Dispersal In A Globally-Invasive Widow Spider

This is a guest post by Dr. Monica Mowery.

Title Image Credit: Sean McCann, CC BY 2.0, Image Cropped

Dispersal and life history of brown widow spiders in dated invasive populations on two continents (2022) Mowery et al., Animal Behaviour, https://doi.org/10.1016/j.anbehav.2022.02.006

The Crux

As I write this, I can hear invasive myna birds chirping in the trees outside, and see yellow pollen from the invasive Acacia trees floating through the air. What makes these species able to thrive far away from their native habitat? Despite the knowledge of how harmful invasive species can be, humans continue to transport species to new environments, both intentionally and unintentionally. Yet even with the explosive growth of both invasive species and invasion ecologists, we still don’t know a lot about which traits make the most successful invaders that can thrive and spread to new places.

One way to investigate this is to compare invasive populations that have just arrived at a new place, with populations that have been in an area for a long time. To better understand invasive species, we need to figure out how traits shift in invasive populations, as some individuals survive transport, establish, and spread to new habitats, expanding their range. When this happens, traits can change, or shift, as the species adapt to the new environment. Such traits, such as body size, number of offspring, and dispersal ability, may be particularly important during range expansion. This study is an investigation into how traits of invasive spiders shift on a broad geographic scale on two continents.

Did You Know: Spiders Can Fly!

Spiders commonly disperse using silk as juveniles, often soon after they emerge from the egg sac. Spiders can disperse short distances by rappelling, attaching silk lines to something close by, or long distances by ballooning, where they release strands of silk, and can even travel hundreds of kilometres. Even Darwin was fascinated by ballooning as spiders ballooned past The Beagle. Scientists continue to discover new aspects of this behaviour, from electric field effects on ballooning (Morley & Robert 2018), to symbiotic bacteria that affect ballooning (Goodacre et al. 2009), to the broader effects of aerial dispersal on distribution range (Buzatto et al. 2021).

What We Did

The brown widow spider, Latrodectus geometricus, is an invasive species with neurotoxic venom, and is related to the better-known black widow. The brown widow is likely from southern Africa, and since the 1930s, has been spreading to many regions on several continents, including Israel and the United States. We compared eight populations of invasive brown widows, four in the United States, and four in Israel, where we knew when the spiders were first detected. In the U.S., the populations ranged from Florida, where brown widows were first found in the 1930s, to over 3,500 kilometres away in California, where the spiders were found after 2000. In Israel, the spiders were first detected in Tel Aviv in 1980, and have since spread south to the Negev Desert.

To test which traits shifted during the invasion process, we collected spiders from all of these populations, then took them back to the lab to measure their tendency to disperse, a key trait of invasive spiders. Besides being transported by humans, spiders can also travel long distances by ballooning, where they release silk and are carried on air currents.

We also measured traits that may affect survival and establishment, such as spider body size, and traits related to reproduction and fecundity (number of clutches and number of eggs in a clutch).


Dispersal arena: how we measured brown widow spiderling dispersal behaviour in the lab. With a light breeze from the fan, dispersing spiderlings climbed the stick, released silk from their spinnerets, and rappelled or ballooned away from the stick.

What We Found

We found that spiders from newly established populations in Israel dispersed at a higher rate and were quicker to disperse than spiders from older established populations. This was not the case in the U.S. populations. Spiders in more recently established populations in Israel were larger than those in older populations, but there were no consistent patterns across U.S. populations.

We also found that spiders from recently established populations varied more in egg sac traits, which may be beneficial in a new, unpredictable environment. Overall, we found patterns of traits shifting with invasion establishment time in Israel, but not the United States.

Problems?

We mentioned that some of the patterns we found in the Israel were not present in the US. This difference could be because of the much greater distance between sites and potential for more introduction events in the U.S., which would lead to more variability in these populations. There is only one solution here – more tests. Luckily (for us at least), the spiders have invaded many far flung locations, so further studies should test for patterns in South America, Australia, and Asia. We found these differences in size in field-collected individuals, and differences in dispersal in the offspring of field-collected individuals. To determine if these traits are plastic or under selection, studies could rear populations in the lab for several generations under constant conditions to reduce the effects of the different environmental conditions in the source sites.

So What?

These results show that traits like dispersal and size are important for invasive spread. Selection throughout the invasion process, and in different invaded habitats, resulted in spiders with different traits. It’s important to know how traits like dispersal vary across invasive ranges, because it provides clues for how species spread and are successful in new places. Once we know which combinations of traits lead to invasion success, we will be able to better predict potential invasive species and manage species that are already spreading.


Dr. Monica Mowery is a Zuckerman STEM postdoctoral fellow in the labs of Yael Lubin and Michal Segoli at Ben-Gurion University of the Negev. She received a B.S. in biology and community health at Tufts University, working on butterfly visual signals and behaviour in Sara Lewis’ lab. Her PhD was conducted in the labs of Maydianne Andrade and Andrew Mason at the University of Toronto Scarborough, where she studied invasion success in widow spiders. You can read more about Monica’s work at her website.

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