2008). Since Foster’s Rule was proposed, as many exceptions as adherents to

1 this rule have been characterized (Meiri et al., 2006). Some publications have examined Foster’s original findings in greater detail describing how the absence of competitors or differences in prey size are correlated with apparent insular body size change (Case, 1978). Others have focused on trying to offer explanations for why some groups of organisms tend to be exceptions using quantitative measures such as degree of divergence in size (Lomolino, 1985)

Among the species used by Foster to establish insular body size trends, island rodent populations were consistently larger than their mainland counterparts (Foster, 1964). Subsequent studies with deer mice (Peromyscus maniculatus) on the Gulf Islands of British Columbia

(Redfield, 1976) and woodrats (Neotoma) in Baja California, Mexico (Smith, 1992) supported

Foster’s Rule. Other studies examining the tri-colored squirrel (Callosciurus prevosti) in

Southeast Asia (Heaney, 1978) and Hubert's multimammate mouse (Mastomys huberti) in

Senegal (Ganem et al., 1995) did not.

With studies providing evidence of island rodent populations having both larger and smaller body size than their mainland counterparts, other studies in the literature have tried to identify and model the factors that could explain why some rodent populations seem to adhere to

Foster’s Rule and why others do not. Researchers have turned to statistical models such as decision-tree models, a method of analysis that allows layering of multiple predictor variables to predict the direction of a particularresponse (in this context, increase or decrease in insular rodent body size) (Durst & Roth, 2015). For insular body size, ecological and environmental factors including precipitation, climate, island area, predation, population, diet and size of competitor species were ranked by their degree of influence in predicting the direction of body size (Durst & Roth, 2012).

Critically, the modeling performed by these two recent studies was broadly comparative for a range of rodent species and predictive of general trends in rodent body size. In my research,

I focused on California Channel island subpopulations of deer mice to see if diet composition based on resource availability within general feeding behaviors influenced body size. My stable isotope analysis seeks to take a closer look at the importance of varied consumption of resources such as protein from prey animals or carbohydrates from plant resources on island rodent species size. The natural and manmade environmental differences between individual California Channel

Islands provide a unique model for comparing resource availability. Understanding historical and modern human activity on each island is critical for interpretation of stable isotope analysis.

Anacapa Island, Santa Cruz Island, Santa Rosa Island, and Santa Barbara Island have experienced heavy ecological alteration due to past and current human presence with occupation dating back more than ten thousand years. In contrast, among the islands I studied mice from, human activity on San Miguel Island has been historically limited due to past naval explosive testing and harsh winds creating safety concerns for recreation and tourist activity (Chiles, 2015).

Additionally, stable isotope analysis of Anacapa Island deer mice living before and after

Anacapa Island’s eradication of black rats (Rattus rattus) in 2001 and 2002 illustrated how resource competition impacts body size and confounds Foster’s Rule (Howald et al., 2009).

In my study, I analyzed 13C and 15N stable isotopes in hair samples taken from adult female and male deer mice from five of the eight California Channel Islands (Anacapa Island,

Santa Cruz Island, Santa Rosa Island, Santa Barbara Island, and San Miguel Island) and mainland sites at Sedgwick Reserve and Grant Park. I expected that natural geographical differences between islands, such as higher grass coverage on less windy islands or an abundance of steep cliffs favored by nesting seabirds, will lend to differences in stable isotope

3 concentrations between samples due to differential resource access. For example, mice on islands with fewer steep cliffs might consume fewer “seabird resources”, which include seabird hatchlings and eggs. A comparison of stable isotope analysis characterizing dietary differences and body size metrics such as mass and body length clarified the relationship between varied resource access due to environmental differences and rodent body size. Inter-island environmental differences could explain why some island rodent populations follow Foster’s

Rule and other groups do not.

Biologists study the dietary patterns and resource use of individual animals by analyzing

13C and 15N stable isotope signatures from body tissue samples (Meier-Augenstein & Kemp,

2012). In the literature, animal tissue stable isotope data is typically supplemented with a survey and stable isotope analysis of local flora and fauna that these animals may consume. The comparison of animal tissue and ecological reservoir data may show patterns indicating what the animals ate. The mice I used for my study, however, were not originally collected for this dietary analysis and body size comparison. Therefore, an extensive survey of local flora and fauna as rodent food sources was not performed. Without this information, it was only possible to have broad comparative insight into the type of resources consumed by deer mice, such as if certain island subpopulations were consuming more protein as opposed to carbohydrates. To formulate ideas about what other organisms or plants California Channel Islands deer mice were consuming, 13C and 15N data from literature utilizing stable isotope analysis to study other rodents’ diets were used to approximate potential food sources (Harper, 2006; Hobson et al.,

1999; Millus & Stapp, 2008; Stapp & Polis, 2003; Stapp et al., 1999). Since previous studies have found general trends within 13C and 15N signatures based on the nitrogen-fixing and carbon processing of various plants and animals (Meier-Augenstein & Kemp, 2012; Peterson & Fry,

1987), I made the assumption that flora and fauna from different locations throughout the world would display relatively similar 13C and 15N values to comparable plants and animals on the

California Channel Islands.

Stable Isotope Analysis Background

Researchers use stable isotope analysis to uncover both process and source information.

Process information from stable isotopes distributions comes from the physical and chemical reactions that cause fractionation and the release and exchange of elemental isotopes into the environment (Meier-Augenstein & Kemp, 2012). Fractionation is defined as the separation of isotopes from each other through various chemical, physical, and biological processes such as consumption of other organisms, photosynthesis, and geological cycles. In studies where certain processes such as decomposition, diffusion of carbon dioxide into plant leaves, or water evaporation are not the research focus, they can be treated as fractionation noise and mathematically adjusted for by using values established by previous researchers (Ben-David &

Flaherty, 2012b; Peterson & Fry, 1987). Using literature data to remove isotope fractionation noise, biologists utilize stable isotope analysis to study ecosystem trophic levels and diets of regional animal populations.

Common elements studied using stable isotope analysis are carbon, nitrogen, sulfur, hydrogen, and oxygen. Using mass spectrometry to determine the concentration of non-decaying isotopes, researchers can identify certain characteristics of an organism based on the nature of elements being examined. My study focused on 13C and 15N ratios because they reflect specific aspects of dietary process and source information that were crucial for understanding rodent resource use.

Previous studies (e.g. Fry and Peterson 1987, Stapp and Polis 2003) have utilized 13C concentration in various body tissues to study diet diversity in various animals. For example, 13C concentration is especially high in marine carbonate sediments, compared to terrestrial sediments, due to the exchange of CO2 between the atmosphere and ocean surface (Meier-

Augenstein & Kemp, 2012). This high 13C concentration is propagated through higher trophic levels. Plankton extract and greatly fractionate inorganic carbon in the form of CO2 dissolved in the water during photosynthesis (Peterson & Fry, 1987). These plankton are then consumed by marine organisms. In turn, these marine organisms will be consumed by other marine organisms or terrestrial animals close to marine food sources. The consumption of marine food sources is reflected within animal tissue as elevated 13C levels. This phenomenon has been considered in studies such as Stapp and Polis’ paper which examined to what extent various rodent populations in the Gulf of California and Mexico supplemented their diet with coastal food sources such as seabird eggs. Utilizing carbon stable isotope analysis data, they showed that rodents closer to the coast had higher 13C concentrations than in-land populations and therefore, ate more from marine food sources (Stapp & Polis, 2003).

Nitrogen identifies both dietary composition sources and trophic levels based on protein consumption. Previous studies have shown that 15N increases around 3.4% with each trophic level (Minagawa & Wada, 1984). Therefore, researchers can identify the relative trophic levels

15 15 of animals in an ecosystem using N values. Most of the stable N isotope originates from N2 gas in the atmosphere and a small portion originates from decomposed material in the soil. In terrestrial ecological systems, 15N initially enters food webs through animal consumption of plants that have fixed nitrogen through the nitrogen cycle. In the intertidal and aquatic systems, nitrate serves as the source of nitrogen taken up by plankton. In previous literature, nitrogen has

6 been primarily used to establish trophic level ordering because it consistently increases with increasing trophic level (Adams & Sterner, 2000).

Along with trophic level, other studies have utilized nitrogen to indicate the source of dietary materials because 15N fractionation levels from marine sources are higher than terrestrial ones (Peterson & Fry, 1987). In Paul Stapp and Gary Polis’ 2003 study, 15N stable isotope data was used to establish the trophic level of coastal and in-land populations of the mice. Stapp and

Polis caught more deer mice near island coasts rather than further inland. The mice caught along the coasts of small islands with abundant marine resources had higher 15N values than those caught inland. These marine resources were derived from roosting seabirds consuming fish and supported higher densities of deer mice despite land area limitations of islands that cap terrestrial productivity and resource availability. In my investigation of insular rodents, I predicted that elevated 15N values would indicate increased marine resource consumption by California

Channel Islands deer mice as Stapp and Polis did in their study of insular rodents in the Gulf of

California (Stapp & Polis, 2003). This increase in 15N would occur both directly through the consumption of seabird material such as eggs and hatchlings as well as through propagation of additional nitrogen through trophic levels from seabird guano fertilizing island plants and causing overall elevation 15N values for island flora and fauna (Millus & Stapp, 2008).

Based on the uses of 13C and 15N stable isotope values outlined in previous studies, I have several specific hypotheses for what my stable isotope data could indicate for the sampled

Channel Islands mice. I based these predictions on the fact that 13C ratios may reflect the incorporation of aquatic food sources in the deer mice’s diets and that 15N ratio data may reflect whether mice subpopulations consumed more plant or animal matter (primarily seabird material, for deer mice on the California Channel Islands).

In my study, increased 13C and 15N values indicates greater protein consumption as seabird resources are the primary source of protein for deer mice on the California Channel

Islands. Consumption of these seabird resources would raise deer mice’s 15N values in comparison to deer mice that do not consume seabird resources. Elevated protein ingestion due to availability of seabird resources indicated by elevated 13C and 15N values in stable isotope value could contribute to larger rodent body size. Reduced 13C and 15N values would show decreased protein consumption. If correlated with smaller average body size, lower 13C and 15N values could indicate lower seabird egg and hatchling density on a specific California Channel island or the presence competition for resources, in the case of Anacapa Island before the eradication of black rats.

Materials and Methods

Deer Mice Collection

Ninety-six deer mice (Peromyscus maniculatus) were collected, weighed, and measured by Dr. Paul Durst at eight different collection sites spread between five California Channel

Islands (Anacapa Island, Santa Cruz Island, Santa Rosa Island, San Miguel Island, and Santa

Barbara Island) and two mainland California locations (Grant Park and Sedgewick Reserve)

(Figure 1, Table 1). Sample collection took place over two years and specimens were kept frozen before sampling in 2014 and 2015. Santa Cruz Island had one collection location on the eastern portion of the island and one on the western portion. All eastern Santa Cruz Island deer mice and two western Santa Cruz Island deer mice were captured in March 2010. Santa Barbara Island deer mice and the majority of Sedgwick Reserve samples were captured in July 2010. San

Miguel Island deer mice were caught in June 2011. Deer mice from Anacapa Island, Santa Rosa

Island, and four additional Sedgwick Reserve deer mice were caught in July 2011. Grant Park deer mice and western Santa Cruz Island deer mice were collected in May 2012.

Figure 1. Map of all deer mice collection sites plotted using geographic coordinates provided by Dr. Durst. Site Latitude Longitude Anacapa Island 34.015658 -119.363365 Santa Cruz Island (East) 34.048428 -119.562102 Santa Cruz Island (West) 33.995853 -119.718275 Santa Rosa Island 33.999626 -120.062610 San Miguel Island 34.037581 -120.342599 Santa Barbara Island 33.480387 -119.030643 Sedgwick Reserve 34.692658 -120.042181 Grant Park 34.289878 -119.287559

Table 1. Coordinates for each deer mice collection location. To facilitate comparison between deer mice living on Anacapa Island before and after black rat (Rattus rattus) eradication as a case study for the presence and absence of competitor rodent species, stable isotope analysis was also performed on hair taken from older samples. Ten deer mouse and nine rat pelts collected on Anacapa Island in August 1940 by Dr. Jack Christian von Bloeker were requested from the Natural History Museum of Los Angeles County (LACM).

These study skins were not frozen, unlike the whole deer mice collected by Dr. Durst.

Hair Sample Processing and Analysis

After removal from a -80°C freezer, deer mice were thawed slightly before hair sample collection. An approximately roughly 3.25 cm by 1.25 cm (weighing approximately 0.25 gram) square of fur was removed from the side of each mouse between its front and back legs using standard laboratory scissors. If hair was inaccessible due to freezing of deer mouse specimens, additional fur was taken on different sites along the body to increase the weight of the cut hair to around 0.25 grams, an amount that buffered hair sample loss during the washing and drying process. Scissors were washed with water and wiped clean to prevent sample cross- contamination.

Once collected, hair samples were then washed using a 2:1 chloroform:methanol solution to clean off possible contaminants such as dust. Once the solution was added, aliquots were capped and then vortexed using a bench vortex mixer. Samples were then removed from their tubes and placed onto a vacuum filter and water was passed through to remove the chloroform:methanol solution (Ben-David & Flaherty, 2012a). The samples were then placed in a 38°C oven for an hour with their tubes left uncapped to completely dry them.

Stable isotope analysis was performed by the Duke Environmental Stable Isotope

Laboratory (DEVIL) at Duke University’s Nicholas School of the Environment Earth and Ocean

Sciences Division. After drying, 1.00 milligrams (+/- 0.1mg) of hair for each sample were loaded into standard stable isotope tin capsules based on techniques described by Merav Ben-David and

Elizabeth Flaherty (Ben-David & Flaherty, 2012a) and DEVIL’s guidelines for sample preparation1. To ensure sample loading tin capsules were not ripped by long hairs, hair samples were cut into smaller (approximately 1 mm) segments with standard laboratory scissors. The scissors were wiped down between each sample trimming with Kimwipes soaked in methanol.

After rodent hair was placed within the tin capsules, the tin capsules were flattened, folded, and sealed using offset tweezers. Individual capsules were then placed in a 96-well plate and this plate was then sent to DEVIL for analysis.

Sample Analysis and Data Collection

DEVIL sample processing returned a .csv file with 13C and 15N values for each rodent tagged by sample identification codes I provided. Within each stable isotope analysis run for deer mice collected by Dr. Durst and the Anacapa Island deer mice and rats collected by Dr. von

Bloeker Jr., internal standards were created by duplicating several samples. These were used to ensure that sample combustion within the mass spectrometer occurred consistently across all of the samples within one 96-well plate (Ben-David & Flaherty, 2012a). Values for duplicated samples were either the same or very close.

Data Analysis

Mathematica, Excel, and RStudio were used to plot and analyze data. Excel was used to create plots comparing mice weight and 15N ratios as well as 13C and 15N ratios for my samples.

Built-in default RStudio packages (stats and graphics) were used to create box plots comparing the range of 13C and 15N values for mice on each of the islands and to visualize outliers. A post-

1 https://nicholas.duke.edu/devil/sampleprep.html 11 hoc Tukey Honestly Significant Difference (HSD) test was performed utilizing an online statistical calculator.2 A Tukey HSD test and Wilcoxon-Mann-Whitney null hypothesis tests were utilized in quantitative analysis of stable isotope results because results were not normally distributed.

In addition to the 15N and 13C stable isotope ratios acquired for the samples directly processed with the help of DEVIL at Duke University, data from several other stable isotope studies was also integrated to provide reference values for food sources in various categories such as C3 plants, C4/CAM plants, intertidal animals, terrestrial animals, and bird material (eggs and hatchlings). Stable isotope ratios for different rodent species such as voles, Keen’s mice, and

Norway rats were also included in the hopes of identifying trends between California Channel

Islands rodents and rodents throughout the world (Harper, 2006; Hobson et al., 1999; Millus &

Stapp, 2008; Stapp & Polis, 2003; Stapp et al., 1999). Data points from graphs found in previous stable isotope studies were digitized using Mathematica functions and graphed alongside my data

(averages and individual data points).

Designation of California Channel Islands as “with seabirds” and “without seabirds” for

Wilcoxon-Mann-Whitney analysis was done using two sources of information. Dr. Durst’s field observations included notes documenting the abundance of seabird rookeries. Information available through data collected by the U.S. National Park Service supplemented Dr. Durst’s observations (Collins, 2011).

Results

Preliminary analysis explored the idea of utilizing 13C and 15N isotope data gathered in other publications as stand-ins for food source data and comparison to other rodents. Data points

2 http://astatsa.com/aOneWay_Anova_with_TukeyHSD/ 12 from previous studies (Harper, 2006; Hobson et al., 1999; Millus & Stapp, 2008; Stapp & Polis,

2003; Stapp et al., 1999) were classified into six categories: C3 plants, C4/CAM plants, birds, rodents, terrestrial food source, and intertidal food source. Publications’ individual 13C and 15N ratio values for each resource category were averaged and plotted against average 13C and 15N ratios for the mice I sampled.

Based on observation of the plotted data, there was substantial overlap of average values for California Channel Island deer mice and various food source averages. This overlap made it impossible to link 13C and 15N values for specific food sources from other publications to values for California Channel Island deer mice (Figure 2). None of the California Channel Island deer mouse subpopulations were eating one specific food source. Overlap of California Channel

Island deer mice’s 13C and 15N values with averages for multiple different food sources aligns with qualitative observations that California Channel Island deer mice, like other rodents, are omnivorous (Landry, 1970).

Santa Barbara

Anacapa

Santa Cruz San Miguel Santa Rosa Old Anacapa Sedgwick Grant Park Santa Cruz West

Figure 2. This figure shows average 13C and 15N stable isotope ratios used in our study plotted with average 13C and 15N values taken from previous publications. Data collected from our samples are designated by black stars labelled with collection location names. Values taken from other publications were categorized to demonstrate the use of 13C and 15N to trace trophic levels and dietary components of rodents through comparison to food sources (C3 plants (n=15), C4/CAM plants (n=7), terrestrial food sources (n=20), intertidal food sources (n=17), birds (n=9)) and other rodents (n=33).

To investigate the correlation between protein consumption and body mass, body masses of the sampled California Channel Islands’ mice were plotted with 15N ratio values (Figure 3).

The correlation between deer mice 15N values and body size was not significant (R2 = 0.0838, p=0.10).

Body mass correlation with Nitrogen ratio 30.0

25.0

20.0

15.0

N 15

10.0

5.0

0.0 0 5 10 15 20 25 30 35 Body Mass (g)

Anacapa Santa Barbara San Miguel Santa Rosa Santa Cruz East Santa Cruz West Grant Park Sedgwick

Figure 3. This figure shows all individual mass values plotted with nitrogen ratio values. Mice mass did not significantly predict 15N ratios with no significant trend tracing increasing/decreasing mass to increasing/decreasing mass ratios (R2 = 0.0838, p=0.10). A variable number of mice were sampled from each California Channel Islands and mainland collection site: Anacapa (n=8), Santa Barbara (n=15), San Miguel (n=8), Santa Rosa (n=13), Santa Cruz East (n=13), Santa Cruz West (n=19), and Grant Park (n=8), Sedgwick (n=12).

Anacapa Island and Santa Barbara Island were designated as islands with seabirds due to their abundant seabird rookeries and substantial documented presence of nesting seabirds. Santa

Cruz Island, San Miguel Island, and Santa Rosa Island, were the islands without seabirds. This designation was reflected in the clustering of Santa Barbara Island and Anacapa Island deer mice’s 15N values (Figure 3). This prompted greater investigation of the effect of seabird presence and protein availability on mice body size and 15N values. A mass comparison of mice on islands with and without seabird populations was performed using a Wilcoxon-Mann-

Whitney non-parametric test for significance. Wilcoxon-Mann-Whitney tests were used because

15 they allow for comparison of two non-normal data sets while two-sample t-tests assumes normal data distribution. Mass of mice on islands with seabirds and without were significantly different, according to a Wilcoxon-Mann-Whitney test (p=0.0086, Figure 4). Higher availability of protein-rich seabird resources may significantly increase mice size.

Mice mass comparison between California Channel Islands with and without seabirds

Islands with seabirds Islands without seabirds

Figure 4. This figure illustrates the distribution of mass values for California Channel Islands deer mice on islands with (n=23) and without (n=73) seabirds.

A Wilcoxon-Mann-Whitney non-parametric test also determined that 15N values for mice on islands with seabirds and islands without seabirds were significantly different (p = 5.70e-12,

Figure 5). This suggests that the presence and abundance of seabirds is correlated with different amounts of protein consumed, between mice on islands with and without seabirds. Further data analysis was done to elucidate whether protein consumption differed purely due to resource availability or if competition was also a significant contributing factor.

Nitrogen ratio comparison between California Channel Islands with and without seabirds

Islands with seabirds Islands without seabirds

Figure 5. This box plot illustrates the distribution of 15N values for California Channel Islands deer mice on islands with (n=23) and without (n=73) seabirds.

After finding significant difference in 15N and mass for mice on islands with and without seabirds, I further differentiated California Channel Islands deer mice subpopulations based on the presence and absence of black rats on individual islands. Black rats represented rodent competitors. With the mice collected by Dr. Durst, three combinations of seabird presence/absence and competition presence/absence could be created from island grouping: islands with seabirds and with no other rodents, islands with no seabirds and with no other rodents (no black rats), islands with no seabirds and with an additional non-deer mouse rodent.

Three separate Wilcoxon-Mann-Whitney tests among all the different combinations of these groups did not yield significant differences for either 15N or mass values.

One combination of comparison, islands with both seabirds and with other rodents did not exist within the original samples studied. To perform a comparison of mice body sizes and

15N values due to the presence of both seabirds and competitive species, older mouse specimens from Anacapa Island dating to before the eradication of black rats on Anacapa Island were

17 acquired from LACM. Thus, these mice were present on Anacapa Island with both black rats and seabirds. Pre-eradication Anacapa Island presents the only comparative scenario where both seabirds and non-deer mice rodent were on a single California Channel Island.

Anacapa Island’s widescale black rat eradication in 2001 and 2002 allows for comparison of mice on an island with abundant seabird resources, in the presence and absence of competitor rodent species. These older samples did not have recorded weights but had body length measurements and the body length measurement excluding tail length was used as a point of comparison between modern and older Anacapa Island samples. A significant difference in body length was evaluated using a Wilcoxon-Mann-Whitney test (p=0.0013, Figure 6).

Body length comparison between modern Anacapa Island mice and historic (1940) Anacapa Island mice samples

Figure 6. This figure shows the distribution of body length values for new (n=8) and old (n=10) (1940) Anacapa Island mice. New refers to Anacapa Island mice captured after black rat eradication in 2001 and 2002. Old refers to Anacapa Island mice captured before black rat eradication.

15N ratios for new and old Anacapa Island mice hair samples were also significantly different, as demonstrated with a Wilcoxon-Mann-Whitney test (p=0.0039). This significant comparison implies differential protein consumption between historic and modern Anacapa

Island mice before and after the eradication of competitive invasive black rats. This 15N ratio comparison showed significance even with the two outliers amongst the 1940 Anacapa Island mice (Figure 7).

Nitrogen ratio comparison between modern Anacapa Island mice and historic (1940) Anacapa Island mice samples

Figure 7. This figure shows the distribution of 15N values for modern (n=8) and old (n=10) (1940) Anacapa Island mice. New refers to Anacapa Island mice captured after black rat eradication in 2001 and 2002. Old refers to Anacapa ISland mice captured before black rat eradication.

A Tukey Honest Significant Difference (HSD) test was used to further compare 15N values for mice on the different islands. Anacapa Island rats from 1940 were also compared with the rest of our samples’ 15N values (Figure 8, Table 2). Anacapa Island mice’s 15N values were significantly different from all other sample groups. Santa Barbara, the other island with a substantial seabird population, was also significantly different from all other island and historic sample groups (Table 2). Anacapa Island mice from the 1940s, however, did not have significantly different 15N values from modern Santa Cruz Island, San Miguel Island, and Santa

Rosa Island deer mice. In contrast, Anacapa Island rats from the 1940s showed significant differences in 15N values from Santa Barbara Island and Santa Cruz Island deer mice (Table 2).

15N Values of all Mice and Rats

15N

Santa Sedgwick Anacapa Grant Santa San Santa Old Old Park Barbara Cruz Miguel Rosa Anacapa Anacapa Island mice Island rats

Figure 8. This figure illustrates the distribution of 15N values for all sample categories. Anacapa (n=8), Grant Park (n=8), Santa Barbara (n=15), Santa Cruz (n=32), Sedgwick (n=12), San Miguel (n=8), Santa Rosa (n=13), Old Anacapa Island mice (n=10), and Old Anacapa rats (n=9).

Anacapa Grant Santa Santa Sedgwick San Santa Old Old Island Park Barbara Cruz Miguel Rosa Anacapa Anacapa Island Rat mice Anacapa 0.0010 0.0027 0.0010 0.0010 0.0010 0.0010 0.0010 0.0010

Grant Park 0.0010 0.79 0.90 0.39 0.46 0.019 0.0010

Santa 0.0010 0.0010 0.0010 0.0010 0.0010 0.0010 Barbara Santa Cruz 0.90 0.90 0.90 0.11 0.0020

Sedgwick 0.90 0.90 0.24 0.011

San Miguel 0.90 0.90 0.40

Santa Rosa 0.73 0.11

Old 0.90 Anacapa Mice Old Anacapa Rat

Table 2. P-value table for comparison of 15N values for all possible island pairs. P-values from a Tukey HSD test for significance are shown in the table. Values less than 0.01 are bolded. 0.01 was used as the cut-off for significance due to the testing of multiple sets of data groups, to reduce the likelihood of a false positive. Values less than 0.05 are underlined.

Discussion

The results of my stable isotope analysis show significant differences in 15N values and body size between various California Channel Islands deer mouse subpopulations. While this data presents promising results for further clarifying why Foster’s Rule for insular body size seems to apply for certain populations of insular rodents and not others, limitations of this analysis must be considered. Significant inter-island differences in mice body size and stable isotope values for deer mice collected at separate times could be explained by other factors.

These factors include climate cycles that affect resource availability in different years and varying extent of human activity on different islands. I examined the effect of the El Niño-

Southern Oscillation (ENSO) in considering the impact of climate cycles. Human activity on the

California Channel Islands can be broadly categorized to ranching, tourism, commercial-scale fishing, and military operations (Chiles, 2015). Climate cycles and the extent of human activity affect both the comparison of stable isotope data and body mass on islands with and without seabirds and the evaluation of stable isotope data and body length differences for Anacapa deer mice in the presence and absence of competitor rodents (black rats).

Based on Dr. Durst’s observations and data collected by the U.S. National Park Service

(Collins, 2011), Anacapa Island and Santa Barbara Island were classified as islands with seabirds. Santa Cruz Island, San Miguel Island, and Santa Rosa Island were designated as islands without seabirds. When these two groups of California Channel Islands were compared, significant difference was found between the 15N values and body mass of mice on islands with seabirds versus mice on islands without seabirds. ENSO weather effects and varying human activity on the different islands complicate definitively implicating the presence of seabirds as a contributing factor to increasing rodent body size on islands as Foster’s rule predicts will occur.

Beyond ENSO weather effects on regional ocean temperature and precipitation over land

(Appendix 1), most literature for El Niño effects on seabird populations typically focuses on the breeding patterns and offspring health of a regional seabird population during primary ENSO event years and years with residual anomalous water warming effects (Ribic et al., 1992).

Tropical seabird species’ behavior changes in relation to ENSO effects have been studied in more detail than other species in non-tropical areas such as Southern California. Cubaynes et al. tracked 11000 red-footed boobies and found that during El Niño years, red-footed boobies were more likely to skip breeding during their single breeding season in the face of environmental and resource limitations and first-time breeders also tended to be older during these years (Cubaynes et al., 2011). Black et al. found that reproductive behaviors of seabirds were also impacted during El Niño years with lay dates for Caussin auklets shifting earlier (Black et al., 2010).

Devney et al. showed that beyond broad El Niño effects on ecosystem productivity and lowering the availability of aquatic prey, seabirds’ inherent breeding dynamics are impacted. Changing water temperatures that preceded El Niño events affected seabird foraging and breeding due to their reliance on aquatic food sources. In contrast, in-shore tropical bird species were not as affected because they did not rely on aquatic food sources (Devney et al., 2009). Additionally, the regional distribution of seabird species also changes during El Niño and La Niña years

(Jaksic, 2004).

With the documented effects of El Niño and La Niña events on tropical seabirds, it is possible that California Channel Islands seabird species are also affected by ENSO. The exact effect of ENSO on the reproductive behavior and species distribution of seabirds on the

California Channel Islands is an area of future study. For the mice collected by Dr. Durst, mice from eastern Santa Cruz Island, Santa Barbara Island were collected during 2010, an El Niño

24 year, while mice from western Santa Cruz Island, Anacapa Island, and San Miguel Island were collected during 2011 and 2012, La Niña years. Thus, differences in ENSO ocean current behavior and atmospheric conditions could influence the changes in body mass and 15N values seen in my analysis because samples collected during years with different ENSO effects were grouped together to compare the effect of varied resource availability. The designation of islands with or without seabirds could be attributed to seabirds localizing to certain islands and not others during El Niño or La Niña years. For the comparison of deer mice body length and 15N values in the absence and presence of competitor rodents, Dr. von Bloeker’s deer mice were collected during an El Niño year while Dr. Durst’s deer mice were trapped during a La Niña year. Differential resource availability due to ENSO weather effects on seabird behavior and mice consumption of seabird resources could have played a part in the significantly larger body length and higher 15N values of the modern Anacapa deer mice.

La Niña

El Niño

Figure 11. This figure illustrates El Niño and La Niña events and the arrows indicate years in which the California Channel Islands deer mice used in my stable study were captured.3

Along with directly impacting seabird behavior, ENSO also affects the behavior of fish consumed by seabirds. Fish species are affected differently due to variation in their lifespans and reproductive strategies (Jaksic, 2004). Studies examining fish behavior have tended to focus on economically important fish species and their response to El Niño and La Niña events. Waluda et al. found that jumbo flying squid catches off the coast of Peru decreased during both El Niño and

La Niña years with abnormally warm and cool waters, respectively (Waluda et al., 2006). Kumar et al. studied the response of tuna in the Indian Ocean to El Niño and La Niña events and found decreased tuna catches during strong El Niño and La Niña years (Kumar et al., 2014). Within these studies of fluctuation in fish numbers during El Niño and La Niña years, however, it is

3Figure adapted and modified from Wikipedia: https://en.wikipedia.org/wiki/El_Ni%C3%B1o%E2%80%93Southern_Oscillation 26 important to note that large-scale industrial fishing is concurrently altering fish populations and species distribution. Fishing may skew the ability match flux in fish populations with flux in stable isotope and mass values of California Channel Islands deer mice.

The California Channel Islands’ rich and extensive history of fishing by both domestic and international groups underlies the importance of considering the effects of fishing in addition to ENSO weather events when studying organisms on the California Channel Islands. Different groups from early Native American populations to the Italian fishermen and Chinese squid-driers employed fishing strategies of varying scales that impacted populations at different levels

(McEvoy, 1986). The presence of diverse groups of fishermen targeting various forms of marine life affect seabird populations variably, based on whether the seabirds are omnivorous or exclusively consume one fish species (Anderson & Reyes, 2017).

Previous publications have struggled to disentangle the effects of El Niño events and commercial fishing in decreasing fish and seabird populations. Schoenherr et al. studied brown pelican and cormorant nesting in the California Channel Islands during select El Niño years:

1983, 1990, 1992, and 1998 (Schoenherr et al., 1999). During those years, these researchers found fewer nesting pairs but noted that sustained decline of these seabird populations over time could be indicative of overall declining fish populations due to commercial fishing (Schoenherr et al., 1999; Smith & Eppley, 1982) rather than rhythmic population fluctuations due to ENSO seasonal changes (Ribic et al., 1992).

Beyond speculation of how ENSO weather effects and commercial fishing indirectly affect California Channel Islands deer mice through changing the availability of seabird material for mice consumption, elucidating exact seabird resource consumption by the California Channel

Islands deer mice would require a comprehensive flora and fauna survey or stomach content

27 analysis. An evaluation of flora and fauna could also indicate if land clearing and repurposing for ranching and farming and altered the studied California Channel Islands differently, creating different food sources for the deer mice. In particular, Santa Cruz Island experienced heavy alteration when the Caire family planted trees all over the island to “civilize” the land during the early 20th century (Chiles, 2015). Additionally, the presence of seabirds on islands may increase

15N values of California Channel Islands deer mice due to guano fertilizing plants on the islands

(Millus & Stapp, 2008). This effect would confound a significant increase in 15N values due to direct seabird material (egg and hatchling) consumption.

In my analysis of Anacapa Island deer mice from 1940 and those collected by Dr. Durst in 2011, I found that the body lengths and 15N values differed significantly. As with the different collection years for Dr. Durst’s deer mice, ENSO weather effects could have also influenced the comparison of body length and 15N values for Dr. von Bloeker’s deer mice. 1940 was recorded as a strong El Niño year while 2011 was a La Niña year (Figure 11).

In addition to ENSO weather effects possibly contributing to the variation in rodent size through seabird material consumption, the method of eradicating black rats from Anacapa Island further complicates the comparison of 15N values and body lengths. During the black rat eradication in 2001 and 2002, researchers utilized live trapping to capture deer mice. Captured deer mice were then held on an on-island captive-holding facility. This was performed to create a population of deer mice that was then released after the black rats were eradicated, since deer mice would also attempt to eat the rodenticide-poisoned bait (Howald et al., 2009). This procedure of capturing a re-release population of Anacapa Island deer mice could have created an artificial bottleneck (Nei et al., 1975) for larger mice that were not able to escape the bait traps placed by researchers. While body size measurements of Anacapa Island deer mice before

28 and after black rat eradication have not been taken (a possible future direction of study), genetic studies have found loss of certain alleles within the Anacapa Island deer mice before and after the black rat extermination (Ozer et al., 2011). With the possibility of artificial selection for larger mice, the idea of increased protein consumption from seabird materials leading to increasing body size may not be applicable.

ENSO weather effects and human activity raise questions about significant differences in

15N values and body masses of deer mice on California Channel Islands with and without seabirds and regarding significant differences in 15N values and body lengths of Anacapa Island deer mice caught before and after competitive black rats were removed from the island. Future exploration may separate the effects of ENSO from commercial fishing to help explain if flux in seabird populations affected my results or if greater protein availability from seabird resources correlated with greater deer mouse body size. The effects of the black rat eradication and artificial selection of deer mice on Anacapa present an interesting case study for body size changes in the absence of competitor species and in relation to an artificial genetic bottleneck.

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ENSO refers to the El Niño and La Niña current oscillation that is associated with changes in the ocean and the atmosphere. In years without El Niño or La Niña effects, westward winds blow warm water in the Pacific Ocean from the coast of South America towards the eastern coast of Australia. This allows for cool deep water upwelling off the coast of South

America as less dense warm water is displaced and denser cold water from the deep ocean rises.

This cold water is rich in carbon dioxide and this feeds large algal blooms that appear of the coast of South America. El Niño and La Niña occur irregularly (every 3 to 7 years) and La Niña follows occurrences of El Niño (IPCC, 2007; NOAA, 2016).

During El Niño years, westward winds that move warm water from the coast of South

America west die down. This prevents cool deep water upwelling off the coast of South America as well as distinct atmospheric effects throughout the world (NOAA, 2016) (Figure 9). In the La

Niña years that follow, westward winds over the western Pacific Ocean increase in intensity causing a bulge of warm water to form off the coast of eastern Australia. Cool deep water upwelling occurs off the coast of South America but the accumulation of warm water in the western Pacific creates different atmospheric conditions in regions throughout the world

(NOAA, 2016) (Figure 10).

Figure 9. This figure illustrates global atmospheric effects that occur during El Niño years.4

4 Figure adapted from NOAA website: https://www.climate.gov/news-features/featured-images/global-impacts-el- ni%C3%B1o-and-la-ni%C3%B1a 35

Figure 10. This figure illustrates global atmospheric effects that occur during La Niña years.5

With ENSO’s widescale effects on atmospheric conditions over land as well as ocean water conditions, El Niño and La Niña also affect animals such as the fish that consume

5 Figure adapted from NOAA website: https://www.climate.gov/news-features/featured-images/global-impacts-el- ni%C3%B1o-and-la-ni%C3%B1aa 36 organisms that eat algae in the ocean and the seabirds that feed on the fish. El Niño effects on seabirds have been better studied during more recent El Niño events since the 1980s. Older El

Niño and La Niña events, in the absence of reliable weather data, were verified using seabird nesting data. "Muttonbirding" diaries collected by the Rakiura Maori of New Zealand documenting the practice of harvesting seabird chicks on the shores of New Zealand were mapped to Southern Oscillation Index and Pacific Decadal Oscillation. When compared to weather data, La Niña events typically followed more successful muttonbirding seasons with a greater number of and relatively larger seabird chicks trapped. El Niño events tracked to less successful muttonbirding seasons with fewer and relatively smaller seabird chicks caught

(Humphries & Möller, 2017).

Acknowledgements

I would like to acknowledge and thank Dr. Mercer, Dr. Roth, and Dr. Durst for their contributions and all of their help with this project over these last four years. Thank you to Dr.

McShea for providing feedback during the thesis writing process. This project was made possible by National Science Foundation Department of Environmental Biology (NSFDEB) Award

#1311597. Additionally, thank you to the Los Angeles County Museum Thank you to the

Natural History Museum of Los Angeles County (LACM) for lending study skins, making the competition comparison possible. I would like to thank DEVIL for processing the stable isotope analysis samples and providing me with all the data I utilized for this project. Additionally, thank you to my Biology 495 class for providing peer review.