A limited list of prey may weave an interwoven food web by encouraging wolf spiders of multiple species to prey on each other, even cannibalizing themselves, a study from the University of Nebraska-Lincoln said.
Ecologists have long known that predators with similar diets can coexist by effectively dividing up a community’s food sources to mitigate competition and, ideally, leaving enough prey for everyone. But species analyzes of Nebraska wolf spiders suggest that when the diversity of their mutual prey decreases, the eight-legged predators may maintain ecological balance, in part, by eating each other.
The decline in prey diversity should spell bad news for weaker predators, who are then put in more direct competition with their stronger counterparts, said Stella Oeterwall, who led the study while doing her PhD in Nebraska. Predators that are at least occasionally successful in killing and eating their more competitive peers, she said, could benefit in two ways that collectively act as a “balancing mechanism.”
“Some of your diet is now coming from that other predator, rather than the common prey you’re competing for,” said Uiterwaal, now a postdoctoral researcher at Washington University in St. Louis. “You also reduce the population size of this top predator, so you have fewer of them to compete with.”
The study arose from what Uiterwaal observed while studying and later teaching at Cedar Point Biological Station, a field site adjacent to a lake in southwestern Nebraska. There, she and some colleagues, including doctoral advisor John DeLong, realized that the native wolf spider species wealth seemed to defy an ecological principle by occupying more or less the same niche of the same habitat.
“We’ve noticed that there are many different species of wolf spiders that seem to be doing the same thing,” Oeterwall said. “And there’s this classic ecological notion that species can’t do exactly the same thing. If that happens, they won’t be able to survive in the environment for very long.”
So the researchers spent two summers collecting samples from eight species of wolf spiders and their potential prey. Wanting to get as accurate as possible in enumerating that prey, Uiterwaal took to placing hollow wooden boxes on unexpected plots of land, then using a custom vacuum cleaner to suck up every ground-bound insect and plane inside. Several captured creatures also appeared on the wolf spider’s menu: flies, grasshoppers, crickets, butterflies, moths, aphids, and yes, other spiders.
“You name it, and they’ll eat it,” Oeterwal said. “We’ve even seen spiders there eating frogs.”
However, cataloging the actual diets of 605 wolf spiders would require more sophisticated techniques. One such technique involved analyzing the DNA of the spiders’ digested food for barcodes: DNA sequences unique to each type of prey the spiders consumed. Uiterwaal also applied a mathematical method — one it developed — that helped the team ascertain how much of each prey the spider had consumed.
Contrary to the team’s expectations, the diet of any species of wolf spider is mostly similar to other species.
“All of these spiders eat basically the same things — which I wasn’t expecting, because you find these spiders in slightly different places, they look different, and they have different behaviours,” Oeterwall said. “You’d expect that to be reflected in their diet in some way. But it turns out they overlap a lot.”
Their discovery has left the possibility of a spider world that eats and eats it. Just one problem: Given the challenges of differentiating the DNA of a wolf spider species, Uiterwaal knew the barcode would struggle to capture any spider-on-spider predation. To explain this, she and the team analyzed the ratio of lighter versus heavier nitrogen atoms, or isotopes, in the tissue samples of each wolf spider they collected. As heavier nitrogen atoms persist and accumulate throughout the food web, predators tend to have more of these isotopes than their prey—which means researchers can use them to estimate an animal’s rank in the local food web.
It also means that if wolf spiders eat each other regularly, isotope analysis on the food web is more likely to rank them higher than the barcode-based method. That’s exactly what Utterwall and her colleagues found. In fact, the average ranking far exceeded what the team expected. In many food webs, plants rank at 1, herbivores at 2, and the predators of those herbivores are 3, with predators rank at 4. The terrestrial food web seems to stretch.
Average order of wolf spider species at Cedar Point Biological Station? It’s nearly a 6. A given spider has a rating of 8.5—a particularly high place for a predator that, Otterwall said, “isn’t exactly what anyone would call the top of the food chain.”
“It points to this level of complexity and predation that is probably really important in determining how the whole system works,” said DeLong, an associate professor of biological sciences at Nebraska. “Instead of thinking about these short food chains where everything is very vertical, it’s really this iterative thing where everyone eats everyone, it kind of piles on itself.
“The implication of how the food web is organized is really different than what we imagined going into this.”
The team was in for another surprise. Uiterwaal decided to analyze how much certain factors—the sex and size of a predator, the characteristics of its environment, and the abundance and diversity of its prey—might influence the likelihood of one wolf spider preying on another. Previous lab experiments have suggested that all of these variables may be at play. But the team found that only prey diversity, or the lack thereof, was associated with wolf spiders attacking themselves.
Although a number of reasons can help explain the difference, Oeterwall said the differences between the laboratory and the wild are probably a good place to start.
“I think foraging in the field is very different than foraging in a petri dish in the lab, where you don’t have all these other things to worry about,” she said. “You’re worried about being eaten by other spiders or other predators[in the wild]. You probably have parasites to deal with. You’re also trying to find mates, and find the right temperature areas for you. You have all these other things going on that might These are overwhelmed by the effects we see in the lab, when we’re just playing with one specific variable.
“The fact that these predictions we have from the lab won’t (necessarily) translate well into complex real-world situations — it’s not just about spiders. That would be true of any system.”
DeLong credited his former advisor, whom he called a “real hero”, for conducting such an ambitious study and managing to uncover nuances that laboratory work might miss.
“It collected really different types of data,” he said, “to paint a different kind of story than anyone had told before.”
The team detailed its findings in Journal of Animal Ecology. Uiterwaal and DeLong co-authored the study with Nebraska’s Amber Squires and Bennett Grappone, Cornell University’s Brian Dillard, UCLA’s Mercedes-Benz Sora Kim and Ariadne Castaneda. The researchers received support from the National Science Foundation.