Untangling interactions: do temperature and habitat fragmentation gradients simultaneously impact biotic relationships?

Future climate and land-use intensification modify arthropod community structure

Changes in the atmospheric concentration of carbon dioxide ([CO2]), nutrient availability and biotic diversity are three major drivers of the ongoing global change impacting terrestrial ecosystems worldwide. While it is well established that soil nutrient heterogeneity exerts a strong influence on the development of plant individuals and communities, it is virtually unknown how nutrient heterogeneity and global change drivers interact to affect plant performance and ecosystem functioning. We conducted a microcosm experiment to evaluate the effect of simultaneous changes in [CO2], nutrient heterogeneity (NH), nutrient availability (NA) and species evenness on the biomass and nutrient uptake patterns of assemblages formed byLolium perenne,Plantago lanceolataandHolcus lanatus. When the nutrients were heterogeneously supplied, assemblages exhibited precise root foraging patterns, and had higher above‐ and belowground biomass (average increases of 32% and 29% for above‐ and belowground biomass, respectively). Nutrient heterogeneity also modulated the effects of NA on biomass production, complementarity in nitrogen uptake and below: aboveground ratio, as well as those of [CO2] on the nutrient use efficiency at the assemblage level. Our results show that nutrient heterogeneity has the potential to influence the response of plant assemblages to simultaneous changes in [CO2], nutrient availability and biotic diversity, and suggest that it is an important environmental factor to interpret and assess plant assemblage responses to global change.

Direct quantification of terrestrial biosphere responses to global change is crucial for projections of future climate change in Earth system models. Here, we synthesized ecosystem carbon-cycling data from 1,119 experiments performed over the past four decades concerning changes in temperature, precipitation, CO2 and nitrogen across major terrestrial vegetation types of the world. Most experiments manipulated single rather than multiple global change drivers in temperate ecosystems of the USA, Europe and China. The magnitudes of warming and elevated CO2 treatments were consistent with the ranges of future projections, whereas those of precipitation changes and nitrogen inputs often exceeded the projected ranges. Increases in global change drivers consistently accelerated, but decreased precipitation slowed down carbon-cycle processes. Nonlinear (including synergistic and antagonistic) effects among global change drivers were rare. Belowground carbon allocation responded negatively to increased precipitation and nitrogen addition and positively to decreased precipitation and elevated CO2. The sensitivities of carbon variables to multiple global change drivers depended on the background climate and ecosystem condition, suggesting that Earth system models should be evaluated using site-specific conditions for best uses of this large dataset. Together, this synthesis underscores an urgent need to explore the interactions among multiple global change drivers in underrepresented regions such as semi-arid ecosystems, forests in the tropics and subtropics, and Arctic tundra when forecasting future terrestrial carbon-climate feedback.

Ongoing ocean global change due to anthropogenic activities is causing multiple chemical and physical seawater properties to change simultaneously, which may affect the physiology of marine phytoplankton. The coccolithophore Emiliania huxleyi is a model species often employed in the study of the marine carbon cycle. The effect of ocean acidification (OA) on coccolithophore calcification has been extensively studied; however, physiological responses to multiple environmental drivers are still largely unknown. Here we examined two-way and multiple driver effects of OA and other key environmental drivers-nitrate, phosphate, irradiance, and temperature-on the growth, photosynthetic, and calcification rates, and the elemental composition of E. huxleyi. In addition, changes in functional gene expression were examined to understand the molecular mechanisms underpinning the physiological responses. The single driver manipulation experiments suggest decreased nitrate supply being the most important driver regulating E. huxleyi physiology, by significantly reducing the growth, photosynthetic, and calcification rates. In addition, the interaction of OA and decreased nitrate supply (projected for year 2100) had more negative synergistic effects on E. huxleyi physiology than all other two-way factorial manipulations, suggesting a linkage between the single dominant driver (nitrate) effects and interactive effects with other drivers. Simultaneous manipulation of all five environmental drivers to the conditions of the projected year 2100 had the largest negative effects on most of the physiological metrics. Furthermore, functional genes associated with inorganic carbon acquisition (RubisCO, AEL1, and δCA) and calcification (CAX3, AEL1, PATP, and NhaA2) were most downregulated by the multiple driver manipulation, revealing linkages between responses of functional gene expression and associated physiological metrics. These findings together indicate that for more holistic projections of coccolithophore responses to future ocean global change, it is necessary to understand the relative importance of environmental drivers both individually (i.e., mechanistic understanding) and interactively (i.e., cumulative effect) on coccolithophore physiology.

vii Acknowledgements ix Authorship declaration xi Chapter I: Introduction 1 1.1 A perspective of climate change in ecological research 1 1.2 How does climate change affect us? 2 1.3 The multiple axes of real-world complexity 4 1.3.1 Biodiversity and the importance of interactions among species 4 1.3.2 Biotic complexity: from species to networks 5 1.3.3 Synergistic global environmental change drivers 7 1.4 Thesis objectives and outline 8 Chapter II: Plant-mediated and non-additive effects of two global change drivers on an insect herbivore community 12 2.1 Abstract 12 2.2 Introduction 13 2.3 Material and Methods 15 2.3.1 Study site 15 2.3.2 Experimental design 16 2.3.3 Vegetation survey 18 2.3.4 Data analysis 18 2.4 Results 20 2.4.1 Plant community response to the environmental change drivers: ....... 22

Soil enzymes are crucial in mediating ecosystems' responses to environmental drivers, so that the comprehension of their sensitivity to drivers of global change can help make predictions of future scenarios and design tailored interventions of biomanipulation. Drivers of global change usually act in combination of two or more, and indirect effects of one driver acting through modification of another one often occur, yet most of both manipulative and meta-analysis studies available tend to focus on the direct effect of one single driver on the activity of specific soil enzymes. One of the biggest challenges is, therefore, represented by the difficulty in assessing the interactions between different drivers, due to the complexity of disentangling the single direct effects from the indirect and combined ones. In this review, after elucidating the general mechanisms of soil enzyme production and activity regulation, we display the state-of-the-art knowledge on direct, indirect and combined effects of the main drivers of global change on soil enzyme activities, identify gaps in knowledge and challenges from research, plus we analyse how this can reverberate in the future of biomanipulation techniques for the improvement of ecosystem services. We conclude that qualitative but not quantitative outcomes can be predicted for some interactions such as warming + drought or warming + CO2 , while for other ones, the results are controversial: future basic research will have to center on this holistic approach. A general trend toward the overall increase of soil enzyme activities and acceleration of biogeochemical cycles will persist, until an inflection will be caused by factors such as future shifts in microbial communities and changes in carbon use efficiency. Applied research will develop toward the refinement of "in situ" analytical systems for the study of soil enzyme activities and the support of bioengineering for the better tailoring of interventions of biomanipulation.

Human impacts are causing an unprecedented change of biodiversity across scales. To quantify the nature and degree of the biodiversity change, there have been a number of meta-analysis studies investigating the effects of global change drivers (land use, climate, etc.). However, these studies include few primary literature studies of soil biodiversity. Soil biodiversity is important for a variety of ecosystem services that are critical for human wellbeing. Yet, we know little about how soil organisms may respond to changing environmental conditions. Although studies have investigated the impact of global change drivers on soil biodiversity, they lack sufficient depth in the number of drivers and/or taxa included. Additionally, the previous focus on aboveground organisms has also resulted in a bias towards certain global change drivers in the primary literature. For example, climate change and land use change are more often studied, whilst pollution is typically understudied as a global change driver. Building on previous studies, we will conduct a meta-analysis to compare the effects of global change drivers (land use, habitat fragmentation/loss, fire, climate change, invasive species, pollution, and nutrient enrichment) on soil fauna (micro- to macro-invertebrates). This project aims to fill the current gaps in the literature, and actively participate in incorporating soil biodiversity into future global biodiversity assessments, by creating the first global open-acess dataset on the impacts of multiple global change drivers on soil fauna.

Nitrogen (N) deposition and climate change are both known to threaten global biodiversity. However, we still have a limited understanding of how interactions between these global change drivers affect individuals and populations of specialist species, such as geophytes, within their natural habitat. We explored possible interactive effects of N, drought, and warming on population vitality (mean leaf length, leaf density, flowering probability) and morpho-physiological traits (e.g., leaf and bulb size, N allocation to leaves and bulbs) of the globally rare forest geophyte Gagea spathacea (Liliaceae) in deciduous forests of northern Germany by applying experimental N addition across a climate gradient over a 5-year period. Mean leaf growth and leaf density were not affected by N addition but were enhanced by warmer and drier conditions in the months before leaf emergence. N addition increased N allocation of individual plants towards their subterranean bulbs. Importantly, effects of N addition on morpho-physiological traits depended on warming and drought, with N-fertilized plants showing increased leaf length and decreased specific leaf and bulb N concentration after drier autumns and warmer winters. This indicates that N deposition may partially compensate for increased N demands during warming-induced growth, although this growth-promoting interaction effect is not (yet) reflected in population vitality. Our results highlight the importance of considering multiple global environmental change drivers and a whole plant perspective (above- and belowground traits) to predict long-term growth responses of (endangered) forest spring geophytes and to develop adapted long-term protection strategies.

<p><strong>Anthropogenic climate warming is a significant driver of global ecosystem change. High-altitude alpine habitats with low annual mean temperatures warm proportionally faster than the global average. These ecosystems contain cold-tolerant, alpine-specialist plants that are disproportionately sensitive to the impacts of climate warming. Climate warming is predicted to facilitate upward shifts in species ranges, increasing invasion pressure and disrupting species interactions. Plant functional trait responses can provide insights into the possible impacts of climate warming on species that may alter alpine communities, which are providers of valuable and unique ecosystem services. Research on plant functional trait responses to warming has focused on easily quantified morphological traits. Knowledge of plant chemical traits is less common, limiting understanding of coordinated shifts in plant function in response to drivers of global change. My thesis aimed to understand how alpine plants respond to global drivers of ecosystem change, such as warming and shifts in plant community composition, by altering their morphological traits and tissue chemistry and how this affects changes in ecosystem service provision.</strong></p><p>I measured the responses of 18 plant functional traits, including those related to leaf morphology, leaf and root tissue chemistry, aboveground biomass, and plant height, to warming and dominant species removal. Traits were measured across six species and two experiments based in Tongariro National Park, New Zealand: Warming and Removal in Mountains (WaRM) and Warming and Invasion (WIN). I hypothesised that trait responses would primarily indicate species benefit from warming and dominant species removal treatments. Plant functional trait responses were highly species-specific, with native species being the most responsive overall and often indicating a shift towards more acquisitive resource strategies, particularly at high-elevation sites. Under warming, high-alpine species Poa cita and Rytidosperma setifolium exhibited positive trait responses. The generative height of Poa cita increased under warming, and Rytidosperma setifolium demonstrated increases in leaf mass, length, and area, as well as vegetative height and aboveground biomass when the dominant species was also removed. Gaultheria colensoi had mixed responses, with decreased leaf size but increased tissue nutrients. Dracophyllum subulatum was measured in both WaRM and WIN experiments; leaf size increased under warming at the WIN site but decreased in the WaRM site, indicating higher stress levels. These trends were inversed at each site when warming and removal treatments were combined. Surprisingly, the traits of invasive species, Calluna vulgaris and the asters were largely unresponsive to treatments. Axes 1 and 2 from principal component analyses were also largely unresponsive to treatments despite individual trait responses, indicating a possible decoupling of trait correlations under warming and species removal.</p><p>Overall, my findings indicate that warming may alleviate temperature limitation to more acquisitive growth strategies in native alpine species, but that competition from invasive species may limit growth responses. These effects may be modulated by plant functional and mycorrhizal type, elevational range, site characteristics, and the length of the warming and invasion period. Alpine ecosystems will need careful management to navigate trade-offs in native plant survival and invasive species removal. An awareness of these trade-offs in future management and conservation plans may enhance outcomes for the culturally, scientifically, aesthetically, and recreationally valuable indigenous and endemic species found in Tongariro National Park.</p>

Current global change substantially threatens pollinators, which directly impacts the pollination services underpinning the stability, structure and functioning of ecosystems. Amongst these threats, many synergistic drivers, such as habitat destruction and fragmentation, increasing use of agrochemicals, decreasing resource diversity, as well as climate change, are known to affect wild and managed bees. Therefore, reliable indicators for pollinator sensitivity to such threats are needed. Biological traits, such as phenotype (e.g. shape, size and asymmetry) and storage reserves (e.g. fat body size), are important pollinator traits linked to reproductive success, immunity, resilience and foraging efficiency and, therefore, could serve as valuable markers of bee health and pollination service potential. This data paper contains an extensive dataset of wing morphology and fat body content for the European honeybee (Apis mellifera) and the buff-tailed bumblebee (Bombus terrestris) sampled at 128 sites across eight European countries in landscape gradients dominated by two major bee-pollinated crops (apple and oilseed rape), before and after focal crop bloom and potential pesticide exposure. The dataset also includes environmental metrics of each sampling site, namely landscape structure and pesticide use. The data offer the opportunity to test whether variation in the phenotype and fat bodies of bees is structured by environmental factors and drivers of global change. Overall, the dataset provides valuable information to identify which environmental threats predominantly contribute to the modification of these traits.

Abstract: I investigated how habitat area, habitat connectivity, and landscape context affect the species richness and abundance of trap‐nesting bees, wasps, and their natural enemies. The research was done on 45 orchard meadows ranging in size from 0.08 to 5.8 ha and differing in habitat connectivity and the surrounding landscape matrix. Effects of the surrounding landscape matrix were restricted to circles with a radius of 250 m. Only the species richness of natural enemies increased with landscape diversity in the surrounding matrix. Population densities of bees increased with habitat connectivity. Total species richness and species richness of bees, eumenid wasps, and natural enemies significantly increased with habitat area. Significant species‐area relationships existed for insect groups but not for plants, thereby confirming the hypothesis that higher trophic levels are more affected by habitat fragmentation than lower trophic levels. The slope of species‐area relationships was steeper for mutualistic bees than for predatory wasps and natural enemies. In contrast to expectations, the rate of parasitism did not depend on habitat area but only on the local and regional abundance of hosts. My results suggest that the area and connectivity of habitat fragments is most important for the conservation of habitat specialists, whereas generalists may profit from a diverse surrounding landscape matrix.

The proverb ‘the enemy of my enemy is my friend’ is axiomatic in community ecology and the study of tri-trophic interactions. This is particularly relevant for studies of plant–herbivore–parasitoid interactions. Parasitoid wasps eavesdrop on plant volatiles emitted after plants are attacked by herbivores (De Moraes et al. 1998). This facilitates host finding by parasitoids, attack by parasitoids reduces population size of herbivore hosts (Rosenheim 1998), and the plants and parasitoids each benefit by the interaction (van Loon et al. 2000). One takes for granted the observation that most plant secondary compounds likely evolved as plant defences against natural enemies, but this was formally proposed as a mechanism only 52 years ago (Frankel 1959). Similarly, plant–insect herbivore–parasitoid interactions are key to understanding plant population dynamics and community ecology, although the conceptual framework for this is also relatively new (Karban et al. 1999). A closer look at life in the tangled bank reveals highly complex ecological interactions and evolutionary outcomes among plants, insect herbivores and the third trophic level. Taking just the case of plant–insect–parasitoid interactions, one finds that most plants are attacked by many herbivore species (co-infections), which are in turn attacked by many parasitoid species, which are attacked by hyperparasitoids, and so on. When examined in detail, such interactions can be driven by complex, unimaginable mechanisms—for example, a phage virus toxin encoded in the genomes of bacterial symbionts of aphids renders hosts resistant to parasitoid wasps (Oliver et al. 2009). Such a complex (and wonderful) ecological and evolutionary milieu might be viewed as an intellectually intractable morass. For example, two herbivorous insect species using a common host plant may compete with or facilitate one another, or may not affect the other’s fitness at all—and—the direction and strength of such interactions are likely to be highly context-dependent. Can we build a robust framework for predicting the direction and strength of plant–herbivore– parasitoid interactions in real ecological communities? Advances in network theory and competition theory are likely to be helpful in understanding how such systems work from an ecological perspective, and potentially how they evolve (Chesson & Kuang 2008; Guimaraes et al. 2011). But are there more fundamental traits of plants that can be leveraged to gain insight into the direction and strength of ecological interactions within these complex communities? In this issue of Functional Ecology, Soler et al. (2012) provide fresh insight into how co-infections robustly modulate plant defence signalling, and then tests whether this influences tri-trophic interactions in a natural ecological community. Widespread signalling cross talk, primarily via the three plant hormones jasmonic acid (JA), salicylic acid (SA) and ethylene (ET), allows plants to coordinate and fine-tune their defences against the diversity of potential natural enemies. Plants deploy distinct, but often overlapping defence strategies against biotrophic microbes, phloem-feeding insects and leaf-chewing herbivores (Stout et al. 2006). Numerous studies have examined the physiological and molecular genetic basis of plant signalling and cross talk, mostly in model plant species (Arabidopsis thaliana, tomato and tobacco). Herbivores and many fungi typically induce the JA pathway and many bacterial and some fungal pathogens the SA pathway, and ET seems to bemodulator of cross talk between the two pathways. While extremely insightful, these studies have led to some typological thinking—that herbivores induce the JA pathway and that bacteria induce the SA pathway. This hazard is apparent when one considers that the causative agent of bacteria speck disease (the Gram-negative bacterium Pseudomonas syringae) in plant leaves will induce either the JA pathway or the SA depending on the presence or absence of single avirulence or virulence genes (Cui et al. 2005). This indicates that it may well be difficult to make predictions about how particular natural enemies induce plant defences, and whether this scales up the trophic cascade to predators. Nonetheless, there are widespread, repeatable and ecologically relevant cross-talk phenotypes present across host plant taxa. For example, the SA pathway and JA pathway are largely mutually antagonistic in plants. How such antagonism affects the their trophic level, as well as interactions among co-infecting natural enemies is relatively unknown, but clearly likely to be extremely important ecologically and evolutionarily (Thaler 1999). Only a handful of elegant studies (e.g. (Rodriguez-Saona et al. 2005)) have examined how plant defence hormone signalling is affected by attack from multiple herbivore species, and how this translates into effects on the herbivores and their parasitoids in the third trophic level. Soler et al. (2012) provide an important step forward by studying co-infection of Brassica oleracea plants by a leaf-chewing herbivore and a phloem-feeding herbivore. The authors cast the effects of *Correspondence author. E-mail: whiteman@email.arizona.edu

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 timing of life-history events in insects can have important consequences for both survival and reproduction. For insect herbivores with complex life histories, selection is predicted to favor those combinations of traits that increase the size at metamorphosis while minimizing the risk of mortality from natural enemies. Studies quantifying selection on life-history traits in natural insect herbivore populations, however, have been rare. The purpose of this study was to measure phenotypic selection imposed by elements of the first and third trophic levels on variation in two life-history traits, the timing of egg hatch and pupal mass, in a population of oak-feeding caterpillars, Psilocorsis quercicella (Lepidoptera: Oecophoridae). Larvae were collected from the field throughout each of two generations per year for three years and reared to determine the effects of the date of egg hatch on both the risk of attack from parasitoids and the pupal mass of the survivors. The direction and strength of phenotypic selection attributed to aspects of the first and third trophic levels, as well as their combined effects, on the date of egg hatch was measured for each of the six generations. Heritabilities of and genetic correlations between pupal mass and the date of adult emergence from diapause (the life-history trait expected to have the largest influence on the timing of egg hatch, and thus larval development) were estimated from laboratory matings. In four of the six generations examined, significant directional selection attributed to the first trophic level was detected, always favoring early-hatching cohorts predicted to experience higher leaf quality than late-hatching cohorts. Directional phenotypic selection by the third trophic level was detected in only one of three years, and in that year the direction of selection was in opposite directions during the two successive generations. The combined effect of selection by both trophic levels indicated that the third trophic level acted to either reduce or enhance the more predictable pattern of selection attributed to the first trophic level. In addition, I found evidence of truncation selection acting to increase the mean and decrease the variance of pupal mass during the pupa-adult transition in the laboratory. Pupal mass and diapause duration were found to vary significantly among full-sibling families; upper bounds for heritability estimates were 0.57 and 0.30, respectively. Furthermore, these two traits were found to be positively genetically correlated (families with larger pupae had longer diapause durations), resulting in a fitness trade-off, because larger pupae enjoy higher survival through metamorphosis and female fecundity but emerge later, when average leaf quality for offspring is generally poorer.

Tropical ecosystems face escalating global change. These shifts can disrupt tropical forests' carbon (C) balance and impact root dynamics. Since roots perform essential functions such as resource acquisition and tissue protection, root responses can inform about the strategies and vulnerabilities of ecosystems facing present and future global changes. However, root trait dynamics are poorly understood, especially in tropical ecosystems. We analyzed existing research on tropical root responses to key global change drivers: warming, drought, flooding, cyclones, nitrogen (N) deposition, elevated (e) CO2, and fires. Based on tree species- and community-level literature, we obtained 266 root trait observations from 93 studies across 24 tropical countries. We found differences in the proportion of root responsiveness to global change among different global change drivers but not among root categories. In particular, we observed that tropical root systems responded to warming and eCO2 by increasing root biomass in species-scale studies. Drought increased the root: shoot ratio with no change in root biomass, indicating a decline in aboveground biomass. Despite N deposition being the most studied global change driver, it had some of the most variable effects on root characteristics, with few predictable responses. Episodic disturbances such as cyclones, fires, and flooding consistently resulted in a change in root trait expressions, with cyclones and fires increasing root production, potentially due to shifts in plant community and nutrient inputs, while flooding changed plant regulatory metabolisms due to low oxygen conditions. The data available to date clearly show that tropical forest root characteristics and dynamics are responding to global change, although in ways that are not always predictable. This synthesis indicates the need for replicated studies across root characteristics at species and community scales under different global change factors.