Difference in seasonal activity pattern between non-native and native ants in subtropical forest of Okinawa Island, Japan Abstract Biological invasions by non-native tramp ants are reported throughout the world, particularly in island ecosystems. In Okinawa Island, a subtropical island in south-western Japan, many tramp ants including the invasive ant, Anoplolepis gracilipes, already occur. In this study activity patterns of the ground-foraging tramp and native ants were investigated for one year by using food bait traps at the forest edges where both categories of ants coexist. On a seasonal time scale, activity patterns were different between exotic and native ants. Native ants are active and seemingly more dominant from spring to summer whereas tramp ants become more dominant from autumn to winter. These results suggest there might be temporal niche partitioning between tramp and native ants, and native ants might be able to dominate tramp ants in their suitable seasons.
The positive association between disturbances and biological invasions is a widely observed ecological pattern in the Anthropocene. Such patterns have been hypothesized to be driven by the superior competitive ability of invaders or by modified environments, as well as by the interaction of these factors. An experimental study that tests these hypotheses is usually less feasible, especially in protected nature areas. An alternative approach is to focus on community resilience over time after the anthropogenic disturbance of habitats. Here, we focused on ant communities within a forest to examine their responses after disturbance over time. We selected the Yanbaru region of northern Okinawa Island, which is a biodiversity hotspot in East Asia. We compared ant communities among roadside environments in forests where the road age differed from 5 to 25 years. We also monitored the ant communities before and after disturbance from forest thinning. We found that the species richness and abundance of exotic ants were higher in recently disturbed environments (roadsides of 5-15 years old roads), where the physical environment was warmer and drier. In contrast, the roadsides of 25-year-old roads indicated the potential recovery of the physical environment with cooler and moister conditions, likely owing to regrowth of roadside vegetation. At these sites, there were few exotic ants, except for those immediately adjacent to the road. The population density of the invasive species Technoymex brunneus substantially increased 1-2 years after forest thinning. There was no evidence of the exclusion of native ants by exotic ants that were recorded after disturbance. Our results suggest that local ant communities in the Yanbaru forests have some resilience to disturbance. We suggest that restoration of environmental components is a better strategy for maintaining native ant communities, rather than removing exotic ants after anthropogenic disturbance.
In social insects, information on the presence of a queen is known to affect the reproductive behaviour of workers. In the queenless ant Diacamma sp., information of the presence of the gamergate, the functional queen, is transmitted exclusively by direct physical contact between the gamergate and workers. Periodic contacts between the gamergate and each worker are therefore necessary to maintain worker sterility. However, how the difference in the contact interval influences reproductive behaviour of workers is unclear. In the present study, by artificially controlling the exact interval of physical contact between workers with the gamergate, it is shown that the intercontact interval influences the worker's reproductive condition (i.e. a longer interval leads to more developed ovaries). This system provides a good opportunity to study the mechanisms underling the colony size dependence of individual as well as colony characteristics.
Concerns about widespread human-induced declines in insect populations are mounting, yet little is known about how land-use change modifies the dynamics of insect communities, particularly in understudied biomes. Here we examine how the seasonal patterns of ant activity, key drivers of ecosystem functioning, vary with human-induced land cover change on a subtropical island landscape. Using trap captures sampled biweekly from a biodiversity monitoring network covering Okinawa Island, Japan, we processed 1.2 million individuals and reconstructed activity patterns within and across habitat types. We determined that communities inside the forest exhibited more variability than those in more developed areas. Using time-series decomposition to deconstruct this pattern, we found that ant communities at sites with greater human development exhibited diminished seasonality, reduced synchrony, and higher stochasticity compared to those at sites with greater forest cover. We demonstrate that our results cannot be explained by variation in either regional or in situ temperature patterns, or by differences in species richness or composition among sites. We conclude that the breakdown of natural seasonal patterns of functionally key insect communities may comprise an important and underappreciated consequence of global environmental change that must be better understood across Earth's biomes.
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