The Web of Life (22-3-17) Carnivorous Plants

 Take A Load Off

Carnivorous plants made a wily evolutionary move: able to compensate for living in lousy soils by literally taking in animals. Directly or indirectly, the animals are a meal ticket.

Pitcher plants are carnivorous. They lure insects to the top of their specialized leaf traps with sweet nectar, where many lose their grip on the slippery rim, falling into a fluid-filled trap below.

The victims desperately try to climb out, only to discover with their last breaths that their prison is filled with stretchy fibers. The more an insect struggles, the more entangled it becomes.

Digestive enzymes in pitcher fluid break down the sunken prey, harvesting the hardest nutrient to get enough of: nitrogen. Pitcher fluid is itself a microbial ecosystem.

In the instance of the North American purple pitcher plant, the pitcher is an expansive ecosystem: a self-contained food web, home to an array of mosquito larvae, midges, rotifers, protozoa, and bacteria, many of which can survive only in this unique habitat. The little pitcher animals shred prey that fall in, with the microbiome feeding on the remainders. Finally, the pitcher plant absorbs the leftover bits.

Having the animals creates a processing chain that speeds up all the reactions. Then the plant dumps oxygen back into the pitcher for the insects. It’s a tight feedback loop. ~ American biologist Nicholas Gotelli

The rim of the southeast-Asian-native Raffles’ pitcher plant is not always slippery, letting some insects escape. It is a loss-leader strategy.

Individual scout ants search for profitable food sources. Finding a pitcher full of sweet nectar, they report back at the colony and recruit many more foragers.

By turning off their traps for part of the day on ‘dry’ days, these pitcher plants increase their overall capture. (Raffles’ pitcher plants have selective dry days where they keep their lips dry for up to 8 hours.) If the trap was always slippery, ant scouts would be in the pitcher rather than recruiting pitcher-plant food.

What looks like a disadvantage at first sight turns out to be a clever strategy to exploit the recruitment behavior of social insects. ~ German botanist Ulrike Bauer

Insect-trapping pitcher plants are small-timers compared to their cousins: the giant pitchers in the misty mountains of Borneo. These plants lure tree shrews, small rats, and bats – not to their deaths, but for their defecations. The plants are precisely sized for their clientele.

Tree shrews are enticed by the sweet nectar placed on the inside lid at the top of a giant pitcher plant. The pitcher is arranged so it can support the weight of these animals, and to situate clients so that they must crouch to take their treat.

The posturing puts a shrew or similarly sized rat so that it may easily defecate while dining, dropping nitrogen and other nutrients into the pool below. These pitcher plants get most, if not all, of their nitrogen needs met by their clientele. Swapping easily made sweets for hard-to-get fertilizer is a bargain.

Pitching for shrews is a smart strategy. There are fewer ants on the mountainsides where these plants grow.

Tree shrew droppings are especially nitrogen rich, as they have short guts that do not extract all the nutrients from their food. Plus, the speedy throughput means frequent deposits.

Taking a different tack, the Borneo pitcher Nepenthes hemsleyana provides a cozy lodging for woolly bats without the need of providing sweets. The pitcher’s orifice has an extended concave surface which distinctively reflects bat echolocation calls, making the plant easy for bats to locate amid the cluttered forest.

Pitcher walls have a girdle of thick tissue that lets the bats snugly wedge themselves in. To accommodate multiple bats, there is less fluid. What fluid there is maintains a comfortable humidity, preventing dehydration of its guests. The temperature is stable, providing a respite during the heat of midday; altogether a restful place to unload – which is exactly what the host has in mind.

Bats that lodge in pitchers are generally healthier than those that have to rough it. And the plants benefit: they economize by producing less sweets than those that attract insects. Breaking down the bodies of victims is time-consuming. Feces provides a readier source of nitrogen and other nutrients than tough, scrawny insects.