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INTRODUCTION
1. Rationale and approach
The body of ecological literature has escalated over the last 100+ years. Initially it was –
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descriptive ecology followed quickly by basic quantitative methodologies, and then vegetation mapping that started
the revolution
then population and community approaches with specialisations in wildlife management and environmental economics
became fashionable
followed by ecosystem studies and biogeochemical recycling/productivity - with programmes on co-evolution and
convergence between climatically similar continental systems, and behaviour and capture/relocation studies of
vertebrates
then ecophysiology, followed by molecular and mathematical specialisations became more fashionable as the
technological revolution enabled ecologists to ask and answer many different and extremely complex questions.
At about this time fieldwork became “a luxury rather than a necessity” and many ecological studies became laboratory and/or
computer-based. Today ecology encompasses a whole range of disciplines, and is multi-, inter- and trans-disciplinary, with a
focus on –
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Common Property resource management and Intellectual Property rights (a socio-political dimension, e.g. PLAAS)
Biodiversity Conservation and Global Climate Change (a conservation and biosphere dimension, e.g. SANBI)
sophisticated GIS, and data collection and retrieval systems aimed at the monitoring
and evaluation of impacts locally, regionally an globally (where policy issues and legislation are key drivers, e.g. Richard
Knight’s group here at UWC)
and there are other specialisations… (e.g. such as the BotSoc, Wildlife Management Association, IUCN, Agriculture and
Forestry, DWAF and WfW, DEAT, Cape Nature and SANParks, etc.)
Despite all this we have yet to master adequate monitoring and implementation (despite the huge knowledge base), at not only
local but at global levels. We as Homo sapiens have still to deal with the NUMBER ONE issue which is over population and over
consumption by our own species (currently at about 40% of total global net productivity – which is in the region of 60%
terrestrial productivity).
Through time, and especially over the last few hundreds years, we have created a bewildering variety of ecological issues/crises
with unforeseen downstream environmental impacts. As our theoretical ecological knowledge had expanded, our ability to apply
this knowledge has diminished (exponentially?).
EXAMPLES
1. land clearing
2. creating health risks
3. DDT and other chemicals
4. Aliens
PROBLEMS
1. Fundamentalism
2. Lack of understanding of spatial and temporal scales
3. Dislocation and the “new ethics”
Ecological study must recognise all levels and approaches as facets of an extremely complex and critical important SCIENCE. To
study ecology there must be some common theme - and many text books use different models/themes. I will attempt to follow
a mechanistic theme i.e. that natural selection and evolution explains most of the patterns and complexity of the diversity of life.
Your comprehension will be greatly enhanced by field observation and study – your own experience. BUT this is a seven week
course so by definition this course can do little more than scratch the surface.
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2. The order of the Natural World
The Greek for house is “oikos”, our immediate environment. Today “ecology” denotes the study of the natural environment,
particularly the inter-relationships between organisms and their surroundings. AND the environment of each organism has
different physical attributes, even though several may live in a single location. Different environments have different stresses.
Ecologists are interested in patterns in nature beyond those embodied in organisms – the diversity and complexity of species
assemblages, energy and nutrient flows, the structure and function of systems, etc. AND ecologists explain the patterns
observed.
BUT first one must see the patterns, and pose the right scientific research questions! Hence our fist practical at Kirstenbosch.
The Natural World is diverse and complex: EXAMPLES Erica spp., beetles, leaf shapes and sizes, etc.
The natural world is dynamic, but it is also stable and self-replenishing: EXAMPLES deaths comes in different ways to different
species and even the same organisms (age, predators, parasites, etc.), stable systems are in fact constantly turning over and
nutrients are being recycled, and populations that could be huge are not.
Man is very much an integral part of the Natural World: EXAMPLE We have had a major global impact and are continuing to do
so. Stripped of our CULTURE we are a most unimpressive species. Yet today we control and manipulate nature, BUT do not
control the natural physical forces – will that ever happen?
3. How we perceive and understand our environment
As we embark on a new discipline we need to take stock.
The organism is the fundamental unit of ecology. These are usually well defined units/entities, with a physical boundary from
the rest of the world. Organisms are controlled by a system of internal controls that maintain an intimate and dynamic
relationship with the environment. The population is a collection of similar entities, and social behaviour confers a structure on
the population.
Communities embrace diverse species, but communities lack the discreetness and organisation of organisms.
Organisms fall into natural groups we call species. These are recognised according to their similarities and today can be
genetically confirmed. Taxonomists are those that do the classification.
Organisms are well designed with respect to their environments. Birds have wings, lions have sharp teeth and cows have broad
teeth. Design impacts other adaptations too. But as a whole the species morphology is a well integrated entity.
The natural world can be conceived as a set of patterns. We constantly recognise patterns in our surroundings, organisation and
interrelationships in the complexity that surrounds us. Some are able to do this better that others, and as ecologists we have to
develop and hone these skills. We are able to recognise these patterns because of the predictability of their elements. So we
can ANTICIPATE NATURE FROM PAST EXPERIENCE. The more accurately we are able to predict/anticipate the better the
ecologists we become.
THE PHYSICAL ENVIRONMENT
4. Life and the physical environment
We often contrast living (biotic) and non-living (abiotic) as opposites. Although these realms of the natural world are mostly
readily distinguishable, they do not exist one apart from the other. Life is dependent on the physical world, but the impact of life
on the physical world is mostly more subtle.
Yet the biotic and abiotic worlds are interdependent, and life (and the occurrence/distribution of organisms is dependent upon
the physical world). And life is like an internal combustion engine transforming energy to perform work – the difference between
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the two is that in the physical world the energy level throughout the system always follows the path of least resistance. While in
biological systems energy is used purposefully by the organism to maintain itself OUT OF EQUILIBRIUM with the physical forces
of gravity, heat flow, diffusion, and chemical reaction (e.g. a boulder rolling downhill versus a bird flying). In the living world the
ultimate source of energy is sunlight, and REPRODUCTION is uniquely a biological function.
The biotic and abiotic parts of an ecosystem are linked by a constant exchange of material through NUTRIENT CYCLES - driven
by the energy from the sun. Thus the earth is a giant “heat machine”. And the variation of solar radiation with latitude creates
major global patterns in temperature and rainfall.
5. Soil development
We tend to take the dirt under our feet for granted. This is VERY UNWISE, as most of the vital mineral exchange between the
biosphere and the inorganic world occurs in the soil.
As with climate, soil formation is determined by physical and chemical processes with a huge variety of results dependent on
local conditions (which effect rate of formation, and kind of soils formed). Soils both influence vegetation types, AND vegetation
types also influence soil types (e.g. podsolisation). HOWEVER, the most important factor determining soil type is parent material
(both its physical and chemical composition, and the latter affects pH - namely hydrogen ion concentration which is a key factor
that mobilises cations).
Basically there are two rock types – volcanic and/or sedimentary that weather to form different soil types. AND different soil
types have different water holding capacities, and different soil-air space capacities (e.g. capillarity and/or colloidal differences,
the processes of calcification and salanisation occur when evaporation exceeds percolation, etc.).
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Thus clays are formed from rocks that are fine grained (shales, mudstones, etc.), or from granites and other rocks of
volcanic origin that are rich in clay minerals (mostly aluminosilicates - with K+ and Mg++ - that weather into fine particles that
are ionically charged – forming lattice clay structures).
Sands are coarse-grained soils high in silica.
Silts are particles intermediate between clay and sand.
Loams are a mixture of clays and sands.
Limestones have high CaCO3.
Podsols are humic soils.
Laterites are iron (Fe3+) rich soils that were formed under wet tropical conditions.
Once formed soils remain in a dynamic state due to physico-chemical and biotic influences (and THE most important agents of
change are the soil fauna and decomposers/detritovores).
NOTE: Not all soils are formed in situ, as they can be transported by wind (aeolian), water (alluvium) and gravity (colluvium) –
and today by people (manuport).
6. The diversity of natural communities
Humans are orderly and we have classified animals and plants, and ecosystems/vegetation types.
Thus vegetation has been classified on
i. Floristics: i.e. species - following different approaches such as
 Floral Kingdoms globally (e.g. Good), and
 Regionally using the techniques evolved by the Zurich-Montpellier phytosociological school
 Acocks (1953) for SA using his Veld Types map (1949)
ii. Structure, such as forests, woodlands, grasslands, etc.
iii. Structure and function, based on criteria such as leaf-size, sclerophylly, plant part shedding strategies,
etc. and sometimes represented by Dansereau and/or Raunkiaer’s scheme
iv. Holdridge’s classification by temperature and rainfall.
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ADAPTATION
7. Environment and adaptation
All organisms encounter a wide variety of environmental conditions and patterns - from a huge variety of sources/pressures that
are physical, chemical and biotic (plus human). These factors all impact the structure and function of individuals, populations,
communities, ecosystems and the biosphere. Some local examples for individual species (i - iv), populations (v - vi),
communities (vii - viii), ecosystems (ix - xii) and biosphere (global warming) are:
INDIVIDUALS
i.
Some fire survival strategies of fynbos species
 Reseeders – e.g. Protea lepidocarpodendron and Leucospermum conocarpodendron
 Resprouters - e.g. Protea nitida and Hypodiscus aristata
ii.
Some rooting strategies for the low available plant nutrients in the heathland soils
 Proteoid and restioid roots
 Mycorrhizal infections – endo-, ecto- and vesicular-arbuscular
iii.
The breeding strategy of the long-tailed sugarbird
iv.
Fynbos ants and myrmecochory
POPULATIONS
v.
Baboons social structures have evolved to allow them to survive under the harshest of conditions – predator avoidance,
and troop structure with an alpha male, role of females and how young are taught
vi.
Hyrax and Black Eagles
COMMUNITIES
vii.
Heathlands – sclerophylly, leaf anatomy (morphological and/or functional) iso-bilateral, size, pollination syndromes, seed
dispersal mechanisms, etc.
viii.
Forests – quasi-organism with “own” species composition, fleshy fruits, etc.
ECOSYSTEMS
ix.
Succulent Karoo
xii.
Renosterveld, etc.
8. Natural selection and design in nature
The close correspondence between an organism and its environment is no accident. Only those organisms well suited to their
environments survive (survival of the fittest by natural selection - Darwin), and these traits have evolved over evolutionary time.
Examples:
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Darwin’s finches on Galápagos…
Savanna ecosystems, and the consumers of plant productivity by invertebrates and mammals - their population structure
and inter-relationships along the trophic chain/food web; grazers/browsers, carnivores/scavengers, decomposers, etc., and
fire as the ultimate primary consumer.
Cryptic colouration and the evolution of melanistic forms of the peppered moth in the UK.
Cultivated plants and domesticated animals.
9. The nature of adaptation
The diverse environments globally have resulted in a vast array of adaptations from those animals in the deep ocean trenches,
to mountain top communities, to deserts and wetlands. All adaptations must be integrated
 The shape of a bird’s beak closely corresponds to what it eats, etc.
 Legs of mammals have other functions than locomotion – such as grooming, scratching, fighting, etc. (e.g. zebras cannot
scratch, so have evolved a special type of tail to swish, and a skin that can be shaken vigorously by underlying muscles)
 Spelaeogriphus lepidops and cave crickets
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The questions that are raised about the nature of adaptations are:
i.
Are all species well adapted?
Successful populations are by definition successful. However, evolution is not forward looking and natural selection cannot
anticipate, so human induced changes are causing havoc as they are occurring so rapidly.
ii.
Adaptation can never be perfected
Because in every new population there are unfit heredity traits, and evolution is opportunistic. Because the environment is
continually changing populations have to continually re-adjust.
iii.
Who measures fitness?
Survival.
iv.
Cultural changes is analogous to genetic evolution
These are passed through teaching and learning that parallel/supplement evolution
ORGANISMS IN THEIR PHYSICAL ENVIRONMENTS
10. Basic properties and the requirements of life
Life is an extension of the physical world so has to obey physical and chemical constraints of water, energy and nutrients (C, H,
N, O and others too). AND a basic requirement of life is that it maintains the organism OUT OF EQUILIBRIUM WITH THE
OUTSIDE WORLD. This comes at an energetic cost.
WATER A KEY RESOURCE
For example:
 Terrestrial animals obtain oxygen from the atmosphere and at the same time have to guard against loss of water
 Marine fish drink water, and have to pump out the excess salt to maintain their ionic balance so important to their existence
(osmo-regulation)
Life processes take place in an aqueous medium – all organisms are composed mostly of water! And the unique physical and
chemical properties of water are the basis of life (heat, liquid, change in density at 4 0C [so ice forms on top of water], and it is a
universal solvent [NaCl in water goes to Na+ and Cl- in a reversible equation]).
NUTRIENTS FOR LIFE
Life depends on the availability of inorganic nutrients, and a wide variety is required. Firstly there are the macro elements of H,
O and C (obtained from photosynthesis and the basis of sugars/carbohydrates), and N, P, S, K, Ca, Na, Mg and Fe (from the
soil). The other elements required are considered to be microelements.
Table. Major nutrients and their functions (micronutrients are elements such as Co, Cu, Mo, Zn, etc.)
ELEMENT
N
P
K
S
Ca
Na
Mg
Fe
FUNCTION
Structural component of proteins/nucleic acids
Structural component of nucleic acids, phospholipids and bone
Major solute in animal cells
Structural component of many proteins
Regulator of cell membrane permeability, structural component of bone/cell walls
Major solute in extracellular fluids of animals
Structural component of chlorophyll and many enzymes
Structural component of haemoglobin and many enzymes
LIGHT - THE ENERGY DRIVING FORCE
Green plants are the primary source of energy capture by photosynthesis. The oxygen released in the process is a further key to
life as we know it. Not all light is useful in photosynthesis nor is all the light visible (different wave lengths are epitomised by
rainbows), and at the ultra-violet end it is damaging and at the infrared end it is warming (the role of the atmosphere and
ozone).
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OXYGEN AVAILABILITY LIMITS BIOLOGICAL ACTIVITY
Respiration is a key metabolic process that enables stored energy to be released. So systems depleted of oxygen become
anaerobic/anoxic and metabolism can cease (blood, water, soil air space system, etc.). Organisms have specially adapted
organs for oxygen capture (gills, lungs, root hairs, etc.).
Metabolism and size, also heat and size, etc.
11. Aquatic and terrestrial environments
Life evolved in the sea, and the colonisation of the land was a massive step (took hundreds of millions of years). Today
terrestrial life has attained a higher degree of organic diversity. In fact it seems that the more harsh the environment the more
diverse the life (cf. coral reefs and the Benguela up-welling system, heathlands and non-heathlands [e.g. fynbos and forest at
Kirstenbosch]).
To contrast the aquatic and terrestrial environments we need compare the properties of water and air! They differ in
 Buoyancy and viscosity
 Light is attenuated very quickly in water (so photosynthesis is limited to 10m where 50% of light penetrates, and only 7%
to 100m)
 Oxygen is usually much more scarce in water
 Water loss is a critical problem for terrestrial organisms and there are many adaptations that both animals and plants have
evolved to minimise water loss
12. Regulation and homeostasis
A changing external environment is the norm. There are annual, diurnal and unpredictable cycles of change that have to be
survived to thrive. Organisms respond to these cycles in many different ways – we shiver in the cold, sweat in the heat and get
sun-tanned, etc. All these responses are directed towards maintaining an optimum functional level (=homeostasis). How
effectively organism’s respond is about how effective they are able to regulate their metabolism to maintain homoeostasis.
Responses are about costs and benefits, and the better able organisms are to regulate themselves the more efficient they are.
These are essentially ecophysiological responses and are better dealt with in a course of ecophysiology, than to take up too
much time in this seven-week course in ecology
Suffice to say that homoeostasis requires energy, responses are a type of negative feedback loop, and the level of regulation
depends on the requirement to adjust.
13. Organisms in heterogeneous environments
Environmental variation is a fact of life, and can be is inconsistent in time and in space. The type of response is dictated by the
rate and amount of environmental change. Some fluctuations are predictable and can be seasonal, daily or tidal – and there
may be obvious cues, some other changes are unpredictable.
Of particular importance to us in South Africa is the graininess of the environment. Graininess may be spatial, temporal, and/or
in pitch/flux. For example:
 This may result in different shapes and sizes of deep soils in fynbos (and the depth may not be consistent either), AND the
size of the patches may be a metre square or a few metres square. The resulting vegetation pattern will, therefore,
represent the underlying graininess (fine- or coarse-grained)
 The animals that depend on the fynbos will be dispersed accordingly
 Another factor is that SA is mostly south of the Tropic of Capricorn so aspect is important too
Other responses can be behavioural and/or metabolic. For example:
 As far as animals are concerned there are diurnal, nocturnal and crepuscular species
 Longevity is another issue
 Dimorphism can be yet another response
 Migration
 Aestivation/dormancy
 Changes in reproductive activities
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Fire
Day length
Etc.
14. Environment, adaptation, and the distribution of organisms
No single organism can tolerate all environmental conditions found on earth, each thrives in its range (some much broader than
and others that may be very narrow – ECOLOGICAL AMPLITUDE OF NICHE SPACE/SIZE).
On a local scale environment plays an overwhelming role in distribution, confining populations to areas and habitats in which the
species thrives and is perpetuated. On a regional and global scale geographic barriers and historical (and pre-historical) factors
impact distributions. The success of introduced aliens attests to this. There are a wide range of other factors that also confine
populations and some examples are:
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
ix.
x.
xi.
Geographic such at the distribution of afro-montane forests from Ethiopia to the Cape Peninsula
The distribution range of a species may also be geographically contained (e.g. Plants like Afrocarpus falcatus,
Mimetes fimbriifolius, and Acacia karoo and animals like elephant, bontebok and the micro-frog Microbatrachella
capensis)
Sometimes some behavioural/morphological factor contains species distributions like the Southern Race of the
White Rhino, Riverine Rabbit
The case of Apodytes dimidiata in KZN coast to Drakensberg populations
Soils – Limestone Fynbos, Mopane woodlands
Moisture gradients - elephant versus hippopotamus
Dispersal Kigelia pinnata versus Brachystegia spp.
Gradients of climate, soils, moisture e.g. Cyperus textilis
Shade and sun leaves; Ilex mitis
Intertidal algae and molluscs
Etc.
ORGANISMS IN THEIR BIOLOGICAL ENVIRONMENTS
15.
Predators and their prey
Animals must capture food to survive and reproduce, and they themselves must avoid capture. So what are the attributes of
prey species? There is a huge diversity of predator-prey interactions that generalisations are no easy.
However, regardless of any interactions predators nearly always limit the distribution of prey species to a narrower range than
limited by physiological constraints. Because as individuals are taken more must be produced. GRAPH page 206
Thus when an efficient predator removes individuals it makes room for other species who have similar niches but are not eaten
as much. Factors that influence relationships between predator and prey as things such as relative sizes. We may think of
cheetah and impala, where the prey is a little smaller than the predator. But some predators feed on huge quantities of minute
prey such as Blue Whales and krill (mainly water dwelling species as not too many dense species in the air). Thus as the ratio of
prey to predator gets closer to one so the specialisation must increase to kill the prey. Eventually there is a limit beyond which
the prey is too big.
AND at the other end of the spectrum are the “live-in” predators, or parasites, where the survival of the predator depends on
the survival of the host! This is analogous to herbivores, e.g. elephant and vegetation, while beetle larvae and seed predation is
a little different.
While we often view herbivore relationships differently from classic predator/prey relationships functionally they are similar.
The nature of the “prey” has profound anatomical and behavioural effects (evolutionary pressures). Such as dentition, limbs,
gape size, eye-sight, etc. Also the quality of the diet impact digestive systems, from ruminants, carnivores to frugivores.
Prey also adapt to such pressures and develop strategies to hide, flee, or fight to escape predation, etc. They also can develop
cryptic colouration, become mimics, evolve “eyes”, become toxic and even mimic toxic beasts, etc.
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Plants have evolved a host of mechanisms to avoid being eaten (structural, chemical, even cryptic, etc.), while others want to
be eaten!
Many insects on the other hand have evolved mutualistic associations and become extremely specialised. The array is seemingly
infinite.
Competitors
Competition has always been a very difficult and controversial ecological concept. It has been defined as the use of resources
that reduces the availability of those resources for another species (light, space, food, water, etc.). It can be INFRASPECIFIC
and/or INTERSPECIFIC. GRAPH page 234
Many species can co-exist together using many of the same resources (e.g. forest trees). Lake Baikal in Siberia has 239 spp. of
gammarid crustacean and Lake Malawi has >200 spp. of cichlid fish. And in all these examples it seems that there are minute
differences that allow for such radiation/coexistence of spp. DIAGRAM page 235
Thus competition is an ecological driver. It impacts reproduction, body size, etc. DIAGRAM page 242
16.
Individuals in groups
In most populations there is a temporal/spatial relationship between individuals. Some individuals are attracted to each other
some their distribution is clumped, while others are repelled/avoid one another resulting in a more even distribution. Such
patterns can be key ecological drivers. In animals such behavioural differences are key to management of populations.
18.
The sexes
To reproduce sexes must get together, and as such sexual behaviour is often view as the opposite to competition! As such this
can create an entirely different environment for evolution – not just about genetic exchange, but also association.
Behaviourists have waxed eloquently about courtship, mating systems and territoriality, etc. and frankly for me this is not what I
consider to be “mainstreaming” ecology.
19.
Life history patterns
They are key to the understanding of managing systems. However, all patterns tend to be specific and require details
knowledge when one is considering population/ecosystem management issues. Thus I am simply going to flag this as a key
issue and when important – collect the detailed data, for example malaria and bilharzia
ECOLOGICAL GENETICS AND EVOLUTION
20. Charles Darwin and the theory of evolution
Before the publication of On the Origin of Species by Darwin, people widely believed that species arose independently and
uniquely as divine creations of God, and that their forms, being perfect, could not undergo change. However, some thinkers did
begin to entertain the notion that species were not fixed – that they could be, and had been, transformed. But in the mid-1800s
this notion was not fashionable. Lamarck was one person who postulated that certain characteristic could be inherited and
stated that the effects of use and disuse of organs was transmitted from parent to offspring. Thus the long neck of the giraffe
could be explained by postulating generations of giraffes that had to reach progressively higher and higher vegetation and had
to stretch their necks to do so. Thus Lamarck and others paved the way for a theory of evolution by raising the possibility that
species change. It remained for Darwin to present convincing evidence for evolution and to discover the mechanism of
evolutionary change though NATURAL SELECTION, and this at a time when heredity was poorly understood. General acceptance
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of the theory was slow, and it was only when there was a better understanding of genetics that the theory was revitalised. And
today Darwin’s notion of the survival of the fittest is generally acknowledged.
21. The nature of inheritance
Mendel’s experiments with the garden pea were both simple and elegant, and his ability to recognise the problem and to design
suitable experiments was part of his genius. However, discovery and understanding do not lie only in experiments; they lie also
in the analysis of the experimental results and their interpretation in a new conceptual framework. What Mendel discovered was
that if you cross two plants with the same characteristics their progeny always exhibit the parental form. However, if you cross
plants with different traits only one form of the trait will appear in the progeny (thus one of the traits was lost!). But if you
crossed the progeny with each other then all the traits would reappear in a fixed ratio – thus the traits were not lost but had
rather become latent. Thus he developed the concepts of DOMINANT and RECESSIVE, and showed that there was remarkable
experimental consistency when peas (his experimental plant) were continued to be bred. Mendel postulated that the mechanism
of inheritance was through GENES and that plants had a PHENOTYPE and a GENOTYPE.
At the same time Mendel was working on inheritance other biologists had discovered chromosomes and the process of MITOSIS
and MEISOSIS, GAMETS and ZYGOTES. And when the science of genetics evolved it was soon discovered that not all inherited
characteristics are “black and white” but that mostly they are more complex and graded – and today we know that characters
are usually controlled by several genes and this has lead to the understanding of continuously varying traits. And today the
science of genetics is well understood and the completing of the mapping of the human genome project had many implications
for us.
22. Genetic variation in natural populations
Although genetic variation provides the material basis for evolution, we have struggled to understand the evolutionary
significance of variability itself. We do not know whether the level of genetic variation in a population is optimised by natural
selection, or is an unavoidable consequence of intrinsic properties of biological systems.
We do know that mutations (genetic mistakes) occur – some small others large. And at the molecular level this is much better
understood now than before. And we do not think that all mutations are caused by the environment, in fact many have been
experimentally demonstrated to be spontaneous. We also now know that many mutations reduce evolutionary fitness.
23. The organisation of variability
While it may be argued that genetic variability provides populations with evolutionary flexibility. Most animals are diploid, and
higher plants are diploid and POLYPLOID. Suffice for me to say in this course is that genetic variability in natural populations is
influenced by adaptations of the life cycle of organisms and mating structures of populations. Most of these adaptations
enhance the fitness of the individual and its progeny. But there is a geographical component of genetic variation within
populations. Individuals that inhabit slightly different environments are exposed to different selective pressures, and these
differences are reflected in the genetic constitution of a population over its geographic range.
24. The spatial occurrence of variation in populations
The study of geographical variation within populations began with early attempts to classify species. It was soon noticed that
there were regional differences between populations of the same species. And over time the typological notion of a species has
changed to a biological definition viewing species as genetically integrated evolutionary units. I will leave this to the taxonomists
to review as this is the core of their discipline.
25. Phyletic evolution and speciation
Reproductive isolation leading to speciation is straight forward, but is not the only evolutionary mechanism. Again this is really
the domain of the systematists and I will leave it there.
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26. The history of evolution
Phyletic evolution can be followed through a sometimes adequate and sometimes fragmentary fossil record. Perhaps for the
ecologist the most conspicuous feature of evolution is its heterogeneity. Phylogenetic groups, and even whole faunas, have
gone through periods of rapid evolution and diversification, followed by quiescent and even periods of extinction.
GENETICS OF POPULATIONS
27. Demography
This is the study of birth and death processes that determine the age structure of populations and the rate of change in
population size. The basic statistics of demography are the probability of death and the fecundity of individuals of each age in
the population. These are the factors that reflect the adaptations of the individual and its ability to procure resources, survive
extreme environmental conditions, and avoid predators. From these factors we can calculate the intrinsic growth potential of a
population.
Demography is important to understanding natural selection, competition, predation, and even ecosystem stability, because
populations, or genetically distinguishable subpopulations, having different birth and death statistics by reason of their various
adaptations. They also have different inherent population growth potentials. This growth potential, under a given set of
conditions, is an expression of evolutionary fitness, competitive ability, and predator efficiency. The response of the intrinsic
growth potential to varying conditions is a measure of the inherent stability of a population. Demography also is an important
concept for ecologists because it links the structure and functioning of organisms directly to population processes, and thereby
bridges two levels of biological organisation. Processes on each level of biological organisation summarise the integrated
processes on the levels below.
Although we may measure population growth by the number of individuals added to a population, the size of this increment
depends on the original size of the population. Two populations, one of ten individuals and one of a hundred individuals, both
exposed to the same environment, would not be expected to increase by the same absolute amount. It stands to reason that
increase is proportional to initial size. Populations grow by multiplication (proportional or geometric increase) rather than by
addition (absolute or arithmetic increase) because all individuals potentially can contribute to population growth by reproducing.
If a population’s size remains constant over a long period, the age structure of the population parallels the survival of individuals
because the number of individuals alive at a given age decreases according to the mortality rate for that age. A serious
restriction to using age structure as a basis for survival curves is that one must assume that individuals alive at each different
age are the descendants of an equal number of newborn. In other words, that the number of newborn does not change from
year to year. This condition is rarely met in natural populations. The influence of birth rate on population age structure can be
visualised in various human populations (p. 433).
Age-specific survival is normally estimated for natural populations from the survival of individuals marked at birth, or from the
deaths of individuals of known age. But both methods have serious limitations. The first requires that individuals do not
emigrate from the area in which they were marked; otherwise they would be erroneously counted as dead. The second method,
commonly used in studies of bird populations, requires that large numbers of individuals be marked to obtain sufficient
recoveries of dead individuals.
When the ages of individuals can be determined by their appearance, without knowing their dates of birth, the construction of a
survival table becomes a relatively simple task. Among plants, for example, the age of forest trees in temperate zones can be
determined from growth rings in their wood. Among animals, the age of fish can be determined from growth rings in their scales
or ear bones. The age of mountain sheep can be estimated from the size of their horns.
28. Natural Selection
A genetic change that influences fecundity and survival alters the fitness of individuals that bear the mutation. By applying the
methods of population genetics theory, which is firmly grounded in demography, we can predict the success of a new gene and
assess its effect on the evolution of a population.
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Natural selection tends to increase the average fitness of a population.
29. Fitness and Evolution in Natural Populations
Genetic variation and natural selection are intimately tied to the ecology of populations in two ways. First, the genetic
characteristics of a population, and the changes in these characteristics, reflect the genotype-environmental interaction - the
outcome of which is the fitness of the individual. Second, the genetic mechanism, and the stored genetic variation of the
population, determines the ability of a population to respond to change in the environment. We experience the impact of
evolutionary response within our own lifetimes in the adaptation of insects and micro-organisms to pesticides and antibiotics,
and in the adaptation of plants to the heavy-metal contaminated soils of mine dumps.
Unless the environment changes very rapidly, as it often does in the wake of human meddling, genes with extremely deleterious
effects on fitness normally would be rare - having long since been weeded out of the population. The environment is thought to
change slowly relative to generation time and the evolutionary potential of populations. Yet, most phylogenetic lines that have
ever begun to wend their way up the evolutionary tree of life are now extinct. Somewhere along the line their evolutionary
potential failed them, and they were replaced by more successful forms.
30. The Evolution of Population Characteristics
Most adaptations are apparent in the individual. Form, physiology, and behaviour are expressed in the phenotype, and the
purpose of such adaptations is clearly to promote the survival and productivity of the organism.
ECOLOGY OF POPULATIONS
31. Population Growth and Regulation
Knowledge of the growth and regulation of populations is a prerequisite to understanding the structure and dynamics of the
biological community and the ecosystem. All species have a high potential for population growth under optimum conditions. In
“On the Origin of Species”, Darwin wrote: “There is no exception to the rule that every organic being naturally increases at so
high a rate that, if not destroyed, the earth would soon be covered by the progeny of a single pair”. This potential is never
realised fully by natural populations as their numbers are regulated, often within narrow limits.
What is a population?
Ecologists apply the term population size very loosely, to pragmatically defined assemblages of individuals of one species. The
walls of cages and aquaria delimit laboratory populations. Edges of study areas conveniently define populations in nature, for
which density (individuals per unit area) is used as an index to population size. For species whose numbers and geographical
ranges are restricted, and for species that are easily counted, it is possible to measure the size of the entire population. In
practice this is accomplished only for populations of endangered species (Ground Hornbills, Wild Dogs) and commercially
important species (ducks, deer, and so on in the USA where these are hunted for sport). Usually, the ecologist must restrict
their studies to the local population within a well-defined study area, or to the artificial population in the laboratory. So long as
the movement of individuals into and out of the study area is small compared with the replacement of individuals by
reproduction within the area, local studies may afford reasonable insight into the processes that influence population size. One
point will become clear above all others - although ecologists have sought fundamental principles applicable to all populations,
their diverse views of population regulation have been determined by the systems they have studied (thus there is apparently
no ‘norm’).
The growth potential of populations is great
We can best appreciate the capacity of a population for growth by following its rapid increase when introduced to a new region
with a suitable habitat (e.g. alien trees in the Fynbos). In 1859 twelve pairs of the European rabbit were released on a ranch in
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Victoria, Australia. Within six years the population of rabbits increased so rapidly that 20,000 were killed in a single hunting
drive.
Fluctuation in size, not constancy, is the rule for natural populations
Because birth and death are sensitive to changes in climate and food supply, population fluctuations follow directly upon
variation in the environment. The degree of variation in the size of a population depends both on the magnitude of fluctuation in
the environment and on the inherent stability of the population (p. 509). Short-lived organisms are more sensitive to short-term
variation in the environment, and their populations often fluctuate dramatically (p. 510).
In seasonal environments, reproduction occurs only when climate and resources combine to produce favourable conditions.
Seasonal changes in temperature, moisture, and nutrients additionally affect mortality, either directly or indirectly. When life
span is so short that many generations occur within a year, seasonality can greatly influence population size.
Ecologists are not completely agreed on the cause of population cycles. The populations vary much too regularly for the cycles
to be caused solely by variation in the environment. The most reasonable explanation ties the cycles to inherent population
properties of predator/prey interactions.
32. Competition
The tendency of populations to increase exponentially expresses the unrestricted biotic potential of the individual. The
realisation of this potential diminishes as population density increases is the essence of population regulation. The growth rate
of a population is dependent upon the fecundity and longevity of individuals. The depressing influence of density on growth rate
reflects the detrimental effect of competition on the survival and reproductive performance of individuals.
Laboratory experiments demonstrate that two species cannot coexist if they require similar resources.
Field observations and laboratory experiments convey opposite impressions of nature. We frequently observe many ecologically
similar species coexisting in nature, clearly using many of the same resources. Closely related species of trees (or in the Cape
Erica spp.) grow in the same habitat, all needing sunlight, water, and soil nutrients.
By contrast, closely related species rarely coexist in the laboratory. If two species are forced to live off the same resource,
inevitably one persists and the other dies out. The results of laboratory competition experiments led to the formulation of the
competitive exclusion principle, which states that two species cannot coexist on the same limiting resource (but this has not
been experimentally demonstrated in nature).
The word “limiting” is included in the statement of the competitive exclusion principle because only resources that limit
population growth can provide the basis for competition. Non-limiting resources, like atmospheric oxygen, are superabundant
compared to the needs of organisms, and their use by one organism does not make them less available to others.
How can the principle of competitive exclusion, verified by laboratory experiments, be reconciled with observations of similar
species coexisting in nature? The coexistence of many similar species in nature suggests that competition between species
generally is much weaker in natural communities than in laboratory populations (or ecologists have not fully understood the
mechanisms?). Species avoid competition by partitioning resources and habitats among themselves, a response that species in
simplified laboratory environments cannot make. But the existence of resource partitioning in nature does not mean that
competition does not occur.
In general, however, when species can avoid extensive overlap with other species in the use of resources, competition can be
reduced sufficiently to permit indefinite coexistence. It seems that only in laboratory situations, where species are forced to
exploit a single resource, is competition so intense that only one population can persist.
Does competitive exclusion ever occur in nature? Does competition influence the number of species that can coexist in a
community? Do the numerous ways in which species exploit the environment, and thus avoid competition, allow any conceivable
number of species to be crammed into a community? Or is there some degree of ecological similarity between species that
cannot be exceeded without leading to extinction? And are natural communities saturated with species whose similarities just
approach this level? Ecologists have not resolved these problems; the factors that determine the number of species in a
community constitute an important area of ecological research to some researchers.
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33. Predation
Predation has received no less attention from ecologists than has competition. In its broadest sense – food consumption –
predation is the prime mover of energy through the community, and defines the links in the food chain. Predation also is a basic
factor in population ecology and evolution. Prey populations largely determine the growth rate of predator populations because
they provide the food necessary for growth and reproduction. Conversely, predators tend to reduce the growth rate of
populations of their prey. Whether predators can substantially reduce the equilibrium size of a prey population is, however, a
controversial question. When predators selectively prey on individuals of different ages, they alter the age structure of the prey
population and change its dynamics.
We may ask many questions about predator-prey relationships. To what extent do predators stabilise, or cause cyclic
fluctuations in, prey populations? Do predators limit prey populations below the carrying capacity of the environment for the
prey? If predators are so efficient that they can substantially reduce prey populations, how do they keep from eating too many
of their prey? Do predators act to maximise their returns, that is, do they “manage” prey populations? Surprisingly little is known
about predator and prey relations in nature, and much of what we know is a rather coarse mixture of field observations and
anecdotes, and experiments with laboratory systems and using mathematical models of population processes.
34. Evolutionary responses
In the preceding chapters dealing with competition and predation we assumed that carrying capacities, competition and
predation efficiencies, did not vary. But each of these population traits is the product of the adaptations of individuals in the
population, and each is therefore subject to evolutionary change. Populations of predators, prey, and competitors are a part of
the environment of every species. All such interacting populations select traits in each other that tend to alter their relationship.
When two populations interact both can respond through evolutionary changes to adjust to the new environment. When their
relationship is antagonistic, as it is between competitors and between predator and prey, the species can become locked into an
evolutionary battle to increase their own fitness - each at the other’s expense. Such a struggle can lead to an evolutionary
equilibrium in which both antagonists continually evolve in response to each other, but the net outcome of their interaction does
not change. This bubbling evolutionary pot is stirred by changes in the environment and the appearance of new species in the
community, whether by natural range expansion or by human introduction (p. 640).
35. Extinction
With its extinction in 1914, the passenger pigeon joined a lengthening list of species that have vanished from the Earth (and in
recent times many of these extinctions were driven by human activities).
Extinction is important in the study of ecology for two reasons. First, the extinction of local populations of a species can play a
role in the regulation of population size. Second, the local extinction of one species broadens the ecological opportunity open to
similar species and may enable a species previously excluded by competition to establish itself in the community.
The study of extinction ought to elucidate many aspects of ecology and evolution, but it has largely failed to do so. This failure
can be traced directly to the facts that extinction is so infrequent and difficult to predict that direct observation is not feasible,
that extinction is merely the final event in a long sequence of subtle evolutionary and ecological processes leading to the demise
of a population - and that the fossil record is too incomplete to record the details of a dwindling lineage. And, of course fossils
open only a tiny obscured window on biological communities of the past.
Most of what we know about extinction comes from observations of the phenomenon on three different levels. First we can
observe the extinction of small local populations when local climatic conditions become severe, or when an efficient predator is
introduced to the community. Such extinctions are ecological rather than evolutionary events, and would not normally lead to
extermination of an entire lineage. Second we can observe or infer the local and sometimes global extinction of a species,
resulting from competition from ecologically similar species. Such replacements usually would cause a change in the structure of
the community, and lead to ecological and evolutionary readjustments among the remaining species. Third fossils record the
extinctions of entire groups of organisms. Dinosaurs, ammonites, and trilobites are among numerous once-successful taxonomic
groups that have disappeared, sometimes abruptly, from the face of the Earth (p. 657).
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ECOLOGY OF COMMUNITIES
36. Community ecology
The community is an association of interacting populations.
The term community has been given such a variety of meanings by ecologists that it borders on being meaningless (Ricklefs
interpretation, but not mine). It usually is restricted to the description of a group of populations that occur together. Throughout
the development of ecology as a science the term has often been tacked on to “associations” of plants and animals that are
spatially delimited and that are dominated by one or more prominent species, or by a physical characteristic. One speaks of a
restio, protea, renosterbos community, meaning all the plants and animals found in the particular place dominated by its
namesake. Used in this way, “community” is unambiguous as it is spatially defined and includes all the populations within its
boundaries.
Organisms and materials move slowly across the boundaries of most terrestrial communities of plants and animals compared to
the turnover of organisms, and flux of materials and energy within the community.
The maintenance of community structure and functioning depends on a complex array of interactions, directly or indirectly tying
all its members together in an intricate web. The influence of a population extends to ecologically distant parts of the
community through its competitors, predators, and insects that feed on foliage or pollinate flowers. By preying upon pollinators,
birds indirectly affect the number of fruits produced, the amount of food available to animals that feed upon fruits and
seedlings, the predators and parasites of those animals, and so on. The ecological and evolutionary impact of a population
extends in all directions through its influence on predators, competitors, and prey, but this influence is dissipated as it passes
through each successive link in the chain of interaction.
Is the local community a natural unit of organisation?
The obvious analogy between community and organism led many early ecologists, notably the influential plant ecologist
Clements to describe communities as discrete units with sharp boundaries (quasi-organisms). This view is reinforced by the
conspicuousness of many dominant vegetation types. The boundaries between these community types can often be so sharp as
to be crossed within a few yards along a gradient of climate conditions.
An opposite view of community organisation was held by Gleason who suggested that the community, far from being a distinct
unit like organism, was merely a fortuitous association of organisms whose adaptations enabled them to live together under he
particular physical and biological conditions found at a particular location (p. 672) – the ‘gradient or continuum concept’.
Clements’ and Gleason’s concepts of community organisation predict different patterns in the distribution of species over
ecological and geographical gradients. On one hand Clements believed that the species belonging to a community were closely
associated with each other; the ecological limits of distribution of each species coincided with the distribution of the community
as a whole. This type of community organisation is commonly called a closed community. On the other hand, Gleason believed
that each species was distributed independently of others that co-occurred in a particular association, an organisation referred
to as an open community. We would draw the boundaries of an open community arbitrarily with respect to the geographical and
ecological distributions of its component species, which may extend their ranges independently into other associations of
species.
Closed communities represent natural ecological units with distinct boundaries. The edges of the community, called ecotones,
are points of rapid replacement of species along the gradient/continuum.
We observe distinct ecotones between abrupt change in the physical environment – for example, at the transition between
aquatic and terrestrial communities, between distinct soil/geological types, or between north-facing and south-facing slopes of
mountains – or when one species or life form so dominates the environment of the community that the edge of its range signals
the distributional limits of many other species.
Sharp physical boundaries create sharp ecotones.
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37. Community organisation
Because it is impossible to measure the interactions of each species with every other species in a community, ecologists have
sought simpler indexes of community organisation. The most widely used being the number of species in the community.
Additional Studies show that more species live in a tropical habitat than in a temperate habitat with similar vegetation.
One may ask whether a study of the number of species can teach us anything about the organisation – the inner workings – of
the community. Many have argued that the number of species varies in direct proportion of the productivity of the habitat,
which is influenced by temperature, sunlight, rainfall, and nutrients. If this were true variation in number of species would seem
to be a trivial problem. But this explanation, and others like it, is unsatisfactory on several counts. First there are too many
exceptions. For example, salt marshes are among the most productive habitats, yet they support relatively few species. Second
any complete theory of species number must explain how the number of species in a particular habitat is regulated; how the
species are formed; where they come from; and how interactions between species set an upper limit to their number. A simple
correlation is not a sufficient explanation. Third species number is too important an attribute of community structure to dismiss
easily. It is possible that many species can exploit different kinds of resources more efficiently than a few species can, because
each species may become specialised to fill a different ecological role. It is also likely that the environmental heterogeneity
created by many species provides further opportunity for the diversification of life forms. That is diversity tends to breed more
diversity.
More species occur in tropical communities than in temperate and arctic communities (p. 687/8)
In virtually all groups of organisms the number of species increases markedly towards the equator. Within a given belt of
latitude around the globe the number of species varies widely among habitats according to productivity and degree of structural
heterogeneity. For example, censuses of birds in small areas (usually 10-50 ha) of relatively uniform habitat reveal about six
species of breeding birds in grasslands, 14 in shrublands, and 24 in deciduous forest. But because not al communities fit this
pattern, other factors may influence the number of species. Marshes are productive yet have relatively few species of birds.
Deserts, by contrast, are relatively unproductive but have many more species of birds than grasslands, which are more
productive but structurally much simpler. Numbers of species, therefore, appears to increase with productivity and structural
heterogeneity of the vegetation.
Species can be added to a community by increasing specialisation, ecological overlap, and environmental heterogeneity
Added species can be accommodated within a community in several ways. Consider the distribution of several species along a
resource continuum – perhaps the size of prey items or the height at which feeding takes place within a forest. If the variety of
resources were increased, species could be added to the longer resource continuum without diminishing the ecological range of
other species. If the productivity of the habitat did not increase in direct proportion to the variety of resources, the added
species would reduce the productivity of the others.
When an ecologist considers the number of species in a small patch of uniform habitat, the degree of specialisation with respect
to resource within that habitat is an important consideration. Suppose a region has five distinctive habitats, each of which could
support no more than 10 species. If all species were habitat generalists the whole region would only support 10 species. If,
however, the species were habitat specialists and could only maintain populations in one habitat, the region would support 50
species.
This simple example illustrates two kinds of diversity – local or alpha diversity and regional or beta diversity.
38. Community development
Communities are constantly changing. Organisms die and others are born to take their places. Energy and nutrients continually
pass through the community. Yet the appearance and composition of most communities do not vary. If a community is
disturbed – a forest cleared for agriculture – the community is slowly rebuilt. Pioneering species adapted to the disturbed habitat
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are successively replaced by others until the community attains its former structure and composition. The sequence of changes
on a disturbed site is called succession and the final association of species achieved is called a climax. By 1916 University of
Minnesota ecologist Clements had outlined the basic features of succession, supporting his conclusions by detailed studies of
change in plant communities in a wide variety of environments.
Succession follows an orderly pattern of species replacements
The creation of any new habitat invites a host of invading species to exploit the resources made available. The first colonisers
are followed by others, slower to take advantage of the new habitat but eventually more successful than the pioneer species. In
this way, the character of the community changes with time. Successional species themselves change the environment. For
example, plants shade the surface, contribute detritus to the soil, and alter soil moisture. These changes often inhibit the
continued success of the species that cause them, and make the environment more suitable for other species, which then
exclude those, responsible for the change.
The transition from abandoned fields to mature forest is only one of several successional sequences leading to the same climax.
Forests are the end point of several different successional series, or seres, each having a different beginning.
Primary succession develops on newly exposed landforms
Ecologists classify seres into two groups, according to their origin. The establishment and development of plant communities in
newly formed habitats previously without plants – sand dunes, lava flows, rock bared by erosion or exposed by a receding
glacier – is called primary succession. The return of an area to its natural vegetation following a major disturbance is called
secondary succession.
Successional stages in time resemble gradients of communities in space
The sequence of species in the sere parallels change in the physical environment as it is modified by the developing vegetation.
Many of the stages in a time sequence through a sere may be found along geographical gradients in vegetation, often called
ecoclines. The ecocline represents a series of stages of community development stopped at different points by lack of moisture.
Each stage of the ecocline represents the local end point of succession.
The climax is the local end point of a sere
Succession traditionally is viewed as leading inexorably toward an ultimate expression of plant development, the climax
community. These observations led to the concept of the mature community as a natural unit, even as a closed system. The
nature of the local climax was thought to be determined solely by climate. Aberrations in community composition caused by
soils, topography, fire, or animals were thought to represent interrupted stages in the transition toward the local climax –
immature communities.
In recent years, the concept of the climax as an organism or unit has been greatly modified, to the point of outright rejection by
many ecologists, with the recognition of communities as open systems whose composition varies continuously over
environmental gradients.
Succession results from variation in the ability of plants to colonise disturbed habitats and from changes in the environment
following the establishment of new species
Two factors determine the positions of species in a sere: the rate at which species invade a newly formed or disturbed habitat,
and changes in the environment over the course of succession. Some species disperse slowly, or grow slowly once they have
become established, and therefore become dominant late in the sequence of associations in a sere. Rapidly growing plants that
produce many small seeds, carried long distances by the wind or by animals, have an initial advantage over species that are
slow to disperse, and they dominate early stages of the sere.
When does succession stop?
Succession continues until the addition of new species to the sere and the exclusion of established species no longer change the
environment of the developing community. Conditions of light, temperature, moisture, and, for primary seres, soil nutrients,
16
change quickly with the progression of different growth forms. The change from grasses to shrubs and trees on abandons fields
bring a corresponding modification of the physical environment. Conditions change more slowly, however, when the vegetation
reaches the tallest growth form that the environment can support. The final biomass dimensions of the climax community are
limited by climate independently of events during succession. Once forest vegetation is established, patterns of light intensity
and soil moisture are not changed by the introduction of new species of trees, except in the smallest details.
The time required for succession to proceed from a cleared habitat to a climax community varies with the nature of the climax
and the initial quality of the soil
The character of the climax is determined by local conditions
Clements’s idea that a region had only one true climax (the monoclimax theory) forced botanists to recognise a hierarchy of
interrupted or modified seres by attaching names like subclimax, preclimax, and postclimax. This terminology naturally gave way
before the polyclimax viewpoint, which recognised the validity of many different vegetation types as climaxes, depending on the
habitat. More recently, the development of the continuum index and gradient analysis has fostered the broader pattern-climax
theory of Whittaker, which recognises a regional pattern whose composition at any one locality depends on the particular
environmental conditions at that point.
Many factors determine the climax community, among them soil nutrients, moisture, slope, and exposure. Fire is an important
feature of many climax communities, favouring fire-resistant species and excluding others that otherwise would dominate.
Grazing pressure also can modify the climax. Grassland can be turned into shrubland by intense grazing. Herbivores kill or
severely damage perennial grasses and allow shrubs unsuitable for forage to establish. Most herbivores graze selectively,
suppressing favoured species of plants and bolstering competitors that are less desirable as food.
The characteristics of dominant species change during succession
Succession in terrestrial habitats entails a regular progression of plant forms. Plants characteristic of early stages and late stages
of succession employ different strategies of growth and reproduction. Early-stage species are opportunistic and capitalise on
high dispersal ability to colonise newly created, or disturbed habitats rapidly. Climax species disperse and grow more slowly, but
as seedlings are shade tolerant and of a larger size they have a competitive edge over early successional species. Plants of
climax communities are adapted to grow and prosper in the environment they create, whereas early successional species are
adapted to colonise unexploited environments (p. 738).
The allocation of a large proportion of production to shoot biomass in early successional plants leads to rapid growth and
production of large crops of seeds. Because annual plants must produce seeds quickly and copiously they never attain large
size. Climax species allocate a larger proportion of their production to root and stem tissue to increase their competitive ability;
hence they grow more slowly. The progression of successional species is therefore accompanied by a shift in the compromise
between adaptation for great dispersal power and adaptation for great competitive ability.
The biological properties of a developing community change as species enter and leave the sere. As a community matures, the
ratio of biomass to productivity increases; the maintenance requirements of the community also increase until production no
longer can meet the demand, at which point the net accumulation of biomass in the community stops.
As plant size increases with succession, a greater proportion of the nutrients available to the community are tied up in organic
materials. Furthermore, because the vegetation of mature communities has more supportive tissue, which is less digestible than
photosynthetic tissue, a larger proportion of their productivity enters the detritus food chain rather than the consumer food
chain. Other aspects of the community change as well. Soil nutrients are held more tightly in the ecosystem because they are
not exposed to erosion; minerals are taken up more rapidly and stored to a greater degree by the well-developed root systems
of forests; the environment near the ground is protected by the canopy of the forest; conditions in the litter are more favourable
to detritus-feeding organisms.
Ecologists generally agree that communities become more diverse and complex as succession progresses. It is not known
whether the increase in the diversity of a community during its early stages of succession is related to increased production,
greater constancy of physical characteristics of the environment, or greater structural heterogeneity of the habitat.
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The study of succession leads us to formulate four basic ecological principles. First, succession is one-directional. Second,
successional species alter the environment by their structure and activities, often to their own detriment and to the benefit of
other species. Third, the climax community is not a unit; rather it represents, at any given place, a point on a continuum of
possible climax formations. The nature of the climax is influenced by climate, soil, topography, fire and the activities of animals.
Fourth, the climax may be a changing mosaic of successional stages maintained by wind, frost, fire or other sources of mortality
acting locally within the community.
39. The Niche Concept in Community Ecology
Community attributes, such as number of species and their relative abundances, are superficial measures that reflect
characteristics of a habitat and the interactions among species that live there. By relating such attributes to variation in the
environment, ecologists have attempted to sort out the factors that regulate community structure and function. We have,
however, become increasingly aware that simple correlations cannot elucidate community organisation. Furthermore, the study
of organisation at the community level is made difficult by the nature of communities themselves.
The niche concept expresses the relationship of the individual to all aspects of its environment.
Hutchinson defined the niche concept formally. Ideally, he said, one could describe the activity range of each species along
every dimension of the environment, including such physical and chemical factors as temperature, humidity, salinity and oxygen
concentration, and such biological factors as prey species and resting backgrounds against which an individual may escape
detection by predators. Each of these dimensions can be thought of as a dimension in space. If there are n dimensions, the
niche is described in a one-dimensional space, of which it occupies a certain volume. Of course, we cannot visualise a space
with more than three dimensions; the concept of the n-dimensional niche is therefore an abstraction. But it is possible to handle
multidimensional concepts mathematically and statistically.
Each species occupies a part of the n-dimensional volume – its niche – that represents the total resource space of the
community. We may think of the total niche space of a community as a volume into which are fit the niches of all the species of
a community, as one would pack balls of various sorts into a box. The number of species in the community depends upon the
total amount of niche space and the average size of each species’ niche. Community ecologists are interested in the factors that
determine both these quantities. But one must first find ways to measure the niche.
The overlap between the niches of any two species is determined by their breadths and the distances between the optima of
each species. Because niches are not likely to be symmetrical, it may be more fruitful to measure niche breadth and overlap
directly, and to ignore the distance between optima (p. 744).
Some resource dimensions may be continuously variable, such as temperature and humidity, but others are discrete and
discontinuous, such as soil types. Measurements of niche breadth and overlap are diminished by our ignorance of the resource
dimension itself. All individuals are specialised to some degree with respect to the population as a whole. Each individual spends
its life within a limited geographic area and restricts its activities to a portion of the total habitat space occupied by the
population. This specialisation arises in part because the individual has limited time and dispersal range, but it may be based on
learned or inherited behavioural variation, morphological variation based on genetic polymorphism, sexual dimorphism, or age.
In addition to this between-individual component to the niche of a population, niche space shifts in a daily and seasonal rhythm,
following the ever-changing resource space of the habitat. Niches are, therefore, so difficult to measure that field observations
on the ecological relationships of the species in a community probably could not be relied on as adequate descriptions. At best,
we can hope to obtain only fragmentary measures that indicate relative degrees of niche breadth and overlap within
communities.
ECOLOGY OF ECOSYSTEMS
40. Primary production
Organisms need energy to move, grow, and maintain the functions of their bodies. Energy to support these activities enters the
ecosystem as light, which plants convert to chemical energy during photosynthesis. The rate at which plants assimilate the
energy of sunlight is called primary productivity. It is important to realise that primary production underlies the entire trophic
structure of the community. The energy made available by photosynthesis drives the machinery of the ecosystem. The flux of
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energy through populations of herbivores, carnivores, and detritus feeders, and the biological cycling of nutrients through the
ecosystem are ultimately tied to the primary productivity of plants.
Photosynthesis is an uphill process: it requires an input of energy to drive the chemical reaction. The mere presence of the
ingredients for photosynthesis is not, however, sufficient to make the reaction occur. Carbon dioxide, water vapour, and sunlight
occur together in the earth’s atmosphere, yet we are not deluged by a constant rain of glucose. The probability of the
appropriate chemical reaction occurring there is very low. In plant cells, pigments and enzymes bring molecules and energy
together so as to make the chemical reactions of photosynthesis highly probable.
Plants are made up of ore than just glucose. Various biochemical processes use glucose both as a source of energy and as a
building block to construct other, more complex organic compounds. Rearranged and joined together, sugars become fats, oils,
and cellulose, the basic structural material of the plant cell wall. Combined with nitrogen, phosphorus, sulphur, and magnesium,
sugars are used to produce an array of proteins, nucleic acids, and pigments. Plants cannot grow unless they have all these
basic building materials. Chlorophyll contains an atom of magnesium in addition to nitrogen and carbon atoms, just as
haemoglobin contains an atom of iron. Thus, even though all other necessary materials might be present in abundance, a plant
lacking magnesium cannot produce chlorophyll, and thus cannot grow.
The amount of water required by photosynthesis is a drop compared to the bucket needed to balance transpiration. Water
makes up the bulk of plant tissues and is the essential medium that makes nutrients available.
We must distinguish two measures of production. Gross production refers to the total energy assimilated by photosynthesis. Net
production refers to the accumulation of energy in plant biomass, that is to say, plant growth and reproduction. The difference
between the two measures is accounted for by respiration by the plant.
41. Energy flow in ecosystems
Plants manufacture their own “food” from raw materials. Hence they are referred to as autotrophs, literally self-nourishers.
Animals obtain their energy in ready-made food by eating plants or other animals. Animals are, therefore, referred to as
heterotrophs, meaning nourished from others. The specialisation of living forms as food producers and food consumers creates
an energetic structure – called the trophic structure – in biological communities, through which energy flows and nutrients cycle.
The food chain from grass to caterpillar to sparrow to snake to hawk delineates the path of organic materials and the energy
and nutrient minerals they contain. Each link in the food chain, each trophic level in the community, dissipates most of the food
energy it consumes as heat and motion. Hence, with each step in the food chain, the total amount of usable energy passed to
the next higher trophic level becomes smaller (p. 781).
Autotroph and heterotroph distinguish organisms that convert inorganic forms of energy to organic forms from those whose sole
source of energy is organic matter.
Herbivore and carnivore refer to eaters of plants and animals, respectively ,although carnivore is sometimes reserved for meat
eaters. Other terms such as insectivore and piscivore (fish eater) are used. Herbivores also come in a variety of forms –
nectivores, frugivores, grazers, browsers, and so on – depending on what they eat and how they eat it. Predators distinguish
organisms that consume prey from parasites, organisms that feed on living prey, or hosts. Hence seed eaters are properly called
predators because they destroy the tiny plant embryo in the seed, and many grazers and browsers are properly called parasites,
because they consume leaves or buds without killing the tree. Mosquitoes and vampire bats would fit equally well in the
category of browsers.
Trophic levels provide a simple framework for understanding energy flow through the ecosystem. But energy takes complicated
paths through the trophic structure of the ecosystem..
Many carnivores eat fruit and other plant materials when their normal prey is not readily available. Some carnivores eat other
carnivores as well as herbivores. Most herbivores, however, particularly those that eat leafy vegetation, are so specialised in
their food habits that they are incapable of carnivorary.
The great variety of feeding relationships within the community links species into a complex food web. Each species feeds on
many kinds of prey. Within the context of this complexity, trophic levels become abstract concepts, useful for understanding
general patterns of structure and energy flow through the ecosystem, but usually quite useless as categories for assignment of
individual species.
The trophic structure of a community can be described in terms of pyramids of productivity, biomass, and numbers
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The amount of energy metabolised by each trophic level decreases as energy is transferred from level to level along the food
chain. Green plants, the primary producers, constitute the most productive trophic level. Herbivores are less productive, and
carnivores still less. The productivity of each trophic level is limited by the productivity of the trophic level immediately below it
and by the efficiency with which energy is transformed into biomass. Because plants and animals expend energy for
maintenance, less and less energy is made available, through growth and reproduction, to each higher trophic level. Of the light
energy assimilated by a plant, 30 to 70 per cent is used by the plant itself for maintenance functions and the energetic costs of
biosynthesis. Herbivores and carnivores are more active than plants and expend even more of their assimilated energy on
maintenance. As a result, the productivity of each trophic level is usually no more than 5 to 20 per cent of that of the level
below it. The percentage transfer of energy from one trophic level to the next is called the ecological efficiency or food chain
efficiency of the community (p. 784).
As energy availability decreases, the biomass and numbers of individuals on each trophic level usually decrease. The biomass
structure of the community resembles the pyramid of productivity in most terrestrial communities.
Detritovores can be a major part of the food web
Many ecologists attribute a distinctive role to organisms that consume dead plant and animal matter. First, as we have seen, all
organisms “decompose’ organic matter. Terrestrial mammals consume 20 to 200 grams of food for every gram of body weight
produced. The remainder is metabolised and used as a source of energy. Most ingested food is returned to the environment in
an inorganic form: carbon dioxide and water are exhaled; water and various mineral salts are excreted in sweat and urine.
Second, detritovores have no special purpose different from any other organism in the overall function of the ecosystem. The
individual detritovores obtains energy and nutrients in its food just like herbivores and carnivores. And just like all other
organisms, detritovores leave behind indigestible remains, breakdown products of metabolism, and excess minerals that they
cannot use. Detritovores eat the garbage of the ecosystem for the same reason herbivores and carnivores at fresh food: to
make a living. It is all a matter of taste.
42. Nutrient cycling
Nutrients, unlike energy, are retained within the ecosystem where they are continually recycled between living organisms and
the physical environment.
43. Stability of ecosystems
Stability is the inherent capacity of a system to tolerate or resist changes caused by outside influences.
We may define the inherent stability of a system as the ratio between variation in the environment and variation in the system
itself, but this definition is difficult to apply to populations or communities.
The biological significance of stability is even more elusive than its description and measurement.
Virtually all human activity disturbs natural communities. Natural biological communities do not yield enough harvestable food to
support dense human populations, and man selects the most “desirable” components of natural communities, alters their
evolution through artificial selection to suit his purposes, and maintains these populations of crops, livestock, pulp trees, city
parks, and backyards in a continually disturbed state, while the population is constantly exposed to conditions they are not
adapted to cope with. The price man pays for exploitation of natural resources is the price of maintaining their stability by
constant management: curbing pest infestations, maintaining soil fertility, and cleaning out weeds.
Our concern with the constancy of the natural world and with the basis for stability in natural communities is understandable.
Measures of community structure and function – number of species, number of trophic levels, rates of primary production,
energy flow, and nutrient cycling – reflect ecological interactions among populations and between individuals and their physical
environments. The ability of the community to resist ecological perturbations no doubt reflects the homeostatic mechanisms of
individuals and the growth responses of populations.
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Some terms need defining
Ecologists use stability and related terms in so many different ways that it will be necessary to provide explicit definitions here:
stability is the intrinsic ability of a system to withstand or to recover from externally caused change
constancy is a measure of the degree of variation of some attribute of a system
predictability is a measure of regularity in patterns of change
Seasonal fluctuation in the environment usually is predictable; day-to-day variation often is not.
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