Microhabitat selectivity underpins regional indicators of fish abundance and replenishment

Species with specialized resource use can display strong spatial heterogeneity in abundance according to the availability of their preferred habitats. If these preferences shift with ontogeny, then a wide range of habitats may need to be protected in order to support both adult populations and their replenishment. We explored whether microhabitat selectivity interacts with habitat availability to provide an effective suite of indicators for regional fish abundance and replenishment, using offshore rocky reefs in south-eastern Australia as a case study. We examined generalized additive mixed models (GAMMs) in a full subsets approach to infer the best predictors for adult and juvenile fish density in four diverse families (Labridae, Odacidae, Pomacentridae, Serranidae), based on rapid underwater visual surveys across transects (⿼500 m²), wave exposures (0.3⿿1 km), and sites (0.3⿿48 km). We then examined whether these regional fish-habitat models aligned with the microhabitat electivity of individuals (at scale of <1 m²). Microhabitat selection by reef fishes at the local scale underpinned the most effective habitat indicators for regional heterogeneity in fish abundance, and pointed to critical nursery habitats that support hotspots of juvenile recruitment. Strong species-habitat relationships, such as these, can be combined with broad-scale habitat mapping to assess the potential carrying capacity of focal areas, spatial management zone placements, and nursery habitats that warrant special protection. A number of emerging threats to these key habitat types indicates an urgent need for habitat-based protection and monitoring as a key part of holistic marine ecosystem conservation and management.     

GROWTH RATE VARIATION IN THE N:P REQUIREMENT RATIO OF PHYTOPLANKTON1

Theoretical considerations predict that the cell N:P ratio at transition from nitrogen limitation to phosphorus limitation of phytoplankton growth (critical ratio, R_c) varies, as a function of population growth rate. This prediction is confirmed by experimental, data from the literature along with new experimental data for the marine, prymnesiophyte Pavlova lutheri (Droop) Green. R_c passes through a maximum at intermediate growth rates for the three phytoplankton species for which data, are available, but there is significant interspecific variability in its value. There is no theoretical or experimental evidence to support the idea that the ratio of subsistence N and P cell quotas is equal to R_c over the range of growth rates, or that the subsistence quota ratio equals the ratio of the N and P cell quotas minus a storage fraction. Examination of N:P composition ratios can be used to determine which nutrient is limiting, but cannot be used to determine relative growth rates or competitive advantage between species limited by the same nutrient. Growth rates are determined by environmental conditions and by the cell quota of the limiting nutrient, without reference to the cell quota of the non-limiting nutrient.     

Stoichiometry in an ecological context: testing for links between <Emphasis Type="Italic">Daphnia</Emphasis> P-content,growth rate and habitat preference

We used laboratory experiments with ten Daphnia taxa to test for links between Daphnia P-content, growth rate and habitat preference. The taxa represent a wide range of body sizes and most show distinct preferences for one of three habitats: shallow lakes, deep, stratified lakes or fishless ponds. Previous studies show that taxa from shallow lakes and fishless ponds experience high predation risk and rich food resources, whereas taxa from deep lakes experience low predation risk, strong food limitation and potentially P-deficient resources. Thus, we predicted higher P-content and higher maximal growth rates in taxa from ponds and shallow lakes and lower P-content, lower maximal growth but reduced sensitivity to P-limitation in taxa preferring stratified lakes. In each of 25 experiments, a clonal Daphnia cohort was cultured for 4 days on a P-sufficient (molar C:P ratio 70) or a P-deficient (C:P 1,000) diet of a green alga at a high concentration (1 mg C l^–1). The P-content of adult Daphnia fed the P-sufficient diet ranged from 1.52 to 1.22% mass. Small-bodied taxa from shallow lakes had higher P-content than larger-bodied taxa from deep lakes or fishless ponds. However, we found a nonsignificant negative correlation between P-content and growth on the P-sufficient diet, rather than the positive relationship predicted by the growth rate hypothesis. The P-deficient diet resulted in declines in both growth rate and P-content compared with the P-sufficient controls and the extent of the declines differed between taxa. Taxa from ponds showed a marginally greater decline in growth with the P-deficient diet compared with taxa from shallow or deep lakes. However, contrary to stoichiometric theory, no relationship was found between a species P-content and growth depression on the P-deficient diet. Although we found evidence for habitat adaptations, our results show that factors other than Daphnia P-content are important in determining differences between Daphnia species in both maximal growth rate and sensitivity to P-limited growth.     

Colony size and not nest density drives reproductive output in the Common Tern Sterna hirundo

Density is known to be an important factor in population size regulation. Several mechanisms of density limitation have been identified in colonial birds. We studied competition in Common Terns Sterna hirundo to assess whether the factor limiting reproductive output was competition for nest‐sites, which is dependent on local nest density, or density‐dependent competition for food resources, which is dependent on overall colony size using the same foraging area. We found strong associations of both colony size and nest density with reproductive output in five colonies of Common Terns in three different habitats (one marine, two freshwater). Based on detailed long‐term datasets of six separate sub‐colonies of the Banter See colony that differed in nest density, we found that reproductive success was not related to nest density but to overall colony size, possibly a result of resource depletion and food competition. We also found carry‐over effects of colony size during rearing on post‐fledging return rate. These results have important implications for the conservation management plans aimed at recovering declining populations of Common Terns.     

雌先熟植物茄参 （茄科）的繁育系统

对分布于云南西北部的茄参（Mandragora caulescens）的繁育系统进行了研究。茄参的花期为5月底到6月初,单花持续时间为（9.9±2.8）d,其性表达方式为雌性先熟。有三种昆虫拜访茄参的花,白背熊蜂蜂王是最有效的传粉昆虫,但其访花频率很低;苍蝇和蚂蚁的访花频率较高,但苍蝇传粉效率十分有限;蚂蚁是窃蜜者。授粉实验证明茄参具有部分自交亲和能力,但由于雌性先熟,不具备主动自交能力。人工添加异交花粉显著提高了结实率和结籽数,证明茄参存在花粉限制。茄科的大部分植物是严格自交不亲和的,但茄参族的茄参是自交部分亲和,这和茄科天仙子族的部分成员类似。我们认为这两个独立起源于新世界向欧亚大陆扩散事件的族,各自从自交不亲和向自交亲和转变可能与高山地区恶劣的自然条件有关,这种繁育系统的进化模式是否具有普遍性值得进一步研究。     

Optimal pollinator attraction strategies in Trollius ranunculoides Hemsl. (Ranunculaceae) at different altitudes: increased floral display or promotion of nectar output?

For alpine plant species, patterns of resource allocation to functional floral traits for pollinator attraction can be highly significant in adaptation to low pollinator abundance and consequent pollen limitation. Increased pollination can be achieved either through a larger floral display or production of more pollen rewards. In this study, variation in resource allocation to different components for pollinator attraction was studied along an altitudinal gradient in Trollius ranunculoides, an obligate self‐incompatible out‐crosser of the Qinghai–Tibet Plateau. We compared resource allocation to conspicuous yellow sepals (which mainly provide visual attraction) and degenerate petals (which provide the major nectar reward) between populations at four altitudes. Furthermore, we investigated the contribution of sepals and petals to pollinator attraction and female reproductive success in an experiment with sepal or petal removal at sites at different altitudes. At the level of single flowers, resource allocation increased to sepals but decreased to petals with increasing altitude. Consistent with these results, sepals contributed much more to visitation rate and seed set than petals, as confirmed in the sepal or petal removal experiment. Sepals and petals contributed to female reproductive success by ensuring visitation rate rather than visitation duration. To alleviate increasing pollen limitation with increasing altitude, resource allocation patterns of T. ranunculoides altered to favour development of sepals rather than petals. This strategy may improve pollination and reproductive success through visual attraction (sepal) rather than nectar reward (petal) over a gradient of decreasing pollinator abundance.     

Experimental test of nest-site limitation in mature mixed forests of central British Columbia,Canada

Nest-site availability limits cavity-using populations in many harvested forests; however, little is known about the extent of nest-site limitation in mature forests with a full complement of excavator species and intact processes of cavity creation and loss. To examine the role of nest-site availability in limiting cavity-using populations in mature mixed conifer forests in central British Columbia, Canada, we conducted an 11-year before-after control-impact experiment in which we increased nest-site availability via nest box addition. Our 7 sites (3 treatments, 4 controls) had low cavity densities (<2/ha) prior to treatment and cavity occupation rates were also low (<10%/yr), which is a relationship often cited in the literature as evidence of non-limitation in cavity-nesting populations. Following nest box addition at our treatment sites, which tripled the availability of cavities, total density of bird and mammal nests more than tripled. Density of mountain chickadee (Poecile gambeli) nests increased 9-fold on treatment sites and returned to pre-treatment levels following box removal, suggesting that chickadee populations were limited by cavity availability at our study sites. Density of red squirrel (Tamiasciurus hudsonicus) and northern flying squirrel (Glaucomys sabrinus) nests and roosts also increased significantly at treatment sites following box addition and declined following box removal. We noted little change in chickadee or squirrel nest density at control sites monitored concurrently. Squirrels preferred large-sized over small-sized boxes, and significantly enlarged the entrance areas of small boxes by chewing, suggesting that there may have been a shortage of suitable nest and roost sites for them in our study area. We contend that low cavity occupancy rates may not accurately reflect nest-site availability for cavity nesters in mature forests, and that cavity size may influence the true availability of cavities on the landscape. © 2011 The Wildlife Society.     

NITROGEN LIMITATION IN DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE) LEADS TO INCREASED SUSCEPTIBILITY TO DAMAGE BY ULTRAVIOLET‐B RADIATION BUT ALSO INCREASED REPAIR CAPACITY1

Chl fluorescence was used to measure the photosynthetic capacity of the green alga Dunaliella tertiolecta in order to investigate interactions between susceptibility to acute UV‐B radiation (UVBR, 280–320 nm) exposure and decreased nitrogen availability. Under UVBR exposure the decline in the fluorescence parameters F_v/F_m (the maximum effective quantum yield Φ_PSIIe‐max) and F_v′/F_m′ (the operational quantum yield of PSII, Φ_PSIIe) were enhanced with higher UVBR fluxes, with the data well described by the Kok model, inferring that a dynamic balance existed between damage and repair with the repair proportional to the pool size of inactivated targets. When UVBR exposure was coupled with nitrogen limitation, the inhibition of photosynthesis was intensified. Under the more severely N‐limited conditions, the damage rate increased. Unexpectedly, repair rates were also stimulated under N‐limited conditions, although this was insufficient to counteract the increase in damage, so the overall effect of N limitation was an enhancement of UVBR‐induced inhibition of photosynthesis.     

人工引入传粉者对油茶丰产的意义及潜在的生物安全

动物传粉在全球生态系统中提供重要服务,全球农业生产对动物传粉服务的需求稳定增长。油茶是我国特有的重要木本油料植物,栽培历史悠久,由于严格自交不亲和而高度依赖动物传粉。基于我国油茶产业发展的新形势,从资源分配和花粉限制等生态因素系统分析了油茶产量受限制的原因,提出油茶的传粉者限制效果等同于花粉限制效果,人工引入潜在的高效传粉者是解除油茶资源限制措施和传粉生态限制措施之间冲突、提高油茶产量的有效途径。论述了人工引入传粉者传粉服务的优势和挑战。对人工引入传粉者,特别是外来传粉者所面临的潜在风险和控制途径进行了梳理,提出从传粉生态服务解决油茶产量和质量问题的新思路。全文旨在为人工引入传粉者在油茶丰产中的作用提供理论与科学依据,以期促进油茶产业健康有序发展。     

The phosphorus‐rich signature of fire in the soil–plant system: a global meta‐analysis

The biogeochemical and stoichiometric signature of vegetation fire may influence post‐fire ecosystem characteristics and the evolution of plant ‘fire traits’. Phosphorus (P), a potentially limiting nutrient in many fire‐prone environments, might be particularly important in this context; however, the effects of fire on P cycling often vary widely. We conducted a global‐scale meta‐analysis using data from 174 soil studies and 39 litter studies, and found that fire led to significantly higher concentrations of soil mineral P as well as significantly lower soil and litter carbon:P and nitrogen:P ratios. These results demonstrate that fire has a P‐rich signature in the soil–plant system that varies with vegetation type. Further, they suggest that burning can ease P limitation and decouple the biogeochemical cycling of P, carbon and nitrogen. These effects resemble a transient reversion to an earlier stage of ecosystem development, and likely underpin at least some of fire's impacts on ecosystems and organisms.