Grazing buffers the effect of climate change on the species diversity of seedlings in an alpine meadow on the Tibetan Plateau

Climate change predominated by warming over the past decades has affected plant biodiversity, distribution, and ecosystem functioning in alpine grasslands. Yet, little is known about the interactive effect of climate change and grazing on biodiversity and ecosystem functioning. Here, we conducted a vegetation translocation experiment (ten soil‐vegetation blocks were translocated from high‐altitudinal site 3,245 m to low‐altitudinal site 3,045 m) combined with grazing treatment in an alpine meadow on the Tibetan Plateau. The results showed that (a) translocation induced effect of climate change from harsh, high‐altitudinal site to benign, low‐altitudinal site significantly promoted species richness, and density of asexual and sexual seedling, with an increase in the proportion of asexual recruitment to sexual recruitment; (b) grazing decreased the proportion of asexual seedling to sexual recruitment within community, led to a shift in the dominant plant functional groups from graminoids and legumes to forbs; and (c) grazing partly offset the increased species richness of seedling, but not seedling density, induced by climate change. These findings suggest that moderate grazing may buffer the effect of climate change on the plant community composition, and thus, functional role in alpine meadows. Further understanding the influence of climate change on grassland ecosystems needs to consider the non‐additive effect of grazing and climate change to sustainability of grassland services.     

Grazing weakens temporal stabilizing effects of diversity in the Eurasian steppe

Many biodiversity experiments have demonstrated that plant diversity can stabilize productivity in experimental grasslands. However, less is known about how diversity–stability relationships are mediated by grazing. Grazing is known for causing species losses, but its effects on plant functional groups (PFGs) composition and species asynchrony, which are closely correlated with ecosystem stability, remain unclear. We conducted a six‐year grazing experiment in a semi‐arid steppe, using seven levels of grazing intensity (0, 1.5, 3.0, 4.5, 6.0, 7.5, and 9.0 sheep per hectare) and two grazing systems (i.e., a traditional, continuous grazing system during the growing period (TGS), and a mixed one rotating grazing and mowing annually (MGS)), to examine the effects of grazing system and grazing intensity on the abundance and composition of PFGs and diversity–stability relationships. Ecosystem stability was similar between mixed and continuous grazing treatments. However, within the two grazing systems, stability was maintained through different pathways, that is, along with grazing intensity, persistence biomass variations in MGS, and compensatory interactions of PFGs in their biomass variations in TGS. Ecosystem temporal stability was not decreased by species loss but rather remain unchanged by the strong compensatory effects between PFGs, or a higher grazing‐induced decrease in species asynchrony at higher diversity, and a higher grazing‐induced increase in the temporal variation of productivity in diverse communities. Ecosystem stability of aboveground net primary production was not related to species richness in both grazing systems. High grazing intensity weakened the temporal stabilizing effects of diversity in this semi‐arid grassland. Our results demonstrate that the productivity of dominant PFGs is more important than species richness for maximizing stability in this system. This study distinguishes grazing intensity and grazing system from diversity effects on the temporal stability, highlighting the need to better understand how grazing regulates ecosystem stability, plant diversity, and their synergic relationships.     

放牧和模拟增温对藏北高寒草地植物群落特征及生产力的影响

气候变化和放牧活动对草地植物物种多样性和生产力具有重要影响。为探索藏北高寒草地植物物种多样性和生产力对增温、放牧及其交互作用的响应, 于2011年在藏北高原开始建立增温实验平台, 2016年起增设放牧、增温+放牧实验, 连续2年(2016-2017年)观测了植物群落特征、群落组成、生产力和物种多样性。结果表明, 增温和放牧对高寒草地植物高度和净初级生产力具有显著交互作用。在放牧条件下, 增温对植物高度无显著影响; 但在不放牧条件下, 增温却显著增加了植物高度。在放牧条件下, 增温对净初级生产力的影响存在年际差异, 2016年增温对生产力无显著影响, 2017年增温显著降低了植物净初级生产力; 但在不放牧条件下, 增温对植物净初级生产力无显著影响。增温和放牧对高寒草地植物物种丰富度、盖度、重要值及多样性均无显著交互作用。植物盖度在增温和放牧条件下显著降低, 杂类草物种比例显著增加, 但物种多样性均无显著变化。研究表明, 增温和放牧显著改变高寒草地群落结构。未来气候变化条件下, 放牧活动加剧有可能导致高寒草地生产力降低。     

An elevational shift in butterfly species richness and composition accompanying recent climate change

The geographic ranges of many species have shifted polewards and uphill in elevation associated with climate warming, leading to increases in species richness at high latitudes and elevations. However, few studies have addressed community‐level responses to climate change across the entire elevational gradients of mountain ranges, or at warm lower latitudes where ecological diversity is expected to decline. Here, we show uphill shifts in butterfly species richness and composition in the Sierra de Guadarrama (central Spain) between 1967–1973 and 2004–2005. Butterfly communities with comparable species compositions shifted uphill by 293 m (± SE 26), consistent with an upward shift of approximately 225 m in mean annual isotherms. Species richness had a humped relationship with elevation, but declined between surveys, particularly at low elevations. Changes to species richness and composition primarily reflect the loss from lower elevations of species whose regional distributions are restricted to the mountains. The few colonizations by specialist low‐elevation species failed to compensate for the loss of high‐elevation species, because there are few low‐elevation species in the region and the habitat requirements of some of these prevent them from colonizing the mountain range. As a result, we estimated a net decline in species richness in approximately 90% of the region, and increasing community domination by widespread species. The results suggest that climate warming, combined with habitat loss and other drivers of biological change, could lead to significant losses in ecological diversity in mountains and other regions where species encounter their lower latitudinal‐range margins.     

Experimental evidence of reduced diversity of seedlings due to climate modification in a Mediterranean-type community

We are still lacking in experimental evidence of the effects of climate change on the richness of plant species under field conditions. We report a decrease in the species richness of recruited seedlings in a Mediterranean shrubland in experimentally induced drought and warming over 4 consecutive years. Drought decreased the number of emerging seedlings and their respective species richness. Warming also decreased seedling species richness, but it did not affect the number of emerging seedlings. Species that produce fewer recruits are more likely to disappear in drier or warmer scenarios. However, when the effect of induced climate treatment was greatest, the more abundant species in control stands were not necessarily the ones least affected by treatment; in other words, species‐idiosyncratic responses may occur. These results show that demographic processes are sensitive to minor climate changes, with probable consequences on the diversity and structure of the future plant communities.     

Recent vegetation changes at the high‐latitude tree line ecotone are controlled by geomorphological disturbance,productivity and diversity

Aim We test how productivity, disturbance rate, plant functional composition and species richness gradients control changes in the composition of high‐latitude vegetation during recent climatic warming. Location Northern Fennoscandia, Europe. Methods We resampled tree line ecotone vegetation sites sampled 26 years earlier. To quantify compositional changes, we used generalized linear models to test relationships between compositional changes and environmental gradients. Results Compositional changes in species abundances are positively related to the normalized difference vegetation index (NDVI)‐based estimate of productivity gradient and to geomorphological disturbance. Competitive species in fertile sites show the greatest changes in abundance, opposed to negligible changes in infertile sites. Change in species richness is negatively related to initial richness, whereas geomorphological disturbance has positive effects on change in richness. Few lowland species have moved towards higher elevations. Main conclusions The sensitivity of vegetation to climate change depends on a complex interplay between productivity, physical and biotic disturbances, plant functional composition and richness. Our results suggest that vegetation on productive sites, such as herb‐rich deciduous forests at low altitudes, is more sensitive to climate warming than alpine tundra vegetation where grazing may have strong buffering effects. Geomorphological disturbance promotes vegetation change under climatic warming, whereas high diversity has a stabilizing effect.     

Effects of experimental warming on biodiversity depend on ecosystem type and local species composition

Climatic warming is a primary driver of change in ecosystems worldwide. Here, we synthesize responses of species richness and evenness from 187 experimental warming studies in a quantitative meta‐analysis. We asked 1) whether effects of warming on diversity were detectable and consistent across terrestrial, freshwater and marine ecosystems, 2) if effects on diversity correlated with intensity, duration, and experimental unit size of temperature change manipulations, and 3) whether these experimental effects on diversity interacted with ecosystem types. Using multilevel mixed linear models and model averaging, we also tested the relative importance of variables that described uncontrolled environmental variation and attributes of experimental units. Overall, experimental warming reduced richness across ecosystems (mean log‐response ratio = –0.091, 95% bootstrapped CI: –0.13, –0.05) representing an 8.9% decline relative to ambient temperature treatments. Richness did not change in response to warming in freshwater systems, but was more strongly negative in terrestrial (–11.8%) and marine (–10.5%) experiments. In contrast, warming impacts on evenness were neutral overall and in aquatic systems, but weakly negative on land (7.6%). Intensity and duration of experimental warming did not explain variation in diversity responses, but negative effects on richness were stronger in smaller experimental units, particularly in marine systems. Model‐averaged parameter estimation confirmed these main effects while accounting for variation in latitude, ambient temperature at the sites of manipulations, venue (field versus lab), community trophic type, and whether experiments were open or closed to colonization. These analyses synthesize extensive experimental evidence showing declines in local richness with increased temperature, particularly in terrestrial and marine communities. However, the more variable effects of warming on evenness were better explained by the random effect of site identity, suggesting that effects on species’ relative abundances were contingent on local species composition. Synthesis A global research priority is to understand the consequences of climate change for biodiversity. A growing number of experimental studies have manipulated climatic drivers, in particular changes in temperature, in local communities. In the first quantitative meta‐analysis of community‐level studies across freshwater, marine and terrestrial experiments, species richness declined consistently with experimental warming. This effect was insensitive to warming intensity, duration, and multiple environmental and procedural covariates. However, evenness responses were weakly negative only in terrestrial systems and more variable across ecosystem types. Linear mixed model analyses revealed that the identity of local sites explained nearly 50% of variance in evenness effect sizes, compared to only 10% for richness. This result provides evidence that local species composition strongly constrains changes in relative species abundances in response to warming.     

Plant diversity and insect herbivores: effects of environmental change in contrasting model systems

There is increasing concern over the potential impact of anthropogenic factors (e.g. increasing nutrient inputs, global climate change) on the rate of loss of diversity in ecosystems. Such losses may affect ecosystem processes. In addition, a change in diversity of one group of organisms may influence the diversity of species of the next trophic level. We examined the extent to which plant species richness influences that of insect herbivores in two systems: a long‐term field experiment on heather moorland and a model community in the Ecotron controlled environment facility. We examined the response of these two plant communities to environmental change, specifically increased levels of nutrients, grazing and atmospheric CO₂. We measured the indirect effects of changes in these factors on insect herbivores, both above‐ and below‐ground. In the moorland system, grazing was the largest influence on plant community structure. The community was dominated by one species, Calluna vulgaris, and loss of cover under heavy grazing allowed competing species to invade. However, grazing regime was not a major influence on the species richness of the insect herbivore community. Site was more important: there were a greater number of Hemiptera species on sites with more mineral soils than on peat sites, possibly because a greater variety of grass and herb species was present on the former sites. In the Ecotron, below‐ground factors were also important drivers of community change: elevated CO₂ increased carbon availability in the soil and there were simultaneous changes in the community composition of soil biota. Above‐ground, some plant species increased in abundance and others decreased, leading to interaction‐specific effects on the insect herbivores. In two very different studies of the effects of environmental change on the interactions between plants and their herbivores, several similar conclusions can be drawn: (1) effects are likely to be site‐ and interaction‐specific; (2) outcomes are likely to be strongly dependent on the initial state and the dominant species of the plant community; and (3) indirect effects, often mediated by below‐ground factors, may have a bigger influence on insect‐plant interactions than more direct effects of above‐ground factors.     

Projected latitudinal and regional changes in vascular plant diversity through climate change: short-term gains and longer-term losses

A simple approach is suggested to project potential changes in the diversity of vascular plants. We use the current (recent past) relationship between plant diversity and geographic variation in the climate, as well as elevation range, to project changes in regional species richness (at 100 × 100 km resolution), concentrating on six climate scenarios for 2020, 2050 and 2080. The results show an overall trend towards increased vascular plant species richness. Increases in richness by 2050 and 2080 are expected over approximately three-quarters of the land surface, but decreases are expected in other regions. The magnitudes of richness gains and losses both increase over time, as the level of warming grows. The latitudinal pattern of change suggests that richness increases will be greatest at high latitudes, where plant productivity and diversity are largely limited by temperature. Richness decreases are not projected consistently in any latitudinal band, but are most likely to be observed at 5–40ºN, where declines in precipitation drive most projected decreases in richness.     

Random species loss underestimates dilution effects of host diversity on foliar fungal diseases under fertilization

With increasing attention being paid to the consequences of global biodiversity losses, several recent studies have demonstrated that realistic species losses can have larger impacts than random species losses on community productivity and resilience. However, little is known about the effects of the order in which species are lost on biodiversity–disease relationships. Using a multiyear nitrogen addition and artificial warming experiment in natural assemblages of alpine meadow vegetation on the Qinghai‐Tibetan Plateau, we inferred the sequence of plant species losses under fertilization/warming. Then the sequence of species losses under fertilization/warming was used to simulate the species loss orders (both realistic and random) in an adjacently novel removal experiment manipulating plot‐level plant diversity. We explicitly compared the effect sizes of random versus realistic species losses simulated from fertilization/warming on plant foliar fungal diseases. We found that realistic species losses simulated from fertilization had greater effects than random losses on fungal diseases, and that species identity drove the diversity–disease relationship. Moreover, the plant species most prone to foliar fungal diseases were also the least vulnerable to extinction under fertilization, demonstrating the importance of protecting low competence species (the ability to maintain and transmit fungal infections was low) to impede the spread of infectious disease. In contrast, there was no difference between random and realistic species loss scenarios simulated from experimental warming (or the combination of warming and fertilization) on the diversity–disease relationship, indicating that the functional consequences of species losses may vary under different drivers.