The trade-off between feeding, mate seeking and predator avoidance in copepods: behavioural responses to chemical cues

The behavioural responses of adult Temora longicornis to chemicalsignals from predators and potential sexual partners was studiedusing an automated three-dimensional (3-D) tracking system anda 2-D high-resolution filming set-up. The animals adapted theirswimming speed and turning behaviour in reaction to chemicalsignals to increase or decrease encounter probability with otherorganisms. Female copepods were shown to react to the smellof males with little ‘hops’, quite distinct fromboth their normal smooth swimming motion and escape responses.These hops are likely to be signals intended to increase theencounter probability with males. Swimming behaviour was shownto be a trade-off between conflicting motivations such as predatoravoidance on the one hand, and the search for food and sexualpartners on the other.     

Evidence of the trade-off between starvation and predation risks in ducks

The theory of trade-off between starvation and predation risks predicts a decrease in body mass in order to improve flight performance when facing high predation risk. To date, this trade-off has mainly been validated in passerines, birds that store limited body reserves for short-term use. In the largest avian species in which the trade-off has been investigated (the mallard, Anas platyrhynchos), the slope of the relationship between mass and flight performance was steeper in proportion to lean body mass than in passerines. In order to verify whether the same case can be applied to other birds with large body reserves, we analyzed the response to this trade-off in two other duck species, the common teal (Anas crecca) and the tufted duck (Aythya fuligula). Predation risk was simulated by disturbing birds. Ducks within disturbed groups were compared to non-disturbed control birds. In disturbed groups, both species showed a much greater decrease in food intake and body mass during the period of simulated high risk than those observed in the control group. This loss of body mass allows reaching a more favourable wing loading and increases power for flight, hence enhancing flight performances and reducing predation risk. Moreover, body mass loss and power margin gain in both species were higher than in passerines, as observed in mallards. Our results suggest that the starvation-predation risk trade-off is one of the major life history traits underlying body mass adjustments, and these findings can be generalized to all birds facing predation. Additionally, the response magnitude seems to be influenced by the strategy of body reserve management.     

Seasonal migration determined by a trade-off between predator avoidance and growth

Migration is a common phenomenon in many organisms, terrestrial as well as aquatic, and considerable effort has been spent to understand the evolution of migratory behaviour and its consequences for population and community dynamics. In aquatic systems, studies on migration have mainly been focused on commercially important fish species, such as salmon and trout. However, seasonal mass-migrations may occur also among other freshwater fish, e.g. in cyprinids that leave lakes and migrate into streams and wetlands in the fall and return back to the lake in spring. In a conceptual model, we hypothesized that this is an adaptive behaviour in response to seasonal changes in predation (P) and growth (G) and that migrating fish change habitat so as to minimise the ratio between predation mortality and growth rate (P/G). Estimates from bioenergetic modelling showed that seasonal changes in the ratio between predator consumption rate and prey growth rate followed the predictions from the conceptual model and also gave more precise predictions for the timing of the habitat change. By quantifying the migration of more than 1800 individually marked fish, we showed that actual migration patterns followed predictions with a remarkable accuracy, suggesting that migration patterns have evolved in response to seasonally fluctuating trade-offs between predator avoidance and foraging gains. Thus, the conceptual model provides a mechanistic understanding to mass-migration in prey fish. Further, we also show that the dominant prey fish is actually absent from the lake during a major part of the year, which should have strong implications for the dynamics of the lake ecosystem through direct and indirect food-web interactions.     

Morphological and life-history differentiation between littoral and pelagic forms of pumpkinseed

Trophically dimorphic pumpkinseed populations were investigated in five lakes in Ontario, Canada to determine: (1) whether the morphological traits that distinguish littoral from pelagic forms are consistent among populations; (2) whether the pharyngeal jaw apparatus is diminished in pelagic pumpkinseeds because of a lower proportion of hard-bodied prey in their diets; (3) whether there is life-history differentiation between littoral and pelagic forms. Pumpkinseeds captured from the littoral and pelagic zones differed significantly in morphology in four of the five lakes, but the number of external measures that differed significantly within the differentiated populations ranged from zero to six. Littoral pumpkinseeds generally had longer heads, more rearward placement of dorsal and pectoral fins, longer pectoral fins and deeper bodies than pelagic pumpkinseeds. Littoral and pelagic pumpkinseeds were more readily differentiated by internal morphometric measures, with littoral individuals having larger molars and wider spacing between gill rakers than pelagic individuals. Littoral and pelagic differences in age at maturity, size at maturity and gonado-somatic index were present only in one of three populations assessed for these traits, suggesting that morphological divergence is not necessarily accompanied by life-history differentiation.     

Induced pigmentation in zooplankton: a trade-off between threats from predation and ultraviolet radiation.

Ultraviolet (UV) radiation is harmful to all life, and the ongoing depletion of the ozone layer is likely to affect interactions among both terrestrial and aquatic organisms. Some organisms have evolved adaptations to reduce radiation damage, such as the various types of protective pigmentation of freshwater zooplankton. However, strong pigmentation also increases vulnerability to visually hunting predators. Hence, where both UV radiation and predation are intense, zooplankton may be sandwiched between conflicting selective pressures: to be pigmented and to be transparent at the same time. Here, I show that the level of pigmentation in copepods is up to ten times higher in lakes without predatory fishes than where fishes are present. Moreover, animals from the same population exposed to either UV light or predator scent showed a 10% difference in pigmentation after only four days, suggesting that pigmentation is an inducible trait. Hence, individual copepods are not passive victims of selective predation or radiation damage, but adjust the level of pigmentation according to the prevailing threat. The ability to adjust pigmentation level rapidly may be especially useful in situations where risk assessment is difficult due to strong seasonal and spatial variation in risk variables, such as in Arctic regions. With progressive thinning of the ozone layer, the ability of some but not other organisms to adjust protection against UV radiation may lead to counter-intuitive, large-scale alterations in freshwater food webs.     

Oviposition-site shift in phytophagous mites reflects a trade-off between predator avoidance and rainstorm resistance

Predators can reduce prey population densities by driving them to undertake costly defences. Here, we report on a remarkable example of induced antipredator defence in spider mites that enhances the risk to rainstorms. Spider mites live on the undersides of host plant leaves and usually oviposit on the leaf undersurface. When they are threatened by predatory mites, they oviposit on three-dimensional webs to avoid egg predation, although the cost of ovipositing on webs has not yet been clearly determined. We prepared bean plants harbouring spider mite (Tetranychus kanzawai) eggs on either leaf surfaces or webs and exposed them to rainstorms outdoors. We found that fewer eggs remained on webs than on leaf surfaces. We then examined the synergistic effect of wind and rain by simulating both in the laboratory. We conclude that ovipositing on webs comes at a cost, as eggs are washed off the host plants by wind and rain. This may explain why spider mite populations decrease drastically in the rainy season, although they inhibit leaf undersides where they are not directly exposed to rainfall.     

Shifting the balance between foraging and predator avoidance: the importance of food distribution for a schooling pelagic forager

It is widely held that when predator avoidance conflicts with other activities, such as feeding, avoidance of predators often takes precedence. In this study, we examine how predation risk and food distribution interact to influence the schooling behavior and swimming speed of foraging juvenile walleye pollock, Theragra chalcogramma. Fish were acclimated to either spatially and temporally clumped, or spatially and temporally dispersed food for 3 weeks. Fish were then monitored while feeding in the absence and presence of predatory sablefish, Anoplopoma fimbria. Fish foraging for clumped food swam rapidly in a loose school when predators were absent, but swam more slowly and adopted more cohesive schooling in the presence of predators, trading-off foraging opportunity for decreased vulnerability to predators. Fish foraging for dispersed food swam about slowly and did not engage in cohesive schooling in either the absence or presence of predators. These fish accepted greater predation risk in order to continue foraging, suggesting that the cost of schooling, in terms of decreased foraging opportunity, was greater when food was dispersed than when it was clumped. This lower responsiveness to predators among fish receiving dispersed food demonstrates that predator avoidance does not always take precedence over other activities, but rather, that a balance is maintained between predator avoidance and feeding, which shifts as food distribution changes.     

Sex differences in the trade-off between feeding and mating in the guppy

Male guppies Poecilia reticulata court almost continuously and often indulge in sneaky mating attempts. Female guppies, on the other hand, devote much of their time to foraging. This paper demonstrates that both sexes reduce their feeding rate in mixed-sex groups relative to single sex ones and suggests that the decrease is less pronounced for females than for males. Thus males make feeding and mating decisions that are dependent on hunger while female foraging behaviour is constrained by sexual harassment. The observed sex differences in behaviour are a consequence of asymmetrical mating costs in males and females.     

Exchange between littoral and pelagic waters in a stratified lake due to wind-induced motions: Lake Kinneret, Israel

We used field data and three-dimensional numerical simulations to investigate the exchange processes between the lake boundaries (littoral) and lake interior (pelagic) due to wind-induced motions in Lake Kinneret, Israel. The field data showed large-scale metalimnion oscillations with amplitudes up to 10 m in response to westerly diurnal winds, the existence of a well-defined turbidity intrusion into the metalimnion of the lake and a benthic boundary layer (BBL). The observed vertical and horizontal movements of the turbidity were explained by the vertical and horizontal advection associated with basin-scale wave motions and a residual circulation set up by the basin-scale motions and the wind field. The horizontal advective transport in the metalimnion, associated with the velocities induced by the basin-scale mode-two Poincaré wave, provided the primary control for the exchange between the lake boundaries and lake interior on daily time scales. Detailed comparison of simulation results with both temperature and velocity profiles revealed excellent agreement for time scales from hours to days. The validated numerical model was used to extract the residual motions that provided the secondary exchange flows, in the various regions of the lake on time scales from days to weeks. The residual motion in the surface layer consisted of a combination of an anticlockwise gyre in the western half of the lake and a weak clockwise gyre along the eastern boundary that were forced by the curl of the wind field. In the metalimnion, residual motions were predominantly forced by the basin-scale internal wave motions and an anticlockwise gyre established itself throughout the whole basin. Lastly in the BBL, residual motions consisted of an anticlockwise gyre, but the geographic distribution was patchy. Together these results provide new insight into a proper characterization of the processes underlying the flux paths of water and particles between the lake boundaries and lake interior. Handling editor: L. Naselli-Flores     

Effect of light, prey density, and prey type on the feeding rates of Hemimysis anomala

Hemimysis anomala is a near-shore mysid native to the Ponto-Caspian region that was discovered to have invaded Great Lakes ecosystems in 2006. We investigated feeding rates and prey preferences of adult and juvenile Hemimysis in laboratory experiments to gain insight on the potential for Hemimysis to disrupt food webs. For both age groups (AGs), we measured feeding rates as a function of prey abundance (Bosmina longirostris as prey), prey type (B. longirostris, Daphnia pulex, and Mesocyclops sp.), and light levels (no light and dim light). Mean feeding rates on Bosmina increased with prey density and reached 23 ind. (2 h)^?1 for adults and 17 ind. (2 h)^?1 for juveniles. Dim light had little effect on prey selection or feeding rate compared to complete darkness. When feeding rates on alternate prey were compared, both AGs fed at higher rates on Bosmina than Daphnia, but only juveniles fed at significantly higher rates on Bosmina relative to Mesocyclops. No significant differences were observed between feeding rates on Mesocyclops and on Daphnia. Hemimysis feeding rates were on the order of 30–60% of their body weight per day, similar to predatory cladocerans that have been implicated in zooplankton declines in Lakes Huron and Ontario.     

Bacterioplankton in the littoral and pelagic zones of subtropical shallow lakes

We measured bacterioplankton (phylotypes detected by fluorescent in situ hybridisation, morphometric forms, abundance and production) in samples collected in summer in the littoral and pelagic zones of 10 subtropical shallow lakes of contrasting area (from 13 to 80,800 ha). Compared to the pelagic zones, the littoral zones were overall characterised by higher macrophyte dominance and lower concentrations of total phosphorus and alkalinity and higher concentrations of dissolved organic carbon (DOC) and humic substances. Similarities of bacterial production and biomass turnover and density of active phylotypes and morphotype proportions were related to similarities in a set of environmental variables (including nutrients, humic substances content, predator density and phytoplankton biomass), and some additionally to lake area. Horizontal heterogeneity in bacterioplankton variables (littoral versus pelagic) increased with lake area. Bacterioplankton biomass and production tended to be lower in the littoral zone than in the pelagic zone despite higher concentrations of DOC and humic substances. A likely explanation is higher predation on bacterioplankton in the littoral zone, although allelophatic effects exerted by macrophytes cannot be excluded. Our results indicate that organic cycling via bacterioplankton may be less efficient in the littoral zone than in the pelagic zone of shallow lakes.     

Reproduction under predation risk and the trade-off between current and future reproduction in the threespine stickleback

An increasing number of studies show that animals adjust their reproductive effort to the risk of predation. However, to maximize lifetime reproductive success this adjustment should depend on the animals'' current and future reproductive potential. Here I tested this hypothesis by allowing threespine stickleback males (Gasterosteus aculeatus), differing in current and future mating probabilities, to reproduce in pools in both the presence and absence of predators. As expected, males adjusted their reproductive effort to the risk of predation. Fewer males bred, and all males developed less nuptial coloration in the presence of predators. However, males with a low current mating probability took less risk than males with a higher mating probability, whereas all males increased risk taking when future reproductive opportunities decreased. The results thus support the hypothesis that males are able to assess both the risk of predation and their current versus future mating probability, and adjust their reproductive decisions accordingly. The study further suggests that predation risk may have less effect on sexual selection than previously assumed, as the males which refrained from reproducing in the presence of predators were mainly males with a low mating probability.     

Resource allocation between growth and endothermy allows rapid nestling development at low feeding rates in a species under high nest predation

Nestling development is among the most energy‐demanding periods of a bird’s lifetime and altricial species require extensive parental energy investment in the form of feeding and heating. In the present study I analyze the relation and trade‐offs between nestling growth, development of thermoregulation and feeding rate in blackcap Sylvia atricapilla, a species suffering from high nest predation. Nestlings were characterized by rapid growth but they achieved only 80% of adult mass prior to fledging. Body mass showed highest relative growth rate before nestlings achieved homeothermy. The onset of endothermy, indicated in day 7, coincided with 90% of nestling fledgling weight, indicating that the two processes are separated in time. A strong negative correlation between feeding rate and growth rate demonstrates that blackcap nestlings develop their bodies under relatively low feeding rates and more feeding is needed for maintenance of body temperature than for body growth. The study indicates high cost of endothermy for parents – endothermic nestlings received over 100% more feedings than ectothermic ones. The findings are discussed in the light of adaptation of the species to high predation risk.     

Combined effects of fish and macroinvertebrate predation on zooplankton in a littoral mesocosm experiment

Whether macrophytes offer an effective refuge for zooplankton in all shallow lakes is subject to debate. To explore potential constraints between different predator threats and the related habitat choice by zooplankton, we conducted a mesocosm experiment in 12 large-sized pools mimicking the nearshore environment with part of its length being covered by submersed macrophytes (Egeria densa) and holding a mixed zooplankton community. Four treatments were used: (i) young zooplanktivorous fish (3 silverside, Odontesthes bonariensis) in the “open-water” zone; (ii) macroinvertebrate predator (31 grass shrimp, Palaemonetes argentinus) in the vegetated zone; (iii) both, fish in the open-water and shrimp in the vegetated zones; and (iv) control with no predators. Our results show specific effects of each predator on the abundance, composition, and size of cladocerans. Regarding distribution, in control and shrimp mesocosms, no differences were found between the two zones, while cladocerans were clearly more abundant in the vegetated side in the presence of fish. When both fish and shrimp were present, cladocerans preferred the vegetated zone too, but in a smaller proportion, and their abundance was less. The presence of predatory macroinvertebrates in vegetated littoral zone reduces the refuge value of this habitat, at least for cladocerans.

     

Indeterminate growth is selected by a trade-off between high fecundity and risk avoidance in stochastic environments

 We developed a stage-structured model to describe optimal energy allocation among growth, reproduction, and survival. Our model includes stochastic fluctuations in survival rate at age 0 but constant survival rate at older ages. Many mammals and birds cease to grow after maturity (i.e., determinate growth), whereas organisms in a number of other taxa grow beyond maturation (i.e., indeterminate growth). We discuss the conditions under which each of the following strategies is optimal: (I) semelparity, (II) iteroparity with determinate growth, and (III) iteroparity with indeterminate growth. Our model demonstrates that iteroparity with indeterminate growth is selected for when a nonlinear relationship exists between weight and energy production; this strategy is also often selected for in stochastic environments, even with a linear relationship between weight and energy production. The optimal strategy in stochastic environments is to maximize the long-term population growth rate, which does not correspond with maximization of total fecundity. The optimal life history is determined by a balance between spreading a risk and increasing the number of offspring. Our model suggests that optimal life history strategy depends on the magnitude of environmental fluctuations, the advantage of investing in growth, the cost of survival, and the nonlinearity between weight and energy production. Received: February 20, 2002 / Accepted: September 20, 2002 Acknowledgments We thank Drs. Y. Matsumiya, K. Morita, K. Shirakihara, and Y. Watanabe for encouragement and helpful advice. We also thank the responsible editor and anonymous reviewers for helpful comments. This work was supported by a Japan Society for the Promotion of Science grant to H.M. Correspondence to:Y. Katsukawa     

Selective feeding by larval dytiscids (Coleoptera:Dytiscidae) and effects of fish predation on upper littoral zone macroinvertebrate communities of arctic lakes

Carbon and nitrogen stable isotopic data from the primary producers in mangrove ecosystems are needed to investigate trophic links and biogeochemical cycling. Compared with other mangrove species (e.g. Rhizophora mangle) very few measurements have been conducted on the white mangrove, Laguncularia racemosa. The carbon and nitrogen stable isotopic and elemental compositions of L. racemosa were analyzed and compared from Florida and Belize. ¹³C values of L. racemosa from Florida (mean = –26.4) were slightly higher than those from Twin Cays, Belize (mean = -27.4), which may be due to higher salinity in some parts of the Florida site. There was no difference between the ¹⁵N values from L. racemosa from these two sites (Florida mean = 0.6; Belize mean = 0.3), which are indicative of nitrogen derived from nitrogen fixation in a planktonic marine system. However, higher ¹⁵N values from L. racemosa at Man of War Cay in Belize (11.4 and 12.3), which is fertilized by roosting marine birds (14.0), illustrate that L. racemosa can sensitively reflect alternative nitrogen sources. Although the isotopic data could not distinguish between Avicennia germinans, R. mangle and L. racemosa in Belize the L. racemosa had considerably higher C/N ratios (46.5 – 116.1) compared with the Florida samples (42.2 – 76.0) or the other mangrove species. Unlike some previous findings from R. mangle, substrate characteristics (e.g. salinity, NH₄ ⁺, and H₂S) were not related to the isotopic or elemental composition of L. racemosa. ¹³C, ¹⁵N and C/N were analyzed for ecosystem components from L. racemosa habitats at Twin Cays, including other plants (e.g. R. mangle, A. germinans and seagrass), detritus, microbial mats and sediments. Results from mass-balance calculations show that mangrove detritus composes very little of the sediment, which is principally composed of microbial biomass (80 – 90%). Detritus at some sites is also influenced by sources other than that from L. racemosa, including seagrass leaves.