Global shifts towards positive species interactions with increasing environmental stress
TL;DR: A synthesis of 727 tests of the stress-gradient hypothesis in plant communities across the globe shows that plant interactions change with stress through an outright shift to facilitation (survival) or a reduction in competition (growth and reproduction).
Abstract: The study of positive species interactions is a rapidly evolving field in ecology. Despite decades of research, controversy has emerged as to whether positive and negative interactions predictably shift with increasing environmental stress as hypothesised by the stress-gradient hypothesis (SGH). Here, we provide a synthesis of 727 tests of the SGH in plant communities across the globe to examine its generality across a variety of ecological factors. Our results show that plant interactions change with stress through an outright shift to facilitation (survival) or a reduction in competition (growth and reproduction). In a limited number of cases, plant interactions do not respond to stress, but they never shift towards competition with stress. These findings are consistent across stress types, plant growth forms, life histories, origins (invasive vs. native), climates, ecosystems and methodologies, though the magnitude of the shifts towards facilitation with stress is dependent on these factors. We suggest that future studies should employ standardised definitions and protocols to test the SGH, take a multi-factorial approach that considers variables such as plant traits in addition to stress, and apply the SGH to better understand how species and communities will respond to environmental change.
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TL;DR: It is suggested that other topics also need addressing, including better assessment of the wider benefits of intercropping in terms of multiple ecosystem services, collaboration with agricultural engineering, and more effective interdisciplinary research.
Abstract: Intercropping is a farming practice involving two or more crop species, or genotypes, growing together and coexisting for a time. On the fringes of modern intensive agriculture, intercropping is important in many subsistence or low-input/resource-limited agricultural systems. By allowing genuine yield gains without increased inputs, or greater stability of yield with decreased inputs, intercropping could be one route to delivering ‘sustainable intensification’. We discuss how recent knowledge from agronomy, plant physiology and ecology can be combined with the aim of improving intercropping systems. Recent advances in agronomy and plant physiology include better understanding of the mechanisms of interactions between crop genotypes and species – for example, enhanced resource availability through niche complementarity. Ecological advances include better understanding of the context-dependency of interactions, the mechanisms behind disease and pest avoidance, the links between above- and below-ground systems, and the role of microtopographic variation in coexistence. This improved understanding can guide approaches for improving intercropping systems, including breeding crops for intercropping. Although such advances can help to improve intercropping systems, we suggest that other topics also need addressing. These include better assessment of the wider benefits of intercropping in terms of multiple ecosystem services, collaboration with agricultural engineering, and more effective interdisciplinary research.
738 citations
Cites background or methods from "Global shifts towards positive spec..."
...…conditions (e.g. semiarid, arctic, alpine or heavily grazed systems), net beneficial (facilitative) interactions are more common, but are outweighed by negative interactions in productive environments (e.g. mesic grasslands) as plant growth conditions improve (He et al., 2013; Li et al., 2013)....
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...…our knowledge, explicitly included a search for the type of patterns predicted by the SGH. Based on the approaches adopted by recent meta-analyses for seminatural and natural systems (He et al., 2013), we suggest that such an analysis would be relatively straightforward for intercropping systems....
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TL;DR: This review examines how spatial and temporal differences in resource availability or climatic conditions can influence these interactions between species and how these interactions influence the growth of mixtures.
405 citations
03 Feb 2016
TL;DR: This review indicates that while the effects of tree-species diversity on growth and other forest functions are now receiving a lot of attention, far less is known about the effects on growth or forest functioning and direct measurements of the processes could greatly contribute to the understanding of structural diversity effects.
Abstract: Through complementarity interactions, mixed-species forests can be more productive than monocultures, and this effect can increase with tree-species richness. However, this is not always the case. This review examines the processes and stand structural attributes that can influence diversity-productivity relationships (DPRs); how they influence resource availability, resource uptake, and resource-use efficiency; and also describes some important differences between tree-diversity versus grassland-diversity experiments. The size of the complementarity effects caused by these processes and stand structures varies along spatial and temporal gradients in resource availability and climate. These spatial and temporal dynamics have now been examined in many studies, and the general patterns are summarized using a simple framework; complementarity is predicted to increase as the availability of resource “X” declines (or climatic condition X becomes harsher) if the species interactions improve the availability, uptake, or use efficiency of resource X (or interactions improve climatic condition X). Importantly, this framework differs from the stress-gradient hypothesis to account for a wider range of inter-specific plant interactions (not only facilitation) by considering contrasting methods used to quantify species interactions while accounting for stand structure. In addition, complementarity (as opposed to facilitation) for a given species combination can increase as growing conditions improve in forests, contrary to predictions of the stress-gradient hypothesis with regards to facilitation. This review indicates that while the effects of tree-species diversity on growth and other forest functions are now receiving a lot of attention, far less is known about the effects of structural diversity on growth or forest functioning. Direct measurements of the processes, as opposed to focusing mainly on growth responses, could greatly contribute to our understanding of structural diversity effects.
340 citations
TL;DR: The quality of published meta-analyses in plant ecology was uneven and showed little improvement over time, and adoption of a checklist of quality criteria for meta-analysis for use by research synthesists, peer reviewers and journal editors is suggested.
Abstract: Summary
The number of published meta-analyses in plant ecology has increased greatly over the last two decades. Meta-analysis has made a significant contribution to the field, allowing review of evidence for various ecological hypotheses and theories, estimation of effects of major environmental drivers (climate change, habitat fragmentation, invasive species, air pollution), assessment of management and conservation strategies, and comparison of effects across different temporal and spatial scales, taxa and ecosystems, as well as research gap identification.
We identified 322 meta-analyses published in the field of plant ecology between 1996 and 2013 in 95 different journals and assessed their methodological and reporting quality according to standard criteria. Despite significant recent developments in the methodology of meta-analysis, the quality of published meta-analyses was uneven and showed little improvement over time.
We found many cases of imprecise and inaccurate usage of the term ‘meta-analysis’ in plant ecology, particularly confusion between meta-analysis and vote counting and incorrect application of statistical techniques designed for primary studies to meta-analytical data, without recognition of the violation of statistical assumptions of the analyses.
Methodological issues for meta-analyses in plant ecology include incomplete reporting of search strategy used to retrieve primary studies, failure to test for possible publication bias and to conduct sensitivity analysis to test the robustness of the results, as well as lack of availability of the data set used for the analyses.
The use of meta-analysis is particularly common in community ecology, ecophysiology and ecosystem ecology, but meta-analyses in ecophysiology are more likely not to meet standard quality criteria than papers in other subdisciplines. Fewer meta-analyses have been conducted in plant population ecology.
Synthesis. Over the past two decades, plant ecologists have embraced meta-analysis as a statistical tool to combine results across studies, and much has been learned as a result. However, as the popularity and usage of meta-analysis in the field of plant ecology has grown, establishment of quality standards, as has been done in other disciplines, becomes increasingly important. In order to improve the quality of future meta-analyses in plant ecology, we suggest adoption of a checklist of quality criteria for meta-analysis for use by research synthesists, peer reviewers and journal editors.
289 citations
TL;DR: It is shown that net positive effects have a long history of being considered ecologically or evolutionarily unstable, and recent evidence of its potential stability is presented, and it is presented that these increases are ubiquitous across ecosystems.
Abstract: 'Summary' 403
I. 'Introduction' 403
II. 'Facilitative mechanisms increasing diversity' 405
III. 'Facilitation as an evolutionary driver in proximate interactions' 410
IV. 'Why has facilitation been just recently added to ecological theory?' 411
V. 'Facilitation and the plant functional trait programme' 412
VI. 'Predictability and testability' 412
VII. 'Conservation, restoration and management' 413
VIII. 'Conclusions and next steps' 413
'Acknowledgements' 413
References 414
Summary
Models describing the biotic drivers that create and maintain biological diversity within trophic levels have focused primarily on negative interactions (i.e. competition), leaving marginal room for positive interactions (i.e. facilitation). We show facilitation to be a ubiquitous driver of biodiversity by first noting that all species use resources and thus change the local biotic or abiotic conditions, altering the available multidimensional niches. This can cause a shift in local species composition, which can cause an increase in beta, and sometimes alpha, diversity. We show that these increases are ubiquitous across ecosystems. These positive effects on diversity occur via a broad host of disparate direct and indirect mechanisms. We identify and unify several of these facilitative mechanisms and discuss why it has been easy to underappreciate the importance of facilitation. We show that net positive effects have a long history of being considered ecologically or evolutionarily unstable, and we present recent evidence of its potential stability. Facilitation goes well beyond the common case of stress amelioration and it probably gains importance as community complexity increases. While biodiversity is, in part, created by species exploiting many niches, many niches are available to exploit only because species create them.
278 citations
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TL;DR: The metafor package provides functions for conducting meta-analyses in R and includes functions for fitting the meta-analytic fixed- and random-effects models and allows for the inclusion of moderators variables (study-level covariates) in these models.
Abstract: The metafor package provides functions for conducting meta-analyses in R. The package includes functions for fitting the meta-analytic fixed- and random-effects models and allows for the inclusion of moderators variables (study-level covariates) in these models. Meta-regression analyses with continuous and categorical moderators can be conducted in this way. Functions for the Mantel-Haenszel and Peto's one-step method for meta-analyses of 2 x 2 table data are also available. Finally, the package provides various plot functions (for example, for forest, funnel, and radial plots) and functions for assessing the model fit, for obtaining case diagnostics, and for tests of publication bias.
11,237 citations
"Global shifts towards positive spec..." refers methods in this paper
...We conducted mixed/random-model effect size calculations and publication bias tests using metafor package (Viechtbauer 2010), and other analyses in R v2....
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Book•
27 Apr 200910,518 citations
"Global shifts towards positive spec..." refers background or methods in this paper
...Difference score was calculated as (effect size at high stress) – (effect size at low stress), and variance of the difference score as (variance at high stress) + (variance at low stress) (Borenstein et al. 2009)....
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...Using mixed-effect models (Borenstein et al. 2009), we estimated mean effect sizes at each of the low and high stress levels for the entire dataset and its subsets....
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...These results based on small sample sizes should be viewed with caution, and are areas that deserve further study, as the estimate of the betweenstudies variance will have poor precision when sample sizes are small (Borenstein et al. 2009)....
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...…before and after adjusting the publication bias quantitatively using the trim and fill method, a nonparametric data augmentation technique that estimates the number of studies missing due to the suppression of the most extreme results on one side of the funnel plot (Borenstein et al. 2009)....
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...We summarised survival data in 2 9 2 contingency tables and calculated odds ratios as the log of the ratio of the odds of survival with and without neighbours (Borenstein et al. 2009)....
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TL;DR: In this paper, a new global map of climate using the Koppen-Geiger system based on a large global data set of long-term monthly precipitation and temperature station time series is presented.
Abstract: Although now over 100 years old, the classification of climate originally formulated by Wladimir Koppen and modified by his collaborators and successors, is still in widespread use. It is widely used in teaching school and undergraduate courses on climate. It is also still in regular use by researchers across a range of disciplines as a basis for climatic regionalisation of variables and for assessing the output of global climate models. Here we have produced a new global map of climate using the Koppen-Geiger system based on a large global data set of long-term monthly precipitation and temperature station time series. Climatic variables used in the Koppen-Geiger system were calculated at each station and interpolated between stations using a two-dimensional (latitude and longitude) thin-plate spline with tension onto a 0.1°×0.1° grid for each continent. We discuss some problems in dealing with sites that are not uniquely classified into one climate type by the Koppen-Geiger system and assess the outcomes on a continent by continent basis. Globally the most common climate type by land area is BWh (14.2%, Hot desert) followed by Aw (11.5%, Tropical savannah). The updated world Koppen-Geiger climate map is freely available electronically in the Supplementary Material Section.
10,518 citations
TL;DR: A new digital Koppen-Geiger world map on climate classification, valid for the second half of the 20 th century, based on recent data sets from the Climatic Research Unit of the University of East Anglia and the Global Precipitation Climatology Centre at the German Weather Service.
Abstract: The most frequently used climate classification map is that o f Wladimir Koppen, presented in its latest version 1961 by Rudolf Geiger. A huge number of climate studies and subsequent publications adopted this or a former release of the Koppen-Geiger map. While the climate classification concept has been widely applied to a broad range of topics in climate and climate change research as well as in physical geography, hydrology, agriculture, biology and educational aspects, a well-documented update of the world climate classification map is still missing. Based on recent data sets from the Climatic Research Unit (CRU) of the University of East Anglia and the Global Precipitation Climatology Centre (GPCC) at the German Weather Service, we present here a new digital Koppen-Geiger world map on climate classification, valid for the second half of the 20 th century. Zusammenfassung Die am haufigsten verwendete Klimaklassifikationskarte ist jene von Wladimir Koppen, die in der letzten Auflage von Rudolf Geiger aus dem Jahr 1961 vorliegt. Seither bildeten viele Klimabucher und Fachartikel diese oder eine fruhere Ausgabe der Koppen-Geiger Karte ab. Obwohl das Schema der Klimaklassifikation in vielen Forschungsgebieten wie Klima und Klimaanderung aber auch physikalische Geographie, Hydrologie, Landwirtschaftsforschung, Biologie und Ausbildung zum Einsatz kommt, fehlt bis heute eine gut dokumentierte Aktualisierung der Koppen-Geiger Klimakarte. Basierend auf neuesten Datensatzen des Climatic Research Unit (CRU) der Universitat von East Anglia und des Weltzentrums fur Niederschlagsklimatologie (WZN) am Deutschen Wetterdienst prasentieren wir hier eine neue digitale Koppen-Geiger Weltkarte fur die zweite Halfte des 20. Jahrhunderts.
7,820 citations
"Global shifts towards positive spec..." refers methods in this paper
...Climate was assigned using the climatic zones of the K€ oppen–Geiger system (Kottek et al. 2006)....
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...Climate was assigned using the climatic zones of the K€oppen–Geiger system (Kottek et al. 2006)....
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Book•
01 Jan 1979
TL;DR: In this paper, the authors present plant strategies in the established phase and the regenerative phase in the emerging phase, respectively, and discuss the relationship between the two phases: primary strategies and secondary strategies.
Abstract: PLANT STRATEGIES. Primary Strategies in the Established Phase. Secondary Strategies in the Established Phase. Regenerative Strategies. VEGETATION PROCESSES. Dominance. Succession. Co-Existence. References. Index.
5,687 citations