Distribution, Abundance and Rarity

Distribution, Abundance and Rarity
Within their geographic range, most organisms show:
– High abundance in a few areas
– Low abundance in many areas
Distributions are most commonly related to environmental gradients
Zones of Tolerance
V. E. Shelford, tiger beetles and
– moisture
– temperature
– light
Critical maximum, critical
minimum define “zone of
Highest abundance where
most conditions are optimal
Range of Tolerances
Differing organisms have
differing tolerances
Two useful prefixes:
– steno = narrow
– eury = wide
Wide tolerance = wide distribution?
Narrow tolerance = narrow distribution?
Grinnell (1917) - Niche as Habitat
– place that an organism lives
– physical conditions encountered or needed for survival
Elton (1935) - Niche as Function
– what an organism does
– the “role” of the organism in the environment
– also known as the ecological niche
Hutchinson (1957)
Fundamental niche
– n-dimensional hypervolume
– sum of all “zones of tolerance”
– describes where an organism could occur
Realized niche
– subset of fundamental niche
– the space actually occupied by the organism
– fundamental niche reduced by environmental limitations
Populations are not always directly connected
Since habitat is patchy, populations may also be patchy
Dispersal between separated populations influences population persistence
Commonness, Rarity, and Extinction Vulnerability
Three attributes:
– Geographic range (large to small)
– Habitat tolerance (broad to narrow)
– Population size (large to small)
Given these attributes, which combinations would lead to rare species?
How would this affect extinction risk?