Beyond Tracking

Galapagos Giant Tortoise Feeding Ecology

Having discovered some of the mechanisms governing migration and other movements, we wanted to place our new knowledge into a wider ecological and conservation context. We knew that food availability influenced tortoise migrations, but we had little data on tortoise diets. Previous studies had been conducted in the 1980s before the explosion of introduced species on Santa Cruz Island.

We spent several hundred hours observing tortoises and recording all feeding activity, noting the plant species eaten and other details such as bites per minute. We also studied how plant communities change along the elevation gradient.

Tortoises eat at least 96 different plant species. Young mature leaves are preferred, and fruit also makes up a large part of the diet.

Galapagos tortoises consume many plant species that were introduced to Galapagos by people often preferring these over native and endemic plant species. Many of these species were brought to Galapagos as food sources for people and livestock and are highly nutritious so it is not surprising that tortoises feed on them. We found that the physical condition of tortoises may even be improved when feeding on plant species introduced to Galapagos by people.

An unfortunate consequence of feeding heavily on fruits from non-native plant species introduced to Galapagos, such as guava, is that as tortoises eat the fruit, they are also ingesting the seeds they contain. We found that an average pile of tortoise poo contains several hundred seeds of the highly invasive guava tree.

We also found that it might take two to three weeks for a seed to pass through a tortoise’s digestive tract and during this time, a migrating tortoise may travel several kilometers. For this reason, tortoises are capable of dispersing huge numbers of seeds over large distances. This accelerates the spread of these species, which can be highly invasive and destructive to native Galapagos plant communities.

On the other hand, tortoises are also dispersing the seeds of many native species and potentially maintaining them on the islands. Tortoises can genuinely be called the “Gardeners of the Galapagos.”

This story is written up more fully in the following journal articles:

Galapagos Giant Tortoises and Human Interactions

As Galapagos develops economically and the human population rises, it will be increasingly important to understand the dynamics of tortoise-human interactions.

Like migratory species all over the world, long distance migration by Galapagos tortoises means that many tortoises leave the protective security of the Galapagos National Park and enter private farmland in the highlands of Santa Cruz and other inhabited islands. This has the potential to lead to challenges both for farmers and for tortoises – several giant tortoises can destroy a field of newly planted maize, for example. On the other hand, the presence of tortoises is compatible with cattle farming.

Furthermore, the private lands of Galapagos are dedicated to different uses, from arable agriculture, to livestock production, to tourism and urban development, all of which have different implications for tortoise conservation and the relationship between tortoise and people. As Galapagos develops economically and the human population rises, it will be increasingly important to understand the dynamics of tortoise-human interactions.

Additional research started in 2017 involves in-depth studies within farmlands to better understand how tortoise movements and behavior might depend on land use and habitat fragmentation, and which strategies could be implemented together with land owners and stakeholders to solve these potential conflicts and their consequences.

We have integrated research on this issue into the Galapagos Tortoise Movement Ecology Program. First, a brief stakeholder workshop was initiated in 2018 to bring private landowners, researchers, and members of the Galapagos National Park Directorate and local institutions together to generate a constructive dialogue to share experiences and discuss strategies and solutions to potential conflicts between people and tortoises.

Attitudes toward tortoises were almost exclusively positive or benign, with landowners recognizing the importance of tortoises to the entire economy of Galapagos, and the non-trivial recognition that the “tortoises were here first.” However, if tortoises are perceived to limit rather than enhance economic opportunity, this situation could change.

As a follow-up to this work, we facilitated a study on tortoise ecology in private lands conducted by PhD student Kyana Pike. The study involved analysis of how tortoise ranging and behavior in private lands are influenced by land use type and human infrastructure such as roads, fences, and natural and artificial ponds.

Global Positioning System (GPS)-tagged tortoises spend an average of 150 days per year in private lands, and a tortoise uses an average of four farms, up to a maximum of 24 different farms. Use of multiple farms under multiple different land uses by a single tortoise indicates that finding shared solutions to tortoise conservation and conflict mitigation on Santa Cruz island will require cooperation across the agricultural zone.

Contrary to our expectations, fences in their current configuration seem to present few serious barriers to tortoise movements. Most fences are in a poor state of repair, however even fences designed to keep tortoises out of crop areas are relatively porous.

This important research is in its early days and we will be reporting on outputs in detail in the future. Initial scientific publications on this work are published in the following journal articles: