The biological expression of an organism to live within certain
prescribed physical, chemical, and biological parameters. For aquatic
organisms living in cold, highly oxygenated water, the limits for life
are defined by the amounts of dissolved oxygen, fluctuations in ambient
temperature, and a variety of other related conditions that, taken together
characterize the world’s trout rivers. Exceeding a tolerance limit
for any one factor effectively eliminates all organisms from that zone
that share that characteristic.