Marine Iguanas: The Effects of Modern Tourism*

(*): Student research from Scientific Gorlok, a publication from the Biological Sciences Department at the College of Arts & Sciences, Webster University. Download full May 2018 issue (PDF) here.

The Galápagos archipelago is a chain of isolated islands off the coast of Ecuador, home to a vast number of endemic species. This isolation means that some of the species evolved without the presence of a predator, or they lacked escape responses to potential threats (Berger et al, 2007). One such example of this is of the New Zealand Robin. In a study conducted by Maloney and McLean (1995), island robins (naïve) exhibited similar responses to a control and a simulated predator. Mainland robins (experienced), on the other hand, reacted strongly to a simulated predator, but weakly to the control. The lack of predator recognition demonstrated in this study can make for severe changes and complications with the introduction of predators. To combat this, the Galápagos National Park has set forth strict rules and regulations with aims to reduce human-wildlife interactions. However, human presence itself may be enough to create a profound negative impact upon wildlife. One such species is the Marine Iguana.

Introduction

Marine Iguanas are endemic to the Galápagos and inhabit every island, but our observations were limited to the populations on San Cristóbal and Española. They live in colonies along the rocky shores of the coast, and forage in intertidal waters for algae (Wikelski and Trillmich, 1994). During foraging, their body temperature decreases and is restored by periods of sunbathing on rocks along the shoreline. Marine Iguanas are known to be territorial and display aggressive territorial behavior, particularly among males. Headbobs are used to deter others from invading their space or rock. These areas of foraging and sunbathing often overlap with beaches frequently visited by humans. Does the increased human presence affect the behavior of the iguanas? If so, is the effect limited to their foraging habits, or is it present in their territorial behaviors?

Aside from human presence, the Marine Iguanas also face introduced predators, such as feral dogs and cats, and an invasive species of rats. There have been accounts of feral dogs feeding on iguanas on Floreana and San Cristóbal since the late 1830’s- coinciding with the first settlement of humans (Barnett and Rudd, 1983). Because predators are introduced by humans, we assume that there is a positive correlation between their population size and human population size. According to data collected by the National Galápagos Park, the Petrel population on Floreana dramatically decreased because of rat predation in 1981, and this was seen again in 1996 on a different island.

The predation risk allocation hypothesis as presented by Lima and Bednekoff (1999) gives more insight to the question at hand. This model predicts animal behavior and foraging in relation to perceived predatory dangers. It includes factors such as foraging efforts, energy, and time. Human presence and predation are the perceived dangers. The hypothesis implies these perceived threats would create a higher sense of risk, leading to decreased time foraging and time spent regulating temperature. It is reasonable to suggest that the iguanas will move away from the areas of high risk to low risk, so more time can be spent foraging and sunbathing. If this does occur, how would the interactions within the population be affected? Would a change in density create a difference in territorial aggressiveness?

This brings us to our question of interest: how has the increased human presence and increased predation affected the interactions of Marine Iguanas within a population? To answer this, we collected observational data on two islands that have drastically different human presence (our proxy for predator density). The observation sites of our study are in close proximity with, and even have some overlay of beaches that are frequently populated by humans. Following the background research and relevant studies, we expect to find a difference in interactions between the two islands.

Methods

For our observational research, we focused on behavioral displays within marine iguana populations and the populations size of two different islands: San Cristóbal and Española (Figures 1 and 2, respectively). The goal of this was to determine whether or not the iguanas on an island with a greater human presence and increased predation interact with one another differently than on an island with much less human presence and decreased predation. Population size and makeup were observed and compared between the two islands.

  Figure 1: San Cristóbal Island of the Galápagos Islands, Ecuador is the easternmost island and is home to approximately 6,000 residents. San Cristóbal is thought to be one of the oldest islands of the archipelago. Being one of only two islands with an airport, San Cristóbal is especially susceptible to invasive species. We did our experiment here on a trail off the beach of La Lobería, a common spot for tourists and locals.
Six locations were surveyed per island; each researcher observed two separate location on each island for a total of twelve data collection sites (Figures 1 and 2). Because of limited observational space, two of the transects observed on Española were approximately the size of a standard transect when combined. This was accounted for when analyzing the data. At each location, the observers positioned themselves for optimal viewing of the entire transect. Transects were assigned randomly, and observations were recorded for five minutes. The Marine Iguana population of each location was quantified and categorized into two groups: males and females/juveniles. Iguanas were sexed by physical characteristics, such as body size and dorsal spine size. Females and young males are visibly smaller, and fully developed males have longer dorsal spines that are also typically thicker in growth. Size was used to distinguish juveniles from adult males. Behavioral displays of territorial aggression, hostility, and defenses were also observed and recorded. These behavioral displays were defined as head-bobbing towards other iguanas. 
Figure 2: Española Island of the Galápagos Islands, Ecuador is located at the southeastern corner of the archipelago. Española is also said to be one of the oldest islands of the archipelago at approximately four million years. Española is uninhabited allowing it to have many endemic species such as the Española marine iguana that we studied. We did our experiment at Punta Suarez, one of only two visitor sites on the island.

We performed an Analysis of Covariance (ANCOVA) in order to test whether or not the presence of humans has an effect on the population of iguanas. Locations of observation on San Cristóbal were given values 1-6 based on their proximity to a beach with a high concentration of swimmers and surfers. Each location between the beach and the nesting ground was given a value between one and six, with one being the closest to La Lobería, and six being the farthest from the beach and closest to the nesting ground. The population at each location was separated into male and female populations, in order to determine if sex modified iguana behavioral response to predators.

Figure 3: Moving to our locations to do the transcets on San Cristóbal Island, Galápagos Islands by La Lobería beach.

The second statistical analysis we performed was a mixed effects Analysis of Variance (ANOVA), comparing the effects of male population on both islands to number of territorial headbobs. For this, island was used as a random variable, and male population was used as the fixed, independent variable.  Male population size was split into two categories: less than five males, and five or more males.

Results

We failed to reject our null hypothesis that proximity to humans has no effect on iguana population. A p-value of 0.003 was found for population, which is well below the threshold value of 0.05, supporting our hypothesis that humans influence iguana defensive behaviors (Figure 4).

Figure 4: An ANCOVA was performed, Population Size vs Proximity to Humans, to compare the effect of human presence on populations of marine iguanas stratifying for males vs females as well. Values 1-6 are based on the distance we stood from the beach with 6 being the furthest from the beach and 1 closest to the beach. Española was added to the end to see if the trend would continue as the human presence should be even less there. A p-value of 0.0030 was found when looking at just population size giving us significant results. When sex was taken into account, a p-value of 0.0813 was found making the results only marginally significant when we account for the small sample size.

Once sex was accounted for, we obtained a p-value of 0.0813. This value is not below 0.05 but is marginally significant due to the small sample size. When plotted data shows a slope of 2.17 for females and a slope of 0.714 for males. Our results from Española (uninhabited), we see the same trend continues (Figure 4). Due to this, we can say that this may be marginally significant and that proximity to humans may have a greater effect on one sex than the other sex.

Male population size did not influence the frequency of defensive behaviors in Marine Iguanas (Figure 5).  We obtained a p-value of 0.3340. This value is well above the threshold value of 0.05 and therefore not significant.

Figure 5: A violin plot was made based on a mixed effect ANOVA comparing male population to territorial behaviors. Male population was split at five to compare populations of more than 5 and less than 5. We found a non-significant p-value of 0.3340.

Discussion

The main objective of this study was to gain insight into the ways in which human presence is affecting iguana population and behavior. To do this, we collected results from two Islands: Española and San Cristóbal. San Cristóbal has a population of approximately 5500 (Galápagos Conservancy) and a large tourism market, while Española is uninhabited and the only human presence is from single day tourists accompanied by national parks guides.

The result and the failure to reject our hypothesis lead us to conclude that the colonization and presence of humans is changing the location and size of marine iguanas. All of our results on San Cristóbal were gathered between La Lobería beach and a Seagull nesting ground, on the southern side of the island, just east of the main populated area. La Lobería is a commonly frequented beach for surfers, snorkelers, and swimmers on the island of San Cristóbal (Figure 1). Due to this, the beach was used as a landmark for human presence. A trail goes from La Lobería to the seagull nesting ground. This trail was what we used to collect data from between La Lobería and the nesting ground.

From our results, we performed two main statistical analyses. First, we performed an ANCOVA comparing the distance of a population from humans, to the total population at each location and compared the sex of individuals at each location. The separating of males from females and juveniles was important for this due to the territorial nature of mature males. This may lead to a difference in response to humans between males and females.

The second aspect of the ANCOVA analysis was comparing the population of males, to the population of females and juveniles in relation to human presence. Our results from this were marginally significant. Additionally, when observing our plotted results from Española (Figure 2), we saw that the same trend continued, with the females reacting more strongly to humans than males (Figure 4). When taking into account our small sample size, it is reasonable to consider that there may be a significant relationship between these two factors. However, more research is necessary to make any legitimate conclusions.

The second statistical analysis we performed was a mixed effects ANOVA, comparing the effects of male population on both islands to number of territorial headbobs. For this, island was used as a random variance, and the data from both islands was included in the test. Male population was used as the independent variable and was split into two categories: less than five males, and five or more males (Figure 5). Our results for this were not significant and it could be concluded that the frequency of territorial behavior is likely not affected by human presence and is rather determined by different factors.

Two main conclusions can be made from the results of this study. The incredible conservation efforts of the Galápagos Islands must be continued and even improved in order to ensure the health of the iguana populations on the different islands, especially inhabited islands. Additionally, much more research must be done on the different factors that play into the stability of iguana populations on the islands, in order to improve conservation efforts going forward.

Figure 6: A notably large male marine iguana we encountered on the trail to La Carolla beach.

MORE TO EXPLORE

  • Barnett, B.D., and Rudd, R.L. (1983) Feral dogs of the Galapágos Islands: Impact and control. International Journal for the Study of Animal Problems (4)1: 44-58.
  • Berger, S., Wikelski, M., Romero, M., Kalko, E.K.V., & Rödlbe, T. (2007) Behavioral and physiological adjustments to new predators in an endemic island species, the Galápagos marine iguana. Hormones and Behavior (52)5: 653-663 https://doi.org/10.1016/j.yhbeh.2007.08.004
  • Lima, S.L., & Bednekoff, P.A. (1999) Temporal variation in danger drives antipredator behavior: The predation risk allocation hypothesis. The American Naturalist (153)6: 649-659 DOI:10.1086/303202
  • Maloney, R.F., & McLean, I.G. (1995) Historical and experimental learned predator recognition in free-living New-Zealand robins. Animal Behavior (50)5: 1193-1201 https://doi.org/10.1016/0003-3472(95)80036-0
  • Wikelski, M., and Trillmich, F. (1994) Foraging strategies of the Galápagos marine iguana (Amblyrhynchus cristatus): Adapting behavioral rules to ontogenetic size change. Behavior (128)3-4: 255-279

THE AUTHORS & NOTES FROM THE FIELD

Hannah Rowold is a senior biology and psychology double major at Webster University. She has plans to attend MGH Institute of Health Professions to pursue a doctoral degree in Occupational Therapy.

One of the biggest joys of the trip was connecting with others over something we share a love for. I met some wonderful people who were extremely passionate about their work and the Islands. There was an incredible amount of knowledge between them all, and they were genuinely excited to share that and their experiences with us. It didn’t matter that we consistently followed up an answer with two more questions, there was an inexhaustible enthusiasm they expressed in whatever information they were giving us.

Peter Conowall is a junior Chemistry major with minors in Biology and Political Science. He hopes to pursue a career in aquatic research and conservation.

In our second full day on San Cristóbal, we visited Guadalupe’s Farm.  It was a small farm run by a man named Milton and his wife Guadalupe.  The main purpose of their farm is to find ways to grow native, introduced, and invasive species so as to take advantage of the benefits of all plants while not allowing any one species to take over completely.  Milton and Guadelupe have found ways to ensure that the different species live in coexistence with each other.  What really makes their land so unique though is the three small rivers that run through their land.  Only one other island has any freshwater on it.  These rivers also have a large freshwater crustacean that is endemic to only them.  We had the wonderful opportunity to hike down into the valley on his land and go swimming in two different pools of these rivers.  It was truly incredible to see how these two individuals had embraced everything from the land, including invasive plants, and found ways to utilize them in a positive way.

Kayla Dye is a senior Biological Sciences major with an emphasis in Health and Medicine who aspires to go to Med School next fall.

This is me and a Galápagos tortoise at the Giant Tortoise Breeding Center on San Cristóbal, Island.  We have been learning about Galápagos tortoises since grade school and finally being able to see one, in a natural environment, in the Galápagos was the most amazing experience ever.  I studied a lot about them before we went so when I got the opportunity to see them and how they interact first hand I definitely freaked out.  It was definitely something I will never forget and definitely something I never thought I would be able to experience.  I mean come on, it’s one of the oldest living animals out there!  Aside from the Galápagos tortoise we alsoencountered sharks while we were snorkeling just off the coast of Española Island, most importantly the hammerhead shark, and though no one got a picture of them, wow, were they absolutely breath-taking.  Our trip to the Galápagos consisted of one once in a lifetime opportunity to the next, even the trip itself was one.  Every experience was just as amazing and memorable as the next.