Showing papers in "Journal of Ecology in 1976"

Dynamic structure of wave-regenerated Abies balsamea forests in the north-eastern United States.

Abstract: In 1947 A. S. Watt began his presidential address to the British Ecological Society by saying, 'It is now half a century since the study of ecology was injected with the dynamic concept, yet in the vast output of literature stimulated by it, there is no record of an attempt to apply dynamic principles to the elucidation of the plant community itself and to formulate laws according to which it maintains and regenerates itself.' The intervening decades have not changed the situation much; except for Watt's own paper and a very few others, this field is still largely unexplored. The main difference is that advances in nearly all other areas of ecology have rendered our ignorance about long-term ecosystem dynamics even more striking by contrast. Despite the fact that the concepts of succession and climax have been among the central themes of plant ecology since the late nineteenth century, our knowledge of how ecosystems vary and maintain themselves through time has increased only slightly in the last seventy-five years. Much of this neglect has been due to the rather oppressive influence on the minds of many ecologists of the notion of 'climax'. For years, it was generally assumed that in the absence of disturbance the vegetation on any site would eventually reach a self-reproducing steady-state equilibrium, in which all system properties would be relatively constant through time (cf. Clements 1936). Although discussion of this point focused mainly on species composition, it would seem to follow that general system properties such as net primary productivity, decomposition rates, and soil chemistry would also be constant in such a stable system. In many cases this theoretically stable 'climax' vegetation-type came to be thought of as the 'natural' vegetation for the area; the idea that the frequency of natural disturbance might make this concept of 'stable climax' a totally unrealistic model for natural ecosystems was rarely considered. Thus, ecologists spent much time and effort searching for, describing, and classifying 'climax' ecosystems in this special sense, even though there was often little or no evidence that stable systems of this sort would ever come into existence under natural conditions. In fact, many studies have indicated that natural disturbance plays a far more vital role in ecosystem dynamics than that attributed to it by the classic climax theory. Cooper (1913) found that the native vegetation of Isle Royale was not the homogeneous, all-aged stand that would have been predicted by the classic theory, but rather an irregular matrix of small wind-throw areas of various ages, with regeneration generally restricted to recent wind-throws. It was, he said, 'a mosaic or patchwork which is in a

The depletion of carbon dioxide from lake water by phytoplankton

Abstract: It has long been known that dense populations of phytoplankton tend to deplete the carbon dioxide present in natural waters, with an associated raising of pH. The ecological significance of these reactions, as possible growth-limiting or retarding factors, is far from clear. This situation is remarkable in view of the intensive study given to the relationship between rates of photosynthesis and carbon dioxide supply in some unicellular algae, the numerous measurements of carbon assimilation by phytoplankton in nature, and the attention bestowed on pH as an ecological factor. Recently a controversy has developed around the relative importance of carbon and phosphorus supply in the enrichment ('eutrophication') of inland waters (e.g. Kuentzel 1969, 1971; King 1970; Kerr, Paris & Brockway 1970; Kerr et al. 1972; Schindler 1971; Schindler et al. 1973; Goldmann et al. 1972; Goldman, Oswald & Jenkins 1974), partly sustained by a shortage of detailed information on the dynamics and availability of inorganic carbon in freshwaters. In particular, the exchange of carbon dioxide between atmosphere and water is rarely estimated, the interrelations of various forms of carbon dioxide are rarely assessed, analytical methods are usually unspecific, and the relevant physiological responses of characteristic common species of freshwater phytoplankton are largely unknown. The present work developed from observations on depletions of carbon dioxide and upward pH shifts, occasionally very pronounced, in two English lake waters. Examples are described in relation to the algal crops involved and the stratification of environmental factors. The general limnological background of these environmental events, including details of diurnal changes, will be described more fully elsewhere. Further analysis is based upon systematic laboratory measurements of photosynthetic activity in relation to episodes of algal production in nature, extended to experimental manipulations of media, population density, and time-sequences of carbon dioxide consumption. As the physiological work is based upon natural populations of known history, it is hoped that ecological inferences can be drawn without undue extrapolation between the properties of different species and different media.

The Effects of Different Forms of Land Use on the Ecology of a Semi-Arid Region in South-Eastern Rhodesia

Abstract: Natural vegetation in the drier parts of Africa probably evolved under light utilization by shifting populations of indigenous herbivores. The pattern and intensity of utilization have changed since the introduction of domestic livestock although, prior to colonization by the white man, utilization remained light because both human and domestic animal populations were small and the African tribesmen were nomadic (Maloiy & Heady 1965). The recent rapid increase in human and livestock populations (Casebeer 1968; Hornby 1968) has led to serious and extensive damage over large tracts of land. In Rhodesia, over-utilization of vegetation began shortly after 1914 and by 1966 some 490% of the Tribal Trust Lands (where there is communal ownership of land) were either overgrazed or had very little herbaceous cover (Cleghorn 1966). Some of the most badly degraded of these areas are in the south-eastern 'lowveld' of Rhodesia. The term 'lowveld' denotes the hot, semi-arid regions of tree/bush savanna, below an altitude of c. 800 m, in which extensive livestock ranching is the main form of land use. The natural vegetation is generally the only source of food for livestock and, by virtue of its cover, it is vital for the conservation of soil and water resources. The area is too dry for planted pastures and improvement of the protective cover and forage value of the range must be achieved by manipulating the natural vegetation. The aim of this study, made in 1970-2, was to determine the composition, trends in composition and primary productivity of an area of lowveld vegetation in which different forms of land use were being applied, and to establish key factors in its ecology. The emphasis has been on overall interrelationships of the vegetation, rather than on detailed studies of particular aspects. In order to reduce inter-site variability, the study was restricted to basalt-derived soils.

Foraging behavior of solitary bees : Implications for outcrossing of a neotropical forest tree species

Abstract: Most flowering plants of neotropical forests display adaptations for animal pollination (see references cited by Frankie, Baker & Opler (1974a) and by Frankie (1975, 1976)), but, since little information is available on the foraging behaviour of the animals involved, there has been considerable speculation as to whether widely spaced, conspecific plants in these forests are selfor cross-pollinated (Corner 1954; Baker 1959; Fedorov 1966; Ashton 1969; Bawa 1974). Bees constitute one of the major groups of visitors to a large proportion of tree species occurring in certain lowland forests (Janzen 1967; Frankie 1975). The potential for interplant movement of bees in Central America has been emphasized by Janzen (1971, 1974), who found that certain euglossine bees in Lowland Wet forests may forage over distances of several kilometres. Janzen has also suggested that members of other bee genera (e.g. Bombus, Centris, Ptiloglossa and Xylocopa) may forage over similar distances. Experimental studies on the nature of breeding systems of tree species in a Costa Rican Dry forest indicate that of the tested species (many of which are bee-pollinated), most have obligate outbreeding systems (Bawa 1974). Bawa's findings strongly imply that bees and other anthophilous animals effect pollination through inter-plant movement. However, patterns of pollinator foraging behaviour in relation to breeding systems have yet to be experimentally investigated for any neotropical plant species. In this paper experimental data are presented on the breeding system of a papilionaceous tree, Andira inermis (Swartz) H.B.K., and on the movements of its pollinators. Early experiments (detailed below) showed that A. inermis is self-incompatible. The flowers show clear adaptation for pollination by bees. The specific questions asked in this study were as follows. (i) What kinds of bees visit Andira flowers and what are their patterns of inter-tree movements ?

The Ecotone Between Spartina Foliosa Trin. and Salicornia Virginica L. in Salt Marshes of Northern San Francisco Bay: II. Soil Water and Salinity

Abstract: Soil salinity has received more attention than any other environmental factor considered to influence plant zonation in tidal salt marshes. The word 'salinity' implies a concentration of total salts, NaCl, Cl-, or other specific salt ions in solution. Depending upon methods of measurement, 'salinity' has been given other meanings such as the amount of Clper unit dry weight of soil. Wide temporal variations in salt marsh soil salinities also make comparisons of data in the literature difficult. In general literature on tidal salt marshes in Britain and other parts of Europe (Chapman 1939, 1960; Gillham 1957a, b; Adriani 1958), and in Australia (Clarke & Hannon 1969) indicates that soil salinity increases landward to a maximum at or just above mean high water (MHW) and then gradually decreases. Some workers have found similar salinity gradients in North Americah east coast marshes (Steiner 1934; Kurz & Wagner 1957), but others have measured a gradual decline in salinity from the most seaward edge of the marsh (Ganong 1903; Penfound & Hathaway 1938; Reed 1947). The few soil salinity measurements made in marshes on the Pacific coast of North America indicate that Salicornia virginica L. tends to grow in more saline places than other salt marsh species (Purer 1942; Mall 1969). Past studies have been hampered in achieving good seasonal or yearly evaluations of soil salinities because scattered, unpredictable, year-round rainfall may produce wide variations in soil salinities from day to day. Also, variations in soil composition may further complicate salinity gradients by altering soil moisture and salt retention characteristics. Finally, there has been a general lack of good statistical sampling of salt marsh soil salinities.

Life history evolution in seven milkweeds of the genus Asclepias.

Abstract: The demographic characteristics of plants and animals have attracted the widespread attention of evolutionary ecologists only in the last decade, in spite of the early theoretical work of Cole (1954), the empirical work of Salisbury (1942) and the obvious close relationship between features of the life history and Darwinian fitness. Recent discussions of life history evolution, e.g. those of Gadgil & Solbrig (1972), Pianka (1972) and Southwood et al. (1974), have focused on competition as an agent of natural selection and have used the terminology of r-selection and K-selection introduced by MacArthur & Wilson (1967). Wilbur, Tinkle & Collins (1974) argue that a discussion of the selective effects of competition alone cannot explain the diversity of life history patterns in several wellstudied species of plants and animals and they advocate the consideration of predation and environmental uncertainty in addition to competition. The purpose of this paper is to illustrate the arguments of Wilbur et al. by an empirical study of the life histories of seven species of milkweeds (Asclepias spp.). First the species are compared with respect to growth form, seed number and seed size. These life history adaptations are then correlated with independently measured indices of the effects of competition, predation and environmental uncertainty on populations of each species in south-east Michigan. This study is based on the premise that reproductive adaptations represent evolutionary adjustments of life history characteristics such that the lifetime contribution of a plant to the next generation is maximal. Milkweeds provide excellent material for the study of evolutionary ecology. Asclepias is a widespread genus with 108 species in nine subgenera in North America and the Antilles (Woodson 1954). All species which have been studied are isoploid (Moore 1946) and have only a low level of self-fertility (R. Wyatt, personal communication). Pollination is effected by large insects, primarily hymenopterans and lepidopterans (F. C. Evans, personal communication), which transfer pollinia (Macior 1965; Willson & Rathcke 1974). Most species have narrow microhabitat tolerances, are perennial and do not propagate vegetatively. Seeds are large compared to other forbs of meadows and wood margins (Stevens 1932; Salisbury 1942); each seed has a tuft of comose hairs which assist in dispersal by wind and, in the case of A. incarnata L., by water. All of the species produce alkaloid compounds (Punyarajun 1965), which deter herbivorous insects, are distasteful to livestock (Whiting 1943), and include cardiac glycoscides which are poisonous to most vertebrates (Parsons 1965; Reichstein et al. 1968; Duffey 1970). Milkweeds are occasionally eaten by deer or by a small number of insect species (Table 1), most of which are aposematic, toxic and have a history of co-

Changes and Variability in the Field Layer of a Coppiced Woodland in Norfolk, England

Abstract: Drury & Nisbet (1973) suggest that a wide range of studies needs to be carried out to clarify what mechanisms control species replacement in different ecological situations. The literature on deciduous woodland succession in lowland England is surprisingly small. Successional patterns are described in the pioneer work of Salisbury (1916) and in that of Adamson (1921, 1932), Watt (1924) and Tansley (1939). Watt (1947) drew attention to the long-term cyclical changes involved in patchy regeneration of undisturbed beech woodland. More recently Horn (1971, 1975) has presented data which throw light on the mechanisms involved in replacement of species in the succession of trees in temperate woodland in general. However, there is a lack of information on the processes involved in field layer changes as a woodland canopy develops through time.

Vegetation Structure in the Mediterranean Scrub Communities of California and Chile

Abstract: One successful approach towards refining the form-environment interaction in natural communities has been to examine the extent of convergence in structural and functional attributes of phylogenetically distinct biotas of climatically similar areas. This approach has been used in tropical areas (Richards 1952; Sarmiento 1972), in semideserts (Riley & Young 1966) and in the areas of Mediterranean climate (Naveh 1967; Specht 1969). The latter have long been cited as examples of evolutionary convergence (Grisebach 1872). Characterized by hot dry summers and cool moist winters, they are commonly located at middle latitudes on the west coasts of continents (specifically the west coasts of California, central Chile, South Africa, southern Australia and the Mediterranean Basin itself). These areas of similar climate display distinct biotas. The coastal mountains of southern California and central Chile were chosen as

An Investigation of Shoot Interactions in Mercurialis Perennis L., A Rhizomatous Perennial Herb

Abstract: Most investigations into the development of plant populations have used annual species grown either under partly controlled field conditions or in glasshouses. Such an approach may enable the effect of single variables upon the population to be investigated and overcomes the problems of defining the individual when vegetative reproduction occurs. However, it is well known that extrapolation to field conditions of results from such experimental populations can be very misleading. In particular, in the case of vegetatively reproducing perennial species, the identity of the basic population unit is obscure when adult plants and their rametst are growing together. Structures such as shoots or tillers which are interconnected may not function independently. Integration has been demonstrated, for example, between tillers of plants of Lolium multiflorumt even though each appears to be capable of independent status in terms of carbon economy (Marshall & Sagar 1968). The same has been shown for developing plants of wheat (Quinlan & Sagar 1962). Sonneveld (1962) and Struik (1965) have outlined schemes of translocation and redistribution of assimilates which imply integration and interdependence between different parts of plants at different times during their growth. Despite this, the tiller, grass tuft or herb shoot has often been arbitrarily chosen as the basic unit in plant ecological studies, particularly those concerned with distribution of particular species within stands of vegetation (e.g. Whitford 1949). Clearly, direct analogy is not possible between these units and individual plants in experimental populations. If tillers, tufts or shoots are not physiologically independent entities, it is important to determine the role they play in the development of a population. It is the purpose of this study to determine the extent to which shoots of the woodland perennial herb Mercurialis perennis growing in natural stands are comparable with discrete units in populations. At maturity, a dense, pure stand of M. perennis forms a virtually closed herb canopy with few gaps through which light can penetrate directly to the ground. This complete ground cover develops early in the growing season and is important in excluding competitor plants from areas dominated by M. perennis (Wardle 1959; Pigott & Martin 1964; Wilson 1968). In addition, M. perennis does not exhibit clonal senescence over

Density-dependent seed germination strategies in colonizing versus non-colonizing plant species.

Abstract: Germination is a critical stage in the life of a plant. While much is known about the physiological processes which occur at that time, the adaptive aspects of germination strategies have received much less attention (Kozlowski 1972; Heydecker 1973). However, population biologists are aware of the importance of these strategies and have emphasized their critical role in the dynamics of plant populations (Harper 1965; Cohen 1967). This study deals with an aspect of germination which has to do with both physiology and ecological strategy: how the rate of seed germination is affected by clustering of seeds into dense arrays, and the relationship between this response to density and the propensity of a species to be colonizing or weedy as opposed to being a member of an indigenous, closed plant community. A few reports on density-dependent percentages and/or rates of germination are scattered in the literature, e.g. for pollen (Brewbaker & Majumder 1961), fungus spores (Toth 1973) and seed (Ballard 1958; Palmblad 1968). The responses of germination to greater density have included both increases (Ballard 1958; Brewbaker & Majumder 1961; Linhart & Pickett 1973; Toth 1973) anct decreases (Palmblad 1968; Toth 1973). However, comprehensive reviews of seed biology (Kozlowski 1972) and seed ecology (Heydecker 1973) do not discuss this phenomenon.

Distribution of Chromium and Nickel in Plants and Soil from Serpentine and Other Sites

Abstract: Serpentine soils in various countries contain high levels of total chromium and nickel, and these are considered by some authors to be at least partly responsible for the characteristic infertility of the soil and paucity of vegetation (Robinson, Edgington & Byers 1935; Figiwara & Kikuchi 1950; Lounamaa 1956; Soane & Saunder 1959; Fernandez, Taboadela & Ojea 1965; Paribok & Alexeyeva-Popova 1966; Menezes de Sequeira 1968). Evidence which supports this conclusion can be of three types. First, foliar symptoms in crop plants such as oats grown in pot or field trials have been used to demonstrate toxicity, especially of nickel (Hunter & Vergnano 1953; Soane & Saunder 1959; Anderson, Meyer & Mayer 1973). Secondly, studies of the availability of chromium and nickel in the soil have been reported using a range of extractants, such as dilute acetic acid or ammonium acetate (Robinson et al. 1935; Soane & Saunder 1959; Spence & Millar 1963; Menezes de Sequeira 1968; Anderson et al. 1973). Generally it has been found that the level of available chromium is very low, often below the detection limit, whereas that of available nickel is much higher. Thirdly, there are reports of very high total contents of these elements in the indigenous plants (Lyon et al. 1968; Wild 1970; Lyon et al. 1971; Ritter-Studnicka & Dursun-Grom 1973; Brooks, Lee & Jaffre 1974; Wild 1974a,b). In view of the low level of availakble chromium and high level of available nickel, it would be expected that plants should contain higher concentrations of nickel than of chromium; in many species this is true, but exceptions can be found. Certainly marked differences between species in the mean element concentrations are found at particular sites (Lyon et al. 1968, 1971; Wild 1974b). In this paper we present data on the concentrations of available and total chromium and nickel in certain serpentine soils in Scotland and Austria, together with data on the concentrations of these elements in a range of indigenous species. We have already reported on the calcium and magnesium concentrations in the same soils and plants (Shewry & Peterson -1975). In this paper we also present the results of extracting chromium and nickel from the soils with a range of solutions and the results of studies on the binding and interconversion of 51Cr3+ and 5 -CrO4'in both serpentine and nonserpentine soils. Inter-element correlations within the soil and within plant species are also discussed. Extensive accounts of the vegetation on the serpentine soils in the Scottish areas concerned have been given by Spence (1957, 1959, 1970) and Proctor & Woodell (1971), while relevant experimental studies have been reported by Proctor (1971a,b).