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by Naomi Stewart
Deep in the recesses of Canada’s wetlands, nestled in sphagnum bogs and pregnant with the water of the last rain is Sarracenia purpurea, our only native carnivorous plant. The provincial flower of Newfoundland and Labrador, S. purpurea is more commonly known as the (purple) pitcher plant, due to its elongated, hollow, carafe-like shape, a shape ideal for catching and holding rainwater. The pitcher plant is found in wetlands, with a vast range extending from Alberta straight through to the Atlantic coast and scattered as far south as Virginia. The purple pitcher plant is the only species of the genus that can handle cold temperate regions – quite a hardy plant.
Unlike some of its more aggressive carnivorous relatives that trap insect prey quickly and then use a plethora of deadly digestive enzymes to break down the chitin found in their exoskeletons, the pitcher plants attack depends on a combination of drowning and a community of organisms called inquilines. An inquiline is an animal that lives un-harmfully in the home of another animal, a behaviour known as commensalism.* As S. purpurea ages beyond its first year, it loses the digestive enzymes that helped it kick-start its life, and instead comes to depend mutualistically upon this unique inquiline community to provide them with nutrients in exchange for this home, as described below.
The purple pitcher plant has a small hood flap at the top that allows rainwater to fall into it without drying out. The plant emits a sweet nectar scent to seduce passing insects, luring them in with the possibility of refreshments. The bright red and purple colouring of the veins, which stand out in contrast to the green vegetation found in wetlands, also draws in certain prey.
Small, slippery hairs line the inside of the hollow-tubed pitcher plant and point downwards, so that any enamoured insects that have come to investigate the sweet smell cannot grasp a hold to climb back out. Prey eventually drown in exhaustion in the pool of collected water (called “phytotelma”) at the bottom. At this point, the inquiline team gets to work eating and digesting the drowned corpses, which are generally small insects like ants, but can also include larger organisms like spiders (I’ve even heard anecdotal evidence of small frogs being stuck in large pitcher plants). Through the physical breakdown of the prey, as well as the release of their excrement, the inquiline community extracts nutrients from the insects that the pitcher plant cannot access due to their lack of digestive enzymes.
The inquiline community of the pitcher plant is a fascinating combination of bacteria, rotifers, midges, and protozoa. Common species include Habrotrocha rosa (rotifer), Metriocnemus knabi (midge), and Wyeomyia smithii (mosquito larva). Several of the inquiline community species, including the midge and mosquito larva, are species only found in the pools of water found in S. purpurea. The wispy mosquito larva is of particular importance as they are the top predator in this carnivorous food web, and their presence (and pitcher size) increases and enhances the diversity of the rest of the inquiline community. (It is not entirely certain why this is, but theories point to the effluence of the larval community providing rich nutrient resources to the bacteria).
To have developed such an individualized food web and miniature ecosystem within its own body, and be the exclusive host to several species, is a unique and beautiful role to play within North American wetlands. As we near the end of a long winter, we can look forward to the opportunities to slowly paddle down quiet, still Canadian lakes, pull upside along a chartreuse patch of sphagnum, and admire the curvaceous, dangerous beauty of S. purpurea, efficient predator with its internal army of inquilines.
For more information, check out: http://www.sarracenia.com/faq/faq5538.html.
*Interesting note: commensalism comes through French from the Latin commensalis meaning ‘to eat at the same table’ (from com- (together/with), and -mensa (table/meal).
Naomi Stewart is a project associate with the United Nations University – Institute of Water, Environment, and Health, in Hamilton, Ontario. Previously a managing editor at the Water Quality Journal of Canada, and a research assistant at Environment Canada in water quality and monitoring, she has a lifelong interest in all natural science communications, but particularly water-based and agricultural sciences. She learned about the awesome S. purpurea through an undergraduate research course in Algonquin Park, where she canoed around and harvested the pitcher plants to study the inquiline community. You can read more popular science at her Deciphering Science blog: http://uncloaked.wordpress.com