外来鱼类革胡子鲶与本地鲶的功能反应及生物学特性的比较

珠江水系是中国遭受外来水生动物入侵最严重的水域之一,分布有许多种外来鱼类,然而这些外来鱼类对本地鱼类资源的影响尚缺乏有效的评估。比较功能反应被认为是量化外来物种生态影响的有效方法之一。本研究采用控制实验对外来鱼类革胡子鲶(Clarias gariepinus)和本地鲶(Silurus asotus)的功能反应进行了比较。实验以鲮(Cirrhinus molitorella)、广东鲂(Megalobrama hoffmanni)和草鱼(Ctenopharyngodon idellus)的幼鱼为饵料,按照1、2、4、8、16和32条的饵料密度进行投喂,设置3个重复组,研究了革胡子鲶和鲶的功能反应类型,建立了功能反应的曲线,并拟合了功能反应的参数;同时分析了革胡子鲶和鲶的食物转化效率并比较了二者的生物学特性。结果表明:革胡子鲶和鲶的功能反应类型均为Type II型。与鲶相比,革胡子鲶具有更高的单位资源消耗率即更强的袭击率(P0.05),而两种鱼对猎物的处理时间则无显著区别(P=0.272);革胡子鲶的食物转化效率显著低于鲶(P0.05),为鲶的0.57倍,两种鱼的相对生长率则无显著差异(P=0.762)。本研究为系统评估革胡子鲶的生态影响提供基础,同时为量化其他外来鱼类的资源消耗率及潜在的生态影响提供借鉴。     

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.     

防御性被捕食者可利用量的波动可调节六斑月瓢虫的功能反应曲线（英文）

【目的】本研究评价了在被捕食者波动条件下,在花生蚜Aphis craccivora Koch和萝卜蚜Lipaphis erysimi(Kaltenbach)上六斑月瓢虫Menochilus sexmaculatus(Fabricius)4龄幼虫的功能反应。萝卜蚜是一种含烯丙基异硫氰酸酯的防御性被捕食者。我们推测,在试验的被捕食者条件下,在最适量被捕食者或丰富的被捕食者条件下饲养的幼虫比在缺乏被捕食者条件下饲养的幼虫表现出更好的功能反应。而在萝卜蚜上六斑月瓢虫的功能反应曲线的斜率比在花生蚜上要低。【方法】将六斑月瓢虫4龄幼虫从饲养条件(缺乏被捕食者/最适乏被捕食者/丰富的被捕食者)突然转移到不同的试验条件(极为缺乏被捕食者/缺乏被捕食者/次适量被捕食者/最适量被捕食者/丰富的被捕食者)下,外推出其功能曲线。【结果】尽管被捕食资源发生了改变,但是六斑月瓢虫4龄幼虫在花生蚜上表现出Ⅱ型反应,而在萝卜蚜上表现出改变的Ⅱ型反应。此外,在5个试验条件下,不管是哪种被捕食者,在最适量被捕食者/丰富的被捕食者条件下饲养的4龄幼虫均表现出正常的捕食率。但是在缺乏被捕食者条件下饲养的4龄幼虫突然转变到最适量/丰富的被捕食者实验条件下时,它们通过取食比平时更多的被捕食者的生物量来补偿食物的短缺。不过,它们在丰富的被捕食者条件下的取食量最高,在极为缺乏被捕食者的试验条件下的取食量最低。而且,在花生蚜上记录到较高的攻击率和较低的猎物处理时间。【结论】结果说明,萝卜蚜可利用量的波动可改变六斑月瓢虫幼虫的功能反应。然而,对于六斑月瓢虫而言,花生蚜比萝卜蚜是更为合适的被捕食者。     

黄瓜新小绥螨对二斑叶螨的捕食效应研究

本文在25±1℃、相对湿度80％ ±5％和光照(L:D=16 h:8 h)条件下研究了黄瓜新小绥螨Neoseiulus cucumeris对二斑叶螨Tetranychus urticae不同螨态的捕食功能反应、捕食选择性及个体间干扰反应,以期为黄瓜新小绥螨的大田释放和利用提供理论依据.结果表明,黄瓜新小绥螨对二斑叶螨各螨态捕食功能反应符合HollingⅡ型圆盘方程,其对二斑叶螨卵(a/Th=47.9965)的控制作用强于若螨(a/Th=27.6906)和成螨(a/Th=9.3963).黄瓜新小绥螨对二斑叶螨卵和若螨表现为正喜好性(Ci>0)、成螨表现为负喜好性(Ci<0).在相同猎物密度下,黄瓜新小绥螨对二斑叶螨成螨捕食率随着自身密度增大而降低,捕食者自身密度干扰反应方程为E=0.2225P-0.617.     

根田鼠攻击行为模式及其进化稳定对策分析

在捕食和附加食物交互作用条件下 ,测定根田鼠种群攻击水平与种群参数之间的关系 ,应用对策论分析攻击行为的模式。通过观测 1 6 5对实验个体的攻击行为 ,发现体重和繁殖特征决定根田鼠个体的攻击水平。体重较大个体及繁殖个体的攻击频次最高。攻击水平的性别差异显著。Kruskal- Wallis单侧秩方差检验结果表明 ,雄体攻击水平显著高于雌体。攻击水平与种群密度呈负相关关系 ,但不显著。与留居个体比较 ,扩散个体的攻击性弱。高质量的附加食物可降低根田鼠的攻击水平 ,捕食者的存在不直接影响攻击水平。根田鼠的攻击行为属仪式化非伤害行为。ESS的鹰 -鸽对策模型分析结果与实验结果一致 :不同处理种群 ESS集的鹰型对策者比例有显著差异 ,环境条件愈差 ,取胜的价值愈大 ,种群中鹰型对策者比例愈高。有捕食者、无附加食物( P,- F)种群的环境条件最差 ,鹰型对策者比例最高 ,为 0 .4 0 ;次为无捕食者、无附加食物 (- P,- F)种群 ,为 0 .33;有捕食者、有附加食物 ( P, F)种群 ,为 0 .1 8;无捕食者、有附加食物 (- P, F)种群的环境条件最优越 ,鹰型对策者比例最低 ,为0 .1 7。验证了捕食和食物交互效应能改变田鼠类动物攻击行为格局及对策模式的假设     

八斑鞘蛛对多种猎物的选择捕食作用研究

研究八斑鞘蛛在多种猎物共存时的日捕食量,功能反应,捕食作用率。在有棉铃虫和棉蚜共存且密度互补时,八斑鞘蛛对棉铃虫的功能反应属Holling Ⅲ型反应;一种猎物密度变化,其他种猎物密度固定时,功能反应呈Holling Ⅱ型反应。研究了捕食作用率与猎物共存种类,相对丰盛度,捕食者本身数量的关系。 计算机(IBM-PC)模拟结果表明:捕食者个体间的相互干扰、温度、猎物内禀增长率对系统稳定性有一定影响。     

四种鲤科鱼类对捕食胁迫行为响应的种间比较

为了比较早期捕食胁迫经历和当前环境中存在的捕食者对鱼类行为的影响, 并考查这些影响是否存在种间差异, 研究分别考查了测定环境(有、无捕食者存在)对有、无捕食胁迫经历的鳊(Parabramis pekinensis)、草鱼(Ctenopharyngodon idellus)、鲫(Carassius auratus)和中华倒刺鲃(Spinibarbus sinensis)等4种鲤科鱼类探索性、活跃性和勇敢性的影响。结果发现: 早期捕食胁迫经历与当前环境条件对鱼类行为产生截然不同的影响, 且存在较大的种间差异。无捕食胁迫经历的鳊、草鱼和中华倒刺鲃均会对陌生的捕食者乌鳢(Channa argus)做出行为响应, 提示这3种鱼可能对陌生捕食者具有一定的识别能力, 但这种识别与猎物鱼通过捕食胁迫经历获得的识别仍具有一定差距; 具有捕食胁迫经历的鳊和中华倒刺鲃在空白环境中未表现出反捕食行为, 可能是节约能量的一种策略。总体而言, 草鱼对捕食胁迫经历和测定环境处理反应更为敏感, 而中华倒刺鲃的反应则相对保守。但当周围环境中存在捕食者时, 4种鲤科鱼类均会通过维持较高运动状态的方式来应对捕食者。维持这种应激状态可能对猎物鱼保持与捕食者的距离, 并随时保持警惕较为关键。     

食物、捕食和种间竞争对东方田鼠种群动态的作用

采用2×2×2析因实验设计,在野外围栏条件下,测定食物、捕食和竞争物种黑线姬鼠(Apodemus agrarius)对东方田鼠(Microtus fortis)种群动态作用的格局.食物可利用性、捕食及种间竞争的独立作用对种群最小存活数均具有极显著的效应,除捕食与种间竞争的交互作用接近显著水平外,食物与种间竞争、食物与捕食者以及三者间的交互作用均不显著;三类外部因子对种群补充量的独立作用效应均达到极显著水平,且对种群补充量的作用具有累加效应;食物可利用性、捕食及种间竞争对种群繁殖成体的比例具有极显著的作用;三类外部因子对种群幼体与成体的比例具有极显著的作用.对种群年龄结构而言,与捕食者及种间竞争比较,食物可利用性是相对较弱的影响因子,在任何捕食与种间竞争交互作用条件下,食物的作用均不显著;三类外部因子均能显著地影响东方田鼠的体重增长率,但三者的交互作用对其影响不显著;MANOVA结果表明,捕食对成体存活率的作用最强烈,其次,为食物可利用性,种间竞争的作用最弱,但三者的交互作用效应不显著.对幼体的存活时间,除捕食的作用接近显著水平外,食物可利用性及种间竞争的作用均不显著.结果提供了食物可利用性、捕食和种间竞争对东方田鼠种群动态作用的充分证据,验证了食物、捕食和种间竞争对田鼠类种群动态具有独立或累加效应的总假设.     

日本沼虾可通过同种个体释放的报警信息感知捕食压力

许多水生动物在遭受被捕食的压力时,会释放报警信息,传送警示,其同种个体通过感知报警信息,可对捕食者作出恰当的反捕食反应。日本沼虾(Macrobrachium nipponensis)是目前淡水养殖业中重要的养殖对象和最有发展前途的品种之一,能否有效识别外来捕食者,对其生存尤为重要。为探究日本沼虾对同种个体释放的报警信息的识别能力,本实验以克氏原螯虾(Procambarus clarkii)为捕食者,研究日本沼虾对同种个体视觉识别克氏原螯虾后释放的报警信息的识别情况。结果表明:实验组日本沼虾的躲避数量(3.20±0.94只,n=80)显著高于对照组(2.81±0.80只,n=80)(重复测量方差分析,F_(1,156)=7.753,P=0.006);初反应阶段实验组日本沼虾跨越中线的次数(28.90±21.19,n=40)显著低于对照组(69.08±21.58,n=40)(非参数检验,Z=-5.498,P0.01)、终反应阶段实验组跨越中线次数(19.93±12.44,n=40)也同样显著低于对照组(36.33±21.17,n=40)(配对检验,t_(39)=6.618,P0.01);日本沼虾可通过同种个体释放的报警信息感知捕食压力,并采取提高遮蔽物的使用率、降低活动水平等反捕食反应;日本沼虾对报警信息的识别将能提高其对危险的警惕性,从而提高生存率。     

捕食者对空心莲子草叶甲种群的生物胁迫

广食性捕食者广泛捕食植食性昆虫,常被用于有害生物的生物防治,也因此影响植食性昆虫对杂草的生物效果。空心莲子草叶甲(Agasicles hygrophila)(鞘翅目:叶甲科Chrysomelidae)作为入侵恶性杂草空心莲子草(Alternanthera philoxeroides)(苋科:莲子草属Alternanthera)的专性天敌,从美国的弗罗里达州引入中国,在释放地防治空心莲子草取得了较好的防治效果。虽然空心莲子草叶甲在引入地均已建立田间种群并有一定程度的自然扩散,但丰富的食物资源,并未使空心莲子草叶甲的自然种群数量变得繁荣,因此其未能有效抑制空心莲子草的扩散蔓延。在野外调查时发现空心莲子草生境中存在大量广食性捕食者。这些广食性捕食者是抑制空心莲子草叶甲种群数量扩张的生物胁迫因子吗?为此,选择捕食性昆虫龟纹瓢虫(Propylaea japonica)(鞘翅目:瓢虫科Coccinellidae)、蜘蛛类捕食者拟水狼蛛(Pirata subpiraticus)(蜘蛛目:狼蛛科Lycosidae)与斜纹猫蛛(Oxyopes sertatus)(蜘蛛目:猫蛛科Oxyopidae)为捕食者,分别以空心莲子草叶甲各虫态为猎物,构建简单的捕食者-猎物系统,在室内检测了上述3种捕食者对空心莲子草叶甲各虫态在不同密度下的日捕食量,以期了解捕食者对空心莲子草叶甲的捕食作用,客观评估空心莲子草叶甲的生物防治效能。研究结果表明:捕食者龟纹瓢虫、斜纹猫蛛与拟水狼蛛均捕食空心莲子草叶甲的卵粒及1龄、2龄幼虫,斜纹猫蛛与拟水狼蛛捕食3龄幼虫,捕食者的捕食量均随着猎物密度的升高而增加,寻找效应降低。三者均不捕食成虫。除拟水狼蛛对3龄幼虫的捕食用Holling II模型拟合不呈显著相关关系外,其余捕食反应均拟合Holling II模型并显著相关。通过拟合方程得出捕食者对空心莲子草叶甲卵粒的理论日最大捕食量为:斜纹猫蛛10.9粒,拟水狼蛛为6.2粒,龟纹瓢虫为5.6粒;对1龄幼虫的理论日最大捕食量为:斜纹猫蛛为17.1头;拟水狼蛛为35.8头,龟纹瓢虫为10.4头;对2龄幼虫的理论日最大捕食量为:斜纹猫蛛为6.6头,拟水狼蛛为11.2头,龟纹瓢虫为2.9头;对3龄幼虫的理论日最大捕食量为:斜纹猫蛛捕食12.3头,拟水狼蛛为1.1头。研究结果证实了捕食者可通过捕食作用降低空心莲子草叶甲种群密度,削弱空心莲子草叶甲对空心莲子草的控害效能,是空心莲子草叶甲种群存活的生物胁迫因子。建议在提高空心莲子草叶甲田间种群数量,达到对空心莲子有效的持续控制效果方面开展进一步研究。     

Predation by adult and larval lady beetles (Coleoptera: Coccinellidae) on initial contact with lady beetle eggs

Na?ve adults and larvae of the native lady beetles Coleomegilla maculata (DeGeer), Cycloneda munda (Say), Hippodamia convergens Guérin-Méneville, Olla v-nigrum (Mulsant), and the exotic lady beetle Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae) were tested for their initial response to eggs of these five lady beetle species and for egg consumption on first contact and after 3 h. Additionally, field-collected O. v-nigrum and H. axyridis adults were tested. C. maculata, H. axyridis, and O. v-nigrum adults responded similarly to all egg species on first contact. Higher numbers of C. munda adults did not eat C. maculata, H. convergens, and O. v-nigrum eggs on first contact compared with numbers that did eat C. munda and H. axyridis eggs. H. convergens adults always ate C. munda eggs but hardly ate H. axyridis eggs on first contact. Results showed that over the 3-h interval, egg predation by those predators feeding on first contact was always higher, except for adults and larvae of C. maculata, than for those that did not feed on first contact. Thus, acceptance of eggs on initial contact does impact egg survival. It is likely that eggs of all native species tested (i.e., C. maculata, C. munda, H. convergens, and O. v-nigrum), but not exotic H. axyridis eggs, are suitable food for C. maculata, H. convergens, and O. v-nigrum, whereas only C. munda eggs serve as suitable food for C. munda. In direct contrast, all egg species tested would likely serve as suitable food for the exotic H. axyridis.     

Intraspecific variation in the consumption of exotic prey – a mechanism that increases biotic resistance against invasive species?

1. Hyper-successful exotic species can both displace the native prey that formerly made up a native predator's diet and represent an abundant potential prey resource for native predators. Little is known about how this drastic change affects native predators, or their short- and long term potential to regulate the exotic species.
2. We compared zebra mussel consumption by pumpkinseed sunfish ( Lepomis gibbosus ), redbreast sunfish ( Lepomis auritus ) and rock bass ( Ambloplites rupestris ) from populations that were either previously exposed to zebra mussels or naive to them.
3. Fish from populations with longer exposure to zebra mussels consumed many more zebra mussels than fish from populations with shorter or no previous exposure to zebra mussels.
4. Our experiment does not allow us to identify the mechanisms that underlie the patterns we found, but we discuss several plausible scenarios and their ecological implications.
5. Predator adaptation to exotic prey may be an important but overlooked factor in invasion biology. The initial response to exotic prey by a native predator may be a poor estimate of its ability to present biotic resistance to the invasion over the long term.     

Non-lethal predator effects on the feeding rate and prey selection of the exotic zebra mussel Dreissena polymorpha

Predators may induce changes in prey feeding that indirectly influence both the impact of prey on resource abundances and their interactions with other species in their community. We evaluated whether clearance and excretion (faeces plus pseudofaeces) of phytoplankton by zebra mussels were affected by the presence of predatory cues from roach, Rutilus rutilus, and signal crayfish, Pasifastacus leniusculus . We found that non-lethal effects of predators can alter zebra mussel clearance rate and thus the impact of zebra mussels on phytoplankton. Risk cues released by both predators had similar negative effects on clearance rate of zebra mussels and cascading positive indirect effects on phytoplankton resources. Predation risk had a stronger effect on zebra mussels' clearance rate of cyanobacteria and diatoms than cryptophytes and chrysophytes. The presence of predators did not significantly affect the rate at which zebra mussels expelled and excreted phytoplankton, although there was a tendency for more chlorophyll to be expelled and excreted in the presence of predators. Our results contribute to the growing evidence that predators indirectly affect resource dynamics and food web structure through their non-lethal effects on consumers. Our results suggest that exotic species such as zebra mussels can show behavioural responses to both native (e.g. roach) and exotic (e.g. crayfish) predators.     

Innate Predator Recognition and the Problem of Introduced Trout

Innate predator recognition typically only occurs when there is an evolutionary history between predator and prey. Predator introductions thus can pose a substantial threat to native fauna that rely heavily on inherent identification of predators. In permanent aquatic habitats prey often encounter a variety of predatory and non-predatory fish species, and the ability to distinguish between the two is essential to avoid wasted time and energy spent in unnecessary antipredatory efforts. Here, we present a study evaluating the ability of lab-reared larvae of an endangered fully aquatic salamander (hellbenders: Cryptobranchus alleganiensis ) to recognize chemical cues from native and introduced fish predators. We recorded responses of hellbender larvae to chemical stimuli from native and non-native predatory fishes, a non-predatory fish and a blank control. Eastern hellbender larvae ( C. a. alleganiensis ) significantly reduced activity in response to chemical stimuli from native predators ( Micropterus salmoides , Micropterus dolomieu , Ambloplites rupestris , Sander vitreus , and Cottus carolinae ), but responses to non-native rainbow ( Oncorhynchus mykiss ) and brown ( Salmo trutta ) trout were not significantly different from responses to the non-predatory control (redhorse sucker, Moxostoma spp.). Responses of larval Ozark hellbenders ( C. a. bishopi ) to brown trout were similar to that of the native fishes and different from the blank control, but responses to rainbow trout did not differ from the blank control. The generally weak responses of larval hellbenders to chemical cues from introduced predatory trout could lead to increased predation in the wild, which may have exacerbated the decline of hellbender populations.     

Trophic ecology of the pikeperch (Sander lucioperca) in its introduced areas: a stable isotope approach in southwestern France

During the last decades, non-native predatory fish species have been largely introduced in European lakes and rivers, calling for detailed information on the trophic ecology of co-existing native and non-native predators. The present study describes the trophic ecology of the introduced pikeperch (Sander lucioperca) in two southwestern French rivers, using stable isotope analysis. Pikeperch could be categorized as a top-predator, and had a significantly higher trophic position (TP, mean±SE=4.2±0.1) compared to other predatory fish such as the native pike (Esox lucius, TP=3.7±0.1) and the introduced European catfish (Silurus glanis, TP=3.8±0.1). Most studies of resource use in freshwaters consider predatory fish as ecologically equivalent; however, this study showed that the pikeperch occupied a higher trophic niche compared to other predatory species in the Lot and Tarn rivers (Garonne River basin). This apparent specialization may thus have consequences upon interspecific relationships within the predatory guild and upon the functional organization of biological communities. To cite this article: D. Kopp et al., C. R. Biologies 332 (2009).     

Variations in piscivory of invasive largemouth bass <Emphasis Type="Italic">Micropterus salmoides</Emphasis> associated with pond environments

Introductions of non-native predatory fishes can be a major driver of aquatic biodiversity loss. The largemouth bass Micropterus salmoides (L.) has been introduced throughout much of the world, thereafter negatively affecting native faunal communities owing to its predatory impact. To investigate the environmental factors affecting the predatory performance of invasive bass, we examined the stomach contents and habitat characteristics of bass in 15 irrigation farm ponds in northeastern Japan. The food habits of the bass populations differed among the studied ponds: the predominant prey items were fishes among bass in seven of the ponds, whereas aquatic invertebrates (mainly insects and zooplankton) were the predominant taxa in the diets of bass in the eight remaining ponds, with the onset of piscivory related to body size. The results of multivariate analysis indicated that the extent to which the bass consumed fish was positively associated with fish prey abundance and negatively associated with percentage of aquatic vegetation coverage. We suggest that the extent of aquatic vegetation coverage strongly influenced the predation efficiency of bass in the ponds. These findings might be employed to assess a pond ecosystem’s vulnerability to invasive largemouth bass and to reduce the predator’s impact on native fish species by improvements to the habitat.     

Non‐lethal predator effects on the performance of a native and an exotic crayfish species

1. I tested the hypothesis that the potential for non‐lethal effects of predators are more important for overall performance of the fast‐growing exotic signal crayfish (Pacifastacus leniusculus Dana) than for the slower growing native noble crayfish (Astacus astacus L.). I further tested if omnivorous crayfish switched to feed on less risky food sources in the presence of predators, a behaviour that could reduce the feeding costs associated with predator avoidance. 2. In a 2 month long outdoor pool experiment, I measured behaviour, survival, cheliped loss, growth, and food consumption in juvenile noble or signal crayfish in pools with either a caged predatory dragonfly larvae (Aeshna sp.), a planktivorous fish that do not feed on crayfish (sunbleak, Leucaspius delineatus Heckel), or predator‐free controls. Crayfish had access to multiple food sources: live zooplankton, detritus and periphyton. Frozen chironomid larvae were also supplied ad libitum outside crayfish refuges, simulating food in a risky habitat. 3. Crayfish were mainly active during hours of darkness, with signal crayfish spending significantly more time outside refuges than noble crayfish. The proportion of crayfish outside refuges varied between crayfish species, time and predator treatment, with signal crayfish spending more time in refuges at night in the presence of fish. 4. Survival in noble crayfish was higher than in signal crayfish, and signal crayfish had a higher frequency of lost chelipeds, indicating a high level of intraspecific interactions. Crayfish survival was not affected by the presence of predators. 5. Gut‐contents analysis and stable isotope values of carbon (δ¹³C) and nitrogen (δ¹⁵N) indicated that the two crayfish species had similar food preferences, and that crayfish received most of their energy from feeding on invertebrates (e.g. chironomid larvae), although detritus was the most frequent food item in their guts. Signal crayfish guts were more full than those of noble crayfish, but signal crayfish in pools with fish contained significantly less food and fewer had consumed chironomids compared with predator‐free controls. Length increase of signal crayfish (35%) was significantly higher than of noble crayfish (20%), but signal crayfish in pools with fish grew less than in control pools. 6. This short‐term study indicates that fish species that do not pose a lethal threat to an organism may indirectly cause reductions in growth by affecting behaviour and feeding. This may occur even though prey are omnivorous and have access to and consume multiple food sources. These non‐lethal effects of predators are expected to be particularly important in exotic crayfish species that show a general response to fish, have high individual growth rates, and when their feeding on the most profitable food source is reduced.     

Infochemical-mediated intraguild interactions among three predatory mites on cassava plants

Carnivorous arthropods exhibit complex intraspecific and interspecific behaviour among themselves when they share the same niche or habitat and food resources. They should simultaneously search for adequate food for themselves and their offspring and in the meantime avoid becoming food for other organisms. This behaviour is of great ecological interest in conditions of low prey availability. We examined by means of an olfactometer, how volatile chemicals from prey patches with conspecific or heterospecific predators might contribute to shaping the structure of predator guilds. To test this, we used the exotic predatory mites Typhlodromalus manihoti and T. aripo, and the native predatory mite Euseius fustis, with Mononychellus tanajoa as the common prey species for the three predatory mite species. We used as odour sources M. tanajoa-infested cassava leaves or apices with or without predators. T. manihoti avoided patches inhabited by the heterospecifics T. aripo and E. fustis or by conspecifics when tested against a patch without predators. Similarly, both T. aripo and E. fustis females avoided patches with con- or heterospecifics when tested against a patch without predators. When one patch contained T. aripo and the other T. manihoti, females of the latter preferred the patch with T. aripo. Thus, T. manihoti is able to discriminate between odours from patches with con- and heterospecifics. Our results show that the three predatory mite species are able to assess prey patch profitability using volatiles. Under natural conditions, particularly when their food sources are scarce, the three predatory mite species might be involved in interspecific and/or intraspecific interactions that can substantially affect population dynamics of the predators and their prey.     

Risk of predation and feeding rate in tropical freshwater fishes: field evidence

Summary In a previous study (A. Prejs and K. Prejs in press) we found that at the end of the dry season small fish species trapped together with piscivorous fish in small, shallow pool varied in their level of food intake. The two smallest of these species fed little, whereas larger species fed at high rate. By examining the species and size composition of the diet and gape limitations of predatory species, I found that the decreased feeding rate of small fish was in response to a high risk of predation by gape-limited predators. The reduction of feeding rate was very substantial when compared to that of the same species in predator free area.