When an invasive animal species arrives in a previously uncolonized territory, it may outcompete local species by breeding more successfully, growing faster, dispersing more easily or having better defenses against predators or diseases. It is not often that a new arrival becomes established because it eats the local residents. But this is what appears to be happening with brown widow spiders (Latrodectus geometricus) in the southern states of the U.S.
Brown widows are thought to have evolved in Africa, although they were first described in the 1800s from specimens identified in South America. Today, they have a cosmopolitan tropical and subtropical distribution, with established populations in Hawaii, Florida, some Caribbean Islands, parts of Australia, South Africa, Japan, and Cyprus. In the U.S., the spiders remained restricted to the Florida peninsula for decades, but after 2000 they began to show up in other Gulf Coast states. Brown widows are now known from South Carolina to Texas, and are also well-established in California.
The problem with the arrival of brown widow spiders in these states is that they are rapidly replacing the indigenous black widows (Latrodectus hesperus). The two species of spiders appear to have very similar ecological niches, feeding on the same types of prey – insects and small vertebrates such as lizards and mammals. It is possible that prey resources are limited and that competition between the two species occurs, with the brown widows winning out.
However, a new research study, conducted by scientists at the University of Southern Florida (USF), suggests that this is not a simple case of one species being better at hunting or at occupying the habitat. Instead, the brown widows appear to be targeting the indigenous black widows and killing them out.
“We have established brown widow behavior as being highly aggressive towards the southern black widows, yet much more tolerant of other spiders within the same family,” said Louis Coticchio, who led the study as part of his undergraduate research at USF.
Coticchio spent the first part of his career as a zookeeper specializing in venomous animals in California and returned to Florida to earn a degree in biology, channeling a passion for spiders into his research projects. While collecting wild spiders in Florida, he noticed brown widows displacing black widows but not other, related species.
“I had a sneaking suspicion that Florida in particular provided plenty of food and habitat for both the brown and black widow, and that there was possibly some other area such as behavioral differences that were playing a role,” said Coticchio. “My observations in the field showed that brown widows appeared to be much more tolerant of other species outside of their genus, and so if resources were the main factor, then we should have seen the same behavior with other spiders competing for the same resources, but that did not seem to be that case.”
Coticchio devised a three-part study to investigate the reasons why brown widows were able to displace black widows. He collaborated with spider expert Richard Vetter of UC Riverside and Dr. Deby Cassill, an associate professor in the Department of Integrative Biology at USF.
Initially, the researchers used mathematical modeling to evaluate the risk factors that spiders face in terms of their survival. They found that both brown and black widows are far more likely to die by predation than by starvation. In other words, “competition for scarce resources is not a significant cause of mortality among spiderlings for either species.”
Next, the experts compared rates of growth and fertility between brown and black widows, finding that brown widow females were 9.5 percent larger than black widow females, and reached reproductive maturity 16 percent earlier. Male brown widows also reached reproductive maturity earlier (21 percent) than male black widows, but the brown males were considerably smaller (25 percent) than the black males. One consequence of these differences in life history patterns is that brown widow females are about twice as fertile as black widow females, with brown widows often producing multiple egg sacs in comparison with black widows that produce just one.
Finally, the researchers placed captive female brown widow spiders in close proximity with other species of spiders and monitored what happened. When brown widows were paired with red house spider (Nesticodes rufipes) females, peaceful cohabitation occurred in 50 percent of instances, while in 40 percent the brown widows were killed and eaten by the red house spider females. Brown widows cohabitated with triangulate cobweb spiders (Steatoda triangulosa) in 80 percent of pairings and were killed in just 10 percent.
But the situation was totally different when brown and black widow females were paired. In the case of sub-adult spiders, brown widows killed and consumed the black widows in 80 percent of pairings. In pairings of adults, black widows were killed in 40 percent of trials, while they defensively killed brown widows in 30 percent of trials and cohabitated in the remaining 30 percent.
Throughout the experiments, brown widow spiders regularly ventured into black widow webs, the researchers say. Red house spiders and triangulate cobweb spiders also showed such “bold” behavior, but black widows were never observed as aggressors. Stereotyping black widow spiders as fearsome and aggressive is clearly not appropriate.
However, characterizing brown widows as “aggressive” is also somewhat misleading. They may be the aggressors against black widow spiders but humans have no need to fear them. Their venom is less dangerous than that of a black widow and they life peacefully in close proximity with humans. In fact, they tend to be “very shy when harassed by humans or larger animals that are not considered prey,” according to Coticchio. “They will run or roll up into a ball and play dead when being attacked or harassed by most other animals outside of their prey range.”
It is not yet understood why brown widow spiders behave in this way towards black widows. Certainly, it is of concern because this may lead to a shift in species composition – already, the increased incidence of brown widows in the southern U.S. is matched by noticeable declines in the numbers of black widows, potentially threatening the status of the indigenous black widows in their normal range.
“One question I would love to answer is how brown widows interact with other species of spiders, more specifically black widows, in Africa, where brown widows are believed to have originated,” said Coticchio. “I would love to see if their behavior and displacement of black widows is something that they have adapted here in North America, or if this behavior is something they exhibit naturally, even in areas where they have coevolved with black widows for much longer periods of time.”
The research is published in the journal Annals of the Entomological Society of America.
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