Population Regulation:

general theory and history

Charles J. Krebs

Department of Zoology

University of British Columbia

1

Background

Statistical methods to estimate animal numbers were developed in 1930s

Almost no data were available on natural populations before 1950s- major improvements in counting

methods in 1960-1990s

2

Two Central Questions

(1) What stops population growth and

why ?

(2) What factors determines average

abundance of a population ?

- both these questions require

mechanisms

3

Whooping Crane Recovery in North America4

5

Stojanovic, D., et al. 2018. Further knowledge and urgent action required to save Orange-bellied

Parrots from extinction. Emu 118:126-134.

Orange-bellied Parrot

6

7Population Regulation – A Simple Model

Density-dependent Birth Rate Model8

Density-dependent rate

Density-independent rate

Population Regulation

Equilibrium view – describe how birth

and death rates are related to

population density

Density-dependence is the key

- but problems arose …

9

Complication #1

Equilibrium view – many populations

show no sign of an equilibrium ??

Population density is a shorthand but not

a cause, not a mechanism of change

- conservation is not possible without

understanding the mechanisms

10

Complication # 2

elegant

theory

messy

reality

11

12Mallard Female Survival

Sedinger, J.S. and Herzog, M.P. (2012) Harvest and dynamics of duck populations. Journal of Wildlife Management 76: 1108-1116.

Density-dependent survival but very weak

Mallard Population Changes13

A Second View of Populations

Equilibrium view – “Balance of Nature”

- 1850s to 1970s – stability rules

Non-equilibrium view

- 1970s – change is common

- need to understand causes of change

- focus on mechanisms

14

A Comprehensive Theorymechanisms

causing

change

individual

differences15

European Rabbit in Australia16

Introduced

In 1859 for

hunting

Rapidly

became

a pest

Example 1 – Disease can limit populations17

Initial epidemickilled 99% of the rabbits

MyxomatosisIntroduced in 1950

18 A Complication - Evolution

- The rabbits became

more resistant

- The virus became

less lethal

19Population Regulation – A Simple Model

Population density

Rate

of

po

pu

lati

on

gro

wth

-15

-10

-5

0

5

10

15

population

declines

population

increases

The conservation dilemma

Allee Effect for Threatened Species20

extinction zone

Breeding Success in Shearwaters in New Zealand21

Cause = feral pig

predation on nests

Two Conservation Paradigms22

Small Population Paradigm

- the population consequences of small

population size ?

Declining Population Paradigm

- detect a decline, diagnose the cause,

halt the decline by effective management

Caughley, G. (1994). Directions in conservation biology. Journal of Animal Ecology 63,

215-244. doi: 10.2307/5542

23Small

Population

Paradigm

Start

Loss of genetic

variation

Reduction in fitness

Decline in

density

Prairie Chicken in Illinois USA24

Small, isolated

population in

farmland

Gradual loss

of egg fertility

Translocated

birds from

western states

in 1992

Declining Population Paradigm25

(1) Detect a decline via monitoring

(2) Identify the causes of the decline

(3) Specify detailed hypotheses and

predictions

(4) Test these hypotheses and take action

Marion Island Petrel Recovery26

12 species of petrels nesting on this Indian Ocean

island

1948: 5 domestic cats introduced to control house mice

1977: 3000 cats on island, killed 48,000 great-

winged petrels, petrels now becoming rare

1977: Feline panleukopenia introduced, cats reduced

1986-90: shooting of cats, by 1991 no cats detected

1992: petrels recovered, but house mice increasing ….

Causes of Population Decline / Extinction27

(1) Overkill – fishing or hunting- 10% ? of losses, larger animals

(2) Habitat destruction and fragmentation- 40% of losses ?

(3) Introduced species- 35-40% of losses ?

(4) Chains of extinctions- specialized species, 5% ?

Overharvesting – Northern Cod28

29 Overharvesting – Northern Cod

Total closure of the fishery in eastern

Canada in 1992

Prediction: fishery will recover over the

following 25-30 years

Cod numbers have slowly recovered as of

2020 due to fishing and grey seal predation

30 Introduced Species - Predators

Lake trout were a major freshwater fish in the Great Lakes of North America

Building of ship canals opened up the Great Lakes to the sea and marine predators

The sea lamprey became established in freshwater during the 1930s

Sea Lamprey Predation on Lake Trout 31

Dilemmas #1 – Killing for Conservation32

Invasive species– can we reduce their

impact ?

- very difficult to eradicate

- best to prevent colonization

Native species that impact other native

species – how to manage these ?

33

Map of 11 study sites,

Northern Spotted Owl,

1983-2013

Dugger, K. M. et al. (2016).

The effects of habitat, climate,

and Barred Owls on long-term

demography of

Northern Spotted Owls.

Condor 118:57-116.

33

34

Northern Spotted Owl Population Trends34

Killing for Conservation (cont’d)

Northern Spotted Owl declines can probably

be stopped by killing Barred Owls, a

natural invader

Barred Owls are everywhere within the range

of the Northern Spotted Owl so killing

would have to continue to be effective

Do we want to do this killing or not ?

35

Human Population Dynamics36

No population can increase without limit

Humans are no exception

Stable populations can exist in two states:

- (1) high birth rates = high death rates

- (2) low birth rates = low death rates

The movement from (1) to (2) is the

Demographic Transition

Human Population Growth37

Human Population Projections38

Current path

39 Demographic Transition

Mexico1.2% increase2.1 children

Sweden0.3% increase1.8 children

2020

Human Population Growth40

Current human population = 7.83 billion- 2020 rate of increase = 1.1%- Total Fertility Rate = 2.3 children

Carrying capacity of the earth - how many people can the Earth support ?- no clear agreement- best guess: ecological footprint

Ecological Footprint of Humans41

The carbon footprintis the largest part ofhuman ecological

footprint

Coda

No population increases without limits

The problem is to find the mechanisms

causing population change

With the experimental approach, we are

making progress

But “the devil is in the details” …

42

Thanks for listening !

43