Predator Population Size Research Articles

Effects of predation and weather on the breeding success of Avocets Recurvirostra avosetta

We investigated breeding success of Avocets Recurvirostra avosetta on the Wadden Sea coast of Schleswig-Holstein, Germany, during the years 1988–1997. Annual variations in hatching success were mainly determined by the presence of Red Foxes Vulpes vulpes in an embanked part of the study site and by the occurrence of storm tides on the saltmarshes. Hatching success did not correlate with the population size of avian egg predators. Annual chick survival rates were positively associated with June temperatures. The influence of predators on chick survival rates was not detectable. Annual breeding success (number of fledglings per number of breeding pairs) was significantly correlated with June temperature and chick survival, but not with hatching success.

Egg cannibalism in Baltic sprat Sprattus sprattus

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 196:269-277 (2000) - doi:10.3354/meps196269 Egg cannibalism in Baltic sprat Sprattus sprattus Friedrich W. Köster*, Christian Möllmann Institute of Marine Sciences, Dept of Fishery Biology, Düsternbrooker Weg 20, 24105 Kiel, Germany *E-mail: fkoester@ifm.uni-kiel.de ABSTRACT: Throughout the last 2 decades a shift from a cod- to a sprat-dominated system occurred in the upper trophic levels of the Central Baltic Sea. This was caused by a decline in the cod stock, due to recruitment failure and high fishing intensity, resulting in a decrease in predation pressure on sprat. Concurrently with the lowest cod stock size on record, sprat reached biomass values of above 2 × 106 t in 1992, being relatively stable afterwards. Besides predation mortality through cod and in recent years also an increasing fishing pressure, cannibalism on eggs may be a compensatory process limiting the reproductive success of sprat and hence contributing to the population regulation in the Central Baltic. Based on sprat stomach sampling on 21 cruises between March 1988 and July 1996 cannibalism on sprat eggs was investigated in the Bornholm Basin, one of the main spawning areas of Central Baltic sprat. Using a model of gastric evacuation to estimate daily food intake rates and a Virtual Population/Extended Survivor Analysis for computing predator population sizes, egg cannibalism rates were estimated. These were compared to egg abundance data from ichthyoplankton surveys and to preliminary estimates of seasonal egg productions. The study revealed significant interannual differences in the intensity of sprat egg cannibalism with considerable predation in 1990 to 1992 (>15% of the egg abundance during peak spawning and >60% of the seasonal production) and a reduction in most recent years (<16% of the corresponding abundance and production). As a possible reason for these differences a combination of changes in the vertical overlap of predator/prey and variability in the food environment were identified. Shortcomings of the applied methods and the possible impact of cannibalism on the reproductive success and population development of sprat in the Central Baltic Sea are discussed. KEY WORDS: Baltic sprat · Egg cannibalism · Population development · Stomach content analysis · Daily food intake Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 196. Publication date: April 18, 2000 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 2000 Inter-Research.

Cross‐correlation analysis of fluctuations in local populations of pear psyllids and anthocorid bugs

Summary1. To test whether predatory anthocorids migrate into pear orchards when populations of pear psyllids are building up, a cross‐correlation analysis was carried out on their population numbers. Predator and prey population sizes were assessed weekly in 3 consecutive years (1991–93) by sampling pear leaves for eggs and nymphs of psyllids and pear tree branches for adult psyllids, as well as adults and nymphs of predatory anthocorids. The time‐series consisted of numbers (per leaf or branch) averaged over preselected pear trees in an orchard and, in addition, over other trees selected along the hedgerows flanking the orchard.2. The fluctuations in populations of adult and juvenile anthocorids showed strong cross‐correlations with those of the eggs and nymphs of pear psyllids, but less correlation with adults of pear psyllids, as expected based on their increased ability to escape from predation. The psyllids always appeared first on the pear trees, resulting in positive phase shifts. The first peak of adult anthocorids on pear trees was always later than the first peak in the hedgerows, and the first peak of nymphal anthocorids on pear trees was always later than the first peak of adults on these pear trees. In each of the 3 years, anthocorids were rarely observed in the pear orchard during the first part of the growing season (April–June), but during the second half of the growing season (July–August) there was a strong numerical response of the anthocorid populations to increasing population densities of pear psyllids.3. These results provide support for the hypothesis that the numerical response of the predators to prey density is caused initially by migration of anthocorids into the pear orchard and then by a reproductive response. The migrants originate from the hedgerows and other trees elsewhere, where they feed on aphids during the first part of the growing season.

The Adaptive Dynamics of Consumer Choice.

This article uses mathematical models to investigate the consequences of noninstantaneous choice between two prey types by a predator. The models are characterized by three features: sustained cycles in predator and prey population sizes, a trade-off between the predator's consumption rates of the two prey and adaptive adjustment of the consumption rates at a rate proportional to the change in predator fitness per unit change in consumption rates. Adaptive adjustment of consumption rates frequently prevents the system from achieving an ideal free distribution of predator foraging effort and frequently results in chaotic dynamics or alternative attractors. The process of switching to consume the more common prey can reduce the minimum density of one or both prey. These phenomena occur for a variety of different models, including models in which both prey are in the same or in distinct habitats. An ideal free distribution can often be attained by adding prey refuges or immigration, provided these have a large enough effect on the dynamics of the system. The dynamics of switching can be important in determining indirect interactions between prey species and other aspects of food web dynamics.

The Adaptive Dynamics of Consumer Choice

This article uses mathematical models to investigate the consequences of noninstantaneous choice between two prey types by a predator. The models are characterized by three features: sustained cycles in predator and prey population sizes, a trade‐off between the predator's consumption rates of the two prey and adaptive adjustment of the consumption rates at a rate proportional to the change in predator fitness per unit change in consumption rates. Adaptive adjustment of consumption rates frequently prevents the system from achieving an ideal free distribution of predator foraging effort and frequently results in chaotic dynamics or alternative attractors. The process of switching to consume the more common prey can reduce the minimum density of one or both prey. These phenomena occur for a variety of different models, including models in which both prey are in the same or in distinct habitats. An ideal free distribution can often be attained by adding prey refuges or immigration, provided these have a large enough effect on the dynamics of the system. The dynamics of switching can be important in determining indirect interactions between prey species and other aspects of food web dynamics.

The Ruffe Population of Loch Lomond, Scotland: Its Introduction, Population Expansion, and Interaction with Native Species

The ruffe ( Gymnocephalus cernuus), never before recorded in Scotland, was discovered in Loch Lomond in 1982. During the 1980s ruffe became well established and expanded its range throughout the loch and through the slow-flowing influent and effluent tributaries, only being excluded from tributaries with high flow rates. Recoveries offish entrained at a pumping station, abstracting water for domestic supply, provide a measure of the rate of population expansion between 1982 and 1996 and the current status of ruffe in Loch Lomond. Between 1982 and 1992 the ruffe population grew exponentially. Since 1992 the population appears to have stabilized to some extent but at a very high level. Gill-netting surveys conducted throughout Loch Lomond have shown that the high abundance of ruffe recorded at this site is representative of a large population throughout the loch. The diet of ruffe feeding in Loch Lomond differed both spatially and seasonally. Although prey choice was diverse, it was primarily composed of benthic macro-invertebrates, but at times included underyearling fish (including ruffe) and eggs of other fish species. The most important of these is the powan ( Coregonus lavaretus), a broadcast spawner of national conservation value. To examine the possibility of dietary overlap with perch ( Perca fluviatilis) and brown trout ( Salmo trutta), the diets of these three species were compared. The data showed very little evidence of overlap in diet, suggesting that feeding resource competition between ruffe and trout and ruffe and perch does not occur between adults of these species. Evidence for the disruption of predator-prey relations by introduced ruffe is reviewed. Ruffe are now the primary prey species for cormorants ( Phalacrocorax carbo), herons ( Ardea cinerea), and northern pike ( Esox lucius) feeding in Loch Lomond. Calculations based on predator population sizes and consumption rates suggest that potentially up to 17 metric tons of ruffe, representing over 2 million individuals, may be being consumed annually by these predator species. The effects on native fish of this shift in diet in these predator species is discussed, as is the ability of ecologists to predict the impact of invasions of new species in aquatic communities.

Timid Consumers: Self-Extinction Due to Adaptive Change in Foraging and Anti-predator Effort

We model the evolution of anti-predator ability in a prey species whose predator has a saturating functional response. Increased anti-predator ability is assumed to require decreased food intake. The anti-predator ability is specified by the value of a continuous trait, whose evolutionary dynamics are determined by the rate of change of fitness as a function of the trait value. This situation can produce a perpetual evolutionary increase in the anti-predator trait, with a concomitant decrease in foraging (or other fitness-enhancing activities); the end result of the process is often extinction of the prey species. This outcome is most likely if the predator population size is maintained by some alternative prey species. Conditions that can promote or counteract this tendency toward self-extinction are discussed.

Benthic Invertebrate Responses to Size and Density Manipulations of the Common Mummichog, Fundulus Heteroclitus, in an Intertidal Salt Marsh

Most studies of the effects of predation on the organization and structure of prey communities have ignored predator population size structure and density. In the present investigation, we designed a series of field experiments to test the effects of these predator population properties on the intertidal benthic invertebrates of a salt marsh. The common mummichog, Fudulus heteroclitus, was the primary predator in our experiments. Three size—classes of mummichogs (small, 40—50) mm Total Length [TL]: medium, 50—69 mm TL; large &gt;70 mm TL) were maintained at each of three densities (1, 2 and 4 fish/m2) in 10—m2 enclosures on the salt marsh. Water—filled pits were included in each cage as low—tide refuges for the fish. Each fish treatment combination was replicated three times. In addition, there were three uncaged 10—m2 plots (open controls) and three caged plots from which Fundulus were excluded (closed controls). Three cylindrical core samples (0.005 m2) were extracted from the sediment to a depth of 10 cm in each of the 33 experimental plots 1, 4, and 10 mo after initiation of the experiment in June 1978. Densities of the small, surface—dwelling invertebrates which make up most of the benthic community (Manayunkia aestuarina, Streblospio benedicti, Leptochelia rapax, ostracods, hydrobiid snails, and Gemma gemma) were highest in enclosures containing large fish, and lowest in closed controls and in enclosures containing small fish. Intermediate invertebrate densities occurred in enclosures containing medium—sized mumichogs. The effects of fish size were enhanced at high fish densities (2—4 fish/m2), although fish density alone had little effect on invertebrate abundance. We considered several hypotheses that could explain the findings. Small epibenthic predators—disturbers like the grass shrimp, Palemonetes pugio, were commonly observed in the experimental enclosures. The gut contents of fish taken from enclosures indicated that the shrimp were ingested only by medium and large size—classes of F. heteroclitus. We suggest that infaunal densities increased in response to the control of small epibenthic predator—disturbers (i.e., P. pugio) by the larger F. heteroclitus.

Prey Species Replacement along a Gradient of Nutrient Enrichment: A Graphical Approach

In this paper I develop a graphical method for visualizing and analyzing the effects of nutrient enrichment on the equilibrium characteristics of 1—predator, many—prey systems. The system considered consists of 1 nonswitching, prey—limited predator feeding on an array of prey species which complete for a single resource. The replacement of 1 prey species by another as eutrophication proceeds is an integral part of this model. The model predicts that prey species which can survive at low nutrient densities should be replaced at higher nutrient densities by species with high maximum rates of increase and by species more resistant to predation. This prediction may be reversed when the predator's population size is controlled by mechanisms which inhibit the predator's ability to respond numerically to increased prey density. Both predictions can be understood in terms of shifts in the relative importances of competition and predation as the nutrient status of a system changes. The graphical method presented should be particularly adaptable to empirical studies of species change along gradients of nutrient enrichment.

Response of predator-prey systems to nutrient enrichment and proportional harvesting

A predator-prey System is modelled by a pair of ordinary differential equations, and the qualitative effects of prey nutrient enrichment and predator harvesting at a rate proportional to the predator population size are studied. Some theoretical analysis concerning the stability of equilibrium points and the existence of stable limit cycles are included. Three models are examined as examples, and for two of them computer simulations are included to illustrate the changes in qualitative behaviour under nutrient enrichment and increase of harvesting effort. The essential difference between this study and our previous work on constant-rate harvesting (Brauer et al. 1976) is that, here, extinction of predators in finite time is impossible although the predator population may tend to zero as l→∞

On chances of survival under predation

The paper presents a simplified version of a prey-predator model: it is assumed that the predators develop according to a birth-and-death process with immigration, independently of the size of predator population. The latter is eliminated at a rate proportional to the size of predator population, while the increases occur only at fixed breeding times, according to a branching process scheme. It is shown that the prey population cannot be stable, in the sense that it has to grow indefinitely or become extinct. The paper gives conditions under which the latter event is bound to occur with probability one.