Oecologia

Species with moderate intraspecific trait variability are locally abundant within an environmentally heterogeneous subtropical forest Abstract Species with large intraspecific trait variability (ITV) have larger niche breadth than species with low ITV and thus are expected to be more abundant at the local scale. However, whether the positive ITV–abundance relationship holds in heterogeneous local environments remains uncertain. Using an individual-based trait dataset encompassing three leaf traits (leaf area, specific leaf area, and leaf dry mass content) of 20,248 individuals across 80 species in an environmentally heterogeneous subtropical forest in eastern China, ITV for each trait of each species was estimated by rarefaction. Resource-based niche breadth and marginality (the absolute distance between the mean resource states used by a species and the mean plot-wise resource states) were estimated simultaneously by the K–S method and the outlying mean index, respectively. Species with moderate ITV were often locally abundant, while species with large or small ITV were locally rare. This unimodal relationship between ITV and species abundance persisted when traits were analyzed separately and for all tree size classes. There was also a hump-backed relationship between niche breadth and marginality, and ITV was positively associated with niche breadth. The combined results suggest either a trade-off between the benefit from expanding niche breadth to adapt to multiple habitats and the disadvantage of reducing competitive ability, or a scarcity of favorable resources. Our results do not support the traditional thought that ITV positively correlates with species abundance in heterogeneous local environments. Instead, our study suggests that moderate—rather than large—intraspecific trait variability increases species abundance at local scales.

The paradox of nest reuse: early breeding benefits reproduction, but nest reuse increases nest predation risk Abstract Many animals build new nests every breeding season instead of saving time by reusing old ones. One hypothesis is that nest reuse leads to increased predation risk if predators memorize nest locations and revisit these sites. Here we examine patterns in the prevalence of facultative nest reuse. Further, we relate nest reuse and timing of breeding to nest predation risk, clutch size and nestling survival. We analyse 1570 breeding attempts of the Eurasian sparrowhawk (Accipiter nisus) from Denmark (1977–1997) and from two sites in Norway (1985–2017). The probability of reuse varied between study areas, increased in replacement clutches, and was lower in adults compared to 1-year-old breeders. Pairs reusing nests laid their first egg on average 2.6 ± 1.0 SE days later than those building new nests, suggesting they are compensating for an already late breeding schedule. Indeed, reuse increased nest predation risk, but we discovered no other productive effects of reuse. In non-predated nests, late breeders had both smaller clutches and lower nestling survival. We propose that nest predation is a contributing driver to the behaviour of building a new nest each year, whereas nest reuse is a strategy to compensate for delayed onset of breeding, mainly used by inexperienced males.

Long-term acclimation to near-future ocean acidification has negligible effects on energetic attributes in a juvenile coral reef fish Abstract Increased levels of dissolved carbon dioxide (CO2) drive ocean acidification and have been predicted to increase the energy use of marine fishes via physiological and behavioural mechanisms. This notion is based on a theoretical framework suggesting that detrimental effects on energy use are caused by plasma acid–base disruption in response to hypercapnic acidosis, potentially in combination with a malfunction of the gamma aminobutyric acid type A (GABAA) receptors in the brain. However, the existing empirical evidence testing these effects primarily stems from studies that exposed fish to elevated CO2 for a few days and measured a small number of traits. We investigated a range of energetic traits in juvenile spiny chromis damselfish (Acanthochromis polyacanthus) over 3 months of acclimation to projected end-of-century CO2 levels (~ 1000 µatm). Somatic growth and otolith size and shape were unaffected by the CO2 treatment across 3 months of development in comparison with control fish (~ 420 µatm). Swimming activity during behavioural assays was initially higher in the elevated CO2 group, but this effect dissipated within ~ 25 min following handling. The transient higher activity of fish under elevated CO2 was not associated with a detectable difference in the rate of oxygen uptake nor was it mediated by GABAA neurotransmitter interference because treatment with a GABAA antagonist (gabazine) did not abolish the CO2 treatment effect. These findings contrast with several short-term studies by suggesting that end-of-century levels of CO2 may have negligible direct effects on the energetics of at least some species of fish.

Volatiles of pathogenic and non-pathogenic soil-borne fungi affect plant development and resistance to insects Abstract Plants are ubiquitously exposed to a wide diversity of (micro)organisms, including mutualists and antagonists. Prior to direct contact, plants can perceive microbial organic and inorganic volatile compounds (hereafter: volatiles) from a distance that, in turn, may affect plant development and resistance. To date, however, the specificity of plant responses to volatiles emitted by pathogenic and non-pathogenic fungi and the ecological consequences of such responses remain largely elusive. We investigated whether Arabidopsis thaliana plants can differentiate between volatiles of pathogenic and non-pathogenic soil-borne fungi. We profiled volatile organic compounds (VOCs) and measured CO2 emission of 11 fungi. We assessed the main effects of fungal volatiles on plant development and insect resistance. Despite distinct differences in VOC profiles between the pathogenic and non-pathogenic fungi, plants did not discriminate, based on plant phenotypic responses, between pathogenic and non-pathogenic fungi. Overall, plant growth was promoted and flowering was accelerated upon exposure to fungal volatiles, irrespectively of fungal CO2 emission levels. In addition, plants became significantly more susceptible to a generalist insect leaf-chewing herbivore upon exposure to the volatiles of some of the fungi, demonstrating that a prior fungal volatile exposure can negatively affect plant resistance. These data indicate that plant development and resistance can be modulated in response to exposure to fungal volatiles.

Soil-mediated impacts of an invasive thistle inhibit the recruitment of certain native plants Abstract Invasive plants may outcompete and replace native plant species through a variety of mechanisms. Recent evidence indicates that soil microbial pathways such as pathogen accumulation may have a considerable role in facilitating competition between native and invasive plants. To assess microbe-mediated pathways of invasion, we tested the impacts of invaded and non-invaded field soils on plant establishment using naturally occurring populations of the common Eurasian invader Cirsium arvense (Canada thistle) in Southern Ontario, Canada. Linked field and greenhouse experiments were used to quantify differences in the germinability and early growth rates of native plant species, depending on exposure to the microbial community in invaded or non-invaded soils. The invaded microbial community significantly reduced early growth rates for two of the seven native species surveyed, and decreased seed germination for another. In contrast, the germination and growth of invasive Cirsium were not affected by its own soil microbial community. These results demonstrate that the invasion of C. arvense can reduce the performance of some native plant species through changes to the soil microbial community. Different effects on different species suggest that this invader may also change the relative importance of certain natives in the invaded community. If these effects influence plant abundance in the field, microbially mediated interactions in the soil may aid the invasion of C. arvense and facilitate the disruption of invaded communities.

Interspecific competition for frugivores: population-level seed dispersal in contrasting fruiting communities Abstract Indirect interactions among plant species mediated by frugivorous animals can be central to population and community dynamics, since the successful seed dispersal of species may depend on facilitative or competitive interactions with heterospecific plants. Yet, empirical evidence on these interactions is very scarce and mostly available at small spatial scales, within populations. Because lipid-rich fruits are known to be preferred by migratory birds, here we test our prediction of competitive inferiority of a carbohydrate-rich fruited species (the hawthorn Crataegus monogyna) compared to lipid-rich co-fruiting species in a Mediterranean region where the bulk of seed dispersal relies on migratory birds. We assessed avian seed dispersal in both relative (fruit removal rate) and absolute terms (seed dispersal magnitude) in seven hawthorn populations distributed across an altitudinal gradient encompassing three contrasting fruiting contexts: hawthorn is scarce in the lowlands, common in the midlands, and the dominant fruit species in the highlands. We found evidence of seed dispersal reduction due to interspecific competition in the lowland populations, where lipid-rich fruits dominate. Besides, DNA barcoding analysis of bird-dispersed seeds revealed that only a small subset of the local frugivore assemblages consumed hawthorn fruits in the lowland communities. Instead, the consumers of hawthorn fruits resembled the local frugivore assemblages where hawthorn fruits were more dominant and frugivore choices more limited. Our study suggests mechanisms by which the rarity or dominance of plant species might be jointly influenced by environmental constraints (here, precipitation along the altitudinal gradient) and frugivore-mediated indirect interactions among plants hindering or facilitating seed dispersal.

Foraging strategies of individual silky pocket mice over a boom–bust cycle in a stochastic dryland ecosystem Abstract Small mammals use multiple foraging strategies to compensate for fluctuating resource quality in stochastic environments. These strategies may lead to increased dietary overlap when competition for resources is strong. To quantify temporal contributions of high (C3) versus low quality (C4) resources in diets of silky pocket mice (Perognathus flavus), we used stable carbon isotope (δ13C) analysis of 1391 plasma samples collected over 2 years. Of these, 695 samples were from 170 individuals sampled ≥ 3 times across seasons or years, allowing us to assess changes in dietary breadth at the population and individual levels across a boom–bust population cycle. In 2014, the P. flavus population increased to 412 captures compared to 8 captures in prior and subsequent years, while populations of co-occurring small mammals remained stable. As intraspecific competition increased, the population-wide dietary niche of P. flavus did not change, but individual specialization increased significantly. During this period, ~ 27% (41/151) of individuals sampled specialized on C3 resources, which were abundant during the spring and previous fall seasons. Most of the remaining individuals were C3–C4 generalists (64%) (96/151), and only 9% (14/151) specialized on C4 resources. In 2015, P. flavus population density and resource availability declined, individual dietary breadth expanded (84% generalists), no C3 specialists were found, and specialization on C4 resources increased (16%). Our results demonstrate a high degree of inter-individual plasticity in P. flavus foraging strategies, which has implications for how this species will respond to environmental change that is predicted to decrease C3 resources in the future.

Low levels of intraspecific trait variation in a keystone invertebrate group Abstract The trait-based approach to ecology promises to provide a mechanistic understanding of species distributions and ecosystem functioning. Typically, trait analyses focus on average species trait values and assume that intraspecific variation is small or negligible. Recent work has shown, however, that intraspecific trait variation can often contribute substantially to total trait variation. Whilst many studies have investigated intraspecific variation in plants, very few have done so for invertebrates. There is no research on the level of intraspecific trait variation in ants (Hymenoptera: Formicidae), despite the fact that there is a growing body of literature using ant morphological trait data and demonstrating that these insects play important roles in many ecosystems and food webs. Here, we investigate the intraspecific variability of four commonly used ant morphological traits from 23 species from the Maloti-Drakensberg Mountains of southern Africa. In total, we measured 1145 different individuals and made 6870 trait measurements. Intraspecific variation accounted for only 1–4% of total trait variation for each of the four traits we analysed. We found no links between intraspecific variation, phylogeny and elevation. On average, six individuals generated robust species means but under biased sampling scenarios 20 individuals were needed. The low levels of intraspecific morphological variation that we find suggest that the approach of using mean species traits is valid, in this fauna at least. Regardless, we encourage ant trait ecologists to measure greater numbers of individuals, especially across gradients, to shed further light on intraspecific variation in this functionally important group of insects.

The importance of productivity and seasonality for structuring small rodent diversity across a tropical elevation gradient Abstract Photosynthetic productivity is a key determinant of the abundance and distribution of biodiversity around the world. The effect of this productivity on the distribution patterns of mammals is frequently invoked; however, it is seldom measured directly. In this study, we used Sherman live traps set in dry and rainy seasons across a 2300-m elevation gradient in southwestern Mexico to assess small rodent species distributions, and to relate these patterns to habitat structure, climate, and a well-accepted measure of photosynthetic productivity: the normalized difference vegetation index (NDVI). While habitat structure and climate helped explain some of the patterns observed, NDVI proved to be the most important contributing variable for most of the distribution models. We also found that partitioning the gradient-distribution model by trapping season revealed strong differences in terms of the effect of NDVI and the other explanatory variables. For example, lower elevations were associated with seasonal and year-round reductions in rodent diversity and were composed almost exclusively of granivore-based species assemblages. By contrast, the middle and upper elevations were more species rich, less affected by seasonality, and characterized by omnivorous species. Our results suggest that the positive productivity–diversity relationship found may be due, at least in part, to increased food resources and niche opportunities at more productive elevations. Increased diversity at the higher elevations may also be partially due to reductions in competition that result from productivity increases, as well as from the broader spectrum of feeding guild representation that it and the lack of seasonality allow.

Seed mass equalises the strength of positive and negative plant–plant interactions in a semi-arid grassland Abstract The interplay and balance between positive and negative interactions are important and recurrent topics in plant ecology. If facilitation occurs because benefactors ameliorate stress, such balance may be driven by seed mass because stress tolerance and competitive ability of plants, particularly seedlings, are positively correlated with seed mass. Thus, small-seeded, stress-intolerant species may require facilitation, but not large-seeded ones. This would equalise the magnitudes of opposite-signed interactions because in small-seeded species, positive effects of facilitators and negative effects of competitors should be strong, while both effects should be weak in large-seeded species. To test this idea, we assessed the effects of interactions with four associated species on different components of the performance of ten focal species. As expected, the largest facilitative and competitive effects were recorded in small-seeded species, and positive interactions had similar magnitudes to negative ones for any given seed mass, especially when performance was integrated into lifelong fitness. Furthermore, the fact that small-seeded species seem to be strongly facilitated may explain why they are not outcompeted by large-seeded species. This is an alternative to other hypotheses explaining the coexistence of plants with different-sized seeds. The close balance between opposite-signed interactions in the presence of stress may also explain why interactions have strong effects on individuals (that interact with only a few species), but seemingly weaker effects on populations, where interactions between many species would cancel out.

Alexandros Sfakianakis
Anapafseos 5 . Agios Nikolaos
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6948891480

alsfakia@gmail.com