Roy Turkington

Impact of food and predation on the snowshoe hare cycle.

Snowshoe hare populations in the boreal forests of North America go through 10-year cycles. Supplemental food and mammalian predator abundance were manipulated in a factorial design on 1-square-kilometer areas for 8 years in the Yukon. Two blocks of forest were fertilized to test for nutrient effects. Predator exclosure doubled and food addition tripled hare density during the cyclic peak and decline. Predator exclosure combined with food addition increased density 11-fold. Added nutrients increased plant growth but not hare density. Food and predation together had a more than additive effect, which suggests that a three-trophic-level interaction generates hare cycles.

Population changes of the vertebrate community during a snowshoe hare cycle in Canada’s boreal forest

We measured the density changes of 22 species of vertebrates during a snowshoe cycle in northern Canada. Hares were the dominant herbivore in the system and changes in their numbers were correlated with changes in numbers of arctic ground squirrel, spruce grouse, ptarmigan, lynx, coyote, great horned owl, goshawk, raven and hawk owl. Hare numbers were not correlated with numbers of red-backed vole which showed peaks during the low, increase, and early decline phases of the hare cycle. Hawk owls were the only predator whose numbers correlated with changes in red-backed voles while boreal owls and weasels were correlated with densities of Microtus. Red squirrel, American kestrel, red-tailed hawk, northern harrier, wolverine, magpie, and gray jay showed no correlation with hare or vole numbers. We conclude that species in the boreal forests of Canada do not exhibit the strong synchrony found between voles and other members of the vertebrate community in northern Fennoscandia. We discuss some of the possible reasons for these differences.

Coordinated distributed experiments: an emerging tool for testing global hypotheses in ecology and environmental science

There is a growing realization among scientists and policy makers that an increased understanding of todays environmental issues requires international collaboration and data synthesis. Meta-analyses have served this role in ecology for more than a decade, but the different experimental methodologies researchers use can limit the strength of the meta-analytic approach. Considering the global nature of many environmental issues, a new collaborative approach, which we call coordinated distributed experiments (CDEs), is needed that will control for both spatial and temporal scale, and that encompasses large geographic ranges. Ecological CDEs, involving standardized, controlled protocols, have the potential to advance our understanding of general principles in ecology and environmental science.

Ecological Implications of Specificity between Plants and Rhizosphere Micro-organisms

Publisher Summary This chapter discusses the ecological implications of specificity between plants and rhizosphere micro-organisms. The importance of specificity during infection or subsequent plant growth on plant competitive ability and subsequent community structure has a great potential influence. The plant population biology and rhizosphere microbiology elucidates the role of rhizosphere micro–organisms and evolved specificity on plant competition, species distribution, and community structure in different environments. This chapter discusses the specificity in relation to the effectiveness of mutualistic plant–micro–organism associations to evolve the fitness of both organisms. The specificity at this level of association is demonstrated between plant species, cultivars, and genotypes and isolates of rhizosphere bacteria or mycorrhizal fungi. Specificity between plants and growth-promoting micro-organisms occurs at either of two stages of these associations: during infection of the root system to form root nodules or mycorrhizas, or during subsequent growth of the infected plant host. The major categories of rhizosphere micro–organisms capable of altering plant growth and the proposed mechanisms of action for plant growth–promoting rhizobacteria are also tabulated. Infection specificity can be demonstrated with root nodule bacteria and less so with mycorrhizal fungi and may result from the specific association of effective plant–microbe combinations.

RESPONSE OF CRESTED WHEATGRASS (AGROPYRON CRISTATUM L.), PERENNIAL RYEGRASS (LOLIUM PERENNE AND WHITE CLOVER (TRIFOLIUM REPENS L.) TO INOCULATION WITH BACILLUS POLYMYXA

Abstract The effect of inoculation of crested wheatgrass ( Agropyron cristatum L.), perennial ryegrass ( Lolium perenne L.) and white clover ( Trifolium repens L.) with the soil diazotroph Bacillus polymyxa was studied. Plant growth responses to inoculation varied from slightly negative (perennial ryegrass) to highly positive (white clover and crested wheatgrass) when root, shoot and plant dry weights were measured. Root-to-shoot ratios were also increased in the latter two species. Seedling emergence in crested wheatgrass was shown to be enhanced by inoculation with the bacterium. Possible mechanisms of the growth response include suppression of pathogenic organisms in the rhizosphere by the inoculant strain, root-associated nitrogen fixation, solubilization of phosphorus compounds or the bacterial production of plant growth-promoting substances. Use of sterilized soil in experiments and a lack of antibiotic production by the Bacillus render the first possibility unlikely. However, in vitro assays showed the bacterium to possess an active nitrogenase, to be capable of solubilizing organic but not inorganic phosphate compounds, and to produce indoleacetic acid, but not detectable amounts of cytokinins or gibberellins. Experimentation with A. cristatum suggested solubilization of P may be involved in enhanced seedling emergence, but neither N fixation nor phosphate solubilization were the primary contributing factors to the observed growth response. The data support the contention that production of growth-promoting compounds similar in activity to indoleacetic acid by the bacterium is the likely stimulus for the observed increase in plant productivity.

INTERACTIVE EFFECTS OF NUTRIENTS AND DISTURBANCE: AN EXPERIMENTAL TEST OF PLANT STRATEGY THEORY'

The relative impacts of nutrient availability, disturbance intensity, and in- terspecific competition on species distribution and growth were studied by applying five levels of nutrients and five intensities of clipping to simple artificial communities (ofDactylis glomerata, Holcus lanatus, Lolium perenne, and Trifolium repens) in a factorial design for 16 mo. This was done to test the hypothesis that the effects of interspecific competition would decline along gradients as the amount of nutrients decreased and the degree of disturbance increased. Nutrients and disturbance, alone and in interaction, produced sig- nificant effects on percent cover of all species, and all species achieved maximum percent cover in the undisturbed regime. The experimental design permitted us to distinguish between percent cover reductions caused directly by nutrients and disturbance from those caused by interspecific competition. When grown in mixture, interspecific competition produced marked changes in the distribution patterns of percent cover that were in addition to the direct impacts of nutrients and disturbance. The absolute reduction in percent cover due to interspecific competition was greatest in the undisturbed regime, and for many mixtures this competitive effect was significantly reduced or even eliminated in the highly disturbed areas. Along the nutrient gradient the responses were variable and neighbor dependent, but for all species, the absolute reduction in percent cover due to interspecific competition was generally most severe where percent cover in pure stands was highest. When the data were standardized for differences in percent cover in pure stands and competition was assessed as the percent reduction in percent cover between pure and mixed stands, then the effect of interspecific competition on Trifolium declined along both gra- dients. The three grasses had a reasonably constant competitive effect across the disturbance gradient, but along the nutrient gradient the highest competitive effects were generally achieved where percent cover in pure stands was highest. We argue that this standardized index is less appropriate because it obscures the obvious differences in interspecific com- petition effects that are clearly operating within these communities.

THE MUTUALISM–PARASITISM CONTINUUM IN ECTOMYCORRHIZAS: A QUANTITATIVE ASSESSMENT USING META‐ANALYSIS

Context dependency is deemed to position the outcomes of species interactions along a continuum of mutualism to parasitism. Thus, it is imperative to understand which factors determine where a particular interspecific interaction falls along the continuum. Over the past 20 years research on the ectomycorrhizal symbiosis has resulted in sufficient independent studies to now generalize about the factors and mechanisms that affect host response to ectomycorrhizas. Using meta-analysis we quantitatively evaluated the role of biotic (partner identity and colonization levels of ectomycorrhizal fungi) and abiotic (phosphorus levels) factors in determining host biomass, height, and shoot:root responses to ectomycorrhizal associations. On average, seedlings across multiple host genera increased in total biomass when inoculated with ectomycorrhizal fungi regardless of the identity of the fungal associate; host genera differed in the magnitude of response for both total biomass and shoot:root ratio. Association with different fungal genera modified only host allocation of biomass to shoots and roots. Neither level of colonization on inoculated seedlings nor the level of contamination on control seedlings relative to colonization levels by target fungi on inoculated seedlings was important in explaining variation in effect sizes for any growth response. None of our proposed factors (identity of partners, colonization level, magnitude of contamination, or duration of association) explained variation in effect sizes for shoot height, although in general seedlings were taller when inoculated with ectomycorrhizal fungi. Phosphorus additions did not influence effect sizes. Although the general trend across studies was for a positive response of hosts to ectomycorrhizal inoculation, publication bias and methodological issues effectively reduce and distort the spectrum on which we evaluate host responses to ectomycorrhizal inoculation. Our results indicate that the variation in ectomycorrhizal fungi perceived by the host may be of a discrete (presence/absence of ectomycorrhizal fungi) rather than continuous nature (variation in identity or abundance of ectomycorrhizal fungi).

The effects of NPK fertilization for nine years on boreal forest vegetation in northwestern Canada

. Plant productivity is limited by mineral nutrient availability in many boreal forest ecosystems. This study is an analysis of the growth responses of components of a boreal plant community (cryptogams, herbaceous and woody perennials, the dominant shrubs Salix glauca (grey willow) and Betula glandulosa (bog birch) and the dominant tree Picea glauca (white spruce), to the addition of an NPK fertilizer over a nine-year period. The study was carried out in a low-nutrient boreal forest ecosystem in the Yukon territory in northwestern Canada. The following predictions were tested: (1) that there would be an overall increase in abundance (measured either as cover, density, or dry mass) of all components of the vegetation, (2) that vegetation composition would change as more competitive species increased in abundance and (3) that initial community changes in response to fertilization would be transient. In general, all predictions were found to be true. Species composition changed rapidly in response to fertilizer. Graminoids (e.g. Festuca altaica) and some dicots (e.g. Mertensia paniculata and Achillea millefolium) increased in cover, while other dicots (e.g. Anemone parviflora), dwarf shrubs (e.g. Arctostaphylos uvaursi), bryophytes and lichens declined. There was a significant increase in the growth rate of the two dominant shrubs and of Picea, but not in the cone crop or seed production by Picea. Surveys after 1 or 2 years showed responses by the vegetation but more stable patterns of response did not emerge until after 5 or 6 years. There were consistent and directional changes in the percent cover of some of the herbaceous species on control plots. Growth rates of Salix and Betula varied considerably from year to year, independently of treatment. Long-term studies are essential if we are to understand the role of nutrient limitation in this ecosystem.

Vegetation dynamics and neighbour associations in pasture-community evolution

(1) This paper reports analyses of vegetation patterns and species associations conducted over 3 years in three different-aged pasture communities with similar management histories. (2) Relative abundances of species differed among the three pastures but there were no significant differences in species diversity or composition. Ordination of percentage-cover data from all surveys, however, established a trend of increasing community stability, or constancy, with increasing pasture age. (3) Soil analyses showed little correlation with species-cover and no trends with pasture-age. (4) Patterns of association in time between species were investigated with contact sampling. Four different patterns were identified. Between-species associations in younger communities were predominantly temporary, while older communities had more associations which were persistent over the study period. (5) These data form the basis of a qualitative model of pasture-community evolution which attributes within-community changes in time to biotic interactions between neighbours. The model serves as a hypothesis for experimental studies to be reported later.

Within-population variation in localized and integrated responses of Trifolium repens to biotically patchy environments

SummaryGenets of Trifolium repens (white clover) were collected from three patches of old permanent pasture dominated by Agrostis capillaris, Holcus lanatus or Lolium perenne. Plants derived from the genets were grown with plants of one grass species present on one side of each T. repens, and a different grass species on the other side, in all combinations of two of the three grasses. Different modules (a node with its associated internode, leaf, and axillary bud) on the same clover plant responded independently to the microenvironment provided by their own neighbouring grasses. In contrast, all apical meristems on the plant reacted similarly, showing a unified response and integrating the effects of the different microenvironments experienced by the whole clover plant. This is consistent with what is known both physiologically about the nutrition of meristems and modules, and ecologically about the exploratory growth habit of the species. Averaged over all associated grasses, there was no significant variation in the final dry weight of the different clover genets but these differed in their growth habit response to different grasses. In response to Agrostis as a neighbour, each meristem of T. repens rapidly produced many small modules. New modules were produced more slowly and were larger when Holcus or Lolium was the neighbour. The same pattern of differences occurred among clovers sampled from different backgrounds. Either genetic differences paralleled plastic responses, or plastic changes in phenotype that developed in response to different neighbours in the field persisted in the greenhouse. Plants taken from backgrounds of different grass species showed different responses to growing with those grass species. The differences were manifest primarily in a “positive leading diagonal” effect of Holcus or not-Holcus. They were the result primarily of differences in the dry weight per module and the probability of development of the axillary bud into a branch. This confirms earlier results, and implicates the central importance of branching as a means of local response to the microenvironment.