Skip navigation
Go to home page > News > Our blog > Galapagos giant tortoises and climate change may help invasive plants to spread
Galapagos giant tortoise eating fruit
10/08/2017 Climate Invasive species

Galapagos giant tortoises and climate change may help invasive plants to spread

One of the greatest threats to the native biodiversity in Galapagos is the ever increasing distribution and number of invasive species.

Photograph of Esther Gillespie

Esther Gillespie

Volunteer at Galapagos Conservation Trust

Unfortunately, new research has just revealed that Galapagos giant tortoises may facilitate the spread of some invasive plant species in the future. This is due to the killer combination of their impressive long distance seed-dispersal services and climate change.

Galapagos giant tortoises are the largest terrestrial reptiles on Earth and can live for more than 150 years, weighing up to 300kg. The tortoises are referred to as ‘Galapagos gardeners’ for several reasons; they are biological bulldozers that carve out paths in the thick vegetation, eating large amounts of plant matter which makes them a major vehicle for seed dispersal. This is essential for many native species! The tortoises are a keystone species in Galapagos, and have created different habitat zones in the Archipelago through their long distance migrations and seed dispersal. Some of the seeds from certain species of plants can only germinate after digestion in tortoises.

However, unsurprisingly, the tortoises cannot tell the difference between native plant species and invasive plant species. A team of top biologists, including Dr Stephen Blake who leads the Galapagos Tortoise Movement Ecology Programme, have been investigating whether Galapagos tortoises are helping to spread invasive plant species across the Islands.

Galapagos giant tortoise eating guava
Galapagos giant tortoise rocking some guava lipstick… © Christian Ziegler

The tortoises are a keystone species in Galapagos, and have created different habitat zones in the Archipelago through their long distance migrations and seed dispersal.

Invasive species are those that thrive outside their native range, causing detrimental damage to the balance of native flora and fauna within local ecosystems. There is now a total of 870 introduced plant species found on Galapagos – outnumbering the native species!

The team of scientists 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 as you move into the more 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 delicious for tortoises.

Guava fruit
Guava © Sakurai Midori

Guava and passion fruit occur in the humid highlands of Santa Cruz, but their seeds are being dispersed into the lowlands by migrating tortoises. The tortoises transport large quantities of guava and passion fruit seeds into habitats that are currently unsuitable for the germination and survival. The scientists refer to this as ‘wasted’ seed dispersal. The team investigated the seed dispersal of guava and passion fruit in tortoise dung piles and the distribution of adult plants along two elevation gradients of Santa Cruz to assess current levels of ‘wasted’ seed dispersal.

At the moment the wasted seed dispersals are not having any significant negative effects as the seeds cannot germinate in the current conditions of the arid lowlands. However, the scientists were interested to see whether this would be altered with climate change. So they used the results from their studies to create computer models to foresee the consequences of climate change.

Passion flower in Galapagos
Passion flower in Galapagos © Amy Dolman

870

introduced plant species are now found in Galapagos

It has been predicted that climate change will increase the competitive advantage of most invasive species over natives. The team found that the current levels of ‘wasted’ seed dispersal in lowlands will remain the same for passion fruit but decrease for guava. This means that the guava seeds being dispersed in the lowlands by the tortoises will have a higher chance of germinating and surviving under future climate conditions. This is because the future climate conditions are predicted to be more humid, due to increased hot season temperature and rainfall – making the currently arid lowlands a much more suitable habitat for guava plants. Therefore, the tortoises will be facilitating rapid range expansion of guava on Santa Cruz island. This is assuming that tortoise migratory behaviour continues in the same way.

Eradication of guava seems to be unattainable under existing management constraints and it will become more problematic in the future as the combination of climate change and long distance dispersal by tortoises will cause a rapid increase in the geographical range of guava on the island.

Dr Steve Blake tagging a giant tortoise
Dr Steve Blake tagging a giant tortoise © Christian Ziegler

It’s all sounding rather gloomy at the moment. However, the work being done by the Galapagos Tortoise Movement Ecology Programme, partly funded by GCT, is trying to improve our understanding of the ecology of these great reptiles in order to inform future conservation strategies, to try to prevent further spread of invasive plant species. Through tracking the tortoises they can also highlight the areas of lowland on Santa Cruz that may become vulnerable to the spread of guava in the future and need more management.

Plant species dispersed by Galapagos tortoises surf the wave of habitat suitability under anthropogenic climate change - PLOS ONE

Plant species dispersed by Galapagos tortoises surf the wave of habitat suitability under anthropogenic climate change

PLOS ONE – July 2017
Diego Ellis-Soto, Stephen Blake, Alaaeldin Soultan, Anne Guézou, Fredy Cabrera, Stefan Lötters

Read more

How you can help

By adopting a Galapagos giant tortoise, you can help protect these extraordinary creatures for many tortoise-lifetimes to come.

Related articles

Galapagos giant tortoise eating cactus pad
20th Nov 2023
Island restoration Research

Galapagos giant tortoises: An update from the field

We spoke to Dr Patrick Moldowan, who joined the Galapagos Tortoise Movement Ecology Programme as Principal Investigator earlier this year, about the team’s recent work in the field.
Read more
14th Nov 2023
Wildlife facts

Meet the six Galapagos species you can adopt with GCT

One of the ways we encourage people to support the wildlife of Galapagos is through our species adoptions, which help fund several projects across the Archipelago.
Read more
Plastic waste found in Galapagos giant tortoise faeces
13th Nov 2023
Plastic pollution

New research shows that Galapagos giant tortoises are ingesting plastic waste

A new study published in the journal Environmental Pollution has found that giant tortoises on Santa Cruz island are ingesting items including medical face masks, glass and plastic bags.
Read more
Floreana mockingbird
3rd Oct 2023
Island restoration Rewilding

12 missing species set to return to Floreana

Today is the day when the next phase of the restoration of Floreana island begins, as the project moves into the eradication phase, the largest and most complex eradication ever attempted on an inhabited tropical island.
Read more

Get the latest news from Galapagos

Join our mailing list to receive our monthly email newsletter, bringing you the latest news on Galapagos and our work to protect the Islands.

Hidden
Share This Page