Floristic and ecological diversity of Ranunculus aquatic habitats in the sub-Atlantic range: implications for conservation

Ranunculus communities have been identified by the European Community Directive on Conservation of Natural and Semi-Natural Habitats as a key habitat in need of protection. The ecological requirements and floristic composition of three Ranunculus communities were investigated in North-Eastern France. We also aimed to determine the relationships between environmental parameters, Ranunculus abundance and species richness. R. fluitans communities clearly differed from the other communities by a higher evenness and species richness and by the occurrence of specific species. In contrast, R. penicillatus and R. peltatus communities displayed similar floristic content. Three different sub-associations were however identified within these communities. If alkalinity was determinant in isolating R. fluitans communities from the other stretches, the ecological ranges of the three communities strongly overlapped while considering parameters linked with other resources or with physical constraints. Further, no significant correlations were highlighted between environmental parameters, Ranunculus abundance and species richness. With respect to conservation, these observations may indicate the particular difficulty in defining each Ranunculus habitat on a European scale, and identify several key points which need to be taken into account when evaluating their status.     

Revealing the mechanisms of sexual isolation in a case of sympatric and parallel ecological divergence

Two ecotypes of a marine intertidal snail (Littorina saxatilis), living at different microhabitats and shore levels, have evolved in sympatry and in parallel across the Galician rocky shore. These ecotypes differ in many traits (including size) due to differential adaptation. They meet, mate assortatively, and partially hybridize at the mid shore where the two microhabitats overlap. The partial sexual isolation observed is claimed to be a side‐effect of the size differences between ecotypes combined with a size assortative mating found in most populations of this species. We investigated this hypothesis using three complementary experimental approaches. First, we investigated which of the different shell variables contributed most to the variation in individual sexual isolation in the field by using two new statistics developed for that purpose: (1) pair sexual isolation and (2) r_i, which is based on the Pearson correlation coefficient. We found that size is the most important trait explaining the sexual isolation and, in particular, the males appear to be the key sex contributing to sexual isolation. Second, we compared the size assortative mating between regions: exposed rocky shore populations from north‐westwern Spain (showing incomplete reproductive isolation due to size assortative mating) and protected Spanish and Swedish populations (showing size assortative mating but not reproductive isolation between ecomorphs). Most of the variation in size assortative mating between localities was significantly explained by the within‐population level of variation on size. Third, we performed a laboratory male choice experiment, which further suggested that the choice is made predominantly on the basis of size. These results confirm the mechanism proposed to explain the sexual isolation in the Galician hybrid zone and thus support this case as a putative example of parallel incipient speciation. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94 , 513–526.     

Development of European bitterling in the gills of freshwater mussels

The development of bitterling embryos within the unique environment of a freshwater mussel's gills requires a departure from typical cyprinid embryological development. Eggs are large (2·6×1·7 mm), illustrating the low risk of predation, and elliptical; a response to unionid gill morphology and a way of increasing the transfer rates of respiratory and excretory products to and from the tissues. The yolk sac develops elongated lateral processes during early ontogeny; these secure the embryo into the host's interlamellar space. Once the larva is capable of movement (8·2 mm) the lateral processes are lost and the larva becomes less dependent of the host's gills for the provision of oxygen. Hatching (3·3 mm) and pigmentation of the blood (6·4 mm) occur relatively early; this may increase the rate of oxygen supply to the tissues. Pigmentation of the eyes and appearance of the melanophores occurs relatively late (7·4 mm and 7·9 mm, respectively); embryos are not required to detect or avoid predators. Bitterling larvae generally emerge from the host once the yolk sac has been consumed (10·5 mm); this may mark a change in respiratory and nutritional requirements.     

PROBLEMS WITH SAMPLING DESERT TORTOISES: A SIMULATION ANALYSIS BASED ON FIELD DATA

Abstract: The desert tortoise (Gopherus agassizii) was listed as a U.S. threatened species in 1990 based largely on population declines inferred from mark-recapture surveys of 2.59-km² (1-mi²) plots. Since then, several census methods have been proposed and tested, but all methods still pose logistical or statistical difficulties. We conducted computer simulations using actual tortoise location data from 2 1-mi² plot surveys in southern California, USA, to identify strengths and weaknesses of current sampling strategies. We considered tortoise population estimates based on these plots as “truth” and then tested various sampling methods based on sampling smaller plots or transect lines passing through the mile squares. Data were analyzed using Schnabel's mark-recapture estimate and program CAPTURE. Experimental subsampling with replacement of the 1-mi² data using 1-km² and 0.25-km² plot boundaries produced data sets of smaller plot sizes, which we compared to estimates from the 1-mi² plots. We also tested distance sampling by saturating a 1-mi² site with computer simulated transect lines, once again evaluating bias in density estimates. Subsampling estimates from 1-km² plots did not differ significantly from the estimates derived at 1-mi². The 0.25-km² subsamples significantly overestimated population sizes, chiefly because too few recaptures were made. Distance sampling simulations were biased 80% of the time and had high coefficient of variation to density ratios. Furthermore, a prospective power analysis suggested limited ability to detect population declines as high as 50%. We concluded that poor performance and bias of both sampling procedures was driven by insufficient sample size, suggesting that all efforts must be directed to increasing numbers found in order to produce reliable results. Our results suggest that present methods may not be capable of accurately estimating desert tortoise populations.     

Carry-over effects of the social environment on future divorce probability in a wild bird population

Initial mate choice and re-mating strategies (infidelity and divorce) influence individual fitness. Both of these should be influenced by the social environment, which determines the number and availability of potential partners. While most studies looking at this relationship take a population-level approach, individual-level responses to variation in the social environment remain largely unstudied. Here, we explore carry-over effects on future mating decisions of the social environment in which the initial mating decision occurred. Using detailed data on the winter social networks of great tits, we tested whether the probability of subsequent divorce, a year later, could be predicted by measures of the social environment at the time of pairing. We found that males that had a lower proportion of female associates, and whose partner ranked lower among these, as well as inexperienced breeders, were more likely to divorce after breeding. We found no evidence that a female''s social environment influenced the probability of divorce. Our findings highlight the importance of the social environment that individuals experience during initial pair formation on later pairing outcomes, and demonstrate that such effects can be delayed. Exploring these extended effects of the social environment can yield valuable insights into processes and selective pressures acting upon the mating strategies that individuals adopt.     

Toads in the backyard: why do invasive cane toads (Rhinella marina) prefer buildings to bushland?

Like many invasive species, cane toads (Rhinella marina) in Australia concentrate in the disturbed habitats created by human activity, rather than in pristine areas. We surveyed cane toads in the wet–dry tropics of the Northern Territory to assess the abundances, body sizes, sexes, behaviour, hydration state and feeding rates of toads around buildings compared to those in areas remote from buildings, and conducted experimental trials to assess the effects of building-related variables (lights and increased toad densities) on the foraging success of toads. Toads around buildings were smaller than bushland conspecifics, and adult sex-ratios were female-biased. Toads were more sedentary around buildings than in the bush, but their feeding rates (based on direct observations and faeces production post-capture) were similar. That similarity, despite twofold-higher densities of competing toads around building, reflected the strong enhancement of feeding rates due to artificial lights attracting insects (in our experimental trials, a threefold increase regardless of the number of competing toads). Toads collected from around buildings were apparently in better hydric condition. Thus, access to water also may attract toads to buildings. The relative scarcity of adult male toads around buildings likely reflects waterbody-centred reproductive activities, whereas the concentration of females and juveniles around buildings is driven largely by access to the insects attracted by artificial light. We conclude that buildings enhance the persistence of cane toad populations and may facilitate their spread.