Pitcher plants eat their spider competitors

Sarracenia purpurea, the purple pitcher plant, is popular plant among carnivorous plant enthusiasts. It’s native range is across the eastern United States and Canada. It’s a small, hardy plant so if you have a sheltered location and the right soil you can often leave it out to fend for itself. If you leave it to fend for itself then you’ll find mostly ants in the pitcher, but that’s not all. New research by Milne and Waller looks at the relationship between spiders and purple pitcher plants. They find that when it comes to spiders, the pitcher plants have a preference.

They examined over one and a half thousand captured spiders, to see what the pitchers were taking. Of these about two-thirds were sheet web‐spiders (Linyphiidae) and wolf spiders (Lycosidae). However, they certainly weren’t two-thirds of the local spider population.

The authors point out that different spiders have different talents, for example: “Pisaurids (nursery web spiders and fishing spiders) were rarely captured but had high abundances in the environment. This may be due to the unique ability of many pisaurids (e.g., Dolomedes) to dive into and walk on top of water…, thereby avoiding capture if they did fall into pitchers.”

In the case of Linyphiids, Milne and Waller compare the spider’s habit of building webs at about the same height as a S. purpurea trap. To make matters worse, a Linyphiid generally likes to make a web of about the same size as the interior of a S. purpurea trap. If the plant is attracting prey and sending them down, then the inside of a pitcher is an excellent place to build a trap, right up to the moment you lose your footing and fall in yourself.

It would suggest that the victims are simply the spiders that hand around near the pitchers, but Milne and Waller found evidence suggesting there was more going on that that.

For example, while being a resident on or around the plant increased risk of death for spiders, not all residents were easily captured. Agelenids, funnel-weaving spiders, were rarely found in traps, despite being covering the pitcher plants with their webs.

Milne and Waller also set out some pitfall traps and compared what fell into cups of soapy water with what they found in the pitchers. They found more ground-running spiders in their pitfall traps and fewer sheet weavers. There is clearly something more going on than random falls in.

Another explanation might be the nectar the pitcher plants produce. Linyphiids weave webs that connect to pitcher plants, and then sit by the edge of their webs. It means they’ll be exposed to the nectar offering that the pitcher plant produces and, sooner or later, there’s some nectar that proves a bit too tempting. It might also tempt other spiders, if they were around but they wouldn’t have the habit of sitting in the right place to be tempted that the Linyphiids have.

This study is a helpful comparison of the variety of spider prey in traps with the wider local spider population. It will be a useful source to refer back to as more research examines interaction between spiders and pitcher plants. This approach might be particularly interesting where plants like S. purpurea have become invasive species.