October 23, 2017

Colonising the plant world

Book Review: Insects on Plants: community patterns and mechanisms by D. R. Strong PhD, J. H. Lawton PhD, Sir Richard Southwood PhD, DSc, FRS. Blackwell Scientific Publications, 1984.

Published in The Ecologist Vol. 15 No. 3, May 1985.


The authors consider that the study of how various phytophagous (plant eating) insects colonise different plants under different ecological conditions will cast light on 3 questions which they say “dominate ecommunity (sic) ecology at the present time”. These are:

  • “How predictable are natural communities?”
  • “How important is competition between component species in determining community structure?”
  • “What proportion of co-existing species in contemporary communities are co-evolved?”

That such a study is important seems clear, if one considers the sheer diversity of phytophagous insects; there are at least a third of a million species, a figure which is all the more impressive if one realises that there are only 8,500 species of birds and 4,500 species of mammals. As the authors point out there are nearly ten times more species of butterflies and moths than of all birds and mammals combined.

Beetles – which make up one of the 9 orders of insects – also come “in a bewildering variety of shapes and forms”, so much so that when the great biologist J. B. S. Haldane was asked by a clergyman what his studies had taught him about the Lord, he answered “his inordinate fondness for beetles”. In addition, “for every species of phytophagous insects there is also approximately one predatory parasitic or saprophagous insect species”.

The authors describe how plants “have fundamentally influenced the evolution of phytophagous insects”. This must clearly be so, for if the millions of different species of phytophagous insects had not learnt over the ages to become efficient predators or parasites of the plants on which they depended for their sustenance, they could not have survived.

The authors also point to the perhaps less obvious principle that “phytophagous insects have probably played a very important part in the evolution of plants”. They consider that it is likely “that phytophagous insects had a strong hand in generating the bewildering biochemical diversity and the rich variety of growth forms, leaf shapes and seasonal phenologies of modern land plants”. This they regard as only a guess but to me it seems to be quite obvious.

For this extraordinary diversity of plant life to have survived, it must clearly have learned to limit the impact on it, of its predators. These we know it has succeeded in doing in all sorts of ways; some of them are extremely ingenious.

Further on in the text the authors state what would seem to be the co-evolution principle quite emphatically:

“Reciprocal adaptation and counter-adaptation between plants and insect phytophages has been an important mechanism driving a steady increase in plant and insect diversity over the broad sweep of the fossil record.”

An interesting section of the book, Chapter 3, deals with the way in which the diversity of the insects living off a plant species increases, as the latter becomes better established in a particular ecosystem. Thus the authors consider

“biochemically unusual species of introduced trees such as Eucalyptus spp. in Europe and North America and Quercus spp in South Africa, Australia and New Zealand, present formidable barriers to colonisation by unadapted phytophages and in consequence have impoverished faunas.”

The authors, however, do not consider that this factor is all that important in determining insect diversity. More Important is “the size of the geographic range of the host plant”. Widespread species of plants, they consider, are hosts for more species of insects than rare plants. The reason seems clear:

“Widespread species of plants grow in more habitats and over a wider range of climatic zones than rare plants. Hence different species of insects are found in different parts of the ranges of widespread plants. Secondly, widespread plants present more conspicuous ‘targets’, for colonising organisms. Thirdly, small populations, on plants with restricted ranges, may be more prone to extinction.”

Thus as the range of a plant expands, there is a tendency for the number of insects it supports to increase. This is illustrated by the recent history of the rosebay willowherb (fireweed) Chamerion angustifolium in Britain. It was formerly a rare plant, growing in certain localities only but its range has undergone a massive expansion since the First World War. At the same time the number of insect species living off it has increased since 1919 from 13 to 30.

This information is obviously of practical value. One can expect introduced trees to remain immune to attack by local insects for a long time. Slowly, however, local insects, previously feeding on other trees will learn to colonise it. Thus

“in Britain, the North American ornamental cypresses (Chamaecyparis, Cupressacae) have recruited one of the two native species of mirid bug found on the native co-familial juniper, and one of the two native species of shield bug (the second shield bug is probably extinct in Britain). Similarly, Nothofagus species (southern beech, Fagaceae) introduced into Britain from South America and Australia, have acquired breeding populations of at least six species of Typhlocybinae (leaf-hoppers), all of which normally feed on related native British trees in the same family (Fagus (beech) and Quercus (oak)).

The authors then use this information as an argument against the ecological view of the ecosystem as a natural system, that evolves according to set rules by orderly stages (succession) towards a situation which maximises overall stability (the climax). This was the view of early ecologists at the turn of the century, (Clements, Shelford, Phillips etc) a view which has unfortunately gone out of fashion since the fifties when ecology was systematically transformed into a hard science so as to make it scientifically respectable and to make its teachings conform with the world view of modernism. In this way ecology ceased to be holistic and became as reductionistic as any other scientific discipline.

Significantly too, in this respect the authors tell us:

“plants recruit insects from a variety of hosts, spanning the full gamut from close relatives to those with no obvious structural or biochemical affinities but merely close physical proximity. The result is a fauna on most plants that is a pot-pourri of the co-evolved, the preadapted and the opportunistic, in varied and at present unpredictable, proportions.”

In this way they make clear their opposition to the old Clementsian idea of the ecosystem as a sort of superorganism with a definite structure and function.

The authors, keen to accentuate the importance of the individual components of the plant community, as opposed to that of the community itself, are necessarily committed (as are the neo-Darwinists who seek to explain evolution in terms of the same reductionistic and mechanistic world view) to accentuate the importance of competition as a determinant of what community structure they accept, and to under-playing co-operation in all its various forms.

They are however, honest enough to admit that competitive interactions between phytophagous insects do not appear to have played as important a role in determining insect diversity as they had previously thought. This, however, does not invalidate the thesis that competition is the most important factor involved. This they justify in the following way:

“Failure to detect significant interspecific interactions in contemporary communities does not necessarily mean that competition has no part to play in structuring that community. It may instead have left its mark in ‘the ghost of competition past’.”

To use Connell’s phrase. This sort of argument makes nonsense of the principle of empirical verification which is supposed to underlie scientific method.

Another important section deals with the question of coevolution which the authors describe as “reciprocal evolutionary change in interacting species”. We have seen that they have already accepted the principle as being paramount but in Chapter 7 they purposefully seek to underplay its importance.

The authors admit that this model appears plausible but they intimate that it is probably untrue. It may be true in certain cases, but these may be the exception rather than the rule. They then quote Caughley and Lawton, and also Monro who

“divided grazing systems into two types, representing the end points of a continuum: there are non-interactive systems, where the herbivores have no measurable impact on the performance of their food plants, and interactive systems where they do.”

They then tell us that rare species fall into the former category. However, one can give the authors of this book a dose of their own medicine and suggest that the fact that these insects have “no measurable impact on the performance of their food plants” does not mean that they do not interact. The interaction may simply not be of the sort that is measurable by the crude equipment at present available, or of a type whose effects are subtle and only discernible over a long period – (The Ghost of interaction past).

The contrived nature of the arguments used by the authors to under-play the importance of co-evolution, shows to what extent they have been imbued with the narrow scientific world-view of which reductionism is a necessary feature. Thus they tell us, in general co-evolution cannot occur because the removal of herbivores appears to have immediate beneficial effects on the plants on which they live. To quote them:

“Over and above the effects imposed by single species of herbivores, several recent experiments have shown that if entire suites of herbivores attacking a plant are removed, plant performance markedly improves.”

Also

“The effect of eliminating, or markedly reducing, herbivores has been well shown by Waloff and Richards’ (1977) study on scotch broom (Sarothamnus scoparius) a shrub that lives for up to about a decade. Bushes protected by insecticides had higher reproductive rates and, beyond five years of age, survived better than those exposed to the natural level of herbivory.”

Another argument equally illustrates their reductionistic approach. Thus they tell us that

“When groups of herbivores act in concert in this way, tight reciprocal coevolution between the host plant and one or two species of phytophagous insects seems particularly unlikely. As soon as plant performance is influenced by several herbivore species, conflicting selection pressures may be generated that restrict or prevent coevolution.”

It is clear that the authors are accustomed to considering inter-relationships between a single predator or parasite and a single prey or host species at a time. In a real ecosystem as opposed to a highly simplistic mathematical model of an ecosystem, adaptation must inevitably occur to the ecosystem as a whole – composed, as it is likely to be, of a large diversity of biotic factors or a-biotic factors rather than to a single component of the ecosystem ,such as an individual species, whatever might appear to be the latter’s importance to the life cycle of the species adapting to it.

There is no reason whatsoever why adaptation to an ecosystem as a whole should provide the optimum adaptation to any of the eco-system’s individual components. In fact it almost certainly will not and it is for this reason that the argument provided by the authors is so totally unacceptable.

The unacceptability of other arguments, proposed by the authors to underplay the importance of coevolution, becomes apparent as soon as adaptation is used holistically. Their conclusion that “the insect fauna of plants is a pot pourri of the co-evolved, the pre-adapted and the opportunistic in varied and unpredictable proportions” simply reflects the extent to which they have been imbued with the paradigm of modernism and hence refuse to see an ecosystem as a highly organised natural system, capable of co-ordinated self regulatory behaviour.

Their statement that “coevolution most certainly does not provide a general mechanism to explain the contemporary structure of phytophagous insect communities” reflects the same set of prejudices. Only those committed to an extreme reductionistic approach can consider evolution as being anything else than a vast co-evolutionary enterprise. A species cannot evolve by itself in a biotic and climatic vacuum, it can only co-evolve as an integral part of an ecosystem, and for the purpose (a term that would make Southwood shudder) of contributing to the latter’s stability.

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