At Brian Cannon's Ellicott City home, the spring peepers were making a racket. But when he took his 10-year-old son Brendan to a nearby pond at the end of Montgomery Run Road to listen for frogs this summer, the two didn't hear a thing.
There was "not a chirp, not anything," said Cannon, who had signed up for the national Frogwatch USA monitoring program.
But there were lots of goldfish and some 2-foot-long koi lurking beneath the surface of the water, which were eating the frog eggs before they could hatch.
This summer, a number of Maryland ponds are endangered by harmful non-native animals and plants, and many of the invasions are the result of careless actions by people.
"You don't take your pets and dump them in a pond," said Susan Muller, a natural resources technician with the Howard County Department of Recreation and Parks, referring to the goldfish at Montgomery Run. She said the fish there probably were released by someone who didn't want them anymore, or who thought they would be attractive in the wild.
From the shore, people walking by can see a bright orange ripple moving through the water, made up of a dozen fish swimming together. Muller said there probably are hundreds more in the shallow pond.
At Font Hill Wetland Park near Ellicott City, a bushy water plant called hydrilla is filling up the pond, forming a dense brown and green layer just under the water's surface. Muller said it probably was dumped there from someone's aquarium.
"Eventually it is going to choke out all the oxygen," harming fish and other plants, Muller said. Last year, the department treated the Font Hill hydrilla with a herbicide, but "it has come back with a vengeance," she said.
Non-native 3-foot-long carp have been spotted at Guilford Pond near Savage. Muller is seeking volunteers to catch and eat them rather than let them go to waste.
"The issue [of invasive species] is everywhere," said Carol Holko, an entomologist with the Maryland Department of Agriculture.
Plants that originated in other parts of the world may choke out native species on land and fill up waterways, reducing the habitat and food sources for insects, birds and animals, she said.
Invasive animals - many of which have no indigenous natural predators - may damage habitats and compete with native species for food.
Invasive species also have an economic impact by intruding on agricultural areas and by requiring state and county resources to combat them in parks and open spaces.
The Maryland Invasive Species Council reports that dealing with invasive species cost Maryland $1.8 million in 2000 - the last year for which it has statistics. And the council has just begun to scratch the surface of the problem: The group has more than 75 species of concern on its latest list.
In Howard County, staff of the Department of Recreation and Parks' Natural Resources Division control invasive plants at the site of department tree plantings.
They are also removing acres of prolific multiflora rose from the Blandair property in Columbia. But "we don't have the resources to do that much," said Mark Raab, superintendent of the division.
Human carelessness "is certainly one of the more common pathways" for invasive species to spread, Holko said. "People will release an animal with the best intentions."
One of the most prominent examples is the aggressive, land-traveling northern snakehead fish, which became the subject of international attention last summer. A man released two of them in a Crofton pond and the state Department of Natural Resources had to poison the pond to make sure the fish did not spread to other waterways.
Invasive plants can also receive a boost from people, Holko said. Species sold for use in landscaping can spread their seeds to other areas, where they take hold and take over, she said.
One such invader that concerns Muller is purple loosestrife. Each plant - which has a distinctive purple flower - produces over a million seeds and survives better than its native competition.
At Muller's request, Cannon took Brendan and members of his Webelos Scout den to a pond on Breconshire Road in the Burleigh Manor area last weekend to pull purple loosestrife.
Cannon, a military analyst, said he likes to get the Scouts and his family involved in environmental projects. "It has real educational value for the kids," he said.
With limited funding for professionals, volunteers are one of the area's best tools for managing invasives, Muller said. A program at the Middle Patuxent Environmental Area is particularly well-organized, she said, and similar efforts are under way in other counties.
Even people who do not volunteer can be more careful with their pets and plantings.
People don't release invasives maliciously, Raab said. "They are just ignorant of the fact that they are harmful."
The Maryland Invasive Species Council recommends that people learn to recognize invasive plants on their property and pull, cut or spray them before they can go to seed.
The organization also suggests asking for native plants from nurseries and finding other places - such as pet stores and animal rescue programs - to take unwanted exotic pets.
"We just want people to think about the consequences of what they are doing," Muller said.