This study aims to examine the following three issues concerning the impact of global warming on the populations of the water strider Aquarius paludum in the Kochi-Nankoku area (33º30’N) of Kochi prefecture, Japan, based on the data collected from 2012 to 2015. 1. Has the number of generations increased? 2. Has aestivation appeared in summer adults? 3. Have overwintering adults stopped dispersing between the water surface and overwintering land sites which had been located at warm places on lands (for example under the leaves) far away from water habitats? Instead, have they overwintered on the shores of water habitats, because these habitats were warm in winter in recent years? In response to the first question, sampling data showed that the number of generations may have increased from three (1989-2002 strains) and four (2004-2008) to five (2009-2015) per year in Kochi (33°N). In response to question 2, some adults (21.6-30.0%) adopted aestivation in June-August in 2012-2014. However, maximum daily summer temperatures were lower than usual in 2014, and no adults adopted aestivation in 2015. In response to question 3, the ratio of adults with well-developed flight muscles decreased from 45% in 1995 to 24-36% in 2009-2013, 8.6% in 2014 and finally 0% in 2015 in overwintering adults collected from the field during fall. High summer precipitation and lower maximum summer temperatures in the previous year seemed to decrease their tendency to disperse by flight in the following year of 2015. Most overwintering adults appeared to have stopped migration between water bodies and overwintering sites on lands far from the water bodies, and overwinter near the shore instead in 2015. The use of Aquarius paludum as a biological indicator (a biological model) may be possible in the future, as this species can respond and change their life history traits quickly in accordance with the global change.
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