what are the limitations that keep population sizes from continuing to grow exponentially?

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.