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Sep. 13, 2022

What makes invasive species able to spread and thrive far away from their native habitat? Humans continue to transport species to new environments, intentionally and unintentionally, which can cause ecological and economic damage. Yet even with the explosive growth of invasive species, we still don't know much about which traits make the most successful invaders able to thrive and spread to new places. 

​"To better understand invasive species, we need to figure out how traits shift in invasive populations. Some individuals survive transport and establish and spread to new habitats, thus expanding their range. When this happens, traits can shift as species adapt to the new environment. For example, dispersal ability and defenses against predators and parasites may be crucial during range expansion," explains Dr. Monica Mowery. Dr. Monica Mowery is a Zuckerman STEM postdoctoral fellow in Prof. Michal Segoli's lab at the BIDR, Marco and Louise Mitrani Department of Desert Ecology, Ben-Gurion University. 

The brown widow spider, Latrodectus geometricus, is an invasive species with neurotoxic venom related to the better-known black widow. The brown widow is likely from southern Africa. Since the 1930s has been spreading to many regions around the world, including Israel and the United States, mainly through human-mediated transport. By studying a globally invasive species like the brown widow, it is possible to use the invasion as a natural experiment to answer ecological and evolutionary questions and better manage other global invaders. In the U.S., brown widows were first found in Florida in the 1930s, and by 2000 had reached California, 3,500 kilometers away. In Israel, the spiders were first detected in Tel Aviv in 1980 and had more recently spread south through the Negev Desert to Eilat and north to Haifa. Brown widows live at high densities in urban habitats like parks and playgrounds and may displace local spider species. As a result, they can negatively affect humans and the ecosystems they invade. 

Dispersal propensity is a crucial factor in invasion success but is rarely studied in the field. In addition to spreading with humans, spiders commonly disperse on their own using silk as juveniles, often soon after they emerge from the egg sac. Spiders can disperse short distances by rappelling, attaching silk lines to nearby objects, or long distances by ballooning, releasing strands of silk, and even traveling hundreds of kilometers. Even Charles Darwin was fascinated as spiders ballooned past the HMS Beagle. In a recently published study, Mowery found that brown widow spiderlings from a newly invasive population in the Negev Desert dispersed aerially under an extensive range of temperatures, wind speeds, and during the day and night (Mowery et al. 2022, Ethology). This study is important to understand the importance of ballooning vs. human-mediated dispersal during the species' range expansion. 

In another new study in Animal Behaviour (Mowery et al., 2022), Mowery, in a collaboration between Prof. Yael Lubin at Ben-Gurion University, Dr. Ally Harari at the Volcani Center, and Maydianne Andrade and Andrew Mason at the University of Toronto Scarborough, tested how traits of invasive spiders shift on a broad geographic scale on two continents. They compared eight populations of invasive brown widows, four in the United States and four in Israel, for which the first detection dates are known. They collected spiders from these populations to test which traits shifted during the invasion process. They measured their tendency to disperse, traits that may affect survival and establishment, such as spider body size, and traits related to reproduction and fecundity in the lab. 

"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. population. 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 traits shifting with invasion establishment time in Israel, but not in the United States. The lack of differences in the U.S. might be explained by the large geographic distances, the long time scale of invasion, and likely occurrence of multiple invasions," explains Mowery. 

Interactions with native competitors and predators may affect invasion success. After colonization, invasive species spread to new locations, where they may have an advantage over native species. One possible advantage is the avoidance of parasites or better defenses against them. In a recent study in Behavioral Ecology, Mowery, Valeria Arabesky, Prof. Yael Lubin, and Prof. Michal Segoli tested this idea by comparing the defenses of the invasive brown widow and a native white widow spider. They found that a common natural enemy, a parasitoid wasp, prefers native white widow spider egg sacs compared to the invasive brown widow spider. More and larger wasps emerged from the native widow egg sacs, indicating that the species is a better host for the wasp. The lower suitability of brown widow egg sacs for the wasp parasitoid may explain the rapid invasive spread of brown widow spiders worldwide. 

"Once we know which combinations of traits lead to invasion success, we will be able to predict potential invasive species better and manage species that are already spreading," concludes Mowery.

Above: Brown widow spiderlings (Photo Credit: Alfred Daniel)

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Above: Spiky brown widow egg sac (Photo Credit: Monica Mowery)

The studies were supported by a Zuckerman STEM Postdoctoral fellowship, a Mitacs Globalink fellowship, and the Animal Behavior Society George W. Barlow award to Monica Mowery. Research at the University of Toronto Scarborough was supported by an NSERC Discovery grant (#2017-06060) and a Leaders Opportunity fund grant from the Canada Foundation for Innovation (#203764) to Maydianne Andrade.

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