The Winds of Evolution

Yesterday marked the beginning of what is predicted to be an abnormally active hurricane season. The effects this will have on at-risk communities while unknown is concerning. The human toll and economic damage of hurricanes are often devastating. These naturally occurring catastrophic events also impact and profoundly influence the environment and many non-human species. Here are three stories about hurricanes that focus on their biological and evolutionary impacts. 

Story 1 – Big Footed Lizards

Paul Hirst (Phirst) / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.5)

Anole lizards have long textured toepads that are adept at gripping. This is a useful skill as these lizards spend much of their lives above the ground using the tropical tree branches as an interconnected highway of sorts. For a long time, scientists assumed that this trait had evolved gradually over time as these lizards became more arboreal (tree-living). 

Then in 2017 hurricanes Irma and Maria hit the research sites of biologist Colin Donihue who was studying Caribbean anole lizards. This allowed him to compare populations before and after a hurricane. What Dr. Donihue found was that those lizards who had managed to survive had both larger feet and smaller hind legs – two traits that would help an individual cling to a tree, rather than get blown into the ocean or plummet to the ground. (Pause here to image a lizard holding onto a tree while 80+ mph winds swept through.) 

However, adaptions often have trade-offs and traits that can help an animal weather a violent storm might not be that helpful the other 99.9% of the time. To determine if this long footedness would stick. Dr. Donihue and his team returned a year later to measure the feet of the hurricane survivor’s descendants. They found that this next generation also had long toepads and short back legs. Their next step was to gather and collate information from historical weather reports and from museum specimens to see if this pattern was repeated anywhere else. What they found was surprising both in its scale and consistency – across the world regions that had experienced repeated hurricanes had Anole lizards with significantly longer toe-pad lengths!

Story 2 – Social Warrior Spiders

First a little spider sociology. Most spiders are solitary, but some do live in groups. The species Anelosimus studiosus does both. Some are social and live in colonies of 500+ individuals while others live alone. In general, social spiders are less aggressive than their asocial counterparts – a trait that helps them live together but also makes them less likely to fight off predators. However, the Anelosimus studiosus spider again tries to cover all the bases. In its colonies, there are ‘warrior’ workers who are aggressive and ‘nanny’ workers who are more docile. 

In a situation similar to the Anole lizards above, scientists had the opportunity to compare Anelosimus studiosus colonies before and after the 2018 hurricanes Florence and Michael and observed more warrior members in the post-hurricane colonies. When they came back a year later, these colonies still had more warriors than their pre-hurricane ancestors. They also identified areas of high and low hurricane frequency using historical weather records and then surveyed colonies in these areas. They discovered that areas with frequent hurricanes had a much higher warrior to nanny ratio than areas that had not experienced hurricanes. 

This may be because aggressive colonies fare better when resources are scarce – such as during the immediate aftermath of a hurricane. However, this increased species aggression might not continue indefinitely – even if hurricanes become more frequent. Aggressiveness within Anelosimus studiosus appears to have a ‘hard stop’. When colonies have too many warriors or when these warriors become hyper-aggressive, they begin attacking other spiders in the group and the colony quickly self-destructs.

Story 3 – Invasive Grass with an Extra Edge

Darkone (talk · contribs) / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.0)

The common reed – Phragmites australis – is a perennial grass found throughout the world. In the US it’s commonly considered an exotic species although recent genetic studies have found evidence of a subspecies (Phragmites australis subsp. Americanus) that may be native. However, while this subspecies is a slow grower, Phragmites australisis as a whole is an aggressive colonizer that can quickly outcompete native vegetation by blocking out light and releasing harmful toxins that kill other plants. 

In places where the common reed invades, native plants like wild rice, cattails, and wetland orchids quickly disappear and the reed becomes a monoculture. This in turn reduces animal diversity, slows nutrient cycling, and alters the wetland’s hydrology. Unfortunately once established getting rid of it is difficult. Eradication of the species often requires multiple years of control burns because its roots grow so deeply. 

The rapid expansion of Phragmites australisis into the wetlands of North America was originally thought to be a classic tale of invasion due to its’ introduction into a predator/competitor free environment and human disturbance. However, when scientists took a deeper look, they found an unexpected pattern. When they overlaid the plant’s expansion history with maps showing the last 100 years of hurricanes there was a strong correlation. The species expanded much faster in areas of high hurricane occurrence. Researchers then created a computer model of the expansion of the common reed and used this to estimate that hurricanes could explain ~80% of its recent spread. 

Again, it appears the reed’s extensive roots systems might be the key to its persistence. In this case, deep roots help these plants survive the huge disturbances caused by hurricanes (high winds, soil erosion, salinity changes, debris buildup, etc.) and then thrive in the open soils created by these same disturbances and annihilating other less hardy local vegetation. Such findings suggest that stopping Phragmites australisis expansion will be incredibly difficult as land conservation can do little to protect against natural disturbances and hurricane frequencies are /likely/predicted to only increase. However, such information is also helping scientists develop new management strategies.

Interested in finding out more?

Here are the three articles discussed above.

Donihue, C.M., Herrel, A., Fabre, A. et al. Hurricane-induced selection on the morphology of an island lizard. Nature 560, 88–91 (2018). https://doi.org/10.1038/s41586-018-0352-3

Little, A.G., Fisher, D.N., Schoener, T.W. et al. Population differences in aggression are shaped by tropical cyclone-induced selection. Nat Ecol Evol 3, 1294–1297 (2019). https://doi.org/10.1038/s41559-019-0951-x

Bhattarai GP, Cronin JT (2014) Hurricane Activity and the Large-Scale Pattern of Spread of an Invasive Plant Species. PLoS ONE 9(5): e98478. https://doi.org/10.1371/journal.pone.0098478