Common toads and frogs include the Gulf Coast toad ("Incilius nebulifer"), Blanchard's cricket frog ("Acris blanchardi"), green tree frog ("Hyla cinerea"), squirrel tree frog ("Hyla squirella"), spring peeper ("Pseudacris crucifer"), eastern narrow-mouth toad ("Gastrophryne carolinensis"), American bullfrog ("Lithobates catesbeianus"), bronze frog ("Lithobates clamitans"), and southern leopard frog ("Lithobates sphenocephalus"). Two other frequently encountered tree frogs, Cope's gray tree frog ("Hyla chrysoscelis") and the gray tree frog ("Hyla versicolor"), are identical in appearance and can only be distinguished by subtle differences in their calls or by laboratory analysis. The East Texas toad, commonly seen in the Big Thicket, has been controversial among herpetologist and taxonomist, some arguing that it is a distinct species ("Anaxyrus velatus") while others argue that it is a hybrid between Woodhouse's toad ("Anaxyrus woodhousii") found to the west and Fowler's toad ("Anaxyrus fowleri") found to the east. Two rare and highly secretive frogs are the pickerel frog ("Lithobates palustris") and the southern crawfish frog ("Lithobates areolatus") which is a fossorial species, spending much of its time in crayfish burrows and other small cavities in the ground. One invasive species, the small Rio Grande chirping frog ("Syrrhophus cystignathoides"), is native to the Rio Grande Valley and Mexico but, it does not appear pose an ecological threat to other species as invasive species often do.