“Everything goes ‘round and ‘round” is a common theme in my ecology classes and even on guided hikes. It is hard to understand how anything works in an ecosystem without understanding nutrient cycles. Nutrients like carbon, nitrogen and phosphorous all have to come from somewhere – you cannot get something from nothing! – though the cycles by which they travel through habitats are often hard to see. However, in a vernal pool, everything is laid out in a very clear way, once you know where to look.
Vernal pools are the temporary pools of water in forests that come from snow melt and rain water in the spring. They typically dry up by mid or late summer, but while they exist they are essential for the reproduction of several amphibians, as well as many insects. These temporary pools cannot support fish and so they are relatively free of predators – though amphibian eggs can still be consumed by a number of birds, insects, and mammals.
Due to their seasonal flooding and drying, vernal pools become nutrient-rich hotspots, often serving as “hot moments” for biogeochemical cycling, or the cycling and recycling of essential elements such as carbon, oxygen, nitrogen, and phosphorus in nature. The decaying leaves from the previous fall provide abundant nutrients for microbes and algae. The microbes and algae become food for tadpoles and invertebrates like mosquito larvae and isopods. These in turn attract more wildlife, such as birds and small mammals seeking a meal.
When the amphibians have gone through metamorphosis, they leave the dwindling pool and head into the forest, where they spend the remainder of the season and then overwinter in the forest floor. This migratory pattern transfers carbon and nitrogen and other elements into the forest, enriching it and serving as a direct pathway for nutrients to enter cycles within the forest – as the animals eat, defecate, and are themselves sometimes eaten. If they survive until the following spring, the animals born in a vernal pool will return to reproduce.
Every fall the leaf litter is renewed, and every spring the precipitation pools, creating the perfect environment for invertebrates and bacteria, to break down the organic matter and make the nutrients available for other animals and plants throughout the forest. Every summer the animals follow their migration pathways into the forest and every spring they return to the vernal pool. The cycles form more interlocking rings than we can count, but we can witness the processes if we look carefully.
