Shin-ichi Suda

Dragonfly species richness on man-made ponds: effects of pond size and pond age on newly established assemblages

We studied the abundance and species richness of adult dragonflies in 11 artificial ponds which were recently established (within 2 years). We found that the adult dragonfly assemblage patterns were influenced by pond size as well as pond age. The species richness was positively correlated with the pond size, which was because the distributional patterns of species were significantly nested according to pond area. The species richness was highly correlated with pond age in association with the vegetation cover within ponds. It was suggested that the species richness was enhanced by the increasing immigration rate of species which favor well-vegetated ponds.

The sensitivity of dragonflies to landscape structure differs between life-history groups

Contrasting life-history strategies of long versus short pre-reproductive phases are known in adult dragonflies (Odonata) of temperate regions. Because the long-phase species spend a longer time in terrestrial habitats such as grasslands or woodlands during their pre-reproductive phase, we hypothesized that long-phase species would be more sensitive to landscape structure than short-phase species. To test this hypothesis, we conducted periodic censuses of adult dragonflies at small man-made ponds. We compared the two above functional groups in terms of the degree to which species occurrence depended on landscape structure. The difference among the two groups was not significant, but occurrence of long-phase species tended to depend on landscape structure. Long-phase species responded to landscape structure at larger spatial scales and showed stronger spatial autocorrelation in their occurrence among sampling ponds than short-phase species.

Impacts of weeping lovegrass (Eragrostis curvula) invasion on native grasshoppers: responses of habitat generalist and specialist species

We investigated invasion impacts of a grass species (Eragrostis curvula) on native grasshoppers by periodic censuses of these insects on gravelly floodplains of the Kinu River, Japan. Our hypothesis was that there are greater impacts on natives when they are habitat specialists, as opposed to habitat generalists. The study area comprised two main habitat types: gravelly areas and riparian grasslands. Among 12 grasshopper species identified, five were more abundant in one of the habitat types and all of them were significantly negatively affected by coverage of weeping lovegrass, whereas seven occurred at the both habitat types simultaneously and a significantly smaller portion of species (two of the seven) was negatively affected by the alien plants. The results suggest that habitat specificity is related to the grasshopper species’ sensitivity to the plant, indicating that habitat specialist herbivores living on open gravelly floodplains are likely highly vulnerable to this plant invasion.

Invasion of weeping lovegrass reduces native food and habitat resource of Eusphingonotus japonicus (Saussure)

To examine the relative importance and interactions of trophic and non-trophic effects of plant invasions on herbivores, we investigated how the invasion of weeping lovegrass Eragrostis curvula impacted Eusphingonotus japonicus, a food generalist grasshopper endemic to gravelly floodplains with sparse vegetation in Japan. Field observations and laboratory experiments showed that the grasshopper fed mainly on herbs endemic to the gravelly floodplains, which were negatively impacted by the alien grass. The alien grass was not consumed as food. Field censuses also showed that the abundance of the grasshopper was positively correlated with the coverage of endemic herbs in a plot where weeping lovegrass was not dominant. Dominance of the grass (i.e., habitat modification for E. japonicus) negatively impacted the grasshopper and weakened the positive relationship between the grasshopper and endemic herbs. These results suggest that an understanding of the relationship between non-trophic and trophic effects is essential to predict the impacts of plant invasions on herbivores.

Identifying priority habitats and monitoring species for conservation and restoration of lentic Odonata habitats: assemblage nestedness on Amami-Oshima Island, Japan

We investigated Odonata faunal and habitat characteristics (forest cover, emergent, submerged, floating-leaved and floating plant covers, pond area, NO3−, chemical oxygen demand, and presence/absence of a nonnative fish) in 10 ponds on Amami-Oshima Island. In total, 26 species of six odonate families were found, and we detected significant nestedness of species composition among the ponds (22 species in the most species-rich pond, and 8 species in the most species-poor pond). Species found only in the most species-rich ponds were: Anax nigrofasciatus nigrofasciatus, Acisoma panorpoides panorpoides, Agriocnemis famina oryzae, Rhyothemis severini, Anasiaeschna martini, Hemicordulia okinawaensis, Lyriothemis elegantissima, and Hydrobasileus croceus (hereafter referred to as the rare species). These rare species are generally known to preferentially inhabit ponds with lush emergent plants and/or to prefer cooler habitats shaded by forest cover, such as Anax nigrofasciatus nigrofasciatus, Anac. martini, He. okinawaensis, and L. elegantissima. In contrast, the common species also found in species-poor ponds were: Ischnura senegalensis, Pantala flavescens, Anax parthenope julis, Ictinogomphus pertinax, and Tramea verginia, which are known to prefer an open water surface as spawning habitat. These differences in habitat preference between the rare and common species may be among the major reasons for the significant positive effects of percent forest cover and emergent plants on Odonata species richness. These results suggest that nestedness helped identify precise habitat characteristics and rare species that should be considered for conservation and restoration of lentic habitats on Amami-Oshima Island.