100 Influential Papers - page 8

Varley, G.C. (1947)
The natural control of population balance in the
knapweed gall-fly, (
Urophora jaceana
).
Journal of
Animal Ecology, 16, 139-187.
Density dependence is one of the central concepts in ecology : it
is critical to the growth, persistence and dynamics of populations.
This idea introduced by AJ Nicholson, was extended by Nicholson
and V.A.Bailey [
Proceedings of the Zoological Society of London
,
3
,
551-598 (1935)] to show theoretically how natural enemies, and
in particular, parasitoids, influence the stability and dynamics
of populations. Varley’s 1947 paper based on his Cambridge
thesis of 1938 provided the first and definitive field test of the
concepts of Nicholson and Bailey on the stability and persistence
of host-parasitoid interactions. Varley documented the ecological
effects on the interaction between the knapweed gall-fly and
a range of parasitic natural enemies. By carefully investigating
different ecological processes acting on
Urophora jaceana
,
Varley showed that three factors control population size: direct
density dependent early larval mortality and delayed density
dependent parasitism by two different parasitoids. While the
use of mathematical theory and statistical methods of analysis
(to account for both deterministic and stochastic factors) have
advanced enormously since Varley’s study, the necessity for
clear, carefully planned field experiments to test concepts has
never been in dispute. Varley’s paper remains a testament to this
approach of challenging contemporary theory with experimental
and observational tests.
Michael Bonsall
S ECT I ON ONE
POPU L AT I ON DYNAM I C S
01
02
03
Elton, C. & Nicholson, M.
(1942)
The ten year cycle in the
number of lynx in Canada.
Journal of Animal Ecology, 11,
215-244.
Modern population ecology seems far removed from the
early days of the subject. Today, sophisticated theory is
brought together with rich, long-term datasets to help
us understand how and why populations are responding
to environmental change. We are now able to explore
simultaneously the coupled ecological and evolutionary
dynamics of populations, and delve into the detail of
individual life histories and the factors that shape them.
The ‘roots’ of modern population ecology are much more
modest by comparison. The earliest papers were purely
descriptive, simply reporting the observed patterns of
population change over time. In many ways, the paper
by Elton & Nicholson is unremarkable because it simply
reports data on Lynx numbers in Canada. Its importance
really lies in the cyclical patterns it documents; patterns
that demanded a mechanistic explanation, and which
subsequently stimulated the development of a rich array
of theoretical population dynamics models. We now know
that population cycles can be generated by a wide range
of ecological mechanisms, but none of this work would
have happened without the pioneering studies of early
ecologists such as Elton & Nicholson. Observations of
ecological systems matter! In an age of austerity, we would
do well to remember this.
Ken Norris
08
1
Birch, L.C. (1948)
The intrinsic rate of natural increase of an insect population.
Journal of Animal Ecology, 17, 15-26.
Charles Birch (1919-2009) wrote this paper while visiting Elton’s Bureau of Animal Population at Oxford. By 1948 he
had joined Sydney University. As of 2012, this paper has 1,445 citations. It urges that ’The intrinsic rate of increase [
r
,
as introduced by Lotka for human populations] is a basic parameter which an ecologist may wish to establish for an
insect population’. The paper shows, in detail that looks excruciating by today’s standards, how
r
is calculated from
age-specific survivorship and fecundity data (
l
x
and
m
x
) from Birch discusses the ecological
usefulness of
r
, and gives a very detailed illustration of its calculation from data for the rice weevil
Calandra oryzae
. Notably, this is one of
the first quantitative analyses of the demographic behaviour of an animal population, albeit one which was growing exponentially under
laboratory conditions. More generally, Birch was a major figure in ecology’s earlier days. With Andrewartha he wrote The
Distribution and
Abundance of Animals
(Chicago University Press, 1954), the world-wide leading ecology textbook of the 1950s and 1960s. In the late 1960s
he founded the Social Responsibility in Science Movement in Australia (to which I owe my subsequent career). He also was an influential and
ego-free leader of anti-Vietnam-war activism in Australia.
Robert May
0
e
rx
l
x x
m
dx
1.
C Elton
LC Birch
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