100 Influential Papers - page 9

POPU L AT I ON DYNAM I C S
04
Varley, G.C. & Gradwell, G.R. (1960)
Key factors in population studies.
Journal of Animal Ecology, 29,
399-401.
This paper provided the first methodology that was
widely used to dissect the immediate causes of changes
in population size. Following Morris’s concept of key
mortality factors, Varley and Gradwell developed a
quantitative approach based on Haldane’s logarithmic
method for comparing the contribution of successive
mortality factors to total mortality. The method was
originally applied to time series for winter moth
populations in WythamWood, where variation in winter
disappearance was the key factor responsible for most
of the variation in numbers. It was subsequently used by
others to explore the dynamics of other invertebrates,
birds and mammals. It proved particularly useful in
relatively long-lived species where it provided a way of
identifying the relative impact of mortality in different
categories of individuals on changes in population size.
Today, this approach has been largely abandoned in
favour of more powerful and sophisticated techniques
and it is appreciated that complex patterns of dynamics
can emerge from systems where population growth
rate depends on population density in a non-linear
manner. Nevertheless, both the new methods and our
understanding of the demographic impact of mortality
at different stages owe much to Varley and Gradwell’s
original paper and the approach to population dynamics
that they pioneered.
Tim Clutton-Brock
06
Roland, J. (1994)
After the decline-
what maintains low
winter moth density
after successful
biological control?
Journal of Animal Ecology, 63, 392-398.
In the 1950’s the winter moth,
Operophtera brumata
became
well known to population ecologists as the subject of one of
the first life table analyses of an insect population [Varley,
G.C. and Gradwell, G.R.
Journal of Animal Ecology
,
29
, 399-401
(1960)]. At about the same time winter moth was discovered
to be defoliating oak woodlands in Nova Scotia in eastern
Canada, and 20 years later it moved west to Victoria, British
Columbia. The tachinid fly,
Cyzenis albicans
was introduced to
Canada as a biological control agent. Because parasitization
was low for British winter moth populations, Varley thought
this fly would not be a successful biocontrol agent. But it
was. Using both a life table approach and experiments,
Jens Roland carried out one of the most comprehensive
experimental investigations of a successful biological control
programme. His work showed that winter moth populations
can have two distinct density equilibria, a higher one in the
absence of a specialized parasitoid and a lower one regulated
by ground beetle predation on pupae. The important
messages from this work are that 1) successful biological
control agents do not have to be density dependent, 2)
factors that limit and regulate populations can be different,
and 3) parasitoids that are relatively rare in the native habitat
can be successful control agents.
Judith Myers
Solomon, M.E. (1949)
The natural control of animal populations.
Journal of Animal Ecology, 18, 1-35.
Although influential, this paper had a slow start, only 327 citations being recorded for it between 1949 and 1993. Since 2000, however, it
has averaged about 28 citations a year. To the modern reader this paper comes across as wordy and discursive, more like a popular article
than a scientific paper. This does not, however, mean that the science and the man behind the article were not first class. Journals had less
pressure on their space in those days and scientists had more time to think and read. If only it were so now. This paper has had an immense
influence on the study of population dynamics, although for my generation who were undergraduates in the 1970s, Solomon’s little Study in
Biology book,
Population Dynamics
, published in 1969, was our main, if not only, encounter with his work. Today we recognise this paper as
the first one to formalise the term ‘functional response’ and the first to draw together the disparate conceptual strands of pre-war population
dynamics in one coherent whole. This was an inspirational paper and inspired early modern mathematical ecologists such as Holling, Watt,
Gradwell and Varley and through them more recent past BES presidents such as Mike Hassell, Bob May and John Lawton. A truly influential
piece of writing.
Simon Leather
1
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GC Varley
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