Showing papers in "Journal of Animal Ecology in 1992"
TL;DR: If theory says that species interactions are a necessary but not sufficient condition for local saturation in ecological time, then the principal direction of control for species richness is from regional to local, which suggests that the key to community structure may lie in extrinsic biogeography rather than in intrinsic local processes.
Abstract: 1. Are local ecological communities ever saturated with species? That is, do they ever reach a point where species from the regional pool are unable to invade the local habitat because of exclusion by resident species? 2. We review the theoretical evidence for saturation in various community models and find that non-interactive models predict the absence of saturation as expected, but that interactive models do not uniformly predict saturation. 3. Instead, models where coexistence is based on niche space heterogeneity predict saturation, whereas those where coexistence is based upon spatial heterogeneity yield mixed predictions. 4. Thus, theory says that species interactions are a necessary but not sufficient condition for local saturation in ecological time. 5. We then argue that unsaturated (Type I) assemblages are likely to be ubiquitous in nature and that even saturated (Type II) assemblages may not show hard limits to richness over evolutionary time-scales. 6. If local richness is not often saturated, then regional richness is freed from local constraint, and other limits on regional richness (which, in turn, limit local richness) become important, including phylogenetic diversification over evolutionary time-scales. 7. Our speculations inevitably suggest that the principal direction of control for species richness is from regional to local. If correct, then the key to community structure may lie in extrinsic biogeography rather than in intrinsic local processes, making community ecology a more historical science.
1,023 citations
709 citations
TL;DR: Theoretical framework for the localization and assessment of resources has been proposed in this paper, where three search mechanisms have been proposed: orientation cues, internal sensory information, and external sensory information.
Abstract: One Introduction.- 1 Theoretical framework.- 1.1 Animal abilities (internal constraints).- 1.2 Resource availability and distribution (external constraints).- 1.3 Efficiency and optimization.- 1.4 Trade-offs: competition and risks.- 1.5 Plasticity in searching behaviour.- 1.6 Summary and conclusions.- Two Information for the Localization and Assessment of Resources.- 2 Orientation cues: information for searching.- 2.1 Orientation based on external sensory information.- 2.2 Orientation based on internally-stored or internally-derived information.- 2.3 Stochastic influences or 'noise'.- 2.4 Summary and conclusions.- 3 Scanning mechanisms.- 3.1 Vision.- 3.2 Mechanoreception.- 3.3 Contact chemoreception and olfaction.- 3.4 Audition.- 3.5 Summary and conclusions.- 4 Initiating factors: when to search.- 4.1 Searching rhythms.- 4.2 Resource stimulus.- 4.3 Physiological state.- 4.4 Summary and conclusions.- 5 Assessment mechanisms: resource, patch and habitat selection.- 5.1 Assessment of resources.- 5.2 Assessment of patches and habitats.- 5.3 Summary and conclusions.- Three Search Mechanisms.- 6 Locating patches and distant resources.- 6.1 Searching when no environmental cues are available.- 6.2 Searching based on environmental cues.- 6.3 Summary and conclusions.- 7 Restricting search to a patch.- 7.1 Looping/spiralling or zigzag motor patterns.- 7.2 Patch-edge recognition.- 7.3 Variable move lengths.- 7.4 Changes in arrival-departure directions.- 7.5 Summary and conclusions.- 8 Foraging in the most profitable patches and leaving when profitability declines.- 8.1 Simple patch-departure mechanisms.- 8.2 Counting and keeping track of time to decide when to leave a patch.- 8.3 Can animals really estimate capture rate?.- 8.4 Summary and conclusions.- 9 When to return to a resource patch.- 9.1 Cropping.- 9.2 Traplining.- 9.3 Leaving patches early.- 9.4 Summary and conclusions.- 10 Learning to forage efficiently.- 10.1 Conditioning.- 10.2 Sampling between patches.- 10.3 Switching between patches conditional on what other individuals are doing.- 10.4 Sampling and spatial memory.- 10.5 More on spatial memory.- 10.6 Summary and conclusions.- 11 Exploratory behaviour.- 11.1 Summary and conclusions.- 12 Central place foraging.- 12.1 Distance travelled and patch choice as related to metabolic costs.- 12.2 Distance travelled as a function of predation risks.- 12.3 Group effects.- 12.4 Summary and conclusions.- Four Sources of Variability.- 13 External environment.- 13.1 Abiotic factors.- 13.2 Biotic environment.- 13.3 Influences of other individuals.- 13.4 Alternatives to searching.- 13.5 Summary and conclusions.- 14 Internal environment.- 14.1 Deprivation: time-dependent effects on searching.- 14.2 Status-dependent changes in searching behaviour.- 14.3 Summary and conclusions.- 15 Genetic factors.- 15.1 Variability in searching traits.- 15.2 Interactions between internal and external environment and genes.- 15.3 Summary and conclusions.- 16 Ontogenetic and maternal influences.- 16.1 Maternal effects.- 16.2 Juvenile behaviours.- 16.3 Early adult experiences.- 16.4 Summary and conclusions.- Five Methodology.- 17 Analysing search tracks.- 17.1 Data collection.- 17.2 Analysis of tracks.- 17.3 Summary and conclusions.- 18 Computer simulations of search behaviour locomotory patterns.- 18.1 Summary and conclusions.- References.
684 citations
TL;DR: In the British biota, turnover at this scale is more the product of range and habitat restriction than of dispersal limitation; and turnover is a relatively minor component of regional diversity, because of the predominance of strong gradients in alpha diversity.
Abstract: 1. We measured beta diversity, or turnover in species composition, in each of 15 taxa (including plants, vertebrates and invertebrates), along two common transects: N-S and W-E arrays of 50 x 50km squares across Britain. Comparing taxa, we asked whether high beta diversity is associated with poor powers of dispersal. Within taxa, we asked whether turnover increases consistently with geographic distance. 2. Beta diversity on this scale was found to be low in all groups. Total (transect) species richness increased by only 3-13% per 50 x 50km square, relative to the average value of local (within-square) richness; or by 0-6-6% per square, relative to the maximum value of local richness. Among taxa, beta diversity showed no tendency to be higher in poorer dispersers. 3. In nearly all taxa, beta diversity as defined by Whittaker (1960) increased linearly with distance on the N-S transect. However, this was shown to be largely the effect of gradients in alpha (local, within-square) diversity. Moreover, distance is highly correlated with environmental (climatic) dissimilarity, providing an alternative explanation for distance effects. 4. We conclude that in the British biota, turnover at this scale is more the product of range and habitat restriction than of dispersal limitation; and that turnover is a relatively minor component of regional diversity, because of the predominance of strong gradients in alpha diversity.
533 citations
TL;DR: The present study suggests that patch removal can by itself favour 'weedy' species, even in the absence of other, concomitant changes.
Abstract: 1. We take a previously studied model for two species one of which is competitively inferior coexisting in a patchy environment, and examine the effects of removing patches (that is, of decreasing the amount of available habitat). 2. Habitat destruction or patch removal reduces the number (and proportion) of patches occupied by the superior competitor, but can result in an increase in the total number of patches occupied by the inferior competitor (even though there are fewer patches in total). 3. It has long been appreciated that disturbance and destruction of habitat can create edge effects and other ecological changes favouring 'weedy' species. The present study suggests that patch removal can by itself favour such species, even in the absence of other, concomitant changes. 4. An implication relevant to conservation biology is that habitat loss can bring about changes in community composition in remaining patches, even if such patches themselves undergo no intrinsic changes whatsoever.
489 citations
TL;DR: The proportion of grouse with high levels of parasite infection increased with the intensity of predator control as measured indirectly through keeper density, and this suggests that predators selectively prey on heavily infected grouse.
Abstract: An extensive post-mortem survey of grouse revealed that birds killed by predators in spring and summer had significantly greater burdens of the caecal nematode Trichostrongylus tenuis than grouse shot during the autumn. Furthermore, grouse that appeared to have died through the effects of parasites carried greater worm burdens than grouse killed by predators. The proportion of grouse with high levels of parasite infection increased with the intensity of predator control as measured indirectly through keeper density. These two empirical observations suggest that predators selectively prey on heavily infected grouse. The interactions between parasites and predators were examined experimentally by reducing the worm burdens of female grouse with an oral anthelmintic
453 citations
TL;DR: Analysis of the population dynamics of red grouse, Lagopus lagopus scoticus and the parasitic nematode, Trichostrongylus tenuis showed that the empirical data collected are consistent with it being the cause of the cycles observed in grouse populations in the North of England.
Abstract: The population dynamics of red grouse, Lagopus lagopus scoticus and the parasitic nematode, Trichostrongylus tenuis were explored to determine whether interactions between the parasite and host were sufficient to generate cycles in grouse abundance. Two alternative models were used that explicitly consider the dynamics of either the free-living, or arrested larval stages of the parasite. Providing that the life expectancy of the free-living larvae is more than 2-4 weeks, the parasite can readily establish in gousse populations
386 citations
TL;DR: A remarkable range of dynamical behaviour is exhibited by a mathematically explicit model with constant host reproductive rate, deterministically unstable local dynamics and dispersing hosts and parasitoids that only move to nearest-neighbour patches in a density-independent way.
Abstract: We consider models for host-parasitoid interactions in spatially patchy environments, where in each generation specified fractions of the host and parasitoid subpopulations in each patch move to adjacent patches. In most previous work of this general kind, the movement is not localized in this way, but involves «global» mixing of the populations prior to dispersal. A remarkable range of dynamical behaviour is exhibited by a mathematically explicit model with constant host reproductive rate, deterministically unstable local dynamics and dispersing hosts and parasitoids that only move to nearest-neighbour patches in a density-independent way
384 citations
TL;DR: Red grouse and the parasitic nematode, Trichostrongylus tenuis, in northern England had an aggregated distribution within the adult grouse population, even though the degree of aggregation was relatively low compared with other parasite systems.
Abstract: Intensive population studies were conducted for 10 years on red grouse (Lagopus lagopus scoticus) and the parasitic nematode, Trichostrongylus tenuis, in northern England. Winter loss was the key factor determining changes in grouse numbers, although breeding losses were also important. T. tenuis had an aggregated distribution within the adult grouse population, even though the degree of aggregation was relatively low compared with other parasite systems
320 citations
TL;DR: Within the distribution area of the freshwater pearl mussel, the maximum observed life span attained in a population varies from 30 to 132 years and the maximum shell length from 80 to 145 mm.
Abstract: Within the distribution area of the freshwater pearl mussel (Margaritifera margaritifera L.), the maximum observed life span attained in a population varies from 30 to 132 years and the maximum shell length from 80 to 145 mm. For both life-history traits, the growth constant k is a major determinant. The growth constant describes the shape of the average individual growth curve, the lower its value, the slower is the asymptote approached. Maximum size and life span increase as the growth constant declines and the maximum age almost equals the time necessary to complete the growth pattern
TL;DR: The data show that it is not correct to equate parental effort to the number of chicks raised, as territories of different quality require different levels of effort for successful reproduction.
Abstract: 1. Oystercatchers that breed on the saltmarsh of the Frisian island of Schiermon-nikoog occupy two different types of territory. In 'resident' territories the nesting area and the feeding area are adjacent. In 'leapfrog' territories the nesting area and the feeding area are separated by distances of 200-500 m. 2. Between 1984 and 1989, residents fledged on average 3.5 times as many chicks per year as leapfrogs. 3. The discrepancy in fledgling production arose primarily through increased mortality of leapfrog chicks due to starvation. This was indicated by low body weights of leapfrog chicks found dead and successful enhancement of leapfrog chick growth through supplementary food. In addition, the presence of a sibling reduced the probability of fledging for leapfrog chicks, but not for resident chicks, providing further evidence that leapfrog chicks competed for a limited food supply. 4. Leapfrog parents fledged fewer chicks, not because of poor feeding conditions in their feeding territory (intake rates of leapfrogs exceeded those of residents), but because they failed to transport a sufficient amount of food to the chicks. To supply their chicks with the same amount of food as did residents, leapfrog parents should have spent c. 4000 s per low water period in transport flights: no leapfrog parent ever reached this level of effort. 5. Our data show that it is not correct to equate parental effort to the number of chicks raised, as territories of different quality require different levels of effort for successful reproduction.
TL;DR: It is concluded that the adaptations Colobus polykomos show to periodic shortages of high energy foods are behavioural rather than physiological, and it is concludedthat the adaptations they show are behaviouralrather than physiological.
Abstract: 1. Colobines are widely described as folivorous and a prediction of low basal metabolic rate appears to be supported by low activity levels, but energy budget estimates based on data from a field study of Colobus polykomos question these assumptions. 2. Annual diet and activity budgets are shown to change seasonally: unripe tree seeds are the preferred food type, with increased time spent feeding during the peak of seed production. Time spent resting is highest during wet seasons, when seeds are not available and only leaves are eaten. 3. The high energetic value of seeds compared to fruit flesh, young and mature leaves, is demonstrated. 4. Energy budgets are estimated for adult males and females (non-reproductive, pregnant and lactating), using daily energy expenditure (DEE) as maximum, and average daily metabolic needs (ADMN) as minimum values. 5. A simple model is employed to demonstrate that a diet of 100% seeds supplies sufficient calories for all estimates of energy intake and expenditure, whereas a diet of 100% leaves is unable to meet DEE and only the highest estimates of feeding rate (grams of leaves min-1) approach ADMN. 6. The advantages of detoxification, and the limitations of energy production for small-bodied fore-gut fermenters are outlined. 7. Evidence that C. polykomos is hypometabolic is considered, and it is concluded that the adaptations they show to periodic shortages of high energy foods are behavioural rather than physiological.
TL;DR: The regional distribution was constrained by the inability of the butterfly to colonize suitable habitat that was further than a fex kilometres from existing metapopulation; some of these sites are now populated by P.argus following successful introductions.
Abstract: Nine metapopulations of the butterfly Plebejus argus were mapped in North Wales in 1983 and 1990. The metapopulations occured in three biotopes; five were in heathland, one in mossland and three in limestone grassland. Within metapopulations, local distributions of the butterfly corresponded to the spatial distributions of suitable habitat : P. argus was able to colonize virtually all suitable habitat patches that were less than 1 km from existing, populated patches. In contrast, the regional distribution was constrained by the inability of the butterfly to colonize suitable habitat that was further than a fex kilometres from existing metapopulation; some of these sites are now populated by P.argus following successful introductions
TL;DR: The extent to which density-related changes in early growth might be responsible for fluctuations in over-winter survival and population size is investigated.
Abstract: In the Soay sheep population of Hirta (St. Kilda), high winter mortality occurs every 3-4 years following summers when population density exceeds 2.2 sheep ha −1 . During these die-offs, more than 50% of adults, 70% of yearlings and 90% of lambs die and population density falls by around 65%. This paper investigates the extent to which density-related changes in early growth might be responsible for fluctuations in over-winter survival and population size
TL;DR: The degree and duration of anorexia in overwintering salmon is not preprogrammed but is regulated by energy reserves, the salmon effectively having a 'defended energy level' below which appetite is increased until lost energy reserves have been restored.
Abstract: 1. Juvenile Atlantic salmon (Salmo salar L.) that do not migrate to sea within the next year spend the winter under stones in the stream bed. In a laboratory study, we show that this coincides with a loss of appetite even in the presence of excess food: an index of appetite dropped by an average of 96% between early September and late December. 2. This period of anorexia leads to the virtual cessation of growth under environmental conditions that would otherwise allow growth to occur. 3. Juvenile salmon normally exhibit a gradual loss of fat reserves during the winter. This depletion was experimentally accelerated by depriving fish of food for 3 weeks. The fish responded by increasing their appetite; appetite was negatively correlated with the estimated energy reserves of individual fish. 4. Those fish previously deprived of food regained their appetite and made up their lost fat within 4 weeks of resuming feeding; their appetite then decreased to the level of control fish. 5. The parallel changes in habitat choice and appetite in autumn may be a consequence of the decreased profitability and increased predation risk when feeding in winter. The degree and duration of anorexia in overwintering salmon is not preprogrammed but is regulated by energy reserves, the salmon effectively having a 'defended energy level' below which appetite is increased until lost energy reserves have been restored.
TL;DR: The nutritional requirements of egg formation in lesser blackbacked gulls Larus fuscus L., are investigated through a series of feeding experiments providing additional food of different quality.
Abstract: The use of supplementary feeding experiments to investigate the relationship between food supply and clutch and egg size in birds has yielded disparate results, some authors showing an advancement in laying date and/or increased clutch size in response to additional food whereas other studies have found no effect. Here we investigate the nutritional requirements of egg formation in lesser blackbacked gulls Larus fuscus L., through a series of feeding experiments providing additional food of different quality
TL;DR: In the moth family Noctuidae, delayed density dependence was slightly but significantly more frequent in woodland populations than in non-woodland populations, consistent with a higher frequency of outbreak species among forest Lepidoptera.
Abstract: 1. The incidence of density dependence was examined in 5715 time-series of annual abundances of 447 species of moths and aphids in the UK using three techniques (Ricker equation, Bulmer's test and Pollard et al.'s test). 2. Delayed density dependence (lag 2 years) was not found more frequently than expected by chance. In the moth family Noctuidae, delayed density dependence was slightly but significantly more frequent in woodland populations than in non-woodland populations, consistent with a higher frequency of outbreak species among forest Lepidoptera. 3. The incidence of significant density dependence increased with the length of the time-series (from 10 to 24 years), and density dependence was detected more frequently in species which did not show a significant trend in abundance over time. 4. The incidence of density dependence was 79% and 88% in moth and aphid time-series which were longer than 20 years (Bulmer's test). 5. There were only slight differences between localities in the average degree of density dependence when the effects of time-series length and trend were removed. 6. Significant differences were found between species in the average degree of density dependence, especially in noctuid moths and aphids. In noctuids, the average degree of density dependence increased with body size.
TL;DR: The influence of ant-attendance on patterns of parasitism was studied for two parasitoid species attacking the black bean aphid Aphis fabae, Trioxys angelicae and Lysiphlebus cardui on the host-plants Evonymus europaeus (L.) and Cirsium arvense (L).
Abstract: The influence of ant-attendance on patterns of parasitism was studied for two parasitoid species attacking the black bean aphid Aphis fabae (Scop.), Trioxys angelicae (Haliday) and Lysiphlebus cardui (Marshall) on the host-plants Evonymus europaeus (L.) and Cirsium arvense (L.). Foraging Trioxys angelicae females were heavily attacked by honeydew-collecting ants. However, ants did not treat Lysiphlebus cardui females in an aggressive way. Ant attendance provided a considerable protection for A.fabae against T.angelicae, but did not prevent a high parasitization success of L.cardui
TL;DR: It is proposed that density-dependent fecundity occurs because, as density increases, proportionally more poor-quality sites (with small clutches) are occupied, and not because the clutch size in all territories decreases.
Abstract: In some studies but not in others the average clutch size decreases with density. We propose that density-dependent fecundity occurs because, as density increases, proportionally more poor-quality sites (with small clutches) are occupied, and not because the clutch size in all territories decreases. This mechanism will only cause density dependence if the habitat is heterogeneous at the scale of the home range or territory of an individual. Density-dependent fecundity will be found in some populations but not in others because of differences in the scale of habitat heterogeneity
TL;DR: The results indicate that resource limitation may occur among mobile detritivorous invertebrates in low-order streams and it is speculated that leaf litter may be limiting as a habitat for shredders as well as a source of food.
Abstract: 1. The retention characteristics of stretches of four low order streams in south-east England were manipulated over a period of 2 years by the addition of small traps which accumulated leaf litter. Unmanipulated control stretches were retained in each stream. Our aim was to test the prediction that the local density of 'shredders', invertebrates which comminute autumn-shed leaves, would increase in response to leaf litter availability. One stream naturally retained a high leaf litter biomass, two had few natural leaf packs and one was intermediate. 2. After manipulation had begun, simultaneous samples of the benthos and leaf litter were taken from control stretches and the traps and intervening stream bed ('intertraps') in the manipulated stretches. 3. The effects of manipulation were slight in the highly retentive site, in terms of both litter standing stock and shredder abundance. The other three sites showed rapid responses, with increased leaf litter and shredder abundance in traps compared with controls. 4. In the least retentive site, manipulation significantly increased the biomass of coarse particulate organic matter in the intertrap samples. In one other site, there was also an increase in shredder density in the intertraps. 5. The changes associated with manipulation were consistent despite the differences in pH among streams, which determined the species assemblage of shredders at each site, but not the pattern of response to increased retention. 6. The results indicate that resource limitation may occur among mobile detritivorous invertebrates in low-order streams. The nature of the limiting resource is discussed; it is speculated that leaf litter may be limiting as a habitat for shredders as well as a source of food.
TL;DR: The hypothesis that wing wear is a proximate factor responsible for an increase in mortality in older workers in foraging bumble bees is supported and provides one functional reason for foragers to adopt a currency that maximizes the ratio of net benefit to cost (i.e. 'efficiency').
Abstract: 1. The insect wing appears to degenerate with use, so that wing wear increases with increased flight activity. Wing degeneration may affect an insect's mortality. 2. The rate of mortality is known to increase with age in worker bumble bees. This study examines whether wing wear can account for mortality in foraging bumble bees. 3. Workers in eight wild-foraging colonies of bumble bee (Bombus melanopygus Nylander) were divided into two treatment groups: clipped (the outer margin of each forewing trimmed, reducing wing surface area by an average of 18%) and unclipped controls. The mortality and behaviour of foraging individuals was assessed with colony watches and night censuses. 4. Treatment did not detectably affect the proportion of foraging bees, the lengths of foraging or within-nest bouts of foragers, or pollen load sizes, but wing-clipping did cause bees with the greatest amounts of initial wing wear to stop foraging. 5. Mortality was positively related to natural wing wear among unclipped bees. In addition, relative to control foragers, foragers with clipped wing margins experienced significantly reduced life expectancies. 6. These results support the hypothesis that wing wear is a proximate factor responsible for an increase in mortality in older workers. The wing-wear hypothesis therefore provides one functional reason for foragers to adopt a currency that maximizes the ratio of net benefit to cost (i.e. 'efficiency').
TL;DR: The most important factor causing breeding failures was farming operations, which accounted for 85% of all nest losses and difference in hatching success between habitats was an effect of different farming practices.
Abstract: Nest site choice and reproductive success of lapwings was studied during 1988-90 at a farmland site dominated by cereal crops in central Sweden. The most important factor causing breeding failures was farming operations, which accounted for 85% of all nest losses. Difference in hatching success between habitats; unsown tillage (9% hatching success), sown tillage (78%), fallow fields (31%) and grassland (67% hatching success) was an effect of different farming practices
TL;DR: It is tested the hypothesis that meadow voles from populations experiencing severe spring declines should exhibit severe stress responses as indicated by high free corticosterone levels.
Abstract: Spring declines are a common feature of small mammal demography. We tested the hypothesis that meadow voles from populations experiencing severe spring declines should exhibit severe stress responses as indicated by high free corticosterone levels. Three populations in southern Ontario were live-trapped intensively in the spring of 1985, all animals were bled each time, and measurements of total corticosterone, corticosterone-binding globulin (CBG), and total androgen (testosterone and dihydrotestosterone) were obtained by radioimmunoassay
TL;DR: The role of lion Panthera leo predation in the dynamics of blue wildebeest Connochaetes taurinus and zebra Equus burchelli populations was investigated through simulation models.
Abstract: The role of lion Panthera leo predation in the dynamics of blue wildebeest Connochaetes taurinus and zebra Equus burchelli populations was investigated through simulation models. The data used in the models were from intensive observations over 4 years in the south-east of the Kruger National Park. Population estimates of wildebeest and zebra were made from aerial surveys, sex and age ratios from ground counts. Lion numbers were determined from observations of marked and radio-collared animals. Predation was studied by following lions for continuous periods of up to 336 h. Two models were constructed
TL;DR: The adaptive value of superparasitism (oviposition in a previously parasitized host) when solitary parasitoids deplete a patch simultaneously is investigated, using a deterministic game theoretical model in which encounter rates with host types change over time due to depletion.
Abstract: 1. The adaptive value of superparasitism (oviposition in a previously parasitized host) when solitary parasitoids deplete a patch simultaneously is investigated, using a deterministic game theoretical model in which encounter rates with host types, which differ in the number of parasitoid eggs they contain, change over time due to depletion. 2. Parasitoids should never superparasitize when they search a patch alone. 3. When parasitoids search in the presence of conspecifics, the evolutionarily stable strategy (ESS) is to reject parasitized hosts at first but later, after a threshold has been reached, to accept them. Different thresholds exist for hosts containing different numbers of eggs. 4. The thresholds are expressed in gross rate of offspring gain. 5. With an increasing number of parasitoids per patch, females should begin to superparasitize at higher rates of gain. 6. The gross rate of gain at which a parasitoid leaves the patch (the leaving threshold) influences the threshold rate at which they should begin to superparasitize. Patch time allocation and host acceptance decisions are interdependent. 7. Both the handling time and the pay-off from an egg laid in a parasitized host strongly influence the threshold rate at which parasitoids should begin to superparasitize. This threshold is only weakly influenced by the number of hosts in the patch. 8. The model predicts that patch time per female and degree of superparasitism are influenced by the number of females in a patch. 9. The total number of offspring gained per unit time in the patch decreases with the number of females, even when the patch area per female and the number of hosts per female are kept constant; thus, superparasitism leads to interference. 10. Experimental evidence is in qualitative agreement with the predictions of the model.
TL;DR: It is argued that pre-consumption cues signalling patch quality are of less use to sheep than the information gained about a patch during grazing, but that this is initially distorted by recent experience; and that information constraints on the maximization of long-term intake rate will occur during frequent switching within heterogeneous vegetation.
Abstract: 1. The proposition that large herbivores make foraging choices which maximize intake rate was examined in two experiments on discrimination by sheep grazing pairs of swards (patches) varying in composition of two botanical components, grass and clover, and varying in sward surface height. 2. Patch selection was influenced by sward height and by clover content, and their effects were additive. Tall patches were preferred, and intermediate contents of clover were selected in preference to low or high contents. This would reinforce the patchy distribution of the grass and clover components of mixed swards. Sheep did not exploit local variation in clover content within patches. 3. There was no evidence that patch choice was influenced by differences in the intake rate obtained from the two alternatives at hand, although on average the highest intake rates were obtained on tall patches with intermediate clover contents, coincident with the pattern of preference. 4. During frequent switching between patches, components of the functional response such as bite depth, bite weight and intake rate were modified by patch composition and also by the composition of the alternate patch. This short-term carry-over effect must increase the discrimination difficulties on heterogeneous vegetation. 5. It is argued that pre-consumption cues signalling patch quality are of less use to sheep than the information gained about a patch during grazing, but that this is initially distorted by recent experience; and that information constraints on the maximization of long-term intake rate will occur during frequent switching within heterogeneous vegetation.