Native pickerelweed (Pontederia cordata) shows off its purple flowers and lush green foliage along the shore of Capaum Pond. Photo credit: K.A. Omand.

The answer to that question might surprise you. Ponds are hotspots for insects, birds, amphibians, and reptiles. The combination of wildlife and ever-changing lush landscapes also attracts the human eye. We are fortunate on Nantucket; while sometimes we have “too much of a good thing” in terms of over-exuberant pond vegetation, most of our aquatic plants are native to our region. Many provide vital habitat and food for microscopic zooplankton and insects, in turn feeding larger organisms like turtles, fish, and a wide variety of birds. Pond shore plants like native blue flag iris (Iris versicolor), magenta flowered water willow (Decodon verticillata), and purple flowered pickerelweed (Pontederia cordata) provide a feast for the eyes and host a multitude of insects from dragonflies to native bees.

Our native blue flag irises (Iris versicolor, pictured, and Iris prismatica) grow in large clumps and produce spectacular flowers, but do not spread aggressively throughout the pond shore, as does the invasive yellow iris (Iris pseudacorus). Photo Credit: D.I. O’Dell.

Among humans, pond lovers also seem to be in the majority. In recent years, small garden ponds have been a popular feature in landscape plans. So yes, it is possible to have your very own pond right in your backyard. But what will live in that pond? Usually a garden pond will be planted with a mix of exotic waterlilies and some dramatic emergent plants that will grow along the edge, like irises or papyrus. Fish such as koi (colorful jumbo relatives of goldfish) usually become the designated inhabitants of the pond, along with the occasional bullfrog introduced “from America,” but our native spring peepers, green frogs, mallards and herons also quickly find their way to these small oases.

Non-native yellow iris (Iris pseudacorus) growing along the edge of a Nantucket pond. These introduced iris will gradually spread to fill an entire wetland or cover the whole pond shore. Photo Credit: K.A. Omand.

After all, the idea that these garden ponds are completely contained or separate from nature is an illusion, just like anything else humans construct. In addition to the benefit of attracting wildlife, we are often introducing species that have never before been found on Nantucket, or even in our region. Sometimes humans introduce these species intentionally―like the showy water lilies or yellow irises―but other times they take us by surprise, arriving as stealthy hitchhikers.

Some non-native species are invasive. That means that when they are introduced to a new habitat, they undergo a population explosion, often crowding out native species and altering how an ecosystem functions. Some species may benefit, but many species find their homes and food chains forever disrupted. Introduced invasive species can be a real challenge for humans, too. They can choke our waterways, lower property values, and cause other economic damages.

Curly pondweed (Potamogeton crispus) was once commonly used in aquariums, but grows very rapidly and has been banned in many states due to its tendency to infiltrate lakes and ponds. Photo Credit: K.A. Omand.

Curly pondweed (Potamogeton crispus) is a good example of a hitchhiker or unintended introduction. These days, aquatic gardeners are unlikely to intentionally plant this species, as it is well known for its ability to spread rapidly to fill all available space in a pond or aquarium, and then die off mid-summer, fouling pond water and causing oxygen levels to plummet. Instead, this species makes its way around the world by “accidents,” released into natural ponds when people dump aquariums, or hidden as a weed in ornamental plants often ordered online. Finding this plant in a private pond on Nantucket is a concern, because even though the pond is not connected to another body of water, it could be spread by humans or animals. A previous record of this plant on Nantucket turned out to be a case of mistaken identity, but next time we might not be so lucky.

Parrot’s feather water milfoil (Myriophyllum aquaticum), was introduced accidentally to a fishpond connected to Miacomet Pond and is being actively managed by the Nantucket Islands Land Bank. Photo Credit: K.A. Omand.

Parrot’s feather (Myriophyllum aquaticum) is another example of a recent hitchhiker, first found in an island garden pond in 2007. The homeowner said the plant appeared after a batch of koi fish were added to the pond, so it may have been present as seeds or plant fragments in the container used to transport the fish, or it may have been growing in an aquatic plant pot that was added to the ornamental pool.

If you have a garden pond or are thinking of installing one in your yard, the best plan is to choose your plants and animals carefully, checking to make sure that your selections are not included on invasive plant or animal lists. The Massachusetts Prohibited Plants List is a great place to check plants you might use in your landscaping or water garden to make sure they aren’t invasive. More specifically, there is an online guide to selected invasive aquatic plants and a few animals for Massachusetts.

Interested in more Nantucket local info? If you notice plants or animals in your pond that you didn’t add yourself, please check out the Nantucket Biodiversity Initiative website. The “Invasive Plant Species Committee” and “Nantucket Aquatic Threats” pages will help you learn more about these topics and find out who to contact on Nantucket if you suspect you have an unwanted pest species. Above all, keep your pond to yourself—never “transplant” any pond plants or animals to other ponds!