Structural complexity in relation to the habitat preferences,territoriality, and hatchery rearing of juvenile China rockfish (<Emphasis Type="Italic">Sebastes nebulosus</Emphasis>)

Conservation efforts require an understanding of the basic behavior and ecology of target species. However, limited information exists for a wide range of taxa, including declining species of rockfish (genus Sebastes). First, we observed captive juvenile China rockfish (S. nebulosus) to determine how they interact with their environment and conspecifics. Juveniles exhibited site fidelity and territoriality. These aggressive interactions occurred within the context of size-biased dominance, centered on competition for structurally complex habitat. Given the apparent importance of structure and the absence of structure in typical hatchery environments, we then asked how the absence of structure affects future behavior. When barren-reared and structure-reared juveniles were combined into a structurally complex aquarium, barren-reared fish displayed less structure use and less site fidelity than structure-reared fish. However, after 1 to 2 weeks, barren-reared fish began to use structure and showed site fidelity that eventually equaled that of structure-reared juveniles, showing that those behavioral effects of the rearing environment were not permanent. Though these short-term effects may still impact survival after hatchery release, we were unable to detect significant effects on vulnerability to a predator (lingcod, Ophiodon elongatus) in laboratory trials.     

The effect of prior experience on a prey’s current perceived risk

The prior experience of prey may influence how they assess the level of predation risk associated with an information source. Here, I present the results from a set of experiments that demonstrate how the prior experience of green frog (Rana clamitans) tadpoles can influence their risk assessment during exposure to the chemical cue of predatory larval dragonflies (Anax spp.) consuming conspecific tadpoles. At the short-term scale, green frog tadpoles perceived a higher level of risk when consecutive cue exposures overlapped, but only when the total chemical cue concentration was weak. Weaker chemical cue concentrations may be less reliable than stronger cue concentrations, and overlapping cue exposures may increase the degree of certainty that tadpoles have in their perceived risk. When consecutive cue exposures did not overlap, tadpoles assessed the risk associated with each cue exposure independently. Predator-conditioned tadpoles responded longer during exposure to the Anax chemical cue than nonconditioned tadpoles, which suggests that a tadpole’s long-term experience eventually does influence its risk assessment. In general, the results suggest that a prey’s prior experience may influence its current perceived risk by influencing either the degree of certainty in or the level of its perceived risk. Understanding how the prior experience of prey influences their current risk assessment requires that the rate of decay of the value of prior experience should be identified at two timescales as an indicator of the current level of predation risk.     

Interactive effects of episodic hypoxia and cannibalism on juvenile blue crab mortality

We hypothesized that as the spatial extent of hypoxic bottom water increased, (1) adult blue crab predator densities would increase in shallow habitats as they avoided hypoxia, and that (2) juvenile blue crabs, which use shallow unvegetated habitat as a predation refuge from adult conspecifics, would experience increased mortality rates during crowding by cannibalistic adult blue crabs. These hypotheses were tested along a depth gradient of sandy-mud shoreline in the Neuse River Estuary (NRE), North Carolina, USA using a combination of (1) hydrographic measurements to characterize the spatial extent of hypoxia, (2) beach seines to quantify the density of adult blue crab predators in relatively shallow water as a function of 1, and (3) tethering experiments to quantify relative rates of predation on juvenile blue crabs as a function of 1 and 2. During our seven tethering experiments, the NRE study site experienced a range of DO scenarios including normoxia, chronic hypoxia, and hypoxic upwelling. No known predators of juvenile blue crabs, other than adult conspecifics, were collected in any of our shallow-water seines. During the transition from normoxia to chronic hypoxia, blue crab predator densities in shallow refuge habitats increased 4-fold, and relative mortality rates of juvenile blue crabs in shallow habitats increased exponentially with the density of adult conspecifics. Conversely, during hypoxic upwelling events, the density of adult blue crabs in shallow water declined, which may explain why the relative mortality of juvenile crabs did not increase significantly with the increasing spatial extent of hypoxia. Thus, juvenile blue crabs may be relatively safe from adult conspecifics during hypoxic upwelling events, but not during chronic hypoxia. These experimental results highlight the need to consider the effects of dynamic water quality on mobile consumers emigrating from degraded habitats when considering indirect trophic impacts beyond the immediate area of impact.     

Experimental studies on direct and indirect interactions in a three trophic-level stream system

Summary We used a complete block design to experimentally study direct and indirect interactions in a three trophic-level freshwater system consisting of a top predator, the green sunfish, Lepomis cyanellus, an intermediate predator, small-mouthed salamander larvae, Ambystoma barbouri, and prey, hatchling isopods, Lirceus fontinalis. This system occurs naturally in small stream pools in central Kentucky; experiments were done in laboratory pools. Salamander larvae ate isopods and thus had a direct, negative effect on isopod survival. Accordingly, isopods responded to the presence of salamander larvae by increasing their tendency to bury themselves in the sand substrate. Fish ate salamanders and thus had a direct, negative effect on salamander survival. Salamanders responded to fish presence by increasing their time spent under plexiglass plates that simulate refuge rocks. The overall effect of fish on isopods depended on the presence of salamanders. In the absence of salamanders, fish predation on isopods had a direct, negative effect on isopod survival; isopods thus responded to the presence of fish by burying themselves in the sand. With salamanders present, fish had a positive overall effect on isopod survival; i.e., direct, negative effects of fish on isopods were outweighed by indirect, positive effects. Indirect positive effects of fish on isopods came through a reduction in salamander predation rates on isopods in the presence of fish. The mechanism involved both a decrease in the number of salamanders (a trophic-linkage indirect effect; cf. Miller and Kerfoot 1987) and a reduction in the feeding rate of individual salamanders on isopods (a behavioral indirect effect). The decrease in individual salamander feeding rates on isopods was due to reductions in both salamander activity and in spatial overlap between salamanders and isopods in the presence of fish. The latter effect reflected the fact that salamanders and isopods used different refuges from fish; salamanders went under refuge plates, whereas isopods primarily buried themselves in sand. Estimates of the relative importance of various direct and indirect effects of sunfish on isopods suggested that positive, behavioral indirect effects were of roughly the same magnitude as direct, negative effects, both of which were more important than were trophic-linkage indirect effects. Contrary to expectations, the presence of isopods did not affect the refuge use or survival of salamanders in the presence of fish.     

捕食风险的种群动态效应及其作用机理研究进展

捕食者不但可以通过直接捕杀猎物而控制猎物的种群数量,还可以通过捕食风险效应影响猎物种群的繁殖和动态,并且在某些情况下,捕食风险效应对猎物种群动态的控制作用甚至大于捕食者的直接捕杀.关于捕食风险效应对猎物动物繁殖产出和种群动态变化的作用及其机理方面的野外研究越来越受到国内外学者重视.本文介绍了近年来捕食风险效应的研究进展,重点关注了美国黄石国家公园中捕食者对马鹿(Cervus elephus)、加拿大育空地区的捕食者对白靴兔(Lepus americanus)的捕食风险效应等案例研究,以阐明捕食风险效应对猎物种群动态影响的重要性,以及关于捕食风险效应影响猎物种群繁殖和动态机理的两个假说(捕食者敏感食物假说、捕食应激假说).并结合我国在捕食者与猎物之间关系的研究现状,提出了进一步在野外开展捕食风险效应对濒危有蹄类猎物种群动态影响研究的建议,阐释了开展这些研究的重要意义.     

Predator–prey interactions and the cannibalism of larvae of Armigeres subalbatus (Diptera: Culicidae)

Predation and cannibalism can affect the co-existence of mosquito species or the assemblage of mosquito species (i.e., community structure). In this study, predatory feeding patterns and cannibalism of larvae of Armigeres subalbatus mosquitoes were quantified under laboratory conditions using Aedes albopictus and Culex uniformis larvae as prey organisms. Rate of consumption of prey larvae and the rate of cannibalism of 1st to 4th instar larvae of Armigeres subalbatus were reported at 24 h intervals with and without alternative food supplement. Both the 3rd and 4th instar larvae of Ar. subalbatus showed a substantial predation on the larvae of Ae. albopictus (33.6 ± 4.4) and Cx uniformis (47.3 ± 7.6). The cannibalism of the predatory larvae was strongly correlated to the larval density (r = 0.9). The predator and the prey density and the given food type were significant factors that determined the rate of pupation, death and the emergence as adults. A significant relationship was shown for the co-occurrence of predatory larvae and the prey larvae at the 85 natural breeding habitats observed at the field (χ² = 74.4, p < 0.001). Nine breeding sites (10.6%) were positive for both prey and predatory larvae. Our study showed robust information about the predatory and cannibalistic behavior of Ar. subalbatus larvae emphasizing their role in managing the population structure of mosquito communities.     

The potential for hyperparasitism to compromise biological control: Why don’t hyperparasitoids drive their primary parasitoid hosts extinct?

Predation or parasitism on species introduced as biological control agents is a common explanation for failure of biological control programs. Although there is clear evidence from some biological control programs that hyperparasitism can impact a parasitoid biological control agent, it is not clear whether hyperparasitoids have the potential to cause control failure. We performed glasshouse experiments using cages containing 48 plants to address whether the hyperparasitoid Asaphes suspensus can potentially eliminate a population of the primary parasitoid Aphidius ervi, a biological control agent of the pea aphid Acyrthosiphon pisum. Although As. suspensus has a low intrinsic rate of increase, only one-half that of A. ervi and one-third that of pea aphids, it was nonetheless capable of eliminating the A. ervi population within seven A. ervi generations. In contrast, in the absence of As. suspensus, A. ervi eliminated the pea aphid population. Field surveys, however, found that As. suspensus does not eliminate entire natural populations of A. ervi in lucerne crops, probably due to the high frequency of disturbance that favours high intrinsic rates of increase and short generation times. Nonetheless, the ability of As. suspensus to eliminate A. ervi in cages despite its low intrinsic rate of increase underscores the potential for hyperparasitism to disrupt biological control. Small populations are expected to be particularly susceptible to hyperparasitism, such as when releases of a new biological control agent are made.     

Plant–plant interactions,environmental gradients and plant diversity: A global synthesis of community-level studies

Previous syntheses on the effects of environmental conditions on the outcome of plant–plant interactions summarize results from pairwise studies. However, the upscaling to the community-level of such studies is problematic because of the existence of multiple species assemblages and species-specific responses to both the environmental conditions and the presence of neighbors. We conducted the first global synthesis of community-level studies from harsh environments, which included data from 71 alpine and 137 dryland communities to: (i) test how important are facilitative interactions as a driver of community structure, (ii) evaluate whether we can predict the frequency of positive plant–plant interactions across differing environmental conditions and habitats, and (iii) assess whether thresholds in the response of plant–plant interactions to environmental gradients exists between “moderate” and “extreme” environments. We also used those community-level studies performed across gradients of at least three points to evaluate how the average environmental conditions, the length of the gradient studied, and the number of points sampled across such gradient affect the form and strength of the facilitation-environmental conditions relationship. Over 25% of the species present were more spatially associated to nurse plants than expected by chance in both alpine and dryland areas, illustrating the high importance of positive plant–plant interactions for the maintenance of plant diversity in these environments. Facilitative interactions were more frequent, and more related to environmental conditions, in alpine than in dryland areas, perhaps because drylands are generally characterized by a larger variety of environmental stress factors and plant functional traits. The frequency of facilitative interactions in alpine communities peaked at 1000 mm of annual rainfall, and globally decreased with elevation. The frequency of positive interactions in dryland communities decreased globally with water scarcity or temperature annual range. Positive facilitation-drought stress relationships are more likely in shorter regional gradients, but these relationships are obscured in regions with a greater species turnover or with complex environmental gradients. By showing the different climatic drivers and behaviors of plant–plant interactions in dryland and alpine areas, our results will improve predictions regarding the effect of facilitation on the assembly of plant communities and their response to changes in environmental conditions.     

Too cold to prosper—winter mortality prevents population increase of the introduced American slipper limpet Crepidula fornicata in northern Europe

The distribution of the introduced American slipper limpet Crepidula fornicata (L.) on the Atlantic coast of Europe shows a clear latitudinal gradient. In the south, C. fornicata may reach abundances of several thousand individuals per meter squared accompanied by diverse ecological and economic effects. In contrast, abundances at northern waters as in Germany, Denmark and Norway barely reach 100 ind. m⁻², indicating one or several limiting factors for population increase. By studying a population in the northern Wadden Sea of Germany, we examined four factors that we regarded as likely to potentially limit population increase of C. fornicata: (1) high predation by the main benthic mollusc predators, (2) high infestation by parasites, (3) low reproductive output and growth in cold waters and (4) high winter mortality during freezing winters. Our results do not indicate biotic restrictions: The main benthic mollusc predators, shore crabs (Carcinus maenas) and sea stars (Asterias rubens) strongly preferred the dominant blue mussel (Mytilus edulis) in choice experiments and no infestation with parasitic trematodes were observed. Further, there was no indication of limitation in reproduction and growth since abundant larvae were observed in the water column and the period of reproduction (April to September) as well as growth rates (recruits: 9-14 mm first summer) match with data from southern areas. In contrast, population dynamics of C. fornicata was strongly affected by cold winters: During the two winters investigated, mortality amounted to 56-64% with up to 97% on single mussel beds contrasting to 11-14% yearly mortality in areas without frost in southern Europe. Negative winter effects are also suggested by remarkably low larval abundances after an exceptionally severe winter. Our results suggest that winter mortality is the main limiting factor for population increase of C. fornicata in the study area. We propose that milder winters as a corollary of global warming may allow for an increase in the abundance of northern populations combined with a northward shift of the concomitant negative ecological and economic effects.     

Patterned vegetation and rainfall intermittency

We study a mathematical model for the dynamics of patterned dryland vegetation in the presence of rainfall intermittency, adopting a spatially explicit approach. We find that most results found for constant precipitation carry over to the case of intermittent rainfall, with a few important novelties. For intermittent precipitation, the functional forms of the water uptake and consequently of the vegetation growth rate play an important role. Nonlinear, concave-up forms of water uptake as a function of soil moisture lead to a beneficial effect of rainfall intermittency, with a stronger effect when vegetation feedbacks are absent. The results obtained with the explicit-space model employed here are in keeping with those provided by simpler, implicit-space approaches, and provide a more complete view of vegetation dynamics in arid ecosystems.     

Where have all the weta gone? Results after two decades of transferring a threatened New Zealand giant weta, Deinacrida mahoenui

There have been numerous transfers of the large-bodied orthopteran, the Mahoenui giant weta (Family Anostostomatidae: Deinacrida mahoenui), over the past 19 years but there has been limited follow-up monitoring to ascertain establishment and breeding of transferred populations. Recent surveys carried out at all the locations where this weta were transferred, found weta at four of the seven transferred sites. The most important factor determining the success of past transfers is the absence of introduced mammalian predators, particularly rats, at a site. At two sites, Mahurangi Island Scenic Reserve and Warrenheip, weta appeared to be flourishing and have successfully established new populations in the absence of rats. If mammals are present at a site, the occurrence of dense prickly gorse to protect giant weta from predation is another important factor in their survival. Weta were found at very low densities (only single specimens were captured) at Mangaokewa Scenic Reserve and Tikikaru (private land) and it is likely their populations are not viable in the long term. Further efforts to establish Mahoenui giant weta populations should be in mammal-free sanctuaries containing native forest. More intensive post-transfer monitoring using radio-transmitters would enable better understanding of their behaviour after transfer. In addition, long-term regular monitoring of transferred populations is required, particularly those where the likelihood of rat re-invasion is high.     

Habitat deterioration and efficiency of antipredator strategy in a meadow-breeding wader,Temminck's stint (Calidris temminckii)

Many populations of waders breeding on open shores and shores with short vegetation especially on the Baltic coasts have recently become endangered. The declines have taken place simultaneously with human-induced loss and deterioration of habitats due to eutrophication and overgrowth. To investigate mechanisms by which habitat changes could affect breeding success and ultimately population dynamics, we studied an endangered coastal population of Temminck's stint. We hypothesized that the rate of nest predation has become higher because the nest defence strategy (early detection of predator and early departure from the nest), which originally evolved in open habitats, is less effective on shores with reduced visibility. As predicted, nests survived better on wide than on narrow shores. Predation made a major contribution to this trend, although successful and predated nests did not differ in concealment at a microhabitat scale. The better the visibility from the nest, the longer was the flushing distance, but only in response to alarm calls or behaviour of other species, not when they were absent. Temminck's stints seem to obtain information about an approaching predator visually from sentinels. Therefore, it is essential that there is at least moderate visibility around the nest. We conclude that habitat characteristics – visibility from the nest and sentinel birds – affect the effectiveness of the nest defence strategy of Temminck's stint. These should be taken into account when seeking causes and mechanisms for declines of Temminck's stint and other waders of open and shortly vegetated shores. Received: 5 January 1998 / Accepted: 20 April 1998     

Living in the city: resource availability, predation, and bird population dynamics in urban areas

This article explores factors that shape population structure in novel environments that have received scant theoretical attention: cities. Urban bird populations exhibit higher densities and lower diversity. Some work suggests this may result from lower predation pressure and more predictable and abundant resources. These factors may lead to populations with few winners and many losers regarding access to food, body condition, and reproductive success. We explore these hypotheses with an individual-energy-based competition model with two phenotypes of differing foraging ability. We show that low frequency resource fluctuations favor strong competitors and vice versa. We show that low predation skews equilibrium populations in favor of weak competitors and vice versa. Increasing the time between resource pulses can thus shift population structure from weak to strong competitor dominance. Given recent evidence for more constant resource input and lower predation in urban areas, the model helps understand observed urban bird population structure.     

相互作用的集合种群研究动态

在集合种群水平上,两个或更多物种可以生活在同一个斑块网络中而没有相互作用.但在很多情况下,种间的相互作用会影响种群的迁移率、灭绝率和侵占率,从而调节相应物种的集合种群动态.这方面的研究主要有集合种群水平上物种之间的竞争、捕食以及在没有任何环境异质性的条件下物种在空间上聚集分布的产生和维持等.综述了近年来关于集合种群水平上的竞争,捕食者和猎物系统以及捕食与复杂空间动态的最新研究成果.     

三种壁虎染色体核仁组织者的银染色观察与蹼壁虎的核型

本文以骨髓细胞为材料, 采用常规制片法和一步银染色技术,首次报道了无蹼壁虎、铅山壁虎和多疣壁虑的NORs,并用α-巯基乙醇短期培养无蹼壁虎的骨髓细胞,分析其核型。结果表明,3种壁虎都仅显示一对NORs,也未见有融合现象。其中无蹼壁虎的NORs位于第18对染色体的次缢痕区,而铅山壁虎和多疣壁虎分别位于第19对和第14对上。另外,我们从C带分析中发现,铅山壁虎和多疣壁虎的NORs位点均为结构异染色质区。经α-巯基乙醇培养的无蹼壁虎的染色体明显“拉长”。具次缢痕的染色体可从以往报道的第 19 对调整为第18对,其次缢痕区也比直接制作的标本大而清晰。     

Long-term tree population dynamics and their implications for the conservation of the Kakamega Forest, Kenya

Long-term studies of tree population dynamics play an important role in identifying the conservation needs of tropical forest ecosystems. We examined changes in tree population structure and composition over an 18-year period (1981–1999) in three plots located at the center of the Isecheno study site in the Kakamega Forest, Kenya, a forest with a history of logging and other anthropogenic disturbance. DBH size class distribution took the shape of an inverse J curve in both 1981 and 1999 and did not differ significantly between the two study periods. Stem density increased significantly during the study with most of the increase occurring in the smallest stem size class (10–14 cm DBH). Nearly all of the most common species in 1981 remained among the most common in 1999, though the density of pioneer species decreased by 21% during the study. Our results suggest that forest in the study plots remained relatively undisturbed and in good condition over the study period. Forest in the plots also appeared still to be recovering from the selective logging of large trees that took place at Isecheno in the 1940s. In addition to our longitudinal study, we compared tree population parameters at three additional Isecheno sites spread over a distance of 1 km that have experienced different histories of disturbance: (i) a lightly human disturbed site (LHD), (ii) a heavily human disturbed site (HHD), and (iii) a cattle disturbed site (CD). While all three sites were selectively logged in the 1940s, the main signs of disturbance today are footpaths at the LHD site, tree stumps at the HHD site, and wide cattle paths at the CD site. Not surprisingly, of the disturbed sites, the LHD site was in the best condition. Trees at the HHD site exhibited extremely poor recruitment into the small size classes, a condition that can probably be attributed to human exploitation of small trees for poles. The CD site appeared to be at an earlier successional stage than the other disturbed sites with its low mean DBH, high overall stem density, and high pioneer species stem density. Browsing and trampling of vegetation by cattle may be the source of the light gaps that have led to the abundance of pioneer species at this site. We conclude that conservation measures applied to central Isecheno, including the establishment of a forest station nearby and ranger patrols, appear to have succeeded, but that the prognosis for the Kakamega Forest in general is bleak if protection efforts are not increased in other parts of the forest, where anthropogenic disturbance remains high. We also note the considerable variation in tree population structure and composition that can occur within a small area depending on the local history of disturbance.     

Predator avoidance by the freshwater snailPhysella virgata in response to the crayfishProcambarus simulans

Summary We examined predator avoidance behavior of the freshwater snailPhysella (=Physa) virgata in response to the crayfishProcambarus simulans. In both laboratory and field enclosure experiments snails crawled above the waterline for 2 h or longer, then returned to the water.Physella virgata react to chemical signals given off by crayfish actively foraging on conspecific snails; they do not react to inactive crayfish. Low dissolved oxygen, crowding, and disturbance of the sediments did not elicit crawlout behavior. Crawling above the waterline, by reducing the probability of encounter between vulnerable, thin-shelled snails and crayfish, is an adaptive response to predation.     

Does fragmentation of Urtica habitats affect phytophagous and predatory insects differentially?

Effects of habitat fragmentation on the insect community of stinging nettle (Urtica dioica L.) were studied, using 32 natural nettle patches of different area and degree of isolation in an agricultural landscape. Habitat fragmentation reduced the species richness of Heteroptera, Auchenorrhyncha, and Coleoptera, and the abundance of populations. Habitat isolation and area reduction did not affect all insect species equally. Monophagous herbivores had a higher probability of absence from small patches than all (monophagous and polyphagous) herbivore species, and the percentage of monophagous herbivores increased with habitat area. Abundance and population variability of species were negatively correlated and could both be used as a predictor of the percentage of occupied habitats. Species richness of herbivores correlated (positively) with habitat area, while species richness of predators correlated (negatively) with habitat isolation. In logistic regressions, the probability of absence of monophagous herbivores from habitat patches could only be explained by habitat area (in 4 out of 10 species) and predator absence probability only by habitat isolation (in 3 out of 14 species). Presumably because of the instability of higher-trophic-level populations and dispersal limitation, predators were more affected by habitat isolation than herbivores, while they did not differ from herbivore populations with respect to abundance or variability. Thus increasing habitat connectivity in the agricultural landscape should primarily promote predator populations. Received: 4 February 1998 / Accepted: 4 May 1998