Origins and diversification of Indo-West Pacific marine fauna: evolutionary history and biogeography of turban shells (Gastropoda, Turbinidae)

The present study aimed to assess the consequences of tectonic events and temperature regime on the diversification of Indo‐West Pacific (IWP) turban shell species. Bayesian and parsimony methods were used to construct a phylogenetic hypothesis using sequence data from three genes for the turban shell genus Turbo and for a larger data set including representatives of all known genera in the subfamily Turbininae. Phylogenetic analyses indicate that all IWP Turbo species form a single clade approximately 68 Myr in age, predating the closure of the Tethys Sea and therefore predating the physical separation of the IWP from other biogeographical regions. All but one of the IWP subgenera tested in Turbo also predate the closure of the Tethys Sea. Fossil evidence for Marmarostoma, the largest subgenus, confirms that at least some Turbo lineages currently restricted to the IWP were previously more widespread. The combination of the phylogeny with the fossil evidence suggests that present day diversity in IWP Turbo is the result of the evolutionary persistence within the IWP of several, morphologically distinct lineages, some of which were more widespread in the Oligocene. Some IWP lineages show significant increases in diversification in the early Miocene, probably as a result of the increased availability of both shallow‐water habitats due to tectonic plate movements and increased carbonate platforms in the central IWP resulting from coincident diversification of zooxanthellate corals. The IWP is therefore behaving as both a cradle of diversity (with new species originating in situ) and a museum of diversity (with lineages that predate its isolation also being maintained). Bayesian and parsimony analyses of the subfamily recovered five clades and mapping the temperature regime (tropical or temperate) of each species onto the molecular tree using parsimony suggested that temperate habitat is an ancestral character in at least four of the clades. This result was also supported by Bayesian reconstruction of ancestral states. Astralium (the fifth clade) may also prove to have a temperate origin, but this could not be determined with certainty given the available data. The origin of diversity in tropical regions is of particular interest because it has been suggested that the tropics are the source of many evolutionary novelties and have provided a species pool, from which temperate regions were populated. The present study suggests that Turbininae may be an exception to this rule. The tree shape also suggests that temperature has had an effect on speciation rates; temperate Turbininae are apparently evolving more slowly or suffering more extinction than their tropical sister clades, which show greater diversity. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 92 , 573–592.     

A modular concept of plant foraging behaviour: the interplay between local responses and systemic control

In this paper we examined the notion that plant foraging for resources in heterogeneous environments must involve: (1) plasticity at the level of individual modules in reaction to localized environmental signals; and (2) the potential for modification of these responses either by the signals received from connected modules that may be exposed to different conditions, or by the signals reflecting the overall resource status of the plant. A conceptual model is presented to illustrate how plant foraging behaviour is achieved through these processes acting in concert, from the signal reception through signal transduction to morphological or physiological response. Evidence to support the concept is reviewed, using selective root placement under nutritionally heterogeneous conditions and elongation responses of stems and petioles to shade as examples. We discussed how the adoption of this model can promote understanding of the ecological significance of foraging behaviour. We also identified a need to widen the experimental repertoires of both molecular physiology and ecology in order to increase our insight into both the regulation and functioning of foraging responses, and their relationship with the patterns of environmental heterogeneity under which plants have evolved.     

Interspecific competition between two field populations of grass-feeding bugs

Abstract. 1. We tested the hypothesis that interspecific competition between two grass-feeding stenodemine bug species ( Notostira elongata Geoffroy and Megaloceraeu recticornis Geoffroy) was capable of causing population-scale mortality in the field.
2. N.elongata nymphs were added to two field enclosures in which the M.recti-cornis population was to hatch. In one of these enclosures the grass Arrhena-therum elatius L., on which only M.recticornis can feed, was also added. Population changes of the bug species were then monitored. An unmanipulated enclosure and an unfenced plot of grassland were used as controls.
3. The results demonstrated that N.elongata lowered the survival of M.recticornis and that this effect was removed by the addition of A.elatius . The results also suggested that N.elongata suffered less from the effects of competition with M.recticornis when the latter had access to the refuge foodplant.
4. Interspecific competition was rare among members of the grass-feeding stenodemine guild in the area studied. Only one out of fifteen possible pairs competed, and this competition occurred only under special circumstances.     

A numerical model of nitrogen flxation and its application to Lake Valencia,Venezuela*

SUMMARY. 1. A numerical model for calculation of daily and annual nitrogen fixation in lakes is presented. The model is based on empirically-derived equations for the rates of nitrogen fixation by heterocysts (nitrogen-fixing cells) in relation to light and on functions for the vertical and tetnporal distributions of heterocysts and light in a lake. 2. Applications of the model to Lake Valencia, Venezuela, between December 1980 and December 1981 indicated that nitrogen fixation is largely a surface phenomenon in this lake: 80% of diurnal fixation occurred within 1m of the water surface. 3. Nitrogen fixation is largely restricted to periods of lake stratification, when the phytoplankton have sufficient light for growth, but dissolved inorganic nitrogen is scarce. Nitrogen fixation was maximal late in the stratification period of 1981: 85 % of fixation occurred within the last 3 months of the 9-month period. 4. The annual nitrogen fixation in Lake Valencia is 26 kg ha^?1, which is comparable to the nitrogen fixation in temperate eutrophic lakes with seasonal blue-green algal blooms. However, nitrogen fixation accounted for only 23% of the total nitrogen supply to Lake Valencia in 1981.