Takehisa Yamakita

Re-Structuring of Marine Communities Exposed to Environmental Change: A Global Study on the Interactive Effects of Species and Functional Richness

Species richness is the most commonly used but controversial biodiversity metric in studies on aspects of community stability such as structural composition or productivity. The apparent ambiguity of theoretical and experimental findings may in part be due to experimental shortcomings and/or heterogeneity of scales and methods in earlier studies. This has led to an urgent call for improved and more realistic experiments. In a series of experiments replicated at a global scale we translocated several hundred marine hard bottom communities to new environments simulating a rapid but moderate environmental change. Subsequently, we measured their rate of compositional change (re-structuring) which in the great majority of cases represented a compositional convergence towards local communities. Re-structuring is driven by mortality of community components (original species) and establishment of new species in the changed environmental context. The rate of this re-structuring was then related to various system properties. We show that availability of free substratum relates negatively while taxon richness relates positively to structural persistence (i.e., no or slow re-structuring). Thus, when faced with environmental change, taxon-rich communities retain their original composition longer than taxon-poor communities. The effect of taxon richness, however, interacts with another aspect of diversity, functional richness. Indeed, taxon richness relates positively to persistence in functionally depauperate communities, but not in functionally diverse communities. The interaction between taxonomic and functional diversity with regard to the behaviour of communities exposed to environmental stress may help understand some of the seemingly contrasting findings of past research.

Hydroides dianthus (Polychaeta: Serpulidae), an alien species introduced into Tokyo Bay, Japan

Calcareous tube polychaetes (family Serpulidae) are notorious biofoulers that are easily transported and introduced to allochthonous habitats. Here we report the recent introduction of Hydroides dianthus (Verrill, 1873) to eastern Japan as its first occurrence in East Asia, probably from European or American coasts. Specimens had been found on artificial hard substrata together with congeners H. ezoensis, H. exaltatus and H. fusicolus in Tokyo Bay, Japan in 2006. The origin, vector, source of introduction and possible impact of H. dianthus on Japanese coasts is discussed from a perspective based on worldwide Hydroides transport.

Scale dependency in seagrass dynamics: how does the neighboring effect vary with grain of observation?

Although the importance of spatial scale in ecology has been increasingly recognized, the effects on ecological processes of changing the grain size of the observation have rarely been tested for empirical populations. A seagrass bed is an ideal system to study scale-dependency because it occurs in two-dimensional shallow soft-bottoms and can be monitored on a broader scale by using remote-sensing techniques. To investigate the grain dependency of seagrass spatial dynamics, we analyzed the effect of neighboring vegetation on the annual transition between vegetated and unvegetated states in a seagrass meadow in Futtsu, Tokyo Bay. The presence or absence of seagrass vegetation was observed at different grains from aerial photographs taken annually over 17xa0years. We detected the presence of a neighboring effect both in the increasing process (transition from the unvegetated to the vegetated state) and the decreasing process (vice versa) of vegetation. In the increasing process, the intensity of the neighboring effect was positive with the small grain, but the effect decreased to 0 with grain of ca. 20xa0m. In the decreasing process, the neighboring effect was negative with the small grain and increased to 0 with grain of ca. 30xa0m. The observed grain dependency in the neighboring effects also varied among different positions of the bed and among different years. The grain dependency in the increasing process cannot solely be explained by shoot elongation of the seagrass, which can cause positive neighboring effects only at small grain (≤6xa0m). The neighboring effect at the greater grain can be regarded as an emergent property.