Spider populations become more aggressive after hurricanes, study finds
LOS ANGELES - Hurricanes not only bring devastating weather and leave destruction in their wake — the storms also bring higher numbers of aggressive spiders, according to a new study published in the journal Nature Ecology and Evolution.
By closely watching tropical cyclones and anticipating their trajectory, scientists were able to sample spider populations before and after cyclones made landfall, therefore providing a picture of how these “black swan” weather events (meaning they are unpredictable events beyond what is normally expected with severe impacts) change spider populations.
The researchers discovered that following cyclones, colonies with more aggressive foraging responses were able to produce more offspring than affected colonies which were more docile, and more of the “spiderlings” born into aggressive colonies were able to survive into early winter. These changes, they found, have long-term affects on the aggressiveness of spider populations in a specific region.
Because a major weather event like a hurricane destroys the natural habitats of many animals and insects, it is generally the most aggressive creatures that survive after being forced to live off of the resources they have gathered or to re-home themselves.
Researchers from McMaster University in Hamilton, Ontario, Canada, the University of California at Santa Barbara, and the University of California at Davis decided to observe multi-female colonies of the group-living spider, Anelosimus studiosus, that occur along the Gulf and Atlantic coasts of the United States and Mexico.
They chose this particular species of spider because it is characterized by a behavioral dimorphism, meaning that individuals tend to exhibit one of two traits quite strongly due to their genetics — in this case, spiders are either docile or they are aggressive. Natural colonies of this spider tend to have a mix of both phenotypes, and the phenotype that makes up the majority in any specific colony determines the collective aggressiveness of the colony.
“Colony aggressiveness determines the speed and number of attackers that respond to prey, prey sharing efficiency and wastage, tendency to cannibalize males and eggs, and susceptibility to infiltration by predatory and parasitic foreign spiders,” the study’s authors wrote. “Like individual-level aggressiveness , colony aggressiveness in A. studiosus is transmitted down colony generations, from parent to daughter colony, and is a major determinant of spiders’ survival and fecundity in habitat-and site-specific manners.”
They observed 240 colonies as well as their paired controlled reference sites before and after each of three tropical cyclones in 2018: Subtropical Storm Alberto, Hurricane Florence and Hurricane Michael.
For each storm, researchers traveled to sites within the projected storm path to study and record data on the behavior and demographics of naturally-occurring colonies along the storm’s path, each of which was paired with a reference sit far outside the projected storm path but within the same general region.
After 48 hours, they would return to each colony to see which had persisted through the storm.
Not only did researchers find that the more aggressive colonies affected by tropical cyclones were able to produce more offspring which lived longer than their docile counterparts, they found that docile spiders were dominating in control areas.
What’s more, researchers found that “the regional variation in colony aggressiveness was correlated with the local history of cyclone disturbance events. The proportion of spiders that respond to prey was positively correlated with the number of cyclone strikes (category 1-5) in the county over the past 100 years.”
In other words, this isn’t a short term effect of tropical cyclones — these kind of “black swan” weather events have the ability to change regional variation across populations for generations to come.
“As the environmental impacts of cyclones only promise to grow as sea levels rise, the pressure to understand the environmental impacts of these storms has never been more pressing,” the study’s authors conclude. “These data provide compelling evidence for tropical cyclone-induced selection driving the evolution of an important functional trait and show that black swan events contribute to within-species diversity and local adaptation."