Abstract:

Seed dispersal is a key process determining the spatial pattern of plant populations. Myrmecochory is a term referring to a plant-ant mutualism in which ants disperse plant seeds while using the nutritionally valuable elaiosomes as food. In this study, our goal was to understand how ants affect seed dispersal and seedling distribution patterns of a myrmecochorous plant, Globba lancangensis. G. lancangensis is a small perennial herb of Zingiberaceae mainly distributed in southwest Yunnan Province. It propagates primarily by seeds but also by a small number of bulbils. There are 31.2±2.13 (Mean±SE, n=32) seeds in each fruit. The seeds are 2-3 mm in length and bear a white-colored elaiosome. We conducted an observational study on the seed dispersal of G. lancangensis in October 2002 in its natural habitat in Xishuangbanna, Yunnan Province of China (21°59′ N, 100°16′ E; 1 180 m asl; annual mean temperature, 18.3 ℃; annual mean rainfall, 1 339 mm). A total of ten species of ants were identified that transported the seeds of G. lancangensis and their activities were recorded. The mean dispersal distance of seeds was （0.47±0.03） m (n=216), ranging from 0.01 m to 3.35 m. Odontoponera transversa, Pheidole sp. and Pachycondyla luteipes were the most important seed dispersing ant species, occurring 61%, 50% and 28% of the time, respectively, with mean dispersal distances of (0.60±0.09) m, (0.20±0.01) m and (0.32±0.05) m, respectively. As a whole, ants promoted a decrease in the degree of aggregation of G. lancangensis seeds. Ants of Pheidole sp. collected a total of 43% of the seeds of G. lancangensis but consumed most of the seeds collected, suggesting that Pheidole sp. is a seed harvester ant　O. transversa did not remove seeds with artificially removed elaiosomes and appeared to be the most sensitive ant species to seed elaiosomes. Overall, O. transversa made the most significant contribution to seed dispersal, as demonstrated by the highest visiting frequency (61%) and the greatest mean dispersal distance (0.60±0.09) m of all ants. These results imply that a tight mutualism exists between O. transversa and G. lancangensis. To understand whether ants influence the spatial pattern of seedlings, we compared the spatial distribution of seedlings of G. lancangensis in its natural habitat with that of two other species in the Globba genus, G. barthei and G. racemosa; the latter two species propagate mainly by bulbils which are dispersed by abiotic means. Nine 25 m×25 m plots, three plots for each species, were established and the location of every Globba seedling inside the plots was recorded. The mean distance to the nearest neighbor of G. lancangensis was significantly higher than both G. barthei ((36.8±1.45) cm vs. (29.8±2.70) cm, Mean±SE; t73,33=2.11, p=0.037) and G. racemosa ((36.8±1.45) cm vs. (28.7±3.16) cm, t73,31=2.33, p=0.022). We used Z-values to describe the degree of aggregation of the populations of the three species. The Z-value of G. lancangensis (-1.70±0.19, Mean±SE) was significantly larger than the Z-value of both G. barthei (-2.58±0.37, t73,33=2.36, p=0.020) and G. racemosa (-3.28±0.53, t73,31=3.54, p=0.001), indicating that G. lancangensis was significantly less aggregated. These results suggest that ants make a significant contribution to dispersing G. lancangensis seedlings by increasing the distance to its nearest neighbor and decreasing the degree of aggregation in its natural habitat.