Surrogates for Ecosystem Engineers? A new method to restore the Galapagos Islands

What do you see when you picture an engineer? Perhaps you picture a human in a hard hat watching a crane lower wood onto a new building? Maybe you see a person frantically typing at a computer while designing the latest technology. In ecosystems, engineers look a bit different. 

Ecosystem engineers are organisms that modify the habitat they live in. One famous ecosystem engineer is the beaver. Beavers build dams which alter the path of the water and consequently change the entire ecosystem, affecting many other organisms. Ecosystem engineers also provide a way to naturally restore degraded ecosystems. In the beaver example, reintroducing beavers to an area will ultimately improve the habitat for the plants and animals living there. Reintroduction of ecosystem engineers is becoming a common method to return habitats to their pre-disturbed state, and perhaps, is a strategy that would make Charles Darwin proud. 

In the late 1800s, Charles Darwin, known as the father of evolutionary biology, landed on the Galapagos islands off the west coast of Ecuador. There he found an assortment of finches, iguanas, and giant tortoises. Despite being the crown jewel of evolutionary biology, Santa Fe Island is now heavily degraded due in part to the introduction of exotic herbivores, like goats, and the extinction of an important ecosystem engineer — the Santa Fe tortoise — over a century ago. The Santa Fe tortoise was an herbivore and ate some of the woody vegetation which allowed the tree cactus, found only in the Galapagos to grow. The tortoise also smashed vegetation with its massive body as it meandered through the brush which helped redistribute nutrients and maintain the savannah-like ecosystem.

With the Santa Fe tortoise extinct, it seems impossible that ecologists can use the ecosystem engineer to help restore the ecosystem, right?

Wrong! Sort of. 

In a new study, Wachington Tapia and colleagues are evaluating whether a similar tortoise species from a neighboring island can match the ecosystem engineering prowess of the extinct Santa Fe tortoise. The researchers released 732 Española tortoises on Santa Fe Island in 2015 and have been monitoring the ecosystem to determine whether these tortoises alter the vegetation to match the pre-disturbance conditions more closely, and the potential risks with an emphasis on the endemic pallid iguana.

They crossed their fingers for a successful species introduction

To have a successful introduction of tortoises and show that they are indeed ecosystem engineers, the researchers looked at a few key metrics.

1. The tortoises must survive and have a similar health to those on Española Island.
2. They must change their habitat. In this case, the researchers hoped that the tortoises would inhibit woody vegetation and cactus growth, which would help the island return to its historic savannah type ecosystem.
3. The tortoises should not impose a negative effect on the native species that already live on the island, particularly the iguanas. Since the tortoise and the iguana are both herbivores, there is a chance that they may compete for food, and one may outcompete the other.

So far, tortoises have been successful ecosystem engineers

The first measure of success is that the tortoises must survive and indeed, the tortoises on Santa Fe Island were able to survive and flourish in their new home, although they grew slightly slower than their Española Island counterparts.

The tortoises were also successful in changing the vegetation and consequently the habitat on Santa Fe Island. In areas where tortoises and iguanas were allowed to roam, there was less woody plant regeneration from 2015 to 2020. This is likely due to the massive size of the tortoise. Like driving a tractor through a forest, when something that large barrels across an ecosystem it is difficult for woody shrubs to grow. This is a step in the right direction in establishing the grassy savannah habitat characteristic of the Galapagos. Additionally, the endemic tree cactus is now thriving thanks to the reduction in woody plants and the presence of the tortoise. This is evidence that the tortoises are reviving their role of ecosystem engineers on Santa Fe Island. 

Finally, the reintroduction of tortoises to the island did not negatively impact the existing iguana population, with iguana counts actually increasing with each survey. The evidence showed that the eating behavior of both the iguana and the tortoise shaped the distribution of the plants in the habitat and that they were able to coexist. This is another win for the restoration ecologists! 

This study ended in a success story, but it is important to understand that not all species introductions are beneficial. The scientists did substantial research beforehand to ensure that the Española tortoise was similar to the extinct Santa Fe tortoise and would likely pose few threats to the existing endemic species. They monitored where the tortoises were and the effect on the ecosystem and were prepared to pull back if they caused any damage.

This study showed that in the absence of an ecosystem engineer, a surrogate species may be used to replace it and be used to restore a degraded ecosystem. Since this study was initiated, the new tortoises on Santa Fe Island have not bred and it will take many years to establish a breeding population and reach island-wide occupancy. The population has been doubling its coverage every year since 2015, and researchers hope the population will continue to spread across the island in the years to come. Given the success of this first phase of reintroductions, the researchers are moving onto phase two which involves continued monitoring of the tortoises and their habitat. 

Source:
Tapia, W., Goldspiel, H.B. and Gibbs, J.P. (2021), Introduction of giant tortoises as a replacement “ecosystem engineer” to facilitate restoration of Santa Fe Island, Galapagos. Restoration Ecology. Accepted Author Manuscript e13476.

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