Showing papers in "Journal of Animal Ecology in 1977"

Habitat, the template for ecological strategies?

Abstract: The very etymology of Ecology, from the greek 'Qikos', 'the household', implies that ecologists should devote some attention to the 'house' or habitat of the population or community they are studying. However, as Charles Elton (1966) has so forcibly pointed out, 'definition of habitats, or rather lack of it, is one of the chief blind spots in Zoology'. Elton himself has provided us with a qualitative classification of habitats, while another past President, Alex Watt (1947) highlighted the dynamic nature of habitats by his phrase, 'pattern and process'. Elton referred to the need to quantify habitat characteristics. In this Address I will attempt some quantification;however, you will all be aware that in doing this I will not be able to emulate those former Presidents who have been able to provide a definitative synthesis of a field or of their own studies, my offering can be but a small beginning, an indication of the type of characteristics we should quantify. In considering ecosystem patterns and environment R. M. May (1974) writes 'it is to be emphasized that although patterns may underlie the rich and varied tapestry of the natural world, there is no single simple pattern. Theories must be pluralistic'. Indeed, the complexity of the subject is daunting and in any attempt to formulate some type of general framework, one is continually beset with exceptions. In stressing the need for a framework I am echoing a plea of my predecessor Amyan Macfadyen (1975) who cited K. E. F. Watt's (1971) vivid image 'if we do not develop a strong theoretical core that will bring all parts of ecology back together we shall all be washed out to sea in an immense tide of unrelated information'. In some ways I think we may see ourselves at a similar point to the inorganic chemist before the development of the periodic table; then he could not predict, for example, how soluble a particular sulphate would be, or what was the likelihood of a particular reaction occurring. Each fact had to be discovered for itself and each must be remembered in isolation. It is noteworthy that from Dobereiner's early efforts in 1816 it took more than fifty years before Mendeleeff ormulated his Periodic Law (1869) and even after this there were various attempts at rearrangement. Another parallel may be drawn with astronomy before the development of the Hertzsprung-Russell diagram that relates the evolution and the properties of stars. Again in our own subject biology, the situation is somewhat analagous to that before the formulation of the Linnean system of classification; but now from this system of classification, we are able to organize our knowledge of, for example, the functional morphology of organisms and we can even make assumptions, with a high probability

An Ecological Theory on Foraging Time and Energetics and Choice of Optimal Food-Searching Method

Abstract: In the past few years there has been much theoretical work on optimal foraging behaviour in animals. It relates to the assumption that animals tend to optimize their feeding activities. Reviews appear in Schoener (1971), MacArthur (1972), Emlen (1973) and Pianka (1974). Interest has been paid particularly to optimal choice of diet for predators with a choice among different prey types (Charnov 1976a; Emlen 1966, 1968, 1975; MacArthur 1972; MacArthur & Pianka 1966; Marten 1973; Pulliam 1974; Rapport 1971; Schoener 1969a, 1969b, 1971, 1974), to strategies of movement and to exploitation of different habitat patches (Charnov 1976b; Emlen 1973; Krebs, Ryan & Charnov 1974; MacArthur & Pianka 1966; Royama 1970; Schoener 1974; Smith 1974a, b; Smith & Dawkins 1971; Tullock 1970). Interest has also been focused on relations between various features of morphology, energetics of foraging, and feeding strategies (e.g. Feinsinger & Chaplin 1975; Gill & Wolf 1975; Hainsworth & Wolf 1972a, b, 1975; Heinrich & Raven 1972; Wolf, Hainsworth & Stiles 1972; Wolf, Stiles & Hainsworth 1976). Various components of the time expenditure for foraging were considered by Holling (1965, 1968) in constructing models on functional response of predators to density of their prey (see also the recent reviews by Hassell, Lawton & Beddington (1976) and Beddington, Hassell & Lawton (1976)). The present approach differs from those of earlier papers in that particular emphasis is laid here on the energy cost of locomotion during foraging and its effect on the foraging time. One possible goal is to construct a general mathematical model of foraging time and energetics. The moment an animal starts to move about in search of food to cover its energy needs, it suffers an extra energy drainage that must also be replaced. Thus, for example, the energy cost of locomotion for foraging of some birds may readily come up to 5-10 times the basal metabolic rate. Luckily, the energetic cost of foraging is usually easily outweighed by the energy gains. However, this is not always necessarily so. When food density decreases, an animal has to spend progressively more time searching for food, and it may at times have difficulty finding enough time to cover its energy budget. If it were to continue with the same foraging technique it would eventually find it difficult even to make foraging gains catch up with energy losses due to foraging activities, let alone provide also the energy required for non-foraging periods. In this paper, I want to assess the consequences to an animal of the energy and time expenditures for foraging. Further, I will explore whether or not an animal may gain a selective advantage by employing different search methods in different seasons in a fluctuating environment. More precisely, I look for answers to the following main questions.

Movement patterns and egg distribution in cabbage butterflies

Abstract: variety of movement patterns which would have produced very diverse spatial distributions in other circumstances (see Appendix). To predict the outcomes of movement in all the environments which a species normally encounters, the rules of movement for individual animals must be known, and to understand the biological significance of those rules, it is necessary to know how the resultant distributions affect survival and reproduction. This paper describes movement and egglaying of female cabbage white butterflies, Pieris rapae L., in Canada and Australia. P. rapae was chosen because individual females are conspicuous and easily tracked; their objectives (oviposition sites and nectar) are known; their success in finding those objectives can be observed; and the fate of different distributions and densities of eggs and larvae can be studied in the field. The behavioural descriptions are built into simulation models. These solve two methodological problems: defining what measurements will adequately describe the behaviour, and putting those measurements together to predict searching success in any particular circumstances. If the behaviour model is realistic, the parameters define its important properties, and the model itself becomes a tool to predict and explain egg distributions.

On the inadequacy of simple models of mutual interference for parasitism and predation

Abstract: Mutual interference between searching arthropod parasites (i.e. parasitoids) is a beguilingly simple process, with apparently far-reaching implications for the stability of parasite-host interactions. The widely held view is that as parasite density increases, behavioural interactions (mutual interference) between searching individuals cause a reduction in the parasites' searching efficiency. In consequence, interference tends to confer stability on parasite-host interactions by reducing the death rate per parasite imposed on the hosts (and so the recruitment of parasites in the next generation) when parasite densities are high. The stabilizing effects of interference have recently been incorporated into a wide variety of ecological models (e.g. Austin & Cook 1974; Bulmer 1977; De Angelis, Goldstein & O'Neill 1975; Hassell & May 1973; Rogers & Hubbard 1974). It is often assumed (e.g. Beddington 1975) that similar considerations apply to predator-prey interactions. This paper is concerned primarily with parasitism, but possible extensions to predation are considered in the Discussion. The paper is divided into three sections. First, we re-examine the empirical model of interference proposed by Hassell & Varley (1969). This model postulates a linear relationship in logarithmic form between parasites' searching efficiency and density, with the degree of mutual interference given by the gradient. The model is already known to be oversimplified in one important respect. Thus, both a priori considerations (Royama 1971) and more detailed behavioural models (Beddington 1975; Rogers & Hassell 1974) require that the relationship should be curvilinear, with interference becoming negligible at low parasite densities. We later show that the behavioural models themselves indicate pitfalls in extrapolating from laboratory experiments conducted at higher parasite densities than prevail in the field. The second part, dealing with a host-parasite model written in difference equations, suggests that behavioural considerations largely preclude the possibility of significant mutual interference when the parasite is at or close to its equilibrium density. The behavioural models discussed assume that parasites search randomly. This assumption is relaxed in the last section. The implications of this change are that due to differential exploitation of the richer patches, searching efficiency measured over the whole environment is found to decline with parasite density (indicating interference) when there are no behavioural interactions. We call this effect 'pseudo-interference'. Accordingly we argue that 'interference' in the field can arise from both behavioural interactions and the effect of non-random search for aggregated hosts.

Predation by Weasels (Mustela nivalis) on Breeding Tits (Parus Spp.) in Relation to the Density of Tits and Rodents

Abstract: Weasels are usually thought of as ground predators, subsisting largely on mice and voles, but they are equally adept at climbing trees to plunder the nests of birds. With the widespread introduction of nest boxes throughout Europe for bird population studies, the weasel soon emerged as a regular and sometimes highly destructive predator of eggs, young, and occasionally the parent birds (Perrins 1965; Graczyk, Galinski & Klejnotowski 1970; Larsen 1974; Flegg & Cox 1975). An interesting feature common to the long continuing studies is that predation tends to fluctuate widely from one year to the next, but usually the investigation has been too brief to establish the reasons for this. In Wytham Wood near Oxford, data on the breeding success of four Parus species of tits in next boxes have been collected continuously since 1947, and the causes of nest failure, including predation by weasels, have been carefully recorded. Long term records of rodent abundance are also available for Wytham (Southern & Lowe 1968; Southern 1970); these monitored populations of Bank Voles Clethrionomys glareolus (Schr.) and Wood Mice Apodemus sylvaticus (L.) which are the commonest small mammals in the wood and the main prey species of the weasels there, comprise respectively 36 and 14% of their diet (King 1971, 1975). A k-factor analysis by Krebs (1970) showed that in years of high nest density, predation on Great Tits Parus major L. was proportionately more severe; a detailed examination of the fate of nests in relation to the distances between occupied nest boxes showed that, within the range measured, there was no threshold density below which the birds were completely safe (Krebs 1971). The present study is likewise confined to the breeding season of the tits which occupies the three months from April to June. Using data mainly from 1947-72, the first aim of the study is to assess the intensity of the weasels' predation on tits in relation to the abundance of both tits and rodents. The second objective is to examine differences in the timing of predation within the breeding season. Perrins (1965) found consistent differences between the risk of predation to early and late layers, and attributed this variation to a change in the behaviour of both prey and predator. This paper explores the possibility that differences in the timing of predation may also arise from variations in prey density.

Dispersal in Dipterans: Its Costs and Consequences

Abstract: Dispersal may be an important factor in the persistence of a population living in a heterogeneous environment (Andrewartha & Birch 1954; Southwood 1962; den Boer 1968; Roff 1974a, b). But though an animal which disperses may have a higher fitness than the non-disperser when measured over many generations (Van-Valen 1971; Roff 1975) the short term benefits of dispersal often appear to be less than the costs; the possible increase in mortality, the risk of not finding suitable habitats to colonize and the possible reproductive cost due to either the time spent in dispersing or the energy expended which may have gone into reproduction. The last cost, the energy expended in dispersing, is the most tractable to study, particularly in a group of organisms, like the dipterans, in which it is known that dispersal activity is energetically expensive. A significant portion of the insect's energy budget may be used in flying (Weis-Fogh 1952; Hocking 1953; Sotavalta & Laulajainen 1961; Yurkiewicz 1965). If egg production and flight share the same energy reserves, flight will diminish the number of eggs an insect can produce. Energy expended in flight is related to the wingbeat frequency which depends upon body size and wing area (Greenewalt 1962). Within any particular genus these parameters will be highly correlated and hence the rate of energy consumption can be related directly to a single parameter such as body length. If flight duration and egg production are negatively correlated and the suitability of oviposition sites fluctuates over time and space, then an insect's fitness will be modified by its size in relation to the spatio-temporal patterning of oviposition sites. The specific questions this study was designed to answer were: (a) what is the cost of flight in terms of egg production? (b) Is the size of a fly an important parameter in determining its dispersal ability? As a model dipteran I chose Drosophila melanogaster (Meigen) because its flight energetics are well known and it is easily reared and experimented with. Flies were obtained from the wild and maintained in the laboratory for about a year prior to the study.

The Body Composition of Adult Perch, Perca fluviatilis in Windermere, with Reference to Seasonal Changes and Reproduction

Abstract: The present paper describes the seasonal cycle of sexually mature fish in terms of their main body constituents (water, fat, protein and ash). The energy values of both the somatic and gonadic tissues have been determined for a 'standard' fish of each sex and predictive equations have been formulated. The importance of understanding body composition changes in growth and production studies has long been accepted. Much of the work in this field has been done under laboratory conditions (e.g. Gerking 1955; Brett, Shelbourn & Shoop 1969; Niimi 1972a; Jezierska 1974; Elliott 1976). Others have measured the chemical composition of various body components from wild populations and some of these have been related to seasonal and reproductive cycles. Many references to work on chemical composition of fish can be found in Love (1970). Studies on the perch (Percafluviatilis L.) in Windermere have been extensive and well documented (Bagenal 1970). Le Cren (1951) describes in detail the length-weight relationship and seasonal cycle in gonad weight and condition of this perch population. Iles (1974) referring to this work infers from it that 'maturation involved to some degree at least, the translocation of material previously stored in the body'. This study tests that inference and extends the work further.

Effect of food limitation during the breeding season on the size, body components and egg production of female sticklebacks (gasterosteus aculeatus)

Abstract: structure. Gasterosteus aculeatus L., the three-spined stickleback, is a suitable subject. The female spawns several times over a period of 2 to 3 months in spring and summer (Baggerman 1957), and egg production is a function of food availability (Wootton 1973a). The energy cost of egg production by jfemales on an ad libitum ration has been calculated (Wootton & Evans 1976) but the effect of a restriction of food supply during the breeding season on egg production and growth can also be quantified. Such an approach will eventually allow the development of a model predicting growth and egg production in natural populations of sticklebacks from information on the rate of food consumption.

The Energetics of Prey Selection by Redshank, Tringa totanus (L.), in Relation to Prey Density

Abstract: The prey species mostly taken by the wading bird redshank, Tringa totanus (L.), on the shore belong to a number of taxa and include the amphipod crustacean Corophium volutator (Pallas), the decapod crustaceans Carcinus maenas (L.) and Crangon vulgaris (Fabricius), the gastropod mollusc Hydrobia ulvae (Pennant), the bivalve mollusc Macoma balthica (L.), and the polychaete worms Nereis diversicolor (O.F. Miller) and Nephthys hombergi (Lamarck) (Goss-Custard 1969; Prater 1970; Burton 1974; GossCustard & Jones 1976). The diet in a particular situation is affected by factors such as mud temperature and substrate which seem to affect the behaviour of the prey and thus their availability to the birds (Goss-Custard 1969, 1970a). This paper examines the influence of prey density on diet. The aims of the study were (1) to describe the diet at different prey densities, (2) to test whether the birds select between prey rather than simply take every available item they encounter and (3) if selection occurs, to test the hypothesis that the birds prefer the species which maximize the net rate at which they collect energy.

Non-equilibrium 'island' communities: diptera breeding in dead snails

Abstract: units or ecological 'islands' scattered through other habitats. Communities develop within them that are characterized by the absence of green plants and the presence of a diverse fauna of arthropods as well as multitudinous microorganisms. These communities have at their disposal a limited amount of energy, which is gradually used up by the activities of the community members. The successional changes that take place in these 'islands' are usually rapid, occurring on a scale of days (or even hours) rather than years. The changes are largely the results of the activities of the organisms themselves. The physical environment often has relatively' little direct effect. Because of the rapidity of the successional changes, there is usually only a single generation of any species (excluding microorganisms) before the habitat unit has become either unsuitable or exhausted. Dispersal is then necessary to find other suitable units for colonization. These ecological 'islands' are not self-sustaining, and the communities in them can never reach an equilibrium at least on the scale of the individual 'island'. Thus they differ in several respects from the geographical islands considered by MacArthur & Wilson (1967), the habitat islands on the mainland (Vuilleumier 1970; MacArthur 1972), and the host plants considered as islands by Janzen (1968, 1973). Dead snails form small habitat units of the type considered above.; They are usually soon exploited by a variety of Diptera. The fly larvae feed on and complete their development within a single snail. Each isnail is essentially an isolated unit. Interchanges with the external environment are usually limited to the import of Dipteran eggs (or larvae in the case of the ovoviviparous Sarcophagidae), and the export of mature larvae, ready to pupate in the soil, or of adult flies. The community that develops in the snail is shortlived and each species normally has only a single generation before the food supply is exhausted. The number of species and individuals is limited, compared to the communities of larger carrion, and it is possible- to make a complete census of all those attaining the pupal or puparial stage. It is also easy to set up many replicate habitats and study the variation among the communities in different snails in a small area and at different times of the year. This paper is concerned with the community in the dead snails from the aspect both of the single snail and the set of snails exposed to attack at one time. It examines the patterns of abundance of species and individuals among the snails, and successional and seasonal changes in these patterns. It then looks at the effects of predation, parasitism and competition on the communities. The life history strategies used by the different species and

Coexistence in a Guild of Wandering Spiders

Abstract: The spider fauna of soil and litter may be separated into several distinct groups, based on the methods used in prey capture. Balogh & Loksa (1948) divided spiders into three groups: web spinners, cursorial forms and saltatorial forms. They called these groups syntrophia (after Balogh 1946), groups of organisms exploiting similar resources in the same way. In recent ecological literature, the term 'guild' has been applied to such groups (Root 1973). Cursorial, or wandering spiders constitute a guild in that they are all non-specific predators of arthropods (and thus exploit a single resource or similar resources) and are all hunting spiders (exploiting resources in a similar manner). These spiders are known to move through the litter and run down or pounce on prey when they are encountered (Turnbull 1973). They spin no webs, but may inhabit permanent or temporary retreats or burrows. This guild includes the families Clubionidae, Gnaphosidae, Lycosidae, Pisauridae, Thomisidae, and some representatives of the Agelenidae and Hahniidae (Breymeyer 1966). Members of most of these families are similar in general morphological features, mode of prey capture and size. The crab spider species (Thomisidae) in this study are different from those of other families morphologically (the first two pairs of legs are laterograde rather than prograde), but forage in a similar way. As many as thirty to fifty species of wandering spiders may be found in a single forest, and there appears to be a high degree of similarity of species composition over many areas (Cannon 1965; Peck 1966). The wandering spider guild may constitute upwards of 43/o of grounddwelling spider species in a forest (Drew 1967), and accounts for a majority of the spider biomass (Moulder & Reichle 1972). Recently, Enders (1975, 1976) has raised some questions about co-existence and limiting similarity of closely related spider species. He has shown that spiders employing different foraging strategies rely on different means of reducing competition to coexist. As part of an investigation of leaf litter spider communities (Uetz 1975), the spatial and seasonal distribution of foraging activity in the wandering spider guild was studied by pitfall trapping. The ten species studied include three pairs of congeners and groups of similar size, providing an opportunity to examine the role of spatial and temporal stratification and body size differences in allowing coexistence of species.

DENSITY DEPENDENCE IN THE LIFE-CYCLES OF ANIMALS AND ITS IMPORTANCE IN K- AND r-STRATEGIES

Abstract: Density-dependent mortality causes proportionate increase in mortality or decrease in fecundity as population density increases. The term was first suggested by Smith (1935) in 1935 to replace Howard & Fiske's (1911) older terminology. Density-dependent mortalities are able to regulate a population and keep it stable under certain conditions but occasionally they will over-compensate for changes in population density (Varley, Gradwell & Hassell 1973). Published data concerning the life-histories of a variety of animals can often be reanalysed to reveal the nature of the density-dependent process at work. The strength of any density-dependent mortality and the position in the life-cycle at which it acts on any particular animal species could be described as that animal's 'population regulation strategy'. This 'strategy' can be related to various features in the animal's biology, such as the type of habitat in which it lives. The aim of this study is to investigate the nature of some of these relationships and thus get a better understanding of the significance of this 'population regulation strategy' in terms of the animal's overall ecology. The timing and strength of density-dependent mortalities are compared with the type of animal (insect or non-insect), the permanence of the habitat in which the animal lives, whether it is more life a Kor r-strategist, the agencies through which the mortalities act and, lastly, the reproductive rate. There are a number of models available for describing density dependence. These usually describe mortalities and/or emigration although birth and immigration rates may also be of the same type. The simplest model is

Changing spatial relationships in a population of apodemus sylvaticus with the onset of breeding

Abstract: Few small mammal ecologists would dispute the validity of the home range concept, but the biological details embodied in this concept have been the elusive subject of study for the past forty or more years. A great deal is now known about how home range size varies with species and season (Miller 1958; Getz 1961; Brown 1969; Mazurkiewicz 1971; Maza, French & Aschwanden 1973; and many more), but much less is known about the biological reasons underlying these size variations, which Sanderson (1966) stressed as being the next focus of study. Of particular relevance to this problem are the spatial relationships between individuals in a population. Just as the behavioural interactions between individuals vary during the year (Healey 1967; Turner & Iverson 1973), so will the spatial organization in response to the changing living requirements. Martinsen (1968) showed that for chipmunks the differential use of the total home range area changed greatly in response to reproductive and food demands. The results presented in this paper reveal that the activities of the majority of woodmice (Apodemus sylvaticus Linnaeus) in a woodland habitat are centred on distinct home range areas which show significant changes at the onset of breeding to alter the spatial relationships within the population as a whole.

Populations and production of benthic animals in two contrasting shallow lakes in norfolk

Abstract: marked and rapid loss of aquatic vegetation from most of the broads, with an increase in turbidity, caused primarily by excessive eutrophication (Mason & Bryant 1975c; Mason 1976a). A corresponding loss in the diversity of the fauna of the open broads has occurred. The littoral zones, in contrast, remain very diverse in animals and emergent aquatic plants and are highly productive (Mason & Bryant 1974, 1975b, c; Mason 1976a, b). With this change from a primary production system dominated by angiosperms (which are highly seasonal) to one dominated by algae (with an extended growing season in the broads), a change in the pattern of production in the zoobenthos is to be expected. Algae, whether living or dead, are probably more readily utilizable as food by benthic animals, while the decrease in faunal diversity may lead to reduced competition for those species able to tolerate the new conditions. The present study compares the populations (from November 1971 to June 1975 inclusive) and production (from June 1973 to May 1975 inclusive) of the macroinvertebrates of a broad which has been subject to recent cultural eutrophication (Alderfen Broad) with those inhabiting a broad which still retains clear water and a good growth of macrophytes (Upton Broad).