Showing papers in "Oikos in 1986"

Size variability and competition in plant monocultures

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Predation on ground-nesting birds' nests in relation to predator densities and habitat edge

Abstract: To study which predators are responsible for nest-losses among ground-nesting birds in boreal forest and how predators utilize different habitats, especially forest/farmland edge, artificial nests were exposed to predators in central Sweden in 1981. A setup which forced nest-robbers to leave foot-prints showed that different avian and mammalian species robbed nests in relation to their relative densities. Among both avian and mammalian nest-robbers there were habitat specialists and generalists. Predators appeared not to develop a memory for the experimental nests. In forest, birds were relatively more common than mammals as nest robbers than in farmland. Differences in the role of different predator species as nest robbers in forest vs. farmland habitats reflected their choice of habitat. No predator seemed to have developed specific nest-robbing skills and loss of individual nests was considered as a random event. Predation rates were higher when nests could be detected from a distance. However, the relevance of the results for real nests remains largely unknown. Variations in predation pressure in relation to different types of ecotones is discussed in view of the recent debate on how patterns of predation become altered as the size of patches of pristine environments are reduced as a consequence of human influences. It is suggested that the main factor affecting the rate of predation in patchy environments is the steepness of productivity gradients between an habitat island and the surrounding matrix rather than patch size itself.

Effects of light and nutrient stress on leaf phenolic chemistry in salix dasyclados and susceptibility to Galerucella lineola (Coleoptera)

Abstract: The amounts of phenolic compounds present in Salix dasyclados Wimm. leaves, and the susceptibility of the tissue to the leaf beetle Galerucella lineola F., were determined in laboratory grown plants stressed with respect to light and nutrient availability. Plants of clonal origin were grown in units with circulating nutrient solutions under (1) low light with free access to nutrients (low carbon supply), (2) high light with free access to nutrients (optimal conditions), and (3) high light with suboptimal nutrient supply (low nutrient supply). Concentrations of phenolic compounds in plants with low carbon supply were only about one-third compared with the other two treatments. The relative availability of carbohydrates for construction of carbon-based defensive compounds, such as phenolics, may explain the different susceptibility of willow leaves when grown under the specified environments. Beetles consumed about five times as much leaf tissue on plants low in carbon and rich in nitrogen as in the other two treatments.

Man as an Intertidal Predator in Transkei: Disturbance, Community Convergence and Management of a Natural Food Resource

Abstract: The indigenous peoples of Transkei, South Africa, remove shellfish from the shore to supplement their diet. Paired exploited and protected rocky shores were compared in terms of community structure and of the size structures of component species. Relative abundance of algae and sessile, inedible invertebrates is greater, and modal and mean sizes of exploited species are smaller at exploited sites than at protected sites. Disturbance through selective predation increases species richness, in line with the \"intermediate disturbance hypothesis\", and also leads to community convergence towards a common state. The recovery potential of these systems is high and the resource could best be managed on a \"rotational cropping\" basis.

Asymmetrical competition between age classes as a factor causing population oscillations in an obligate planktivorous fish species

Abstract: A population of the obligate planktivorous vendace, Coregonus albula, exhibited an oscillation cycle with a periodicity of two years. A hypothesis based on asymmetrical competition between age classes favouring small vendace is put forward to explain the oscillations. The hypothesis is based on one central argument, namely that small vendace will have a competitive advantage during summer due to their lower metabolic requirements and the low abundance and mean size of available resources. This argument is supported by data on seasonal changes in growth relationships and resource availability. The oscillation cycle is suggested to be due to the fact that the individual size of 1-yr-old vendace belonging to a strong year class will be relatively small at the beginning of summer due to high within age class competition. At the same time they have, as an effect of their high population number during their first summer, competed heavily with that year's one-year-old vendace with a resulting low reproductive output of the latter. One year old vendace of a strong year class will therefore grow rapidly during summer for two reasons: (1) their own small size at the beginning of the summer and (2) low competition intensity from the few young of the same year. This will result in a high reproductive output and a strong year class will thus give rise to another strong year class. Oscillating vendace populations have life histories with strong inter age class competition and long juvenile versus mature life periods, and are practically semelparous. These factors have been found to be the prerequisite for the occurrence of oscillations in theoretical models on age structured populations. The absence of oscillations in some vendace populations can be explained by high interspecific competition, reduced intraspecific competition and/or prolongation of the mature life period, factors that will violate the conditions necessary for the appearance of os

Should egg size and clutch size decrease with age

Abstract: We analyse a model in which females gather resources for eggs in a pre-reproductive phase, and are then, in the reproductive phase, subject to mortality during the production of successive clutches of offspring. We find that if clutch size is constrained, egg size should decline with maternal age. Where egg fitness shows diminishing returns with egg size, and maternal mortality is age-independent, the decline in egg size will be linear, and the rate of decline will be proportional to maternal mortality. All females with the same survival prospects follow the same trajectory of decline, but larger females enter the trajectory at a larger egg size. Similarly, if egg size is constrained, clutch size declines with maternal age, the rate of decline increases with maternal mortality, and larger females lay larger clutches initially. Support for the models is found in a wide variety of species, and in particular in a number of recent studies of butterflies.

Centrifugal community organization

Abstract: Pimm has invented a useful classification of community organizations based upon ecological preferences. We add the category of centrifugal structure to it. If the primary preference of all species is shared, but their secondary preferences are distinct, they are said to be organized centrifugally. The biological basis for centrifugal structure may be that habitats represent combinations of different environmental variables and, although the ideal combination is the same for many species in a guild, each species is adapted to tolerate relative deprivation of a different component of the mixture. We have modelled the isolegs of centrifugal organization. (Isolegs are the lines of equal optimal behavior drawn in a state space.) Their principal distinguishing feature is their negative slope. Because of this, niche shifts in such communities should occur backwards: removing a competitor produces narrower niches in those left. Another feature which distinguishes it is its inability to generate a 'ghost of competition past': competition should always be a strong dynamical influence. We examined the relationship of two species of gerbils in the Negev Desert, Israel, and determined that they both exhibit negative isolegs. Other aspects of their habitat selection are consonant with a picture of centrifugal organization. But it remains unknown how general or important is this pattern of community organization.

Two distinct roles for predators in freshwater assemblages

Abstract: Disagreements over the importance of predation in affecting freshwater community structure seem partially attributable to differences in use of essential terms and to a failure to distinguish between the roles of predators in causing and maintaining structure. From analysis of 18 predator-prey, community experiments, I have drawn the following conclusions. Most field experiments show, at best, that predators may depress community density but none demonstrate that these reductions are non-transitory (e.g., evident in the following year). Furthermore, relatively few studies report effects of freshwater predators on species richness or evenness. There is no compelling evidence that predators are the dominant factor in maintaining community structure (i.e., density-dependent regulation of diversity through reduced competitive exclusion). However, it does appear that they contribute to community regulation and may sometimes be an important factor. In contrast, it seems certain that some predators have a role in the causation of diversity through coevolution, size-selective predation, and restrictions on species distributions and community composition. Often these effects are only evident when predators are introduced into systems where they have not coevolved or at least co-occurred with prey for many prey generations.

The Response of an Intertidal Concholepas concholepas (Gastropoda) Population to Protection from Man in Southern Chile and the Effects on Benthic Sessile Assemblages

Abstract: In this paper we document the changes experienced by the carnivorous Concholepas concholepas snail population after the establishment of a marine reserve in May 1978. Our data indicate that the release from human predation results in a striking change of the Concholepas population structure, due to the increment of individuals larger than 40-50 mm peristomal length. This evidence strongly indicates that the usual absence of larger size classes from the intertidal is not a consequence of physiological limitations of the muricid, as was previously thought. Significant density differences were detected only in 1984, but they were due to substantial reduction of the snails in the harvested control area.

A comparative field study of the breeding behaviour of a stream and a pond population of Gammarus pulex (Amphipoda)

Abstract: The breeding system of the freshwater amphipod crustacean Gammarus pulex (L.) contains a precopulatory phase during which a male carries a female. The seasonal changes in this behaviour were studied in a stream and in a pond population. The length of the guarding phase varied considerably over the seasons. In the stream, larger animals of both sexes were at an advantage in reproductive competition and the advantages were greatest at the peak of the breeding period. Animals of both sexes infected with a cystacanth of the parasite Polymorphus were less likely to be paired than were non-infected individuals. In the pond, the life cycle was more similar to that of a lake population of G. lacustris Sars than to the stream population. Pairs were more vulnerable to fish predation than were unpaired males. It is suggested that fish predation prevented all year breeding.

Population trends of North Finnish land birds in relation to their habitat selection and changes in forest structure

Abstract: Long-term trends in the 22 most abundant land bird species breeding in northern Finland were related to their habitat selection patterns. Independent data sets were used to describe long-term population trends (POP), edge preferences (EDGE) and the effects of the changing age structure of the forests on bird density (AGE). AGE combined the present age preferences of birds with data on the changing age structure of North Finnish forests from the 1950s to the 1970s. EDGE and AGE correlated positively though not significantly with POP. When EDGE and AGE were taken into account simultaneously, a significant correlation with POP emerged (67% of the variance accounted). Southern species were an exception; their population trends seem to depend on changes in southern Finland. The sedentary species of old forests have plummeted during the recent decades in northern Finland. The results for this group (Parus montanus, P. cristatus, P. cinctus, Certhia familiaris, Perisoreus infaustus) agreed well with the finding that fragmentation and changes in the age structure of the forests are mainly responsible for the recent trends among the abundant forest birds in the north. Our results seemingly indicate a close tracking of the environmental resources by the bird community. However, as the geographical scale of the study is broad (northern Finland as opposed to one population site) and the temporal scale decades rather than years, a simple saturation hypothesis is not an inevitable inference from the data; and if there is close tracking of the environmental resources, it is rather in terms of populations and not of the whole community. An alternative hypothesis is provided by changing numbers of high-quality and low-quality population sites; in this view, the regional pattern is a result of complicated dynamics in a mosaic of local populations.

Stress and the coarse control of growth and root-shoot partitioning in herbaceous plants

Abstract: From a priori reasoning and the integration of many experimental observations, a new, ecologically hased scheme is derived to explain the way in which plant growth and resource partitioning is broadly controlled by external environmental conditions. We suggest that the partitioning of resources between belowand above-ground plant parts is jointly controlled by the absolute amounts of belowand above-ground environmental stress, by the below-/above-ground environmental stress ratio, and by the growth potential of the species itself. Under particular conditions, any shortfall from the species's innate growth potential may be explained in terms of the total of, and ratio between, belowand above-ground stress. Hyperbolic relationships link the variables under consideration. Though the equations ultimately derived are not open to straightforward experimental validation, they are likely to be of value in predicting 'perceived stress' both aboveand below-ground, as an experiment involving three contrasted species indicates.

Seasonal changes in the social organization of male stoats, Mustela erminea: an effect of shifts between two decisive resources

Abstract: A change in male social organization is expected when the most important resource during the breeding season (receptive females) has different dispersion and predictability characteristics as compared with the most important resource during the nonbreeding season (usually food). Male stoats showed a marked seasonal shift in their social organization from a pattern of intrasexual territories during the non-breeding season (autumn and winter) to a non-territorial pattern with extensive and overlapping ranges during the mating season (spring and summer). This shift could not be explained by changes in food availability or distribution, but was correlated instead with the onset of breeding. The proposed hypothesis, that a change in social structure is caused by a shift of decisive resources, will explain seasonal shifts in the social organization of several other mammalian species.

Rodent cycles in relation to food resources on an alpine heath

Abstract: Populations of microtine rodents in Lapland, N Sweden, show strong cyclic fluctuations. We examine relations between the density of rodents and their food resources during two four-year cycles on alpine heath. The grey-sided vole Clethrionomys rufocanus, the most common species, varied more than 100-fold in density between peak and bottom years. Breeding ceased earlier in peak than in increase years. The voles weighed 26-44% more in peak years than during the declines or early increase phases. Breeding in C. rutilus and Microtus oeconomus followed a similar pattern. Lemmus lemmus, however, reproduced longer than the other species during peak years. Reduction in the quantity of available food through grazing was rejected as a proximate cause for the rodent cycles. During the second population peak, voles ceased breeding and began to decline while preferred food plants were still abundant. Nor was there any consistent relationship between plant nutrients (N, P, Ca, Mg, K, C) and the rodent cycle. Nutrient levels in food plants seemed to depend mainly on processes other than rodent grazing. Hence, factors other than food quantity and nutrient levels seemed to be important in the short-term causation of the cycles. Exclosure experiments showed that rodents markedly reduced the phytomass of several plants, influenced the species composition of the alpine heath, and reduced the flowering frequency of food plants.

Above and below ground dry mass of the three main vascular plants on hummocks on a subarctic peat bog.

Abstract: Conventional harvesting in combination with an indirect method based on 14C allocation, was used to quantify the supply of organic matter, especially of fine roots, to successive peat levels down to 25 cm depth, in a raised area of Sphagnum fuscum about 30 m across. The three species studied were Andromeda polifolia, Empetrum hermaphroditum, and Rubus chamaemorus. The above ground biomass of these three species was estimated to be 23 g m'2 64 g m-2 and 12 g m-2 respectively; the below ground coarse root and rhizome biomass was 150 g m-2, 213 g m-2 and 220 g m-2 respectively, and the total fine root (<0.5 mm) biomass was 1100 g m-2, 600 g m-2 and 350 g ml2 respectively. Andromeda and Rubus had thus 98%, and Empetrum 90% of their living biomass below ground. The two dwarf shrubs had their living below ground biomass concentrated in the 0-15 cm peat layer, with their bulk at 2.5-7.5 cm, while Rubus was evenly distributed throughout the whole profile, to 25 cm depth.

Experimental study of avian predation on coexisting grasshopper populations (Orthoptera: Acrididae) in a sandhills grassland

Abstract: An experiment was performed to examine the impact of avian predation on populations and assemblages of insect herbivores in a grassland system. Here, I examine the effect of avian predators on grasshopper populations in regard to overall density, densities of individual species, species diversity, and overall taxonomic composition. Predation pressure was examined by comparing each of these attributes of grasshopper populations in 10 m x 10 m exclosures where birds were excluded with equally sized plots where birds were allowed to forage normally. In general, avian predation significantly affected grasshopper populations in a sandhills grassland. Overall densities were reduced 27.4% in the predation treatment, most species occurred in lower numbers (though some exceptions were observed), and fewer species coexisted. Specific differences in overall taxonomic composition between the treatments were not detected using MANOVA. The relevance of these results to understanding community organization of herbivorous insects is discussed; the compensatory mortality hypothesis is not supported although other plausible hypotheses cannot be further evaluated with the present data.

Pheromone dialects in European turnip moths Agrotis segetum

Abstract: Female pheromone gland extracts from cultures of Agrotis segetum originating from Sweden, France, Hungary and England were analysed for pheromone components and precursors (fatty acids). The pheromone blends were similar in moths from the Swedish, English and Hungarian populations, whereas the French diverged with a much higher amount of (Z)-7-decenyl acetate relative to the homologous pheromone components (Z)-7-dodecenyl acetate and (Z)-9-tetradecenyl acetate. -from Authors

Predatory risk to insects foraging at flowers

Abstract: Interactions between nectar-seeking insects (primarily bumble bees Bombus spp. and honey bees Apis mellifera L.) visiting common milkweed (Asciepias syriaca: Asclepiadaceae) inflorescences and their commonest predator at the inflorescences, the crab spider Misumena vatia (Clerk) (Thomisidae), were recorded. Densities of Misumena were low. On average a given insect would only be attacked once each 4-6 d by a spider, and predicted periods until capture of the different species ranged from 21 to 109 d, far longer than any individual visited milkweed. Insects showed no sign of avoiding spiders.

Is there a self-thinning rule for animal populations?

Abstract: There is a well established (though imperfectly understood) '-3/2 power rule' for self-thinning in plant populations, but no comparable rule for animal populations. Here, we search for such a rule applicable to sessile and especially mobile animals. Energetics allometries suggest a '-4/3 power rule', but animal self-thinning is seen to depend on organism-resource interactions in a way not found in plants, because of the peculiar nature of light as a resource. Nevertheless, new and encouraging experimental support for a -4/3 power rule is presented.

A comparative study of decomposition, oxygen consumption and nutrient release for selected aquatic plants occurring in an estuarine environment

Abstract: The rates of decomposition and nutrient regeneration were compared among six aquatic plants representing examples from phytoplankton (Chlorella sp.), macroalgae (Ulva lactuca), submersed vascular macrophytes (Myriophyllum spicatum, Potamogeton perfoliatus, and Ruppia maritima) and marsh grasses (Spartina alterniflora). These plants, which were obtained from the Choptank River estuary, Maryland, (except for Chlorella which was a laboratory culture) were placed in 1 mm mesh bags and incubated in aquaria with ambient water under dark, aerated, temperature controlled (20 + 3°C) conditions for 93 d. The rank in decomposition rates based on both decrease in original mass (decrease in chlorophyll a for Chlorella) and associated oxygen consumption was phytoplankton > macroalga > submersed macrophytes > emergent macrophyte, and rates were directly proportional to the initial nitrogen content of the plant tissues. Nitrogen content of all the plant tissues increased during decomposition, yet reductions of C:N ratios were only observed for those plants with initial C:N > 20. N:P ratios generally increased due to a much higher leaching for P (10-40% of initial P) compared with N (1 to 10% of original N). The leached P was equally distributed between dissolved inorganic and organic forms. Generally, the magnitude of P and N leaching rate was not related to respective initial nutrient concentrations of the plant, nor to the plant's structural integrity (C:N ratio). Total N and P dissolved in the water column plus that in plant material remaining in the mesh bags at the experiment's termination accounted for 7 to 48% of their original respective quantities for submersed macrophytes compared with 8294% for Spartina.

Reproductive costs in the prawn Palaemon adspersus: effects on growth and predator vulnerability

Abstract: Reproductive costs in the prawn Palaemon adspersus : effects on growth and predator vulnerability

Movement patterns of male stoats Mustela erminea during the mating season: differences in relation to social status

Abstract: Male stoats were tracked during the mating season using radio-telemetry. Three different movement patterns were apparent during spring. Old males (> 2 yr), roamed over extensive areas and stayed a few days in each place. Yearlings were either stationary, i.e. present in the same area for the entire spring and with a home range of about 2 km2 or they were transients and moved constantly over large areas. The movements of roamers and stationary males were more strongly influenced by the distribution of females than were the transients' movements. Movement activity showed two peaks during spring, one during late April early May, and then again during early June. Males appeared to avoid confrontations, and a meeting between two individuals was observed on only one occasion. The results of this study suggest that mate-searching behaviour is determined by social status, and that the reproductive success of a male depends on his behaviour and on the spatial and temporal distribution of receptive females.

Gigantism in Island Populations of Wood Mice (Apodemus) in Europe

Abstract: Many rodents have large body sizes on islands, and there are many hypotheses that try to explain this observed pattern. Using body size data on Apodemus in Europe as an example, I try to evaluate t ...

Predation causing synchronous decline phases in microtine and shrew populations in western Finland

Abstract: Summary. I show thaf microtine and shrew populations in western Finland had synchronous decline phases, although the other population fluctuation phases were not regularly synchronous. Th€ most probable reason in spring and summer is the pred|tion by nomadic birds of prey (which frequentty f€ed on shrews in addition to microtin€s) and to a smaller extent the predation by resident smal! mustelids, since the distast€ of thes€ mammalian predators for shrews has been shown in several sludies. In wint€r small must€lids may play r more important role than partially migrating birds of prey. Occasionally also unfavourable weather conditions in wint€r mev affect these svnchronous declines observed in the trensition zone of Fennoscindia.

Sex ratio variation in relation to maternal condition and parental investment in wild reindeer Rangifer t. tarandus

Abstract: Along small-sized, resource-limited females with high mortality of progeny during parental investment, the prenatal sex ratio favoured males. After weaning, at the end of juvenile male dispersal, the ratio had been skewed in favour of females. Among large-sized, well-fed females without postnatal offspring mortality, the preand post-weaning sex ratio favoured females. The findings are discussed in relation to theories of natural selection and facultative adjustments of sex ratio in mammals.

Life history evolution in a recruitment population: why are adult mortality rates constant?

Abstract: Adult mortality rates often vary much less through time (or with population density) than do clutch size or immature mortality rates. This observation is studied theoretically using a life-history evolution model for a density regulated population. The adaptation model is used as a tool to probe the underlying constraints (tradeoffs or genetic correlations) which may be operative in producing the observed demographic pattern. This exercise leads to some new life-history questions.

Equilibrium diet: optimal foraging and prey coexistence

Abstract: A model of predator-mediated prey coexistence is presented in which the predator is an optimal forager and the superior competitor is also a more desirable prey. A stable three-species equilibrium is possible if the profitability of the poorer competitor is below the equilibrium intake requirement of the predator. In general, the determinants of equilibrium will define this "equilibrium diet". If a positive three-species equilibrium is locally unstable, the system may persist globally due to the addition of the less preferred prey to the diet of the predator during periods of predator starvation. In intrinsically unstable systems optimal foraging may increase stability by shifting losses from decreasing populations to those that are increasing.

Multiple paternity in the adder, Vipera berus

Abstract: Female adders are known to mate with several males during the mating season. The presence of a "copulatory plug", permitting only the first male to fertilize the eggs, has been reported. To test this paradox, female adders were mated under laboratory conditions with males carrying different, electrophoretically detectable markers. Females mated twice do produce bipaternal broods. Furthermore, single mated females can also produce multipaternal broods, showing that sperm from previous year(s) can be used.