Can ‘Tortoise Highways’ Save the Western Santa Cruz Giants From Extinction?

The Galapagos Island tortoises are famous throughout the world. These giants from a bygone age are unique and found nowhere else on Earth. The western Santa Cruz giant tortoise is a subspecies of Galapagos tortoise, and they are critically endangered. They are one of the larger subspecies of Galapagos tortoises — males can grow up to 600-700 pounds and females 300-400 pounds. While this is sadly not unusual for a species with a very limited range, the cause is unusual. The threat to these creatures does not come from tourism or climate change, it comes from a tree. Here we will unravel the mystery of how a tree could become the cause of the Galapagos Islands’ next extinction.

Western Santa Cruz Giant Tortoise Vulnerability

Species that are found in very limited habitats are inherently more vulnerable to extinction than more widespread species. The western Santa Cruz giant tortoise (Chelonoidis porteri) is one of the 13 living subspecies of Galapagos tortoises.  In the 1600s, there were an estimated 250,000 tortoises residing in the islands, but their numbers were decimated over the following centuries as sailors hunted them for meat and for their shells. The IUCN currently lists the western Santa Cruz giant tortoise as critically endangered. There were approximately 3,400 mature individuals alive during the last survey in March 2016.

This tortoise has an extremely limited range. It is only found on the southwestern slopes of Santa Cruz Island. This amounts to an area of around 55 square miles, although there is an area of almost 200 square miles of suitable habitat (forest and shrubland) available if their numbers could recover.

Threats to the Santa Cruz Giant Tortoise

There are a number of issues threatening the survival of this species, but thankfully, hunting is almost entirely a thing of the past. Instead, they are now threatened by having to compete for food (low-growing vegetation such as cacti) with the goats, donkeys, and other herbivores that humans have introduced to the islands.

In addition, their eggs, hatchlings, and juveniles are eaten by other animals, including rats, cats, and ants. Some have even been hit by cars.

Feeding Patterns and Migration Barriers

These tortoises have a seasonal migration pattern that is similar to many other species. They travel from one area to another to access the most nutritionally rich food in a particular season. This provides them with enough energy to allow them to thrive and reproduce. During the rainy season, the fast-growing, nutritious vegetation of the lowland areas is the best option. However, during the dry season, this food source is very sparse, and the better choice is the less nutritious but more reliable vegetation at higher elevations. Typically, these tortoises travel around 12 miles during migrations.  

Unfortunately, they can find their migration routes blocked by barriers, including fences and roads. But a more serious concern is that these routes are also being blocked by trees!

Why Can’t the Tortoises Move Through Trees?

The tree species in question is the Spanish cedar (Cedrela odorata). It was imported to the islands from mainland Latin America in the 1940s as a fast-growing source of timber for the population. A few plantations remain, but the problem is that this has now become an invasive species in the wild. Furthermore, because the cedar contains toxic compounds and is relatively insect-proof, it outcompetes the native and endemic vegetation.

More of a problem is that the tortoises tend to refuse to travel through these cedar forests even though, in theory, they are large enough to push their way through. The exact reasons for this are not yet understood, but it could be because they are put off by the toxic compounds within the trees. It may also be because invasive blackberry bushes tend to grow under the trees. Another suggestion is that they do not like the shading effect that the trees produce.

Disrupting migration patterns long-term means that the tortoises cannot access the diet their bodies have evolved to thrive on and this could further impact their ability to survive and reproduce.

Tortoise Highways

It may seem that the obvious solution is to clear away the cedar trees but the situation is more complex than that. Simply removing them can allow invasive blackberry shrubs to take over the empty space, and they are just as much of a problem for the tortoises. The dense undergrowth they produce is almost impossible for them to move through.

Instead, it may be more effective to focus on maintaining currently uninvaded habitats and researching why some areas have not been invaded by cedars at all. Maintaining tortoise highways through the cedar forests for migrating tortoises is critical.