Exact numbers of plant and animal species occurring in the Big Thicket region are difficult to state objectively due to a number of factors including: the imprecise boundaries of the area; which (if any) accidental and vagrant species to count; whether to count extinct and extirpated species; whether to count introduced and invasive species; migratory vs. resident species; frequent taxonomic changes and taxonomic instability. These and other factors all contribute to what sometimes appear as inconsistent or contradictory counts and numbers.