Meta-analysis of elevational changes in the intensity of trophic interactions: Similarities and dissimilarities with latitudinal patterns

The premise that the intensity of biotic interactions decreases with increasing latitudes and elevations is broadly accepted; however, whether these geographical patterns can be explained within a common theoretical framework remains unclear. Our goal was to identify the general pattern of elevational changes in trophic interactions and to explore the sources of variation among the outcomes of individual studies. Meta-analysis of 226 effect sizes calculated from 134 publications demonstrated a significant but interaction-specific decrease in the intensity of herbivory, carnivory and parasitism with increasing elevation. Nevertheless, this decrease was not significant at high latitudes and for interactions involving endothermic organisms, for herbivore outbreaks or for herbivores living within plant tissues. Herbivory similarly declined with increases in latitude and elevation, whereas carnivory showed a fivefold stronger decrease with elevation than with latitude and parasitism increased with latitude but decreased with elevation. Thus, although these gradients share a general pattern and several sources of variation in trophic interaction intensity, we discovered important dissimilarities, indicating that elevational and latitudinal changes in these interactions are partly driven by different factors. We conclude that the scope of the latitudinal biotic interaction hypothesis cannot be extended to incorporate elevational gradients

High tolerance to simulated root herbivory in hydroponically grown cuttings of Salix phylicifolia

The shortage of information regarding the effects of root damage on forest plants, especially trees and shrubs, is the most critical gap in our knowledge of belowground insect herbivory. This study examines how the tea-leaved willow, Salix phylicifolia, responds to simulated root herbivory and how nutrient availability affects these responses. Hydroponically grown cuttings were used to precisely apply the desired levels of root damage. Root: shoot ratios increased proportionally to the intensity of root damage, whereas specific leaf area, leaf water content and chlorophyll fluorescence were not affected. The removal of 2-64% of roots once in early summer and the removal of 25% of roots three times during the summer did not change biomass production by cuttings. The repeated removal of 50 and 75% of roots decreased shoot biomass and, to a lesser extent, total root production. The tolerance of tea-leaved willow to root damage was greater in low nutrient treatment than in high nutrient treatment, thus supporting the Growth Rate Model. According to this model, in low resource conditions plants do not attain their maximum growth rate and therefore have a higher capacity for regrowth after damage. We conclude that, in the absence of water limitations, tea-leaved willow shows high tolerance to root losses and is unlikely to be affected by realistic levels of root herbivory

Biases in ecological research: attitudes of scientists and ways of control

The properties of the human mind affect the quality of scientific knowledge through the insertion of unconscious biases during the research process. These biases frequently cause overestimation of the effects under study, thereby violating the reproducibility of the research and potentially leading to incorrect conclusions in subsequent research syntheses. We explored the level of knowledge about biases and attitudes to this problem by analysing 308 responses of ecology scientists to a specifically developed survey. We show that knowledge about biases and attitude towards biases depend on the scientist’s career stage, gender and affiliation country. Early career scientists are more concerned about biases, know more about measures to avoid biases, and twice more frequently have learned about biases from their university courses when compared with senior scientists. The respondents believe that their own studies are less prone to biases than are studies by other scientists, which hampers the control of biases in one’s own research. We conclude that education about biases is necessary, but not yet sufficient, to avoid biases because the unconscious origin of biases necessitates external intervention to combat them. Obligatory reporting of measures taken against biases in all relevant manuscripts will likely enhance the reproducibility of scientific results.</p

Latitudinal gradient in the intensity of biotic interactions in terrestrial ecosystems: Sources of variation and differences from the diversity gradient revealed by meta-analysis

The Latitudinal Biotic Interaction Hypothesis (LBIH) states that the intensity of biotic interactions increases from high to low latitudes. This hypothesis, which may partly explain latitudinal gradients in biodiversity, remains hotly debated, largely due to variable outcomes of published studies. We used meta-analysis to identify the scope of the LBIH in terrestrial ecosystems. For this purpose, we explored the sources of variation in the strength of latitudinal changes in herbivory, carnivory and parasitism (119 publications) and compared these gradients with gradients in the diversity of the respective groups of animals (102 publications). Overall, both herbivory and carnivory decreased towards the poles, while parasitism increased. The latitudinal gradient in herbivory and carnivory was threefold stronger above 50-60 degrees than at lower latitudes and was significant due to interactions involving ectothermic consumers, studies using standardised prey (i.e. prey lacking local anti-predator adaptations) and studies aimed at testing LBIH. The poleward decrease in biodiversity did not differ between ectothermic and endothermic animals or among climate zones and was fourfold stronger than decrease in herbivory and carnivory. The discovered differences between the gradients in biotic interactions and biodiversity suggest that these two global macroecological patterns are likely shaped by different factors

Seasonal variations in bird selection pressure on prey colouration

The direction and strength of selection for prey colouration by predators vary in space and time and depend on the composition of the predator community. We tested the hypothesis that bird selection pressure on prey colouration changes through the season due to changes in the proportion of naive juvenile individuals in the bird community, because naive and educated birds differ in their responses to prey colours. Bird predation on caterpillar-shaped plasticine models in two boreal forest sites increased sevenfold from early summer to mid-summer, and the time of this increase coincides with the fledging of juvenile birds. In early summer, cryptic (black and green) models were attacked at fivefold higher rates compared with conspicuous (red and yellow) models. By contrast, starting from fledging time, cryptic and conspicuous models were attacked at similar rates, hinting at a lower selectivity by naive juvenile birds compared with educated adult birds. Cryptic models exposed in a group together with conspicuous models were attacked by birds at a threefold lower rate than cryptic models exposed singly, thus supporting the aposematic commensalism hypothesis. However, this effect was not observed in mid- and late summer, presumably due to the lack of avoidance of conspicuous prey by the juvenile birds. We conclude that selection pressure on prey colouration weakens considerably when naive birds dominate in the community, because the survival advantages of aposematic colouration are temporarily lost for both the conspicuous and their neighbouring cryptic prey

Leaf size is more sensitive than leaf fluctuating asymmetry as an indicator of plant stress caused by simulated herbivory

Fluctuating asymmetry (FA), defined as small, random departures away from perfect symmetry, is frequently recommended as a sensitive and universal indicator of environmental stress imposed by both abiotic and biotic factors. However, the stress sensitivities of FA and of other morphological traits are rarely compared directly. Here, we tested the hypothesis that leaf FA, rather than leaf size, is more sensitive to simulated herbivory. We performed blind measurements of FA and length of the same leaves of juvenile mountain birches (Betula pubescens var. pumila) after removal of 0, 2, 4, 8 and 16% of their leaf area during five consequent years. The leaf length in defoliated birches was 88% of that in control birches, indicating that simulated herbivory was stressful for trees. By contrast, leaf FA did not differ significantly between defoliated and control birches, despite a sufficient (91%) power of statistical analysis. Thus, leaf size, rather than leaf FA, was a more sensitive indicator of stress. This low sensitivity of FA to stress, discovered with the use of blind methods, contrasts a large pool of earlier studies, many of which have been likely influences by confirmation bias. We urge the publication of 'negative' or inconclusive results which currently remain underreported due to publication bias

Predation and parasitism on herbivorous insects change in opposite directions in a latitudinal gradient crossing a boreal forest zone

The latitudinal biotic interaction hypothesis (LBIH) predicts that the strength of various biotic interactions decreases from low to high latitudes. Inconsistency between studies testing this hypothesis may result from variations among different types of interactions and among study systems. Therefore, exploration of multiple interactions within one system would help to disentangle latitudinal patterns across individual interactions and to evaluate latitudinal changes in the overall impact of enemies on prey. We tested the prediction based on the LBIH that the pressure of natural enemies on herbivorous insects decreases with increase in latitude across the boreal forest zone. We also asked whether the impacts of major groups of these enemies exhibit similar latitudinal patterns and whether these patterns are consistent across study years. In 10 forest sites located from 60 degrees N to 69 degrees N in Northern Europe, each summer, from 2016 to 2019, we measured (a) mortality of three groups of leafmining insects caused by birds, ants, parasitoids and unknown factors, (b) bird attacks on caterpillar-shaped plasticine models and (c) birch foliar damage caused by defoliators and leafminers. Latitudinal patterns in both insect herbivory on birch and top-down pressure on herbivorous insects varied considerably and inconsistently among the four study years, so that only some of the year-specific correlations with latitude were statistically significant. Nevertheless, meta-analysis combining correlations across years, preys and enemies revealed general decreases in predation by birds (on both natural and model prey) and ants, but an increase in parasitism rates, from low to high latitudes. We found that the direction of latitudinal changes in the strength of biotic interactions was interaction-specific: predation and herbivory supported LBIH, whereas parasitism exhibited an opposite trend. Consequently, the overall impact of natural enemies on herbivorous insects did not change with latitude and was therefore an unlikely reason for the poleward decrease in herbivory observed in our gradient. Considerable among-year variation in the strength of the latitudinal patterns in all the studied interactions suggests that this variation is a widespread phenomenon

Bird predation does not explain spatial variation in insect herbivory in a forest-tundra ecotone

The contribution of bird predation to the spatial variations in insect herbivory remains imperfectly understood, especially in Arctic ecosystems. We experimentally tested the hypothesis that the differences in insect herbivory between tundra and forest biomes, and between plant life-forms in these biomes, are associated with differences in the intensity of bird predation on defoliating insects. We observed substantial variation in herbivory (0% to 20% of foliage lost) among nine forest, mountain tundra, and lowland tundra sites in the Kola Peninsula (northwestern Russia) and among five woody plant species, but we found no consistent differences in herbivory between biomes and between plant life-forms. Bird attacks on artificial caterpillars were tenfold more frequent in forest than in tundra, while bird exclusion effects on herbivory did not differ between biomes, and the intensities of bird predation measured by these two methods were not correlated. Bird exclusion led to increases in insect herbivory, and this effect was significant in trees and tall shrubs but was not significant in dwarf shrubs in either forest or tundra sites. Bird predation, as measured in bird exclusion experiments, increased with an increase in the level of foliar damage inflicted by insects in forests but not in tundra habitats. We conclude that bird predation generally decreases plant losses to insects in both forest and tundra habitats, but birds are unlikely to shape the spatial patterns of plant losses to insects in Arctic ecosystems

Insect herbivory increases from forest to alpine tundra in Arctic mountains

Current theory holds that the intensity of biotic interactions decreases with increases in latitude and elevation; however, empirical data demonstrate great variation in the direction, strength, and shape of elevational changes in herbivory. The latitudinal position of mountains may be one important source of this variation, but the acute shortage of data from polar mountains hampers exploration of latitude effects on elevational changes in herbivory. Here, we reduce this knowledge gap by exploring six elevation gradients located in three Arctic mountain ranges to test the prediction that a decrease in herbivory occurs with increasing elevation from forest to alpine tundra. Across the 10 most abundant evergreen and deciduous woody plant species, relative losses of foliage to insect herbivores were 2.2-fold greater at the highest elevations (alpine tundra) than in mid-elevation birch woodlands or low-elevation coniferous forests. Plant quality for herbivores (quantified by specific leaf area) significantly decreased with elevation across all studied species, indicating that bottom-up factors were unlikely to shape the observed pattern in herbivory. An experiment with open-top chambers established at different elevations showed that even a slight increase in ambient temperature enhances herbivory in Arctic mountains. Therefore, we suggest that the discovered increase in herbivory with elevation is explained by higher temperatures at the soil surface in open habitats above the tree line compared with forests at lower elevations. This explanation is supported by the significant difference in elevational changes in herbivory between low and tall plants: herbivory on low shrubs increased fourfold from forest to alpine sites, while herbivory on trees and tall shrubs did not change with elevation. We suggest that an increase in herbivory with an increase in elevation is typical for high-latitude mountains, where inverse temperature gradients, especially at the soil surface, are common. Verification of this hypothesis requires further studies of elevational patterns in herbivory at high latitudes.</p