Associational resistance and shared doom: effects of epibiosis on herbivory

The potential for spatial associations between palatable and unpalatable plant species to reduce herbivore pressure on the palatable species has been described as associational resistance, associational refuge or associational defense for numerous terrestrial and marine communities. One of the closest associations between species-epibiosis-has not been thoroughly investigated in this regard. In this study we evaluated how different associations between host seaweeds and epibiotic plants and animals influenced the movement of an omnivorous sea urchin (Arbacia punctulata) to the host and subsequent feeding on the host. A. punctulata showed clear preferences when given pairwise choices between 12 prey species (3 animals, 9 algae). These preferences were consistent and allowed us to rank the six epibiont species and six host species linearly from least to most preferred by A. punculata. Most host-epibiont associations dramatically changed urchin preference, increasing or decreasing urchin grazing on fouled hosts as compared to clean conspecifics. Herbivory on the host increased when the epibiont was more preferred, and decreased when it was less preferred than the unfouled host alga. Taking the host species as a point of reference, we classified epibiosis-caused decrease in herbivory as associational resistance, while epibiont-caused increases in herbivory were defined as shared doom. These epibiont-host-herbivore interactions could select for hosts that facilitate the growth of certain low preference epibionts on their surfaces in situations where the resulting decreases in herbivory would offset the various negative effects of being fouled. In contrast, in situations where herbivores are common, the negative effects of being fouled by palatable epibionts may be much greater than is generally assumed. In our assays, unpalatable hosts fouled by palatable epibionts became much more attractive to urchins and rose several ranks on the urchins' preference hierarchy.     

Optimal foraging versus shared doom effects: interactive influence of mussel size and epibiosis on predator preference

In the western Baltic Sea, the highly competitive blue mussel Mytilus edulis tends to monopolize shallow water hard substrata. In many habitats, mussel dominance is mainly controlled by the generalist predator Carcinus maenas. These predator-prey interactions seem to be affected by mussel size (relative to crab size) and mussel epibionts.There is a clear relationship between prey size and predator size as suggested by the optimal foraging theory: Each crab size class preferentially preys on a certain mussel size class. Preferred prey size increases with crab size.Epibionts on Mytilus, however, influence this simple pattern of feeding preferences by crabs. When offered similarly sized mussels, crabs prefer Balanus-fouled mussels over clean mussels. There is, however, a hierarchy of factors: the influence of attractive epibiotic barnacles is weaker than the factor ‘mussel size’. Testing small mussels against large mussels, presence or absence of epibiotic barnacles does not significantly alter preferences caused by mussel size. Balanus enhanced crab predation on mussels in two ways: Additional food gain and, probably more important, improvement in handling of the prey. The latter effect is illustrated by the fact that artificial barnacle mimics increased crab predation on mussels to the same extent as do live barnacles.We conclude that crab predation preferences follows the optimal foraging model when prey belong to different size classes, whereas within size classes crab preferences is controlled by epibionts.     

Grazing intensity influences the strength of an associational refuge on temperate reefs

Recent studies have emphasized the role of positive interactions in ecological communities, but few have addressed how positive interactions are mediated by abiotic stress and biotic interactions. Here, I investigate the effect of a facilitator species on the abundance of macroalgae over a gradient of herbivory. Grazing by sea urchins can be intense on temperate reefs along the California coast, with benthic macroalgae growing exclusively in physical refuges and interspersed within colonies of the strawberry anemone, Corynactis californica. Field experiments indicated that the net effect of C. californica on turf algae was strongly nonlinear over a gradient in density of sea urchins. At low intensities of urchin grazing, the anemone and macroalgae competed for space, with algae capable of overgrowing C. californica. At intermediate grazing intensities, C. californica provided a refuge for turf algae but not for juvenile kelp. Neither turf algae nor kelp benefited from the presence of C. californica at the highest levels of grazing intensity, as sea urchins consumed nearly all macroalgae. The hump-shaped effect observed for C. californica contrasts with the prevailing view in ecological theory that positive interactions are more common in harsh environmental conditions. The results reported here qualify this view and underscore the need to evaluate positive interactions over a range of abiotic stress and consumer pressure.     

Scale-dependent indirect interactions between two prey species through a shared predator

We investigated the potential for indirect interactions between two prey species, pea aphids ( Acyrthosiphon pisum ) and potato leafhoppers ( Empoasca fabae ), through a shared predator (Nabis spp.), and how these interactions may change across three spatial scales. In greenhouse experiments using small clusters of plants containing pea aphids and/or potato leafhoppers, the predation rates on both pea aphids and potato leafhoppers were independent of the presence of the other species, indicating no indirect interactions. In greenhouse experiments using cages containing 48 plants, when aphids and leafhoppers were confined to separate plants among which nabids could move, pea aphids had a positive effect on the survival of potato leafhoppers from predation. The positive effect of aphids on leafhoppers occurred because nabids spent more time on plants harboring aphids, thereby drawing nabids away from plants containing leafhoppers. Finally, we measured the abundance of nabids in a large-scale experiment designed to manipulate the abundances of pea aphids and potato leafhoppers in alfalfa fields. Fields with high aphid density contained more nabids, thereby suggesting that pea aphids will have a negative indirect effect on potato leafhoppers by increasing the density of nabids within fields. Potato leafhoppers had no indirect effects on pea aphids at any scale. This study shows that indirect interactions between prey species may depend upon spatial scale, because the factors affecting a predator's diet choice on a small scale may differ from those factors affecting a predator's distribution at larger scales.     

Allele fixation in a dynamic metapopulation: Founder effects vs refuge effects

The fixation of mutant alleles has been studied with models assuming various spatial population structures. In these models, the structure of the metapopulation that we call the “landscape” (number, size and connectivity of subpopulations) is often static. However, natural populations are subject to repetitive population size variations, fragmentation and secondary contacts at different spatiotemporal scales due to geological, climatic and ecological processes. In this paper, we examine how such dynamic landscapes can alter mutant fixation probability and time to fixation. We consider three stochastic landscape dynamics: (i) the population is subject to repetitive bottlenecks, (ii) to the repeated alternation of fragmentation and fusion of demes with a constant population carrying capacity, (iii) idem with a variable carrying capacity. We show by deriving a variance, a coalescent and a harmonic mean population effective size, and with simulations that these landscape dynamics generate repetitive founder effects which counteract selection, thereby decreasing the fixation probability of an advantageous mutant but accelerate fixation when it occurs. For models (ii) and (iii), we also highlight an antagonistic “refuge effect” which can strongly delay mutant fixation. The predominance of either founder effects or refuge effects determines the time to fixation and mainly depends on the characteristic time scales of the landscape dynamics.     

Scale-dependent effects of fine sediments on temperate headwater invertebrates

1. Anthropogenic activities can increase fine sediment supply to streams over multiple spatial and temporal extents. Identifying the processes responsible, and the scale at which any effects on stream organisms become evident, are key management needs, but appropriately scaled surveys are surprisingly few. 2. We surveyed macroinvertebrates and superficial fine sediments at two spatial resolutions (reach‐ and patch‐scale) in tributaries of the River Usk, a temperate, montane catchment in rural Wales (U.K.). Land use, habitat and geomorphological character were measured on‐site or derived from an existing database (=Fluvial Audit). We aimed to identify: (i) how in‐stream sediments varied with land use and associated geomorphology; (ii) likely consequences for macroinvertebrates and (iii) any scale‐dependence in relationships between macroinvertebrates and sediment character. 3. At both the reach‐ and patch‐scales, bed cover by fine sediment was related directly to the extent of eroding banks 500 m upstream. In turn, sedimentation and bank erosion were negatively correlated with catchment or riparian woodland extent. 4. At the reach scale, macroinvertebrate composition varied with catchment land use and stream chemistry, with richness declining as rough grazing or woodland was replaced by improved grassland. There was no response to deposited sediment except for weak increase in the relative abundance of oligochaetes. 5. By contrast, at the patch scale, fine sediments were accompanied by pronounced changes in invertebrate composition, and we ranked the 27 most common taxa according to their apparent sediment tolerance. General estimating equations showed that total and Ephemeroptera, Plecoptera and Trichoptera richness decreased significantly by 20% and 25% at the most sediment impacted sites (30% cover) by comparison with sediment‐free sites. 6. We conclude that sediment deposition in the upper Usk system mostly reflects local bank erosion, with riparian woodland likely to mediate this process through bank stability. Fine sediment release had marked ecological effects, but these were detectable only at patch‐to‐patch scales. We suggest that investigation of localized sediment release in streams will benefit from scale‐dependent or scale‐specific sampling, and some effects could go undetected unless sample resolution is selected carefully.     

Mating affects resource selection and modulates associational effects between neighbouring resources

Associational effects occur when the attack rate on a resource depends on neighbouring resources in the environment. These effects are predicted to result from mismatches experienced by the consumer organism in resource selection along hierarchical search levels. As resource selection depends on sensory information used during search behaviour, we expected that different physiological states of an insect might modulate the outcome of associational effects due to differences in resource selection. We used Drosophila melanogaster, as a model organism for olfactory‐guided behaviour in insects, to study the effects of mating induced behavioural changes on associational effects between two alternative resources. We found that mating has no effect on the ability of D. melanogaster to locate resource patches, but rather affects the perception of the resources within the patch. Consequently, we only found associational effects in the experiments with unmated females and not in the experiments with mated females. Our results suggest that the lack of associational effects for mated females resulted from changes in the use of short‐range olfactory cues, leading to random selection among the resources. In conclusion, our results suggest that the physiological state of an insect modulates associational effects by affecting resource selection rates within the patch.     

Scale-dependent effects of landscape context on urban bee diversity

Journal of Insect Conservation - As urbanization continues throughout much of the world, there is great interest in better understanding the value of urban and residential environments to...     

Scale-dependent effects of windthrow disturbance on forest arthropod communities

The effect of disturbance on local communities may operate within the context of the spatial landscape. We examined the scale-dependent effects of windthrow disturbance caused by a large typhoon on three arthropod communities in a temperate forest of Japan. Canopy arthropods were collected by beating foliage, forest-floor arthropods were collected by sweeping the vegetation, and flying arthropods were collected in Malaise traps. To assess the “functional spatial scale” at which arthropods responded to tree-fall disturbance, the gap rate was quantified at different spatial scales by sequentially enlarging the radius of a circular landscape sector in 10-m increments from 10 to 500 m. We then analyzed the responses of order richness and abundance to the gap rate for each arthropod community. The spatial scale of the significant best-fitting model, which was selected from the models fitted to the gap rate at stepwise spatial scales, was regarded as the arthropod-specific functional spatial scale. Arthropod order richness was not dependent on the gap rate. In contrast, arthropod order abundance depended significantly on the gap rate in many orders, but varied in the response direction and functional spatial scale. These order-specific, scale-dependent responses to tree-fall gaps could complicate interactions among organisms, leading to complex community organization. An understanding of the spatial processes that link the use of space by organisms with the spatial scale at which ecological processes are experienced is required to elucidate the responses of populations, communities, and biotic interactions to disturbances in a spatial landscape context.     

Indirect effects between shared prey: Predictions for

Suppression of a target prey by a predator can depend on its surrounding community, including the presence of nontarget, alternative prey. Basic theoretical models of two prey species that interact only via a shared predator predict that adding an alternative prey should increase predator numbers and ultimately lower target pest densities as compared to when the target pest is the only prey. While this is an alluring prediction, it does not explain the numerous responses empirically observed. To better understand and predict the indirect interactions produced by shared predation, we explore how additional prey species affect three broad ecological mechanisms, the predator's reproductive, movement, and functional responses. Specifically, we review current theoretical models of shared predation by focusing on these mechanisms, and make testable predictions about the effects of shared predation. We find that target predation is likely to be higher in the two prey system because of predator reproduction, especially when: predators are prey limited, alternative or total prey density is high, or alternative prey are available over time. Target predation may also be greater because of predator movement, but only under certain movement rules and spatial distributions. Predator foraging behavior is most likely to cause lower target predation in the two-prey system, when per capita predation is limited by something other than prey availability. It is clear from this review that no single theoretical generalization will accurately predict community-level effects for every system. However, we can provide testable hypotheses for future empirical and theoretical investigations of indirect interactions and help enhance their potential use in biological control.     

树种组成决定联合抗性或易感性:以昆嵛山腮扁叶蜂发生为例

以昆嵛山天然赤松(Pinus densiflora)林和寡食性食叶昆虫-昆嵛山腮扁叶蜂(Cephalcia kunyushanica)为研究对象,对比树种组成类型、多样性、立地和林分因子对昆嵛山腮扁叶蜂种群密度的影响,分析了2009-2011年昆嵛山腮扁叶蜂在不同林分类型中种群年度波动变异系数.结果表明:林分类型是影响昆嵛山腮扁叶蜂虫口密度最重要的解释变量,随着赤松与混交亲缘关系越远,虫口密度越小.不同林分类型中,Shannon指数和赤松株虫口密度存在差异,但两者变化趋势完全不同.赤松纯林中赤松株虫口密度均为最高,与赤松-同属混交林没有显著差异,显著高于赤松-同目和阔叶树种组成的混交林;赤松纯林和赤松-同属混交林中昆嵛山腮扁叶蜂种群年度波动大于其他两种林分类型.表明相比树种多样性,树种组成对昆嵛山腮扁叶蜂种群的影响更为重要,赤松与其亲缘关系较近树种混交,昆虫种群稳定性较差,赤松与相邻树种形成对昆嵛山腮扁叶蜂联合易感作用,而与其他亲缘关系较远的树种混交,使害虫种群稳定性增强,进而形成联合抗性作用.     

Scale-dependent effects of landscape pattern on plant diversity in Hunshandak Sandland

The influence of landscape pattern on plant diversity has strong scale-dependent effects. However, the relationship is still unclear for sandy land, which covers more than one-third of the world’s land mass. Aiming at exploring such scale-dependent effects in sandy land, we conducted a case study in Hunshandak Sandland, northern China. Principal component analysis (PCA) and Redundancy Analysis (RDA) was used to disentangle the relations between landscape pattern and alpha and beta plant diversity. Our results show that landscape pattern has an important influence on plant diversity, however, there existed scale effects. Landscape diversity enhanced the alpha diversity, conversely, reduced the beta diversity for all scales. PSSD (Patch Size Standard Deviation) positively related with alpha diversity whilst negatively related with beta diversity on moderate and large spatial scales, same as LPI (Largest Patch Index) on moderate scales. Shape complexity of patches can slightly increase both alpha and beta diversity at large scales. The adjustment of landscape pattern based on different spatial scales can enhance plant diversity. It is useful to improve plant diversity conservation in sandy land.     

Small scale variability of benthic assemblages: biogenic neighborhood effects

In this study, patterns of community development were investigated within vs. outside 'habitats'. These habitats represented five different monospecific assemblages of one of the following species: the brown alga Fucus serratus, the red alga Delesseria sanguinea, the green alga Enteromorpha intestinalis, the seagrass Zostera marina and the blue mussel Mytilus edulis. Natural assemblages were allowed to develop on paired artificial substrata-separated by ca. 1 m-within (treatment) vs. outside (control) of habitats. The same colonizer species settled on treatment and control substrata for given habitats. However, after 5 months of settlement and post-settlement dynamics, their proportional abundance and the structure of treatment and control assemblages differed in many instances. Variability among replicates of a given treatment, seperated by up to 50 m, was large, indicating a patchy spatial distribution of organisms. Despite this spatial heterogeneity among within-treatment replicates, analysis of similarity revealed that in most instances significantly different assemblages developed between treatments on a small spatial scale depending on whether substrata were positioned within as compared to outside a given habitat.Consequently, the algae, seagrass or mussels constituting a habitat seem to control the structure of the benthic assemblage developing in their vicinity by one or more possible mechanisms: reduction of larval advection, exudation of metabolites that influence settlement and/or post-settlement survival, and/or-in the case of mussel assemblages-predation on larvae.In addition to spatial variability in larval supply, stochasticity in succession, substratum heterogeneity, competition and predation effects, this investigation reveals the potential of a further assemblage structuring factor: the impact of neighboring organisms.     

Scale-dependent diversity effects of seed dispersal by a wild herbivore in fragmented grasslands

Dispersal limitation between habitat fragments is a known driver of landscape-scale biodiversity loss. In Europe, agricultural intensification during the twentieth century resulted in losses of both grassland habitat and traditional grassland seed dispersal vectors such as livestock. During the same period, populations of large wild herbivores have increased in the landscape. Usually studied in woodland ecosystems, these animals are found to disperse seeds from grasslands and other open habitats. We studied endozoochorous seed dispersal by roe deer (Capreolus capreolus) in fragmented grasslands and grassland remnants, comparing dispersed subcommunities of plant species to those in the established vegetation and the seed bank. A total of 652 seedlings of 67 species emerged from 219 samples of roe deer dung. This included many grassland species, and several local grassland specialists. Dispersal had potentially different effects on diversity at different spatial scales. Almost all sites received seeds of species not observed in the vegetation or seed bank at that site, suggesting that local diversity might not be dispersal limited. This pattern was less evident at the landscape scale, where fewer new species were introduced. Nonetheless, long-distance dispersal by large wild herbivores might still provide connectivity between fragmented habitats within a landscape in the areas in which they are active. Finally, as only a subset of the available species were found to disperse in space as well as time, the danger of future biodiversity loss might still exist in many isolated grassland habitats.     

Scale-dependent dynamics: Zooplankton and the stability of freshwater food webs

The study of freshwater pelagic communities is entering an exciting and controversial phase. Recent efforts to clarify how food web interactions differ from food chain interactions have emphasized the various, often subtle, repercussions of top predators on communities. Predators can modify community structure not only through directly imposed death rates, but also through direct and indirect effects on prey interactions, behavior, life-styles and morphology (e.g. induction of defenses). In some cases, the effects influence ecosystem properties (material fluxes, turnover rates and primary production). Attempts to trace food web impacts in enclosure and lake studies have revealed important time-dependent system properties. Severe resource limitation of fast variables (phytoplankton and small zooplankton) stabilizes lower trophic levels, whereas the potentially destabilizing effects of fish population oscillations are long compared to the growing season and subject to year-to-year climatic vagaries. The time-scale dependent approach is important because it emphasizes how local (transient) solutions may be more ecologically relevant to stability calculations than overall (global) solutions.     

Scale-dependent relationships between landscape structure and microclimate

This paper examined the scale-dependent relationships between landscape structure (e.g., slope, elevation, and overstory canopy coverage) and microclimate (e.g., air and soil temperatures and soil moisture) at different spatial scales along a 10050 m transect in the Southeastern Missouri Ozarks, USA. The landscape structure and microclimate variables were measured every 10 m along the transect during the growing season, June to September, 1996. We used the simple correlation analysis and "moving window" technique (no overlap between two adjacent windows) to examine the correlation coefficients between landscape structure and microclimate variables at scales or window sizes from 10 m to 2000 m. Because the sample size decreased rapidly in the "moving window" method, we also used the standardized cross-variogram to investigate the relationships between landscape structure and microclimate variables at a larger range of scales from 10 m to 8000 m. We found that the relationships between landscape structure and microclimate were apparently scale-dependent along the transect, suggesting the interactions between landscape structure and microclimate were stronger at some scales than others. The landscape structure variables were poor in explaining the variation of each microclimate variable at fine scales (e.g., 10 m). The correlations between elevation and microclimate variables were, in general, significantly improved with the increase of scales, while the improvement was less significant for slope and canopy coverage. Of the landscape structure variables, elevation, in general, had a higher correlation with the microclimate variables than slope and overstory canopy coverage at most scales examined. Our results suggest that small scales (e.g., < 100 m) are not suitable to study the relationships/interactions between landscape structure and microclimate and larger scales (e.g., > 500 m) are more appropriate though the relationships vary at the larger scales. Both the simple correlation analysis and standardized cross-variogram analysis were effective and, in general, consistent in characterizing the scale-dependent relationships between landscape structure and microclimate. Meanwhile, the standardized cross-variogram had the advantage to examine the relationships at large scales over the correlation analysis because the sample size reduced rapidly in the correlation analysis.