Invasive brown widow spiders thrive in city landscapes

As urban habitats continue to expand, a new study suggests that these human-influenced landscapes may serve as an unexpected sanctuary for invasive species. The research, led by Dr. Monica Mowery and her team at Ben-Gurion University of the Negev, has found that invasive brown widow spiders flourish in the urban environment, largely thanks to a lack of predators and a significant decrease in parasitism.

Published in the journal Oecologia, the study sheds light on the intriguing behavior of the brown widow spider, or Latrodectus geometricus, a species that is increasingly making its presence felt in warm urban climates across the globe. 

“Our results suggest that habitat plays a key role in changing interactions with predators and parasites, and in particular that urban species may benefit from a lower abundance of predators in their habitat,” explained Dr. Mowery.

Focus of the study

To understand the variations in host and parasitoid densities, the researchers embarked on a year-long investigation in the Negev desert, comparing the egg sac parasitism of the urban invasive brown widow spider with the desert-dwelling white widow spider, or L. pallidus. 

The researchers discovered that brown widow spider populations thrived at remarkably high densities in urban environments – with web distances as close as 10 centimeters apart – while experiencing astoundingly low rates of parasitism from their common foe, a parasitoid wasp that specifically targets spider egg sacs.

By contrast, the white widow spiders, despite populating dense communities in the Negev Desert, were heavily infested with parasites. The stark differences observed in parasitism rates suggest that urban environments provide a protective shield of sorts to the invasive spiders.

Bold field experiment 

To investigate further into the effects of habitat on these species, the researchers initiated a field experiment, transplanting spider webs containing egg sacs from both species between urban and natural desert habitats. 

The outcomes were fascinating: not only did the egg sacs in the natural habitats experience higher parasitism, but predation on the white widow spider egg sacs was also significantly more pronounced in the desert ecosystem, possibly due to birds, other spiders, or ants. 

Intriguingly, none of the invasive brown widow’s egg sacs in the transplant experiment fell prey to any predators, indicating that local desert predators may not recognize this newcomer as a viable food source.

Advantages in urban habitats 

This study raises crucial questions about the advantages invasive species gain when they colonize urban, fragmented habitats. These environments seemingly offer a suite of benefits, including reduced predator presence, which could contribute to invasive species thriving and persisting in unfamiliar territories.

Dr. Mowery’s research was a collaborative effort, involving researchers Valeria Arabesky, Tamir Rozenberg, Prof. Yael Lubin, and Dr. Michal Segoli from Ben-Gurion University of the Negev. The study was supported by a Zuckerman STEM Postdoctoral fellowship granted to Dr. Monica Mowery.

This research demonstrates the critical role that urbanization plays in shaping biological interactions. It also highlights the urgent need for continued research on the impacts of urban environments on our natural world.

More about the brown widow spider

The brown widow spider (Latrodectus geometricus) is a venomous spider species belonging to the widow family (Theridiidae). It is closely related to the more well-known black widow spider (Latrodectus mactans) and is found in various parts of the world, including the southern United States, Central America, South America, the Caribbean, Africa, and parts of Asia.

Key characteristics

Appearance

The brown widow spider is typically light to medium brown in color, with an orange or yellow hourglass-shaped marking on the underside of the abdomen. This hourglass marking is a characteristic feature shared with other widow spiders, although the coloration may vary.

Web structure

Brown widow spiders create tangled, irregular, and messy webs, which are often built in sheltered areas like under rocks, eaves, outdoor furniture, and other human-made structures.

Venom

Like other widow spiders, the brown widow possesses venom that is neurotoxic to its prey. While its venom is not as potent as that of the black widow, it can still cause painful reactions in humans. However, bites from brown widow spiders are rarely serious and usually only cause mild to moderate symptoms such as pain, redness, and swelling around the bite area.

Behavior

Brown widow spiders are generally shy and non-aggressive towards humans. They will usually only bite if they feel threatened or disturbed. The majority of bites occur when people accidentally come into contact with the spider, such as reaching into a dark space where the spider has constructed its web.

Reproduction

Female brown widow spiders are larger than males. After mating, the female produces egg sacs that contain hundreds of eggs. The egg sacs are typically white to tan and are suspended within the web. The female guards the eggs until they hatch.

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