A common observation among those studying vernal ponds is that ponds in close proximity don’t share all the same species. As I have discussed before, even when they share the same species the populations are often genetically distinct. I can clearly state the conclusion of the next series of posts by saying that vernal pool species don’t get around much. A newly created vernal pool may fill quickly with species, or not, and two new vernal pools within few meters of one another won’t necessarily have the same species composition. Okay, that’s the end of the story without all the meaningful and fun part. And maybe the reader has had the experiences to reach the same conclusion. Or perhaps after hearing some of my stories…
My first intense interaction with the topic of community structure came about as I was introduced to the ponds around Churchill, Manitoba (made semi-famous by the National Geographic feature on polar bears at their garbage dump, and yes, they are scary animals). There are two broad classes of ponds there. Ponds in the tundra outside of town and those along the granitic outcrops along Hudson Bay. These ponds have a distinct biota (sorry, nothing about amphibians here but they do occur occasionally). Tundra ponds are relatively large (they can be acres but are often smaller) and are acidic due to the peat moss. They are the home to the interesting cladoceran Polyphemus, one of the few predatory members of the group (most of it’s relatives are marine although their aren’t many species of marine cladocerans). These ponds are also the home of Daphnia middendorffiana. The nearby bluff ponds have neither of these species but rather are home to D. magna. There is no overlap in the distribution of the species even when tundra ponds are within a few meters of the bluff habitats.
Stan Dodson and John O’Brien were among the first people to study this relationship in Alaskan ponds (he had D. pulex instead of magna as his other species). In the Churchill ponds, factors driving this distinction in distribution were studied by one of Paul Hebert’s graduate students, Jaimie Loaring (now MacIssac). In both regions the distribution is driven by the predatory copepod, Heterocope septentrionalis. In tundra ponds the copepod lives along with D. middendorffiana but the copepod is absent from bluff ponds where magna is found. The two Daphnia species are not found together.
There are three distribution issues: 1) Why isn’t the copepod found in bluff ponds? 2) How can one species of Daphnia co-exist with the predator but the other can’t? and 3) Why don’t the Daphnia species occur together? The answers will follow.
