Showing papers in "Journal of Ecology in 1961"

Segregation and Symmetry in Two-Species Populations as Studied by Nearest- Neighbour Relationships

Abstract: Much work has been done in recent years on the spatial patterns of natural populations of plants, and it has been customary to study one species only at a time. Suppose, however, an investigator were concerned with a population containing two co-dominant species. Then, besides determining the density and spatial pattern of each of the codominant species considered separately it would also be interesting to know how the individuals of one species were arranged with respect to the individuals of the other species. The two species could be described as 'unsegregated' if any individual plant of one of the species was as likely to be found growing close to members of the other species as to members of its own species. Conversely, the two species could be described as 'segregated' if they had a tendency, however slight, to occur in one-species clumps, so that any individual was more likely to be found near members of its own species than near members of the other. In studying the relationship between two species growing in the same area it has been usual to sample the area with quadrats and then determine whether the number of joint occurrences of the two species in the same quadrat was significantly greater (or less)) than chance expectation; if so, the two species were said to be positively (or negatively) associated. However, as Greig-Smith (1957, Ch. 4) has shown, the result obtained inevitably depends on the size of quadrat used. It is thus meaningless to assert that a pair of species is, say, positively associated, unless at the same time one specifies at what scale this positive association manifests itself. Nevertheless, the degree to which the individuals in a two-species population are, colloquially speaking, 'mingled together' is clearly an intrinsic property of the whole population, and one independent of scale. Furthermore, it should be possible to measure this property in such a way that the result obtained is independent of any arbitrary characteristic (such as quadrat size), of the sampling method. It is for this purpose that the concept of segregation, as described in the previous paragraph, is proposed. The smaller the degree of 'mingled-togetherness' of a pair of species, the greater will be their degree of segregation. A population of two species, A and B, may exhibit two entirely different types of pattern, as shown in Fig. 1. One type of pattern, the 'unsegregated' type, is shown in Figs. la and lb. In Fig. la the two species are present as two co-extensive random populations. In Fig. lb the population as a whole is clumped, but within every clump the individuals of species A and the individuals of species B are present in the same proportions and are arranged independently of each other. Figs. lc, 1d, le and If show examples of the second, or 'segregated', type of pattern.

A Reconsideration of Grassland Classification in the Northern Great Plains of North America

Abstract: INTRODUCTION . 135 DESCRIPTION OF THE AREA 137 CONCEPTS OF VEGETATION ANALYSIS 141 LOCATION OF STUDY AREAS . . . . . . . . . 144 CLIMAX COMMUNITIES . . . . . . . . 145 Fescue Prairie Association . . . . 145 Mixed Prairie Association . . . . . . . 147 Stipa-Agropyron Faciation . . . . . . 147 Stipa-Bouteloua-Agropyron Faciation . . . . . . 150 Stipa-Bouteloua Faciation . . . . . . . 152 Bouteloua-Agropyron Faciation . . . . . . 153 Agropyron-Koeleria Faciation . . . . . . 155 FORBS AND SHRUBS . . . . . . . . . 156 COMPARISON WITH PREVIOUS CLASSIFICATIONS . . . 158 SUCCESSIONAL COMMUNITIES . . . . . . 159 Primary successions . . . . . . . . 160 Secondary successions . . . . . . . . 163 RELATION TO COMMUNITIES IN THE NORTHERN UNITED STATES. . . . 164 SUMMARY . . . . . . . . . 165

Adaptation to Habitat in a Group of Annual Plants

Abstract: Clapham (1946) has emphasized that the persistence of a species at a given site is dependent upon the interaction of plant and environment throughout the whole of the life-cycle, at the stages of germination, establishment, vegetative growth, flowering and the setting of viable seeds. He points out that the elucidation of the way in which a species thus interacts with its environment can most profitably be investigated by experimental methods. This work represents an attempt to investigate experimentally, using relatively simple apparatus, the basis of adaptation to a rather extreme habitat in a well-defined group of annuals on the Derbyshire limestone. As annuals, their survival in any locality depends upon the successful accomplishment every year of all the above stages whatever the seasonal and yearly variations of climate. Despite this variation the life-cycles are remarkably similar and constant: germination invariably takes place in the early autumn, usually in midor late September; vegetative growth then proceeds until some time in December and with the onset of more favourable conditions in the spring, flowering occurs; seeds are shed in May or June but no germination takes place until the following autumn. As will be seen later, this life-cycle fits rather well into the local climatic pattern, but its significance and the way in which it is controlled was not at all obvious. The habitat is very clearly defined as the species are only to be found on shallow soil in open, sunny situations. On the Derbyshire limestone these conditions are provided in abundance on rock ledges, screes, etc., with a more or less southerly aspect where the annuals in question form a distinct community. Sociologically, therefore, the situation is very simple, particularly as competition between individuals or species is almost nonexistent. The following species may be included in the community, varying somewhat in abundance:

Stratigraphic Evidence of Regeneration in Some Irish Bogs

Abstract: The processes by which the mosaics of vegetation and microtopography on raised bog surfaces have originated and are maintained have frequently excited speculation (Gorham 1957; Godwin & Conway 1939; Osvald 1923, 1937, 1949; Pearsall 1956; Ratcliffe & Walker 1958; Tansley 1939). In very general terms opinion is divided between those who see the pool-and-hummock complex as a balanced and self-perpetuating regeneration mechanism and those who regard the pools as erosion features of a bog which, if actively growing, would have a continuous vegetation cover. The importance of peat stratigraphy as a record of the processes which have taken place during the history of a bog has been recognized (Conway 1948, Godwin 1946, Osvald 1923, Tansley 1939) but examples of the detailed studies required are rare and inconclusive (Ratcliffe & Walker 1958). The aim of this work, carried out in Ireland during the summer of 1959, was to collect the most detailed observations practicable in the field on the recent ecological history of a number of bogs as revealed by the stratigraphy of the upper, post'Grenz', peats.

On e. c. pielou's index of non-randomness

Abstract: Using the information supplied by a sample of random quadrat counts and by a second sample of the distances measured from random points to the nearest plants, Pielou (1959) has devised a valuable index of non-randomness of the distribution of a plant population. The index can be used to test for a departure of a plant distribution from randomness, and also, in the case of a non-random distribution, as a measure of the amount of non-randomness in the distribution. In her derivation of the sampling distribution of her index, Pielou considers only the sampling errors of the point-to-plant distances and ignores the sampling errors in the sample of quadrat counts. In this note allowance is made for the errors of both samples in the derivation of the sampling distribution of Pielou's index. EXPECTED VALUE AND VARIANCE OF PIELOU'S INDEX Pielou's index is calculated from the information supplied by a sample of point-tonearest-plant distances and a second sample of quadrat counts. The first sample is obtained by selecting n random points within the sample area and measuring the distance from each of these points to its nearest plant. Pielou's index includes the quantity Co, the n

The Ecology of Juncus Effusus L. in North Wales

Abstract: PAGE INTRODUCTION 83 THE CLASSIFICATION OF J. EFFUSUS COMMUNITIES . 83 THE RELATION OF FEATURES OF THE HABITAT TO THE CLASSIFICATION OF COMMUNITIES . . 89 THE GROWTH RATE OF J. EFFUSUS 91 THE ECOLOGY OF J. EFFUSUS WITH RESPECT TO ITS GROWTH FORMS 91 The clumped habit 91 (a) A comparison of the flora inside and outside clumps 91 (b) The pattern of clumps . 96 The unclumped habit . 97 Pattern in the unclumped community. . . . . . . 98 DISCUSSION . . . . . . . . . . . 98 The species diagram . . . . . . . 98 The role of J. effusus . . . . . . . . . . 100 SUMMARY . . . . . . . . . . . 101 REFERENCES . . . . . . . . . . . 101

The ecology of lough ine x. the hydroid sertularia operculata (l.) and its associated flora and fauna: effects of transference to sheltered water

Abstract: The hydroid Sertularia operculata is abundant on boulders where the current is strong in the Lough Ine Rapids, especially on the Sill. It decreases in abundance towards positions sheltered from the current (Lilley et al. 1953), and has not been found in Codium Bay which lies just inside the lough. Codium Bay is fully sheltered from the powerful inflow stream; nevertheless there is a slow drift of water across it, always from west to east. The drift is maintained during outflow because water is sucked from all directions into the rapids and during inflow because of the anti-clockwise vortex which then occupies the western half of the South Basin (Bassindale et al. 1956). In this paper we are concerned with the restriction of Sertularia to places exposed to strong current. We have investigated this by observing the fate of living Sertularia on boulders transferred to Codium Bay. During the course of our observations we have also studied the changes in the flora and fauna living on and among the Sertularia, as these organisms might be rather closely implicated.

Alteration of the breeding habitat by sea-birds and seals in western australia

Abstract: The principles discussed in this paper apply over a wide area of temperate Australia and New Zealand and are concerned with the degeneration and regeneration of plant communities associated with fluctuating populations of sea-birds and seals. Actual examples are drawn from among twenty-three islands visited in the Indian Ocean off south-west Australia. These islands are scattered along approximately 500 miles (800 km) of coastline from Cape Leeuwin in the extreme south-west of the continent northwards to the coral islands of the Abrolhos Group in latitude 28? S. Their names and relative positions are shown on the accompanying map. The two southernmost islands are composed of granitic gneiss, one with a limestone capping. The three northernmost are composed largely of coralline limestone, the remainder of consolidated dune limestone or aeolianite. The larger of the limestone islands carry deposits of unchanged dune sand and the gneiss is partially covered by shell grit so that soils on all islands are calcareous. In size they range from a few square chains to several thousand acres, but only portions of the larger islands are occupied by nesting sea-birds. Ecological accounts of the various groups are in press or in course of preparation (Gillham and Storr, 1960, 1961); also McArthur (1957). The indigenous plants vary considerably in their tolerance of manuring and trampling by colonial sea-birds, and radical changes in the floristic composition of the vegetation are effected in the rookeries. Birds which nest on the ground surface or on top of the low maritime scrub are the most inimical to the indigenous plants, but burrowing birds also have a selective influence. The principal surface nesters (nomenclature according to Serventy & Whittell (1948)) are large pied cormorant (Phalacrocorax varius), silver gull (Larus novae-hollandiae), * Working in conjunction with the C.S.I.R.O. Wildlife Survey Section.

The Ecology of Lough Ine: IX. The Flora and Fauna Associated with Undergrowth-forming Algae in the Rapids Area

Abstract: at the north and south ends of the Rapids the currents do not exceed about 01 m/sec (Bassindale et al. 1948). Organisms inhabiting the fronds of the large brown algae (Saccorhiza polyschides and Laminaria digitata) are exposed to the current, but those inhabiting sub-littoral boulders or small algae are partly sheltered by the overlying canopy of fronds, which forms a slippage plane (Kitching et al. 1952). Nevertheless there are striking differences in the fauna of boulders off Mark 7 and from more sheltered regions (Lilly et al. 1953), which are probably to be explained in part by secondary influences of the current, such as the control of sedimentation. For this work seven stations were chosen to provide a comparison between the Lough (Codium Bay and Renouf's Bay), the Rapids (Stations Nos. 1, 3 and 5), and Barloge