Protracted estuarine phase in the life cycle of the marine pufferfish Torquigener pleurogramma

The present study was undertaken to elucidate the way in which the Swan Estuary in south-western Australia is used by the common blowfish Torquigener pleurogramma, a representative of the abundant and widely distributed family Tetraodontidae. T. pleurogramm were collected by beach seine and otter trawl from the Swan Estuary between February 1977 and December 1980 and between May 1984 and February 1986. While T. pleurogramma feeds on a wide variety of organisms in the estuary, the main components of its diet are polychaetes and amphipods for fish <130 mm and bivalve molluscs for larger fish. Numbers of blowfish were inversely correlated with water depth, with densities on the banks (water depth <1.5 m) sometimes reaching 5 fish m^-2, and tended to be greater at night than during the day. The density of T. pleurogramma in the shallows was positively correlated with salinity and inversely correlated with distance from the estuary mouth. Numbers increased greatly in the latter half of 1980 and 1985 as a result of the recruitment of large numbers of the 0+ age class (i.e., fish in their first year of life). Blowfish were represented by seven age classes in the estuary and attained a maximum size of 230 mm (220 g). By the end of their first and second years of life, fish had reached approximately 90 mm (14 g) and 125 mm (39 g), respectively. Sexual maturity was generally not reached until the end of the second year of life. The presence of higher gonadosomatic indices and more mature gonads in fish collected just outside than within the estuary indicate that T. pleurogramma leaves the estuary before spawning. Comparisons between lengthfrequency data, allied with information on the prevalence and intensity of gill parasites, indicate that assemblages in estuarine and neighbouring inshore waters remain distinct for many months and that growth within the estuary is faster than in inshore marine environments.     

Counterintuitive effects of large-scale predator removal on a midlatitude rodent community

Historically, small mammals have been focal organisms for studying predator-prey dynamics, principally because of interest in explaining the drivers of the cyclical dynamics exhibited by northern vole, lemming, and hare populations. However, many small-mammal species occur at relatively low and fairly stable densities at temperate latitudes, and our understanding of how complex predator assemblages influence the abundance and dynamics of these species is surprisingly limited. In an intact grassland ecosystem in western Montana, USA, we examined the abundance and dynamics of Columbian ground squirrels (Spermophilus columbianus), deer mice (Peromyscus maniculatus), and montane voles (Microtus montanus) on 1-ha plots where we excluded mammalian and avian predators and ungulates, excluded ungulates alone, or allowed predators and ungulates full access. Our goal was to determine whether the relatively low population abundance and moderate population fluctuations of these rodents were due to population suppression by predators. Our predator-exclusion treatment was divided into two phases: a phase where we excluded all predators except weasels (Mustela spp.; 2002-2005), and a phase where all predators including weasels were excluded (2006-2009). Across the entire duration of the experiment, predator and/or ungulate exclusion had no effect on the abundance or overall dynamics of ground squirrels and deer mice. Ground squirrel survival (the only species abundant enough to accurately estimate survival) was also unaffected by our experimental treatments. Prior to weasel exclusion, predators also had no impacts on montane vole abundance or dynamics. However, after weasel exclusion, vole populations reached greater population peaks, and there was greater recruitment of young animals on predator-exclusion plots compared to plots open to predators during peak years. These results suggest that the impacts of predators cannot be generalized across all rodents in an assemblage. Furthermore, they suggest that specialist predators can play an important role in suppressing vole abundance even in lower-latitude vole populations that occur at relatively low densities.     

The ichthyofaunal composition of the Elbe Estuary: an analysis in space and time

A stow net (90 m² opening) was used, at regular intervals between July 1989 and June 1993, to collect fish over ca. 4 h on both the flood and ebb tides at five sites distributed at regularly spaced distances throughout the Elbe Estuary. Overall, 53.4% of the species were marine, and some of these were abundant, e.g.Clupea harengus, Pleuronectes flesus, Pomatoschistus minutus, Merlangius merlangus and Sprattus sprattus. However, in terms of number of individuals, such species contributed only 9.1%, compared with 90.0% by anadromous species, which was predominantly due to exceptionally high catches of Osmerus eperlanus. The ichthyofaunal compositions on the flood and ebb tides were similar, even downstream where salinities change markedly during each tidal cycle. Although this is probably due in part to groups of fish being swept first upstream on the flood tide and then back downstream on the ebb tide, several species clearly tended to occupy particular regions of the estuary. Each year the species composition changed sequentially from the most downstream site (max. salinity=ca. 31.4‰) to the most upstream site (max. salinity=ca. 1.5‰), mainly as a result of a sequential decline in the number and abundance of marine species and a progressive increase in the contributions of both anadromous and freshwater species. However, the marine species diagnostic of differences in the compositions among regions varied between years. Thus, for example, the diagnostic species for the downstream sites were C. harengus and M. merlangus in 1990/1991 and S. sprattus in 1992/1993, reflecting differences in the recruitment strengths of these marine species in the two years. The ichthyofaunal composition exhibited pronounced annual cyclical changes in each region and thus occurred irrespective of whether or not the salinity in a region underwent pronounced changes during the year. This cyclicity was attributable to intra-annual variations in the times of recruitment of certain marine species, particularly in the downstream region, and to seasonal migrations of diadromous species into each region, as well as to the movements of freshwater species into the upstream region in winter and early spring. Received: 18 November 1999 / Accepted: 25 October 2000     

Age structure,growth rates,movement patterns and feeding in an estuarine population of the cardinalfish Apogon rueppellii

The biology of a population of the cardinalfish Apogon rueppellii has been studied over several years (1977–1983) in the Swan Estuary in south-western Australia, using ramples collected monthly from the shallows by beach seine and from various depths by otter trawl. While the life cycle of this species typically lasts for one year, at the end of which time the mean length is 50 to 60 mm, some individuals survive for a further year and attain lengths up to 104 mm. A. rueppellii shows a marked tendency to move offshore into deeper water during the winter months. This tendency is more pronounced in the 1+ than in the 0+ year class and in larger than smaller 0+ individuals. An inshore movement of A. rueppellii in the spring is followed by spawning and by oral brooding by the males, which leads to the recruitment of large numbers of a new 0+ year class on to the banks during the summer. The offshore movement is correlated with changes in salinity and temperature. The larger catches taken by otter trawl during the day than at night indicate that A. rueppellii exhibits a diel pattern of activity. Mean fecundity ranged from 70 in the 45 to 49 mm size class to 345 in the 90 to 94 mm size class. Measurements of fecundity and the number of oral-brooded eggs demonstrated that the majority of the eggs released by the female are collected and incubated by the males. Copepods are ingested in relatively greater amounts by small than by large A. rueppellii, whereas the reverse situation occurs with larger crustaceans, polychaetes and small fish. The presence of greater amounts of copepods in the diet during the day and of amphipods at night probably reflects the diel activity patterns of the prey.     

Assessing Landscape Condition Relative to Water Resources in the Western United States: A Strategic Approach

The Environmental Monitoring and Assessment Program (EMAP) is proposing an ambitious agenda to assess the status of streams and estuaries in a 12-State area of the western United States by the end of 2003. Additionally, EMAP is proposing to access landscape conditions as they relate to stream and estuary conditions across the west. The goal of this landscape project is to develop a landscape model that can be used to identify the relative risks of streams and estuaries to potential declines due to watershed-scale, landscape conditions across the west. To do so, requires an understanding of quantitative relationships between landscape composition and pattern metrics and parameters of stream and estuary conditions. This paper describes a strategic approach for evaluating the degree to which landscape composition and pattern influence stream and estuary condition, and the development and implementation of a spatially-distributed, landscape analysis approach.     

Echolocation call design and limits on prey size: a case study using the aerial-hawking bat Nyctalus leisleri

The echolocation calls used by Nyctalus leisleri during search phase in open air space are between 9 and 14 ms long, with the peak energy between 24 and 28 kHz. The pulses are shallowly frequency-modulated with or without an initial steep frequency-modulated component. The diet consists primarily of small flies (Diptera), including many chironomids (wingspan 9–12 mm) and yellow dung flies (Scatophaga; wingspan 24 mm), but also of some larger insects such as dung beetles (Coleoptera; Scarabaeoidea), caddis-flies (Trichoptera) and moths (Lepidoptera). The echo target strength of some prey items was measured. Contrary to models based on standard targets such as spheres or disks, the echo strength of real insects was found to be virtually independent of the emitted frequency within the 20–100 kHz frequency range. A model was used to calculate probable detection distances of the prey by the bat. Using narrow-band calls of 13.7 ± 2.7 ms duration, a bat would detect the two smallest size classes of insect at greatest range using calls of 20 kHz. The results may therefore explain why many species of large and medium sized aerial-hawking bats use low-frequency calls and still eat mostly relatively small insects. The data and model challenges the assumption that small prey are unavailable to bats using low-frequency calls.     

贵阳市城区PM10和PM2.5水溶性离子特征分析

分别于2013年10月和2014年2月、5月、7月在贵阳市城区3个环境空气质量监测国控点位（南明区市监测站、云岩区黔灵公园马鞍山和观山湖区贵阳一中）进行PM₁₀、PM_2.5样品采集,并对10种水溶性离子（SO₄^2-、NO₂^-、NO₃^-、NH₄⁺、Cl^-、F^-、Na⁺、K⁺、Mg²⁺、Ca²⁺）的含量进行了分析。结果表明,研究时段内,贵阳市3个点位PM₁₀、PM_2.5平均质量浓度分别为（64.8±25.5）、（46.6±21.2）μg/m³。其中,云岩区黔灵公园马鞍山点位的颗粒物浓度最低,南明区市监测站点位最高。3个点位PM_2.5平均浓度与PM₁₀平均浓度的比值为0.719,表明贵阳市城区PM₁₀中,PM_2.5占主导地位。水溶性离子分析显示,SO₄^2-、NO₂^-、NO₃^-、NH₄⁺、Cl^-、F^-、Na⁺、K⁺主要分布在PM_2.5中,Mg²⁺、Ca²⁺主要分布在PM₁₀中。3个点位PM₁₀和PM_2.5中的水溶性离子均表现为SO₄^2-、NH₄⁺、Ca²⁺浓度较大,F^-、NO₂^-较小,表明3个点位的污染源总体相同,且水溶性离子占PM₁₀、PM_2.5含量的比例达33.6%~48.1%。贵阳市城区大气中的SO₂转化率在5月、7月、10月较高,2月最低,主要是由于5月、7月、10月的高温、高湿、强辐射环境条件促进了SO₂向SO₄^2-的转化。阴阳离子平衡分析表明,贵阳市城区PM₁₀、PM_2.5呈现出偏碱性的特征。水溶性离子主成分分析表明,贵阳市城区PM₁₀中的水溶性离子主要来源于城市扬尘、生物质燃烧尘、煤烟尘、建筑尘以及二次粒子,PM_2.5中水溶性离子的来源与PM₁₀较为相似。     

Weed-biocontrol insects reduce native-plant recruitment through second-order apparent competition

Small-mammal seed predation is an important force structuring native-plant communities that may also influence exotic-plant invasions. In the intermountain West, deer mice (Peromyscus maniculatus) are prominent predators of native-plant seeds, but they avoid consuming seeds of certain widespread invasives like spotted knapweed (Centaurea maculosa). These mice also consume the biological-control insects Urophora spp. introduced to control C. maculosa, and this food resource substantially increases deer mouse populations. Thus, mice may play an important role in the invasion and management of C. maculosa through food-web interactions. We examined deer mouse seed predation and its effects on seedling emergence and establishment of a dominant native grass, Pseudoroegneria spicata, and forb, Balsamorhiza sagittata, in C. maculosa-invaded grasslands that were treated with herbicide to suppress C. maculosa or left untreated as controls. Deer mice readily took seeds of both native plants but removed 2-20 times more of the larger B. sagittata seeds than the smaller P. spicata seeds. Seed predation reduced emergence and establishment of both species but had greater impacts on B. sagittata. The intensity of seed predation corresponded with annual and seasonal changes in deer mouse abundance, suggesting that abundance largely determined mouse impacts on native-plant seeds. Accordingly, herbicide treatments that reduced mouse abundance by suppressing C. maculosa and its associated biocontrol food subsidies to mice also reduced seed predation and decreased the impact of deer mice on B. sagittata establishment. These results provide evidence that Urophora biocontrol agents may exacerbate the negative effects of C. maculosa on native plants through a form of second-order apparent competition-a biocontrol indirect effect that has not been previously documented. Herbicide suppressed C. maculosa and Urophora, reducing mouse populations and moderating seed predation on native plants, but the herbicide's direct negative effects on native forb seedlings overwhelmed the indirect positive effect of reducing deer mouse seed predation. By manipulating this four-level food chain, we illustrate that host-specific biological control agents may impact nontarget plant species through food-web interactions, and herbicides may influence management outcomes through indirect trophic interactions in addition to their direct effects on plants.     

Where,why, and to what extent have rangelands in the Karoo,South Africa,desertified

We used simple questionnaire surveys to obtain rough categorical information on the status of semiarid and arid rangelands in the Karoo, South Africa. We sent questionnaires to extension officers of the Department of Agricultural Development, asking them to rank the condition of the rangeland in their extension districts (usually about 500 farms) on a three- or five-point scale. The questionnaire also sought information on the possible reasons for changes in the condition of the rangelands; vegetation types most at risk on mismanaged rangelands; and changes in the status of plants, birds, and mammals. Extension officers rated 35% of the semiarid rangelands in southwestern South Africa (the Karoo and southern Kalahari Desert) as being in poor condition. There was some congruency between the perceived condition of the agricultural extension district and the percentage that the stocking rate had decreased. Rangeland condition in more arid areas tends to be poor. Stocking rates have decreased disproportionately more in arid areas than in the more mesic areas of southwestern South Africa. Grasses were rated as the most threatened group of plants, and succulents the least threatened group. Overgrazing in the past was considered to be the most important cause of present-day rangeland condition, followed by present overstocking of rangelands. Antelope were considered more threatened by rangeland management than mammalian carnivores. Avian raptors and bustards, gamebirds, and tortoises were all perceived to be threatened by rangeland mismanagement.