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In biology, a species is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. Other ways of defining species include their karyotype, DNA sequence, morphology, behaviour, or ecological niche. 1

Acanthocephala terminalis
Leaf-footed Bug
Accipiter striatus
Sharp-Shinned Hawk
Acer negundo
Box Elder
Acer pensylvanicum
Striped Maple
Acer rubrum
Red Maple
Acer saccharum
Sugar Maple
Acrosternum hilare
Green Stink Bug
Agelaius phoeniceus
Red-winged Blackbird
Agelenopsis pennsylvanica
Grass Spider
Ageratina altissima
White Snakeroot
Aix sponsa
Wood Duck
Ambystoma laterale
Blue-spotted Salamander
Ambystoma maculatum
Spotted Salamander
Amphion floridensis
Nessus Sphinx Moth
Anas platyrhynchos
Anaxyrus americanus
American Toad
Apioperdon pyriforme
Stump Puffball
Apis mellifera
Honey Bee
Aquarius remigis
Common Water Strider
Aquilegia canadensis
Wild Columbine