Image Credit: Yulia Kolosova, CC BY 4.0, Image Cropped

Impacts of herbivory by ecological replacements on an island ecosystem (2022) Moorhouse-Gann et al., Journal of Applied Ecology, https://doi.org/10.1111/1365-2664.14096

The Crux

Turning an ecosystem that has been ruined by humans back into a thriving natural world is a long, difficult task, but it is possible. One method for making it easier is re-introducing species that we’ve wiped out. Often the reintroduction of the functions that these species perform helps restore many other species, and helps the ecosystem returns to a more ‘natural’ state.

But what happens when a really key species has gone extinct? One way of solving this conundrum is introducing a similar species that performs the same function. This sounds like a good workaround, but introducing a non-native species might have unexpected ecological repercussions.

This week’s researchers were based on Round Island, in Mauritius, where two species of giant tortoise (the saddle-backed and the domed Mauritius giant tortoise) had gone extinct. A third species, the Aldabra giant tortoise, was introduced in 2007. The main point of concern on the island is that the tortoise diet may overlap with that of a vulnerable species, the Telfair’s skink. This week’s team wanted to find out whether the tortoise was helping or hindering the island.

What They Did

To figure out whether or not the skinks and tortoises were competing for food, the researchers took faecal samples from both animals, which were sent off for genetic analysis. The resulting analysis was made much easier by the fact that the researchers already had access to a DNA barcode library, which contained genetic material and indentifiers for almost all the plant species on the island!

The second major analysis concerned how the vegetation ha changed on the island, and whether this was down to the tortoises. There was a pre-existing vegetation survey taken in 2003, before the introduction of the tortoises to the island, and this was repeated in 2015 to monitor how the plant communities had changed. This was compared to the post-release monitoring data, which as taken shortly after the introduction of the tortoises. It showed where they were found in high densities on the island, and allowed the researchers to estimate whether or not changes in vegetation occurred because of the tortoises.

Did You Know: Round Island Successes

Completely getting rid of an invasive species is incredibly difficult, but it’s often much easier on islands, where it’s harder for newcomers to supplement the existing population. Round Island has been the site of a few successful eradications, with both goats and rabbit previously having been completely wiped out!

What They Found

There was a fair bit of overlap between the diets of skinks and tortoises. This may sound like it’s cause for concern, but most of the plants that both species ate were not actually preferred by the skink, and simply foods that they ate when they couldn’t find anything else, which means this overlap might not be a problem.

Many plant species showed changes over the period between the surveys, but most these occurred in areas with and without tortoises. There were a few species of plants which tortoises avoided or preferred, and these showed small changes in presence. One bright spot is that the tortoises increased dispersal of palm tree seeds (Latiana lodigesii), which will likely benefit the wider reptile community.

Problems

There are a few difficulties faced here, despite the fact that a place like Round Island has such a good baseline for starting a study like this (barcoding libraries and pre-existing vegetation surveys are the dream). The tortoises are still all fairly young, so will their effect change as they grow older? And when do the effects of removing the previous invaders kick in? Hopefully future vegetation surveys and tortoise monitoring will give answers as to whether the patterns seen here change over time.

So What?

This is a tricky one. We want initiatives like rewilding to work immediately, but the reality is it can sometimes take decades for these impacts to kick in. There are some changes that have been brought about by the tortoises, but it’s hard to know at this stage exactly how beneficial they’ll be for the island.

Perhaps the biggest positive is that there doesn’t seem to be too much of an effect on the skink population at this point. Well-meaning introductions have gone awry before (as an Australian the cane toad has made me immediately suspicious of said introductions), but studies like this show that our understanding of when these changes are appropriate have advanced considerably.

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Dr. Sam Perrin is a freshwater ecologist who completed his PhD at the Norwegian University of Science and Technology who is obviously against all invasive species being introduced but would secretly love some tortoises roaming around Norway. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.