For the first time, scientists have shown precisely how weather conditions cause multiple populations of a species within a large geographical area to have simultaneous increases or decreases in their abundance, a process known as "spatial synchrony." A paper published on Feb. 17 in the journal Nature reveals that occasional severe weather conditions directly cause the rapid increase or decrease in abundance and mobility of an intestinal parasite that infects populations of an important game bird hunted on country estates in Northern England, causing them all to either decline or thrive simultaneously in breeding success. The research is the first to pinpoint the specific role of climate in causing such incidents of spatial synchrony in animals.
"Our study shows that climate events can synchronize the growth trajectory of populations over large areas, having effects on ecological processes that could be large and far reaching, including an increased risk of extinctions in vulnerable populations, said Peter J. Hudson, the Willaman chair in biology and the director of The Center for Infectious Disease Dynamics at Penn State. Other members of the research team include Isabella M. Cattadori, a postdoctoral research associate at Penn State, and Daniel T. Haydon, a lecturer at the University of Glasgow in the United Kingdom.
The researchers coupled their detailed field studies and ecological knowledge with statistical analyses of data that Hudson had obtained from the owners of 100 individual estates in Northern England, where populations of red grouse have been maintained as game birds for more than 100 years. The team used a statistical technique recently developed by Haydon -- a powerful new form of time-series analysis -- to analyze data on the numbers of red grouse that hunters annually harvested since as far back as 1840. The records provide a gauge of the abundance of each of the 100 independently managed populations for each year.
Using this technique, Husdon's team was able to identify the specific years in which the grouse populations were all pushed into the same phase of increase or decrease in abundance. "Our analysis shows that these populations normally fluctuate in size independently, but in some years they all crash together or they all increase together," Cattadori said. "We suggest we have identified the mechanism that causes these populations to be driven into these collective forcing episodes." The researchers also report that synchrony in these grouse populations does not happen gradually over many years; rather, they all suddenly increase or crash together in just two or three years.
Haydon's statistical technique allowed the researchers to gauge from the condition of each population whether its size would be expected to increase, decrease or stay the same the next year under normal conditions, then to compare those predictions with actual population trajectories over the 100-year period. "What happens is that in some years, when these populations should be moving in different directions, they instead suddenly all move in the same direction," Hudson explained.
Hudson's earlier research had demonstrated that infection by the gastrointestinal nematode, Trichostrongylus tenuis, reduces the reproductive success of the red grouse by causing the hen to lay fewer eggs and by reducing the likelihood that those eggs will hatch. His earlier research also had shown that climate conditions can influence both the transmission of the parasite and the survival of the grouse chicks. Warm and wet conditions allow the nematode population to increase and to climb onto the stalks of heather. Wet and relatively cold Mays followed by warm and relatively dry Julys result in an outbreak of nematodes and so, when the grouse eat the infested heather, they rapidly become diseased.
"In this study we now show that large-scale weather conditions directly affect transmission of the parasite and that these effects -- rather than the direct effects on chick survival -- are the major factor driving grouse populations into synchrony," Hudson reported. "We previously had discovered the temporal mechanism -- that the parasites affect the size of individual populations of the red grouse. Now, in this paper, we have discovered the spatial mechanism of synchrony between grouse populations -- that specific climate events either accelerate or decelerate parasite transmission, which is what causes the host populations to become synchronized during either the increasing or decreasing part of their abundance cycles."
Hudson's research warns of the rapid ecological consequences of extreme and short-term fluctuations in weather conditions. "One of the characteristics of global climate change is that we are getting increased variation in temperature extremes -- sometimes we get colder winters followed by warmer summers and then suddenly we get a warm winter, for example," he said. The research indicates that brief episodes of extreme weather conditions may produce important ecological effects, and that these effects could be big, quick and dynamic. "This result is important not just for red grouse but for our understanding of how large-scale global climate events and environmental factors can affect many local ecological processes and local populations of many species in which spatial synchrony is known to occur, including insects, rodents, birds, fish and mammals," Hudson commented.
"One of the major environmental challenges of our age is to identify and understand the important mechanisms by which climate change influences such complex biological processes as fluctuations in the size of populations," said Hudson. "With the increasing variation in extreme weather conditions we now are experiencing, this challenge has become increasingly important and pressing."
This research was supported by the European Union Marie Curie Individual Fellowship and by the Game Conservancy Trust in England.
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