Increase In Prey Abundance Research Articles

Scale-dependent predator-prey interactions: the aggregative response of seabirds to prey under variable prey abundance and patchiness

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 231:279-291 (2002) - doi:10.3354/meps231279 Scale-dependent predator-prey interactions: the aggregative response of seabirds to prey under variable prey abundance and patchiness Per Fauchald*, Kjell Einar Erikstad Department of Biology, Faculty of Science, University of Tromsø,9037 Tromsø, Norway and Norwegian Institute for Nature Research, Division of Arctic Ecology, The Polar Environmental Center, 9296 Tromsø, Norway *E-mail: per.fauchald@nina.no ABSTRACT: The aggregative response of seabirds to their prey has frequently been measured through their spatial coherence during ship board surveys. In this study, we investigate the aggregative response of murres Uria spp. to capelin Mallotus villosus in relation to yearly changes in capelin abundance and patchiness from a 9 yr data set collected in the Barents Sea. A recent analysis on the spatial distribution of capelin and murres in the Barents Sea reported a hierarchical patch structure, whereby small-scale patches were nested within patches at larger scales. In the present study we carry out similar analyses for each of the 9 years. During the 9 yr period, the estimated total biomass of capelin varied between 100000 and 7300000 t. We identified 2 hierarchical levels of patchiness with characteristic scales of 200 to 300 km and 70 km respectively. We found close relationships between yearly variation in capelin abundance, capelin patchiness and the concordance between capelin and murres at both levels. The patchiness of capelin, as measured by the first-order spatial autocorrelation, increased with increasing capelin abundance at the large-scale level, and there was a similar tendency at the small-scale level. At the large-scale level, the spatial concordance between murres and capelin increased with both increasing capelin density and increasing capelin patchiness. At the small-scale level, the concordance between murres and capelin increased with increasing capelin patchiness. Our results suggest that increased prey patchiness and increased prey abundance enhance the possibility of murres finding satisfactory capelin concentrations. The reduction in prey patchiness with low prey abundance might force predators to switch to other prey items, and might therefore have a stabilizing effect on predator-prey interactions. KEY WORDS: Spatial distribution · Hierarchical patch structure · Capelin · Mallotus villosus · Murre · Uria spp.· Barents Sea Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 231. Online publication date: April 23, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

White-breasted Cormorant, Phalacrocorax carbo, breeding season timing and its control factors at a seasonally constant Kenyan lake

Environmental factors and breeding activity of the White-breasted Cormorant were documented over two breeding seasons (1995/96) at a new 900-nest colony near Lake Naivasha, Kenya (00° 46′S, 36° 22′E), a non-seasonal lake. Breeding of this species has been reported to occur year-round near the equator in East Africa. Other studies suggest that the White-breasted Cormorant breeds mainly during April–June throughout sub-Saharan Africa, irrespective of the local climatic regime or differences in photoperiod trend. At Lake Naivasha, breeding also occurred primarily during April–June. There was no evidence of a consistent seasonal increase in prey abundance during this period, and the timing was not linked consistently to any other environmental changes, such as amount of rainfall, lake water level or transparency, daily photoperiod, or ambient air temperature. Onset of the main rains, however, appeared to be a possible stimulant to breeding readiness in the population as a whole. It is suggested that breeding timing at Lake Naivasha may be influenced by a combination of the subspecies' nearly 12-month endogenous reproductive cycle and an annual trigger through a local environmental factor, possibly the onset of the main rains.

The Feeding Ecology of Actinophrys sol (Sarcodina: Heliozoa) in Chesapeake Bay

ABSTRACTThe occurrence of Actinophrys sol, a planktonic heliozoan, in Chesapeake Bay was monitored over a four‐year period (1988–1991). Actinophrys sol was widely‐distributed throughout Chesapeake Bay and could exceed densities of 5,000 cells liter −1. It was most abundant during the warmer months. Feeding experiments were conducted with field populations of heliozoa using 1‐μm fluorescent microspheres to label ciliate prey. Two ciliates, a small Strobilidium sp. (30 μ in diameter) and a Pleuronema sp. (45 μ length), were the primary ciliate‐prey items in the water column when the experiments were conducted, although a wide range of ciliate taxa was ingested. Two other ciliates not present in situ, a Cyclidium sp. (20 μ length) and a Uronema sp. (40 μ length), were also labeled and added at various concentrations to field populations of plankton containing A. sol. Heliozoan ingestion rates on in situ prey at concentrations of 30 Strohilidium and one Pleuronema ml−1 were 0.2 to 0.3 prey heliozoan−1 hour−1. Ingestion rates increased to a maximum of 1.2 prey heliozoan −1 hour−1 with additions of 100 Uronema ml−1. A mean clearance rate of 0.15 ml heliozoan−1 day−1 did not change with increasing prey abundance. The abundance and distribution of A. sol suggests that these sarcodines may exert strong grazing pressure on the planktonic ciliate populations of Chesapeake Bay at certain times of the year, and may be important in shaping the ciliate community composition and distribution.

Macrofaunal associations with seagrass epiphytes

Abstract Attached epiphytes often make important contributions to total primary production in seagrass meadows. Additionally, they may increase the spatial complexity of seagrass habitats. Experiments conducted using artificial seagrass units (ASU) manipulated both epiphytic structure and epiphytic food resources. Previous work suggested that the increase in faunal density associated with epiphytes was related to increases in structure, but our results indicate that the primary impact of epiphytes lies in their trophic role. Data showed that epifaunal density was significantly greater in conditioned ASUs fouled with a live community of epiphytes (12 285 individuals m −2 ) compared to ASUs with artificially created epiphytic structure (5099 inds. m −2 ) and to control ASUs (5955 inds. m −2 ). This response to epiphytic trophic resources was significant for most herbivore/omnivore taxa, but not necessarily for filter feeding or predatory epifauna. However, densities of two predatory taxa (fish and mud crabs) were significantly greater where epiphytic biomass was higher, which may reflect their response to increased prey abundance. Additionally, ASUs conditioned with live epiphytes had greater taxa richness than other ASUs. Epiphytic structure appeared to play only a limited role in determining the density of most mobile epifauna, but epiphytic structure appeared to be important in augmenting the settlement of bivalves. By using ASUs we were able to control aspects of blade length and shoot density, but the pre-experiment conditioning of treatments fouled with live epiphytes may have played a role in determining absolute differences in macrofaunal density among ASU treatments. Overall, our work suggests that the trophic role of epiphytes can have a dramatic impact on associated epifaunal communities, although future investigations are needed to assess this relationship more fully.

Foraging in a Hierarchical Patch System

abstract: This study presents a model of a forager searching for prey in a stochastic hierarchical patch system where high‐density patches at small scales are nested within low‐density patches at larger scales. In order to track the system, the forager uses long travel distances within large‐scale patches and short travel distances within small‐scale patches. The forager estimates its position in the system according to recent experience and changes its search pattern accordingly. The scaling of the prey system was adapted according to marine pelagic systems of krill and schooling fish, and a forager was simulated for varying prey abundance and prey aggregation. The model suggests that tracking efficiency, defined as the forager's mean position within the system, increases for increasing prey abundance. Furthermore, the model suggests that maximum tracking efficiency is found at intermediate prey aggregations and that tracking efficiency decreases for aggregations above and below this point. The model dem...

Trophodynamics of Pleurobrachia pileus (Ctenophora, Cydippida) and ctenophore summer occurrence off the Norwegian North-West coast

Abstract Stomach-content analyses and laboratory experiments on Pleurobrachia pileus (Cydippida) showed an average digestion time of 2.0 h at 12°C and a high potential predation rate with highest daily ration in terms of prey carbon ingested as percent of predator body carbon for the calanoid copepod Calanus finmarchicus, the biggest prey tested Predation rate increased almost linearly with increased prey abundance over the whole range tested (12-1043 I−1 in start concentration) of mainly small-sized copepods. Tests of the importance of prey size showed an individual clearance rate of 6.1 1 day−1 with Calanus prey alone, which was depressed to 29 % of this when smaller prey was also present in high abundance. This is supposed to be an effect of handling time of prey in the feeding process. The laboratory results were used to estimate the impact of this species in Norwegian coastal waters. Abundance data were collected in summer from 56 stations between 63° and 69°N along a cruise track west of Norway. P. ...

FOREST STRUCTURE AND PREY ABUNDANCE IN FORAGING AREAS OF NORTHERN GOSHAWKS

The U.S. Forest Service recently recommended managing forests to increase the abundance of prey for Northern Goshawks (Accipiter gentilis) in the southwestern United States. However, empirical data are needed to indicate whether goshawks select foraging habitat based on prey abundance, on the presence of forest structures that favor their hunting strategy, or both. During the 1993 and 1994 breeding seasons, we indexed prey abundance and measured forest structure at sites used by radio-tagged goshawks and at nearby paired sites that lacked evidence of goshawk use (contrast plots). Goshawks apparently did not select foraging sites based on prey abundance; indeed, abundances of some prey were lower on used than on contrast plots. Goshawks selected foraging sites that had higher canopy closure (P = 0.006), greater tree density (P = 0.001), and greater density of trees >40.6 cm dbh (P < 0.0005) than on contrast plots. These results are consistent with the hypothesis that goshawk morphology and behavior are adapted for hunting in moderately dense, mature forests, and that prey availability (as determined by the occurrence of favorable vegetation structure where prey are present above a low threshold) is more important than prey density in habitat selection. Nonetheless, current management recommendations for increasing prey abundance also can create a forest structure more favorable for hunting by goshawks, particularly if stands are managed for canopy closure values above the prescribed minimum level of 40% canopy.

Searching behaviour and mate recognition by males of the two‐spot ladybird beetle, Adalia bipunctata

Abstract. 1. Adult males of the two‐spot ladybird beetle, Adalia bipunctata, did not show a functional response to increase in aphid abundance and consumed markedly fewer aphids than do the females.2. At high densities of prey, females spent more time in area‐restricted search than when prey was scarce. Males were always less active than females and they did not respond to an increase in prey abundance by a change in searching behaviour.3. After a brief encounter with a female, a male showed area‐restricted searching behaviour. This behaviour occurred in response to encountering a female's elytra and in particular to a chloroform‐soluble component (sex pheromone) present on or in the elytra.4. Males needed to encounter a female in order to respond to her presence, which indicated the pheromone is a contact pheromone.5. The searching behaviour of males appeared to be mainly directed towards locating females; that of females towards locating aphids. This difference between the sexes should be taken into account when quantifying the predatory response of ladybirds to aphid abundance in the field.

Interference and the ideal free distribution

Current models of interference-based ideal free theory often use the coefficient of interference,m, to quantify the intensity of competitive interference. However, it is not clear whatmmeans in terms of behaviour. This is because the Hassell & Varley model, from whichmis derived, only provides a description of the interference process without alluding to the mechanisms underlying the relationship. In this paper a behaviour-based, analytical model is described which finds the ideal free distribution of predators searching for food in a patchy environment. An interference expression from a model developed by Ruxton et al. (1992,Theor. Pop. Biol.,42,235-253) based on the interactions of a group of predators with each other and their prey is used to obtain the ideal free distribution over any finite number of food patches for different prey distributions. Using examples of environments with two and three patches, the effects of the degree of interference and the total prey density on such distributions are also studied. Contrary to many published results, an over-representation of predators in the best patch is always predicted. This effect is enhanced by increasing prey abundance. When interference is the result of wasted time,mcannot exceed 1; in most casesmis expected to be very low.

Changes in Abundance of Two Percids,Perca fluviatilisandGymnocephalus cernuus, along a Productivity Gradient: Relations to Feeding Strategies and Competitive Abilities

Relative abundances of perch (Perca fluviatilis) and ruffe (Gymnocephalus cernuus) were studied in eight lakes ranked along a gradient of increasing productivity. The diets of perch and ruffe in relation to food resources were quantified over a 5-mo period in one lake of moderate and one lake of high productivity. The abundance of perch decreased and that of ruffe increased along the productivity gradient. Large perch ate fish over the whole season in the more productive lake, but rarely ate fish in May and June in the less productive lake. All size classes of ruffe fed mainly on macrobenthos in both lakes. The diversity of macrobenthos was higher in the lake of low productivity, as was the diet breadth of ruffe. The abundance patterns of perch and ruffe are likely related to the simultaneous effect of increasing prey abundance and decreasing light penetration with increasing productivity. Furthermore, the abundance of perch is likely affected by increasing interspecific competition and that of ruffe by decreasing predation pressure as productivity increases.

Changes in Growth and Reproduction of Harbour Porpoises, Phocoena phocoena, from the Bay of Fundy

Changes in the growth and reproduction of harbour porpoises from the Bay of Fundy are described by comparing samples collected in 1969–73 and 1985–88. The most pronounced change was an increase in the length of calves, from 92.1 cm (SE 1.6) in 1969–73 to 108 cm (SE 1.3) in 1985–88. Females in 1985–88 attained sexual maturity at a significantly younger age (3.44 yr) and shorter length (143 cm) than females from the older sample (3.97 yr and 147 cm). These changes may be attributable to an increase in prey availability to individual porpoises. This presumed increase in prey availability may have resulted from a decrease in porpoise density caused by incidental mortality in commercial fisheries, a simultaneous increase in prey abundance, or a combination of these factors.

Ecology and distribution of juvenile cod and haddock in relation to sediment type and bottom currents on eastern Georges Bank

Recently-settled 0-group juvenile cod Gadus morhua and haddock Melanogrammus aeglefinus were observed by submersible dives and research bottom trawls to inhabit primarily a large pebble-gravel deposit located on the northeastern edge of Georges Bank at 70 to 100 m water depth. Pelagic juvenile gadids are widespread on the bank in late spring, but by late July, they have become demersal and are abundant only on the gravel bed. Coloration of the juveniles mimics the appearance of the pebble bottom, possibly making them less vulnerable to predation there than on the Light-colored sand bottom of most areas of the bank. On the basis of these observations, we hypothesize that the gravel habitat favors their survival through predator avoidance and, possibly to a lesser extent, through increased prey abundance. In particular, the pebble-gravel deposit on northeastern Georges Bank supports the largest aggregations of demersal juveniles, and it may be essential to the recruitment success of the Georges Bank gadid population. By September, the young fish are no longer present on the gravel bed because night-time feeding forays off the bottom result in their transport southeastward in the clockwise current gyre on the bank. The study area is characterized by strong, rotary tidal currents. During the day, demersal juveniles remain withn a few centimeters of the bottom in the boundary layer where currents are somewhat reduced and where they maintain their position by swimm~ng continually into the current. At night, regardless of the current, they rise off the bottom to feed on invertebrates in the near-bottom water column. Abundance estimates of demersal juvelules based on research bottom trawls show variability related to fish s u e and time of day. When compared with estimates of juvenile abundance based on submersible observations, the trawl data underestimate the numbers of juvenile fish present. We expect a knowledge of the die1 behavior, geographic location, and most favorable habitat of demersal 0-group juveniles wdl aid in the assessment and management of the Georges Bank gadid population.

Survival, development and predatory effects of mosquito larvae in Venezuelan phytotelmata

ABSTRACTTo assess the influence of microhabitat and season on predator growth and survival and prey community structure, first instars of native Toxorhynchites haemorrhoidalis, a predatory mosquito, were released into Heliconia bracts, bamboo internodes, and the axils of two species of Aechmea bromeliads during wet and dry seasons in a lowland rain forest in eastern Venezuela. Experimental and control microhabitats were compared by complete censusing of macroscopic invertebrates 10 and 20 days after releases.Survival of T. haemorrhoidalis differed significantly among habitats and was reduced by desiccation in dry-season bamboos and in Aechmea nudicaulis and by the presence of a predatory damselfly (Odonata: Zygoptera) in bromeliads. Developmental rate differed among habitats in the wet but not in the dry season; rapid wet-season maturation was associated with increased prey abundance in Heliconia.Larvae of T. haemorrhoidalis consumed particular taxa and size categories of dipterous prey. Predation significantly reduced the abundance and skewed size-class distributions of Culicidae, Ceratopogonidae, Psychodidae, and pooled samples of Thaumaleidae plus Chironomidae. In bamboos, instar distributions of other mosquitoes were skewed in the presence of T. haemorrhoidalis even though a significant reduction in mosquito abundance was not detected. A decrease in species richness of aquatic Diptera was associated with intense predation by Toxorhynchites in Heliconia and A. nudicaulis.Toxorhynchites theobaldi was recognized as the naturally occurring predator in local bamboos, and released larvae caused significant reductions in abundance or size-class frequency shifts among three families of Diptera. Niche segregation between these sympatric Toxorhynchites is maintained by oviposition preferences: the larger species, T. theobaldi, oviposits in bamboos and the smaller T. haemorrhoidalis occurs in plant bracts and axils.

Diel patterns of emergence of crinoids (Echinodermata) from within a reef at Lizard Island, Great Barrier Reef, Australia

Periods of emergence of nine species (88 individuals) of crinoids (Comanthus parvicirrus, Clarkcomanthus albinotus, Comaster multifidus, Oxycomanthus comanthipinna, Oligometra serripinna, Comanthus gisleni, Comanthus wahlbergi, Comatula purpurea, and Oxycomanthus exilis) were monitored at Lizard Island, Queensland, over seven days in March 1983. One species (O. serripinna) was fully exposed and the others partially exposed. Two patterns of emergence were species-specific: emergent both day and night, and emergent only at night. Intraspecific exposure patterns were generally synchronous in six species and asynchronous in three species. The number of species visible was relatively constant during all dives, while at least twice as many individuals were visible at night compared to day. Degree of emergence was determined as the number and length of arms extended into the water column. Except for species fully exposed, degree of crinoid emergence was generally minimal at midday and maximal at twilight and night. Increases in both number of crinoids visible, and the amount of an individual's emergence, corresponded to decreasing light intensities, even during daytime. It is suggested that the patterns of exposure are a response to increased prey abundance at twilight and night, and also a means of avoiding diurnal predators. Gonads on the longest arms of some partially exposed crinoids were not extended into the water, thus protecting the gonads from predators.

Effects of prey concentration, prey size, predator life stage, predator starvation, and season on predation rates of the carnivorous copepod Euchaeta elongata

Adult females of the large carnivorous copepod Euchaeta elongata Esterly were collected from 1977 to 1980 in Port Susan, Washington, USA. Predation rates of the adult females increased with increasing prey abundance when fed the following 4 sizes of copepods: adult females of Calanus pacificus (average prosome length [PL] of 2 650 μm), adults of Aetideus divergens (PL of 1 560 μm), adult females of Pseudocalanus spp. (PL of 1 060 μm), and nauplii of C. pacificus (PL of 410 μm). Saturation feeding levels were reached when adult females of the predator were fed the small adult copepod, Pseudocalanus spp. Maximum biomass ingested of this small copepod was more than the maximum amount ingested of the larger copepods. Predation rates of the predatory copepodids at Stages IV and V also increased with increasing concentration of the 1 060 μm (PL) prey. High feeding rates exhibited by both adults and copepodids at Stage V of the predator indicate their importance as sources of mortality on populations of small copepods. Ingestion efficiency E i (prey wholly consumed [prey attacked]-1) varied as follows: adults of E. elongata were more efficient than copepodids of E. elongata; adults were more efficient than copepodids when ingesting smaller prey; starved adults were more efficient than fed ones; and both adults and copepodids were more efficient at low food concentrations. For adults of E. elongata, there were no marked seasonal variations in predation or respiratory rates that would represent acclimatory responses; however, small adults obtained during winter were more efficient at ingesting prey than were the larger adults gathered in summer. This seasonal variation in the efficiency of ingestion may be a useful indicator of physiological state: high E i values could indicate that predators are starving in winter, and low E i values could indicate that predators are satiated in summer.

Test of a hypothesis of territory regulation in an insectivorous bird by experimentally increasing prey abundance.

Food availability is frequently hypothesized to be important in the regulation of territorial size. Recent theory suggests animals should respond to increased food availability by decreasing their territory. Most demonstrations of this relationship between food and territory are correlative, and few experimental tests of this hypothesis have been conducted.A field-experimental mainpulation was conducted to test three predictions of the hypothesis that food is a proximate stimulus for regulation of territory. The Rufous-sided Towhee, an insectivorous bird that is a permanent resident of chaparral in Arizona, was used to test the predictions that weekly area, total area, and fluctuations in area would be smaller within experimentally manipulated territories. Food resources were increased within five experimental territories and the response was compared with five control territories.The results did not support any of the three predictions. The hypothesis that food is a proximate stimulus for regulating territorial size was rejected. Two alternative hypotheses, that habitat quality or competition are proximate stimuli for regulating territories, could not be rejected.

Effects of an experimental increase in prey abundance upon the reproductive rates of two orb-weaving spider species (Araneae: Araneidae)

A field experiment was performed to determine if food is a limited resource for adult females of two species of orb-weaving spiders, Mecynogea lemniscata and Metepeira labyrinthea. Spiders built webs after being added to open experimental units located in a mixed deciduous-pine forest in Maryland, USA. Each unit was a frame supporting dead branches of the type used by both species for anchoring webs. Spiders on half the units were exposed to natural prey densities only, while each spider on the other units was given laboratory-reared flies in order to increase prey availability above natural levels. Supplemental feeding continued for 2.5 months. At the end of the experiment all egg sacs were removed from the units.Providing additional prey did not increase the survival rate on the units (net effect of mortality, emigration and immigration). However, both species responded to additional prey by significantly increasing the number of eggs produced per female, indicating that food was a limited resource for these species. Median egg production per female increased from 34 to 62 for Mecynogea lemniscata and from 65 to 145 for Metepeira labyrinthea. Egg weight was not affected.Feeding rates and nearest neighbor distances were determined for spiders in non-experimental populations, which permitted evaluation of the experiment's naturalness. The effects of food supply upon the reproductive rates of the two species are discussed in relation to their numerical response and population dynamics.

Effect of temperature on a greenhouse acarine predator–prey population

Populations of the two-spotted mite, Tetranychus urticae, and its acarine predator, Amblyseius fallacis, were propagated on alfalfa in the greenhouse at constant temperatures in the range 65 to 85 °F (18.3–29.4 °C). The predator limited the initial increase in prey abundance only at temperatures above about 70 °F (21.1 °C). At 80 and 85 °F (26.7 and 29.4 °C) fluctuations in prey and predator numbers increased in amplitude as propagation continued. The age structure of the predator population reared at 75 °F (23.9 °C) differed from that of populations propagated at the other temperatures.