Download TCR White The Inadequate Environment

T. C. R. White
The Inadequate
Environment
Nitrogen and the Abundance of Animals
With 41 Figures
Springer-Verlag
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Thomas C. R. White
Dept. of Crop Protection
Waite Agricultural Research Institute
Glen Osmond, South Australia
5064 Australia
ISBN-13:978-3-642-78301-2
e-ISBN-13:978-3-642-78299-2
DOl: 10.1007/978-3-642-78299-2
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© Springer-Verlag Berlin Heidelberg 1993
Softcover reprint of the hardcover 1st edition 1993
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To the memory
of my two mentors
H. G. (Andy) Andrewartha and
G. B (Joe) Rawlings
Acknowledgements
My story is, inevitably, built upon the ideas and research of others from Darwin to Dawkins - as reflected in my references. However,
there are some whose influence has been general rather than specific,
others of whose direct influence I am no longer conscious; still others
whose relevant work I have never encountered. To all these I apologize
for failing to acknowledge them.
The basic outline of my thesis finally crystallized while I was
teaching ecology at the University of the South Pacific in Fiji, and it
was Hermann Remmert's enthusiastic badgering which started me
putting it into book form. But I was only able to properly address the
task after retirement, and with the support of an honorary research
fellowship at the Waite Agricultural Research Institute.
For many years I worked where ready access to the literature was
limited. lSI's Current Contents, and the hundreds of reprints sent to
me by colleagues throughout the world enabled me to keep up.
Too many colleagues to list lent photographs to illustrate the book
- mostly colour slides. Jennie Groom did a grand job producing
black and white prints from the latter. Many of these same colleagues,
and several others, also helped with comments on appropriate sections. Tom Browning read much of an early draft, alerting me to
grammatical slips and lapses into teleology. But I continue to
deliberately split an occasional infinitive, and resort to teleological
analogy if I think this makes for clearer exposition!
However, it has been Piet den Boer, with his detailed and wise
criticisms via our lengthy discussions by post, who has done more
than anybody else to improve both what I have to say, and the way
in which I say it. I am especially grateful to him.
To all, my very sincere thanks, coupled with the usual exoneration
from any responsibility for what I have said. And in this age of word
processors I can blame nobody but myself for typing and spelling errors!
Last, my very special thanks to my wife, Janice, for grammatical
advice, painstaking assistance with final much valuable proof-reading, and, above all, for continuing to believe in the worth of the project, and for keeping me at it on the numerous occasions when I lost
my confidence.
Preface
This is not a text book. Nor is it an objective review. It is a personal
- and thus biased - view of some facets of ecology; a view I
vigorously advocate. More specifically, it is an attempt to report patterns that I perceive threaded through the fabric of the living world,
the common cause of those patterns, and some of the implications
for our understanding of ecology that flow from this perception. As
a result I come to conclusions that sometimes differ from much that
is generally accepted, and with which many will not agree. But I have
not felt obliged to put the case for alternative hypotheses or interpretations. I leave those who support them to expound their own
viewpoints.
In advocating my viewpoint I have taken examples from studies
that cover a wide geographic and taxonomic range. However, I have
not attempted to be comprehensive. Such would have been a futile
task, even had I the energy, or time left, to try to do so. Every story
I follow up uncovers several new ones. Yet there remains a majority
of the literature that I have never seen - and never will.
Apart from this, however, there is simply not room for all the examples that I have found. If I reported these, the book would be double its size, so I have had to knowingly put much good evidence on
one side. For example, I have left out major groups like the mites and
the nematodes, and the sawflies and leafbeetles among the insects.
Some, like the bacteria, get inadequate coverage; others, like the bark
beetles, get but passing mention. And within the groups from which
I have taken examples, there are many more cases that I could equally
have reported.
Furthermore, I have not tried to be even-handed in selecting which
out of so many examples to use. Rather, I have tried to select studies
which tell a reasonably complete story, or which make a particular
point unambiguously. For all these reasons what I have to say is incomplete and patchy, but I hope that what I have chosen illustrates
the generalities sufficiently, and that others will fill in the gaps.
In attempting to write clearly and simply I have eliminated
acronyms, all but common abbreviations, and as much jargon,
technical language, and quantitative detail as possible. I have balanced the latter, however, with a very full bibliography wherein the reader
who so wishes can find these details.
x
Preface
One of the temptations in modern ecological research is to retreat
within the protective jungle of complexity. For some, the natural
world seems so diverse, and interactions within it so varied, that they
believe we cannot expect to find general patterns there. Unlike physics, they say, there can be no hard laws of ecology that will encompass
the ever-growing diversity of life; every case will be different. Many
delight in this diversity, and present evidence for it in detailed descriptions of the structure of, and interactions within, ecosystems, communities, food webs, and populations.
However, I believe that the task of ecologists is to do more than
describe - no matter how precisely and quantitatively. Like other
scientists we must search for rules that are more general than others,
and which explain the complexity and variability we observe. This
book is my attempt to contribute to that process.
My colleague Piet den Boer warns me, however, that we cannot
make science by piling up selected examples. I agree. But by presenting a broad cross-section of examples I can best demonstrate the existence of general patterns in nature. A knowledge of these patterns
allows the derivation of new, testable hypotheses about their cause.
These, in turn, will lead to the making of more science.
This is especially so if they are primary patterns. Feeding on flush
tissues, coprophagy, or the early death of most young, for example,
are some of the primary ecological patterns that I perceive to be
reflecting the common scarcity of nitrogenous food. They are
analogous to primary evolutionary patterns, like the skeletal structure
of vertebrates, which reflect a common ancestry. Understanding them
may also help interpret secondary ecological patterns, like those
found in the interrelations within communities and food webs, or in
guilds of animals associated with vegetational succession.
Others will protest that there are many exceptions which I have not
presented. But, as Peter Price (1991a) recently pointed out, exceptions
cannot destroy the existence of a pattern - they cannot disprove it.
This can be done only when the preponderance of evidence shows
that another pattern - or no pattern - exists. Furthermore, many
exceptions, when more carefully investigated, prove not to be exceptions at all. They are either reconcilable with the rule they seemed to
contradict, or they lie outside of it.
There are two basic concepts that underpin what I have to say: the
oneness of life; and the importance of the individual. And I do not
use either expression in any emotional or anthropomorphic sense;
quite the reverse.
The enormous diversity of form, function, and interaction of life
that we observe today is all part of one pullulating, mindless continuum; the whole evolved from the same simple, early replicators,
and programmed and driven by their descendent genes to produce
even more genes in the future. [Dawkins (1986, 1989) says all this
Preface
XI
much better than I). Natural selection operates upon the consequent
"oversupply" of individual phenotypes, eliminating all that cannot
cope. The death of so many is not "wasteful", nor cause for sentimental concern. Nature has no "thought" for the "welfare" of future
generations. Natural selection, while determining the future, is yet a
process for the moment.
Because this is the way nature works, the individual phenotype
should be paramount in ecological studies. The environment of an
organism is everything that impinges upon it, including those of its
own kind (Andrewartha 1970). As Einstein is purported to have said,
"The environment is everything that is not me". All of nature comprises individual phenotypes each struggling to survive in its own indifferently harsh environment. It is essential that we think about
ecological interactions from the point of view of that individual's
struggle.
All that said, this book is mostly about herbivores. Not because
they are more "important" than carnivores, but because it is not
widely recognized or accepted, as it is for carnivores, that herbivores
are limited by their food - even less that it is nitrogen, not energy,
which is in critically short supply for them.
This is the heart of my thesis. A lack of access to nitrogen in a
form that can be used for the production and growth of young is the
major restriction on the abundance of all animals. This shortage is
reflected in a host of common structural, physiological, and
behavioural adaptations, which all serve the same function of helping
alleviate this "universal nitrogen hunger" (Keeble 1910).
Many years ago my friend and colleague Alan Newsome, who has
worked all his life with mammals, told me that, for him, insects might
just as well be Men from Mars! Mindful that there are many
ecologists like him, I have arranged and subdivided the text so that
those who wish to find out what I have to say can do so without reading about very much other than "their" animals. But I would plead
with them not to stop there. I think they will find that animals which
they have never encountered, or have had little to do with since their
undergraduate days have, ecologically, much in common with the
animals that they work with every day. Then, if I have been half-way
successful in gathering and presenting the evidence, they will begin to
see the generality of my thesis.
Finally, I did not write this book solely for my fellow ecologists.
I hope that many others will find what I have to say both useful and
enjoyable to read. And students, too, both postgraduate and
undergraduate - those who are not yet set in their thinking like we
older ones. To them I say: read this, think about it, and then go and
look at the world from this point of view. The evidence is there.
Adelaide, November 1992
T. c. R. White
Contents
Part I: The Inadequate Environment
Introduction ..........................................
3
Chapter 1. The Environment of All Organisms
Is Inadequate .........................................
5
1.1 Natural Selection Is a Negative Process ...............
6
1.2 Populations Press Against Limits of a Minimum
Resource... .. .... . . .. .... . .... .... .... ..... ...... .
1.3 What Essential Resource Is Most Likely to Be Limiting?
1.3.1 Nitrogen the Most Limiting Chemical ...........
1.3.2 Nitrogen Limiting Plants ......................
1.3.3 Nitrogen Limiting Animals .....................
1.3.4 Energy Not Limiting ..........................
1.4 Competition a Consequence Not a Cause .............
1.4.1 Intra-specific Competition .....................
1.4.2 Inter-specific Competition and "Competitive
Exclusion" ...................................
1.5 Self-regulation Does Not Exist ... . . . . . . . . . . . . . . . . . . . .
16
20
Chapter 2. Plants as Food for Herbivores ................
22
2.1 Why Is the World Green? ...........................
2.2 How Might Plants Be an Inadequate Source of Food? ..
2.3 How and When Might Nutrients in Plants
Be Too Dilute? ....................................
2.4 When Is a Minimum Supply of Nitrogen
Critically Important? ...............................
2.5 How Might Herbivores Counter the Plants' Evolved
Strategies? ........................................
2.6 How General Is Dilution of Nitrogen in Herbivore Food,
and What Adaptations Have Evolved to Counter it? ...
22
23
8
11
11
12
13
14
14
14
23
24
24
26
XIV
Contents
Part II: Herbivores in an Inadequate Environment
Chapter 3. Insects .....................................
31
3.1 Flush and Senesence Feeders ........................
3.1.1 Two Australian Psyllids on Eucalyptus ..........
3.1.2 Two African Scale Insects on Californian Ice
Plants ......................................
3.1.3 Two Aphids on Scots Pine ....................
3.1.4 The Green Spruce Aphid in Scotland. . . . . . . . . . .
3.1.5 Aphids on Sycamore, Apple, Wheat, and Alfalfa
3.1.6 Scale Insects on Euphorbia and Euonymus ......
3.1.7 A Leafhopper with Alternate Generations
on Brambles and Oak ........................
3.1.8 Two Species of Caterpillars Eating Oak Leaves ..
3.1.9 Two Species of Sawflies Mining in Birch Leaves.
3.1.10 A Chewer and a Sucker on Poplar Leaves. . . . . . .
3.2 Leaf-miners .......................................
3.2.1 The Switch from Flush to Senescence Feeding ...
3.2.2 Leaf-miners Which Induce "Green Islands"
in Leaves ...................................
3.2.3 A New Zealand Weevil Mining in Fallen
Beech Leaves ................................
3.2.4 The American Holly Leaf-miner ...............
3.3 Gall Makers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Physiological Galls ...........................
3.3.2 Nutritional Benefits of Galling ................
3.3.3 Adaptive Nature of Galls Debated Anew ........
3.3.4 Double-dipping: Prolonged Growth plus Hastened
Senescence ..................................
3.3.5 Selection for High Nitrogen and Survival
of the Young ................................
3.3.6 Selection of Growing Tissues for Proliferation
of Galls ....................................
3.4 Chewing Insects ...................................
3.4.1 Creaming-off as a Tactic to Increase Access
to Nitrogen: White Butterflies on Crucifers .....
3.4.2 Early Instars Need More Nitrogen: Gypsy Moth
on Artificial Diet ............................
3.4.3 Illustrations from the Life Cycles of Economically
Unimportant Butterflies ......................
3.4.4 Further Examples from Forest Defoliators .......
3.4.5 Pests of Crops also Reveal the Need for Nitrogen
3.4.6 Examples from Biological Control of Weeds ....
3.4.7 Locusts and Grasshoppers ....................
3.5 Sap-Sucking Insects ................................
32
34
37
38
39
41
42
43
44
45
46
48
48
49
49
50
52
53
54
55
57
58
61
62
63
65
65
70
75
77
80
85
Contents
XV
3.5.1 Aphids ......................................
3.5.2 Psyllids ......................................
3.5.3 Scale Insects .................................
3.5.4 Planthoppers, Leafhoppers, and a Mirid .........
3.5.5 Xylem-feeders ................................
3.6 Fruit Flies ........................................
3.7 Wood-eating Insects ................................
3.7.1 The Key Role of Fungi: Increasing Nitrogen
in Wood.....................................
3.7.2 Termites and Woodroaches: Gut Fauna,
Coprophagy, and Recycled Nitrogen .............
3.7.3 Furniture and Longhorn Beetles ................
3.7.4 Woodwasps ..................................
3.7.5 Borers Which Do Not Ingest Wood .............
85
89
92
93
95
97
98
100
104
106
106
Chapter 4. Crustaceans ................................
108
4.1 Microcrustaceans ..................................
4.1.1 Distribution and Abundance of Food Limited
by Nitrogen ..................................
4.1.2 Nitrogen Content of Food also Important .......
4.1.3 Microcrustaceans Feed Selectively for Nitrogen ...
4.1.4 Coprorhexy ..................................
4.2 Macrocrustaceans: Land Crabs, Lobsters, and Shrimps .
4.3 Terrestrial Crustaceans - The Isopods ...............
4.3.1 The Case of a Common Woodlouse.............
4.3.2 The Role of Microorganisms and Coprophagy ....
108
108
110
112
115
116
117
117
119
Chapter 5. Molluscs ...................................
123
5.1 Some Examples of Freshwater Snails .................
5.2 Marine Limpets and the Flow of Nitrogen
Through the Food Chain ...........................
5.3 Death of the Young, Selective Feeding, and Animal
Protein in the Diet .................................
5.4 Detritus Feeders Feed Selectively, and Depend upon
Microorganisms and Coprophagy ....................
5.5 Terrestrial Snails Live with the Same Constraints ......
5.6 Cannibalism by Young Snails, Illustrates Shortage
of Nitrogen .......................................
5.7 Teredo Shipworms Depend upon Microorganisms
Which Fix Atmospheric Nitrogen ....................
123
99
125
127
129
130
131
133
Chapter 6. Mammals ..................................
135
6.1 Large Mammals ...................................
6.1.1 Feral Donkeys in Australia .....................
135
136
XVI
6.2
6.3
6.4
6.5
Contents
6.1.2 Red Deer in Scotland
6.1.3 Antelope, Giraffe, and Greater Kudu in Africa .. .
6.1.4 Deer in North America ....................... .
6.1.5 The Giant Panda in China .................... .
6.1.6 Domestic Stock .............................. .
Rodents ......................................... .
6.2.1 Squirrels .................................... .
6.2.1.1 True Squirrels ......................... .
6.2.1.2 Chipmunks and Ground Squirrels ....... .
6.2.2 Rats and Mice ............................... .
6.2.2.1 The House Mouse ..................... .
6.2.2.2 The Australian Smokey Mouse .......... .
6.2.2.3 The American White-Footed Mouse ..... .
6.2.2.4 American Woodrats ................... .
6.2.3 Voles ....................................... .
6.2.4 Supplemental Feeding of Small Rodents ........ .
6.2.5 Rabbits and Hares ........................... .
6.2.5.1 The European Mountain Hare .......... .
6.2.5.2 The European Rabbit .................. .
6.2.5.3 The North American Snowshoe Hare .... .
Primates ......................................... .
6.3.1 Colobine Monkeys ........................... .
6.3.2 Cercopithecid Monkeys ....................... .
6.3.3 Howler Monkeys ............................. .
6.3.4 The Gorilla ................................. .
Fruit and Flower Bats ............................. .
Marsupials ....................................... .
6.5.1 The Koala .................................. .
6.5.2 Possums and Gliders ......................... .
6.5.3 The Habitat of Possums and Gliders ........... .
6.5.4 Kangaroos and Wallabies ..................... .
137
138
141
142
144
144
144
145
148
150
151
154
155
156
156
162
164
165
166
167
172
172
174
174
175
178
181
181
184
187
189
Chapter 7. Birds ..................................... .
193
7.1 Birds Eating Green Leaves ......................... .
7.1.1 Geese in Europe and North America ........... .
7.1.2 European Grouse, Ptarmigan, and Capercaillie .. .
7.1.3 North American Grouse ...................... .
7.1.4 Partridges and Pheasants ..................... .
7.1.5 Galliforms as Hindgut Fermenters ............. .
7.1.6 Changes in Abundance of Lagopus Species ..... .
7.1.7 The Thkahe ................................. .
7.1.8 The Hoatzin ................................ .
7.2 Birds Eating Nectar and Fruit ...................... .
7.3 Birds Eating Seeds ................................ .
7.3.1 Columbids .................................. .
193
193
200
207
210
211
212
215
217
217
221
221
Contents
XVII
7.3.2 African Queleas, European Finches,
and the Great Tit ...........................
7.3.3 Darwin's Galapagos Finches .................
7.3.4 The Australian Galah .......................
222
226
230
Chapter 8. Reptiles ....................................
233
8.1 The Giant Tortoises of Aldabra Atoll.. . . .. . . .. .. . . ..
8.2 The Green Turtle of the Bahamas Islands ............
8.3 The Marine and Terrestrial Iguanids of the Galapagos
Islands ..........................................
8.4 The Desert Iguanid of California ...................
8.5 The Green Iguanid of Panama .....................
233
236
Chapter 9. Fish .......................................
242
9.1
9.2
9.3
9.4
242
244
245
247
The Carnivorous Young of Fish ....................
Fish Which Eat Detritus ...........................
Fish Which Eat Algae .............................
Gut Microbes and Coprophagy in Fish ..............
237
239
240
Part III: Survival in an Inadequate Environment
Chapter 10. Strategies to Counter Shortage of Nitrogen
10.1 Strategy A: Synchronize the Life Cycle with Availability
of Good Food ....................................
10.2 Strategy B: Concentrate or Prolong Availability
of Nitrogen in Food ...............................
10.3 Strategy C: Eat More Food More Quickly,
and Digest More Efficiently ........................
10.4 Strategy D: Enlist the Help of Microorganisms .......
10.5 Strategy E: Supplement Plant Food
with Animal Protein ..............................
10.6 Strategy F: Apportion and Concentrate the Limited
Food to a Select Few ..............................
253
253
254
255
256
258
260
Chapter 11. Territorial and Social Behaviours .............
261
11.1 Territorial Behaviour in Carnivores ..................
11.1.1 Birds ......................................
11.1.2 Lizards ....................................
11.1.3 Insects ....................................
11.1.4 Spiders ....................................
11.2 Territorial Behaviour in Herbivores ..................
11.2.1 Mammals ..................................
264
264
265
266
267
268
269
XVIII
Contents
.
.
.
.
.
271
274
278
280
281
287
Chapter 12. Cannibalism .............................. .
291
12.1 Cannibalism by Females Producing Young .......... .
12.2 Cannibalism by Growing Young ................... .
12.3 Cannibalism, Warfare, and Protein ................. .
292
295
297
11.2.2 Birds
11.2.3 Insects ...................................
11.2.4 Fish ......................................
11.2.5 Molluscs ..................................
11.3 Surplus Young, Dispersal, and Philo patry ...........
11.4 Social Structures and Dominance Hierarchies ........
Part IV: Predators in an Inadequate Environment
Chapter 13. Vertebrates .................................. 304
13.1
13.2
13.3
13.4
Lions, Lynx, and Feral Cats
Coyotes, Wolves, and Foxes
Stoats, Mice, and Seed Mast
Pelicans, Puffins, and Other
........................
........................
.......................
Sea Birds. . . . . . . . . . . . . ..
304
306
307
308
Chapter 14. Invertebrates ...............................
314
14.1
14.2
14.3
14.4
14.5
314
315
317
319
323
Triclad Worms ....................... , . . .. . ... ....
Spiders and Scorpions .............................
Ground Beetles, Tiger Beetles, and Ant-lions .........
Praying Mantids ..................................
Parasitoids, Parasites, and Diseases ..................
Part V: The Alleviation of an Inadequate Environment: Outbreaks
Chapter 15. What is an Outbreak ......................... 333
15.1 Some Examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
15.2 What Causes Outbreaks? ..........................
15.3 The Paradox of Enrichment and "r" and "K"
Strategists ........................................
333
336
338
Chapter 16. The Interaction of Food, Prey,
and Predators in Outbreaks ............................
341
16.1 Bacteria and Protozoa .............................
16.2 Rabbits, Foxes, Cats, and Dingoes ..................
16.3 The Varying Response of Predators to Changes in Prey
341
342
343
Contents
XIX
16.4 A Natural Experiment: Guano-Algae-LimpetsOystercatchers ....................................
16.5 A Thought Experiment: Hot Spots in a Box
of Wadding ......................................
347
Chapter 17. Cyclic Outbreaks ...........................
350
Chapter 18. The Influence of Weather on the Generation
of Outbreaks .........................................
356
18.1
18.2
18.3
18.4
18.5
18.6
345
Hot Spots Again: Outbreak Centres and Boundaries ..
Spruce Budworm Outbreaks Revisited ...............
Patchy Environments and Metapopulations ...........
The Role of Viral Diseases .........................
The Link to Climatic Oscillations ...................
Major Outbreaks Which Are Independent
of the Weather ...................................
357
359
361
362
363
References ............................................
367
Subject Index .........................................
413
365
Part I: The Inadequate Environment
Introduction
Over 80 years ago the Professor of Botany at the then University College of
Reading in England, Frederick Keeble, published a small book (Keeble 1910).
I discovered this book in 1985! In it he described his simple but elegant and
imaginative experiments with the green and the yellow-brown marine
platyhelminth worms Convoluta roscojjensis and C. paradoxa. He demonstrated, inter alia, that the relationship between these worms and their symbiotic photosynthetic algae is driven by the lack of available nitrogen in the environment.
The algae are mostly free-living photosynthetic flagellates. They exist precariously in an environment notoriously deficient in nitrogen, but some become embedded in the mucilagenous egg capsules of the worms. There, with
access to a much improved supply of nitrogen, they rapidly multiply. Some are
ingested by the newly hatched worms, and divide repeatedly to form a dense
mass of photosynthetic cells throughout the worms' bodies, using the metabolic waste of the worms as their source of nitrogen (the worms have no excretory
system and, in the experimental absence of algae, accumulate vacuoles full of
crystals of nitrogenous waste).
The worms are colourless when they hatch and feed by ingesting single-celled plants and animals in the water. Once their contained algae have multiplied
and started to photosynthesize they reduce, or in the case of C. roscojjensis,
cease their feeding. They continue to grow, and eventually produce eggs, using
the products of algal photosynthesis for their nutrition. (When artificially
prevented from acquiring the algae they whither and die.) After the production
of eggs, however, in this closed system of exchange of nitrogen back and forth
between worm and alga, nitrogen again becomes limiting. The worms then digest their algae before themselves dying.
Keeble pointed out that to label the association of worm and alga as symbiosis is to miss the significance of the association. For the worm it is obligate
dependence upon the alga - parasitism. Without the alga it can neither rid
itself of nitrogenous wastes nor gain further nutrition. For the alga, on the other hand, that some few of its individuals enter this association with a worm
is "... an episode without significance.. ?'. Most of them remain free as green
photo synthesizers or colourless saprophytes "...which batten on the offal of
the sea .. ?'. But for an ingested cell the association with the animal solves the
problem of acquiring enough nitrogen. As Keeble so colourfully expressed it,
"It sacrifices its independence for a life of plenty. This universal nitrogen-hunger is a misery which makes strange bedfellows?' .
4
Introduction
Indeed it does. It is a misery which drives the ecology of all organisms, and
Keeble was at pains to emphasize this fact. "The nitrogen problem" as he called it, "... stems from the shortage of nitrogen which is available for the formation of nitrogenous food for plants", and as such is, "...the problem which besets all living organisms". This nitrogen hunger, he said, is "... no small matter
of mere academic importance". It " ... presents a problem which every living organism must solve. The supply of available nitrogen is a limiting factor of life"
(my emphases).
An awareness of the fundamental role that this limitation of nitrogen plays
in the ecology of all organisms should be as much a part of each ecologist's
intellectual equipment as is the awareness of the fact of evolution by means
of natural selection. What follows is my attempt, (having painfully reinvented
the wheel!), to get this message across to future ecologists. Please do not let
it languish for another 80 years!.
Chapter 1 The Environment of All Organisms
Is Inadequate
This chapter title seems a fairly obvious statement to make about a world in
which only a small fraction of the offspring of all organisms survive to pass
on their genes to the next generation. The world must be a harsh and inhospitable place for this to happen. Yet there still seems to be plenty of space, and
organisms rarely use more than a fraction of the resources in their habitats.
Much of conventional ecology says that this is because there are many and
varied environmental factors, and numerous competitive and social interactions, which regulate numbers of organisms below those which their resources
would allow; which contain the innate tendency of all organisms to increase
progressively. The environment is therefore not limiting or inadequate. Exhaustion of a resource, probably food, would ultimately limit further increase
of any organism, but this does not happen - or happens only rarely when the
regulators temporarily cease to operate. For most of the time there are processes operating which keep populations in balance around equilibrium levels well
below those which would use up all the resources and destroy the environment.
This view has it that the struggle for existence in nature follows remorselessly from the capacity of organisms to increase their numbers exponentially.
But rather the reverse is true. The capacity of all organisms to increase their
numbers exponentially follows remorselessly from the struggle for existence.
Surviving on this earth is, and always has been, especially for the very young,
a struggle, a chancy business. The huge "biotic potential" of all organisms is
the universal illustration of that fact. The capacity to increase exponentially
did not evolve to provide a struggle for existence as a vehicle for evolution. Nor
need it be espoused as the reason why populations must be regulated. It evolved because, only in populations in which females produced many offspring did
sufficient individuals gain access to sufficient resources, and survive in each
generation, to enable the population to persist.
How did this come about? Because this is a finite world, and in a finite
world there was one sure consequence of the evolution of the first self-replicating entities. Sooner or later a resource, essential to their growth and replication, ran out. There was no longer enough for all seeking to use it. No longer
could all replicate. No longer were all potentially immortal; differential survival had arrived. From the moment a resource became limiting the environment
became less adequate, and natural selection began to operate. The struggle for
existence - to gain enough of that resource to replicate - had begun. The
"lucky" few that happened to get enough replicated; those that failed perished.
Then, as now, anything which lessened the chance of an individual being one
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1 The Environment of All Organisms Is Inadequate
of those few ensured the eventual elimination of that individual's genes from
the gene pool. All else in the living world has followed from this.
In such a world balance is an unnecessary idea. The usual and variable
shortage of a resource provides an alternative explanation. The tacit assumption in the balance of nature is that all species of organisms tend to produce
an abundance of progeny that would survive to reproduce and lead to ever increasing numbers unless controlled by negative feedback mechanisms. As a
consequence the mean observable density over a number of generations is seen
as an optimum or equilibrium density. The further the numbers depart from
this mean the greater the density-dependent pressures in the system pressing
them back toward this mean.
However, there is no such thing as a mean population in nature. The mean
is merely a statistic - an arbitrary, albeit frequently useful, abstraction - derived from a series of samples of the numbers in a population in which numbers are continually changing. These numbers are not deviations from a
predetermined mean; they define it. They result from the members of the population struggling, generation after generation, to exploit the limited opportunities afforded by the inadequate environment. Nor is there an "optimum" or
"equilibrium" density of a population in nature - only the maximum number
that can survive each generation in a population that is pressing hard against
the variable but limited supply of resources in its environment.
To use the original engineering analogy of steam generated by heat being
released through a negative feedback valve, ecologists should be looking to see
what generates the steam, not how much steam passes through the regulating
valve. In nature rarely is enough steam generated to make the valve operate.
I (White 1978) and others before (Den Boer 1968) and since (Dempster and
Pollard 1981; Den Boer 1987) have made these points, but the arguments go
on (Wolda 1991)!
1.1 Natural Selection Is a Negative Process
The role of natural selection in deciding which individuals survive does not involve positive, active selection of those lucky few. It is a negative, passive process which eliminates all the rest; in Den Boer's (1985) words "the non-survival
of the non-fit". The environment is inadequate to support all those seeking to
live in it, so most will fail to do so. They will be selected out or selected against.
Wallace (1866) made this very point in a letter to Darwin. He was urging him
to adopt Spencer's term "the survival of the fittest" in place of "natural selection", and expressed concern at the misunderstanding of the latter, and the
constant "...comparing it in its effects to man's selection.. !'. "Nature", he
said, "does not so much select variations as exterminate the most unfavourable
ones". Some years ago I made the same point to a colleague. His response was
"So what? The glass is half empty or half full!" But I think it is important
that we do make the distinction, for two reasons.