But why make the assumption that the population is healthy and that only a few are being harvested?
This requires a person being intelligent and familiar with local conservation issues. I would not for example recommend collecting mountain yellow legged frogs to feed to your pet T. atratus.

Amphibian declines are caused by three things

1) Chitrid Fungus. Many north american species are resistant to this, either because the climate is not suitable for the fungus, or because the anti-fungal compounds in the frog's skin confer protection.

2)Invasive predators/competitors. Bullfrogs feed on and replace native frogs when introduced...

3) Habitat destruction. Need I say more?

Most of the species a person will encounter in their yard though are resistant to the later, because their habitat requirements are very broad. For example: Southern Leopard Frogs can be found anywhere from swamps to residential drainage canals. So long as there is water and vegetation, these frogs do fine and reproduce in large numbers.

One just needs to pay attention, and learn about the prey animals they catch.