The Ecology Book

71

From their calculations, they

deduced that a disease will persist

in a population whose equilibrium

point (the rate of new additions,

balanced by the natural death rate)

is greater than the combined

effects of natural mortality, disease

deaths, recovery, and transmission

rate. While the disease is present,

that equilibrium point will be lower

than if the population were disease

free. If, however, the equilibrium

point of a population affected by

disease is lower than the combined

effects of deaths, recoveries, and

rate of transmission, the disease

will die out. Once a population is

disease free, its equilibrium point

will return to its former level.

Matching the real world

Anderson and May needed to show

that their model was an accurate

predictor of a real-life population.

They did so by using data from

a study of laboratory mice infected

with the bacterial disease

pasteurellosis; the data included

the impact on the population of

adding individuals at different

rates. The observed data confirmed

their predictions, so the two

scientists were able to consider the

effects of hypothetical values. They

found, for instance, that when the

rate of added mice was highest, the

disease had the greatest impact on

population numbers. This suggests

that species with high reproductive

rates (introducing large numbers

of uninfected offspring) are most

likely to have endemic diseases

within the population, and show

depressed numbers compared with

species that breed more slowly.

They also explored the differing

effects on populations of diseases

of different intensities.

Unlike endemic diseases,

in which the population’s level

of infection remains consistent,

epidemics appear in populations

when the growth rate of all infected

and uninfected members is low

compared to the death rate caused

ECOLOGICAL PROCESSES

by the disease. Infection numbers

rise sharply to a maximum, then

drop away. Epidemics also occur

when a disease is not particularly

deadly but slows the population

growth rate; this has occurred with

human diseases such as measles

and chickenpox.

Applying the theory

The characteristics of disease

and its effects on animal and

plant populations are of increasing

ecological importance. Food

producers, for example, benefit from

studies into the nature of parasites

and the dynamics of diseases that

can affect crops and livestock.

Conservationists also employ

epidemiology to predict how exotic

diseases and invasive parasites

might affect fragile ecosystems. ■

Diseases such as

measles and rubella,

with short infections

and lasting immunity,

will tend to exhibit

epidemic patterns.

Roy Anderson

A pathogen strikes when it finds a suitable host in an

environment that favous infection, as shown where the

circles intersect. For instance, diarrheal diseases spread

quickly among sick people in unsanitary conditions.

Venn diagram of ecological epidemiology

Susceptible host

no

disease

Favorable

environment

for pathogen

Pathogen

no

disease

no

disease

no

disease

no

disease

no

disease

disease

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