What are the limitations that keep population sizes from continuing to grow exponentially?
Dec 30, 2015
What are the limitations that keep population sizes from continuing to
grow exponentially?
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)
Factors that limit population growth:
Environmental “disasters”:
Mount St Helen before 1980
Mount St Helen after the volcanic eruptionin 1980
Yellowstone Fire, 1988
Two general classes of limitations:
Limitations that are independent of population size.
Limitations that depend on
population size.
density-independentlimitations
density-dependentlimitations
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):
Density-dependent limitations on growth are often linked to overcrowding:
• essential resources consumed by other species• harm done by other species (being eaten, injured or infected)
(Data from Harper and McNaughton 1962)
Fra
ctio
n of
sow
n se
eds
prod
ucin
g a
mat
ure
plan
t
Density of seeds sown (no per m2)
Poppy
An example of nutrients limiting population growth (self-thinning in plants):
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
Bacteria growing on agar
Penicillum colonies.
Bacteria-free space
An example where toxins limit the growth of another species:
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?
(After Tinkle et al. 1993)
Sagebrush Lizard in Zion National Park:
(After Tinkle et al. 1993)
Sagebrush Lizard in Zion National Park:
Year
1968 1970 1972 1974 1976 1978 1980
To
tal n
um
be
r o
f liz
ard
s
0
50
100
150
200
250
Sagebrush Lizard in Zion National Park:
Total number of lizards
80 100 120 140 160 180 200 220
An
nu
al c
ha
ng
e in
liza
rd n
um
be
r
-100
-80
-60
-40
-20
0
20
40
60
80
(After Tinkle et al. 1993)
• 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.
Wild donkeys in Australia:
(After Choquenot 1991)
site 1: 3.3 animals/km2
site 2: 1.5 animals/km2
Annual growth rate (1986-1987): - 3%
+20%
• 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.
(After Eisenberg 1966)
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)
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
Eisenberg’s manipulative experiment:
Differences in the numbers of young snails or eggs were non-significant between treatments
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?
Sagebrush lizard
A greater proportion of hatchlings died when lizard density was high.
Surviving lizards were also smaller and smaller lizards lay fewer eggs.
Pond snails
Adults lay fewer eggs when adult snail density was high.
The same proportion of adults and juveniles died, independent of adult density.
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
Summary:1. Many factors can change population birth and death rates,
thus population size.
2. Effects on population growth are classified as either density-
dependent 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.