| The strength and outcome of interspecific interactions often vary across the landscape because of differences in community context. Geographic variations in the composition of the seed disperser and seed predator assemblages may result in variations in selective pressures on the fruit and seed traits of plants. Pinus armandii is a species of pine native to China, occurring from central to western China. It grows at 1000-3300 m altitude and has large (about 300mg) and wingless seeds. Nutcrackers (Nucifraga caryocatactes) are the primary seed dispersers of Pinus armandii, and scatter-hoarding rodents are the secondary seed dispersers. To determine whether and how pines have evolved in response to selection by nutcrackers and scatter-hoarding rodents, we conducted studies in Diqing prefecture, northwest Yunnan province where the mountains and rivers divided the pine forest into isolated or partly-isolated populations. Five study sites which have different elevation (Mingyong<2540m>, Shusong<2740m>, Nixi<2830m>, Weixi<3060m>, and Yeri<3300m>) were selected to investigate how the variations in nutcracker and rodent communities affect the selection on the cone and seed traits of pines. The following are the main results:1. All the 9 cone and seed traits are significant differences in the 5 study sites. Among them, variations on cone length, cone mass, seed mass, seed coat thickness are the most significantly different. The first principal component (PC1) accounts for 43% of the variation in cone and seed structure, and represents overall change in cone and seed size and mass, with increasing values indicating longer, wider, heavier cones with heavier seeds and thicker seed coats. The second principal component (PC2) accounts for 29% of the variation, and represents mainly the net energy reward for the seed dispersers, with increasing values indicating more seeds, higher kernel mass/seed mass, and higher total kernel mass/cone mass. Nixi has a lowest PC1(-1.32±0.15, Mean±SE, N= 30) and a highest PC2 (0.37±0.16) while Weixi has a highest PCI (1.02±0.11) and a lowest PC2 (-0.34±0.18). Shusong, Mingyong, and Yeri have medium PC1 and PC2 and there are no significant differences among them. 2. There were significant differences in the seed removal rates by nutcrackers in 5 study sites. Nixi had a highest seed removal rate (0.075±0.0020, Mean±SE, N= 69) and Weixi was the lowest (0.041±0.00085, N=61). Shusong, Mingyong, and weixi had medium seed removal rates, which are 0.050±0.00092 (N=79),0.060±0.0019 (N=80), and 0.057±0.0013 (N=79), respectively. Among them, there were 48.2±3.77%(Mean±SE, N=30) of cones in Nixi had been harvested by nutcrackerss, which was higher than Shusong (33.1±3.02%, F1.58=3.52, P=0.066), and significantly higher than Mingyong (26.2±2.44%, F1.58=11.99, P<0.01), Yeri (29.3±2.72%, F1.58=8.32, P<0.01), and Weixi (16.5±1.68%, F1.58=17.02, P< 0.001). The percentage of cones harvested in Weixi was also significantly lower than Shusong (F1.58=6.84, P< 0.05). There were no significant differences among Shusong, Mingyong, and Yeri.3. The percentages of large seeds that were scatter-hoarded were higher than small seeds, while the percentages of large seeds that were consumed were lower than small seeds at 3 study sites in 2006. Among them, scatter-hoarding rodents cached 20.5±5.5% of large seeds in Weixi, which was significantly higher than that in Nixi (5.5±1.7%, F1.38=6.90, P< 0.05). Rodents also consumed 45.0±8.7% of large seeds in Mingyong, which were significantly higher than that in Nixi (15.5±4.3%, F1.38=9.19, P< 0.005) and Weixi (21.5±6.4%, F1.38=4.71, P<0.05). The seed fates were significantly different in 2006 and 2007. A certain proportion of seeds were cached in 2006 but most of the seeds were consumed in 2007. Most of the seeds were removed less than 10m. In 2006, the mean removal distance of large seeds in Weixi was higher than Nixi and significantly higher than Mingyong (F1.258=8.43, P< 0.005), and the mean removal distance of small seeds in Weixi was also significantly higher than Mingyong (F1.172=10.97, P< 0.005). In 2007, the removal distance of large seeds in Mingyong was significantly lower than Nixi (F1.246= 20.98, P<0.001) and Weixi (F1.243=16.38, P< 0.001), and the removal distance of small seeds in Mingyong was also significantly lower than Nixi (F1.198= 12.62, P< 0.001) and Weixi (F1.169=18.33, P< 0.001). There were no significant differences (F1.1306= 0.508, P=0.476) in the mean removal distance of all the seeds in the 3 sites between 2006 (2.93±0.12m, N=607), and 2007 (2.82±0.11m, N=701).4. Cone and seed traits evolution were divergent among the 5 sudy sites with different seed disperser community compositions, consistent with variation in selection by nutcrackers and scatter-hoarding rodents. In Nixi where nutcrackers were abundant, selection on cone structure by nutcrackers resulted in small cones, small seeds, thin seed coats, and high total kernel mass/cone mass, that increased the foraging efficiency of nutcrackers and improved their potential for seed dispersal. In Weixi where nutcrackers were less abundant, selection by nutcrackers on cone structure was relaxed, and selection by scatter-hoarding rodents resulted in large seeds and thick seed coats, that constrained the evolution of cone and seed traits that facilitate seed dispersal by nutcrackers. Thus, there is a shift from relying on primary seed dispersal by nutcrackers, to an increasing reliance on secondary seed dispersal by scatter-hoarding rodents.As a result, the difference of seed disperser community compositions is the ecological mechanism giving rise to selection mosaics for Pinus armandii with which they interact and potentially coevolve. The conflicting selection has already induced the divergent cone and seed traits in our study. Whether the geographic selection mosaic can finally make the speciation of Pinus armandii might depend on the persistence of the selection pressure and the intensity of the gene flow among the communities. |
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