  Economic incentives to harvest a species usually diminish as its abundance
declines, because harvest costs increase. This prevents harvesting to
extinction. A known exception can occur if consumer demand causes a declining
species' harvest price to rise faster than costs. This threat may affect rare
and valuable species, such as large land mammals, sturgeons, and bluefin tunas.
We analyze a similar but underappreciated threat, which arises when the
geographic area (range) occupied by a species contracts as its abundance
declines. Range contractions maintain the local densities of declining
populations, which facilitates harvesting to extinction by preventing abundance
declines from causing harvest costs to rise. Factors causing such range
contractions include schooling, herding, or flocking behaviors--which,
ironically, can be predator-avoidance adaptations; patchy environments; habitat
loss; and climate change. We use a simple model to identify combinations of
range contractions and price increases capable of causing extinction from
profitable overharvesting, and we compare these to an empirical review. We find
that some aquatic species that school or forage in patchy environments
experience sufficiently severe range contractions as they decline to allow
profitable harvesting to extinction even with little or no price increase; and
some high-value declining aquatic species experience severe price increases.
For terrestrial species, the data needed to evaluate our theory are scarce, but
available evidence suggests that extinction-enabling range contractions may be
common among declining mammals and birds. Thus, factors causing range
contraction as abundance declines may pose unexpectedly large extinction risks
to harvested species.
