The roles of stochasticity and biotic interactions in the spatial patterning of plant species in alpine communities

Questions

Plant community composition can be influenced by multiple biotic, abiotic, and stochastic factors acting on the local species pool to determine their establishment success and abundance and subsequently the diversity of the community. We asked if the influences of biotic interactions on the composition of plant species in communities, as indicated by patterns of plant species spatial associations (independent, positive or negative), vary across a productivity gradient within a single ecosystem type. Do dominant species of communities show spatial patterning suggestive of competitive interactions with interspecific neighbors? Do species that span multiple community types exhibit the same heterospecific interactions with neighbours in each community?

Location

Three alpine communities in the southern Rocky Mountains.

Methods

We measured the occurrence of species in a 1‐cm spatial grid within 2 m × 2 m plots to determine the spatial patterns of species pairs in the three communities. A null model of independent species spatial arrangements was used to determine whether species pairs were positively, negatively or independently associated, and how these patterns differed among the communities across the gradient of resource supply and environmental stress.

Results

Positive associations, indicative of facilitation between species, were most common in the most resource‐poor and least productive community. However negative associations, suggestive of competitive interactions among species, were not more common in the two more resource‐rich, productive communities. The dominant species of these communities did exhibit higher negative than positive associations with neighbours relative to positive patterning. Independent interspecific patterning was equally common relative to positive and negative patterns in all communities. Species that previously were shown to either facilitate other species or compete with neighbours exhibited spatial patterning consistent with the earlier experimental work.

Conclusions

A large number of species exhibit a lack of net biotic interactions, and stochastic factors appear to be as important as competition and facilitation in shaping the structure of the three alpine plant communities we studied.

     

气候变化下生物交互作用对树线生态过程的影响

树线交错带是具有强烈生物交互作用的高寒生态过渡带,生物互作与树线生态过程密切相关。本研究系统综述了气候变化下植物间、动植物间和微生物与植物间互作因子对树线生态过程的影响。植物间互利或竞争作用的强度调控变暖背景下树线生态过程的变化,目前尚缺少树轮生态学证据,且亟待检验高阶互作的适用性;动物采食活动、微生物与植物间互作可通过影响土壤状况、改变树木生长和更新等生态过程动态,增强或削弱树线与气候间耦合关系。迄今为止,地下与地上过程联系如何影响气候变暖下的树线动态尚不明晰,而营养级间互作可能调制树线生态过程对气候响应。青藏高原等高寒区具有开展此类研究的优势与潜力。     

Effect of soil nutrients,neighbor identities and root separation types on intra‐ and interspecific interaction among three clonal plant species

In natural environments, plants frequently interact with both heterospecific and conspecific neighbors. The intensity of belowground plant interaction with neighboring species commonly varies with the availability of soil nutrients in the habitats. According to classical ecological theory, competition between conspecific neighbors may be more severe than competition between unrelated species due to the similar nutrient requirements of close relatives, especially when nutrients are scarce in the habitat. However, many recent studies have shown the opposite pattern, and suggested an alternative mechanism based on species recognition. Taking Zoysia sinica as the focal species, we conducted a controlled experiment to test the results of intraspecific and interspecific interactions among three clonal species Zoysia sinica, Zoysia japonica and Alternanthera philoxeroides, which represent a conspecific, a close relative and a distant relative of the focal species, respectively, and at different root treatments (no separation NS, clone separation CS and ramet separation RS) and two nutrient levels. The results showed that Z. sinica recognized conspecific plants in the NS and CS treatments, and did not show above or belowground competition with these. The performance of the focal plant (Z. sinica) was better when it was grown with a conspecific neighbor as compared to all other types of neighbors. In all root separation treatments, the competition was more intense when Z. sinica grew with a close relative (Z. japonica) than when growing with a distant relative (A. philoxeroides). Generally, competition between plants was more intense at the high nutrient level than at the low nutrient level, suggesting that both soil nutrients and a species recognition mechanism play a significant role for the intra‐ and interspecific interaction and fitness of these three neighboring clonal species.     

Including species interactions in risk assessments for global change

Most ecological risk assessments for global change are restricted to the effects of trends in climate or atmospheric carbon dioxide. In order to move beyond investigation of the effects of climate alone, the climex ^™ model was extended to investigate the effects of species interactions, in the same or different trophic levels, along environmental gradients on a geographical scale. Specific needs that were revealed during the investigations include: better treatment of the effects of temporal and spatial climatic variation; elucidation of the nature of boundaries of species ranges; data to quantify the role of species traits in interspecies interactions; integrated observational, experimental, and modelling studies on mechanisms of species interactions along environmental gradients; and high‐resolution global environmental datasets. Greater acknowledgement of the shared limitations of simplified models and experimental studies is also needed. Above all, use of the scientific method to understand representative species ranges is essential. This requires the use of mechanistic approaches capable of progressive enhancement.     

Habitat shifts of endangered species under altered climate conditions: importance of biotic interactions

Predicting changes in potential habitat for endangered species as a result of global warming requires considering more than future climate conditions; it is also necessary to evaluate biotic associations. Most distribution models predicting species responses to climate change include climate variables and occasionally topographic and edaphic parameters, rarely are biotic interactions included. Here, we incorporate biotic interactions into niche models to predict suitable habitat for species under altered climates. We constructed and evaluated niche models for an endangered butterfly and a threatened bird species, both are habitat specialists restricted to semiarid shrublands of southern California. To incorporate their dependency on shrubs, we first developed climate‐based niche models for shrubland vegetation and individual shrub species. We also developed models for the butterfly's larval host plants. Outputs from these models were included in the environmental variable dataset used to create butterfly and bird niche models. For both animal species, abiotic–biotic models outperformed the climate‐only model, with climate‐only models over‐predicting suitable habitat under current climate conditions. We used the climate‐only and abiotic–biotic models to calculate amounts of suitable habitat under altered climates and to evaluate species' sensitivities to climate change. We varied temperature (+0.6, +1.7, and +2.8 °C) and precipitation (50%, 90%, 100%, 110%, and 150%) relative to current climate averages and within ranges predicted by global climate change models. Suitable habitat for each species was reduced at all levels of temperature increase. Both species were sensitive to precipitation changes, particularly increases. Under altered climates, including biotic variables reduced habitat by 68–100% relative to the climate‐only model. To design reserve systems conserving sensitive species under global warming, it is important to consider biotic interactions, particularly for habitat specialists and species with strong dependencies on other species.     

The role of biotic interactions in shaping distributions and realised assemblages of species: implications for species distribution modelling

Predicting which species will occur together in the future, and where, remains one of the greatest challenges in ecology, and requires a sound understanding of how the abiotic and biotic environments interact with dispersal processes and history across scales. Biotic interactions and their dynamics influence species' relationships to climate, and this also has important implications for predicting future distributions of species. It is already well accepted that biotic interactions shape species' spatial distributions at local spatial extents, but the role of these interactions beyond local extents (e.g. 10 km² to global extents) are usually dismissed as unimportant. In this review we consolidate evidence for how biotic interactions shape species distributions beyond local extents and review methods for integrating biotic interactions into species distribution modelling tools. Drawing upon evidence from contemporary and palaeoecological studies of individual species ranges, functional groups, and species richness patterns, we show that biotic interactions have clearly left their mark on species distributions and realised assemblages of species across all spatial extents. We demonstrate this with examples from within and across trophic groups. A range of species distribution modelling tools is available to quantify species environmental relationships and predict species occurrence, such as: (i) integrating pairwise dependencies, (ii) using integrative predictors, and (iii) hybridising species distribution models (SDMs) with dynamic models. These methods have typically only been applied to interacting pairs of species at a single time, require a priori ecological knowledge about which species interact, and due to data paucity must assume that biotic interactions are constant in space and time. To better inform the future development of these models across spatial scales, we call for accelerated collection of spatially and temporally explicit species data. Ideally, these data should be sampled to reflect variation in the underlying environment across large spatial extents, and at fine spatial resolution. Simplified ecosystems where there are relatively few interacting species and sometimes a wealth of existing ecosystem monitoring data (e.g. arctic, alpine or island habitats) offer settings where the development of modelling tools that account for biotic interactions may be less difficult than elsewhere.     

Effects of traits,species identity and local environmental conditions on the assessment of interactions: insights from an alpine meadow community

Aims The prediction that facilitation is the dominant interaction in physically stressful conditions has been supported by many but not all field studies. In the present paper, we tested the effects of the identity of species, the local environmental conditions and the currencies of performance measurement on such variation.Methods Using contrasting two plots, six species, and up to five multiple traits, we comprehensively explored the effects of the above factors on the assessment of plant interactions in an alpine meadow of the Qing-Hai Tibetan Plateau. Additionally, we attempted to figure out the possible mechanisms underlying the responses observed. The data were analysed by both standard ANOVAs and multivariate statistics.Important findings Our results demonstrated that the response to the removal of neighbours was both species and trait specific, and the effect of the local environmental conditions was dependent on the species involved. The contrast between plots had crucial influence on the net interactions of Kobresia macrantha, but little effect on Elymus nutans. Regarding the abiotic conditions, neighbours had significant impact on soil temperature, moist and solar radiation. The results contribute to advance our knowledge on the potential underlying factors influencing the assessment of facilitation.     

Plant-plant interactions and environmental change

Natural systems are being subjected to unprecedented rates of change and unique pressures from a combination of anthropogenic environmental change drivers. Plant-plant interactions are an important part of the mechanisms governing the response of plant species and communities to these drivers. For example, competition plays a central role in mediating the impacts of atmospheric nitrogen deposition, increased atmospheric carbon dioxide concentrations, climate change and invasive nonnative species. Other plant-plant interaction processes are also being recognized as important factors in determining the impacts of environmental change, including facilitation and evolutionary processes associated with plant-plant interactions. However, plant-plant interactions are not the only factors determining the response of species and communities to environmental change drivers - their activity must be placed within the context of the wide range of factors that regulate species, communities and ecosystems. A major research challenge is to understand when plant-plant interactions play a key role in regulating the impact of environmental change drivers, and the type of role that plant-plant interactions play. Although this is a considerable challenge, some areas of current research may provide the starting point to achieving these goals, and should be pursued through large-scale, integrated, multisite experiments.     

Species distribution models for Peruvian plantcutter improve with consideration of biotic interactions

Biotic interactions have been controversial in distributional ecology, mainly in regards to whether they have effects over broad extents, with the negative view known as the Eltonian noise hypothesis (ENH). In this study, we evaluated the ENH for Phytotoma raimondii, a restricted‐range Peruvian endemic bird species: we developed models based on 1) only abiotic conditions, 2) only host plant distributions, and 3) both abiotic conditions and host plant distributions; models were evaluated with partial receiver operating characteristic test and Akaike information criteria metrics. We rejected the ENH for this case: biotic interactions improved the model. The frequency with which exceptions to the ENH are detected has important implications for distributional ecology and methods for estimating distributions of species.     

Assisted migration within species range ignores biotic interactions and lacks evidence

In the context of climate change, many plant species may have problems adapting or dispersing rapidly enough to keep pace with changing environmental conditions. Given these potential problems, some experts argue against using local plant ecotypes for ecosystem restoration. Instead, they propose to use foreign ecotypes that are adapted to the predicted climate in an approach called assisted migration within species range or predictive provenancing. I argue that such actions may cause a mismatch in biotic interactions and have negative effects on other organisms. As such, assisted migration should only be considered in cases when the local ecotypes would fail to ensure ecosystem services. In fact, there is little experimental evidence on the assisted migration approach so far, and what little there is does not seem to support its use. Even in altered climates, local ecotypes mostly performed equally well or better than foreign ones selected for their adaptations to these climates. The reason is that even if adaptation to climate plays a role, this factor may be overridden by other drivers of local adaptation, such as soil or biotic interactions. Despite assisted migration being a popular concept that is repeatedly commended in scientific literature and propagated among practitioners, it should not be considered a universal tool to improve restoration outcomes during climate change. Given the lack of hard experimental data, I call for large‐scale multispecies experimental studies that will provide the necessary evidence to derive general guidelines and recommendations for management of ecosystems during climate change.     

Advanced snowmelt causes shift towards positive neighbour interactions in a subarctic tundra community

Positive and negative species interactions are important factors in structuring vegetation communities. Studies in many ecosystems have focussed on competition; however, facilitation has often been found to outweigh competition under harsh environmental conditions. The balance between positive and negative species interactions is known to shift along spatial, temporal and environmental gradients and thus is likely to be affected by climate change. Winter temperature and precipitation patterns in Interior Alaska are rapidly changing and could lead to warmer winters with a shallow, early melting snow cover in the near future. We conducted snow manipulation and neighbour removal experiments to test whether the relative importance of positive and negative species interactions differs between three winter climate scenarios in a subarctic tundra community. In plots with ambient, manually advanced or delayed snowmelt, we assessed the relative importance of neighbours for survival, phenology, growth and reproduction of two dwarf shrub species. Under ambient conditions and after delayed snowmelt, positive and negative neighbour effects were generally balanced, but when snowmelt was advanced we found overall facilitative neighbour effects on survival, phenology, growth and reproduction of Empetrum nigrum, the earlier developing of the two target species. As earlier snowmelt was correlated with colder spring temperatures and a higher number of frosts, we conclude that plants experienced harsher environmental conditions after early snowmelt and that neighbours could have played an important role in ameliorating the physical environment at the beginning of the growing season.     

Multi‐species interactions in competitive hierarchies: New methods and empirical test

Question: Are competitive hierarchies, which are typically based on the results of pair‐wise competition experiments, sensitive to the level of species interaction in the underlying competition experiments? Location: Controlled greenhouse study using vegetation typical of old‐fields in East Tennessee, USA. Methods: We extend traditional competitive effect/response methods to incorporate data from competition experiments featuring any level of species interaction (i. e., 2, 3, …, n species interacting simultaneously) and develop an ordinal technique that makes hierarchies more robust to variation in the numerical values of relative yield. We apply these methodological techniques to empirical data from a greenhouse experiment wherein four old‐field plant species were grown in pair‐wise and tri‐wise combination. We also demonstrate how resampling can be used to determine the variability of data and its consequences for development of competitive hierarchies. Results: Different hierarchies were produced when we used different evaluation methods, different levels of species interaction, and different levels of replication. More acute resampling distributions and wider ranges of target/neighbor scores revealed that higher levels of species interaction lead to more distinct hierarchies. Conclusions: Hierarchies developed from interactions among subsets of species may inadequately characterize relationships among the full community because of indirect or higher‐order interactions within multi‐species assemblages. Different evaluation methods can yield different hierarchies, and resampling is an effective tool to determine the sensitivity of resultant hierarchies to the level of replication. In sum, our new methodology can be used to control uncertainty in poorly‐replicated experiments.     

Thermal adaptation affects interactions between a range‐expanding and a native odonate species

1. Increasing temperature and invading species may interact in their effects on communities. In this study, we investigated how rising temperatures alter larval interactions between a naturally range‐expanding dragonfly, Crocothemis erythraea, and a native northern European species, Leucorrhinia dubia. Initial studies revealed that C. erythraea grow up to 3.5 times faster than L. dubia at temperatures above 16 °C. As a result, we hypothesised that divergent temperature responses would lead to rapid size differences between coexisting larvae and, consequently, to asymmetric intraguild predation at higher ambient temperatures. 2. Mortality and growth rates were measured in interaction treatments (with both species present) and non‐interaction controls (one species present) at four different temperature regimes: at an ambient temperature representative of central Germany, where both species overlap in distribution, and at temperatures increased by 2, 4 and 6 °C. 3. The mortality of C. erythraea did not differ between treatment and control. In contrast, mortality of L. dubia remained similar over all temperatures in the controls, but increased with temperature in the presence of the other species and was significantly higher there than in the controls. We concluded that L. dubia suffered asymmetric intraguild predation, particularly at increased temperature. Reduced growth rate of L. dubia in the interaction treatment at higher temperatures also suggested asymmetric competition for prey in the first phase of the experiment. 4. The results imply that the range expansion of C. erythraea may cause reduction in population size of syntopic L. dubia when temperature rises by more than 2 °C. The consequences for future range patterns, as well as other factors that may influence the interaction in nature, are discussed.     

Effects of elevated CO2 on the interspecific competition between two sympatric species of Aphis gossypii and Bemisia tabaci fed on transgenic Bt cotton

Abstract Effects of elevated CO₂ (twice ambient vs. ambient) and Bt Cry1Ac transgene (Bt cotton cv. 33^B vs. its nontransgenic parental line cv. DP5415) on the interspecific competition between two ecologically similar species of cotton aphid Aphis gossypii and whitefly biotype‐Q Bemisia tabaci were studied in open‐top chambers. The results indicated that elevated CO₂ and Bt cotton both affected the population abundances of A. gossypii and biotype‐Q B. tabaci when introduced solely (i.e., without interspecific competition) or two species coexisted (i.e., with interspecific competition). Compared with ambient CO₂, elevated CO₂ increased the population abundances of A. gossypii and biotype‐Q B. tabaci as fed on Bt and nontransgenic cotton on 45 (i.e., seedling stage) and 60 (i.e., flowering stage) days after planting (DAP), but only significantly enhanced aphid abundance without interspecific competition on the 45‐DAP nontransgenic cotton and 60‐DAP Bt cotton, and significantly increased whitefly abundance with interspecific competition on the 45‐DAP Bt cotton and 60‐DAP nontransgenic cotton. In addition, compared with nontransgenic cotton at elevated CO₂, Bt cotton significantly reduced biotype‐Q B. tabaci abundances without and with interspecific competition during seedling and flowering stage, while only significantly decreasing A. gossypii abundances without interspecific competition during the seedling stage. When the two insect species coexisted, the proportions of biotype‐Q B. tabaci were significantly higher than those of A. gossypii on Bt and nontransgenic cotton at the same CO₂ levels, and elevated CO₂ only significantly increased the percentages of biotype‐Q B. tabaci and significantly reduced the proportions of A. gossypii on seedling and flowering nontransgenic cotton. Therefore, the effects of elevated CO₂ were favorable for biotype‐Q B. tabaci to out‐compete A. gossypii under the predicted global climate change.     

湿地植物间竞争和促进互作的研究进展

湿地植物间关系主要包括竞争(负效应)和促进(正效应),是湿地生态研究的核心内容之一,对植物分布、群落和生态系统组成具有重要的调节作用。本文阐述了竞争理论及影响湿地植物竞争能力的主要因素,介绍了促进作用及其在湿地生态系统中的形成机理,详细综述了植物间关系转变的主要理论以及影响湿地植物间关系转变的主要环境因子(水位、温度、盐度、生物因子)。随着环境胁迫强度的增加,植物间关系一般会从竞争转变为促进作用,即优越条件下以竞争为主,胁迫条件下以促进作用为主。针对当前湿地植物间关系的研究多集中于滨海盐生湿地,类型较单一的情况,认为将来需加强对淡水湿地、河口湿地等各类系统的研究。同时,加强湿地植物间关系转变的生物学机制、时间效应,对新研究方法的探索等方面的研究也有助于进一步理解湿地植物间竞争和促进作用的发生及转变机理。