Copyrighted Material
12
Conservation of Small Populations: Effective Population Sizes, Inbreeding,
and the 50/500 Rule Luke J. Harmon and Stanton Braude
Introduction and Background
Population
size
is
extremely
important
in
evaluating
conservation
priorities
for
a
species.

Small
populations
are
at
risk
of
going
extinct
because
of
demographic
stochasticity
and

genetic
drift.
In
this
exercise,
you
will
learn
about
three
of
the
meanings
of
“effective
popu-
lation
size”
and
how
to
estimate
two
of
them.
You
will
then
learn
how
to
apply
these
tech-
niques
to
specific
conservation
situations,
using
the
concepts
of
inbreeding,
the
minimum

viable
population
size,
and
the
50/500
rule.

Effective Population Size
Population
size
has
a
major
impact
on
the
dynamics
of
a
population.
For
example,
in
chap-
ter
11
you
used
simulations
to
see
that
genetic
drift
reduces
allelic
diversity
much
faster
in

small
populations
of
woggles
than
in
large
ones.
Population
size
also
influences
the
chances

of
extinction
through
demographic
stochasticity,
the
random
change
in
population
size

over
time
due
to
random
variation
in
individual
survival
and
reproductive
success.
Such

events
have
a
proportionally
large
effect
in
small
populations.
For
example,
in
a
popula-
tion
of
10
individuals,
one
accidental
death
would
reduce
the
population
size
by
10%.
In

contrast,
if
the
population
were
made
up
of
1000
individuals,
one
accidental
death
would

reduce
the
population
size
by
only
0.1%.
Thus,
small
populations
are
much
more
likely
to

go
extinct
due
to
demographic
stochasticity
than
are
large
populations.

Effective
population
size
(Ne)
helps
us
quantify
how
a
particular
population
will
be
af-
fected
by
drift
or
inbreeding.
Effective
size
takes
into
account
not
only
the
current
census

size
of
a
population,
but
also
the
history
of
the
population.
Effective
population
size
is

the
size
of
an
“ideal
population”
of
organisms
(ideal
refers
to
a
hypothetical
population

in
the
Hardy
Weinberg
sense
with
a
constant
population
size,
equal
sex
ratio,
and
no
im-
migration,
emigration,
mutation,
or
selection)
that
would
experience
the
effects
of
drift
or

inbreeding
to
the
same
degree
as
the
population
we
are
studying.
For
example,
if
our
actual

population
of
50
animals
experiences
the
effects
of
drift
at
the
same
rate
as
an
ideal
popula-
tion
of
20
animals,
the
population
has
a
drift
effective
size
of
20.

There
is
no
such
thing
as
“the effective
size”
of
a
population.
Different
effective
popula-
tion
sizes
help
us
estimate
the
impact
of
different
forces.
The
effective
size
you
estimate
will

depend
on
the
scientific
question
you
are
trying
to
address
(box
12.1).
Estimating
the
ap-
propriate
effective
population
size
is
crucial
in
conservation
biology;
in
most
(but
not
all)

Copyrighted Material
126 • chapter 12
Box 12.1 Different Ways to Measure Effective Population Size
There
are
a
variety
of
population
effective
sizes
that
have
different
mathematical
and

biological
meanings.
The
terms
are
sometimes
confused
or
misunderstood
as
synonymous.

Such
confusion
can
have
serious
implications
for
understanding
and
managing
popula-
tions
of
endangered
or
threatened
species,
as
we
see
below.

Inbreeding effective size, Nef ,
refers
to
the
size
of
an
ideal
population
that
would
allow
the

same
accumulation
of
pedigree
inbreeding
as
the
actual
population
of
interest.
Pedigree

inbreeding
occurs
when
an
offspring
inherits
two
copies
of
a
gene
from
its
parents
which

are
identical
by
descent—that
is,
they
are
both
directly
descended
from
a
single
allele
pres-
ent
in
one
of
the
founders
of
that
population
(perhaps
the
parents
are
cousins
and
each

inherited
the
particular
allele
from
the
same
grandfather).
Nef is
the
measure
of
effective

population
size
that
emphasizes
the
effect
that
small
population
size
has
on
the
chances
of

relatives
mating
with
each
other.
Such
matings
lead
to
a
loss
of
heterozygosity
in
the
popu-
lation.
Thus,
this
effective
size
gives
you
an
indication
of
the
likely
loss
of
heterozygosity

across
all
alleles
in
your
population.

Calculation
of
Nef ideally
requires
pedigree
data.
However,
you
can
estimate
the
in-
breeding
effective
population
size
(Nef)
by
calculating
the
harmonic mean of
the
population

size
over
time
from
the
founding
generation
to
the
penultimate
generation.
The
symbol
t
represents
the
number
of
generations
for
which
we
have
population
size
data.
N(0) is
the

size
of
the
founding
population,
N(1) is
the
size
of
the
population
after
one
generation
etc.

and,
N(t − 1)
is
the
size
of
the
population
one
generation
ago.

t
(a)
Nef =
1 1
 1

+
.
.
.
+
N(0)

+

N(1)
 N(t −
1)

Variance effective size, Nev,
refers
to
the
size
of
an
ideal
population
that
would
accumu-
late
the
same
amount
of
variance
in
allele
frequencies
as
the
population
of
interest;
thus,

this
effective
population
size
indicates
how
rapidly
allele
frequencies
are
likely
to
change.

This
is
important
because
it
also
affects
how
rapidly
isolated
populations
diverge
from
one

another
under
genetic
drift.
Again,
the
symbol
t represents
the
number
of
generations
for

which
we
have
population
size
data.
N(1) is
the
size
of
the
population
after
one
generation,

etc.,
and
N(t)
is
the
size
of
the
current
population.

t
(b)
Nev
=

1 1
 1

+
.
.
.
+
N(1)

+

N(2)
 N(t)

In
addition,
the
following
correction
can
be
used
at
each
generation
if
operational
sex
ratios

are
not
1:1.
This
corrected
population
size
reflects
the
increased
effects
of
both
inbreeding

and
drift
when
the
sexes
are
not
contributing
equally
to
the
allele
pool.

4Nm Nf
(c)
Ns =
Nm
+
Nf
(continued on following page)
Copyrighted Material
population effective sizes • 127
Notice
the
difference
between
formula
(a)
and
formula
(b).
While
the
inbreeding effective
size is
more
sensitive
to
the
number of original founders [N(0)],
the
variance effective
size is
more
sensitive
to
the
number of offspring in the current generation [N(t)].
This
is

because,
as
stated
above,
Nef focuses
on
the
loss
of
heterozygosity
due
to
pedigree
inbreed-
ing
in
the
population;
with
a
small
initial
founding
population,
close
relatives
are
likely

to
mate
with
each
other.
In
contrast,
Nev gives
an
indication
of
the
increase
in
variance

of
allele
frequencies
between
subpopulations
due
to
drift,
and
depends
on
the
number

of
offspring
produced
by
those
founders
and
by
each
subsequent
generation,
up
to
the

present-day
N(t).

These
differences
can
lead
to
large
discrepancies
between
these
two
different
effective

population
sizes
in
real
populations.
For
example,
increasing populations generally
have

a
larger
Nev than
Nef ,
while
declining populations will
generally
have
larger
Nef than
Nev.

Hence,
a
population
coming
through
a
bottleneck
may
have
a
low
inbreeding
effective

size,
but
it
can
have
a
larger
variance
effective
size
if
the
population
bounces
back
rapidly

(as
is
the
case
with
the
Southern
white
rhinoceros
population,
which
you
will
work
on

in
class).

There
are
other
measures
of
effective
population
size
that
focus
on
different
population

genetic
parameters.
For
example,
eigenvalue effective size, Neλ,
focuses
on
the
rate
at
which

unique
alleles
are
lost
from
a
population.
For
this
exercise,
we
will
examine
only
the
two

effective
population
size
estimates
discussed
above.

cases,
effective
population
size
will
be
smaller
than
the
actual
number
of
organisms
in
the

population.
Think
for
a
moment
about
why
this
is
so.
A
conservative
rule
of
thumb
used

by
some
biologists
is
that
Ne is
usually
about
one-fifth
of
the
total
population
size
(Mace

and
Lande,
1991).
Using
such
a
rough
estimate
is
risky
because
Ne can
be
larger than
the

census
size
of
the
population,
depending
on
the
history
of
the
population
and
the
particular

Ne
under
consideration.

Demographic
 stochasticity,
 genetic
 drift,
 and
 environmental
 variation
 can
 interact

to
doom
a
small
population
to
extinction.
This
is
called
an
extinction
vortex,
and
it
is

due
to
a
positive
feedback
loop
(figure
12.1):
the
negative
consequences
of
lower
effective

population
size
make
the
population
smaller,
causing
stronger
negative
effects,
leading
to

an
even
smaller
population
size
(Gilpin
and
Soule,
1986).
For
example,
a
random
envi-
ronmental
change
might
lower
population
size,
leading
to
a
higher
chance
of
population

reduction
due
to
demographic
stochasticity.
This
could
lower
inbreeding
effective
popu-
lation
size
even
more,
leading
to
severe
inbreeding
depression
and
reduced
fertility.
This

further
reduces
the
population
size.
Chains
of
events
such
as
these
mean
that
the
extinc-
tion
probability
for
a
small
population
can
be
extremely
high.
For
example,
Pimm
et
al.

(1988)
showed
that
the
extinction
risk
for
birds
on
small
islands
off
the
coast
of
Britain

rises
with
decreasing
numbers
of
nesting
pairs.
Conservation
biologists
realize
that
an

extinction
vortex
can
begin
when
humans
cause
major
reductions
in
the
population
size

of
a
species.

Calculating Effective Population Sizes
Consider
the
data
in
table
12.1
for
a
population
of
Eastern
fence
lizards,
Sceloporus undula­
tus,
at
Tyson
Research
Center
in
eastern
Missouri:
You
can
estimate
the
inbreeding
effective

Copyrighted Material
128 • chapter 12
More
genetic drift;
less ability
to adapt
More
inbreeding;
depression
Population
more
subdivided by
fragmentation
More
demographic
variation
• Habitat destruction
• Environmental degradation
• Habitat fragmentation
• Overharvesting
• Effects of invasive species
• Environmental variation
• Catastrophic events
• Global climate change
Lower effective
population size
(Ne)
EXTINCTION
FIgure 12.1. Extinction
vortex.
 Population
size
decreases
in
a
positive
feedback
loop,
eventually

resulting
in
the
extinction
of
the
population
(from
Primack,
2000).

size
for
this
population
using
formula
(a)
from
box
12.1.
In
this
case,
t=4
generations;
we

calculate
the
inbreeding effective population size as:

Nef ≈
t
1 1
 1 1

+ + +
N(0)
 N(1)
 N(2)
 N(3)

(12.1)

4 4
=
 =
 =
68.9
≈
69
1
 1
 1
 1 0.058

+ + +
140
 250
 110
 26

Try
this
calculation
yourself—it
can
get
confusing
taking
all
of
these
reciprocals!

This
estimated
effective
population
size
(69)
means
that,
in
terms
of
genetic
inbreeding,

this
population
(with
a
mean
census
size
of
148
lizards
over
5
years)
will
accumulate
the

Table 12.1.
Sceloporus population
at
Tyson
from
1996
to
2000.

Year
 Population
count

1996
 140

1997
 250

1998
 110

1999
 26

2000
 180

Copyrighted Material
population effective sizes • 129
effects
of
inbreeding
at
the
same
rate
as
a
population
that
had
a
constant
size
of
only
69

individuals.

In
contrast,
the
variance effective size,
estimated
with
formula
(b)
in
box
12.1,
is

Nev ≈
t
1 1
 1 1

+ + +
N(1)
 N(2)
 N(3)
 N(4)

(12.2)
4 4
=
 =
 =
70.73
≈
71.
1
 1
 1 1
 0.05655

+
 + +
250
 110
 26
 180

This
effective
population
size
(71)
means
that,
in
terms
of
genetic
drift,
this
population

(with
a
mean
size
of
148
over
5
years)
will
accumulate
the
effects
of
drift
at
the
same
rate

as
a
population
that
had
a
constant
size
of
71
individuals.
Note
that,
in
this
example,
the

mean
size
of
the
population
over
time
reflects
neither
how
it
will
accumulate
inbreeding

nor
how
it
will
experience
drift.
Although
the
difference
between
variance
effective
size
and

inbreeding
effective
size
appears
small
in
this
example,
the
point
is
that
these
are
not
two

ways
of
estimating
the effective
size;
these
are
two
different
effective
population
sizes.
In
the

examples
you
work
with
in
this
exercise,
you
will
see
how
one
population
can
have
very

different
effective
sizes
that
inform
us
about
very
distinct
risks
for
the
population.

Effective Population Size, Inbreeding, and Extinction
Just
as
there
are
a
number
of
different
meanings
of
effective
population
size,
there
are
a

number
of
different
meanings
of
inbreeding.
Because
breeding
with
close
relatives
typically

reduces
the
pool
of
genes
contributing
to
the
next
generation,
one
measure
of
inbreeding
is

F,
inbreeding
as
a
measure
of
drift.
In
addition
to
increasing
the
impact
of
drift,
inbreeding

can
increase
the
proportion
of
deleterious
homozygous
gene
combinations
in
a
population,

which
leads
to
lower
survival
of
young
and
thus
lower
reproductive
output;
this
is
called

inbreeding
depression.
To
begin
to
evaluate
the
potential
impacts
of
small
population
size

on
inbreeding,
we
can
use
the
following
estimation
(from
Soule,
1980):

1
ΔF
=
1−
1
−
 t.
 (12.3)
2Nef
In
this
equation,
ΔF is
the
increase
in
the
inbreeding
coefficient
over
time;
Nef
 is
the
in-
breeding
effective
population
size;
and
t is
the
number
of
generations.
We
can
use
this

equation
to
predict
how
much
the
inbreeding
coefficient
in
a
small
population
will
increase

over
time.
If
ΔF is
0.6
or
higher,
the
fecundity
of
individuals
in
the
population
may
be

reduced,
and
the
population
could
be
at
higher
risk
for
extinction.
For
example,
we
can

calculate
the
increase
in
the
inbreeding
coefficient
after
100
generations
for
the
population

described
above.
For
the
fence
lizards,
Nef
was
69.
This
means
that
ΔF =1
−
(1
−
1/138)100
=

0.52.
We
would
conclude
that
after
100
generations
the
fence
lizards
are
experiencing
some

inbreeding;
however,
because
ΔF <
0.60
it
may
not
threaten
the
population.
Of
course,
the

inbreeding
coefficient
will
continue
to
increase
over
time,
possibly
to
dangerous
levels,
if

the
population
remains
small
and
isolated.
Note
that
the
estimate
of
ΔF ignores
gene
flow—

and
even
moderate
gene
flow
can
greatly
reduce
the
effect
of
genetic
drift,
slowing
the
rate

of
increase
in
the
inbreeding
coefficient
over
time.
So
even
moderate
gene
flow
can
help

maintain
genetic
diversity.
This
is
one
reason
why
many
conservation
biologists
advocate

the
maintenance
of
corridors
connecting
small,
isolated
populations
of
a
species.

Copyrighted Material
130 • chapter 12
Minimum Viable Populations and the 50/500 Rule
You
know
that
demographic
stochasticity
and
genetic
drift
can
negatively
affect
small
pop-
ulations.
Demographic
stochasticity
leads
to
the
random
extinction
of
small
populations,

while
genetic
drift
can
cause
a
reduction
of
genetic
diversity
within
a
population.
These

factors
can
interact
in
an
extinction
vortex
(figure
12.1
discussed
above)
that
can
eventually

lead
to
the
extinction
of
a
population.

To
decide
when
these
factors
might
be
important
for
a
population
of
an
endangered

species,
Shaffer
(1981)
proposed
the
concept
of
the
minimum
viable
population
(MVP).

He
defined
the
MVP
as
the
smallest
isolated
population
(of
a
given
species
in
a
given
habi-
tat)
having
a
99%
chance
of
remaining
in
existence
for
1,000
years,
despite
the
foreseeable

effects
of
demographic
stochasticity,
genetic
drift,
environmental
stochasticity
(random

changes
in
the
environment),
and
natural
catastrophes
(Shaffer,
1981).
Shaffer
chose
the

percentage
and
time
scale
to
represent
what
most
scientists
consider
a
good
chance
for

survival
of
a
species.
Quantitative
objectives
like
the
MVP
provide
specific
guidelines
for

gauging
the
success
of
conservation
programs
(Foose
et
al.
1995).
Populations
smaller
than

the
MVP
are
considered
to
be
at
significant
risk
of
entering
into
the
extinction
vortex
and

becoming
extinct,
so
a
conservation
program
can
be
considered
successful
only
if
it
raises

the
effective
population
size
above
the
MVP.

A
related
concept
is
the
50/500
rule,
proposed
by
Franklin
(1980).
The
“50”
part
of
the

50/500
rule
states
that
populations
with
an
inbreeding
effective
population
size
(Nef)
under

50
are
at
immediate
risk
of
extinction.
This
is
because,
in
such
small
populations,
inbreed-
ing
and
demographic
stochasticity
can
quickly
push
the
population
into
an
extinction
vor-
tex.
The
“500”
part
of
the
rule
means
that
populations
with
a
variance
effective
size
(Nev)

of
less
than
500
are
at
long-term
risk
of
extinction.
In
these
populations,
genetic
drift
may

be
a
strong
force,
leading
to
eventual
loss
of
genetic
variation
(Franklin,
1980).
After
varia-
tion
is
lost,
the
population
will
no
longer
be
able
to
respond
to
environmental
changes,

and
may
be
reduced
in
size
or
go
extinct
if
any
such
changes
occur.
Even
when
properly

understood,
Franklin’s
rule
is
quite
controversial.
Some
authors
question
its
generality,
and

others
suggest
that
the
numbers
are
too
small.
For
example,
Lande
(1995)
suggested
that

any
population
with
Nev less
than
5,000
will
be
subject
to
strong
genetic
drift,
which
will

deplete
the
genetic
variation
in
a
population
and
cause
long-term
extinction
risk.
Recent

experimental
tests
of
the
rule
in
captive
housefly
populations
also
suggest
that
populations

must
be
higher
than
implied
by
the
50/500
rule
in
order
to
survive
and
maintain
genetic

diversity
(Reed
and
Bryant,
2000).
As
a
conservation
biologist,
you
may
not
use
the
50/500

rule,
but
it
is
essential
that
you
understand
where
the
50
and
500
come
from.

Homework
for
this
exercise
takes
approximately
45
minutes.

Case Studies and Data
European Adders
The
European
adder
(Vipera berus)
is
a
small,
venomous
snake
(figure
12.2)
distributed

throughout
Europe
(Arnold
and
Burton,
1978).
This
snake
occupies
a
wide
geographic

range,
but
within
that
range
snakes
are
often
found
in
small,
isolated
populations
sepa-
rated
from
each
other
by
hundreds
to
thousands
of
meters.
Under
natural
conditions,
gene

flow
between
these
subpopulations
is
high,
because
the
male
snakes
disperse
widely
when

Copyrighted Material
population effective sizes • 131
FIgure 12.2. European
adder
Vipera berus.

searching
for
females
during
the
spring
(Madsen
et
al.,
1993).
However,
in
certain
regions

humans
have
disturbed
adder
habitat
through
agriculture
and
urban
encroachment.
Now

we
see
extremely
isolated
small
populations
of
adders,
surrounded
by
large
areas
of
unsuit-
able
habitat
(Madsen
et
al.,
1996).
Consider
the
isolated
population
of
adders
at
Smygehuk,

on
the
south
coast
of
Sweden.
These
adders
are
separated
from
the
nearest
population
by

20
km
of
farmland,
which
is
unsuitable
habitat
for
adders
(Madsen
et
al.,
1996).
Use
the

data
in
tables
12.2
and
12.3
(Madsen
et
al.,
1996)
to
answer
the
following
questions.

Questions to Work on Individually Outside of Class
1.
Table
12.2
gives
data
on
the
population
size
of
the
adders
for
each
year
from
1984
to
1990.

(a)
Plot
the
total
number
of
adult
adders
(y)
over
time
(x).

(b)
Use
these
data
to
calculate
the
inbreeding
effective
size
and
the
variance
effective
size

of
this
population
of
adders.

(c)
Explain
why
the
inbreeding
effective
size
and
the
variance
effective
size
of
this
popu-
lation
differ.

(d)
Recalculate
the
variance
effective
size
of
this
population
with
the
new
information

on
the
sex
ratios
of
snakes
in
the
population
(table
12.3).

(e)
Dr.
Fern
Skipe
has
argued
that
there
is
no
such
thing
as
the
“real”
effective
population

size.
Do
you
agree
with
this
statement?
Why
or
why
not?
Use
the
results
above
in
your

argument.

(f)
Based
on
these
data
and
your
calculations,
make
a
recommendation
to
the
Swedish

government
concerning
this
population
of
vipers.

Copyrighted Material
132 • chapter 12
Table 12.2.
Total
number
of
adult
adders
at
Smygehuk

Year Total
number
of
adults

1984
 138

1985
 40

1986
 34

1987
 42

1988
 37

1989
 41

1990
 34

Table 12.3.
Adult
male
and
female
adders
in
each
year

Year
 No.
of
adult
females
 No.
of
adult
males
 Corrected
population
size

1984
 98
 40

1985
 29
 11

1986
 24
 10

1987
 32
 10

1988
 27
 10

1989
 29
 12

1990
 27
 7

Small-group/In-Class exercise
Please
bring
graph
paper
and
a
calculator
to
class
to
complete
the
next
part
of
this
exercise.

First,
read
the
following
background
information
on
African
rhino
conservation.

African Rhino Conservation
Background The
rhinoceros
is
an
example
of
a
“charismatic
megavertebrate”
that
has

played
a
central
role
in
promoting
worldwide
conservation
efforts.
The
five
extant
species

of
rhino
are
the
last
representatives
of
a
large
group
of
species
that
reached
a
peak
in
diver-
sity
between
25
and
5
million
years
ago
(Estes,
1991).
Two
of
the
five
extant
species
occur

in
Africa:
the
white
rhinoceros
(Ceratotherium simum)
and
the
black
rhinoceros
(Diceros
bicornis).
Despite
their
names,
both
species
are
a
dull
gray
color,
and
can
be
distinguished

by
the
shape
of
their
mouthparts
(figure
12.3).
The
black
rhino
has
a
hook-shaped
triangu-
lar
upper
lip
that
allows
it
to
obtain
its
food
by
browsing
leguminous
herbs
and
shrubs.
The

white
rhino,
on
the
other
hand,
has
a
very
wide,
square
mouth,
and
is
specialized
in
grazing

areas
of
dense
grasses.
All
rhinos
have
poor
eyesight
and
relatively
small
brains,
but
ex-
Copyrighted Material
population effective sizes • 133
FIgure 12.3. Black
(left)
and
white
(right)
rhinoceri.

tremely
sensitive
hearing
and
smell
(Estes,
1991).
Both
African
species
of
rhino
show
geo-
graphic
variation.
The
black
rhino
has
been
divided
into
four
subspecies,
western
(Diceros
bicornis longipes),
eastern
(D. b. michaeli),
southwestern
(D. b. bicornis),
and
south
central

(D. b. minor).
The
western
subspecies
is
the
rarest
and
most
isolated,
with
only
a
few
indi-
viduals
living
in
western
Africa.
The
white
rhino
has
been
divided
into
two
subspecies,
the

northern
(Ceratotherium simum cottoni)
and
southern
(C. s. simum).
These
two
subspecies

occupy
separate
ranges
and
are
more
distinct,
both
morphologically
and
genetically,
than

the
subspecies
of
black
rhinos
(Emslie
and
Brooks,
1999).

Both
species
of
rhino
have
undergone
major
reductions
in
their
ranges
in
the
past
sev-
eral
hundred
years.
Early
colonial
explorers
reported
that
black
rhinos
were
widespread

in
 distribution
 and
 fairly
 common,
 while
 white
 rhinos
 were
 more
 restricted
 in
 range.

After
European
colonization,
southern
white
rhinos
were
very
quickly
reduced
to
near-
extinction,
reaching
a
low
of
just
20
individuals
in
1895.
Since
then,
numbers
of
the
south-
ern
white
rhino
have
steadily
increased;
there
are
over
8,000
alive
today.
The
northern

white
rhino,
on
the
other
hand,
has
shown
a
dramatic
decrease
in
recent
years,
declining

from
over
2,000
in
1960
to
only
25
individuals
in
1998.
Numbers
of
black
rhinos
have
also

declined
since
colonial
times.
Declines
were
especially
severe
between
1970
and
1992,
when

black
rhinos
declined
96%.
The
species
has
recently
shown
some
potential
for
recovery

(Emslie
and
Brooks,
1999).
The
main
reason
for
the
decline
of
all
rhinos
is
hunting
by

humans.
European
colonists
killed
hundreds
of
thousands
of
rhinos
during
the
nineteenth

century.
More
recently,
rhinos
have
been
killed
by
poachers
supplying
markets
in
Asia
and

the
Middle
East
with
rhinoceros
horns.
In
Asia,
rhino
horns
are
used
in
traditional
Chinese

medicine,
whose
practitioners
believe
that
the
horns
lower
fevers,
increase
male
potency,

and
can
cure
a
host
of
diseases.
In
the
Middle
East,
they
are
used
as
handles
for
ornamental

daggers
called
jambiyas
(Emslie
and
Brooks,
1999).
Although
rhino
horns
have
been
used

for
these
purposes
for
hundreds
of
years,
recent
increases
in
demand
put
serious
pressure

on
wild
rhinos,
and
poaching
for
horns
is
the
major
threat
to
African
rhino
populations

today
(Emslie
and
Brooks,
1999).

Black Rhinoceros Diceros bicornis Black
rhinos
were
formerly
the
most
widespread

and
abundant
species
of
rhino
(Estes,
1991),
but
are
now
listed
in
the
IUCN
Red
Book
as

critically
endangered.
Direct
counts
of
black
rhinos
have
shown
declines
of
over
80%
in

Copyrighted Material
134 • chapter 12
7
6
5
4
3
2
1
0
Year
FIgure 12.4. The
black
rhinoceros
has
been
in
severe
decline
in
recent
years
and
is
entering
a
popu-
lation
bottleneck
(data
from
Emslie
and
Brooks,
1999).

the
last
50
years
(Emslie
and
Brooks,
1999).
The
total
population
size
of
black
rhinos
in

1970
was
estimated
at
65,000.
Historically,
black
rhinos
occupied
a
large
range
throughout

Africa.
Today,
black
rhino
populations
are
fragmented,
and
the
species
is
rapidly
declining

in
numbers
(figure
12.4).
The
species
has
been
divided
into
four
subspecies,
each
occupy-
ing
a
separate
geographic
range:
the
western,
eastern,
southwestern,
and
south-central

black
rhinos.
The
ranges
of
each
subspecies
have
different
climates
and
habitats.
They

can
sometimes
be
distinguished
by
characters
such
as
skin
texture
and
horn
length
and

shape.
There
may
also
be
genetic
and
behavioral
differences
across
these
subspecies.
The

western
black
rhinoceros
is
the
most
range-restricted
and
endangered
of
all
the
subspecies

of
black
rhinos:
the
entire
subspecies
is
represented
by
only
a
few
scattered
individuals

in
Cameroon.
This
subspecies,
separated
from
the
rest
of
the
black
rhinos
by
hundreds

of
miles,
may
represent
a
genetically
distinct
lineage,
but
is
threatened
with
extinction
in

the
immediate
future.
The
largest
population
of
black
rhinos
is
in
Kenya,
while
the
main

population
of
southwestern
black
rhinos
is
in
Namibia.
The
south-central
black
rhino
is

the
most
common
subspecies,
with
large
numbers
in
South
Africa
and
Zimbabwe,
and

smaller
populations
in
southern
Tanzania
and
Mozambique.
The
goal
of
your
group
is
to

establish
a
conservation
plan
for
black
rhinos
as
a
whole.
You
need
to
decide
how
much

money
needs
to
be
allocated
to
each
of
the
four
subspecies,
and
which
populations
will

have
the
highest
conservation
priority.

White Rhinoceros Ceratotherium simum White
rhinos
have
always
been
less
com-
mon
and
more
limited
in
distribution
than
black
rhinos
(Emslie
and
Brooks,
1999).
This
is

probably
a
function
of
their
specialized
grazing
diet.
This
feeding
strategy
makes
the
white

rhino
unique,
as
it
is
quite
different
from
that
of
black
rhinos
and,
in
fact,
from
all
other

rhinos
in
the
world
(Estes,
1991).
The
white
rhino
is
one
of
the
largest
purely
grazing
herbi-
vores
that
has
ever
lived.
Southern
white
rhinos
are
separated
from
northern
white
rhinos

by
2,000
km,
and
no
white
rhino
has
ever
been
recorded
in
the
intervening
area.
These
two

Population(intenthousands)
1969 1974 1979 1984 1989 1994
Copyrighted Material
population effective sizes • 135
subspecies
are
genetically
distinct,
with
more
genetic
variation
between
these
two
subspe-
cies
than
among
the
four
subspecies
of
black
rhino
(Smith
et
al.,
1995).

Southern White Rhinoceros Ceratotherium simum simum Southern
white
rhinos

were
on
the
brink
of
extinction
in
1895,
when
overhunting
had
reduced
them
to
just
20

individuals
in
one
population
in
South
Africa.
Their
numbers
have
since
recovered
sub-
stantially
(figure
12.5).
The
recovery
of
the
southern
white
rhino
is
one
of
the
major
success

stories
in
modern
conservation
biology.
Before
their
decline
in
the
ninteenth
century,
their

range
included
much
of
southern
Africa.
After
initial
recovery
of
the
source
population,

many
translocations
were
carried
out;
these
have
successfully
reintroduced
rhinos
into
ar-
eas
where
they
had
been
wiped
out.
Southern
white
rhinos
are
now
the
most
numerous

subspecies
of
rhino;
populations
can
be
found
in
South
Africa,
Botswana,
Namibia,
Swazi-
land,
and
Zimbabwe.
They
have
also
been
introduced
into
Kenya,
Ivory
Coast,
and
Zambia,

all
outside
their
native
range.
The
rhinos
have
functioned
as
a
major
source
of
funding
in

South
Africa,
where
national
parks
have
sold
excess
rhinos
to
private
game
parks
for
as

much
as
U.S.$25,000
apiece.

9
8
7
6
5
4
3
2
1
0
Year
FIgure 12.5. The
southern
white
rhinoceros
has
recovered
from
near
extinction
at
the
turn
of
the

last
century
and
the
current
growing
population
is
descended
from
a
bottleneck
population
of
only

20
animals
(data
from
Emslie
and
Brooks,
1999).

Northern White Rhinoceros Ceratotherium simum cottoni Northern
white
rhinos

have
shown
a
striking
decline
in
recent
years,
and
are
now
perilously
close
to
extinction.

Table
12.4
gives
data
on
northern
white
rhinoceros
populations
by
country
from
1960
to

1998
(Emslie
and
Brooks,
1999).
Although
the
range
of
northern
white
rhinos
once
in-
cluded
parts
of
Uganda,
Chad,
Sudan,
the
Central
African
Republic,
and
the
Democratic

Republic
of
Congo,
they
are
now
restricted
to
a
small
area
in
the
northeast
of
the
Demo-
cratic
Republic
of
Congo
(Emslie
and
Brooks,
1999).
This
population
numbered
only
25

individuals
in
1998,
and
DRC
has
been
suffering
from
tremendous
political
instability
over

the
past
20
years.
Conservation
biologists
have
not
lost
hope
of
preserving
the
subspecies,

Population(inthousands)
1890 1910 1930 1950 1970 1990
Copyrighted Material
136 • chapter 12
Table 12.4.
Northern
white
rhinos
by
country,
1960–1998.

1960
 1971
 1976
 1981
 1983
 1984
 1991
 1995
 1998

Central
African
 Few
 Few
 Few
 Few
 Few
 0?
 —
 —
 —

Republic

Chad
 Few
 Few
 ?
 ?
 0?
 0?
 —
 —
 —

Democratic
Republic
 1,150
 250
 490
 <50
 13–20
 15
 30
 31
 25

of
the
Congo

Sudan
 1,000
 400
 ?
 <300
 <50
 0?
 0?
 0?
 0?

Uganda
 80
 Few
 Few
 Few
 2–4
 0?
 —
 —
 —

Total
 2,230
 650
 500+
 <350
 <70
 15
 30
 31
 25

however,
and
cite
the
recovery
of
the
southern
white
rhino,
which
has
grown
from
a
total

population
of
around
20
individuals
in
1895
to
over
8000
today.
The
northern
white
rhinos

currently
surviving
in
the
Democratic
Republic
of
Congo
represent
the
last
survivors
of
a

unique
lineage
of
rhinos;
their
extinction
would
be
a
great
and
irreversible
tragedy.

Your Job: Help Create Species Survival Plans for African Rhinos (Questions 2–6)
Species
Survival
Plans
(SSPs)
coordinate
the
management
of
rare
and
endangered
species
to

maintain
healthy
breeding
populations,
retain
genetic
variation,
and
minimize
“inbreeding.”

SSPs
often
have
the
conflicting
goals
of
preserving
species
in
a
captive
environment
while
at

the
same
time
minimizing
evolutionary
change
in
the
species
and
minimizing
loss
of
genetic

diversity
from
inbreeding
or
drift
(Templeton,
1991).
These
can
be
significant
forces
affect-
ing
wild
(in
situ)
and
captive
populations
that
are
entering
or
emerging
from
population
bot-
tlenecks.
Your
job
will
be
to
use
real
data
to
help
the
IUCN
Rhino-Rescue
Team
generate
an

SSP
for
the
two
species
of
African
rhinos.
After
you
answer
questions
2–5,
the
entire
class
will

meet
as
a
committee
of
the
whole
to
allocate
funds
for
the
conservation
of
African
rhinos.

2.
Your
instructor
will
assign
you
to
one
of
the
three
rhino
species
or
subspecies
on

which
we
have
data.
First,
you
need
to
assess
the
current
genetic
situation
for
black
rhinos,

or
southern
or
northern
white
rhinos.
This
assessment
should
include
the
inbreeding
and

variance
effective
sizes
for
the
wild
populations.
You
should
be
able
to
project
accumula-
tion
of
inbreeding
in
wild
populations
if
they
are
maintained
at
current
levels.
(Assume

equal
sex
ratios
and
a
generation
time
of
8
years.
For
black
rhinos
and
southern
white
rhi-
nos
you
will
need
to
estimate
census
sizes
from
figures
12.4
and
12.5)
Your
instructor
will

copy
table
12.5
on
the
board
and
you
can
share
your
results
with
the
other
groups.

3.
For
your
species
or
subspecies,
discuss
the
long-range
plan
for
maintaining
the
genetic

health
of
the
population.
Address
the
recommendations
and
the
theoretical
framework
of

Franklin’s
50/500
rule
in
your
plan.

4.
You
should
also
discuss
the
situation
for
your
species
in
the
wild
and
decide
whether

you
want
to
use
the
wild
population
to
supplement
the
captive
zoo
population
or
vice

versa.

5.
Finally,
your
plan
must
include
priorities
for
both
species
and
for
different
popula-
tions
within
each
species.
The
reality
is
that
there
are
limited
funds
available
for
rhino
con-
servation,
and
you
must
generate
guidelines
about
where
resources
should
be
spent.

Copyrighted Material
population effective sizes • 137
Table 12.5.
Population
census
and
effective
sizes
of
African
rhinos.

Census
size,
 Inbreeding
effective
 Variance
effective

1997
 size
(Nef)
 size
(Nev)

Black
rhinoceros

Diceros bicornis
N =
2,600

Southern
white
rhinoceros

Ceratotherium simum simum
N =
8,440

Northern
white
rhinoceros

Ceratotherium simum cottoni
N =
23

6.
Each
group
will
have
a
few
minutes
to
describe
the
situation
for
their
rhinos
and
pro-
pose
an
allocation
of
the
$500,000
which
African
Rhino
Rescue
has
raised.
Once
each
group

has
made
their
brief
presentation
you
will
have
a
chance
to
convince
each
other
(and
your

instructor)
to
take
your
recommendation.

references
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E.
N.,
and
J.
A.
Burton.
1978.
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Foose,
T.
J.,
L.
de
Boer,
U.
S.
Seal,
and
R.
Lande.
1995.
Conservation
management
strategies
based

on
viable
populations.
Pages
273–294
in:
J.
D.
Ballou,
M.
E.
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and
T.
J.
Foose
(eds.),
Population
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York:
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I.
R.
1980.
Evolutionary
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in
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E.
Soule
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B.
A.
Wilcox
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Conservation Biology: An Evolutionary­Ecological Perspective.
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M.
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E.
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E.
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