Galapagos Island’s accidental gardener

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Galapagos giant tortoises may give invasive plant species a helping hand under climate change

One of the greatest threats to the biodiversity in Galapagos is invasive species, many of which have negative effects on the native species. Within the plant communities, non-native species often out compete endemic species for space, water and nutrients. New research has revealed that climate change and Galapagos giant tortoises may make this problem worse in the future, by facilitating the spread of some invasive plant species.

 

Galapagos giant tortoise
Accidental gardener, the Galapagos giant tortoise © Shannon D’Arcy

 

Galapagos Gardeners

Galapagos giant tortoises are referred to as “Galapagos Gardeners” for several reasons; they carve out paths in the thick vegetation, eat large amounts of plant matter and are a major vehicle for seed dispersal. They have helped to create the different habitat zones in the Archipelago through their long-distance migrations across the islands dispersing seeds as they go. Seeds can take up to two weeks to pass through their systems by which time a tortoise could be in a very different habitat.

Galapagos invasive
Guava tree. Under CC Licence

Invasive plants

There are now around 870 introduced plant species found on Galapagos, more than the total number of native plant species. A team of scientists, led by Steve Blake from the Giant Tortoise Movement Ecology Program which GCT support, studied the problem of Galapagos giant tortoises dispersing invasive species’ seeds on Santa Cruz island. In the arid lowlands of the island there is a very low abundance of introduced invasive plant species, however the abundance steadily increases up the gradient to the humid highlands. The two most common invasive plant species on Santa Cruz island that the scientists investigated were guava and passion fruit. Both these plant species were brought to Galapagos by humans for domestic purposes and are a favourite with tortoises.

 

Currently the habitats that the tortoises are dispersing the fruit seeds to are unsuitable for germination and survival. However, using climate change models, the scientists predict that in the future, guava seeds will have a higher chance of germination. This is because future climate conditions are predicted to be more humid with increased hot season temperature and rainfall, making the, currently arid, lowlands a much more suitable habitat for guava plants. Assuming the tortoise migratory behaviour does not change, there could be a rapid range expansion of guava on Santa Cruz.

 

Galapagos flora
Sunflower Trees on Isla Santa Cruz, one species of the unique Galapagos flora that is threatened by the spread of invasive species. © Nicole Aherron

 

Eradication of guava is challenging under existing management constraints. If the climate changes as predicted, plant communities in the lowlands of Santa Cruz could look very different in the future.

 

Written by Clare Simm

Clare is Communications and Marketing Officer at GCT.

Clare has worked in wildlife conservation ever since completing her Masters in Conservation Science. She is responsible for GCT’s communications and marketing and is particularly interested in informing new audiences about why the Galapagos Islands matter.

Please contact Clare with any GCT communications, press or marketing queries.

Read more GCT articles on talk: Wildlife

 

Read more about:

Giant Tortoise Movement Ecology Programme

Science paper on this topic

 

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