What are the limitations that keep
population sizes from continuing to
grow exponentially?
Factors that limit population growth:
1) Some kind of environmental disaster (that kills individuals
outright).
2) Shortage of essential resources (starvation, desiccation).
3) Elevated levels of poisons or stress-inducing factors in the
environment.
4) Interactions with other species (disease, predators,
interspecific competitors)
Environmental “disasters”:
Mount St Helen before 1980
Mount St Helen after
the volcanic eruption
in 1980
Yellowstone Fire, 1988
Two general classes of limitations:
Limitations that
are independent of
population size.
Limitations that
depend on
population size.
density-independent
limitations
density-dependent
limitations
Density-dependent limitations on growth are
often linked to overcrowding:
1) Shortage of essential resources per individual can lower birth rates or
increase death rates (or both):
• the amount of available energy source (food, light)
• the amount of essential nutrients in the food or environment
• water
• space (territory size or quality, nesting sites, predator-free space)
2) Elevated levels of poisons or stress-inducing factors in the environment
can lower birth rates or increase death rates (or both):
• toxic waste produced by individuals in a population
• growth-inhibitors exuded by individuals in a population
• hostile interactions between individuals in a population
3) Interactions with other species can can lower birth rates or increase
death rates (or both):
• essential resources consumed by other species
• harm done by other species (being eaten, injured or infected)
An example of nutrients limiting population growth (self-thinning in
plants):
Fraction of sown seeds
producing a mature plant
Poppy
Density of seeds sown (no per m2)
(Data from Harper and McNaughton 1962)
Examples of territory ownership limiting reproduction:
Bynoe’s Gecko
Pied flycatcher
Desert clicker
An example where toxins limit population growth:
Ethyl alcohol is the waste product of fermentation. Above 15% alcohol by
volume, yeasts cannot ferment.
Some plants produce auto-toxins: allelopathy:
Allelopathy is the exudation of substances by adult plants that
suppress the seed germination or seedling growth.
Seeds
tested
Average dry weight of
seedlings (mg)
Germination ratio:
(+sunfl.) /(control)
Control
soil
+ sunflower leaves
mixed into soil
Sunflower
40
21.5
0.46
Horseweed
43
17.5
0.79
Crabgrass
111.5
13.5
1.015
Jimmyweed
19.5
9
0.675
Wire grass
17
22
0.995
(After Wilson and Rice 1968)
An example where stress of crowding limits population growth:
Crowding alone, in mice and rats, can cause mortality rates to
increase and birth rates to decrease, through stress-related
abnormal behavior.
An example where many species compete for the same
resource: space (barnacles and limpets in the intertidal zone).
Owl limpet
An example where toxins limit the growth of another species:
Bacteria growing on agar
Bacteria-free space
Penicillum colonies.
An example where predators check the population sizes of their prey:
Examples of conditions that favor disease propagation:
So there is checked and unchecked growth.
There are density-dependent and -independent checks on population size.
Imagine a population of ducks at a lake. How would you determine if the
population is a) checked or unchecked, b) whether the population is
checked by density-dependent or -independent mechanisms?
Sagebrush Lizard in Zion National Park:
(After Tinkle et al. 1993)
Sagebrush Lizard in Zion National Park:
Total number of lizards
250
200
150
100
50
0
1968
(After Tinkle et al. 1993)
1970
1972
1974
Year
1976
1978
1980
Annual change in lizard number
Sagebrush Lizard in Zion National Park:
80
60
40
20
0
-20
-40
-60
-80
-100
80
100
120
140
160
180
Total number of lizards
200
220
• Yearlings were larger at low population density.
• Larger lizards laid more eggs.
• Yearlings were larger in high rainfall years.
Food limitations probably were limiting hatchling growth,
exposing them to greater risk of being eaten, and reducing
female fertility.
(After Tinkle et al. 1993)
Wild donkeys in Australia:
site 1: 3.3 animals/km2
Annual growth rate (1986-1987):
- 3%
site 2: 1.5 animals/km2
+20%
(After Choquenot 1991)
•
•
•
6-month old donkeys were 9% larger at low density.
Animals had a 60% higher kidney fat index at low
density.
Male donkeys reached sexual maturity earlier at low
density.
Food limitations (grasses) probably were limiting juvenile
growth and development. Juveniles probably died from
starvation or consequences of malnutrition.
Pond snails in Michigan ponds:
(After Eisenberg 1966)
Eisenberg’s manipulative experiment:
• 12 identical snail-proof cages set up along the margins of a
pond in spring.
• Addition of adult snails in four cages (5x).
• Reduction of snails numbers in four cages (1/5th ).
• No change in four cages (natural density, control).
(After Eisenberg 1966)
Eisenberg’s manipulative experiment:
Average no of
adults
Average no of
young (Jul 7)
Average no of
eggs per 10 min
search (Jul 10)
4 Cages with
subtractions
306
57
347
4 Control
cages
1273
49
435
4 Cages with
additions
4110
37
287
Differences in the numbers of young snails or eggs were nonsignificant between treatments
(After Eisenberg 1966)
Average no
of adults in
spring
Average no of
eggs per 10 min
search
Eggs per
eggmass
Control Cages
984
435
19
+ spinach
1224
9382
42
Food limited egg production: egg production was directly
proportional to the amount of spinach fed per adult.
2) Do birth or death rates limit growth at high density?
A greater proportion of hatchlings died when lizard
density was high.
Surviving lizards were also smaller and smaller
lizards lay fewer eggs.
Sagebrush lizard
Mares had the same pregnancy rate at high or low
density.
Juveniles in the high density population were smaller
and had a 3x higher chance to die.
Wild donkeys
Adults lay fewer eggs when adult snail density was
high.
The same proportion of adults and juveniles died,
independent of adult density.
Pond snails
Summary:
1. Many factors can change population birth and death rates,
thus population size.
2. Effects on population growth are classified as either densitydependent or independent.
3. Density-independent effects = can happen any time,
independent of population size.
4. Density-dependent effects = are correlated with population
size.
5. Next time we see that density-dependent effects are
responsible for creating stable population sizes.