Lectures 27
– Populations Regulation
March
27, 2000
(Chapter
21)
I.
Populations increase
geometrically - but limited by carrying capacity
resources
disease
predation
II.
Popns are dynamic
examples
from Europe
Great Tits varies fourfold
Pied Flycatchers varies
annually by 50%
long-term
trends
Brown-headed Cowbird, Glossy Ibis, Cattle Egret very
recently rare in e. US - now very abundant
Black Ducks & Red-headed Woodpeckers have declined
Finland (233 species) over last 100 yrs
25% down
22% fluctuating
34% increased
20% stable
III.
Popn growth patterns
dN/dt = rN (continuous)
for discrete model:
Nt+1=Nt*r
Nt+2=Nt+1*r, but Nt+1=Nt*r so
Nt+2=(Nt*r)*r, or
Nt+2=Nt*r2, etc.
so we can figure out population doubling time for a given r, or what r
does it take to double a population in t years. How?
Nt=N0*rt : solve for Nt=2N0
2N0=N0rt : divide both sides by N0
2=rt
log(2)=log(rt)
log(2)=t*log(r), so if we know r,
t=log(2)/log(r)
or, if we know t
log(r)=t/log(2)
r=et/log(2)
r varies
large birds = 10 -30%
small birds = 50-100%
Sigmoid
curve
120 starlings in 1890 became 120,000,000 in 50 yrs
8 phaesants on Protection Island in Wash. turned into 1325 in 5
yrs.
Cattle Egrets NA in early 50s 2000 fold from 1956-71
r=.21 for whole period
r=.84 at peak growth period
House Finches released in 1940 = .23 from 62-71
IV.
But we’re not knee
deep in cowbirds - why not?
Regulation vs. limitation
Regulation is density
dependent
mortality and fecundity vary
with popn size
too many Snow Geese, degrade nesting salt marshes – opening spring hunting season
Limitation density independent
storms, etc.
Limits -- 4 factors
habitat
rails and bitterns decline
with loss of marshes
N. Spotted Owl declines as
old-growth forest is cut
Red-cockaded WPs need old-growth pine, open understory, red heart fungus
what does habitat provide?
nest sites
food
habitat modification by
humans
forest -> fields favors
some species
managing forests cuts dead
trees-lose snags,
etc.
deer overbrowse understory
wintering habitat
dense populations on
wintering grounds
1 ha of winter = 5-8 ha
breeding grounds
climate & food supply - often related
Peruvian seabirds and anchovy popns/El Niņo
Guano industry
27-6 million
starvation often over winter = leads to irruptions
disease (and parasites)
Hawaiian honeycreepers decimated by avian malaria and pox
why are island species more
vulnerable? -
resistance is expensive
so it’s lost - just like
flight
Cycles in Red Grouse in
Scotland linked to nematode
Regulation
Social forces - territorial behavior especially
when territories filled in primary habitat overflow forced to
sub-standard
sub standard fills up,
floating population develops
Recruitment
young produced locally
immigrants
Example - Great Tits in Europe - especially Wytham Woods
primary factor is juvenile survivorship over 1st winter
Other factors also play a
limited role
habitat
territorial behavior
dispersal
breeding season fecundity and survival density
dependent (clutch size smaller when pop. high, but not as important as
juvs over winter
How is this demonstrated?
experimentally remove young
and eggs on island
juvenile suvivorship
11->20%
adult survivorship
26->54%
population regulation by food limitation in non-breeding season seems to be the rule
many populations have been through bottlenecks:
Laysan Teal
Red-tails on Socorro Island
genetic evidence shows that
others seem to have been through bottlenecks, even
Red-winged Blackbirds
Black-capped Chickadees
reduce genetic variability
