Vertebrate consumers are known to influence plant recruitment and community assembly based on the species they prefer to eat. The enemy release hypothesis (ERH) predicts that invasive introduced plant species may avoid consumption due to a lack of coevolved predators. Herbivores and granivores may also prefer species due to their functional traits, irrespective of species origin. We hypothesized that in a highly invaded plant community, invasive plants would benefit from enemy release and animal consumers would likely decrease plant diversity if they avoided invasive species in favor of native species (Hypothesis 1). Animals were expected to filter the species pool based on preference for palatable functional traits (Hypothesis 2). A factorial experiment combining cage and native seed addition treatments was established in the Voorhis Ecological Reserve, a California sage scrub ecosystem in Southern California. Seed and seedling preference trials were conducted to gather additional evidence of consumer preference at the study site. Although we found little influence of plant functional traits on consumer selection of species, animals created more functionally similar communities. In preference trials, native rodents and birds were found to prefer native plant species and avoid dominant invasive species, and the effects of animal preference were observed in experimental plant communities. Small vertebrates encouraged the success of invasive plants while limiting recruitment of native species, consistent with the ERH. Thus, birds and small mammals may function as an important filter in sage scrub habitat by reducing community diversity and promoting the dominance of competitively superior invasive plant species.
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Understanding the multi-scale effects of habitat fragmentation on biodiversity is essential for effective conservation in a rapidly changing world. Here, we assessed plant functional composition in edge and interior plots across 28 islands in Thousand Island Lake, China. Plants were categorized into four functional groups based on life form and shade tolerance. Functional composition was defined as the proportions of functional groups in each plot, island, and virtual landscape. Differences between edge and interior plots were analyzed using the Wilcoxon test, and correlations with island area were evaluated using Spearman rank correlation. The relationship between functional composition and total habitat amounts in virtual landscapes was investigated using SLOSS analysis. We found that interior plots had higher proportions of shade-tolerant trees and lower proportions of shade-intolerant trees, regardless of whether richness- or abundance-based metrics were used. The functional composition of shrubs showed minimal variation with plot type, though edge plots had a higher proportion of shade-tolerant shrubs. Island area affected functional composition differently at the island scale, depending on whether richness-based or abundance-based metrics were used. The relationship between total habitat amounts and functional composition exhibited threshold patterns with unstable and stable phases at the landscape scale. Our study reveals a hierarchical structure in the multi-scale impacts of habitat fragmentation on plant functional composition and highlights different responses of functional groups across scales. Thus, we conclude that fragmentation research must consider ecological impacts at multiple scales and from multiple perspectives.
Regenerating forests comprise a significant proportion of tropical forest ecosystems, yet species assembly mechanisms after anthropogenic disturbances remain poorly understood. Locally established ant communities follow predictable paths along forest regeneration gradients, but whether this results from dispersal limitation or habitat filtering is unclear. Social insects, with highly mobile dispersing reproductives (alates) but sessile colony stages, provide a window to disentangle these mechanisms. We compared assemblages of dispersing alates to workers from established colonies of ants and termites along a chronosequence in the Chocó lowland tropical forest, Ecuador. Our study area comprises a regeneration gradient where agricultural land, regenerating forests (1-37 years old), and old-growth forests are interspersed across a 200-km2 landscape mosaic with short distances among land-use types. Alate assemblages of both taxa were independent of forest age but more similar in spatially closer plots. Worker ant assemblages were more similar at similar succession stages and elevations. Termite worker assemblages were more similar at similar elevations but more dissimilar in spatially closer plots. These results indicate that alates can disperse across all succession stages and elevations within our study area, but not all species successfully establish or persist everywhere. For ants, colony distribution is determined by habitat filters associated with forest age and elevation. For termites, colony distribution is influenced by elevation-related habitat filtering. Importantly, we found increasing species diversity with forest age for both taxa, underscoring the importance of advanced natural forest regeneration and old-growth forest conservation for maintaining diverse social insect communities.
Understanding how ecologically similar species coexist remains a central question in ecology. Shifts in species composition along environmental gradients offer opportunities to explore underlying mechanisms, such as condition-specific competition-where competitive dominance reverses across environments. However, broad-scale gradients often involve multiple co-varying abiotic factors, making causal inference difficult, and experimental evidence for competition reversal remains limited. Although temperature is widely regarded as a key driver of salmonid distributions, its mechanistic role remains unclear. We investigated the roles of temperature and species interactions for the coexistence of juvenile Dolly Varden (Salvelinus curilus, colder-adapted) and white-spotted charr (S. leucomaenis, warmer-adapted) in multiple replicated tributaries that minimized spatial-autocorrelation and co-varying physiochemical factors. Structural equation modeling revealed that water temperature significantly influenced Dolly Varden density but not that of white-spotted charr. In addition, Dolly Varden density negatively affected white-spotted charr density, while the reverse effect was not detected, contradicting prior findings of white-spotted charr competitive dominance. A transplant experiment showed that white-spotted charr persisted and grew well in cold tributaries where Dolly Varden numerically dominated. These results suggest that white-spotted charr avoid cold habitats not due to thermal limitations, but due to biotic interactions with Dolly Varden, potentially mediated by food availability. Thus, although temperature shapes species distributions, temperature-specific competition reversal is unlikely to be the dominant mechanism. Because Dolly Varden typically establish themselves in these habitats earlier and attain numerical dominance, we propose that prior residence effects-where early occupants gain competitive advantages-may better explain local coexistence patterns in this system.
Reproductive aging is a widespread process in wild populations, affecting both females and males across many species. It also plays a key role in shaping parental effects as older parents are expected to be unable to invest optimally in reproduction late in life, or such investments may be compromised by the detrimental consequences of aging. In most species, reproductive performance increases over the first breeding attempts, reaches a plateau, and then declines at older ages. Many long-lived species, however, deviate from this pattern, with older individuals maintaining high breeding performance or even improving it, often resulting in offspring of higher quality. Studies examining sex-specific patterns of reproductive aging in long-lived species from natural populations and their consequences for offspring fitness remain scarce compared to the extensive human literature. We used a long-term longitudinal dataset of reproductive data collected from a population of a long-lived seabird, the Scopoli's shearwater, to analyze (i) how a set of parental traits (i.e., reproductive success, body mass and egg size) change with maternal and paternal age, and (ii) potential age-dependent parental effects on the body mass and skeletal traits of the offspring. We found that age strongly affected reproductive success. Early-life improvement was steeper in females than in males, whereas both sexes showed the typical late-life decline expected with advancing age. Adult body mass followed a similar, though partially reversed, pattern: males gained mass more rapidly than females early in life before reaching a plateau. Age also affected female egg volume and width, with older and heavier females laying larger and wider eggs. Parental effects on offspring body mass were mainly driven by pair experience, whereas skeletal traits depended only on chick age and sex. Our study shows that reproductive aging varies across traits in long-lived species, providing support for the asynchronous theory of aging, and reveals unexpected sex-specific patterns of age-related reproductive changes in monogamous species. Moreover, it suggests that advanced parental age is not necessarily associated with negative effects on offspring fitness.
Linear features, such as road verges and hedgerows can function as corridors for pollinators during foraging. Studies have shown that these features can enhance the foraging movement of pollinators such as bees, increasing the pollination success of bee pollinated plants. However, it remains unclear if these effects extend to communities of plants pollinated by other insect pollinators. We conducted a field experiment where we measured the pollination of phytometer plant assemblages linked by linear features. We constructed six 30 m artificial linear features in both urban and rural meadows in Southwest England, and placed plant assemblages at either end of these artificial features, comparing these to plant assemblages without links. Each plant assemblage had seven species known to attract different insect pollinators including bees, flies, and moths. We evaluated the self-incompatibility of plants and used seed set to measure pollination success. Artificial linear features significantly improved the pollination of three bee pollinated species, whereas fly and moth pollinated species were less affected. Despite these differences among pollinator groups, the presence of linear features increased overall seed set in assemblages. We suggest that linear features likely play an important role in maintaining pollination for certain plant species, depending on which insect groups are their key pollinators. This result highlights the importance of considering community-level impacts of corridors on pollination. Given the critical role of bees as pollinators, linear features can provide a practical means to evaluate the ecological functions of habitat corridors, but alternative approaches may be required for flies or moths.
Herbivores evolved mutualisms with bacteria that break down plant fiber and provide usable energy. Bacteria may also provide a source of dietary protein, but to differing extents: foregut fermenters digest rumen bacteria in addition to plant proteins, whereas hindgut fermenters cannot, but may recoup bacterial protein by consuming cecal pellets (coprophagy). Carbon isotope fingerprinting of essential amino acids (EAAs) is a recently refined approach that uses characteristic patterns of carbon isotope ratios among EAAs synthesized by major lineages of life (e.g., bacteria, fungi, and plants) to identify protein sources in consumers. Here, we use carbon isotope fingerprinting to 1) answer whether obligate anaerobic bacteria (i.e., gut bacteria) have the same carbon isotope fingerprint as previously measured aerobic or facultatively anaerobic bacteria; and 2) evaluate this method as a tool in differentiating the contribution of bacterial protein to herbivores with foregut (moose Alces alces and muskox Ovibos moschatus) versus hindgut fermentation (horse Equus ferus caballus) and hindgut fermentation with coprophagy (snowshoe hares Lepus americanus), represented by blood serum/plasma EAAs. We found that obligate anaerobic bacteria had a carbon isotope fingerprint distinct from obligate aerobic and facultative anaerobic bacteria. Moose, muskox, and snowshoe hares had approximately equal proportions of EAAs from diet and gut bacteria, while horses had two-thirds of EAAs from diet and one-third from gut bacteria. Carbon isotope fingerprinting provides a biologically sensitive and field adaptable measure for understanding bacterial contribution to protein nutrition in herbivores with different digestive strategies.
Human-induced changes in nitrogen (N) and phosphorus (P) global cycles and availability significantly impact plant growth and nutritional composition, thereby affecting ecosystem dynamics. However, research on the effects of increased nutrient availability often focuses on plant community-level effects, overlooking interspecific variability and neglecting impacts on higher trophic levels. Using a controlled fertilization experiment with six tree species that naturally occur in the Cerrado biome (Brazilian savannas), we showed that there is substantial interspecific variation in how plants respond to nutrient changes. This reflects the existence of competitive advantage for certain native species under a scenario of increased soil nutrient availability. Such effects propagated to higher trophic levels (herbivores and their predators), also varying between plant host species. The strength and direction of N input effect depended on P levels and the type of herbivores. Large invertebrate leaf herbivores were less affected than phytophagous mites. Impacts on higher trophic levels (predatory mites) were less pronounced than on phytophagous mites. Overall, we show that ongoing soil nutrient enrichment has the potential to alter interspecific competition dynamics in plant communities with consequences for ecological interaction partners. These findings have important implications for conservation and ecosystem management, especially in areas highly exposed to soil nutrient enrichment due to farming and industrial activities.
Although large grazers are well known to alter fire regimes, small herbivore effects on fire have received comparatively little attention. The gopher tortoise (Gopherus polyphemus) is a herbivorous reptile that acts as an ecosystem engineer in upland, fire-dependent ecosystems of the southeastern U.S. Many animals rely on their deep burrows for refuge from extreme temperatures and fire. At the same time, gopher tortoise's burrowing and foraging activities may decrease fire intensity and severity by reducing plant biomass and/or by altering the flammability of the adjacent plant community. We examined the spatial scale and evidence for each mechanism underlying potential fire effects in sandhill at Archbold Biological Station in south-central Florida. We selected 30 existing burrows varying in activity status (active, inactive, abandoned) as well as non-mound control points in relatively open microsites. We characterized plant biomass and community composition within 15 m of mounds and control points and quantified 11 fire-related traits for 23 common plant species. Our analysis of pre- and post-fire drone imagery from an earlier fire in our study area found a localized reduction in fire severity within about 2 m of tortoise burrows. Our analysis of contemporary burrows showed that mounds of both active and inactive tortoise burrows had lower plant and litter cover than abandoned mounds and the vegetation matrix beyond the mound itself. Tortoise effects on community-level flammability were minor and unlikely to modify fire intensity. Overall, the highly localized soil disturbance associated with burrowing is likely the primary means by which gopher tortoises may decrease fire severity surrounding their burrows. Critically, our study highlights how small animals can potentially shape fire behavior via direct reduction of fuel loads.
Nutrient dynamics substantially influences forest productivity, yet the influence of competition on these processes in forests is not well understood. Nitrogen is an important limiting nutrient for tree growth, but how canopy nitrogen dynamics are influenced by tree diversity and competition is unclear. Nitrogen-use efficiency is one mechanism suggested to support niche complementarity, promoting increased productivity as diversity increases, and is influenced by the resorption of foliar nutrients during senescence. We tested how diversity and competition affect canopy nitrogen dynamics by examining nitrogen resorption efficiency (NRE) in over 200 trees from three tree species in a 12-year-old mixed-species hardwood planting that included a full-factorial design of three levels of species diversity, representing intra- and interspecific competition, and two levels of planting density, representing competition intensity. Midseason and senescent foliar samples were collected from black cherry (Prunus serotina), American chestnut (Castanea dentata), and northern red oak (Quercus rubra) and we determined midseason canopy and foliar litter nitrogen concentrations and calculated NRE. Higher midseason canopy nitrogen concentrations were observed in more diverse, low-density plantings, but the response magnitude varied among species. NRE increased as diversity increased; the response, however, varied among species, ranging from a statistically significant increase to no response, but was not influenced by planting density. These findings provide evidence that while the influence of diversity on tree physiological responses depends on the type of competitive interaction (i.e., diversity or intensity), different combinations of species will likely affect processes related with niche complementarity and, ultimately, a positive biodiversity-productivity relationship.
Volatile organic compounds (VOCs) emitted by deadwood are increasingly recognised as key olfactory cues used by saproxylic beetles to locate suitable substrates, yet their role during colonisation remains poorly understood. To address this, we quantified VOC emissions and beetle assemblages while experimentally disentangling the main ecological drivers of the deadwood volatilome (tree species, sun exposure). We exposed 1200 freshly cut branches of oak, beech, spruce, and pine across Central Europe. To mimic natural variation in deadwood and disturbance, bundles were either sterilised (reducing endogenous fungi), inoculated with a brown rot fungus (Fomitopsis pinicola) or a white rot fungus (Fomes fomentarius), or burned. From each bundle, we sampled 448 substances, 89 of which were identified as VOCs, and reared 134 saproxylic beetle species. Broadleaf and conifer species emitted distinct VOC profiles that matched beetle tree-type preferences. In conifers, bark beetles, longhorn beetles, and jewel beetles were associated with different chemical cues, whereas taxonomic separation was not observed in broadleafs. Although treatments altered VOC composition, they did not explain beetle colonisation. Our study shows that VOCs emitted during early decay are associated with distinct beetle assemblages. The VOC composition varied with tree species and treatments, indicating that chemical variation reflects the influence of multiple ecological factors. These findings suggest that tree-species diversity enhances chemical heterogeneity in deadwood, which is linked to broader beetle assemblages. Forest conservation efforts may therefore need to consider the role of chemical variation in deadwood, as it could influence saproxylic colonisation and biodiversity management.
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Atmospheric nitrogen (N) enrichment is known to alter plant community trait composition and diversity. Additional research in low-resource environments is needed, however, as plant communities in these systems may respond differently than those where N is less limiting. Further, fungal endophytes, such as the Epichloë group, alter key mechanisms of species coexistence but have yet to be studied for their effects on community-level functional traits under conditions of global change. We studied the effects of N enrichment and the presence of aboveground fungal endophyte Epichloë amarillans (Epichloë hereafter) within the dominant grass species, Ammophila breviligulata (Ammophila hereafter), on community-weighted mean (CWM) traits and trait diversity of colonizing plant species in a long-term experiment of a low-resource coastal dune system. N enrichment at both real-world and high levels increased CWMs of plant size and specific leaf area (SLA) of colonizing species, and high N enrichment initially suppressed trait diversity (FDis). While N enrichment reduced community-level specific root length (SRL), this effect disappeared when Epichloë was present, indicating a predominant role of the fungal endophyte. We found that N enrichment may alter plant functional traits, even in a low-resource habitat, which may increase productivity, alter erosional dynamics, and affect belowground functioning. The presence of a fungal endophyte, however, altered community response to N enrichment treatments in a key root trait indicating that its presence may affect community-level traits and belowground functioning beyond its host species.
Fertilization and herbivore exclusion are two key forces influencing community structure and species richness in grassland ecosystems. However, the extent to which these factors influence the relative contributions of deterministic and stochastic processes in community assembly remains poorly understood. In this study, we conducted a five-year field experiment that manipulated nutrient availability and herbivore exclusion in an alpine meadow on the eastern Qinghai-Tibet Plateau. The results showed that fertilization, herbivore exclusion, and their combination significantly decreased species richness and drove Raup-Crick dissimilarity close to - 1 relative to the control. In the final year of the experiment, these treatments drove a distinct directional shift in community composition, diverging significantly from the control. Furthermore, all treatments significantly increased community-weighted mean (CWM) height and specific leaf area (SLA), and these shifts in fast resource-acquisition traits were significantly correlated with the intensification of deterministic processes (i.e., Raup-Crick dissimilarity approaching - 1). Consequently, we conclude that in natural alpine meadows (control), community assembly is primarily driven by stochastic processes. In contrast, the experimental treatments diminished the influence of stochasticity, establishing the dominance of deterministic processes through environmental filtering. This shift in assembly patterns was largely mediated by plant functional traits. Because this filtering strongly selected for dominant species possessing fast resource-acquisition traits and exceptional competitive abilities for light, it ultimately led to community homogenization and a decline in species richness. Notably, the combined treatment did not yield a stronger deterministic effect than either treatment alone, indicating a lack of an additive effect.
Population density often modifies the phenotypes of the members of the population. Such density-dependent phenotypic plasticity can affect basic life history traits of the organisms. In insects, a frequently observed expression of such plasticity is the crowding response (CR), where individuals growing at high densities develop faster and attain lower final sizes compared to those at low densities. This plastic change qualitatively differs from the general stress response where lower final sizes are associated with longer development periods. The adaptive significance of CR, as well as the nature of the cues that trigger CR, remains poorly understood. We performed a series of experiments to identify proximate signals leading to CR in the geometrid moth Hypomecis atomaria, a species in which larvae reared in groups consistently pupate earlier and at lower weights than those reared in isolation. Our findings reveal that CR is also induced in complete darkness, suggesting that visual cues of high population densities do not play a decisive role. CR was triggered when the larvae were separated by a mesh barrier, preventing tactile interaction between them. The presence of heterospecific lepidopteran larvae also triggered CR, though to varying degrees. By contrast, neither the presence of dipteran insects in the rearing environment nor human-inflicted tactile stimulation affected the growth schedules of H. atomaria larvae. We conclude that CR is likely induced either by chemical signals or substrate-borne vibrations caused by other larvae. In any case, CR is not a highly specific response to high densities of conspecifics, nor is it a very general reaction to unspecific disturbances. This allows us to narrow down the set of potential adaptive explanations for the phenomenon.
Breeding phenological responses to changing environments affect demographics and population persistence. However, inter-individual variation in adjustment of reproductive timing to predation risk has been overlooked. We thus tested whether increasing natural predation risk enhanced or homogenized reproductive timing of females differing in risk sensitivity. Using behavioral (flight initiation distance, FID) and cognitive (relative head volume) proxies of risk sensitivity, we analyzed laying dates of female common eiders (Somateria mollissima) breeding in SW Finland (Baltic Sea) relative to conspecifics under fluctuating predation pressure, while accounting for breeding experience and body condition. We found that high predation risk was associated with a divergence of female eider breeding phenologies depending on cognitive but not behavioral proxies of risk sensitivity. Accordingly, relatively large-headed females, assumed to be more risk-sensitive, bred later than small-headed ones following years of high adult or nest predation risk-plausibly reflecting extended nest prospecting in risk-sensitive individuals-leading to greater laying asynchrony. In contrast, females displaying longer FIDs (more risk-sensitive) bred earlier than those displaying shorter FIDs, but this response was irrespective of the level of predation risk. Because FID was measured late in incubation, we hypothesize that it may more strongly reflect state-dependent parental investment in the current breeding attempt than risk sensitivity during nest initiation, with higher parental investment promoting delayed escape. Additionally, experienced breeders bred earlier than inexperienced ones. More attention should be given to cognitive traits and behaviors associated with risk sensitivity to better understand variation in individual breeding phenology and synchrony.
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The plateau pika, a keystone species of the Tibetan Plateau, is also widely poisoned as an agricultural pest. An intriguing observation is that pika are more abundant where their main competitor (domestic yak) is also most abundant. A recent study showed that this may be because pika supplement their diet in winter by consuming yak feces. Here we used quantitative fatty acid signatures analysis to explore pika diet and detect the possibility of coprophagy. We collected fat tissue from pika at 3 sites of varying elevation, and compared the abundance of 10 fatty acids (FA) in their body fat with those in their main potential foods (grass, roots and feces). We used an optimization program to establish the best-fit diet to the observed FA distributions, using mouse fat as a negative control. Pika fat FA signatures matched plateau foods far better than mouse fat. Faeces consumption doubled as the elevation of the site increased from 12.8% at the lowest site to 26.8% at the highest. In contrast consumption of roots showed the opposite pattern, comprising 32% at the lowest elevation and falling to virtually zero at the highest. That might reflect the time that the surface ground is frozen inhibiting the ability to dig for roots at higher elevation sites. Our data support the previous suggestion that pika eat yak feces, potentially giving them an over-winter survival advantage increasing their populations where yak are abundant. The full extent of coprophagy however requires more data in a larger population sample.