Predator avoidance in phytophagous mites: response to present danger depends on alternative host quality

We studied whether volatiles released by putative host plants affect the antipredator response of an herbivorous mite, Tetranychus urticae, when the patch was invaded by Phytoseiulus persimilis. Tetranychus urticae laid a lower number of eggs on tomato leaves than on lima bean leaves, suggesting that lima bean is a preferred host food source for T. urticae. In addition, T. urticae preferred lima bean plant volatiles to tomato plant volatiles in a Y-tube olfactometer test. To investigate the antipredator response of T. urticae, we examined the migration of T. urticae from a lima bean leaf disc to a neighbouring leaf disc (either a tomato or lima bean leaf disc) when ten predators were introduced into the original lima bean disc. A Parafilm bridge allowed for migration between the leaf discs. No migrations occurred between leaf discs when there were no predators introduced to the original leaf disc. However, when predators were introduced migrations did occur. When the neighbouring leaf disc was upwind of the original disc, the migration rate of the mite from original lima bean leaf disc to a neighbouring tomato leaf disc was significantly lower than that to a neighbouring lima bean leaf disc. By contrast, when the neighbouring leaf disc was downwind of the original leaf disc, there was no difference in the migration rates between lima bean leaf discs and tomato leaf discs. The number of T. urticae killed by P. persimilis for each treatment was not different, and this clearly shows that the danger was the same in all treatments regardless of the decision made by T. urticae. From these results, we conclude that T. urticae change their antipredator response by evaluating the difference in host plant volatiles in the patch they inhabit.     

Function of the web box as an anti-predator barrier in the spider mite,<Emphasis Type="Italic"> Schizotetranychus recki</Emphasis>

Spider mites inhabiting Sasa bamboo show considerable variation in traits believed to be the result of coevolution between predator and prey. In Schizotetranychus recki Ehara inhabiting the hairy leaves of a dwarf bamboo, Sasa senanensis, all quiescent stages, including eggs, appear within web boxes in the leaf hairs of their host plant, and this habit is thought to be a trait involved in predator avoidance. To test this hypothesis, the survival rates of S. recki eggs inside a web box and those freed of a web box were assessed in relation to six predator species that co-occur with the spider mite in the field. The results clearly show that the webbing behavior (web box) of S. recki has a function in avoiding five predator species. However, one predator species, Agistemus summersi Ehara, preyed more on eggs protected by a web box. This suggests that this predator species has a special trait to overcome the nest barriers.     

Feeding responses of the red fox (<Emphasis Type="Italic">Vulpes vulpes</Emphasis>) to different wild rabbit (<Emphasis Type="Italic">Oryctolagus cuniculus</Emphasis>) densities: a regional approach

We investigate the feeding responses of the red fox (Vulpes vulpes) at a regional scale to different densities of European wild rabbit (Oryctolagus cuniculus) in central–southern Spain. Rabbit abundance indices were obtained in 86 localities during summer 2002. The diet of the fox was studied by analysis of 114 scats collected in 47 of these localities. The feeding response of the fox was examined by a representation of the dry weight percent of rabbit in the diet as a function of the abundance of rabbits; this used data only from those localities where at least 3 scats were collected (70 fox scats from 18 localities). We evaluated the relationship between rabbit abundance and the diversity of the diet of the fox. The feeding patterns of red foxes approximated to Holling’s type III functional response, typical of opportunistic predators. There was a negative relationship between the diversity of the fox’s diet and the abundance of rabbits. Therefore, the fox apparently behaves as a facultative predator, feeding on rabbits when they are abundant and shifting to other prey (and hence a more diverse diet) when rabbits are scarce. These findings are the first step towards understanding the potential role of red foxes in regulating rabbit populations in central–southern Spain.     

Density-dependent predation ofChaoborus flavicans onDaphnia longispina in a small lake: the effect of prey size

Functional response curves of fourth instar larvae ofChaoborus flavicans preying on two size classes ofDaphnia longispina were examined throughout three summer seasons in a small forest lake. Data for each size class were fitted to Holling's disc equation. The parametersa (attack rate) andTh (handling time) were calculated for each prey size from these curves. Attack rate was greater and handling time was shorter for small (0.77 mm) than for large (1.82 mm)Daphnia. In 1:1 mixture of these prey size classes the predation rates ofChaoborus on smallDaphnia at prey densities above 20 l^–1 were greater than predicted from the single size-class experiments. The observed predation rates on largeDaphnia were lower than predicted at all prey densities. Since both single size-class and two size-class experiments were run during the same period of time the difference in observed and predicted predation rates could not be attributed to seasonal changes in prey preference ofChaoborus larvae. In experiments with a concentrated mixture of lake zooplankton (dominated byD. longispina)Chaoborus preference forDaphnia decreased as prey body size increased. There was no obvious correlation between selectivity coefficients and size-frequency distributions ofDaphnia. When medium-sizedDaphnia were omitted from calculations the preference of small over large prey did not differ significantly from the predictions of the single size-class model.     

Effects of vole fluctuations on the population dynamics of the barn owl <Emphasis Type="Italic">Tyto alba</Emphasis>

Many predator species feed on prey that fluctuates in abundance from year to year. Birds of prey can face large fluctuations in food abundance i.e. small mammals, especially voles. These annual changes in prey abundance strongly affect the reproductive success and mortality of the individual predators and thus can be expected to influence their population dynamics and persistence. The barn owl, for example, shows large fluctuations in breeding success that correlate with the dynamics in voles, their main prey species. Analysis of the impact of fluctuations in vole abundance (their amplitude, peaks and lows, cycle length and regularity) with a simple predator prey model parameterized with literature data indicates population persistence is especially affected by years with low vole abundance. In these years the population can decline to low owl numbers such that the ensuing peak vole years cannot be exploited. This result is independent of the length and regularity of vole fluctuations. The relevance of this result for conservation of the barn owl and other birds of prey that show a numerical response to fluctuating prey species is discussed.     

The effects of the functional response on the bifurcation behavior of a mite predator–prey interaction model

 A predator–prey interaction model based on a system of differential equations with temperature-dependent parameters chosen appropriately for a mite interaction on apple trees is analyzed to determine how the type of functional response influences bifurcation and stability behavior. Instances of type I, II, III, and IV functional responses are considered, the last of which incorporates prey interference with predation. It is shown that the model systems with the type I, II, and III functional responses exhibit qualitatively similar bifurcation and stability behavior over the interval of definition of the temperature parameter. Similar behavior is found in the system with the type IV functional response at low levels of prey interference. Higher levels of interference are destabilizing, as illustrated by the prevalence of bistability and by the presence of three attractors for some values of the model parameters. All four systems are capable of modeling population oscillations and outbreaks. Received 12 March 1996; received in revised form 25 October 1996     

Short-term response of arbuscular mycorrhizal association to spider mite herbivory

We examined effects of aboveground herbivory by spider mites (Tetranychus urticae) on colonization and activity of arbuscular mycorrhizal fungi (AMF; Gigaspora margarita) using potted plants (Lotus japonicus). We evaluated changes in arbuscular mycorrhizal (AM) association two ways: (1) conventional trypan blue staining of mycorrhizal hyphae to examine AMF biomass in roots (mycorrhizal colonization) and (2) vital staining for a mycorrhizal enzyme (succinate dehydrogenase, SDH) to examine mycorrhizal activity (SDH activity). Mycorrhizal colonization and SDH activity started to increase 4 days after aboveground herbivory, and returned to the initial levels in the absence of mite herbivory in 7 and 12 days, respectively. These results suggest that the change in AM association in response to mite herbivory is a short-term response.     

The reproductive tactics and activity patterns of solitary carnivores: the Iriomote cat

Felids are generally considered to be crepuscular and nocturnal in their activity, but few studies have attempted to analyze the variability of their activity patterns. We studied the daily activity of the Iriomote cat Prionailurus iriomotensis by radio-tracking on Iriomote Island, Japan. The general activity patterns of Iriomote cats showed slightly prevailing activity during dark periods of the day with particular peaks at dawn and dusk or during the early hours of the night. However, these patterns were clearly dependent on the sex and reproductive status of the cat. Peaks of cats’ activity coincided with those of their main prey. On average, the cats were active for thirteen hours per day. During the mating season, the rhythm of activity in males followed that of breeding females, but not that of non-breeding ones. Males exhibited 11% higher total daily activity and longer active bouts during the mating period than in the remainder of the year. Breeding females had additional mid-day activity peak during the nursing period, but their total time of activity per day was 16% lower than in the period of kittens’ independence. Their active bouts were shorter and more frequent during nursing than at other times. These results suggest that lactating females perform frequent movements to and from the den site to care for kittens. During the non-nursing period, females increased their activity, possibly in response to lowered prey abundance and the need of intensive foraging to recover after lactation. Seasonal and sexual variation of activity patterns in the Iriomote cats confirmed the existence of different reproductive strategies of males and females of these solitary carnivores.     

Rain,prey and predators: climatically driven shifts in frog abundance modify reproductive allometry in a tropical snake

To predict the impacts of climate change on animal populations, we need long-term data sets on the effects of annual climatic variation on the demographic traits (growth, survival, reproductive output) that determine population viability. One frequent complication is that fecundity also depends upon maternal body size, a trait that often spans a wide range within a single population. During an eight-year field study, we measured annual variation in weather conditions, frog abundance and snake reproduction on a floodplain in the Australian wet-dry tropics. Frog numbers varied considerably from year to year, and were highest in years with hotter wetter conditions during the monsoonal season (“wet season”). Mean maternal body sizes, egg sizes and post-partum maternal body conditions of frog-eating snakes (keelback, Tropidonophis mairii, Colubridae) showed no significant annual variation over this period, but mean clutch sizes were higher in years with higher prey abundance. Larger females were more sensitive to frog abundance in this respect than were smaller conspecifics, so that the rate at which fecundity increased with body size varied among years, and was highest when prey availability was greatest. Thus, the link between female body size and reproductive output varied among years, with climatic factors modifying the relative reproductive rates of larger (older) versus smaller (younger) animals within the keelback population.     

Plant regrowth response to a stem-boring insect: a swift moth-willow system

To examine plastic willow regrowth response to herbivory, we studied the effect of a boring insect, the swift moth Endoclita excrescens (Hepialidae: Lepidoptera), which does not remove apical meristems, on shoot growth in three willow species—Salix gilgiana, S. eriocarpa, and S. serissaefolia−by direct observations and experiments in the field. We hypothesized that the stem-boring could initiate new lateral bud activation, and result in secondary shoot regrowth without the removal of the primary apical meristems. There were significantly more lateral shoots on naturally attacked than unattacked stems, and a significant positive correlation between lateral shoot density and the number of swift moth tunnels per tree was observed for all three willow species. Artificial boring, and larval infestation, resulted in an increase in the number of lateral shoots, but did not affect growth of current-year shoots. The length of lateral shoots differed between species, being significantly longer in S. gilgiana than S. eriocarpa and S. serissaefolia. The results of this study show that compensatory regrowth can result even if herbivory does not remove the apical meristem. We argue that this type of plant compensatory response is probably widespread, given that the stem-boring is a common feeding type of insect herbivores.     

Swan foraging shapes spatial distribution of two submerged plants,favouring the preferred prey species

Compared to terrestrial environments, grazing intensity on belowground plant parts may be particularly strong in aquatic environments, which may have great effects on plant-community structure. We observed that the submerged macrophyte, Potamogeton pectinatus, which mainly reproduces with tubers, often grows at intermediate water depth and that P. perfoliatus, which mainly reproduces with rhizomes and turions, grows in either shallow or deep water. One mechanism behind this distributional pattern may be that swans prefer to feed on P. pectinatus tubers at intermediate water depths. We hypothesised that when swans feed on tubers in the sediment, P. perfoliatus rhizomes and turions may be damaged by the uprooting, whereas the small round tubers of P. pectinatus that escaped herbivory may be more tolerant to this bioturbation. In spring 2000, we transplanted P. perfoliatus rhizomes into a P. pectinatus stand and followed growth in plots protected and unprotected, respectively, from bird foraging. Although swan foraging reduced tuber biomass in unprotected plots, leading to lower P. pectinatus density in spring 2001, this species grew well both in protected and unprotected plots later that summer. In contrast, swan grazing had a dramatic negative effect on P. perfoliatus that persisted throughout the summer of 2001, with close to no plants in the unprotected plots and high densities in the protected plots. Our results demonstrate that herbivorous waterbirds may play a crucial role in the distribution and prevalence of specific plant species. Furthermore, since their grazing benefitted their preferred food source, the interaction between swans and P. pectinatus may be classified as ecologically mutualistic.     

Effect of prey size on attack components of the functional response by Notonecta undulata

The number of encounters per prey, the proportion of encounters resulting in attacks, and the proportion of attacks that were successful were observed while fourth-instar Notonecta undulata nymphs preyed on smaller N. undulata nymphs. While encounters per prey and proportion of encounters resulting in attacks increased with prey size, the proportion of attacks that were successful decreased. The increase in encounter rate per prey was due in part to an increase in the predator's reactive distance to prey as prey size increased. While none of the attack parameters varied significantly with prey density, logarithmic regression of the number of encounters per unit search time on prey density suggested that prey density tends to have a positive effect on encounters per first-instar prey but a negative effect on encounters per second-instar prey. A functional response model is presented that incorporates components of the predator's attack rate as exponential functions of prey density and allows for effects of the time the predator may spend evaluating prey encountered but not attacked and time spent attacking prey not captured. Estimates of the attack parameters derived from the experimental data are used in the model to generate functional response curves for fourth-instar N. undulata preying on first- or second-instar conspecifics. The predicted curve for second-instar prey is typical type II but the curve for firstinstar prey is slightly positively density dependent at low prey densities, i.e., type III.     

The effect of prey composition and abundance on the predation rate and selectivity ofMysis relicta

The predation rate and selection of specific prey byMysis relicta in Lake Tahoe depends on both the total and the relative densities of prey classes. Functional responses of mysids to changes in the density of two species of calanoid copepod prey were curvilinear in laboratory experiments. In the field and in 2-prey treatments, preference was usually positive forEpischura and negative forDiaptomus. However, the preference forEpischura was greatest at the lowest combined densities of prey in the laboratory and decreased at densities >100Epischura / m3 in the field. Several hypotheses to explain the inverse relationship between selectivity and prey density are discussed. Field assemblages of prey vary vertically, horizontally and seasonally. Potential effects of this hetrogeneity on both predator and prey populations are examined.     

The effect of flickering U-V light output on the attractiveness of an insect electrocutor trap to the house-fly, Musca domestica

The ultraviolet lamps commonly used in insect electrocutor traps flicker at twice the mains AC frequency. A trap was modified to be powered from a flicker-free (DC) source. When house-flies, Musca domestica L. (Diptera: Muscidae), were released in a test room containing one flickering and one non-flickering trap, 75% of the flies were caught by the trap with the flickering lamps. Mean intensities and emission spectra were identical. The trap with flickering lamps still caught more flies when the intensity of its lamps was lowered to half that of the DC-driven trap. The implications of these results for testing colour preference in attractant lamps are mentioned.

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Zusammenfassung Die ultravioletten Leuchtröhren, die oft in Lichtfallen zur Bekämpfung von Fluginsekten verwendet werden, flimmern bei der doppelten Frequenz des normalen Netzwechselstromes (d.h. 100 oder 120Hz). Dieses Flimmern ist bei den Insekten, die eine hohe Flimmerlichtverschmelzungsfrequenz haben, sichtbar. Das Ziel dieser Untersuchung war, festzustellen, ob die Flimmerfrequenz die Wirksammkeit der Lichtfalle beeinflusst. Eine Rentokil (Modell 143) Lichtfalle wurde umgebaut, damit sie von zwei 12V Autobatterien betrieben werden konnte, um ein flimmerfries Anlockungslicht zu erzeugen. Stubenfliegen wurden in einem 31,8 Kubikmeter grossen Testraum, worin eine nichtflimmernde und eine unmodifizierte, flimmernde Lichtfalle montiert waren, freigelassen. Bei Versuchen, wo die normale Zimmerbeleuchtungsröhren eingeschaltet waren, wurden 75% der Fliegen in der flimmernden Lichtfalle aufgefangenen. Durchschnittswerte für Lichtintensität Beleuchtungsflächen und Ausstrahlungspektrum blieben identisch. Als dieser Versuch im verdunkelten Raum wiederholt wurde, blieben die Verhältnisse der in den beiden Fallen aufgefangenen Fliegen, unverändert. Die Fangrate der Fallen war jedoch im dunklen Raum höher, weil es keine andere für die Fliegen anlockende Lichtquellen gab. Selbst bei der Hälfte der Lichtintensität der von Gleichstrom betriebenen Lichtfalle, wurden 61% der Fliegen in der nichtflimmernden Falle aufgefangen. (Alle Versuchsergebnisse waren statistisch signifikant.) Diese erhöhte Wirksamkeit blieb für beide Geschlechter gleich. Diese Beobachtung stimmt mit den ähnlichen Flimmerfrequenzwerten von männlichen und weiblichen Art Musca überein. Im Fall der Tsetsefliege sind die beiden Geschlechter verschieden flimmerempfindlich, und deswegen würde man Unterschiede in der Reaktion auf flimmernde Lichtfallen erwarten. Von Natur aus werden Fliegen der Art Musca von flimmerden Licht angelockt. Die Konsequenzen dieser Ergebnisse hinsichtlich der Untersuchung der Farbwahl der Anlockungslichtquellen werden in diser Veröfftentlichung erwaähnt.

     

Nocturnal foraging by great skuas Stercorarius skua: implications for conservation of storm-petrel populations

At St Kilda, Outer Hebrides, a large colony of great skuas Stercorarius skua feed extensively on one of the largest colonies of Leach’s storm-petrels Oceanodroma leucorhoa in Europe, but little is known about the dynamics of this predator–prey system. Recently published population estimates of storm-petrels make it possible to estimate the impact of skua predation for the first time. Although skuas in the southern hemisphere catch petrels attending breeding colonies at night, it is not known whether congeners in the northern hemisphere also forage during the hours of darkness. We found (using radio-transmitters) that skuas regularly forage at night and (using light intensifying equipment) observed them catching storm-petrels at night. However, skuas also foraged during daylight hours, and it is unknown whether they might also catch storm-petrels at sea. Data on diet composition reveals that the proportion of storm-petrels in skua diet declined between 1996 and 1997, but remained constant thereafter. Although a large proportion of the storm-petrel prey is likely to consist of non-breeders, numbers consumed suggest that breeders and an unknown quantity of transients may also been eaten. The numbers of storm-petrels eaten are not sustainable and may result in substantial long-term population declines. Under current conditions, maintenance of large populations of both Leach’s storm-petrels and great skuas at St Kilda appears to be mutually exclusive.     

The importance of temporal dynamics of edge effect in reedbed design: a 12-year study on five bird species

Although edge effect is a key topic of conservation biology, we have no data on the temporal dynamics of it. I investigated the distribution of five passerine bird species across reedbed (Phragmites australis) edges during large-scale construction work in the Kis-Balaton marshland, Hungary. The construction provided an experimental approach to study the effects of large timescale changes within a shorter period, because neither the locality nor the vegetation type changed. The water level was increased in the study area, which homogenised the internal structure of reedbed by declining the scattered small willow bushes (Salix) and the grass/sedge layer. The sedge (Acrocephalus schoenobaenus) and reed warblers (Acrocephalus scirpaceus) preferred edges. The sedge warbler, however, declined after inundation, while the reed warbler did not respond. Savis warbler (Locustella luscinioides) sharply declined during the study with changing edge effect. The number of great reed warblers (Acrocephalus arundinaceus) increased during the study, mainly in the reedbed interior, where the stands became patchy with open water. Reed bunting (Emberiza schoeniclus) avoided interiors, and declined over the study. Therefore, there were significant changes in the distribution of reedbed birds across the edge, although the location of edges and the basic habitat, reedbed, did not change. The results highlight the need to incorporate edge effect as a dynamic process in wetland planning.     

Host age, sex, and pathogen exposure level as factors in the susceptibility of Musca domestica to Entomophthora muscae

Adult Musca domestica were exposed to conidial showers from cadavers that were killed by Entomophthora muscae. Of the 4 host ages (14 days, 7 days, 3 days and <1 day), older flies were more likely to die without postmortem signs of infection, while younger flies were more likely to die with signs and produce primary conidia. The youngest fly group also succumbed to infection significantly sooner than older flies. Total mortality did not differ for the 2 sexes, but females died significantly more often with signs in one trial. Males died significantly earlier than did females, but differences were not significant when the sexes were paired by weight. Increased levels of pathogen exposure significantly increased both total mortality and the number of flies dying with signs; it also decreased incubation period. The potential significance of these findings in the epidemiology of the disease is discussed.

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Zusammenfassung Die Mortalitätsrate von Musca domestica nach Behandlung mit Entomophthora muscae Sporen wurde in Laborversuchen in Abhängigkeit von Wirtsalter,-geschlecht und Expositionsdauer festgestellt. Fliegen verschiedenen Alters (14 Tage, 7 Tage, 3 Tage und jünger als 1 Tag) wurden gemeinsam in 237 ml Kartonbehältern gehalten. Die Behälter besassen Netzdeckel, auf denen frisch an einer Entomophthora Infektion gestorbene Fliegen ausgelegt wurden. Auf diese Weise wurden die Versuchstiere 8 Stunden (low exposure) bzw. 48 Stunden (high exposure) lang mit Entomophthora Sporen kontaminiert. Mit Hilfe von am Boden der Versuchsgefässe ausgelegten Objektträgern wurde die relative Menge Entomophthora Sporen festgestellt, der die Versuchstiere ausgesetzt waren. Der Totenfall wurde täglich für die Dauer von 10 Tagen registriert; dabei wurde zwischen Konidienträgern (external signs) und Leichen ohne Konidien (no external signs) unterschieden.Die Versuchsgruppen, die 48 Stunden lang mit Entomophthora Sporen behandelt worden waren, wiesen im Vergleich zu den kürzer behandelten Versuchsgruppen höhere Mortalität und einen höheren Anteil an Konidienträgern auf. Darüber hinaus wurde das Auftreten von Konidienträgern bei den länger behandelten Gruppen früher beobachtet. Im Vergleich zu unbehandelten Kontrollgruppen erhöhte die Behandlung mit Entomophthora Sporen auch die Rate nicht konidientragender Leichen.Bei gleicher Expositionsdauer zeigten Weibchen und Männchen ähnliche Mortalitätsraten. Ein höherer Anteil Konidienträger bei den Weibchen wurde bei einem von zwei entsprechenden Versuchen beobachtet. Konidienträger traten bei Weibchen später als bei Männchen auf. In Bezug auf die Inkubationszeit konnte kein geschlechsspezifischer Unterschied festgestellt werden, wenn Versuchsgruppen mit gleichem Gewicht verglichen wurden. Bei beiden Geschlechtern erfolgte die Bildung von Konidien bei grösseren Individuen später als bei kleineren Individuen.Das Alter der Versuchstiere beeinflusste sowohl die Mortalitätsrate als auch die Häufigkeit und das zeitliche Auftreten von Konidienträgern. Jüngere Fliegen wiesen einen höheren Anteil Konidienträger auf; bei Fliegen mit einem Alter von weniger als einem Tag wurde das Auftreten von Konidienträgern früher beobachtet als bei älteren Individuen.Die untersuchten Faktoren können auch für die Epidemiologie von Entomophthora freilebender Musca domestica Populationen eine wichtige Rolle spielen. In Populationen mit niedrigerem Durchschnittsalter wird sich eine Infektion rascher ausbreiten, da jüngere Fliegen eine kürzere Inkubationszeit aufweisen und auch nach kürzerer Kontaminationsdauer Konidien an den getöteten Individuen ausgebildet werden. Darüber hinaus wird sich eine Infektion dann rascher ausbreiten, wenn die Population mit einer grösseren Sporenmenge infiziert wird, da dann eine kürzere Inkubationzeit und ein höherer Anteil Konidienträger zu erwarten ist.

     

Vulnerability of newly settled plaice (Pleuronectes platessa L.) to predation: effects of habitat structure and predator functional response

Plaice (Pleuronectes platessa) nursery grounds on the Swedish west coast have been subject to increasing cover of annual green macroalgae during recent years, with growth of algae starting at the time of plaice settlement in April to May. A laboratory experiment was performed to investigate how the vulnerability to predation of metamorphosing plaice was affected by the presence of filamentous algae. Predation by shrimps (Crangon crangon) on settling plaice larvae was higher on sand than among algae, whereas predation by crabs (Carcinus maenas) was unaffected by habitat type, suggesting a lower overall mortality of plaice in the vegetated habitat. When predators and prey were presented with a combination of the two habitats, predation by shrimps was as high as that in the sand treatment alone, whereas predation by crabs was lower than that in the two treatments with one habitat. Based on these results, an additional experiment was performed, investigating the functional response of shrimps to six densities of juvenile plaice in a sand habitat with alternative prey present. The proportional mortality of juvenile plaice (12-16 mm total length (TL)) was density-dependent and was best described by a type III (sigmoid) functional response of the predatory shrimps. The results suggested that the combined predation pressure from shrimps and crabs was lower among algae than on sand, but settling plaice and predatory shrimps chose the sand habitat. Plaice densities in the sigmoid part of the obtained functional response curve represented normal to high field densities of plaice on the Swedish west coast, suggesting that shrimp predation could have a stabilising effect on plaice recruitment. The formation of macroalgae mats could therefore lead to a concentration of plaice juveniles in the remaining sand habitat and increased mortality through density-dependent predation by shrimps.     

Web-site selection strategies of linyphiid spiders in alfalfa: implications for biological control

Site-specific foraging can enhance the ability of generalist predators to provide effective and sustainable levels of pest control in agroecosystems. This can result from increased growth rates, higher population densities, and improved capture frequencies of pests at high prey density microsites. We tested the hypothesis that linyphiid spiders would exhibit microhabitat-specific web-site selection strategies in alfalfa. This was predicted to result in high prey densities at web-sites compared to paired non-web-sites through direct, or indirect, selection of prey-rich habitats. A total of 22,242 potential prey items were collected from mini-sticky traps located at 896 microsites. Web-centered mini-sticky traps on the ground, representative of Erigone autumnalis Emerton (Araneae: Linyphiidae) webs, captured similar numbers of potential prey as paired non-web-centered traps nearby. However, aerial sticky traps at web-sites of Bathyphantes pallidus (Banks) (Araneae: Linyphiidae) contained significantly more Diptera and Empoasca fabae (Harris) (Homoptera: Cicadellidae) than paired non-web-centered sticky-traps. Prey activity-densities also varied between web-sites of E. autumnalis and B. pallidus. Diptera were dominant at aerial microsites of B. pallidus whilst Collembola were abundant on ground-based traps of E. autumnalis. These results suggest that in alfalfa, the pressure for selecting prey-rich web-sites by erigonine spiders is low, but B. pallidus exhibits a selective web-location strategy targeted towards high quality dipteran prey. These sites also captured large numbers of E. fabae, a major pest of alfalfa, thus implicating aerial-based linyphiines as valuable predators in biological control.