Chapter 53
Population Ecology
Population EcologyA population– Is a group of individuals of a single species living in the same
general area
Population ecology is the study of populations in relation to environment– Including environmental influences on population density and
distribution, age structure, and variations in population size
Northern Fur Seals
Density and Dispersion
Density– Is the number of individuals per unit area
or volume
Dispersion– Is the pattern of spacing among
individuals within the boundaries of the population
Population DynamicsDensity is the result of a dynamic interplay between processes that add individuals to a population and those that remove individuals from it
Births and immigration add individuals to a population.
Births Immigration
PopuIationsize
Emigration
Deaths
Deaths and emigration remove individuals from a population.
Ins
Outs
Patterns of Dispersion
Environmental and social factors– Influence the spacing of individuals in a
population
A clumped dispersion– Is one in which individuals aggregate in patches– May be influenced by resource availability and
behavior
(a) Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory.
A uniform dispersion– Is one in which individuals are evenly
distributed– May be influenced by social interactions such
as territoriality
(b) Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.
A random dispersion– Is one in which the position of each individual
is independent of other individuals
(c) Random. Dandelions grow from windblown seeds that land at random and later germinate.
Demography
Demography is the study of the vital statistics of a population and how they change over time
Death rates and birth rates are of particular interest to demographers
Life Tables
A life table– Is an age-specific summary of the survival
pattern of a population
Life table of Belding’s ground squirrels
Survivorship Curves
A survivorship curve– Is a graphic way of representing the data in a
life table– Shows a pattern of survival
Survivorship curve for Belding’s ground squirrels
1000
100
10
1
Num
ber
of s
urvi
vors
(lo
g sc
ale)
0 2 4 6 8 10
Age (years)
Males
Females
This survivorship curve shows a fairly constant death rate
Survivorship curves (types I, II, and III)
I
II
III
50 10001
10
100
1,000
Percentage of maximum life span
Num
ber
of s
urvi
vors
(lo
g sc
ale)
Reproductive Rates
A reproductive table, or fertility schedule– Is an age-specific
summary of the reproductive rates in a population
Life History
Life history traits – Traits that affect an organism’s schedule of
reproduction and survival
– They are Trade-Offs between survival and reproduction (parental care, # of offspring, etc.)
– They are very diverse
– They are evolutionary outcomes that are reflected in the development, physiology, and behavior of an organism
Life History
Life history entails 3 basic variables– Age at which reproduction begins– Frequency of reproduction– # of offspring produced per reproductive
cycle
Semelparity, or “big-bang” reproduction
– Reproduce a single time and die
Iteroparity, or repeated reproduction– Produce offspring repeatedly over time
Agave
Rabbit
Life History trade-offResearchers in the Netherlands studied the
effects of parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.)
EXPERIMENT
The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents.
CONCLUSION
100
80
60
40
20
0Reduced
brood sizeNormal brood
sizeEnlarged
brood size
Par
ents
sur
vivi
ng th
e fo
llow
ing
win
ter
(%)
MaleFemale
RESULTS
Some plants produce a large number of small seeds ensuring that at least some of them will grow and eventually reproduce
(a) Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least somewill grow into plants and eventually produce seeds themselves.
Some plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established
(b) Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring.
Population Growth
A Couple Questions for Population Ecologists.– What environmental factors stop a population
from growing?– Why do some populations show radical
fluctuations in size over time, while others remain stable?
Population Change and Population Density
In density-independent populations– Birth rate and death rate do not change with
population density
In density-dependent populations– Birth rates fall and death rates rise with
population density
Population-Limiting Factors
Competition
Territoriality
Health
Predation
Waste accumulation
Intrinsic factors (factors related to the biology/physiology of the particular species)
Competition for ResourcesIn crowded populations, increasing population density intensifies intraspecific competition for resources
100 100
100
0
1,000
10,000
Ave
rag
e n
um
be
r o
f se
ed
s p
er
rep
rod
uci
ng
ind
ivid
ua
l (lo
g s
cale
)
Ave
rag
e c
lutc
h s
ize
Seeds planted per m2 Density of females
0 7010 20 30 40 50 60 802.8
3.0
3.2
3.4
3.6
3.8
4.0
(a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases.
(b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.
TerritorialityIn many vertebrates and some invertebrates territoriality may limit density
Cheetahs are highly territorial
– Chemical communication can warn other cheetahs of their boundaries
TerritorialityTerritories can be small
Gannets exhibit territoriality in nesting behavior
Health
Population density can influence the health and survival of organisms
In dense populations pathogens can spread more rapidly
Predation
As a prey population builds up– Predators may feed preferentially on that
species
Toxic Wastes
The accumulation of toxic wastes– Can contribute to density-dependent regulation
of population size– The Wine Example
• Alcohol content of wine generally peaks out at about 13%. Beyond that level the yeast die off from ethanol toxicity
Intrinsic Factors
For some populations– Intrinsic (physiological) factors appear to regulate population
size
– (Such as an increase in aggressive behavior in a crowded colony of animals leading to a decrease in reproduction rate) Stress has a negative impact on the reproductive system.
Population Dynamics
Population dynamics– Focuses on the interactions between biotic and
abiotic factors that cause variation in population size
Extreme fluctuations in population size are typically more common in invertebrates than in large mammals 1950 1960 1970 1980
Year
1990
10,000
100,000
730,000
Com
mer
cial
cat
ch (
kg)
of
mal
e cr
abs
(log
sca
le)
Metapopulations and Immigration
Metapopulation:
Groups of populations
linked by immigration and emigration
This butterfly exists in many small populations. Individuals migrate between patches (populations) and can also colonize unoccupied patches.
Population Cycles
Many populations undergo regular boom-and-bust cycles. Cycles are influenced by interactions between biotic and abiotic factors
Year1850 1875 1900 1925
0
40
80
120
160
0
3
6
9
Lynx
pop
ulat
ion
siz
e (t
hous
and
s)
Har
e po
pula
tion
size
(t
hous
and
s)
Lynx
Snowshoe hare
Human Populations
Human population growth has slowed after centuries of exponential increase
No population can grow indefinitely– humans are no exception
The Global Human PopulationThe human population increased relatively slowly until about 1650 and then began to grow exponentially
8000 B.C.
4000 B.C.
3000 B.C.
2000 B.C.
1000 B.C.
1000 A.D.
0
The Plague Hum
an
pop
ulat
ion
(bill
ions
)
2000 A.D.
0
1
2
3
4
5
6
The rate of growth began to slow approximately 40 years ago
1950 1975 2000 2025 2050Year
2003
Per
cent
incr
ease
2.2
2
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
1.8
Regional Patterns of Population Change
To maintain population stability (zero population growth)– A regional human population can exist in one
of two configurations– Zero population growth = High birth rates –
High death rates– Zero population growth = Low birth rates –
Low death rates
The demographic transition is the move from the first toward the second state.
Is associated with various factors in developed and developing countries
The transition in Sweden was slow. In Mexico the change was abrupt early and has since
slowed.
50
40
20
0
30
10
1750 1800 1850 1900 1950 2000 2050
Birth rateDeath rate
Birth rateDeath rate
Year
Sweden Mexico
Birt
h or
dea
th r
ate
per
1,00
0 pe
ople
Age Structure
The relative number of individuals at each age
Affects present and future growth trends
Age structure represented in pyramids
Rapid growth Afghanistan
Slow growth United States
Decrease Italy
Male Female Male Female Male FemaleAge Age
8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8Percent of population Percent of population Percent of population
80–8485
75–7970–7465–6960–6455–5950–5445–4940–4435–3930–34
20–2425–29
10–145–90–4
15–19
80–8485
75–7970–7465–6960–6455–5950–5445–4940–4435–3930–34
20–2425–29
10–145–90–4
15–19
Global Carrying Capacity
Just how many humans can the biosphere support?– …no agreement in the scientific community on a number.– But, estimates of global population in 2050 are between 8 and 11 billion (currently about 6
billion)– Will technological advances continue to increase the Earth’s capacity for humans.
Ecological Footprint Concept
Ecological footprint– The aggregate land and water area needed to
sustain the people of a nation. What area is needed to produce the resources consumed and absorb wastes generated.
Ecological capacity– the actual resource base of a country
Looking at the ecological footprint is one measure of how close we are to the carrying capacity of Earth
Ecological footprints for 13 countries– Show that the countries vary greatly in their
footprint size and their available ecological capacity
16
14
12
10
8
6
4
2
00 2 4 6 8 10 12 14 16
New Zealand
AustraliaCanada
Sweden
WorldChina
India
Available ecological capacity (ha per person)
SpainUK
Japan
GermanyNetherlands
Norway
USA
Eco
log
ica
l foo
tprin
t (h
a pe
r pe
rson
)
Red dots are countries that were in ecological deficit in 1997
At more than 6 billion people the world is already in ecological deficit
http://www.myfootprint.org/en/