Differing impacts of two major plant invaders on urban plant-dwelling spiders (Araneae) during flowering season

Plant invasions can have major impacts on ecosystems and influence global species diversity. In Central Europe, Himalayan balsam (Impatiens glandulifera) and American goldenrods (Solidago canadensis and S. gigantea) are important invaders often establishing dense and homogeneous stands, especially in urban and other disturbed habitats. We investigated their impacts on plant-dwelling spiders (abundance, family structure, guild structure) and potential spider prey items during flowering season within an urbanized landscape using a paired design comparing invaded and native reference vegetation plots. In general, flowering American goldenrods and Himalayan balsam had no significant impacts on the spider family composition. Invasion of American goldenrods further had no effect on total spider abundance and potential prey item abundance. In contrast, goldenrods showed a significantly increased crab spider (Thomisidae) abundance while being less inhabited by web builders. Himalayan balsam negatively influenced free hunters and running crab spider (Philodromidae) abundance, while we found no effects on other groups and total spider abundance. For Himalayan balsam, potential prey item abundance was higher than in native vegetation stands. Notwithstanding that our results only represent a snapshot of the system, they suggest that large-scale removal of urban goldenrod stands during flowering season might negatively influence local spider abundance, especially of crab spiders. Management efforts should therefore be accompanied by compensation measures to avoid disruptive effects on local plant-dwelling spider communities.

Dynamic and synergistic influences of air temperature and rainfall on general flowering in a Bornean lowland tropical forest

Supra-annually synchronized flowering events occurring in tropical forests in Southeast Asia, known as general flowering (GF), are “spectacular and mysterious” forest events. Recently, studies that combined novel molecular techniques and model-based theoretical approaches suggested that cool temperature and drought synergistically drove GF. Although the novel approaches advanced our understanding of GF, it is usually difficult to know whether the mathematical formulation reasonably well represents the complex and dynamic processes involved in GF. In the present study, we collected a 17-year set of community-wide phenology data during 1993 to 2011 from Lambir Hills National Park in Borneo, Malaysia, and analyzed it using a model-free approach, empirical dynamic modeling (EDM), that does not rely on specific assumptions about the underlying mechanisms, to overcome and complement the previous limitations. By using EDM, we found that GF in the forest in Lambir Hills National Park is synergistically driven by cool air temperature and drought, which is consistent with the previous studies. Also, we found that cumulative meteorological variables, rather than instantaneous values, drive GF with delayed effects, which is also consistent with the previous studies. Interestingly, the present study showed that effects of cumulative meteorological variables on GF changed through time, which implies that the relationship between GF and meteorological variables may be influenced by other factors such as plant/soil nutrient resource dynamics. Future studies integrating novel mathematical/statistical frameworks, long-term and large spatial scale ecosystem monitoring and molecular phenology data are promising for better understanding and fore-casting of GF events in tropical forests in Southeast Asia.

Biotic and abiotic drivers of dipterocarp seedling survival following mast fruiting in Malaysian Borneo

South-East Asian tropical rain forests experience sporadic, but profuse, seed production after general flowering, leading to the synchronous emergence of various seedlings and subsequent seedling dynamics, which play a crucial role in determining species distribution and coexistence. We examined the relative importance of both biotic (initial height, conspecific seedling density) and abiotic (canopy openness, per cent sand, soil water content) drivers using survival data for 1842 seedlings of 12 dipterocarp species for 1.5 y following mast fruiting in an old-growth Bornean tropical rain forest. More than 30% of all dipterocarp seedlings survived 1.5 y after mast fruiting. When all species were analysed together, we found that initial seedling height, canopy openness and conspecific seedling density affected dipterocarp seedling survival. Negative density dependence indicated that predators were not satiated, but dipterocarp seedlings rather suffered from host-specific natural enemies or intraspecific competition. Species-level analyses of seven dipterocarp species showed large variation in response to biotic and abiotic factors. These results suggest that interspecific differences in the relative importance of biotic and abiotic effects on seedling survival might contribute to species coexistence.