Where did they come from and where did they Galapa-go? The fascinating story of speciation in Bulimulid land snails

While the typical tourist travels 600 miles off the coast of Ecuador to catch a glimpse of the only penguin living north of the equator or the majestic waved albatross, I arrived with a different goal in mind: to find the tiny endemic land snails that are sprinkled across most of the Galapagos islands. Between 6 and 25mm in size, these tiny mollusks hide in leaf piles, plants, and the small pools of moisture on the porous lava rocks.[1] Despite the relative lack of attention afforded to them on the famous archipelago, a closer inspection into their natural history reveals a remarkable story of diversification, adaptation, and evolution.

This past September, I had the amazing opportunity to travel to the Galapagos through the Stanford Sophomore College program. The travel group consisted of fourteen sophomores, Professor Bill Durham, two teaching assistants, and more than thirty alumni. Prior to our trip, we read books, attended lectures, and held discussions about the systems we would encounter. From this, I found that learning as much as you can before visiting a new place transforms the travel experience. Without reading about these snails, I would have walked right by them without a second thought. Instead, our tour group would crouch over tiny puddles, sift through leaf litter, and give shouts of celebration when we found a new snail. Meanwhile, giant tortoises roamed around the trail relatively unnoticed.

IMG_3389

Author, Audrey Bennett, observing a Bulimulid snail.

Although Charles Darwin on his journey to the Galapagos may have overlooked the potential of studying these snails, scientists have recently begun to notice the unnoticed. In particular, their evolutionary history and ecology is fascinating: since their arrival to the Galapagos Islands about 2.77 million years ago, the single colonizer species of the genus Bulimulidae has radiated into 71 different species, making it the largest adaptive radiation on the islands.[2] Adaptive radiation describes the process in which a single ancestor differentiates into different species. As a population begins to use resources in different ways and occupy slightly different habitats, it experiences different selection pressures. Competition between individuals drives some to occupy different ecological niches. As time passes, natural selection may favor the diversification of populations. Think of it like free market economics, where competition between companies provides economic incentive to find new, unexploited parts of the market to develop. In a similar way, a single species of snail, arriving with the same traits and genetic makeup, is only taking advantage of one part of the “market” until individuals begin to find new resources and habitats to minimize competition. Therefore, in specific ecological set-ups, populations that diverge have the opportunity to be much more successful.

However, not all populations behave this way. There are stable populations that are not diverging. For example, alligators have remained practically untouched by significant evolutionary change for at least eight million years.[3] Subsequently, it is interesting to investigate why this group of snails, in this locale, has produced such an amazing example of adaptive radiation. To understand this puzzle, we first need to understand some Bulimulid snail ecology and island biogeography.

The Galapagos Islands are an ideal study site to investigate the mechanisms of colonization (how species arrive and disperse in an area), evolution, and radiation. The significant history of the Galapagos Islands is a testament to this; their isolation and simplified ecology mean that tracking changes in a species’ size, range, traits, and interactions is much easier than on the mainland. The number of islands and islets within the Galapagos (a total of 127!) also provides sources of comparison for scientific studies.[4] Moreover, the Galapagos islands have distinct moisture zones depending on their altitude, with more moist habitats at higher elevations. This creates a diverse range of habitats and resources, with distinct vegetation zones and changes in climate and substrate. While visiting Santa Cruz Island, a more populated island in the archipelago, we traveled from the dry, desert lowlands to the moist, forested highlands within a thirty-minute bus drive. This diversity in habitat allows distinct species to exist in close proximity but utilize different resources, which contributes to the large radiation of these snails over a relatively small land area (the total land area of the Galapagos islands is approximately equal to the state of New Jersey!).[4] If we continue the market analogy, a diverse habitat encourages radiation just as a diverse range of consumer interests allow for different companies to “exist in close proximity” but avoid competing for customers. For instance, the technological needs of people are diverse, which has supported the existence of an impressive number of apps to survive and be sustained. Think about online dating services–different dating apps provide for slightly different niches in the consumer market.

Lastly, the Bulimulid snails’ small size and limited mobility mean that even simple topographical features of the island, like the presence of bumpy lava rocks, can easily separate two populations. As the two populations become reproductively isolated, they begin to exclusively mate and share genes within their subpopulation, which given enough time can lead to divergence into different species. These snails have become incredibly specialized in the ecological niche that they utilize; some species even specialize the type of plant they live on.[2]

Specialization makes sense when competition is driving divergence, but human presence can alter this dynamic. Adaptations that make a species perfectly adapted to a certain niche can now make a species more vulnerable, because if that habitat is threatened, its species is threatened as well. Returning to the economics example, if a worker is very specialized in their trade, but automation makes that job obsolete, the adaptation that once made them very competitive now makes them vulnerable. This pattern has happened with many species across the archipelago with increasing human presence. For the snails, more than 30 of the 71 Bulimulid snail species are now considered endangered.[5] Despite these pressing threats, current conservation plans on the islands do not take snail populations into consideration. For example, many of these threatened species live in the 10% of the Galapagos that does not have national park status.[5]  Additionally, a road to improve access to the airport on Santa Cruz Island required large amounts of gravel from Cerro Maternidad, the locality of B. cavagnaroi. The construction of a gravel pit in this locality has dramatically reduced the range of the species and may alter the microclimate for the remaining population.[5] This exemplifies the danger of specialization–human-caused shifts to the environment can turn a specialized adaptation into a vulnerability. Ideally, future conservation efforts will prioritize protecting the remaining habitats while also funding research to better understand their ecology and natural history.

OLYMPUS DIGITAL CAMERA

Bulimulid snail making its way across a porous rock.

The struggling conservation efforts of these beautiful snails raise interesting questions about the concept of conservation itself. What motivates conservation? What does it take for the public to recognize a species as worthy of protection? Although the snails likely do not contribute to tourism revenue, I struggle with the idea of quantifying the worth of a species, or even establishing the value of a species based on its relation to other species or the local economy. I do recognize that conservation efforts rely on money, and that it is idealistic to think about the value of a species as intrinsic. However, I found that the urge to quantify a species’ value quickly faded as I sat entranced, my face inches away from a small snail making its way across a porous rock. As I’ve tried to turn my focus to small parts of the world that don’t get much attention, I’ve observed that we can find sources of curiosity, fascination, and beauty anywhere… if we just look close enough. There is power in noticing the unnoticed and underappreciated, whether that be snails, ecosystems, or individuals in your community.

Video courtesy of Audrey Bennett.

Article photos courtesy of Audrey Bennett.

References:

  1. Coppois, G., Wells, S. (1987) ‘Threatened Galapagos snails.’ Oryx, Vol. 21, Issue 4, pp. 236-241.
  2. Parent C.E, Crespi B.J. (2006) ‘Sequential colonization and diversification of Galápagos endemic land snail genus Bulimulus(Gastropoda, Stylommatophora).’ Evolution, 60:2311–2328.
  3. Livingston-Florida (2016) ‘Evolution hasn’t revamped alligators in 8 million years.’ Futurity. Retrieved November 7, 2018, fromhttps://www.futurity.org/alligators-evolution-1250572-2/
  4. Coppois, G. (1984) ‘Distribution of bulimulid land snails on the northern slope of Santa Cruz Island, Galapagos.’ Biological Journal of the Linnean Society, 21: 217-227.
  5. Coppois, G., Wells, S. (1987) ‘Threatened Galapagos snails.’ Oryx, Vol. 21, Issue 4, pp. 236- 241.
Advertisements