Linking Precipitation and C3 - C4 Plant Production to Resource Dynamics in Higher Trophic Level Consumers: Lizard Data

In many ecosystems, seasonal shifts in temperature and precipitation induce pulses of primary productivity that vary in phenology, abundance and nutritional quality.  Variation in these resource pulses could strongly influence community composition and ecosystem function, because these pervasive bottom-up forces play a primary role in determining the biomass, life cycles and interactions of organisms across trophic levels.  The focus of this research is to understand how consumers across trophic levels alter resource use and assimilation over seasonal and inter-annual timescales in response to climatically driven changes in pulses of primary productivity. We measured the carbon isotope ratios (d13C) of plant, arthropod, and lizard tissues in the northern Chihuahuan Desert to quantify the relative importance of primary production from plants using C3 and C4 photosynthesis for consumers.  Summer monsoonal rains on the Sevilleta LTER in New Mexico support a pulse of C4 plant production that have tissue d13C values distinct from C3 plants.  During a year when precipitation patterns were relatively normal, d13C measurements showed that consumers used and assimilated significantly more C4 derived carbon over the course of a summer; tracking the seasonal increase in abundance of C4 plants.  In the following spring, after a failure in winter precipitation and the associated failure of spring C3 plant growth, consumers showed elevated assimilation of C4 derived carbon relative to a normal rainfall regime. These findings provide insight into how climate, pulsed resources and temporal trophic dynamics may interact to shape semi-arid grasslands such as the Chihuahuan Desert in the present and future.

在众多生态系统中，温度与降水的季节性变化会诱导初级生产力脉冲的产生，而这类脉冲在物候、丰度及营养品质上均存在差异。这类资源脉冲的变异可对群落组成与生态系统功能产生强烈影响，因为这类广泛存在的上行控制效应（bottom-up forces）在决定各营养级生物的生物量、生活周期及种间相互关系中发挥着核心作用。本研究旨在阐明：在季节与年际尺度上，各营养级的消费者如何响应气候驱动的初级生产力脉冲变化，进而调整其资源利用与同化过程。我们在北奇瓦瓦沙漠（northern Chihuahuan Desert）中测定了植物、节肢动物与蜥蜴组织的碳同位素比值（δ¹³C），以量化利用C3和C4光合作用的植物所产生的初级生产力对消费者的相对重要性。新墨西哥州塞维列塔长期生态研究站（Sevilleta LTER）的夏季季风雨会催生C4植物的生产力脉冲，这类植物组织的δ¹³C值与C3植物存在显著差异。在降水模式相对正常的年份，δ¹³C检测结果显示：消费者在整个夏季对C4来源碳的利用与同化量显著更高，这一结果与夏季C4植物丰度的季节性增长相契合。次年春季，由于冬季降水匮乏且连带导致春季C3植物生长受阻，相较于正常降水年份，消费者对C4来源碳的同化量出现升高。本研究结果揭示了气候、脉冲资源与时间尺度下的营养级动态三者间的相互作用模式，可为理解当前及未来奇瓦瓦沙漠这类半干旱草原的形成机制提供重要参考。