Monday, April 23, 2012

Pitcher plants as ecosystems and not just carnivores?!

Species name: Sarracenia purpurea

Common name: pitcher plant

Location: Nova Scotia

Despite there being numerous species of carnivorous pitcher plants around the world, there is only one species in Canada. This species of pitcher plant occupies boggy habitats from Newfoundland all the way across Canada to British Columbia, and to the north through Yukon, Northwest Territories and Nunavut.

The morphology of different pitcher plants is different, but all of them have one thing in common: they all use a trapping mechanism called the pitfall trap. The leaves of the pitcher plant are modified into receptacles that contain a liquid with enzymes that digest insects unlucky enough to fall into the trap. The top of the trap is coated with a wax-like substance that is very slippery, ensuring a higher success of trapping insects. Why would a plant want to capture insects in the first place? Typically carnivorous plants  live in very nutrient poor environments, and require the vast majority of their nutrients to come from sources that don't include the soil that they sit in. An excellent source of nitrogen and phosphorus, two elements required in relatively large numbers by plants, is the insect body since it's composed of mostly proteins (great sources of nitrogen). Various carnivorous plants employ a huge diversity of trapping mechanisms, but all of them converge on the same purpose which is as a method of nitrogen acquisition.

The idea that pitcher plants only have pitchers to capture insects is actually one of the many misnomers of plant biology. The pitchers of a pitcher plant are actually micro-ecosystems full of complex interactions between plants and insects. In Canadian bog systems, pitcher plants are the main breeding grounds for mosquitos and other small flying insects. Here, the interaction between pitcher plants and insects is much more complex than just digesting insects for nutrients. The adult insects lay their eggs in the pitcher plants, and the larvae of these insects swim in the liquid and feed on other insects that fall into the sweet substance. Their excretions are the main source of nutrients for the plant, who absorb the insect larva poop that sink to the bottom of the pitcher. There are also mutualistic bacteria, protists (what I affectionately refer to as the "garbage can of Biology": if it doesn't fit in any other group, it's a protist), and rotifers ("wheel animals") that live in the liquid of the pitcher. They are often the first line of attack for a pitcher plant, and the plants allow these groups of species to start to break down or decay the insects that fall into the trap, releasing the nutrients from the insect for the plant to absorb.