In ecology there is no such thing as community spirit. Cooperation and altruism, we are told, are illusory, or simply the outcome of the activity of selfish genes. Yet the interdependence of species in communities is often extremely complex. One example is the complexity that characterizes the ecology of beaver ponds once they have been abandoned by the animals that first constructed them. This is discussed in the journal Oikos by John Terwilliger and John Pastor1, who set out to discover why it takes so long for old beaver ponds to revert to forest.
When beavers dam streams and create their own aquatic environment, they inadvertently set in motion a directional process that eventually leads to re-establishment of the forest. The ponds begin to accumulate silt and are abandoned by the beavers. Then, once the dams are breached, the ponds drain and are soon invaded by herbaceous vegetation, especially grasses and sedges. This is how the beavers create grassy openings in the forest — in much the way that high winds and fire create clearings, which are subsequently recolonized by species that prefer open habitats. But with fire and tree falls, healing is usually rapid, resulting in the invasion and growth of a new tree generation within a few years. Old beaver ponds, on the other hand, may remain as damp grassland patches for decades — often 70 years or so — before trees that can tolerate these wet conditions, such as black spruce (Picea mariana), eventually manage to establish a forest cover.
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