White-tailed deer have increased dramatically in the latter 20th century, nearly to the point of threatening certain plant species and the herbivores dependent upon them.
Dozens of varieties of spawning and resident fish are supported by the Delaware River. Resident species like smallmouth bass, channel catfish, hybrid muskellunge, bullhead, white perch, and walleye pike thrive in the river. The river's tributaries maintain stocked trout. Due to improving water quality, large schools of striped bass, shad and herring are again making their seasonal, upstream migration to their spawning areas. The federally listed endangered Shortnose Sturgeon can be found in river segments between Philadelphia and Trenton, and the globally rare Atlantic Sturgeon swims as far upriver as Trenton.
Rare species of (non-harmful) freshwater mussels and sponges grow in Tohickon Creek PA, a stream of very high water quality.