The Manila clam Ruditapes philippinarum was introduced to Poole Harbour (lat 50°N) on the south coast of England in 1988 as a novel species for aquaculture. Contrary
to expectations, this species naturalised. We report on individual growth patterns, recruitment, mortality and production
within this population. On the intertidal mudflats the abundance of clams (>5 mm in length) varied seasonally between 18 and
56 individuals m−2. There appear to be two recruitment events per year and there were 6 year classes in the population. A mid-summer decline
in abundance was partly due to increased mortality but probably also a result of down-shore migration in response to high
water temperatures and the development of anoxic conditions. A winter fishery removes c 75% of clams of fishable size (maximum
shell length ≥40 mm) and c 20% of the annual production. The fishery depresses the maximum age and size attained by the clams
but appears to be sustainable. Clam mortality due to factors other than fishing is highest in late-winter to early spring.
The growth of the clams is intermediate in comparison with many published studies but remarkably good given their intertidal
position. As on the coasts of the Adriatic Sea, where the clam is also non-native, the Manila clam has thrived in a shallow,
eutrophic, lagoon-like system on the English coast. While the Poole Harbour population is currently Europe’s most northerly
reported self-sustaining, naturalised population, given forecasts of increasing air and sea temperatures it might be expected
that this species will eventually spread to more sites around the coasts of Northern Europe with associated economic and ecological
consequences. 相似文献
Sex allocation theory offers excellent opportunities for testing how animals adjust their behaviour in response to environmental
conditions. A major focus has been on instances of local mate competition (LMC), where female-biased broods are produced to
maximise mating opportunities for sons. However, the predictions of LMC theory can be altered if there is both local competition
for resources during development and an asymmetry between the competitive abilities of the sexes, as has been seen in animals
ranging from wasps to birds. In this paper, we test the extent to which asymmetric larval competition alters the predictions
of LMC theory in the parasitoid wasp Nasonia vitripennis. We found that the body size of both sexes was negatively correlated with the number of offspring developing within the host.
Further, we found that when faced with high levels of competition, the body size of females, but not males, was influenced
by the sex ratio of the competing offspring; females were smaller when a higher proportion of the brood was female. This asymmetric
competition should favour less biased sex ratios than are predicted by standard LMC theory. We then develop a theoretical
model that can be parameterised with our data, allowing us to determine the quantitative consequences of the observed level
of asymmetric larval competition for sex allocation. We found that although asymmetric competition selects for less biased
sex ratios, this effect is negligible compared to LMC. Furthermore, a similar conclusion is reached when we re-analyse existing
data from another parasitoid species where asymmetric larval competition has been observed; Bracon hebetor. Consequently, we suspect that asymmetric larval competition will have its greatest influence on sex ratio evolution in species
that have smaller clutches and where local mate competition is not an issue, such as birds and mammals. 相似文献
Sex allocation is an important reproductive decision for parents. However, it is often assumed that females have substantial
control over sex allocation decisions, and this is particularly true in haplodiploid insects, in which females apparently
determine sex by deciding whether to fertilise an egg (and produce a diploid daughter) or not (and produce a haploid son).
Mechanisms by which males may influence sex allocation are not so straightforward, and their potential influence on sex ratios
has been somewhat neglected. Here, we test whether males influence offspring sex ratios in the parasitoid wasp Nasonia vitripennis. We show that some of the variation in observed sex ratios can be attributed to males when comparing the affect of male strain
on sex ratio. We did not find among-male variation in sex ratio with a less powerful experiment using males from only one
strain or an effect of male mating environment. Our data suggest that males can influence female sex ratios and contribute
to the variation around the sex ratios optimal for females. However, the influence is not large, suggesting that females have
more influence on sex allocation than do males. We conclude by considering whether male influences on sex ratio represent
differences in male reproductive competence or deliberate attempts by males to increase their fitness by influencing daughter
production. 相似文献
Anthropogenic climate change is likely to alter the patterns of moisture availability globally. The consequences of these changes on species distributions and ecosystem function are largely unknown, but possibly predictable based on key ecophysiological differences among currently coexisting species. In this study, we examined the environmental and biological controls on transpiration from a pi?on-juniper (Pinus edulis-Juniperus osteosperma) woodland in southern Utah, USA. The potential for climate-change-associated shifts in moisture inputs could play a critical role in influencing the relative vulnerabilities of pi?ons and junipers to drought and affecting management decisions regarding the persistence of this dominant landscape type in the Intermountain West. We aimed to assess the sensitivity of this woodland to seasonal variations in moisture and to mechanistically explain the hydraulic strategies of P. edulis and J. osteosperma through the use of a hydraulic transport model. Transpiration from the woodland was highly sensitive to variations in seasonal moisture inputs. There were two distinct seasonal pulses of transpiration: a reliable spring pulse supplied by winter-derived precipitation, and a highly variable summer pulse supplied by monsoonal precipitation. Transpiration of P. edulis and J. osteosperma was well predicted by a mechanistic hydraulic transport model (R2 = 0.83 and 0.92, respectively). Our hydraulic model indicated that isohydric regulation of water potential in P. edulis minimized xylem cavitation during drought, which facilitated drought recovery (94% of pre-drought water uptake) but came at the cost of cessation of gas exchange for potentially extended periods. In contrast, the anisohydric J. osteosperma was able to maintain gas exchange at lower water potentials than P. edulis but experienced greater cavitation over the drought and showed a lesser degree of post-drought recovery (55% of pre-drought uptake). As a result, these species should be differentially affected by shifts in the frequency, duration, and intensity of drought. Our results highlight the sensitivity of this woodland type to potential climate-change-associated shifts in seasonal moisture patterns and demonstrate the utility of mechanistic hydraulic models in explaining differential responses of coexisting species to drought. 相似文献
The aim of the study was to test hypotheses derived from Nicholson's (1984) theory of work-role transitions within the context of job relocation. These hypotheses concern the adjustment strategies used by those changing jobs. A total of 121 U.K. employees provided data at three points in time during the process of their relocation. Measures included job characteristics, personal change, role innovation, self-esteem and subjective well-being. Data were analysed via non-recursive structural equation models using LISREL VI. Results were generally consistent with the model of work-role transitions. Individual differences in self-esteem were found to influence adjustment strategies over and above the effects of job characteristics. 相似文献
Viruses and bacteria which are characterized by finite lives in the subsurface are rapidly transported via fractures and cavities in fractured and karst aquifers. Here, we demonstrate how the coupling of a robust outcrop characterization and hydrogeophysical borehole testing is essential for prediction of contaminant velocities and hence wellhead protection areas. To show this, we use the dolostones of the Permian Magnesian Limestone aquifer in NE England, where we incorporated such information in a groundwater flow and particle tracking model. Within this aquifer, flow in relatively narrow (mechanical aperture of ~?10?1–1 mm) fractures is coupled with that in pipe cavities (~?0.20-m diameter) following normal faults. Karstic cavities and narrow fractures are hydraulically very different. Thus, the solutional features are represented within the model by a pipe network (which accounts for turbulence) embedded within an equivalent porous medium representing Darcian flowing fractures. Incorporation of fault conduits in a groundwater model shows that they strongly influence particle tracking results. Despite this, away from faulted areas, the effective flow porosity of the equivalent porous medium remains a crucial parameter. Here, we recommend as most appropriate a relatively low value of effective porosity (of 2.8?×?10?4) based on borehole hydrogeophysical testing. This contrasts with earlier studies using particle tracking analyses on analogous carbonate aquifers, which used much higher values of effective porosity, typically ~?102 times higher than our value, resulting in highly non-conservative estimates of aquifer vulnerability. Low values of effective flow porosities yield modelled flow velocities ranging from ~?100 up to ~?500 m/day in un-faulted areas. However, the high fracturing density and presence of karstic cavities yield modelled flow velocities up to ~?9000 m/day in fault zones. The combination of such flow velocities along particle traces results in 400-day particle traces up to 8-km length, implying the need for large well protection areas and high aquifer vulnerability to slowly degrading contaminants.