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Tuatara: Volume 16, Issue 2, July 1968

Definitions and Discussion of Some Concepts and Terms Relating to Terrestrial Ecosystems

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Definitions and Discussion of Some Concepts and Terms Relating to Terrestrial Ecosystems

For Some Years the writers have been trying to arrive at satisfactory definitions of the most useful ecological terms—definitions that are both precise and consistent with one another. There is not available at the present time a publication which brings together definitions of a critically selected group of ecological terms from the fields of botany, zoology and soil science. In the few glossaries that have been independently published in the past, the basic concepts and terms have been obscured by a large number of specialist words, uncritically gathered up for the sake of completeness. Most writers of ecological textbooks have such definitions as they include scattered through the text; only a few have glossaries. We evaluated one of these, published within the last decade, and found that no less than 67 of the 97 terms we have listed below, and consider basic, were not in the glossary. Of the 30 that were, only 15 were defined adequately in our opinion.

We want to make it clear that we bring these definitions forward not with an authoritative attitude, but as a basis for consideration and discussion. Indeed, agreement with all the definitions given here will clearly not be possible; some of our most experienced critics hold quite dissimilar views on how certain terms should be defined. Yet in the interests of communication, we should be aiming at more general agreement than we have at present. We hope that the following definitions will be a step in this direction. We have taken great care, demonstrated by frequent cross-referencing, to ensure that they are at least consistent with one another.

While not wishing to imply that the following agree with all the definitions below, we are grateful to Dr. J. R. Bray, Mr. C. J. Burrows, Mr. A. E. Esler, Dr. J. E. C. Flux, Dr. J. A. Gibb, Prof. D. B. Lawrence, Dr. K. E. Lee, Dr. M. Luxton, Dr. D. R. McQueen, Dr. A. F. Mark, Dr. R. B. Miller, Dr. P. Wardle, Dr. D. Scott, Mr. V. D. Zotov, and particularly to Mr. I. J. Pohlen, for advice and for criticisms of earlier drafts of this list.

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Abundance. (1) The relative number of individuals of a species in a defined area. It is commonly expressed according to a number of classes, e.g. rare, uncommon, common, abundant. Cf. density.

(2) The mean number of individuals of a species per sample area (quadrat), considering only those sample areas in which the species occurs.

Abundance = Total no. of individuals of a species/No. of sample areas in which species occurs

Age Structure. The distribution of the individuals of a population in age classes.

Albedo. The proportion of incident radiation reflected by a portion of the earth's surface, i.e. by vegetation, soil, water, buildings etc.

Association. The term has been used in a variety of ways for vegetation, often with the assumption that associations are floristically homogeneous and exist as integrated units comparable to organisms. We think that since the terms vegetation type and community type (q.v.) analogous to soil type, have no such implications, they are more satisfactory terms to use. “Association’ is also used in soil mapping.

Available Biota. The species and numbers of organisms available for community development by dispersal from neighbouring areas. The term should be distinguished from community biota — the plants and animals that actually survive in the community.

Basal Area. In trees, the cross-sectional area of the trunk at 4.5 ft above the ground (breast height). In tussocks, the cross-sectional area measured about lin. above the ground. The relative basal area of a species is the proportion it forms of the total basal area contributed by all species.

Biomass (of a community or an individual species). The total weight of living organisms per unit area at a given time. Unlike standing crop (q.v.), biomass includes all parts of the organisms.

Boundary. A limit placed in order to divide soil and/or vegetation into classes, for description and mapping. Cf. transition.

Canopy. The layer or layers defined by the uppermost plant crowns or their parts. The concept is applicable to any kind of vegetation. “Canopy’ has often been used for an individual plant crown in vegetation but we think its use should be restricted to the vegetation layer or layers defined by the uppermost crowns.

Canopy Gap. A temporary opening in the canopy (q.v.) resulting from the death, removal, or overturning of plants.

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Carrying Capacity (animals). The upper level of density, beyond which no major increase can occur. Cf. site capacity.

Climax. Vegetation considered to have reached a steady-state condition. Since vegetation is part of the ecosystem which is an open system subject to gains and losses, a state of equilibrium is not reached.

Community. The plants and animals living together in an area (of any size).

The fact that each species is dependent on particular resources of energy, nutrients, water, air, and space, implies that the community is a dynamic system, not to be distinguished from ecosystem (q.v.). Thus “community’ does not represent a separate level of organisation (hieron), lower than “ecosystem’, as stated by some authors (e.g. Odum 1963). Both terms apply to a given volume of the biosphere: which term is used is a matter of emphasis only. The term community focusses attention on the species within the biosphere-volume, whereas the term ecosystem focusses attention on nutrient cycling and energy flow within the biosphere-volume.

Community Type. A conceptual grouping of a number of stands (q.v.), of similar morphology, biotic composition, etc., for purposes of description and mapping. The classes delimited depend on the purpose in view, the scale of mapping, and the criteria used.

Whittaker (1956) and others have pointed out that the concrete communities in which sampling is done, i.e. the stands, should be clearly distinguished from any abstract class of communities, i.e. a community type, so as to avoid the uncritical transfer of information about communitiees in the one sense to communities in the other sense.

See also soil type and vegetation type.

Competition (1) (between animals). ‘Inferred or proven interaction between species which share a … population-limiting resource such as food or shelter.’ (Macfadyen 1957, p. 177). Interference has been suggeseted by some authors (see Macfadyen, loc. cit.) as a more restricted term for some cases of competition.

(2) (between plants). Interaction between individual plants in which growth rates, reproductive efficiency and survival are affected because the combined requirements of light, atmospheric carbon-dioxide, soil moisture, soil oxygen or nutrients are in excess of the rate of supply. Harper (1961) has suggested that the term interference is preferable.

Competitive Displacement Hypothesis. ‘Different species having identical ecological niches (that is, ecological homologues) cannot coexist for long in the same habitat’ (DeBach and Sundby 1963). This is also known as the ‘Volterra-Gause page 101 principle’, ‘Cause's hypothesis’, ‘Grinnell's axiom’, and the “competitive exclusion principle’ (see Gause 1934, Udvardy 1959, Hardin 1960).

Composition (of a community). The species present, usually with an estimate of their abundance, density, cover, basal area, or biomass (q.v.).

Continuum. A sequence (q.v.) in which variation in vegetation and/or soil properties between one stand and the next of the series is continuous rather than discontinuous. The stands may or may not adjoin one another in space.

Cover. The proportion of ground surface covered by the leaves or crowns of a species, or by one or more layers (storeys, tiers) of vegetation, when projected vertically downwards.

Canopy cover of vegetation is the proportion of ground surface covered by the canopy layer (s) when projected vertically downwards. Relative leaf cover and relative crown cover are the respective proportions of the total leaf cover and total crown cover contributed by a particular species. Cover is usually expressed as a percentage.

Crop. The total amount of plant or animal material than can be removed per unit area over a period of time. Standing crop, crop and yield (q.v.) are three scientifically imprecise but useful terms related to the precise terms, biomass, production, and productivity (q.v.). See Westlake 1963.

Density. The number of individual plants, plant parts, or animals of one or more species per unit area. Cf. abundance.

Relative density is the proportion of the total number of individuals of all species in a group (e.g. trees) contributed by a particular species.

Sample area density (quadrat density) = Total no. of individuals of a species/Total no. of sample areas

The reciprocal of sample area density is termed mean area.

Density-Dependent Factor. A factor whose intensity of action is governed by population density. Density-dependent factors are the so-called “biotic factors’ in contrast to the density-independent factors (see state factors) such as climate (but not microclimate).

Dependent Relationship. A relationship in which organisms depend on specific physical conditions or other organisms for their survival as a result of structural, physiological and behavioural adaptations.

Dependent Variable. When applied to ecosystems a dependent variable is a property of the ecosystem, such as population density or soil pH, dependent on the variables, or state factors (q.v.), of page 102 climate, parent rock, relief, ground water, available biota, or time. The latter are wholly or largely independent of population density, soil pH, etc. Cf. density-dependent factor.

Dispersal. The process of transport and spread of plants and animals. Cf. migration.

Dispersion. See pattern.

Distribution. (1) The area occupied by populations of a species, i.e. the geographic distribution or range.

(2) The arrangement of individuals of a population in space.‘In a random distribution the presence of one individual does not either raise or lower the probability of another occurring near by. In a uniform distribution the probability is lower and in a clumped distribution … it is raised.’ (Greig-Smith 1957, p. 12).

Dominant. (1) The plant species with the greatest cover, basal area or biomass in the community.

(2) The plant or animal species that chiefly effects the energy flow within a particular trophic level in the community.

Ecological Pyramids (of numbers, biomass, and energy). Graphical representations of trophic structure (q.v.) in which the first or producer level forms the base and successive levels the tiers which make up the apex. The numbers and biomass “pyramids’ may be partly inverted, but not so the energy pyramid. See Odum and Odum 1959.

Ecosystem. An open system of any size comprising organisms and non-living materials involved together in the flow of energy and the circulation of matter. The vegetation system and the soil system (q.v.), though often described separately, are parts of the ecosystem.

The concept of ecosystem is of “fundamental importance in interpreting the data of dynamic ecology … particularly in relation to quantitative studies of the biological and physical processes affecting productivity and the accumulation, transformation and flow of energy and materials’ (Ovington 1962, p.105). Cf. community.

Differences between ecosystems (or vegetation systems, or soil systems) may be interpreted in terms of the state factors (q.v.) of climate, available biota (including man), parent rock, relief, ground water, and the time during which development has taken place (see Jenny 1946. 1958).

Ecotone. This term has been loosely applied to stands intermediate (in position, and in physiognomy, composition, etc.) between two previously defined communities, often with the assumption that these stands constitute a “tension zone’ in which competition between organisms is more severe than on either side. Another assumption often implied is that an ecotone is a boundary along which two communities compete as integrated wholes analogous page 103 to organisms. We think that spatial transitions (q.v.) can be described and understood satisfactorily without the use of the term ecotone.

Eluviation. The downward movement of particles through the soil in solution, in suspension, or by gravity. The term refers especially to the movement of colloids.

Emergent. A plant, usually in forest or scrub, that has its crown projecting above a lower and more continuous part of the canopy.

Environment. Although the term is commonly used in a broad sense for the surroundings of an organism or organisms, i.e. for the non-living part of the ecosystem, Mason and Langenheim (1957) have emphasised that it has greatest meaning when used in a narrower sense for all “those phenomena that actually enter a relation with a particular organism’. This is called the operative environment.

Environmental Range. The range of physico-chemical-biologic conditions within which an organism can continue to live. The environmental range of a single organism may differ from that of a population, since to maintain the latter, reproduction and the survival of juveniles is necessary.

Evapotranspiration. The loss of water vapour from the ecosystem. It includes both evaporation and transpiration.

Food Chain. A sequence of plant and animal populations through which energy flows as a result of the dependence of one population on another for food. Food webs are formed by the interlacing of food chains.

Frequency. An estimate of the chance of finding individuals of a species within a sample area (quadrat) of a particular size and shape. Frequency is dependent partly on size of individuals, partly on density (q.v.), and partly on pattern (q.v) (Greig-Smith 1957). It is usually expressed as a percentage.

Frequency = No. of sample areas in which species occurs/Total no. of sample areas × 100

Gradient. A gradual change of an environmental factor in space.

Habitat (of a population). The kind of place where a population lives, i.e. the kind of place where one would go to find individuals of the population. Cf. niche.

Home Range. The area that an animal regularly frequents. Cf. territory.

Immobilisation. The temporary withholding of energy or nutrients from the organic cycle (q.v.). Cf. mineralisation and mobilisation.

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Inorganic Regime. Physical and chemical weathering in the ecosystem (wasting regime), together with the gaining, losing, or mixing of inorganic materials (drift regime) (cf. Taylor and Pohlen 1962). Gains include volcanic ash, loess, minerals in rainwater and flush water, colluvial and alluvial deposits. Losses occur during erosion as well as in drainage and surface run-off.

Interference. See competition.

Leaching. The removal of substances from the ecosystem in solution.

Leaf Area Index (L.A.I.). The surface area of the leaves of all plants per unit area of land (Watson 1947).

Leaf Wash. The material excreted or deposited on the surface of leaves, etc., that is transported downwards by rainwater.

Limiting Factor. An environmental factor that limits the growth, density, or distribution of individuals.

Mean Area. See density.

Migration (of animals). The shifting of home range (q.v.), usually concerted and directional, and often over a considerable distance, in response to seasonal or other periodic changes in the physical environment. Cf. dispersal.

Mineralisation. The liberation, by decomposition, of mineral elements from the organic matter in which they are held.

Mobilisation. The liberation of immobilised energy or nutrients into the organic cycle (q.v.). Cf. immobilisation.

Mosaic. Two or more types of vegetation and/or soil displaying a repetitive pattern and treated as a composite whole for purposes of description or mapping.

Niche. The more or less specialised set of relationships which a population establishes with the environmental resources of energy, nutrients, water, air, and space. The concept seems to have the greatest value when attention is being focussed on the differences between species that lead to ecological segregation. Cf. habitat.

Organic Cycle. The flow of energy or materials through the ecosystem, that results from the activities of living organisms. In quantitative studies it is useful to separate energy flow, nutrient cycle, water cycle and carbon cycle.

Organisation (within the community). The network of dependent relationships (q.v.) arising from energy transfers, competition, and other life processes of organisms.

Parent Material (of the soil). The usually unconsolidated mineral or organic material in which soil develops. It should be distinguished from parent rock (including subaerial and alluvial deposits) which is the rock from which the parent material is derived by weathering (see Taylor and Pohlen 1962, p. 19).

Pattern (of a population). The non-random distribution (q.v.) of individual animals, plants, or plant parts (e.g. stems, crowns) within a community.

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With plants attention is focussed, more often than not, on patterns of distribution in the horizontal plane rather than in a vertical direction. Pattern commonly arises either from “exclusion of the species from part of the area by unfavourable environmental factors or by the presence of competing species’, or from “aggregation brought about by inefficient propagule dispersion or by vegetative propagation’ (Greig-Smith 1957, p. 64). To these factors must be added, social aggregation and active disassociation in animals.

Physiognomy. The external features of vegetation.

Population. The aggregate of all individuals of all ages of a particular species in a given area. Population is a concept used for classifying information about individuals (see Mason and Langenheim 1961.

Production. (1) Gross primary production. The total amount of organic matter fixed in an ecosystem per unit area over a period of time. It includes that used in plant respiration during the measurement period. Cf. crop.

(2) Net primary production. The amount of organic matter stored in an ecosystem per unit area over a period of time. It thus excludes that lost by plant respiration during the measurement period.

There is no general agreement yet on the use of productivity terms; the reader is referred to discussions by Macfadyen (1957), Westlake (1963), and Odum (1964).

Productivity. The production expressed as a rate, i.e. the amount of organic matter fixed or stored per unit area per unit time. Productivity should be qualified in terms of the period over which it is averaged. Cf. yield. See Westlake 1963.

Recruitment. The intake of new breeding adults in a population.

Regeneration (in vegetation). (1) The process by which adults of a plant population are replaced by juveniles of the same species.

(2) The process by which vegetation of a given kind redevelops on the same ground following destruction.

Regeneration Gap. An absence or paucity of juveniles in a plant population, usually over a wide area.

Revegetation. The assisted re-establishment of vegetation in an area following loss of the original cover.

Regrowth (of plants). The process in which a plant forms new parts following damage.

Sample. A portion of the whole consisting of one or more sampling units (points, lines, quadrats) from which qualitative or quantitative data are obtained.

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Sequence. A conceptual arrangement of a series of stands (q.v.) in relation to a state factor (q.v.) or complex of correlated factors. The stands may or may not adjoin one another in space.

When stands are arranged in relation to state factors we may distinguish climosequences (climate), lithosequences (parent rock), toposequences (topography), biosequences (available biota), and chronosequences or successions (time) (see Jenny 1958). We may also have sequences where a number of factors are correlated with elevation (see also zone). It is understood that in constructing a sequence for a particular factor, stands are selected with as little variation as possible in any other factor.

Seral Stage. See stage (1).

Sere. See succession (2).

Site. An area of the land surface for which a specific statement can be made of aspect, slope, exposure, ground water, and underlying geological material. The size of a site may vary within wide limits depending on the degree of uniformity required for sampling.

The concept of site has been extended by some authors to include the vegetation it supports and then used as an index of site capacity (q.v.). By others it has been taken to include the climate (see, for example, soil site, Taylor and Pohlen 1962).

Site Capacity. The potential yield (q.v.) of a site. Cf. carrying capacity.

Society. An animal population in which one or more individuals have become specialised for performing particular functions more effectively than the remainder of the population. Integration of the activities of the specialised groups is based on a wide variety of systems of communication. Societies include groups such as herds and flocks as well as social insects.

Soil. (1) The upper weathering part of the earth's crust, commonly but not always differentiated into horizons, of mineral and organic constituents (cf. Joffe 1949, pp. 39-41). According to this definition soil does not include living organisms. The definition can be extended to include accumulations of mineral and organic matter on plants, or in other places above ground. It is the common definition used in pedology, although Taylor and Pohlen (1962) prefer a definition similar to (2) below.

(2) The system below ground level, commonly but not always differentiated into horizons, in which mineral matter, organic residues and living organisms are involved in cyclic exchanges with the vegetation above ground. Soil and vegetation, though often described separately, are parts of a larger open system, the ecosystem (q.v.). (Note: ground level is taken to be the upper surface of the organic horizon.)

Compare these definitions with those given for vegetation.

Soil Horizon. See stratification.

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Soil Profile. A three-dimensional vertical section of the soil through all its horizons. For terms relating to the description of soil profiles see Taylor and Pohlen (1962).

Soil Structure. The manner in which the primary soil particles are aggregated within the soil. The size, shape, arrangement, and stability of aggregates collectively determine the structure of the soil.

Soil Type. The basic soil class used in soil mapping. The soil type (as shown on the map) comprises areas of soil that are alike in characteristics of significance to the nature and functioning of the soil in the natural landscape. It may include a small percentage of areas of different soil, regarded as “inclusions’ within the soil type (see Taylor and Pholen 1962). See also community type and vegetation type.

Stage, (1) Vegetation stage. An arbitrarily delimited part of a vegetation succession (q.v.). Stages may be distinguished by such terms as initial, immature, semimature, and mature. The use of the word “mature’ does not imply complete stability. A seral stage is one in which change is rapid relative to that occurring in semimature and mature stages.

(2) Soil stage. In pedology “stage’ has been used in the time sense like the vegetation stage. It has also been used in a somewhat different cannotation in which it refers to a step in a soil sequence where soils are arranged progressively according to the degree of development of soil horizons. Such “development’ is due in part to differences in the available organisms, for example where one tree species is a podzoliser and another is not. Unfortunately, the terms “young’, “immature’, “semimature’, “submature’, and “mature’ have been applied to stages of both kinds.

Stand. A localised area of vegetation, or vegetation plus soil, selected for sampling.

Standing Crop. The amount of plant or animal material per unit area at a given time than can be removed by normal methods, e.g. the standing timber volume in a forest. Cf. biomass. See Westlake 1963.

State Factors. The principal, relatively independent variables that define the state of the ecosystem: available biota, climate, parent rock, relief, ground water (including chemical composition and temperature variation resulting from geothermal activity), and the time during which development has taken place. Alternatively, relief and ground water may be considered as parts of one factor, topography.

Jenny (1946) suggests that primary water tables conditioned by geology and physiography should be distinguished from page 108 secondary or pedologic water tables that result from a given combination of climate, organisms, topography, and parent rock, acting through time.

State factors can enter functional relationships among themselves, e.g. climate on parent rock, but they may also vary independently in relation to the ecosystem. In so far as variation in one factor may affect ecosystem development it can be regarded as a controlling or trigger factor. However, it cannot be regarded as a sole cause since any property of the ecosystem is a resultant of all the factors acting together.

The functional factorial approach to ecosystem study enables the pattern of vegetation and/or soil in an area to be interpreted on a broad scale. However, this can be satisfactorily done only when precise determination of the various factors is still possible, i.e. when the systems have not been operating for any great length of time. The approach should not be confused with the more dynamic one of relating the growth of particular organisms to particular environmental phenomena (see environment).

See Major 1951, Crocker 1952, Jenny 1958.

Stratification. The manner in which animals, plants, and non-living materials are aggregated into strata within a community. Strata in the vegetation system (layers, storeys, tiers) arise from the aggregation of plant crowns, animals, and epiphytes, at particular levels as a result of inherent differences in light requirement, growth potential, life history, etc.

Strata in the soil system (horizons), apart from any inherited from the parent rock, arise through the accumulation of litter at the surface, through the action of soil processes such as leaching, eluviation, and weathering, through the development of different root systems by different plants, and through the multifarious activities of soil organisms at various depths below the surface.

Structure (in a community). The manner in which individual plants (or particular parts of plants), individual animals, populations, or primary soil particles are aggregated functionally, or in space. See pattern, stratification, trophic structure, age structure, and soil structure.

Succession. (1) The process of directional change in the physiognomy, composition, and structure of a community, usually over a number of years. The change may be due to intrinsic factors (dependent or “biotic’ factors), to a change in one or more extrinsic factors (independent or state factors), or to both.

(2) A conceptual arrangement of a series of stands according to time. Alternative terms for this second usage are sere and chronosequence.

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Territory. The area defended by an individual animal, pair, or family group against other members of the species (and sometimes against members of other species), particularly during the breeding season. Cf. home range.

Topography. The configuration of the land surface, i.e. relief, together with ground water, drainage, and other features of the earth's surface. Relief includes degree of slope, shape of slope, length of slope, aspect, and exposure. See state factors.

Transition. A change from one place to another in vegetation and/or soil. The change may be abrupt, or more or less gradual.

Trophic Level. The functional position of a group of species in a food chain (q.v.). relative to the initial source of energy, i.e. producer level, or primary, or secondary, or tertiary consumer level. It should be noted that many animal species occupy more than one tropic level.

Trophic Structure. The manner in which the total number of organisms, the biomass, or the energy fixed per unit time, is distributed in successive trophic levels (q.v.) within a community.

Vegetation. (1) The cover of plants, above and below ground, commonly but not always differentiated into layers (storeys, tiers). This is the ordinary definition in which animals, etc., are not considered part of vegetation.

(2) The system above ground, commonly but not always differentiated into layers (storeys, tiers), in which plants and animals, organic residues, and mineral matter are involved in cyclic exchanges with the soil below. Vegetation and soil, though often described separately, are parts of a larger open system, the ecosystem (q.v.).

Vegetation Layer (Storey, Tier). See stratification.

Vegetation Type. A conceptual grouping of a number of stands of vegetation, of similar physiognomy, composition, structure, etc., for purposes of description and mapping. The vegetation, as shown on a map, may include a small percentage of areas of different vegetation, too small to be shown separately. See also community type and soil type.

Weathering. The physical and chemical processes by which rocks and minerals are broken down or decomposed.

Yield. The crop (q.v.) expressed as a rate, i.e. the amount of plant or animal material that can be removed per unit area per unit time. Yield should be qualified in terms of the period over which it is averaged. Cf. productivity. See Westlake 1963.

Zone (applied to communities). One of a number of adjacent, more or less parallel community types or groups, forming a sequence where a number of factors are correlated with elevation, such as occurs at the seashore, at lake margins, and on mountains. Zones may constitute either a static pattern, as on a stable coast, or a developmental pattern, as at the edge of an infilling lake.

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The following references are either quoted in the text or are valuable as sources of information concerning ecological concepts.

Allee, W. C.; Emerson, A. E.; Park, O.; Park, T.; and Schmidt, K. P., 1949. Principles of animal ecology. Saunders, Philadelphia and London.

Andrewartha, H. G. and Birch, L. C., 1954. The distribution and abundance of animals. University of Chicago Press, Chicago.

Crocker, R. L., 1952. Soil genesis and the pedogenic factors. Quart. Rev. Biol. 27: 139-168.

DeBach, P., and Sundby, R. A., 1963. Competitive displacement between ecological homologues. Hilgardia 34: 105-166.

Elton, C. S., 1927. Animal ecology. Sidgwick and Jackson, London.

Fosberg, F. R., 1963. The ecosystem concept. In Man's place in the island ecosystem. Bishop Muesum Press, Hawaii.

Cause, G. F., 1934. The struggle for existence. Williams and Wilkins, Baltimore.

Greig-Smith, P., 1957. Quantitative plant ecology. Butterworths Scientific Publications, London.

Hardin, G., 1960. The competitive exclusion principle. Science 131: 1292-98.

Harper, J. L., 1961. Approaches to the study of plant competition. In Mechanisms in biological competition. pp. 1-39. Symposium No. 15 of the Society of Experimental Biology, Cambridge Univ. Press.

Jenny, H., 1941. Factors of soil jormation: a system of quantitative pedology. 3rd ed. McGraw-Hill, New York.

—— 1946. Arrangement of soil series and types according to functions of soil-forming factors. Soil Science 61: 375-391.

—— 1958. Role of the plant factor in the pedogenic functions. Ecology 39: 5-16.

Joffe, J. S., 1949. Pedology. 2nd ed. Somerset Press, New Jersey.

Lack, D., 1947. Darwin's finches. Cambridge Univ. Press.

—— 1954. The natural regulation of animal numbers. Oxford Univ. Press.

Macfadyen, A., 1957. Animal ecology. Sir Isaac Pitman and Sons, London.

Macfadyen, A., and Newbould, P. J., 1964 (Editors). British Ecological Society Jubilee Symposium. Blackwell Scientific Publications, Oxford.

Major, J., 1951. A fundamental, factorial approach to plant ecology. Ecology 32: 392-412.

Mason, H. L., and Langenheim, J. H., 1957. Language analysis and the concept environment. Ecology 38: 325-340.

—— 1961. Natural selection as an ecological concept. Ecology 42: 158.

Odum, E. P. and Odum H. T., 1959. Fundamentals of ecology. 2nd ed. Saunders, Philadelphia and London.

Odum, E. P., 1963. Ecology. Holt, Rinehart and Winston, New York.

Odum, H. T., 1964. Review: A symposium on net production of terrestrial communities. Ecology 45: 415-416.

Ovington, J. D., 1962. Quantitative ecology and the woodland ecosystem concept. Advances in ecological research 1: 103-192.

Taylor, N. H. and Pohlen, I. J., 1962. Soil survey method. N.Z. Soil Bur. Bull. 25.

Urvardy, M. F. D., 1959. Notes on the ecological concepts of habitat, biotope and niche. Ecology 40: 725-728.

Watson, D. J., 1947. Comparative physiological studies on the growth of field crops. Ann. Bot., Land. 11: 41-76.

Westlake, D. F., 1963. Comparisons of plant productivity. Biol. Rev. 38: 385-425.

Whittaker, R. H., 1956. Vegetation of the Great Smoky Mountains. Ecol. Monogr. 26: 1-80.

1 Botany Division, D.S.I.R., Taita.

2 Zoology Department, University of Auckland.

3 Botany Division, D.S.I.R., Taita.