Download Patterns in Ecology

Nordic Society Oikos
Patterns in Ecology
Author(s): John H. Lawton
Source: Oikos, Vol. 75, No. 2 (Mar., 1996), pp. 145-147
Published by: Blackwell Publishing on behalf of Nordic Society Oikos
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OIKOS75: 145-147. Copenhagen1996
JohnLawton'sViewfromthe Park 15
Patterns in ecology
Without bold, regular patterns in nature, ecologists do
not have anything very interesting to explain. Patterns
can exist at various scales in time and space, ranging
from population abundances, through communities,
ecosystems, biomes and the entire biosphere. Robert
MacArthur understood this very clearly. In 1972, in the
Introduction to GeographicalEcology he wrote: 'To do
science is to search for repeated patterns... The best
person to do this [in ecology] is the naturalist who loves
to note changes in bird life up a mountainside, or
changes in plant life from mainland to island, or
changes in butterflies from temperate to tropics.' Yet
despite the obvious importance of patterns, ecology
twenty years after MacArthur is still ambivalent about
pattern seeking. I sense that things are changing (James
Brown's Macroecology, University of Chicago Press,
1995 captures this changing mood as well as any), but
there are still too many people who believe that the
only way to do science is to experimentally manipulate
nature.
It is a curious fact, for example, that ecological
scientists have not gathered together in one place a
database of the significant, interesting, patterns that we
seek to explain, still less a compendium of patterns
linked to theoretical explanations in a hierarchy ranging from 'reasonably well understood' to 'there are
several possible explanations, none very satisfactory', to
'fascinating, but what does it mean?' Some text books
document some of the patterns, more specialised monographs describe others (for instance, for all that I like
what Brown is trying to do, Macroecology is economical with the literature). The result is that there is no one
place to send a new graduate student, from which they
can get a sense of the main empirical framework of our
OIKOS 75:2 (1996)
subject. Compiling such a database will be a difficult
task, but it needs doing.
Paradoxically, ecology's obsession with manipulation
experiments over the last two decades springs, at least
in part, from a reaction against simplistic, selective and
uncritical use of patterns to support (rather than test)
theory; Hutchinson's supposedly constant body-size ratios in guilds of coexisting predators provides a good
example. The need to avoid 'just so stories', and to
impart some rigour into field tests of theory was a
healthy and timely development for our subject, but in
the process the search for patterns worth explaining got
side-tracked. It didn't stop, but it was certainly hard to
get funded - which slows things down somewhat! I
gather from Jim Brown, for instance, that although he
has had considerable grant support for his pathbreaking field manipulation experiments, he has never had
significant grant support for any of his work on largescale ecological patterns. I know of other similar cases,
and it is a disgrace.
Field manipulation experiments, like any approach
to a difficult problem, have both advantages and disadvantages. One rarely discussed disadvantage, pointed
out to me by Clive Jones, is the problem of the scale of
human perception. Very simply, and generalising wildly
but with some justification, human beings operate on
the wrong spatial and temporal scales to discover, by
doing field manipulation experiments, many of the major patterns and rules that determine how assemblages
of 'large' multicellular organisms (higher plants, insects,
fish, birds, etc.) are put together. We find it difficult to
see the wood when trying to manipulate trees.
Consider a fanciful analogy. Imagine you are a fairy,
a bit larger than an atom, sitting in a world made up of
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a mixture of gasses. You would see balls (molecules) of
different sizes and colours whizzing about, occasionally
colliding, and sometimes combining to.yield differentsized balls. Being a curious fairy, with an experimental
bent, you attempt to manipulate the molecules by
building fences in a very local part of this imaginary
world, to keep out the big red balls which appear to be
attacking the smaller blue ones. The grant application
to the king of the fairies says that you wish to: 'Understand and predict the role of large red balls in structuring the assemblage of other balls.' Does it sound
familiar?
The problem, put like this, is obvious. The really
important things determining the behaviour of this
hypothetical, gaseous world, operate at scales either
much bigger, or much smaller than the unaided fairy
can perceive. As scientists we are adept at peering into
smaller scales using a microscope, or some equivalent
piece of clever technology; no doubt the fairy will
quickly realise that he or she needs to study sub-atomic
particles, and devise a means of doing so. It is rather
harder to stand back and realise that the gaseous world
has macroscopic properties, pressure for example, and
predictable, long-term chemical equilibria, when you
are surrrounded by a mass of whizzing, coloured balls,
the individual, local behaviours of which are mainly
random and largely irrelevant. The fairy lacks a macroscope, and has basically posed the wrong question, at
the wrong scale. Ecologists, too, lack a macroscope - a
machine that reveals big patterns that emerge from a
mass of local fuzz, and which operates on time-scales
orders of magnitude longer than a three-year research
grant. But we can search for large patterns in nature,
and they are the next best thing to a macroscope.
Let me suggest a real-life example. The ecological
literature is full of small-scale (each replicate is typically
a few square metres) field manipulation experiments
characterising interspecific interactions between pairs or
a handful of species (usually competitors, or predators
and their prey). Sometimes, and least interesting, the
manipulations last less than a generation of the key
organisms; luckier, or more thoughtful, investigators
(Jim Brown, for instance) manage to continue the experiment for several generations. The aim of the experiment is usually to discover how species A influences
the presence, absence, or abundance of species B, C,
etc. Properly designed and run for long enough, such
experiments are important - this is not diatribe against
good field experiments. But they do not answer the
larger question. For example, why are there 2 species in
the system I am interested in at one locality, 20 at
another and 200 at a third? This bigger question is
highly unlikely to be answered by small-scale manipulation experiments.
One of the boldest amd most interesting patterns to
emerge in ecology over the last decade is the fact that in
many systems (a large majority, it would appear, but
146
not quite all), local species richness (2, 20, or 200
species in the above example) is linearly related to the
number of species in the regional pool, with a slope less
than 1. That is, local assemblages are a 'proportional
sample' of the regional pool (e.g. Ricklefs, R. E. and
Schluter, D. 1993 (eds), Species Diversity in Ecological
Communities. Chicago University Press). Under these
circumstances, small-scale field manipulation experiments that seek to 'explain the structure' or 'understand
the processes controlling diversity' are simply looking
at second, even third-order, phenomena - the noise
round the main regression line. They most certainly do
not say anything very interesting about the processes
(which is what we write in grant proposals) structuring
particular assemblages; they look at the fine tuning.
Many readers will disagree with this view. Good.
Treat it as an hypothesis, and let there be a debate
about the problem. To make it constructive, we might
ask, for example, how the size of the regional pool of
species is determined. The answer will certainly involve
geology, evolution and some very large-scale, slow processes. But what part, if any, do local species' interactions play? If the answer, again, is very little, why do we
pay so much attention to them, at the expense of the
big patterns? A key question in ecology, rarely asked
explicitly, is which local processes amenable to field
manipulation experiments, scale up and have a significant influence on, patterns in species' distributions and
ecosystem processes across landscapes, biomes and the
biosphere?
To search for patterns in nature, is not, of course, the
same as stamp collecting. It is, as MacArthur realised
only too well, central to the scientific enterprise. And to
offer, and test, hypotheses to explain patterns, even
though the tests will not usually involve controlled,
experimental manipulations is also to do science. As I
and others have said repeatedly, if you doubt this, then
astronomy is not a science.
One more thought about patterns is worth articulating. Too often, ecologists seem obsessed with finding a
single explanation for some process or pattern of interest. 'My explanation is right, yours is wrong' may get
papers published (and even grants funded), but for
many of the ecological phenomena I am familiar with,
such polarisation is unwise and unhelpful. For many
phenomena, there are likely to be several contributory
mechanisms, and the question is not so much about
which mechanism is correct, but about the relative
contributions of a plurality of mechanisms. Species-area
relationships, one of the boldest and most robust patterns in ecology, have several explanations, all of them
now known to be valid, but operating with different
force in different systems and on different scales (see
Michael Rosenzweig's Species Diversity in Space and
Time. Cambridge University Press, 1995).
One interesting hypothesis worth considering, is that
like species-area relationships, the most consistent and
OIKOS 75:2 (1996)
clearest ecological patterns will almost invariably have
several explanations, with the various mechanisms all
pulling in the same direction. A corollary is that where
patterns are weak, inconsistent or non-existent, either
nature in these cases is indeed mainly fuzz, or (and
more interesting), conflicting mechanisms are at work,
pulling the patterns in different directions.
There is also a more philosophical message underlying the idea of alternative processes creating the
strongest patterns when they work together. There is no
one correct way to do ecology. Mathematical models,
model ecosystems, field manipulation experiments and
the search for large-scale patterns are all valid approaches, and all have their strengths and weaknesses.
OIKOS 75:2 (1996)
They are simply tools to help us understand nature; like
all tools, each approach does some things well, some
things badly and other things not at all. And all tools
can be used badly by dumb practitioners. Ecology is
likely to be the stronger if it recognises and values a
plurality of approaches to understanding the biosphere,
and if practitioners skilled at using different tools all
pull in the same direction.
John H. Lawton
NERC Centrefor PopulationBiology
ImperialCollege
SilwoodPark
Ascot SL5 7PY, UK
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