Chuan Yan

Linking climate change to population cycles of hares and lynx

The classic 10-year population cycle of snowshoe hares (Lepus americanus, Erxleben 1777) and Canada lynx (Lynx canadensis, Kerr 1792) in the boreal forests of North America has drawn much attention from both population and community ecologists worldwide; however, the ecological mechanisms driving the 10-year cyclic dynamic pattern are not fully revealed yet. In this study, by the use of historic fur harvest data, we constructed a series of generalized additive models to study the effects of density dependence, predation, and climate (both global climate indices of North Atlantic Oscillation index (NAO), Southern Oscillation index (SOI) and northern hemispheric temperature (NHT) and local weather data including temperature, rainfall, and snow). We identified several key pathways from global and local climate to lynx with various time lags: rainfall shows a negative, and snow shows a positive effect on lynx; NHT and NAO negatively affect lynx through their positive effect on rainfall and negative effect on snow; SOI positively affects lynx through its negative effect on rainfall. Direct or delayed density dependency effects, the prey effect of hare on lynx and a 2-year delayed negative effect of lynx on hare (defined as asymmetric predation) were found. The simulated population dynamics is well fitted to the observed long-term fluctuations of hare and lynx populations. Through simulation, we find density dependency and asymmetric predation, only producing damped oscillation, are necessary but not sufficient factors in causing the observed 10-year cycles; while extrinsic climate factors are important in producing and modifying the sustained cycles. Two recent population declines of lynx (1940-1955 and after 1980) were likely caused by ongoing climate warming indirectly. Our results provide an alternative explanation to the mechanism of the 10-year cycles, and there is a need for further investigation on links between disappearance of population cycles and global warming in hare-lynx system.

Trade-off between seed defensive traits and impacts on interaction patterns between seeds and rodents in forest ecosystems

Plants often have two kinds of defensive traits against animal predation: physical and chemical defenses, but the trade-off between them is heavily debated, and their impacts on relationship between plants and animals are largely unknown. We investigated seed traits of 23 tree species and their impacts on seed fates or hoarding behaviors under predation from 16 rodent species in four forest types in China. We provide clear evidence that there is a strong nonlinear trade-off between physical (as measured by seed coat thickness) and chemical (as measured by tannin content) defensive traits in seeds. This trade-off was closely associated with nutritional traits, resulting in coordinated defense syndromes in seeds. The seed fate and hoarding behavior patterns were largely determined by the trade-off-related seed traits and the body mass of rodents, respectively, not by the phylogenetic relations of species. Tree species showed more conservative evolution in seed traits of high starch content, high tannin content, and thin seed coat, but they showed more convergent/divergent evolution in seed traits of high protein content, high fat content, and thick seed coat under rodent predation. Our results suggest that trade-off-related seed traits may play a predominant role in shaping the relationship between plants and animals.

Specific non-monotonous interactions increase persistence of ecological networks

The relationship between stability and biodiversity has long been debated in ecology due to opposing empirical observations and theoretical predictions. Species interaction strength is often assumed to be monotonically related to population density, but the effects on stability of ecological networks of non-monotonous interactions that change signs have not been investigated previously. We demonstrate that for four kinds of non-monotonous interactions, shifting signs to negative or neutral interactions at high population density increases persistence (a measure of stability) of ecological networks, while for the other two kinds of non-monotonous interactions shifting signs to positive interactions at high population density decreases persistence of networks. Our results reveal a novel mechanism of network stabilization caused by specific non-monotonous interaction types through either increasing stable equilibrium points or reducing unstable equilibrium points (or both). These specific non-monotonous interactions may be important in maintaining stable and complex ecological networks, as well as other networks such as genes, neurons, the internet and human societies.

Risk of cache pilferage determines hoarding behavior of rodents and seed fate

Rodents adjust predation and hoarding behavior according to the probability of their hoarding seeds being stolen by competitors. Rodents eat unsafe seeds (seeds more likely to be stolen) immediately and keep safe seeds (seeds less likely to be stolen) for a long time. Rodents spread their seeds in many small “storehouses” when the probability of hoarded seeds being stolen is high and hoard many seeds in one “storehouse” when the probability is low.

Scale-dependent climatic drivers of human epidemics in ancient China

Significance The lack of available data, including written historical sources and natural proxy archives, has constrained us when disentangling the effects of climate change on the prevalence of infectious diseases. We first reconstructed human epidemics in China over the last two millennia and analyzed the impacts of climate change on the prevalence of human epidemics at various time scales. We show that long-term trends of cold and dry conditions indirectly facilitated the prevalence of epidemics through locusts and famines. Nevertheless, temperature showed unstable associations with epidemics on a small time scale. Our study highlights the urgent need to investigate scale-dependent impacts of climate change on the prevalence of diseases. A wide range of climate change-induced effects have been implicated in the prevalence of infectious diseases. Disentangling causes and consequences, however, remains particularly challenging at historical time scales, for which the quality and quantity of most of the available natural proxy archives and written documentary sources often decline. Here, we reconstruct the spatiotemporal occurrence patterns of human epidemics for large parts of China and most of the last two millennia. Cold and dry climate conditions indirectly increased the prevalence of epidemics through the influences of locusts and famines. Our results further reveal that low-frequency, long-term temperature trends mainly contributed to negative associations with epidemics, while positive associations of epidemics with droughts, floods, locusts, and famines mainly coincided with both higher and lower frequency temperature variations. Nevertheless, unstable relationships between human epidemics and temperature changes were observed on relatively smaller time scales. Our study suggests that an intertwined, direct, and indirect array of biological, ecological, and societal responses to different aspects of past climatic changes strongly depended on the frequency domain and study period chosen.

Quantifying the effects of climate and anthropogenic change on regional species loss in China

Human-induced environmental and climate change are widely blamed for causing rapid global biodiversity loss, but direct estimation of the proportion of biodiversity lost at local or regional scales are still infrequent. This prevents us from quantifying the main and interactive effects of anthropogenic environmental and climate change on species loss. Here, we demonstrate that the estimated proportion of species loss of 252 key protected vertebrate species at a county level of China during the past half century was 27.2% for all taxa, 47.7% for mammals, 28.8% for amphibians and reptiles and 19.8% for birds. Both human population increase and species richness showed significant positive correlations with species loss of all taxa combined, mammals, birds, and amphibians and reptiles. Temperature increase was positively correlated with all-taxa and bird species loss. Precipitation increase was negatively correlated with species loss of birds. Human population change and species richness showed more significant interactions with the other correlates of species loss. High species richness regions had higher species loss under the drivers of human environmental and climate change than low-richness regions. Consequently, ongoing human environmental and climate changes are expected to perpetuate more negative effects on the survival of key vertebrate species, particularly in high-biodiversity regions.