Dynamics of a Boreal Lake Ecosystem during a Long-Term Manipulation of Top Predators

We assessed the long-term (16 years) effects of introducing piscivores (northern pike) into a small, boreal lake (Lake 221, Experimental Lakes Area) containing abundant populations of two planktivorous fish species. After the introduction, pearl dace were extirpated and yellow perch abundance was greatly reduced. Daphnia species shifted from D. galeata mendota to larger bodied Daphnia catawba, but the total zooplankton biomass did not increase, nor did the biomass of large grazers such as Daphnia. Phytoplankton biomass decreased after the northern pike introduction, but increased when northern pike were partially removed from the lake. Phosphorus (P) excretion by fish was ∼0.18 mg P m⁻² d⁻¹ before pike addition, declined rapidly to approximately 0.03–0.10 as planktivorous perch and dace populations were reduced by pike, and increased back to premanipulation levels after the pike were partially removed and the perch population recovered. When perch were abundant, P excretion by fish supported about 30% of the P demand by primary producers, decreasing to 6–14% when pike were abundant. Changes in phytoplankton abundance in Lake 221 appear to be driven by changes in P cycling by yellow perch, whose abundance was controlled by the addition and removal of pike. These results confirm the role of nutrient cycling in mediating trophic cascades and are consistent with previous enclosure experiments conducted in the same lake.     

Positive indirect effects of top-predators on the behaviour and survival of juvenile fishes

Top-predators can suppress mesopredator behaviour through risk effects. However, there is limited understanding of whether such behavioural suppression can dampen the lethal and sub-lethal effects of mesopredators on bottom level prey. Here, we document a field experiment that examines whether the presence of top-predator cues (visual and chemical stimuli from a coral trout) can cascade to indirectly influence the behaviour and survival of juvenile fish prey of different species (Pomacentrus amboinensis and P. chrysurus) and size (small = 1.18 cm SL versus large = 1.32 cm SL). Results showed that habitat patches exposed to top-predator cues received fewer visits and foraging attacks from mesopredators, leading to higher space use (~ 46%), feeding rate (~ 95%) and survival (~ 67%) from juvenile fish prey. Survival was always higher for individuals of P. amboinensis and of large-size, independent of the presence or absence of top-predator cues. Our data indicate that predation risk from the top-predator indirectly favoured the persistence and behaviour of juvenile fishes by promoting risk-averse behavioural responses in mesopredators. This study underscores the behavioural mechanisms by which risk effects can cascade through the food web and highlights the consequences that harvesting top-predators may have on the replenishment of bottom prey populations.     

Aquatic Macrophyte Approach to Assess the Impact of Disturbances on the Diversity of the Ecosystem and on River Quality

Disturbance has long been recognized as an important determinant of community characteristics in aquatic systems. The aims of our study were to evaluate the impact of different disturbances on the macrophyte diversity and on river quality. To this end, we investigated the floristic composition for different stretches impacted by disturbances and we tested both diversity indices and the trophic index (IBMR) “Biological Index Macrophytes in Rivers” in the Moselle river (NE of France). The river was divided into four sections of different lengths based on uniformity of morphological characteristics, substrate conditions and flowing velocity: the upper, the wild, the resectioned and the downstream Moselle. Floristic composition and water chemical parameters were analysed from 1999 to 2001. The man‐made increase of nutrient concentration favoured the floristic richness in the last sites of the upper Moselle, whereas river dynamics and floods did not allow the development of vegetation in the wild Moselle. Disturbances caused by industrial sewage and eutrophication allowed the spreading of pollu‐tolerant and riparian alien species. The aquatic macrophyte approach is a useful means to detect impact of disturbances on diversity and on river quality. However, it was not effective in assessing disturbances such as flood overflow or chemical pollution. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Predator‐driven elemental cycling: the impact of predation and risk effects on ecosystem stoichiometry

Empirical evidence is beginning to show that predators can be important drivers of elemental cycling within ecosystems by propagating indirect effects that determine the distribution of elements among trophic levels as well as determine the chemical content of organic matter that becomes decomposed by microbes. These indirect effects can be propagated by predator consumptive effects on prey, nonconsumptive (risk) effects, or a combination of both. Currently, there is insufficient theory to predict how such predator effects should propagate throughout ecosystems. We present here a theoretical framework for exploring predator effects on ecosystem elemental cycling to encourage further empirical quantification. We use a classic ecosystem trophic compartment model as a basis for our analyses but infuse principles from ecological stoichiometry into the analyses of elemental cycling. Using a combined analytical‐numerical approach, we compare how predators affect cycling through consumptive effects in which they control the flux of nutrients up trophic chains; through risk effects in which they change the homeostatic elemental balance of herbivore prey which accordingly changes the element ratio herbivores select from plants; and through a combination of both effects. Our analysis reveals that predators can have quantitatively important effects on elemental cycling, relative to a model formalism that excludes predator effects. Furthermore, the feedbacks due to predator nonconsumptive effects often have the quantitatively strongest impact on whole ecosystem elemental stocks, production and efficiency rates, and recycling fluxes by changing the stoichiometric balance of all trophic levels. Our modeling framework predictably shows how bottom‐up control by microbes and top‐down control by predators on ecosystems become interdependent when top predator effects permeate ecosystems.     

Predator diversity strengthens trophic cascades in kelp forests by modifying herbivore behaviour

Although human-mediated extinctions disproportionately affect higher trophic levels, the ecosystem consequences of declining diversity are best known for plants and herbivores. We combined field surveys and experimental manipulations to examine the consequences of changing predator diversity for trophic cascades in kelp forests. In field surveys we found that predator diversity was negatively correlated with herbivore abundance and positively correlated with kelp abundance. To assess whether this relationship was causal, we manipulated predator richness in kelp mesocosms, and found that decreasing predator richness increased herbivore grazing, leading to a decrease in the biomass of the giant kelp Macrocystis. The presence of different predators caused different herbivores to alter their behaviour by reducing grazing, such that total grazing was lowest at highest predator diversity. Our results suggest that declining predator diversity can have cascading effects on community structure by reducing the abundance of key habitat-providing species.     

Invasion of a Top Predator into an Epipelagic Ecosystem can bring a Paradoxical Top-Down Trophic Control

We apply mathematical modeling to explore different scenarios of invasion of a top predator (carnivorous zooplankton or planktivorous fish) into an epipelagic plankton ecosystem. We use a ‘minimal’ model of three nonlinear ordinary differential equations (nutrient–phytoplankton–herbivores) with the top predator density as a time-dependent parameter. The ecosystem shows different types of response, which can be described in terms of top-down trophic control. Our investigation indicates that under certain conditions the plankton ecosystem model demonstrates a surprising kind of response: in a wide range of realistic ecosystem parameters the invasion of the top predator leads to a prominent increase in the average density of zooplankton and to a resulting decrease of phytoplankton density. This phenomenon is opposite to the ‘typical’ top–down control when the carnivore pressure decreases zooplankton density which, in turn, increases phytoplankton biomass. We call the revealed type of top-down control ‘paradoxical’. Examples of such a response in natural aquatic ecosystems were reported earlier but no clear explanation has been provided hitherto. In this paper, we analyze possible mechanisms of ‘paradoxical top–down control’ and show that it can occur in eutrophic epipelagic ecosystems subject to high rate of cross-pycnocline exchange.     

生态质量与生态系统服务功能

生态系统服务功能是自然生态过程对人类的贡献。文章阐述了生态系统服务功能的定义、内涵及其与生态质量和生态系统健康三者间密不可分的关系。迅猛的人口增长、社会变化和技术发展,已使承载人类社会的生命之舟－大自然受到严重破坏和巨大威胁,减少这些破坏和威胁是整个社会特别是生态学工作者面临的重大挑战。只有深入认识三者之间的关系及其重要性,才能自觉防止和减少带有自我毁灭性的经济和社会活动。     

生态质量与生态系统服务功能

生态系统服务功能是自然生态过程对人类的贡献。文章阐述了生态系统服务功能的定义、内涵及其与生态质量和生态系统键康三者间密不可分的关系。迅猛的人口增长、社会变化和技术发展,已使承载人类社会的生命之舟——大自然受到严重破坏和巨大威胁,减少这些破坏和威胁是整个社会特别是生态学工作者面临的重大挑战。只有深入认识三者之间的关系及其重要性,才能自觉防止和减少带有自我毁灭性的经济和社会活动。     

Soil-mediated indirect impacts of an invasive predator on plant growth

While several studies have shown that invasive plant effects on soil biota influence subsequent plant performance, corresponding studies on how invasive animals affect plants through influencing soil biota are lacking. This is despite the fact that invasive animals often indirectly alter the below-ground subsystem. We studied 18 offshore islands in northern New Zealand, half of which have been invaded by rats that are predators of seabirds and severely reduce their densities, and half of which remain non-invaded; invasion of rats thwarts seabird transfer of resources from ocean to land. We used soil from each island in a glasshouse experiment involving soil sterilization treatments to determine whether rat invasion indirectly influences plant growth through the abiotic pathway (by impairing seabird-driven inputs to soil) or the biotic pathway (by altering the soil community). Rat invasion greatly impaired plant growth but entirely through the abiotic pathway. Plant growth was unaffected by the soil community or its response to invasion, meaning that the responses of plants and soil biota to invasion are decoupled. Our results provide experimental evidence for the powerful indirect effects that predator-instigated cascades can exert on plant and ecosystem productivity, with implications for the restoration of island ecosystems by predator removal.     

Predation on mutualists can reduce the strength of trophic cascades

Ecologists have put forth several mechanisms to predict the strength of predator effects on producers (a trophic cascade). We suggest a novel mechanism – in systems in which mutualists of plants are present and important, predators can have indirect negative effects on producers through their consumption of mutualists. The strength of predator effects on producers will depend on their relative consumption of mutualists and antagonists, and on the relative importance of each to producer population dynamics. In a meta-analysis of experiments that examine the effects of predator reduction on the pollination and reproductive success of plants, we found that the indirect negative effects of predators on plants are quite strong. Most predator removal experiments measure the strength of predator effects on producers through the antagonist pathway; we suggest that a more complete understanding of the role of predators will be achieved by simultaneously considering the effects of predators on plant mutualists.     

Cascading effects of predator diversity and omnivory in a marine food web

Over‐harvesting, habitat loss and exotic invasions have altered predator diversity and composition in a variety of communities which is predicted to affect other trophic levels and ecosystem functioning. We tested this hypothesis by manipulating predator identity and diversity in outdoor mesocosms that contained five species of macroalgae and a macroinvertebrate herbivore assemblage dominated by amphipods and isopods. We used five common predators including four carnivores (crabs, shrimp, blennies and killifish) and one omnivore (pinfish). Three carnivorous predators each induced a strong trophic cascade by reducing herbivore abundance and increasing algal biomass and diversity. Surprisingly, increasing predator diversity reversed these effects on macroalgae and altered algal composition, largely due to the inclusion and performance of omnivorous fish in diverse predator assemblages. Changes in predator diversity can cascade to lower trophic levels; the exact effects, however, will be difficult to predict due to the many complex interactions that occur in diverse food webs.     

梯级开发对河流生态系统和景观影响研究进展

作为水资源和水能开发利用的主要方式,河流梯级开发在满足国民经济各部门对河流水资源开发的要求和推动流域社会经济持续发展的同时,也对整个流域生态系统产生了不可避免的人为影响.本文根据河流梯级开发的过程和流域生态系统的特征,分别综述了梯级开发对坝址区生态系统、流域库区小气候、河岸带生态系统、水生态系统、河流湿地和流域景观生态的主要影响,并提出了研究展望,如加强梯级开发对各生态因子影响后的连锁反应和累积效应研究;在水库群运行和联合调度时期,应加强正负生态效应综合影响作用的研究以及不同时空尺度条件下对流域生态系统演替发展和稳定等方面的研究.     

Architecture of collapse: regime shift and recovery in an hierarchically structured marine ecosystem

By the late 20th century, a series of events or ‘natural experiments’, for example the depletion of apex predators, extreme eutrophication and blooms of invasive species, had suggested that the Black Sea could be considered as a large ecosystem ‘laboratory’. The events resulted in regime shifts cascading through all trophic levels, disturbing ecosystem functioning and damaging the water environment. Causal pathways by which the external (hydroclimate, overfishing) and internal (food web interactions) drivers provoke regime shifts are investigated. Statistical data analyses supported by an interpretative framework based on hierarchical ecosystem theory revealed mechanisms of hierarchical incorporation of environmental factors into the ecosystem. Evidence links Atlantic teleconnections to Black Sea hydroclimate, which together with fishing shapes variability in fish stocks. The hydroclimatic signal is conveyed through the food web via changes in productivity at all levels, to planktivorous fish. Fluctuating fish abundance is believed to induce a lagged change in competitor jelly plankton that cascades down to phytoplankton and influences water quality. Deprived of the stabilising role of apex predators, the Black Sea's hierarchical ecosystem organisation is susceptible to both environmental and anthropogenic stresses, and increased fishing makes fish stock collapses highly probable. When declining stocks are confronted with burgeoning fishing effort associated with the inability of fishery managers and decision‐makers to adapt rapidly to changes in fish abundance, there is overfishing and stock collapse. Management procedures are ineffective at handling complex phenomena such as ecosystem regime shifts because of the shortage of suitable explanatory models. The proposed concepts and models reported here relate the hydroclimate, overfishing and invasive species to shifts in ecosystem functioning and water quality, unravelling issues such as the causality of ecosystem interactions and mechanisms and offering potential for finding ways to reverse regime shifts. We advocate a management approach aiming at restoring ecosystem hierarchy that might mitigate the costly consequences of regime shifts.     

自然生态条件下鼠类数量与天敌数量的平衡关系

近些年,越来越多的科学家将天敌对鼠害的控制作用纳入综合防治的总体方案之中,大部分学者倾向认为,天敌是鼠类种群数量调节的主要因素之一。研究天敌在鼠类种群动态中所起的作用,不仅对鼠害防治工作具有重要意义,而且有助于生态学理论的发展。本文在有关天敌对鼠类种...     

Continental patterns in the diet of a top predator: Australia's dingo

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Antagonistic and Synergistic Interactions Among Predators

The structure and dynamics of food webs are largely dependent upon interactions among consumers and their resources. However, interspecific interactions such as intraguild predation and interference competition can also play a significant role in the stability of communities. The role of antagonistic/synergistic interactions among predators has been largely ignored in food web theory. These mechanisms influence predation rates, which is one of the key factors regulating food web structure and dynamics, thus ignoring them can potentially limit understanding of food webs. Using nonlinear models, it is shown that critical aspects of multiple predator food web dynamics are antagonistic/synergistic interactions among predators. The influence of antagonistic/synergistic interactions on coexistence of predators depended largely upon the parameter set used and the degree of feeding niche differentiation. In all cases when there was no effect of antagonism or synergism (a _ij =1.00), the predators coexisted. Using the stable parameter set, coexistence occurred across the range of antagonism/synergism used. However, using the chaotic parameter strong antagonism resulted in the extinction of one or both species, while strong synergism tended to coexistence. Whereas using the limit cycle parameter set, coexistence was strongly dependent on the degree of feeding niche overlap. Additionally increasing the degree of feeding specialization of the predators on the two prey species increased the amount of parameter space in which coexistence of the two predators occurred. Bifurcation analyses supported the general pattern of increased stability when the predator interaction was synergistic and decreased stability when it was antagonistic. Thus, synergistic interactions should be more common than antagonistic interactions in ecological systems.     

Restoration of a Stratified Lake (Feldberger Haussee,Germany) by a Combination of Nutrient Load Reduction and Long‐Term Biomanipulation

A long‐term biomanipulation has been performed in the stratified Feldberger Haussee since 1985. Prior to manipulation, nutrient load to the lake had declined due to waste water removal. Planktivorous fish were reduced by seining and by enhancement of piscivorous fish. Changes in transparency, nutrients, phytoplankton, zooplankton and fish were documented for both the premanipulation period (1978–1985) and the manipulation period (1986–1998). Transparency increased in response to the manipulation (+54%), but strong year‐to‐year fluctuations were observed. These fluctuations were correlated to chlorophyll a, primary production and the proportion of piscivores in the fish community. We conclude that the success of the restoration was predominantly attributed to bottom‐up forces as a result of the declining nutrient load and an intensified co‐precipitation of phosphorus with calcite. However, the increased predation impact by the piscivorous fish may have caused a reduced nutrient recycling by the planktivorous fish thus contributing also to the improvement in water quality.