The ponds in this courtyard each hold plants, invertebrates (such as snails and larval dragonflies), a few goldfish, and sometimes frogs. Each pond is an example of a ‘mesocosm’, a small semi-natural version of a biological community. A biological community is a group of interacting species in one location. These communities represent one type of real community that exists in wet areas in the Eastern USA: small permanent ponds. These ponds contain invertebrates, algae, and fungi that colonize the ponds naturally. These ponds may also hold frogs (including tadpoles), and fish.
The species living in these ponds are ‘metapopulations’. A metapopulation is a group of populations, with each population in a nearby distinct location. Many wild species live in separated metapopulations such as these ponds. Individuals spend much or all of their life in a single pond, but some move (or are moved) between ponds. A species might go extinct in one pond, but the pond may be recolonized from another pond. Backyard ponds such as these can be an ecological ‘source’ of animals: sending out more individuals than they get back. Some are ecological ‘sinks’, attracting animals, but ultimately failing to help them survive and reproduce. Backyard ponds can be both sources (for some species) and sinks (for other species). For example, an eastern USA native species, the American Bullfrog, may move out to small backyard ponds as a very young froglet, grow up in the small pond, and then move back to a much bigger breeding pond as a full-sized adult. In this case, the pond is a source: they help produce more frogs. But if the pond makes it too easy for owls or neighborhood cats to attack and catch frogs, that pond may be a sink. Individuals keep colonizing the pond, losing many more individuals than they ever produce. Ponds can be a source for some species, and a sink for others.