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Black Widow Spiders Are Acclimating To The Growing Urban Environments In The Southwest US

The southwest United States is home to a variety of venomous animals, such as rattlesnakes, scorpions and on some occasions, Africanized honey bees, or killer bees, as they are more commonly known. Considering that urban and suburban developments are expanding more rapidly in the southwest than in any other US region, animals are being displaced by urbanization at a particularly fast rate in the southwest. Although urbanization leads to environmental degradation and the disruption of native animal habitats, one particularly venomous animal group does not seem to be negatively affected by encroaching urban developments. Unfortunately for city-dwellers in the southwest, this animal happens to be the black widow spider.

Phoenix, Arizona is the country’s second fastest growing city, and as a consequence of this city’s rapid expansion into the natural environment, many desert-dwelling animals are being robbed of the resources they need to survive. However, black widow spiders are having no problem adjusting to big city life. As it happens, black widows are making the best of their new urban landscape. For example, researchers with Arizona State University have found that black widow populations in the city are denser than they are in the natural environment. Not only that, but urban black widows have larger body sizes than their rural counterparts. Black widow populations in the city are 10 to 15 times denser than rural populations, and it has been found that urban black widows are genetically dissimilar to rural black widows. This fact has led researchers to believe that urban black widows in Phoenix did not originate from the rural dessert; instead, it is now believed that they may have originated from another region located outside of the southwest.

Urban black widows are better off than rural black widows for many reasons. Urban landscapes offer black widows superior forms of shelter, a greater abundance of food and prey, and most importantly, typical predators of black widows, most notably parasitic wasps, have a difficult time surviving urban habitats, making the most significant predators a non-issue for urban black widow populations. Naturally, these highly venomous spiders are being spotted regularly within homes and buildings in Phoenix. So if you live in Phoenix or another city in the southwest, check your shoes before putting them on in the morning.

Do you think that urban black widows in Phoenix are mostly represented by a non-native variety of black widows?

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