You open your eyes, blinking away water, you’re on a beach you don’t recognise, and never set out to visit. You look up and along the coast, it’s an island, the flora is alien to you, the climate hotter, and you’re already sweating. An eldritch cry emanates from the forest near you, new wildlife, things you have never seen before skulk around beyond the vines that lay before you.
Lifting yourself up, you decide to escape the blazing sun. You leave the beach and push through the wall of vegetation that veils the forest from the beach. You expect it to be cooler, but it isn’t. The forest is completely new to you, as you move through the undergrowth, unfamiliar insects dart away, flying past plants you’ve never seen before. As you press on through the undergrowth you wonder how long you will have to spend here? How much more time do you have here?
A few weeks pass and you’re still here. You managed to survive, for the days, hours, minutes, and seconds, you endured. You have a food supply, it’s not much, but it’s there. You can survive off what this island provides. You notice, however, that each day, your hard-won spoils are dwindling. Either they are harder to find, or something is finding things before you. A competitor perchance? Can you remember the last time it rained? The last time those plants near the shoreline bore fruit. Are they still even alive?
It’s all gone, either eaten, stolen, or lost forever. There is no food, no water, nothing. You haven’t seen a drop of rain in months. The forest you thought so claustrophobic before is a shadow of its former self. Dry, listless, and barren. You aren’t going to make it. You wonder to yourself, what if it had been different? What if you’d been able to stave off the competition? What if the drought hadn’t come or had come later? You needed longer to prepare. There just wasn’t enough time….
If a species is permitted more time to adapt/acclimatize does this facilitate its success when faced with multiple environmental stressors?
A female Anolis apletophallus, moments before being collected from the mainland as part of the introduction experiment. Photo by D. Nicholson.
But, what if you had more of it? More time. What if, perchance, you had also been a lizard? An anole perhaps? Anolis apletophallus to narrow it down further. Would more time have been the crutch you needed to edge your way through the drought and remain on the island alive? Or more generally speaking, if a species is permitted more time to adapt/acclimatize does this facilitate its success when faced with multiple environmental stressors? If you are an invasive species, could a drought and some competitive species hinder your progress? These are essentially the questions we asked in our recent paper, published in Ecology and Evolution. But rather than shipwrecking our lizards alone on islands, we purposefully introduced them (from mainland Panama) to tiny islands in Lake Gatun, a lake that makes up a substantial part of the Panama Canal. We (Michael Logan – University of Nevada, Reno, Christian Cox – Florida International University, Owen McMillan – Smithsonian Tropical Research Institute, and myself) started this project in 2017, to better understand the processes that drive or restrict a species’ ability to adapt to habitat and climate change over multiple generations. The project itself is hugely interdisciplinary, looking at numerous evolutionary and ecological factors, including microbiomes, behaviour, thermal preference, parasite load, species invasion, morphological change, natural selection, colonization/invasion and population dynamics–the latter concepts were the focus of the later part of my PhD and the paper related to this post.
Each field season (July -November) began by collecting hundreds of lizards from the mainland forests of Gamboa, Panama. This included: morphological measurements, toe pad sizes, upper and lower thermal limits, photographed dewlaps, and DNA samples. Finally, and crucial for my paper, we topped it all off with an individual mark using VIEs (Visual Implant Elastomers). These mainland lizards were then shuttled across the lake and introduced, 70 at a time, to small islands. Release events were then followed by twice weekly mark-recapture surveys of each island to determine which individuals were enduring and what habitat they were using, all of which was compared with populations on the mainland. We also had to collect any new island-born adults and bring them back to the lab on mainland Panama to record all the data I previously mentioned and add them to the census of island lizards. It is also worth noting that the annual survival rate for Anolis apletophallus is approximately 5%, so for the most part, each summer we returned gave us an entirely new generation to work with.
Two of our experimental islands within Lake Gatun. Both islands contain the native competitive species Anolis gaigei, as well as our introduced populations of Anolis apletophallus. Note the 5m boat for scale. Picture by M. Logan.
Due to the amount of work required to complete these introduction, events had to be staggered. Some island populations began their existence in 2017, some in 2018, more in 2019 and so on, giving us multiple island populations staggered across time. Here, however, I am focused on the first eight islands used in the project, four from 2017 and four from 2018. Using individual VIE marks, mark-recapture surveys, and a mark-recapture model, I was able to track the population sizes on each island over time, meaning we could follow any trends and patterns that might arise. Yet having a large, multi-year experimental system like this is risky. Anything and everything can go wrong. In late 2018/early 2019 that “anything and everything” did happen. One of the worst droughts in recent years hit Panama, hugely impacting the local flora and fauna, and drastically lowering the level of Lake Gatun and the Panama Canal itself. Even the weight limit of the supertankers going through the canal had to be restricted to save these numerous metal leviathans that use the canal daily from running aground. To put it another way; 2019 was approximately 1oC warmer than the previous two years of the study and the dry season of 2019 had less than half the amount of rain as the previous year.
What did this drought mean for our island cast-aways? Well, you can probably guess the answer to this question from the narrative at the start. It did not go well. Yet, the damage across different populations was not equal. The four populations we introduced in 2018, a mere three months before the onset of the drought, were essentially obliterated by the time we returned the next year. A few individuals remained, but for the most part, the survivors on these islands were alone. Yet the populations that were introduced to islands in 2017 were still, mostly, surviving. Even the populations that entered the drought with vastly lower population sizes (10-20 lizards) than the 70 individuals of the 2018 islands, managed to survive.
Population sizes and densities across the 8 different island populations; C, D, F & P introduced in 2017 and J, O, S & T introduced in 2018. Dashed lines indicate the two islands with a competitive anole species.
Further to this, two of those 2017 islands even had a competitive species (Anolis gaigei) that all the other islands did not. This competitive element had a detrimental impact on our introduced populations. We found that male and female A. apletophallus had a reduced difference in their habitat usage on the islands with the competitor species (A. gaigei); essentially, we think interspecific competition could be driving interspecific competition. Even when this competitive element was combined with the drought, the populations of lizards exposed to a competitor still managed to cling on, albeit in very low numbers. In fact, as of summer 2022, one of these two-species islands still has a population of A. apletophallus, though the other island was not so lucky.
The ability of a population to establish itself is hugely affected by extreme climate events, such as drought.
Ultimately, we found that after being exposed to a novel environment (in this case islands), the ability of a population to establish itself is hugely affected by extreme climate events, such as drought. However, the amount of time a population spends in a new environment can mitigate the detrimental effects of a climate anomaly. This mitigation in turn is somewhat diminished in the presence of a competitor, but not completely. So, to go back to our earlier narrative, could it be that just having more time on an island is sufficient to avoid wrack and ruin? It looks that way, at least in this case. Let’s, however, look at it in a different way, for a final take-home message. It is often thought that climate anomalies weaken ecosystems and make them more susceptible to invasive species, such as we mimicked here. Yet with our study, we have shown that climate anomalies can mitigate potential invaders if the anomaly hits before the novel species have had time to adapt.
- Island Colonization, Drought, and Competition in Panama - December 14, 2022
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