I’ve learned quite a bit about spiders over the years. (And I have never been able to understand the “burn it with fire!” some folks take towards these 8-legged creatures.) For example, it turns out that some spiders actively hunt fish, while others are vegetarian!
Then, late last year, I came across a couple of articles on an apparent mutualism between spiders and pitcher plants, published in Oecologica and in the Journal of Animal Ecology respectively. It’s not the only mutualistic relationship that pitcher plants are part of: for example, some act as toilets for tree shrews, and gain significant benefits in the process. (My blog buddy Grant also discussed this story.)
Crab spiders are cute little creatures. The family they belong to has around 2100 species worldwide, with 11 or so found here in New Zealand. Unlike most spiders they’re not active hunters and don’t use webs to catch prey; instead, they wait in ambush for dinner to drop by. In the first study, a research team from Singapore investigated two crab spiders that hang out on pitcher plants, to see how their hunting activities might impact on the plant. (In nutrient-poor environments, pitcher plants rely on catching insect prey to obtain the nitrogen that they need for growth.)
The researchers noted that crab spiders live in the pitchers (which are highly modified leaves) of several Nepenthes species. They decided to carry out lab experiments to investigate whether two particular species of spider were a) stealing prey that the plants had already caught (in which case, the spiders would be kleptoparasites), or b) catching – and potentially dropping into the pitcher – prey that the plants might not normally catch. Either way, the plants might still benefit if enough of the spiders’ left0ver meals made it into the pitcher to be digested there, and providing at least some of their nitrogen to Nepenthes.
The team found that the pitchers were were able to catch flies even when no spiders were present, but that having a resident spider increased the overall rate of capture. Both spiders ambushed their prey around the mouth of their host pitchers, but Thomisus nepenthiphilus was a better fly-catcher than Misumenops nepenthicola. I was amused to read that T.nepenthiphilus grabbed its prey, while M.nepenthicola tended to push flies into the pitcher fluids! Presumably it fished them out again afterwards.
Once the spiders had eaten – remember that they feed by sucking liquids from the bodies of their prey – they dropped their leftovers into the pitchers. As you might expect, the corpses were somewhat depleted in nitrogen, but there was still a measurable amount left for the plants; just not as much as if they’d caught the flies directly. The researchers suggested that the apparent loss in total nitrogen availability “can be offset by the increased crab spider-assisted capture rate of flies when environmental prey availability is low.”
That was in the lab. In the wild, crab spiders also take larger prey such as larger flies, moths, wasps, and cockroaches, all of which are occasionally trapped by pitcher plants. Presumably there’d be a greater benefit to the plants when the leftovers of these larger meals are discarded by the spiders. This was investigated further in the second study, which looked at the impact of the crab spider Thomisus nepenthiphilus on the nutrient budget of the pitcher plant Nepenthes gracilis. They found that pitchers where the spider was present contained higher numbers of many prey species, and that the spiders’ feeding reduced the available nutrients in the bodies that they fed upon. However, the size of the prey animals was an important part of the equation:
Overall, T. nepenthiphilus‐assisted prey capture is likely to result in a net nutrient gain for N. gracilis that is proportional to the size of prey consumed by T. nepenthiphilus.
Because of this, the researchers concluded that “resource conversion mutualisms” are more likely to be a thing where high-quality resources are available, due to the nutrient ‘tax’ levied on the pitcher plants by the spiders as they feed.
You might wonder what the spiders are gaining here, apart from a decent meal. I suspect that one of the benefits for them is protection: because they’re operating in the enclosed pitcher, or under its ‘lid’, they’re less vulnerable to predation than if they were lying in wait out in the open. They might also gain from a larger potential prey population, if insects are attracted to the pitchers by scent or colour cues.