Izumi Washitani

Contrasting impacts of invasive engineers on freshwater ecosystems: an experiment and meta-analysis

Invasion by common carp (Cyprinus carpio) and red swamp crayfish (Procambarus clarkii) in shallow lakes have been followed by stable-state changes from a macrophyte-dominated clear water state to a phytoplankton-dominated turbid water state. Both invasive carp and crayfish are, therefore, possible drivers for catastrophic regime shifts. Despite these two species having been introduced into ecosystems world-wide, their relative significance on regime shifts remains largely unexplored. We compared the ecological impacts of carp and crayfish on submerged macrophytes, water quality, phytoplankton, nutrient dynamics, zooplankton and benthic macroinvertebrates by combining an enclosure experiment and a meta-analysis. The experiment was designed to examine how water quality and biological variables responded to increasing carp or crayfish biomass. We found that even at a low biomass, carp had large and positive impacts on suspended solids, phytoplankton and nutrients and negative impacts on benthic macroinvertebrates. In contrast, crayfish had a strong negative impact on submerged macrophytes. The impacts of crayfish on macrophytes were significantly greater than those of carp. The meta-analysis showed that both carp and crayfish have significant effects on submerged macrophytes, phytoplankton, nutrient dynamics and benthic macroinvertebrates, while zooplankton are affected by carp but not crayfish. It also indicated that crayfish have significantly greater impacts on macrophytes relative to carp. Overall, the meta-analysis largely supported the results of the experiment. Taken as a whole, our results show that both carp and crayfish have profound effects on community composition and ecosystem processes through combined consequences of bioturbation, excretion, consumption and non-consumptive destruction. However, key variables (e.g. macrophytes) relating to stable-state changes responded differently to increasing carp or crayfish biomass, indicating that they have differential ecosystem impacts.

A comparative study of the germination characteristics of seeds from a moist tall grassland community.

To account for observed patterns of emergence in the warm temperate grasslands of Japan the seed germination of several dozen species was tested following various types of thermal pre-treatment. An enormous variety of responses was observed among the species, especially in the capacity for immediate germination and the responses to moist storage with chilling. The extent of chilling requirement for the breakage of primary dormancy was largely correlated with the timing of seed dispersal, spring-summer dispersers tending to have a larger chilling requirement. On the other hand, the seedling emergence phenology of the species was closely related to the temperature dependency of the germination of non-dormant seeds. Key-words: Germination, phenology, seedling emergence, seed dispersal, thermal responses

Genetic Diversity and Outcrossing Rate between Undisturbed and Selectively Logged Forests of Shorea curtisii (Dipterocarpaceae) Using Microsatellite DNA Analysis

To evaluate the effect of flowering tree density on genetic variation in a dominant tree species, genetic diversities and outcrossing rates of Shorea curtisii (Dipterocarpaceae) were compared between undisturbed and selectively logged forests using microsatellite markers. The mean heterozygosity and the effective number of alleles per locus in the selectively logged plot were not significantly different from those of the undisturbed plot. However, the average number of alleles derived from the pollen donor was significantly lower in the selectively logged forests, indicating the possibility of a loss of genetic variation in future generations. The average outcrossing rate for 11 trees in an undisturbed plot was 96.3%, which was similar to other reports from dipterocarp species based on isozyme analysis. However, the outcrossing rate in the selectively logged plot was 52.2% for five trees, which was significantly lower than that in the undisturbed plot. This decline of outcrossing rate in the selectively logged plot may result from the much lower density of conspecific flowering trees, and the high rate of selfing may result in inbreeding depression and/or a decrease in genetic variation of this forest in future generations.

Effects of population size and pollinator limitation on seed‐set of Primula sieboldii populations in a fragmented landscape

Female reproductive success, fruit and seed-set and factors affecting them were measured for 3 years (1995–1997) for 24 populations of Primula sieboldii E. Morren, a threatened Japanese plant species in a highly fragmented landscape in southern Hokkaido. The smaller populations (genets <=; 3) set almost no seeds consistently during the study years. In larger populations (genets >= 7), proportions of failed flowers without damage from fungi or herbivores were relatively high, but fruit set per flower varied among the populations in 1995. Correlations of fruit (r = 0.589, P = 0.011) and seed- set (r = 0.688, P < 0.01) with population pollinator availability were highly significant. In 1995, pollination failure would be the most plausible reason for the variation in reproductive success among the populations. However, pollinator availability was relatively high in all populations in 1996 and 1997. In these years, higher proportions of fruit failed because of antagonistic biological interactions. In the populations with poor pollinator availability in 1995, mean fruit and seed-sets were greater in the long-styled morph than in the short-styled morph. Such between-morph differences are thought to be caused by partial self-compatibility of the long-styled morph and the results of hand pollination experiments support this hypothesis.

Effects of common carp on nutrient dynamics and littoral community composition : roles of excretion and bioturbation

We manipulated through an enclosure experiment the presence and absence of small common carp (Cyprinus carpio, L) and their sediment access using a mesh net. The aims of the experiment were to determine carp effects on nutrient dynamics and community composition, and partially separate bioturbation effects from the remaining carp effects such as excretion and predation. During the two months, carp influenced water quality and nutrient dynamics, altered phytoplankton biomass and composition and decreased submerged macrophyte biomass independently of sediment access, indicating that nutrient excretion was the primary mechanism for the carp effects. However, Secchi depth decreased while the concentration of suspended solids increased when carp access to the sediments was allowed, suggesting that bioturbation by carp influenced water transparency. Increase in phytoplankton (dominated by cyanobateria) and suspended solids resulted in reduced submerged macrophyte biomass through reduction of light availability. Zooplankton, numerically dominated by rotifers, increased in carp enclosures. In addition, benthic macroinvertebrates decreased in carp enclosures, regardless of the presence and absence of the netting. Because direct predation effects were not evident, carp probably affected benthic macroinvertebrates through reduction of submerged macrophytes. These results indicate that carp can have dramatic direct and indirect impacts on nutrient dynamics and littoral community structure through excretion and bioturbation. Ecosystem engineering by carp may therefore trigger a shift from a clear water state dominated by submerged macrophytes to a turbid water state dominated by phytoplankton.

Fluctuation of photosynthetic photon flux density within aMiscanthus sinensis canopy

Detailed measurements of diurnal variations in photosynthetic photon flux density (PPFD) were made at seven locations within the canopy of aMiscanthus sinensis grassland to evaluate the light conditions of microsites for heliophilic tree seedlings. Multiple regression analysis revealed that the short-term light fluctuation on a clear day was highly dependent on the wind speed and solar elevation angle, whereas on a cloudy day it was mainly determined by the PPFD incident from above the canopy.The relative PPFD at 40 cm aboveground varied from 0.065 to 0.252, depending on sky conditions and the sensors position in relation to clumped patches ofM. sinensis. On a clear day, the proportion of PPFD readings above 100 μmol·m−2·s−1 contributed by sunflecks ranged between 25.4% and 82.0%. Computer simulation showed that the contribution of sunflecks to the daily carbon gain ofQuercus serrata seedlings may range from 11% to 65%.The mean relative PPFD measured under diffuse light conditions was linearly related to the daily total PPFD and the daily carbon gain by single leaves ofQ. serrata seedlings. This suggests that the relative PPFD under diffuse light conditions provides an appropriate measure of site-specific light availability within a grass canopy.

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.

Citizen science: a new approach to advance ecology, education, and conservation

Citizen science has a long history in the ecological sciences and has made substantial contributions to science, education, and society. Developments in information technology during the last few decades have created new opportunities for citizen science to engage ever larger audiences of volunteers to help address some of ecology’s most pressing issues, such as global environmental change. Using online tools, volunteers can find projects that match their interests and learn the skills and protocols required to develop questions, collect data, submit data, and help process and analyze data online. Citizen science has become increasingly important for its ability to engage large numbers of volunteers to generate observations at scales or resolutions unattainable by individual researchers. As a coupled natural and human approach, citizen science can also help researchers access local knowledge and implement conservation projects that might be impossible otherwise. In Japan, however, the value of citizen science to science and society is still underappreciated. Here we present case studies of citizen science in Japan, the United States, and the United Kingdom, and describe how citizen science is used to tackle key questions in ecology and conservation, including spatial and macro-ecology, management of threatened and invasive species, and monitoring of biodiversity. We also discuss the importance of data quality, volunteer recruitment, program evaluation, and the integration of science and human systems in citizen science projects. Finally, we outline some of the primary challenges facing citizen science and its future.

Plant conservation ecology for management and restoration of riparian habitats of lowland Japan

Abstract Conservation ecology is a new paradigm of ecology that aims at scientific contributions to maintaining earths biodiversity and is committed to ecosystem management indispensable to intergenerational long-term sustainability. Population ecology plays a central role in conservation ecology. Persistence of the metapopulation rather than that of each local population should be pursued in species conservation management. Biological interactions essential to reproduction and soil seed bank components of the population should be investigated and applied to planning for the conservation of a plant population. Gravelly floodplains and moist tall grasslands are among typical riparian habitats containing many threatened plants in Japan. These riparian habitats are now subjected not only to heavy fragmentation but also to intensive invasion of highly competitive alien (nonnative) plants. Extreme habitat isolation may result in reproductive failure or fertility selection in a plant population without pollinators, as exemplified by a nature reserve population of Primula sieboldii. Biological invasions, which are facilitated by extensive changes in the river environment including decreased seasonal flooding, abandonment of traditional vegetation management, eutrophication, and extensive clearing of the land for recreational use, threaten endemic riparian species. To preserve safe sites and growing conditions for threatened plants such as Aster kantoensis, active management to suppress the dominance of alien invader plants is necessary. Population management and habitat restoration should be based on sound information on the population ecology of both threatened and alien invader plants, designed as an ecological experiment to clarify effective ways for management.

Gap-detecting mechanism in the seed germination ofMallotus japonicus (Thunb.) Muell. Arg., a common pioneer tree of secondary succession in temperate Japan

Germination responses ofMallotus japonicus (Thumb). Muell. Arg. seeds to temperature revealed a gap-detecting mechanism in the seed germination of the species. Among various constant and alternating temperatures examined in the range from 12–40°C, only very limited temperature regimes were found to be favourable for seed germination, specifically, alternating temperatures between 18–32°C and 28–40°C. A single several-hour higher-temperature (32–40°C) treatment could also induce the germination of seeds which had been imbibed for several days at a constant temperature in the range of 20–26°C, suggesting that there is a process requiring higher temperature among the overal germination processes. Seeds located at or near the surface of denuded soil would have a good chance of experiencing such a temperature change when several rainy days are followed by fine weather, while seeds beneath close vegetation would not. On the other hand, the pressence or absence of light or a simulated ‘canopy ligh’ had little effect on the germination. Therefore, it was concluded that the seeds ofM. japonicus have a ‘gapdetecting mechanism’ in the form of a higher-temperature requirement of a certain process involved in the overall germination processes.