Resource-dependent sex-allocation in a simultaneous hermaphrodite

Most sex allocation theory is based on the relationship between the resource investment into male and female reproduction and the consequent fitness returns (often called fitness-gain curves). Here we investigate the effects of resource availability on the sex allocation of a simultaneously hermaphroditic animal, the free-living flatworm Macrostomum lignano. We kept the worms under different resource levels and determined the size of their testes and ovaries over a period of time. At higher resource levels, worms allocated relatively more into the female function, suggesting a saturating male fitness-gain curve for this species. A large part of the observed effect was due to a correlated increase in body size, showing size-dependent sex allocation in M. lignano. However, a significant part of the overall effect was independent of body size, and therefore likely due to the differences in resource availability. Moreover, in accordance with a saturating male fitness-gain curve, the worms developed the male gonads first. As the group size was kept constant, our results contrast with expectations from sex allocation models that deal with local mate competition alone, and with previous experiments that test these models.     

Should mothers provision their offspring equally? A manipulative field test

Within‐brood variation in offspring size is universal, but its causes are unclear. Theoretical explanations for within‐brood variation commonly invoke bet‐hedging, although alternatives consider the role of sibling competition. Despite abundant theory, empirical manipulations of within‐brood variation in offspring size are rare. Using a field experiment, we investigate the consequences of unequal maternal provisioning for both maternal and offspring fitness in a marine invertebrate. We create experimental broods of siblings with identical mean, but different variance, in offspring size, and different sibling densities. Overall, more‐variable broods had higher mean performance than less‐variable broods, suggesting benefits of unequal provisioning that arise independently of bet‐hedging. Complementarity effects drove these benefits, apparently because offspring‐size variation promotes resource partitioning. We suggest that when siblings compete for the same resources, and offspring size affects niche usage, the production of more‐variable broods can provide greater fitness returns given the same maternal investment; a process unanticipated by the current theory.     

Do local moisture stress responses across tree species reflect dry limits of their geographic ranges?

Under future climate drought‐induced tree mortality may result in the contraction of species ranges and the reorganization of community composition where abundant and peripheral species exchange their patterns of dominance. Predicting these changes will be challenging because the future suitable habitat may be a mismatch for the current bioclimatic envelope because of discrepancies between the realized and fundamental niche. Here we evaluate the extent of the discrepancy, as applied to tree species in relation to their relative field‐recorded drought sensitivities and their observed range‐wide environmental moisture envelopes. The hypothesis tested was that different species levels of drought‐induced damage at sites where they co‐occur will be positively associated with the minimum moisture availability in the most drought‐prone part of each species current geographic range. We tested the hypothesis using drought damage measurements for 13 Australian Myrtaceae (including Eucalyptus) tree species at a site where all co‐occur, together with 120 years of climate data across their geographical ranges. With limited statistical power the results generated only modest support for the hypothesis suggesting limited capacity to predict future distributions under climate change scenarios. In spite of the poor dispersal capacities of Eucalyptus and allied genera, but consistent with knowledge of breeding systems and genetic variability within Eucalyptus, the findings also suggest that many species have a capacity for rapid adaptive response to climate change, including the vicissitudes of the late Quaternary.     

TOOTH WEIGHTS OF JUVENILE MALE NORTHERN FUR SEALS, CALLORHINUS URSINUS

Tooth weight and body length of juvenile male northern fur seals, Callorhinus ursinus , collected on St. Paul Island, Alaska, from 1948 to 1984 were compared. The relationship between these variables was investigated for individual and combined age classes. A density-dependent increase in mean tooth weight occurred during a decline in fur seal abundance. Consistent, though poorly understood, differences in tooth weight were found among males associated with different rookeries (breeding grounds). Storing the teeth for long periods may affect the weights, but such effects, if any, are too small to account for the changes observed.     

大兴安岭不同纬度兴安落叶松生长对干旱适应性及生长衰退的差异

气候变暖引起干旱事件发生频率和强度增加,由此导致全球部分森林出现衰退和死亡的现象已被广泛报道。然而由于不同地区森林对水分亏缺适应性不同,需要从区域尺度对森林适应干旱事件的特征进行详细研究。在中国东北的森林地区中,大兴安岭在过去几十年中变暖速度最快。以落叶松为主的大兴安岭森林,作为中国唯一的北方森林,具有重要的生态意义。尤其是在全球变暖极大地影响水条件的情况下,了解其对缺水的适应非常重要。通过树木年轮学方法分析比较不同纬度(47°17′—51°17′N)兴安落叶松(Larix gmelinii)径向生长变化率和树木生长对极端干旱的适应性特征(抵抗力(Rt)、恢复力(Rc)和恢复弹力(Rs))。结果表明：不同纬度兴安落叶松对干旱的应对策略呈现共同特征表现为：(1)研究区不同纬度的兴安落叶松生长均与当年生长季7和8月PDSI显著正相关(P<0.05);(2)大径级树木对干旱的抵抗力和恢复弹力更低,对干旱的适应能力更弱。不同纬度兴安落叶松对干旱的应对策略呈现差异性表现：(1)兴安落叶松对干旱的抵抗力和恢复弹力随纬度升高而降低,恢复力随纬度升高而增加;(2)研究区范围内较高纬度的兴安落叶松...     

Effect of mother plant age on germination and size of seeds and seedlings in the perennial sedge Carex secalina (Cyperaceae)

The performance of seeds and seedlings in relation to the age of the mother plant was studied in Carex secalina. Seeds of this sedge can differ substantially in size. We planted 100 C. secalina individuals from three populations in a common garden and followed them for four years. We found that mean seed mass varied with plant age, but the pattern of variation was population-specific, with only one population showing significant reduction in seed mass with age. Similarly, germination frequency changed with age differently in different populations. The relationship between the age of the mother plant and the height of emerged seedlings did not differ between populations. In spite of the fact that plant size and mean seed mass exhibited similar patterns of variation within populations, there was no correlation between these two variables at the level of individuals. This means that the size of C. secalina tufts does not determine how big the seeds will be. Moreover, there was no relationship between mean seed mass and the height of seedlings. Presumably, factors intrinsic to each plant determine the production of either small or large seeds in a population-specific way.     

Periodic habitat loss alters the competitive coexistence between brown trout and bullheads in a small stream over 34 years

1. Changes in the population density of juvenile sea trout Salmo trutta L. and bullheads Cottus gobio L. were compared in a small stream over 34 years. Both species have a similar diet and obviously live in the same general habitat. Habitat loss was most marked in seven summer droughts: severest in 1976, 1983, 1984, 1995, and less severe but followed by autumn droughts in 1969, 1989 and 1993. The contrasting effects of habitat loss on the two species were examined. 2. For both species, the Ricker curvilinear model significantly fit (P < 0.001) the relationship between initial egg density and survivor density for successive life stages, even though egg densities were much lower for bullheads than trout. These analyses provided evidence for density-dependent population regulation and also identified extreme outliers, most being for year-classes affected by summer droughts. 3. The variable effects of changes in habitable area (= % wettable area in sampling section) were quantified by using the residuals, each residual being the absolute value expressed as a percentage of the expected value from the Ricker curve. Significant relationships between the residuals and habitable area showed that habitat loss had a marked effect on survivor density, this being negative for 0+ and 1+ trout, and positive for 0+, 1+ and 2+/3+ bullheads. 4. Therefore, during periods of habitat loss in the summer months, bullhead density increased at the expense of trout density. Low flows and a decrease in wettable area were associated with a marked reduction in habitat quality for drift-feeding trout and an increase in habitat quality, and perhaps also quantity, for benthic-feeding bullheads. This case study shows that, during a major perturbation, the relationship between the densities of two species can change markedly in favour of the less numerous species. The competitive coexistence between the two species is therefore a dynamic process that changes through time with periodic changes in the environment.