Climate anomalies and competition reduce establishment success during island colonization (2022) Nicholson et al., Ecology and Evolution, https://doi.org/10.1002/ece3.9402

The Crux

The colonisation of islands by species on the move has given rise to some of the most fascinating ecosystems around the world. Think the marsupials of Australia, Papua New Guinea’s Birds of paradise, or the multitudes of weird and wonderful creatures that pop up in tiny unexpected landmasses around the globe. On the flipside, invasive species arriving on islands can hit like veritable hurricanes, with similar (though admittedly slightly slower-moving) effects. Yet for these phenomena to take place, a species first has to make it to an island from the mainland. This isn’t always super easy, seeing as islands may be tiny and hard to find, or way out in the middle of nowhere.

But even if they do arrive, whether or not a species is able to persist depends a lot on circumstance. If a large storm or drought hits (increasingly likely with climate change upping the frequency of extreme weather events) just after a species arrives on an island, it might wipe them out before they’ve even gotten started. A competitor already having set up shop there could decrease a species’ survival chances too. Today’s authors were lucky enough to have introduced a new species to a series of islands with and without competitors, all of which were hit by a drought just after one of the introductions. Let’s see how the populations fared.

What They Did

The researchers located eight islands in Panama’s Lake Gatun, an artificial lake created by the construction of the Panama Canal in the early 20th century. They transplanted Panamanian slender anole lizards (Anolis apletophallus) to four of the islands in late 2017, and to the other four in late 2018, just before a severe drought hit. The drought lasted well into 2019. Of the islands that had slender anoles released in 2017, two had a competitor (Gaige’s anole, Anolis gaigei) present, whereas the others were competitor free.

After release, the lizards were monitored using the mark-recapture method. This is a survey technique whereby attempts are made periodically to recapture the released individuals. The success (or lack thereof) of the recaptures over time is turned into an estimate for the number of individuals still alive.

Did You Know: Islands as Study Systems

Islands in general make amazing study systems, as their ecosystems are a little more isolated, and we can often study certain species with a more comprehensive view of their habitat and population. I was lucky enough to visit an island called Vega (it’s just off the coast of Norway) during my PhD, which had a pedigree for the entire moose population dating back decades!

But newly-made, often artifical islands make an even more fascinating system (in my opinion), as they allow us to study colonisation itself. When Krakatoa exploded in the late 19th century, it destroyed nearby islands, and created entirely new ones. This formed the basis for some amazing studies as different species made their way back onto these islands. Likewise, the construction of the Panama Canal formed brand new islands which likewise made brand new study systems and changed how we understood island life.

What They Found

After the drought, the slender anoles were functionally extinct on the four islands to which they were only released in late 2018. On the islands where anoles had an extra year to spread, the population decreased, but they were still alive and kicking.

Of the four islands where the anole populations were doing better, the two islands without competitors had seen populations rising before the drought hit. However on the two islands that contained a competitor species, the population had already been falling before 2019.

Problems

Because of the limitations inherent with most studies, we come back to this a lot, but it has to be sample size here. Island ecosystems provide a unique opportunity to study creatures, but there are a host of reasons why individual islands differ in their ability to support new species. Island area, shoreline complexity, habitat diversity are a few. Only taking eight islands, and then dividing those up into four different categories, make it difficult to tell if what we’re seeing here is causation or correlation.

So What?

This is a fantastic study with some really obvious practical effects. Translocating endangered species to new islands (like the Tasmanian devil) is becoming a new conservation tool, and knowing when these species may need new immigrants to help them out is crucial to their survival. Likewise, knowing when NOT to try and start an island population can help save precious resources.

It can also help us figure out where and when invasive island species are most likely to thrive, so that we can figure out which invasive populations are most crucial to get under control early. I am obviously slightly biased because I love island biogeography, but this is a great study in a fascinating setting. More please!

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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.

Title Image Credit: Dario Taraborelli, CC0 1.0, Image Cropped