The main drivers of global environmental change (CO₂ enrichment, nitrogen deposition, climate, biotic invasions and land use) cause extinctions and alter species distributions, and recent evidence ...shows that they exert pervasive impacts on various antagonistic and mutualistic interactions among species. In this review, we synthesize data from 688 published studies to show that these drivers often alter competitive interactions among plants and animals, exert multitrophic effects on the decomposer food web, increase intensity of pathogen infection, weaken mutualisms involving plants, and enhance herbivory while having variable effects on predation. A recurrent finding is that there is substantial variability among studies in both the magnitude and direction of effects of any given GEC driver on any given type of biotic interaction. Further, we show that higher order effects among multiple drivers acting simultaneously create challenges in predicting future responses to global environmental change, and that extrapolating these complex impacts across entire networks of species interactions yields unanticipated effects on ecosystems. Finally, we conclude that in order to reliably predict the effects of GEC on community and ecosystem processes, the greatest single challenge will be to determine how biotic and abiotic context alters the direction and magnitude of GEC effects on biotic interactions.
The conceptual foundations of habitat fragmentation research have not kept pace with empirical advances in our understanding of species responses to landscape change, nor with theoretical advances in ...the wider disciplines of ecology. There is now real debate whether explicit recognition of 'habitat fragmentation' as an over-arching conceptual domain will stimulate or hinder further progress toward understanding and mitigating the effects of landscape change. In this paper, we critically challenge the conceptual foundations of the discipline, and attempt to derive an integrated perspective on the best way to advance mechanistic understanding of fragmentation processes. We depict the inherent assumptions underlying the discipline as a 'conceptual phase space' of contrasting false dichotomies in fragmentation 'problem space'. In our opinion, the key determinant of whether 'habitat fragmentation' can remain a cohesive framework lies in the concept of 'interdependence': 1) interdependence of landscape effects on species and 2) interdependence of species responses to landscape change. If there is non-trivial interdependence among the various sub-components of habitat fragmentation, or non-trivial interdependence among species responses to landscape change, then there will be real heuristic value in 'habitat fragmentation' as a single conceptual domain. At present, the current paradigms entrenched in the fragmentation literature are implicitly founded on strict independence of landscape effects (e.g. the debate about the independent effects of habitat loss versus fragmentation per se) and strict independence of species responses (e.g. the individualistic species response models underpinning landscape continuum models), despite compelling evidence for interdependence in both effects and responses to fragmentation. We discuss how strong 'interdependence' of effects and responses challenges us to rethink longheld views, and re-cast the conceptual foundations of habitat fragmentation in terms of spatial context-dependence in the effects of multiple interacting spatial components of fragmentation, and community context-dependence in the responses of multiple interacting species to landscape change.
Habitat loss has pervasive and disruptive impacts on biodiversity in habitat remnants. The magnitude of the ecological impacts of habitat loss can be exacerbated by the spatial arrangement – or ...fragmentation – of remaining habitat. Fragmentation per se is a landscape-level phenomenon in which species that survive in habitat remnants are confronted with a modified environment of reduced area, increased isolation and novel ecological boundaries. The implications of this for individual organisms are many and varied, because species with differing life history strategies are differentially affected by habitat fragmentation. Here, we review the extensive literature on species responses to habitat fragmentation, and detail the numerous ways in which confounding factors have either masked the detection, or prevented the manifestation, of predicted fragmentation effects. Large numbers of empirical studies continue to document changes in species richness with decreasing habitat area, with positive, negative and no relationships regularly reported. The debate surrounding such widely contrasting results is beginning to be resolved by findings that the expected positive species-area relationship can be masked by matrix-derived spatial subsidies of resources to fragment-dwelling species and by the invasion of matrix-dwelling species into habitat edges. Significant advances have been made recently in our understanding of how species interactions are altered at habitat edges as a result of these changes. Interestingly, changes in biotic and abiotic parameters at edges also make ecological processes more variable than in habitat interiors. Individuals are more likely to encounter habitat edges in fragments with convoluted shapes, leading to increased turnover and variability in population size than in fragments that are compact in shape. Habitat isolation in both space and time disrupts species distribution patterns, with consequent effects on metapopulation dynamics and the genetic structure of fragment-dwelling populations. Again, the matrix habitat is a strong determinant of fragmentation effects within remnants because of its role in regulating dispersal and dispersal-related mortality, the provision of spatial subsidies and the potential mediation of edge-related microclimatic gradients. We show that confounding factors can mask many fragmentation effects. For instance, there are multiple ways in which species traits like trophic level, dispersal ability and degree of habitat specialisation influence species-level responses. The temporal scale of investigation may have a strong influence on the results of a study, with short-term crowding effects eventually giving way to long-term extinction debts. Moreover, many fragmentation effects like changes in genetic, morphological or behavioural traits of species require time to appear. By contrast, synergistic interactions of fragmentation with climate change, human-altered disturbance regimes, species interactions and other drivers of population decline may magnify the impacts of fragmentation. To conclude, we emphasise that anthropogenic fragmentation is a recent phenomenon in evolutionary time and suggest that the final, long-term impacts of habitat fragmentation may not yet have shown themselves.
Insects sustain key ecosystem functions, but how their activity varies across the day-night cycle and the underlying drivers are poorly understood. Although entomologists generally expect that more ...insects are active at night, this notion has not been tested with empirical data at the global scale. Here, we assemble 331 quantitative comparisons of the abundances of insects between day and night periods from 78 studies worldwide and use multi-level meta-analytical models to show that insect activity is on average 31.4% (CI: -6.3%-84.3%) higher at night than in the day. We reveal diel preferences of major insect taxa, and observe higher nocturnal activity in aquatic taxa than in terrestrial ones, as well as in warmer environments. In a separate analysis of the small subset of studies quantifying diel patterns in taxonomic richness (31 comparisons from 13 studies), we detect preliminary evidence of higher nocturnal richness in tropical than temperate communities. The higher overall (but variable) nocturnal activity in insect communities underscores the need to address threats such as light pollution and climate warming that may disproportionately impact nocturnal insects.
There is strong trait dependence in species‐level responses to environmental change and their cascading effects on ecosystem functioning. However, there is little understanding of whether ...intraspecific trait variation (ITV) can also be an important mechanism mediating environmental effects on ecosystem functioning. This is surprising, given that global change processes such as habitat fragmentation and the creation of forest edges drive strong trait shifts within species. On 20 islands in the Thousand Island Lake, China, we quantified intraspecific leaf trait shifts of a widely distributed shrub species, Vaccinium carlesii, in response to habitat fragmentation. Using a reciprocal transplant decomposition experiment between forest edge and interior on 11 islands with varying areas, we disentangled the relative effects of intraspecific leaf trait variation versus altered environmental conditions on leaf decomposition rates in forest fragments. We found strong intraspecific variation in leaf traits in response to edge effects, with a shift toward recalcitrant leaves with low specific leaf area and high leaf dry matter content from forest interior to the edge. Using structural equation modeling, we showed that such intraspecific leaf trait response to habitat fragmentation had translated into significant plant afterlife effects on leaf decomposition, leading to decreased leaf decomposition rates from the forest interior to the edge. Importantly, the effects of intraspecific leaf trait variation were additive to and stronger than the effects from local environmental changes due to edge effects and habitat loss. Our experiment provides the first quantitative study showing that intraspecific leaf trait response to edge effects is an important driver of the decrease in leaf decomposition rate in fragmented forests. By extending the trait‐based response–effect framework toward the individual level, intraspecific variation in leaf economics traits can provide the missing functional link between environmental change and ecological processes. These findings suggest an important area for future research on incorporating ITV to understand and predict changes in ecosystem functioning in the context of global change.
Many insect species are under threat from the anthropogenic drivers of global change. There have been numerous well‐documented examples of insect population declines and extinctions in the scientific ...literature, but recent weaker studies making extreme claims of a global crisis have drawn widespread media coverage and brought unprecedented public attention. This spotlight might be a double‐edged sword if the veracity of alarmist insect decline statements do not stand up to close scrutiny.
We identify seven key challenges in drawing robust inference about insect population declines: establishment of the historical baseline, representativeness of site selection, robustness of time series trend estimation, mitigation of detection bias effects, and ability to account for potential artefacts of density dependence, phenological shifts and scale‐dependence in extrapolation from sample abundance to population‐level inference.
Insect population fluctuations are complex. Greater care is needed when evaluating evidence for population trends and in identifying drivers of those trends. We present guidelines for best‐practise approaches that avoid methodological errors, mitigate potential biases and produce more robust analyses of time series trends.
Despite many existing challenges and pitfalls, we present a forward‐looking prospectus for the future of insect population monitoring, highlighting opportunities for more creative exploitation of existing baseline data, technological advances in sampling and novel computational approaches. Entomologists cannot tackle these challenges alone, and it is only through collaboration with citizen scientists, other research scientists in many disciplines, and data analysts that the next generation of researchers will bridge the gap between little bugs and big data.
The focus on global insect declines puts insect conservation firmly on the public agenda but could become a double‐edged sword if population trend estimates do not withstand increased scrutiny.
Here, we identify seven key challenges in drawing robust quantitative inference about insect population declines, reflecting errors of baseline, trend estimation and resulting population level inference.
We present a forward‐looking prospectus for the future of insect population monitoring, highlighting opportunities for more creative exploitation of existing baseline data, technological advances in sampling and novel computational approaches.
Habitat loss and fragmentation has long been considered the primary cause for biodiversity loss and ecosystem degradation worldwide, and is a key research topic in landscape ecology.
The glow‐worm Lampyris noctiluca (Linnaeus, 1767) (Coleoptera: Lampyridae) is thought to be declining in the United Kingdom. Yet, much of the evidence for this is anecdotal, with a shortage of ...standardised long‐term data to investigate temporal changes in abundance.
We present an 18‐year time series of standardised transect surveys for glowing adult females at 19 sites within south‐east England (Essex) from 2001 to 2018.
We used generalised additive mixed models (GAMMs) to control for varying sampling effort, temporal autocorrelation, non‐stationarity of seasonal phenology and non‐linearity of temporal trajectories across sites.
We found a significant long‐term reduction in counts of glowing female glow‐worms, after accounting for a significant shift in seasonal phenology across years, and a negative effect of warmer climatic conditions on glow‐worm abundance. Average glowing counts in south‐east England declined by ca. −3.5% per annum from 2001 to 2018, and this result held true even after a range of sensitivity tests to account for potential methodological artefacts in citizen science data collection.
Temporal trajectories in abundance were strikingly out of phase across the 19 sites, suggesting that local‐scale factors in addition to climate are driving greater reduction in numbers at some sites than others.
These standardised surveys present the first quantitative evidence that numbers of glow‐worms could well be declining in the United Kingdom. There is a clear signal of climate warming and drying effects on glow‐worm numbers, but a substantially greater proportion of variation in glowing female counts is explained by local‐scale site factors, such as unmanaged scrub encroachment. Conservation strategies that can mitigate local population losses could be an essential buffer against climate‐driven declines in south‐east England.
The glow‐worm Lampyris noctiluca is thought to be declining. We present standardised transect surveys for glowing adult females at 19 sites within south‐east England (Essex) from 2001 to 2018.
We found a significant long‐term reduction in counts of glowing females, after accounting for a significant shift in seasonal phenology, and negative effect of warmer climatic conditions on abundance.
These surveys present the first quantitative evidence that glow‐worms may be declining in England. Climate warming and local‐scale site factors, such as unmanaged scrub encroachment, could reduce glow‐worm numbers.
Changes in soil fertility during pedogenesis affect the quantity and quality of resources entering the belowground subsystem. Climate governs pedogenesis, yet how climate modulates responses of soil ...food webs to soil ageing remains unexplored because of the paucity of appropriate model systems. We characterised soil food webs along each of four retrogressive soil chronosequences situated across a strong regional climate gradient to show that belowground communities are predominantly shaped by changes in fertility rather than climate. Basal consumers showed hump‐shaped responses to soil ageing, which were propagated to higher‐order consumers. There was a shift in dominance from bacterial to fungal energy channels with increasing soil age, while the root energy channel was most important in intermediate‐aged soils. Our study highlights the overarching importance of soil fertility in regulating soil food webs, and indicates that belowground food webs will respond more strongly to shifts in soil resources than climate change.
Context
Predicting and managing edge effects requires an understanding of the mechanisms that drive them. However, analytical methods that dominate edge effects research are not well suited to ...discriminating mechanisms, because they do not measure ‘indirect’ edge effects: effects that are mediated by covariates in statistical models.
Objective
To discuss the value of indirect effects for improving mechanistic understanding of edge effects.
Methods
We explain how measuring indirect effects improves mechanistic understanding, and provide guidance on how to do so. We also conduct a literature review to examine awareness of indirect effects in empirical studies of mechanisms underpinning edge effects. Finally, we use a recent paper in Villaseñor et al. (Landscape Ecol 30:229–245,
2015
) as a case study to discuss how failure to measure indirect edge effects may limit mechanistic understanding.
Results
Indirect effects provide a means to translate conceptual models of edge effects into mechanistic pathways that are testable and quantifiable. Moreover, failure to measure indirect edge effects can result in impacts of habitat edges being underestimated. However, few studies that we identified in our literature review quantified indirect effects (7 %, n = 72). Worryingly, 11 % of studies did not account for indirect effects despite using statistical models that potentially contained them, possibly resulting in incorrect inference.
Conclusions
A better awareness of indirect effects will help researchers to understand the mechanisms that underpin edge effects, while ensuring that impacts of habitat edges are not underestimated.
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