The Relationships Between Plant Diversity And Insect Diversity In Response To Grazing And Climate Change On Grasslands

The complex trophic relationships play important role in functions and stability ofecosystem. Grasslands contain multiple trophic levels, which perfectly coordinate witheach other and form stable ecological networks. However, species of each trophic levelrespond differently to climate change and human disturbance, leading to more complextrophic interactions in grassland ecosystem. Recently, the interactions between biodiversityand trophic levels, particularly the relationship of multitrophic diversity, are extensivelystudied. Based on the understanding of the effects of grazing and climate change onecosystems and the recognition of trophic interactions, we examined the responses ofinterrelationships across adjacent or multiple trophic levels, from a perspective ofbiodiversity, to climate change and livestock grazing. This knowledge has potentialimportance in greatly enhancing our understanding of the effects of diversity on trophiclevels, and predicting more fully ecological effects of climate change and grazing onecosystem functioning in multitrophic systems. Moreover, it is essential to understandtrophic relationships for maintaining biodiversity, and sustaining the stability of grazinggrassland systems, as well as effectively managing the grasslands.Three manipulated field experiments were conducted in meadow steppe where lies inthe eastern region on the Eurasian Steppe Zone with elevated air temperature, alteredprecipitation, and different large herbivore grazing types as experimental treatments. Bythe investigations and samplings of diversity and composition of plant and insectcommunities, and based on statistical analysis of these data, we obtained some importantresults and conclusions as follow.A field experiment with elevated air temperature was conducted to examine the effectsof experimental warming on plant diversity, insect diversity, and their relationship in ameadow steppe over three consecutive years (2007-2009). Results showed that plantspecies richness tended to increase under warmed conditions, and total aboveground plantbiomass and forb biomass dramatically increased, but grass biomass was not affected bywarming. It indicated that warming induced an increase in dominance of forbs andunchanged proportion of grasses, which altered plant community composition. Warmingresulted in decline in insect species richness by13.3%(P=0.003), and an increase ininsect abundance by12.5%(P=0.002). And furthermore, the magnitude of increase inpredator abundance (41.76%) was higher than that of herbivore abundance (12.11%),indicating the alteration of insect community structure by altering the proportion of predator abundance to herbivore abundance. More importantly, our further synthesisshowed that this positive relationship between insect species richness and plant speciesrichness was enhanced by warming (P=0.021), which indicate that warming strengthenthe dependence of insect diversity on plant diversity. This may attribute to greatersensitivity of insects to elevated temperature and warming-induced alteration in plantcommunity composition. It indicated that plant diversity has strong bottom-up controleffects on insect diversity. This study suggested that while accounting for plant diversityand insect diversity is a major step, understanding the nature and magnitude of theirassociation may be more important in the responses of ecosystems functioning to climatechange in grassland ecosystems. Reformulation of trophic relationship between plantdiversity and insect diversity likely have potential effects on ecosystem functioning andstability in long time scale. Moreover, this study further highlights the effects of warmingon grasslands via alteration of trophic relationship, and may be important for predicting theimpacts of climate change on ecosystem functions.A3-year (2007-2009) natural precipitation manipulation experiment (control,increased rainfall, decreased rainfall) was conducted to examine the effects of alterations inthe amount of precipitation on insect community in a meadow steppe. Results showed thatincreased precipitation treatment significantly stimulated total plant biomass and grassbiomass, whereas decreased precipitation treatment significantly reduced them. Bothincreased and decreased precipitation treatments caused a reduction in plant speciesrichness. The species richness and abundance of herbivore and predator were unequallyreduced, and the magnitude of reduction in predator abundance (61.3%) was higher thanthat of herbivore abundance (14.3%), which suggests potential simplification of insectcommunity structure in altered precipitation plots. Reductions in insect species richnesswere observed in both increased and decreased precipitation plots in2008(P=0.004) and2009(P=0.016), but not in2007(P=0.113), suggesting that the responses of insectspecies richness to altered precipitation has time dependence. This may attribute to changein plant community among different amount of precipitation in different years. Andmoreover, it also indicated that insect diversity had potential lagged response to alteredprecipitation. Thus, the responses of bio-communities did not coherent with climate change,and lagged behind, though climate change occurred. The effects of altered precipitation oninsect diversity may have resulted from altered precipitation associated changes in plantcommunity characteristics, such as biomass, which indicating potential bottom-up effectson insect diversity under altered precipitation. This study is important for understandingthe food web theory under climate change, and highlights that altered precipitation affectecosystem functioning and stability via change in diversity at different trophic levels. Moreover, the lagged responses of insect diversity to altered precipitation emphasesises theimportance of long-term field experiment for the projection of the impacts of climatechange on the functioning of grassland ecosystems.We examined the effects of large herbivores on the relationship between plant andinsect diversity with five grazing treatments (control, cattle, goat, sheep, and a mixture ofthe three grazing types) across three plant diversity levels (low:4-5species, intermediate:8-9species, and high:15-17species) in a meadow steppe. Results showed that the grazingtreatments did not significantly affect plant species richness, but reduced plant biomass (P<0.0001), plant height (P=0.001) and cover (P <0.0001). Grazers affected variation inplant height differently at different plant diversity levels, and this variation increased at thelow plant diversity level and decreased at the high plant diversity level after grazing. Asimilar pattern was observed for insect species richness: grazing had a positive impact atthe low plant diversity level, but had a negative impact at the high plant diversity level.This was attributed to strong responses of insect species richness to plant heightheterogeneity under grazing by large herbivores, implying that plant structuralheterogeneity is more important in influencing insect diversity in grazed grasslands.Therefore, the spatial heterogeneity of vegetation structure should be given more attentionin future work on plant-insect interactions. Furthermore, insect species richness waspositively associated with plant species richness in the absence of grazing (P=0.032), butit decreased with increasing plant diversity in the cattle (P=0.026) and sheep (P=0.032)grazing treatments, and slightly increased, and then dramatically declined in mixed grazingtreatments (P=0.015). Thus, grazing by large herbivores may reverse the positiverelationship between plant diversity and insect diversity by modifying plant structuralheterogeneity. This study further highlights regulatory role of large herbivore grazing ontrophic relationships in grasslands. Moreover, the effects of single and mixed largeherbivores on biodiversity are different in progress of moderate grazing management ingrasslands, and mixed herbivores grazing can result in higher insect diversity at moderateplant species diversity level. Results indicated mixed grazing can maintain higherbiodiversity in plant community with intermediate species richness, which providesvaluable evidence for management of grasslands, and highlights the importance of usinglarge herbivore grazing in management actions, not only to maintain diversity but also tomediate trophic interactions in grasslands.Responses of plant diversity, insect diversity, and their relationship to warming, alteredprecipitation, and large herbivore grazing were analyzed, which present more opportunityto further understanding and recognition for trophic interaction and the effects of climatechange and grazing: warming slightly affected plant species richness, decreased insect species richness, and furthermore enhanced the dependence of insect diversity on plantdiversity; altered precipitation decreased plant and insect species richness, and have laggedeffects. Results showed that climate change strongly affected the relationship betweenplant diversity and insect diversity, which provide new experimental evidence for theimpact of climate change on ecosystems, and valuable theory for predicting the effect ofclimate change. Grazing by large herbivores strongly affected insect diversity, whichdepended on plant diversity. Moreover, livestock grazing reversed the positive relationshipbetween plant diversity and insect diversity, which indicated that large herbivores haveregulatory roles on trophic relationships, and provide further evidence for the recognitionthat large herbivores are the key determinants in grasslands.The results from this study suggested that the relationships between plant diversity andinsect diversity are highly complex, and that novel reformations of trophic relationships inthe long run may potentially contribute changes in structure and functions of ecosystems.This study further emphasized that climate change and grazing are important factors inaffecting trophic interactions in grasslands, and that profound studies on them have criticalecological significance.