| Abstract: | Climate change–induced shifts in species phenology differ widely across trophic levels, which may lead to consumer–resource mismatches with cascading population and ecosystem consequences. Here, we examined the effects of different rainfall patterns (i.e., timing and amount) on the phenological asynchrony of population of a generalist herbivore and their food sources in semiarid steppe grassland in Inner Mongolia. We conducted a 10-y (2010 to 2019) rainfall manipulation experiment in 12 0.48-ha field enclosures and found that moderate rainfall increases during the early rather than late growing season advanced the timing of peak reproduction and drove marked increases in population size through increasing the biomass of preferred plant species. By contrast, greatly increased rainfall produced no further increases in vole population growth due to the potential negative effect of the flooding of burrows. The increases in vole population size were more coupled with increased reproduction of overwintered voles and increased body mass of young-of-year than with better survival. Our results provide experimental evidence for the fitness consequences of phenological mismatches at the population level and highlight the importance of rainfall timing on the population dynamics of small herbivores in the steppe grassland environment.The Earth is facing a great challenge from accelerated climate change. The global surface air temperature has increased by about 1° during the past century and is projected to exceed 1.5 to 2 °C by the end of the 21st century (1). Climate change has caused profound impacts on the Earth’s ecosystems, such as local extinctions (2), range shifts (3), and population fluctuations (4, 5) of many species. Many organisms have advanced the timing of phenological events in response to climate warming, such as earlier leaf-out in plants, earlier emergence of insects, or accelerated egg hatching dates for birds (6). For consumers, phenological events are timed to match peak food resources for breeding; however, the direction of consumer’s phenological response to climate change may differ from the response of species occupying lower trophic levels, leading to asynchrony between resources and consumers (7, 8). With respect to climate change, numerous studies have focused on the impact of temperature and its role in driving phenological asynchrony (9–11) since this is especially critical for species population dynamics and ecosystem functioning. However, relatively little is known about how rainfall mediates asynchrony between resources and consumers and its potential demographic consequences, especially in arid environments.Shifts in rainfall patterns have been greatly affected by climate warming (12) and play a key role in regulating vertebrate population dynamics (13), the species composition of communities, and ecosystem functions and services (14). Both the timing and the amount of rainfall are recognized as distinct but major components that synergistically influence the timing of vegetation phenology, e.g., the timing of plant germination and seed ripening (15, 16). However, it remains unclear whether changes in the timing or the amount of rainfall play the more dominant role in the processes of phenological asynchrony between interacting species despite their distinct effects on aboveground annual net primary productivity (17). It is therefore important to disentangle the independent effects of rainfall timing and amount if we are to predict responses of species’ populations and ecosystems to global climate change.Among small herbivores, rainfall is well recognized to induce a bottom-up increase in abundance via increasing food availability, as observed in Phyllotis darwini and Octodon degus in South America (18, 19), Pseudomys hermannsburgensis and Mus domesticus in Australia (20, 21), Spermophilus dauricus (22) and Cricetulus barabensis (23) in East Asia, Dipodomys merriami in North America (24), and Mastomys natalensis in Africa (25). However, these observations are all based on the correlation between rodent abundance and precipitation; the mechanism underlying the bottom-up effects of precipitation on rodents through plant productivity is often assumed but has been rarely investigated by manipulative experiments. While valuable in their own right, most previous studies have been unable to elucidate fully the role of rainfall as a potential proximate cue in regulating phenology. In natural environments, many biotic factors (e.g., predation and interspecific competition) and abiotic factors (e.g., flooding of burrows) may interact to influence how phenological processes can affect population dynamics. To understand the effects of rainfall on the role of phenological asynchrony in the population dynamics of target species, including the effects of rainfall amount and timing, it is therefore necessary to exclude or control for confounding factors. Conducting more tightly controlled manipulative experiments is a requirement when assessing the fitness consequences of phenological asynchrony (6, 7), although it is very challenging for small rodents owing to the need for large field enclosures that prevent immigration/emigration of individuals and impacts by predators.We conducted a 10-y, large-scale, manipulative experiment to examine the bottom-up effects of changes in rainfall regime (including timing and amount; SI Appendix, Fig. S1) on the phenological asynchrony between plants and herbivores, demographic parameters, and population dynamics of Brandt’s voles Lasiopodomys brandtii. In our study region in Inner Mongolia, an increase in annual rainfall, especially during the early growing season, can markedly enhance annual net primary productivity (26), with more rain increasing the biomass of rye grass Leymus chinensis (27, 28), a major and favored food source for Brandt’s voles (29). Additional rainfall in the early growing season can provide a match between the peak food resources and peak food requirements of young voles. Therefore, we hypothesized that rainfall would change the population density of voles by mediating the timing and peak amount of preferred foods and that rainfall timing (in the early growing season) would be of vital importance in triggering population increases, or outbreaks, of voles in arid steppe grassland. |
