PATTEKN \ N I ) L’RO(‘I1;SS IN (”OIIII’HTITJON
3 1
Vaurie (1951) surmised that differences in bill size and shapr are
correlated with different food habits and constitute a basis upon which
the two species can avoid competition where they are sympat,ric, although he presented no evidence that would substantiate this conclusion.
Unfortunate] y character divergence has received very little serious
attention as an ccological phenomenon and there is only indirect evidence to support the view that it is related to competition, attractive
as this theory may be. Mayr (1963) has correctly emphasized the need
for more quantitative analysis in the evaluation of character divergence.
Sonie of the examplcs cited by Brown and Wilson (l95G) may illustrate
hybridization rather than character displacement. There are cases in
which species differences arc reduced in a region of sympatry, and there
are also cases where the ovcrlap seems to have no effect on the phenotypes of the sympatric populations (e.g. sibling species). As Mayr (1963)
points out, “Only a statistical analysis can bring out the facts needed
for valid generalizations.”
E. I N V A S I O N S
Elton (1958) has documented examples of changes in distribution
resulting from the invasions of species into the geographic ranges of
their cogeners, or of species which in some way require similar resources
and become displaced in the resulting interaction. Thus the persistent
spread of the ubiquitous European starling (Sturnus vulgaris) in North
America has occurred at tlie expense of species such as the bluebird
(Sialia sialis) and the flicker (Colaytes nztmtus) which have been forced
to compete for a limited supply of the nest holes which they require
for breeding.
An overwhelming amount of evidence exists to show that when plants
or animals invade a new area, they seldom do so without disrupting the
ecological balance that existed before they arrived (Elton, 1958). The
effect of competition in nature is perhaps best demonstrated by such
invasions. Island faunas are especially vulnerable to invasion by new
competitors (Mayr, 1963) and there are many well-documented examples. Greenwap (1958) has shown that most of the species of birds that
have become extinct during the past 200 years have been island birds.
However, as Mayr (1963) points gut, sweeping generalizations that
attempt to predict the outcome of invasions from one fauna to another
are not entirely reliable. There are exceptions, both in the fact that
species native to the larger more diversified areas are not necessarily
superior competitors, and to the assumption that such invasions inevitably lead to competitive exclusion. Moreover, there is indirect
3 1
Vaurie (1951) surmised that differences in bill size and shapr are
correlated with different food habits and constitute a basis upon which
the two species can avoid competition where they are sympat,ric, although he presented no evidence that would substantiate this conclusion.
Unfortunate] y character divergence has received very little serious
attention as an ccological phenomenon and there is only indirect evidence to support the view that it is related to competition, attractive
as this theory may be. Mayr (1963) has correctly emphasized the need
for more quantitative analysis in the evaluation of character divergence.
Sonie of the examplcs cited by Brown and Wilson (l95G) may illustrate
hybridization rather than character displacement. There are cases in
which species differences arc reduced in a region of sympatry, and there
are also cases where the ovcrlap seems to have no effect on the phenotypes of the sympatric populations (e.g. sibling species). As Mayr (1963)
points out, “Only a statistical analysis can bring out the facts needed
for valid generalizations.”
E. I N V A S I O N S
Elton (1958) has documented examples of changes in distribution
resulting from the invasions of species into the geographic ranges of
their cogeners, or of species which in some way require similar resources
and become displaced in the resulting interaction. Thus the persistent
spread of the ubiquitous European starling (Sturnus vulgaris) in North
America has occurred at tlie expense of species such as the bluebird
(Sialia sialis) and the flicker (Colaytes nztmtus) which have been forced
to compete for a limited supply of the nest holes which they require
for breeding.
An overwhelming amount of evidence exists to show that when plants
or animals invade a new area, they seldom do so without disrupting the
ecological balance that existed before they arrived (Elton, 1958). The
effect of competition in nature is perhaps best demonstrated by such
invasions. Island faunas are especially vulnerable to invasion by new
competitors (Mayr, 1963) and there are many well-documented examples. Greenwap (1958) has shown that most of the species of birds that
have become extinct during the past 200 years have been island birds.
However, as Mayr (1963) points gut, sweeping generalizations that
attempt to predict the outcome of invasions from one fauna to another
are not entirely reliable. There are exceptions, both in the fact that
species native to the larger more diversified areas are not necessarily
superior competitors, and to the assumption that such invasions inevitably lead to competitive exclusion. Moreover, there is indirect