Download Ecology before ecology: biogeography and ecology in Lyell`s

Journal of Biogeography, 29, 1109–1115
Ecology before ecology: biogeography and ecology
in Lyell’s ‘Principles’
CLASSICS
SERIES
A critical re-assessment of the significance of Charles Lyell (1832) Principles of Geology, Vol. II. John
Murray, London
Charles Lyell’s Principles of Geology (first published in three volumes between 1830 and 1833) has been
described as ‘the most famous geological book ever written’ (Gould 2000, p. 159). The second volume
(1832) is primarily biological in content; indeed a modern subtitle for this volume could be ‘Ecology and
biogeography, a palaeontological perspective’. Most commentators on the biological importance of The
Principles have focused on two aspects; its hostility to Lamark’s evolutionary ideas and the influence of
the book on the young Charles Darwin (e.g. Oldroyd 2002). However, as David Wool (2001) has recently
pointed out, many aspects of the ecology in The Principles appear surprisingly modern, although the term
‘ecology’ was not coined until nearly 50 years after its publication. Care is needed as it is easy to misinterpret such a book by focusing on the ideas which seem prescient to a modern reader while ignoring the
apparently wrong ideas in which they are embedded. Indeed the ecology in Lyell’s Principles is just as
interesting for the absence of now commonplace ideas as well as interpretations that seem bizarre to
modern readers (some of which were also considered strange by many of Lyell’s contemporaries).
The Principles went through 12 editions during Lyell’s life time (Figs 1 and 2) and is unusual in that
although the book had an enormous influence on geology it led to no distinct Lyellian school or
research tradition (Rudwick 1978). This essay concentrates on volume two of the first edition. The
content of The Principles changed quite considerably over the nearly 50 years between the first and
twelfth editions, by concentrating on the first edition my aim is to focus on the ecological
understanding of a leading British scientist in the early 1830s. Using later editions would lead to a
different interpretation of several points (Box 1 or compare this essay with Wool (2001) who relied on
the 9th edition of 1853).
The core of Lyell’s approach in The Principles is a very strict version of uniformitarianism which
stressed that the causes of geological change observed acting today were wholly adequate to explain
past changes and that these causes had always acted at the currently observed rates (Gould 1987;
Rudwick 1998). Therefore Lyell reviews current geological and biological processes in detail, as these
should allow the correct interpretation of past events. This led Lyell to postulate a steady-state world
where biological species became extinct at a rate balanced by the creation of new species (by a
mechanism he left unspecified). Peter Bowler (2000) has argued that this ultra strict version of
uniformitarianism arose from Lyell’s attempt to produce a theoretical approach to geology that was in
accord with the definitions of science current in philosophy at the time. Bowler suggested that in this,
Lyell was behaving in an unusual manner for a scientist in that his theories were partly driven by
philosophical concerns rather than merely using philosophy to justify a scientific position that had been
reached without philosophical input.
THE BALANCE OF NATURE, ADAPTATION AND DISPERSAL
The idea of the balance of nature has a long tradition in western natural history and was normally
explained as divinely ordained (McIntosh 1985). This long-standing idea is clearly present in Lyell who
wrote ‘the various species of contemporary plants and animals have obviously their relative forces
nicely balanced, … In no other manner could it happen, that each species surrounded as it is by
countless dangers should be enabled to maintain its ground for periods of considerable duration’ (Lyell
1832, p. 42). He extends this idea of balance to geological time scales writing that ‘fluctuations of
animate and inanimate creation should be in perfect harmony with each other’ (Lyell 1832, p. 157). As
Gould (1987) has pointed out, Lyell believed that all species arise in perfect adaptation to the current
environmental conditions. It follows from this that, as climate changes so will the species assemblage,
as some become extinct and others are created to suit the changing conditions. Lyell pushed this idea to
its limits with a famous thought experiment that suggested that if the climatic conditions of the
Mesozoic were to return then so too would many of the fauna adapted to such conditions. So ‘huge
iguanodon might reappear in the woods, and the ichthyosaur in the sea’ (Lyell 1830, p. 123). This is
probably the most extreme version of climatic determinism in the history of biogeography and was
treated with ridicule even by many of his contemporaries (Gould 1987; Rudwick 1998).
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1110 D. M. Wilkinson
Figure 1 Charles Lyell (1797–1875), author of Principles of Geology (1830–1833) – ‘the most famous geological
book ever written’ (S. J. Gould, 2000, p. 159). (Reproduced with permission from Oxford University Museum of
Natural History, Oxford).
Lyell never specified the mechanism by which he thought species were created, although according to
Rudwick (1998) he told friends that he thought it was a natural process. However ‘he clearly envisaged a
process of piecemeal production of new species, in appropriate ecological situations that would tend to
be scattered in both space and time’ (Rudwick 1978, p. 233). Lyell thought that new species were
created at a particular geographical location, this has important biogeographical implications as it
suggested to him that dispersal must be of great importance. How else could a species with a single point
of origin come to cover a wide geographical range? From the 10th edition onwards, Lyell accepted the
idea of evolution and this had important effects on his interpretation of biogeography (Box 1).
Lyell devotes three chapters to considering geographical distribution in the context of dispersal;
discussing factors such as the effects of wind, water and animals on seed dispersal and the swimming
abilities of animals such as elephants. He was fully aware that humans were increasingly important in
the dispersal of many species and suggested that they were replacing some of the animals they had
destroyed as seed dispersers. Lyell presented no value judgement on the implications of widespread
human dispersal of organisms; today they are often viewed as a major conservation problem.
HABITAT AND NICHE
Ideas of habitat and niche usually figure prominently in most introductory ecology courses. An informal understanding that particular organisms are usually associated with certain landscapes and
climates must have existed in prehistory. More formal ideas on habitat (usually called station in the
nineteenth century) as distinct from geographical range (habitation) are usually attributed to Augustus
de Candolle around the start of the nineteenth century (Bowler 1992). This distinction between station
and habitation is present in The Principles. Wool (2001) has recently suggested that Lyell’s concept of
station is very similar to modern ideas of niche. If so this would be of great interest as this concept is
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Classics Series No. 1: Lyell’s ‘Principles’ 1111
Figure 2 The Title Page of Principles of Geology, Volume The Second, published by John Murray, Albemarle
Street, London, 1832. John Murray also published Charles Darwin’s On the Origin of Species (1859).
normally considered to have arisen around the start of the twentieth century and had to wait for Elton
(1927) for widespread popularization.
The niche concept suggests that different species within the same guild (e.g. grazing mammals) have
different environmental requirements. As such the total number of individuals an area will support is
likely to be higher if these individuals are drawn from several similar species rather than all belonging
to a single species; consider the many species of ungulates found in some African grasslands. Lyell
(1832, p. 142) has an instructive thought experiment that suggests that his idea of station is much
nearer to habitat than to niche. He wrote ‘if we inclose a park, and stock it with as many deer as the
herbage will support, we cannot add sheep without lessening the number of deer; nor can other
herbivorous species be subsequently introduced, unless the individuals of each species in the park
become fewer in proportion.’ It seems clear that the idea that even similar co-occurring species occupy
different ecological niches had not occurred to him. Indeed his examples of stations include ‘saltmarsh’ and ‘stagnant pool’, clearly habitats not niches. However, this thought experiment does show
that Lyell understood the concept of carrying capacity.
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BOX 1 Lyell’s evolutionary biogeography
Lyell did not endorse Darwin’s views of evolution until the 10th edition of The Principles in 1868 (nine
years after the publication of On the Origin of Species). An acceptance of evolution had important
implications for Lyell’s treatment of biogeography. In the first edition of Principles, Lyell suggested
the local creation of species adapted to the prevailing environmental conditions, so leading to a need
for dispersal if they were to occupy larger ranges. This did not explain why the new species in an area
should closely resemble the recent fossil species in that locality although these may have lived under
different environmental conditions. In the 10th edition, Lyell explains how the idea of evolution
answers this biogeographical problem. The addition of evolutionary ideas makes the biogeography of
islands particularly interesting and he adds a chapter on ‘Insular floras and faunas considered with
reference to the origin of species.’ For example, evolution explained why the biota of Australia was so
different from that of Asia, indeed Lyell illustrates this with a map illustrating Wallace’s line (below).
Since then islands have maintained a prominent place in textbooks of biogeography.
Evolution also helped explain how introduced species can sometimes out compete the indigenous
biota, a problem if species are created perfectly adapted to local conditions as suggested in the first
edition. Lyell (1868, p. 385) wrote ‘organic beings of each great region which man finds in
possession of wide areas are not those which are most fitted of all contemporary species to flourish
there to the exclusion of all others. They appear to be simply the modified descendants of such an
older fauna and flora as happened to preexist… [and]… are powerless to maintain their ground in
the struggle for life if brought into competition with species from different regions which would
never without the aid of man have come into contact with them.
Box Figure Map of Wallace’s line from Lyell (1868); note the inclusion of both zoological and anthropological
data on the same map. In the light of evolution humans are now animals, even if the other species are ‘inferior’!
Also note the typographical error in Lyell’s text, Wallace’s middle initial was ‘R’ not ‘B’. One fathom is equal to
6 feet (i.e. c. 1.8 m).
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TOWARDS INTERMEDIATE DISTURBANCE
The processes that control the number of species found at a site are of great interest for geographical
ecology. The intermediate disturbance hypothesis suggests that environments with low levels of
disturbance have low species richness due to competitive exclusion while high levels of disturbance
eliminates those species incapable of rapid colonization. The result is that species richness peaks in
environments with moderate disturbance levels, this is usually now depicted as a ‘hump-backed’
graphical model, in this form the hypothesis was first described by Philip Grime in the early 1970s
(Wilkinson 1999).
Lyell clearly appreciated the potential for disturbance due to herbivory to increase plant species
richness. This idea is not original to Lyell as he cites Wilcke (cited by Linnaeus in 1749) as describing
how herbivory can lead to the reduction in grasses so that ‘other plants, which were choked by it,
spring up, and the ground becomes variegated with a multitude of different species of flowers’ (Lyell
1832, p. 133). In another example, he describes the tendency for bog moss (Sphagnum) to dominate
peaty systems and points out that ‘such monopolies would be very frequent … if animals did not
interfere actively to preserve an equilibrium in the vegetable kingdom’ (Lyell 1832, p. 131). So while
the intermediate disturbance hypothesis was formalized in its current form in the 1970s, some of the
observations underlying it extend back at least to the eighteenth century.
THE EARTH SYSTEM
In the overview of the history of geology in volume one of The Principles, Lyell identifies James Hutton
as a forerunner of his approach to the Earth. Hutton undoubtedly had a systems approach to geology,
emphasizing the balance between rock formation and erosion in maintaining land over long periods of
geological time. Indeed Lovelock (1995) identifies Hutton as a forerunner of Gaia theory: a strongly
systems-orientated approach to the Earth with a major emphasis on negative feedback loops heavily
influenced by the biota. With such a background it is unsurprising to find topics now central to Earth
Systems Science in The Principles.
A good example is the effect of vegetation on climate, especially rainfall. Lyell wrote that leaves ‘of
all plants are alembics [gourd-shaped distilling apparatus] and some of those in the torrid zone have a
remarkable power of distilling water, thus contributing to prevent the earth from becoming parched
up’ (Lyell 1832, p. 200). Modern computer models suggest that vegetation increases continental
precipitation by 15% (Betts 1999). Once again these views were not original to Lyell but were
widespread at the time with many writers suggesting that the long-term clearance of vegetation in
Europe and the more recent clearances in North America had lead to climatic change (Fleming 1998).
The idea that plants increase rainfall due to the large volumes of water they transpire can be traced
back to early plant physiologists, such as John Woodward, in the late seventeenth and early eighteenth
centuries (Grove & Rackham 2001). Indeed Lyell was sceptical of some of the more extreme claims
made about the role of vegetation in rainfall, suggesting that ‘some writers seem to have attributed too
much importance to the influence of forests, particularly those of America, as if they were the primary
cause of the maintenance of climate’ (Lyell 1832, p. 201). Some topics of great importance in Earth
System Science are noticeably absent, the idea of nutrient cycling being a striking example. However,
within 50 years this important idea would become a topic discussed even in introductory texts (e.g.
Huxley 1880).
ON COMPETITION AND HUMANS AS A CAUSE OF EXTINCTION
Throughout The Principles, Lyell describes many examples of interspecific competition, but mention of
intraspecific competition is striking by its absence. Not until Darwin and The Origin of Species did the
importance of competition between individuals of the same species become clear. The nearest Lyell gets
to this concept in The Principles is a brief discussion of competition between different human races.
Today we would tend to define interspecific competition as taking place between individuals of different species. However in The Principles this distinction is not clear and Lyell often implies competition between the species themselves.
One clear example of interspecific competition described by Lyell is that between humans and many
other species. It was obvious to him that humans were causing some species to become extinct and that
habitat destruction and introduced species were implicated alongside direct hunting. Such an
observation today would usually be accompanied by a statement of regret and a suggestion that
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1114 D. M. Wilkinson
something should be done about it. He was, however, clear that this was not the case and ‘if we wield
the sword of extermination as we advance, we have no reason to repine at the havoc committed’ (Lyell
1832, p. 156); although he admitted that others held different views. A 21st century reader might guess
that Lyell held religious reasons for not lamenting these extinctions, that it was the will of God that
man so uses the creatures of the Earth. However Lyell’s reasons are much more interesting, being based
on secular arguments. He suggests that human caused extinction is a natural process. ‘Every species
which has spread itself from a small point over a wide area, must, in like manner, have marked its
progress by the diminution, or entire extirpation, of some other, and must maintain its ground by a
successful struggle against the encroachments of other plants and animals’ (Lyell 1832, p. 156). This
applies as much to man as ‘the lion, when it first spread itself over the tropical regions of Africa.’
THE IMPORTANCE OF LYELL’S ECOLOGY
Most of the ecological ideas in volume two of The Principles were not original to Lyell, examples
include; interspecific competition, dispersal, habitat and the role of disturbance by grazing. Indeed the
most original biological idea was his extreme version of climatic determinism that had little effect
either on his contemporaries or subsequently. The importance lies in its synthesis of ideas on ecology
and biogeography which illustrates a level of understanding which comes as a surprise to some modern
ecologists (e.g. Wool 2001), being 50 years before the subject of ecology was named and started to
emerge as a recognizable discipline (McIntosh 1985). The effect of this synthesis of ecological ideas on
Lyell’s contemporaries was perhaps limited, as self-conscious ecology did not yet exist and the interactions between organisms and their environment was not yet a major topic of research.
From the vantage point of the early 21st century, the appreciation of the way a mix of hunting,
habitat destruction and species introductions were leading to extinctions is fascinating as we often
consider this to be something not recognized until the 20th century. It is also interesting to see this
described as a natural process and not a problem, a view unlikely to be held by the majority of readers
of this journal.
ACKNOWLEDGMENT
I thank Hannah O’Regan for comments on the manuscript.
DAVID M. WILKINSON
Biological and Earth Sciences,
James Parsons Building,
Byrom Street,
Liverpool John Moores University,
Liverpool L3 3AF, UK
E-mail: [email protected]
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BIOSKETCH
Dave Wilkinson has wide interests in biology and the environmental sciences. His historical researches
have produced papers on nineteenth century Quaternary palaeoentomology, the intermediate
disturbance hypothesis, island biogeography and evolutionary theories for the maintenance of sex. His
theoretical work has concentrated on aspects of evolutionary ecology (especially mutualisms),
biogeography and Earth Systems Science. His more data-based studies have focused on Quaternary
palaeoecology, environmental archaeology (especially the Neolithic in the English Lake District) and
the ecology of protozoa.
2002 Blackwell Science Ltd, Journal of Biogeography, 29, 1109–1115