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FRONTIERS OF BIOLOGY
oughly summarized elsewhere, no attempt will be made to do so here. But
we would direct attention to a growing body of information that stems
from analysis of relatively primitive nervous systems. For example, cater-
pillars are relatively highly specific in their choice of plant food, yet there
are only four taste-receptor cells on each of their maxillary palps. An
acceptance response that initiates feeding, based on the chemical structure
of the available food, is determined by one unique pattern of impulse
discharge coming from the four axons of these cells; all other discharge
patterns lead to rejection of the "tasted" material. Not only is this of great
theoretical significance for behavior, but it can also be a practical lead to
the control of plant pests.
An elegant example is found in the adults of certain moths, which are
normally preyed upon by insectivorous bats. The ears of these moths can
detect the ultrasonic cries made by their predators when they are echo-
locating. Each ear of the moth is supplied with only two acoustic sensory
cells, but arrival of sound of the wavelength used by the bats automatically
triggers evasive behavior on the part of the moth. Through these and a
host of similar examples, the elements of behavior may be observed in
primitive animals. From such understanding, ultimately, we may be able
to build a platform from which to view the behavior of that most complex
of all animals, man.
ECOLOGY
The objective of man's struggle with the environment is not to win but to
keep on playing. The biosphere as a whole is certainly not simple, but it
has been remarkably reliable up to now. The objective of applied ecology
is to keep it so.
Ecologists are generally concerned with the interactions among living
forms and between them and their environments. Our society has placed
upon the ecologists the fearful responsibility of safeguarding the planet
for human habitation. Patently, the working ecologist, on a day-to-day
basis, does not live with that concern. On any day he may be on a lake or
the ocean, examining bottom mud with the use of carbon-dating procedures;
following migratory patterns of birds, mammals, or fish by telemetry; com-
puting the energy balance of a lake, of a terrarium in the laboratory, or of
the southwestern desert; examining the history of a peat bog as recorded
in its fossils; or engaging in a computer simulation of the total ecology of
a rain forest. His tools are not uniquely his; they are whatever applicable
tools science has made available. Clearly, so all-encompassing a set of
115
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116
THE LIFE SCIENCES
concerns cannot be adequately summarized here, and it must suffice to
provide only some indication of the problems that give ecologists concern
and the manner in which these problems are approached.
Some Areas of Ecological Research
ENVIRONMENTAL CHALLENGES TO INDIVIDUAL ORGANISMS
Although the concern of the ecologist is with populations or species, under-
standing must rest on the behavior of individual organisms as well. No one
who has seen a 40-pound salmon Ding itself into the air again and again in
a vain effort to surmount a waterfall can fail to marvel at the strength of
the "instinct" that draws the salmon upriver to the stream in which it was
born. How do salmon "remember" their birthplace and find their way
back, sometimes from thousands of miles? The answer has economic and
political interest because dams athwart the routes of the salmon have cut
heavily into their reproduction, and the diminished numbers of salmon have
affected the fisheries of many nations. Salmon literally smell their way
home. Each home stream, presumably from the plant and mineral oils of
its drainage basin, acquires a unique organic fragrance. Young salmon
"learn" this fragrance in the early weeks of life and retain recognition of
it thereafter. The chemical nature of these odors has not been identified.
How the salmon navigates long ocean distances is unclear, but, as it finds
the main river, it selects that tributary that carries the home odor and con-
tinues upstream.
The energy requirements for this extraordinary journey are prodigious.
A group of salmon, observed while migrating 600 miles up the Fraser River
in British Columbia, took almost three weeks to make the traverse. By
the time the female salmon, which do not feed during migration, had reached
the spawning grounds, they had expended 69 percent of their body fat and
over half of their protein. Energy had been expended throughout this
period at 80 percent of the maximum rate possible for these animals! With
only a small reserve remaining for emergencies, one can readily understand
why only one additional small dam on a river may cause a run of salmon
to collapse.
The continual pressure for survival led those organisms that had ade-
quately adapted to inhabit a remarkable variety of ecological niches. Of
these, none is more remarkable than the animals that inhabit the desert,
where water is scarce. How they survive under those circumstances is a
matter of considerable import. As we have already seen, desert rodents
evolved kidneys capable of secretion of extraordinarily concentrated urine.
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FRONTIERS OF BIOLOGY
At the same time, they "learned" to avoid the harshest aspects of life in the
desert by remaining in their thermostatic, humid burrows during the day,
emerging to feed and explore only at night. The camel and the ostrich each
solved this problem, in part, by acquisition of a heavy coat that serves as
an extremely effective insulator. Others learned to permit their body tem-
peratures simply to approach that of the environment, rising and falling
with the days and nights as with the seasons.
Because of its novelty, one should take note of the mechanism that
permits marine birds to spend virtually the entirety of their lives at sea.
The least saline water available to them is that in the flesh of the fish that
constitute their diet. But each such encounter also entails ingestion of
some seawater as well. Yet the salt concentration of the blood of these
birds is essentially identical with that of terrestrial forms. This is accom-
plished by a unique adaptation, the presence in their nasal passages of a
"salt gland," an organ that, on sensing an increase in the sodium concentra-
tion of the plasma, secretes a highly concentrated salt solution of fixed
concentration and continues to do so until the plasma sodium level has
returned to normal. The mechanism responsible for this function is not
yet known.
Plant species are equally sensitive to the water supply, and the record of
annual rainfall is to be found in the width and density of the annual
deposition of wood. When midday temperatures and radiation exceed a
critical level, which varies with the species, the plant's conducting system
can no longer transfer water from the soil; evaporation from the leaves
continues and water tensions are established within the plants. A suction
force equal to 16 atmospheres has been measured in the tops of redwood
trees under such circumstances. When internal water stress becomes ex-
cessive, the stomata of the leaves close, minimizing water loss but also
shutting off uptake of carbon dioxide and release of oxygen, and hence
halting ohotosvnthesis. Records of the past found in the rings of trees
~ 1 ~ ~ _
. ~ ~ . ~ ~ · ~ ·,1 ~ ~ ~ ~ 1 _ ~1_ ~ ~ A_ ~ I,:
require careful study, combined with associated fossil analyses and exam~-
nation of current climatic conditions, to permit predicting the more im-
portant biological effects of future weather modifications when this becomes
feasible.
THE ABUNDANCE OF LIVING THINGS
The numbers of animals and plants in a given area are primary concerns
of the ecologist. Both absolute numbers, or density, and the ratios among
species are primary variables. Both are subject to external and internal
controls. External controls are exemplified by the weather, application
of pesticides, natural catastrophes, and variation in food supply or in
117
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118 THE LIFE SCIENCES
numbers of predators. Internal controls are exemplified by the changes in
urn patterns oy wn~cn the subject populations respond to all aspects of
their environment. Both have been studied in detail, in the wild and under
laboratory conditions. In natural habitat, both absolute densities and
Prepuces ratios are maintained over remarkably long periods; the objec-
tive of population ecology is to understand the play of forces that achieves
this. These forces vary from species to species, but for all species the
product of the probability of death and the probability of birth, summed
for all ages over a generation, must come to an average value of 1.0 if
constancy is to be maintained. A few examples will suffice. Daphnia, a
small freshwater crustacean, can maintain remarkably dense populations
on an adequate food supply. If a modest grade of removal (fishing) is
applied, the density falls sharply but not linearly. Even with removal rates
as high as 30 percent per day, a small but vigorous population persists.
Patently, the effect of fishing mainly for the young is different from that of
fishing for adults only. This displays the resilience that man exploits; if
we somewhat rarefy a population, we increase its rate of growth, so a
certain degree of rarefaction actually increases the steady yield of animals
harvested. This is a benefit that works both ways; pest controllers are less
than enchanted to find that the more rats they kill the more there are.
Some forms of predation can completely exclude an organism from an
otherwise suitable environment. Native silkworms can be successfully raised
on wild cherry trees as long as each is enclosed in a protective net, with
yields as high as 80 percent from egg to adult. But if the trees are left
unprotected, not one silkworm survives through the larval stage. The
breeding response to adversity is illustrated by the size of egg clutches in
bird species. For example, the habitat of the European robin extends over
a great area, yet these birds maintain a fairly constant density over 35
degrees of latitude. Since the winter death rate of adults is higher in the
colder northern latitudes, the population can remain constant only if this is
offset by an increase in the birth rate. And, indeed, per degree of latitude
this species adds about 0.1 egg to the annual setting.
Some forms of external control are not always obvious. Red tides, the
sporadic blooms of a microscopic dinoDagellate that is highly toxic to fish,
occur occasionally on our eastern coast. The circumstance that leads to
such blooms is not predictable or constant. It usually begins with the for-
mation of a pool of nutrient-rich brackish water in an estuary that has been
temporarily prevented from tidal flushing and becomes enriched with
stream-borne nutrients. When flushed to sea, the toxic water mass main-
tains its integrity for days before dissipation. The nutrients that touch off
such an event are small quantities of primary elements and of a few organic
compounds. A few micrograms of one of these per cubic meter of water
1 1 ~
. . ..
. .
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FRONTIERS OF BIOLOGY 1 l9
appears to make the difference between bloom and no bloom, between en-
vironmental health and disaster
SPECIES INTERACTIONS
Much of the biology of each species is devoted to accommodation to other
species, including such phenomena as defense against disease and predators,
patterns of courtship and mating, body size, life-span, and reproductive
potential. The number of interspecies interactions among the several million
species is vast; systematic studies of these are certain to reveal profitable
ways of utilizing more of the earth's biota for man's benefit. Biological
control tests, new drugs, and repellents are obvious potential applications. It
is this largely untapped potential and not sentimentalism that makes ecol-
ogists protectors of threatened species and of dwindling habitats that harbor
unique combinations of species. It would be tragic if potentially easy solu-
tions to future major problems were lost through ignorance or indifference.
It is repeatedly impressive that seemingly competitive species manage to
accommodate in given ecosystems. For example, study of a spruce forest
containing five species of wild warblers, at first glance much alike in their
habits, revealed their actual ecologies to be surprisingly diverse. Although
they all fed on the insects in the spruce trees, each species had a unique
combination of behavior patterns based on the proportion of the time it
spent hovering, whether it tended to feed on peripheral or central parts
of the trees, how frequently it flew from one place to another, and so on.
Thus, in effect, by exposure to different items of food, they share the re
sources of the forest.
Such understanding can be utilized in practice. For example, the Klamath
weed was accidentally introduced into the livestock ranges of northern
California about 1900. This plant not only replaces valuable foliage, but
it is also highly toxic. But in 1947 a beetle was found in Europe that fed
upon this plant, and it was introduced into the affected areas. Within a few
years, this beetle achieved mass destruction of the Klamath weed popula-
tion. However, along highways and under the shade of trees, the beetle
population is ineffective in controlling the plant. As a result, beetle and
weed populations have achieved an equilibrium with rather low weed
infestation. Meanwhile, the range has been repopulated with useful grasses.
No episode in the deliberate manipulation of ecological systems is more
dramatic than that which began with the introduction of the European wild
rabbit into Britain by the Normans at the beginning of the twelfth century.
With it came the flea, which is the only known host for the myxoma virus.
The virus itself had long been established in the native rabbits of South
America, but it is not pathogenic for that species; yet it induces a rapidly
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120
THE LIFE SCIENCES
debilitating disease in European rabbits. This virus was deliberately intro-
duced into Prance in 1952 and into England in 1953. By 1955 rabbits
were practically extinct over most of Britain. As the rabbits disappeared,
plant species that had previously been highly restricted in distribution by
the grazing of rabbits now flourished and spread. Areas that had previ-
ously supported a covering of low mosses and turf became covered with
deep mats of grass and strands of heather. In turn, the entire insect popu-
lation changed with it. Thus, the total ecosystem was profoundly affected
by removal of only one link in the food chain.
ENERGY FLOW IN ECOSYSTEMS
Only recently has ecology become sufficiently sophisticated to be concerned
with the flow of matter and energy in ecosystems. A growing body of infor-
mation reveals the efficiency of the conversion of solar energy into organic
matter in ponds, ocean areas, open fields, and cultivated farms. In turn,
these provide lessons for man in his future management of the earth. For
example, the average efficiency of an Iowa cornfield is only about 2 percent,
whereas the conversion of solar energy into organic matter by algae in a
fertilized pond may be as high as 20 percent. The use of radioactive carbon
dioxide as a tracer has permitted the construction of balance sheets of
carbon flow, particularly in small lakes; chemical analysis reveals the total
flow of mineral elements as well. Similar techniques have been applied to
small patches of forests.
The consequences of removal of tree cover are extremely dramatic.
For a long period thereafter, the impinging energy is wasted, and the bulk
of the available mineral matter is removed by the leaching action of rain-
fall. Studies of these effects must be continued both on a small scale and
on the larger scale of total drainage basins, forestlands, and deserts. What
has already become apparent is that, although 70 percent of the surface
of the globe is covered by ocean, the total biological yield of the oceans is
approximately of the same order as that of the land area.
STABILITY AND DIVERSITY
Much ecological concern has been addressed to those factors that make
for a stable ecosystem a system whose numbers and balance of species
remain relatively constant over prolonged periods. Examinations of such
systems have involved description of the patterns of the food web, the
distribution and arrangement of different species in space and time, and the
grouping of individuals of several species into higher taxonomic units with
varying functional roles. Such studies have led to a few generalities.
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FRONTIERS OF BIOLOGY
In general, the greater the number of species at any level in the food
chain, the greater the community stability at that level and at lower and
higher levels as well. Amplitudes of fluctuation of herbivore populations
are determined by the number of species of plants they eat and by the
number of predators and parasites that attack them.
The density of food plants determines the extent to which herbivore
populations fluctuate, and the size of the herbivore population, in turn,
determines population fluctuation among predators.
The more diverse the species at any taxonomic level, the more stable the
system. A system of ten equally abundant species is more diverse and
successful than one with a single very common species and nine relatively
rare species.
Although stability may be hard to measure satisfactorily under any cir-
cumstances, it is easier to measure after the fact, when disturbance, the
inverse of stability, has occurred. Such events usually occur spontaneously
or are produced accidentally by advancing civilization. Relevant studies
have recently been performed deliberately. For example, an hourglass-
shaped bog lake was deliberately separated into two lakes by an earthen
barrier across the constriction. Lime was added to one lake, raising the
pH from 5.9 to 7.3. Within a year the transparency of the latter increased
remarkably and, after two years, the well-lighted zone had increased from
2.7 to 7 meters in depth. The Daphnia population was found to replace
itself in one third the time in the lime-treated lake, and new species of
phytoplankton and of rooted aquatic plants began to thrive. Thus the
initial disturbance that had led to acidification of the lake could, in fact,
be successfully reversed, and a more stable, more diverse, and, for man,
more attractive ecosystem was restored.
Many illustrations of the practical applications of ecological understand-
ing are to be found in Chapter 2. The foregoing discussion has served only
to indicate some of the kinds of problems addressed by ecologists. Only
now has the science advanced to a point at which ecologists consider that
they can usefully and successfully construct mathematical models of large
ecosystems e.~., the coniferous forest, the western grasslands, the south
western desert.
The success of these efforts remains to be ascertained.
Meanwhile, applied ecology is man's greatest hope both for protection of
the natural environment and for human survival. Ecological understanding
is required to guide intelligent use of pesticides and of biological mech
anisms for pest control; to indicate when ciear-cutt~ng or slash-and-burn
~nnronc~.h~.~ should he used as forests are put to new use; to give guidance
to the appropriate scheduling for plowing, planting, burning, and normal
agricultural practice; to predict the consequences of the introduction of new
strains of crop plants (already it is clear that, on a single farm, genetic
-en ~
Representative terms from entire chapter:
food supply
