On Wed, 5 Sep 2001, Bill Jolly wrote:
> Can anyone tell me what the prospects are for a species' survival in an
> isolated location once it gets down to, a remnant population of say one
> family of four or five birds?
> Once a group reaches these low levels, is it likely that it can survive
> through a couple of generations by siblings pairing, awaiting the off-chance
> recruitment of new members?
> I've read that laboratory matings of siblings of some small vertebrates have
> continued through many (10 or 20) generations sometimes with only quite
> small genetic hiccups along the way.
I'm sure others on this list are more knowledgable but here is my attempt
at categories of threats to small bird populations. I've ordered them
in what I think is rough order of importance, most important first.
These are threats specific to small populations, not threats to birds
a) demographic fluctuation - population sizes vary randomly due to the
chance events which individuals are subject - predation, injury, disease
etc. This variation can result in extinction of a small population.
b) environmental fluctuations and catastrophes - small populations
are more vulnerable to environmental changes, such as droughts, fires,
storms, bulldozers etc, although this is more a function of range size
than population size.
c) edge effects - often small populations exist in small habitat islands.
A large proportion of these islands will be near the edge of the habitat
which may be harmful to the population, for example by facilitating
d) emigration - if immigration & emigration occur, an imbalance can
produce extinction. Of course, later immigration may restore the
e) allee effects - below a certain population size there may negative
effects due to the absence of other individuals of the same species.
This may be because of reduced cooperative interactions such as communal
nesting or flocking or because potential mates are hard to find.
f) inbreeding depression - repeated matings between closely-related
individuals can cause harmful mutations to affect a large proportion
of offsping. Effects depend on species and circumstances. Arrival of
occasional immigrants can prevent inbreeding depression. A single
unrelated individual arriving every 1-2 generations may be sufficient.
g) loss of genetic variation - genetic drift reduces genetic variation
in small populations. May reduce likelihood of population withstanding
future change, including as environmental change, arrival of pathogens
or introduced species
h) hybridization - a small population may be subject to an increased
degree of hybridization with a closely related species which may be
harmful for several reasons.
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