Migratory Prey Research Articles

Pattern Formation and the Spatial Scale of Interaction between Predators and Their Prey

We study interactions of predators and prey that are characterized by a scale difference in their use of space. Prey are assumed to occupy patches, forming a metapopulation with low migration among patches. Predators are homogeneously distributed over these patches, due to broad-scale foraging behavior or long-range juvenile dispersal. The predator population thus exerts a globally uniform predation pressure on the prey subpopulations. Under these conditions a nonlinear predator functional response depending on local prey density leads to multiple equilibria that can occur for the same parameter values. These equilibria differ in the fraction of prey patches that are (nearly) empty. Equilibria with a larger fraction of empty prey patches are more stable. The system tends to approach equilibria with a sufficiently high number of empty prey patches, so that local and global population dynamics are stable. If unstable dynamics are observed at all, the fluctuations in local prey density exhibit predictable characteristics. Our main conclusion is that a nonlinear functional response of the predator to local prey density can induce the formation of static patterns in prey density and, hence, lead to stable local and global dynamics. It is shown that these results are sufficiently general to carry over to situations in which prey migration between patches does occur or the spatial domain occupied by the prey population is continuous instead of subdivided into patches.

The response of lions and spotted hyaenas to sound playbacks as a technique for estimating population size

Overall, 217 lions (>1 year) Panthera leo (Linnaeus) and 2069 spotted hyaenas (>8 mo) Crocuta crocuta (Erxleben) responded to broadcast vocalizations, call‐ins, during six surveys in the Masai Mara National Reserve, Kenya.Carnivore response varied seasonally and was significantly higher and more uniform when migratory prey were absent. Variation in wildebeest abundance, by affecting food availability, was the only significant predictor of response. Variability in response with overall prey abundance was higher for lions than hyaenas. Lion response occurred within a radius of 2·5 km, was independent of age and sex but dropped virtually to zero whenever lions possessed a carcass at the time of broadcasting.An independent total census recorded 547 lions (all ages) in the Reserve, 9·96% of which were nomads. This proportion increased by 7·94% following the influx of migrants and raised lion (>1 year) density from 0·292 to 0·320 lions/km2. The call‐in estimate of 0·294±0·009 (95% c.l.) lions/km2 practically equalled the total count. The high precision and low bias (−0·224%) of this estimate imply that call‐in surveys are reliable and may be employed to effectively monitor carnivore populations in the long term. Under simple random sampling, covering about 20% of an area would seem adequate to produce reliable density estimates.

Diurnal and tidal vertical migration of pre-settlement King George whiting Sillaginodes punctata in relation to feeding and vertical distribution of prey in a temperate bay

Vertically stratified sampling was undertaken for pre-settlement K n g George whiting Sillaglnodespunctata at 1 site in 1995 and 4 sites in 1996, in Port Phillip Bay, Australia. In 1995, 3 depth strata were sampled: surface, 2.5-3.0 m, and 5.0-5.5 m, in a total water depth of 7 to 8 m. Samphng was conducted on 17 dates and encon~passed all combinations of day and night, and ebb and flood tide. A total of 3, or in one case 4, replicate samples were taken at each depth. On 4 occasions a smaller zooplankton net was deployed at the same time as the ichthyoplankton net. Pre-settlement S. punctata showed 'reverse' &urnal vertical migration, with concentration near the surface during the day and diffusion through the water column at night. A much weaker tidal migration was also detected, with larvae slightly higher in the water column on flood tides. Pre-settlement S. punctata only fed in daylight and zooplankton taxa that were eaten did not show vertical stratification during daytime. In 1996. 4 sites were sampled at a minimum of 10 m depth, and an additional depth stratum, 7.5-8.0 m, was sampled. Smaller numbers of larvae were collected in 1996 compared with 1995. All larvae collected in daytlme were in the surface stratum, whilst at mght larvae were spread through the water column. Although diurnal vertical migration was not a response to prey migrations, daytime ascent of larvae may have occurred to find adequate light levels for visual predation. The generality of tidal migration is unclear, and may have been a result of local factors. Both diurnal and tidal migration would be expected to strongly influence transport of pre-settlement S. punctata to seagrass beds in Port Phillip Bay.

Seasonal Migration of Predators and Prey‐‐A Study of Pythons and Rats in Tropical Australia

Although seasonal migrations of large predatory mammals that follow migrating prey are well documented, no equivalent phenomenon has been described previously in terrestrial reptiles. We surveyed and radio—tracked water pythons (Liasis fuscus) in Fogg Dam and its adjacent floodplain in the wet—dry tropics of northern Australia to document patterns of movement, with particular emphasis on the ways in which the snakes exploit their major prey species, the dusky rat (Rattus colletti). The distribution and abundance of these rodents vary seasonally. During the dry season the rats live in soil crevices in the floodplain, but wet—season flooding forces them to higher ground, primarily to natural levee banks. Python and rat abundances on the floodplain adjacent to Fogg Dam were significantly correlated through time: both reached a maximum during the dry season, and fell dramatically during the wet season. Activity of pythons was centered around Fogg Dam during the dry season, but all of the radio—tracked snakes moved away from this area during the wet season. Most pythons migrated to the vicinity of levee banks on the floodplain up to 12 km away from their dry—season range. By migrating seasonally, water pythons can efficiently utilize a migratory prey species that would otherwise be unavailable for much of the year.

The relationship between food intake and predation risk in migratory caribou and implications to caribou and wolf population dynamics

We examined the hypothesis that spring migration in barren-ground caribou (Rangifer tarandus) enhances access to high quality food, reduces predation risks or both. We related our findings to the hypothesis that one of the consequences of migration is that prey populations cannot be regulated by predation because predators are unable to respond numerically to changes in abundance of migratory prey. In the Northwest Territories, migration to calving grounds by pregnant cows reduced the risk of predation on neonates. Wolf (Canis lupus) densities on calving grounds averaged only 22% of winter range densities because most wolves denned near tree line. The quality and quantity of food that was available to cows that migrated to calving grounds was lower than for bulls and other caribou that lagged far behind the pregnant cows during spring migration. Fecal nitrogen levels were higher in bulls than in cows in late May and early June but there were no differences in mid or late June. Areas occupied by bulls in late May had a greater biomass of live sedges than on the calving ground in early June. It appears that although food in July is abundant and nutritious, insect harassment prevents efficient feeding. Body fat reserves in both sexes declined to almost zero by mid-July, the lowest level of the year. Insect numbers declined in August and body fat levels increased to the highest level of the year by early September. Because the timing of caribou's return to the hunting ranges of tree line denning wolves was related to caribou density, our data were inconsistent with the suggested consequence of migration. Tree line denning by wolves and density-dependent changes in caribou migration suggests a mechanism for population regulation in caribou and wolves. We suggest that the process is as follows; when caribou numbers increase, some density-dependent factor causes range expansion in August (e.g., competition for food) causing caribou to return earlier to the hunting ranges of tree line denning wolves, more denning wolves have access to caribou, wolf pup survival increases and wolf numbers increase. The effect on caribou population growth will depend on the timing and magnitude of the wolf numerical response.

The commuting system of Serengeti spotted hyaenas: how a predator copes with migratory prey. III. Attendance and maternal care

Abstract. In the Serengeti, spotted hyaenas spent 46-62% of the year foraging by commuting between their clan's territory and the nearest herds of migratory herbivores. During the first 12 months of their life, cubs were stationed at the communal den and were dependent on milk from their mother ('denning female'). This study assessed the impact of the commuting system on maternal care in terms of den attendance and commuting effort, and its fitness consequences in terms of cub growth and survival. Duration of absence intervals decreased while frequency of maternal visits and cub growth rates increased with prey abundance. Mothers of singletons attended their cubs at the same rate as mothers of twins. Growth rates of singletons were higher than those of twins at each level of prey abundance. Cubs growing at higher long-term rates had a significantly better chance of survival. When migratory herbivores were absent from the clan's territory, denning females visited the den more often than non-denning females and males because the mean duration of their commuting trips was shorter (3-4 days versus 6-10 days). The higher attendance by denning females required an increase in travelling effort by a factor of 2·6-3·2 above the level of non-denning females and males. Because maternal effort was highest when cub growth was at a minimum, maternal fitness would be maximized by commuting as little as possible.

The commuting system of Serengeti spotted hyaenas: how a predator copes with migratory prey. II. Intrusion pressure and commuters' space use

Abstract. In the Serengeti, Tanzania, spotted hyaenas, Crocuta crocuta Erxleben, regularly left their clan's territory to feed on the nearest migratory herds ('commuting trips'). The mean commuting distance was 40 km. Commuting locations matched the movements of the migratory herds throughout the year. The distribution of commuters was compatible with the hypotheses that commuters (1) minimized contact with resident clans and (2) utilized areas where they were assured of locating migratory herds. The proportion of clan members commuting declined as prey abundance in the clan's territory increased. Territories were visited by non-residents (commuters) throughout the year but the number of foraging commuters increased substantially when large migratory herds entered a territory. During encounters between residents and intruders, residents adjusted their behaviour according to the context of the encounter: residents ignored commuters 'in transit', responded aggressively to commuters at kills, and engaged in prolonged clashes with neighbouring clans. A comparison with the space use systems of other carnivores indicates that the commuting system of spotted hyaenas in the Serengeti is unique. It is hypothesized that the development of the commuting system was contingent on several adaptive behavioural, morphological and physiological specializations.

The commuting system of Serengeti spotted hyaenas: how a predator copes with migratory prey. I. Social organization

Abstract. The social organization and space use of spotted hyaenas, Crocuta crocuta Erxleben, in the Serengeti, Tanzania is described. In contrast to Kruuk (1972, The Spotted Hyena. Chicago: University of Chicago Press), spotted hyaenas in the Serengeti live in large stable groups (clans); median clan size for seven clans was 47 adults and subadults. Clans defended permanent territories that contained the communal den. The size of one well-known territory was 55·5 km 2. Territories contained low densities of resident herbivores (mean 3·3 animals/km 2) throughout the year, but experienced very high densities (mean 219 animals/km 2) of migratory herds, principally wildebeest, Connochaetes taurinus , zebra, Equus burchelli, and Thomson's gazelle, Gazella thomsoni, for only 26% of the year. The most important prey killed inside territories were migratory species. When the migratory herds were away from a clan's territory, clan members regularly left the territory to forage on nearby migratory herds ('commuting trips'). Clan sizes were maintained above the carrying capacity of the territory, estimated from populations of resident herbivores, by year-round exploitation of migratory herbivores. This suggests that, in contrast to many other carnivores, group sizes of Serengeti hyaenas are not limited by resources on the territory and feeding ranges are decoupled from territoriality.

Diel dive depth in penguins in relation to diel Vertical migration of prey: whose dinner by candlelight?

Diel dive depth in penguins in relation to diel Vertical migration of prey: whose dinner by candlelight?

Role of Predatory Fish in Community Dynamics of an Ephemeral Stream

In enclosed runs in a third-order ephemeral stream in west-central Kentucky, the effects of predatory sunfish (Lepomis) on benthic macroinvertebrates, benthic algae, and detritivory were compared (1) before stream intermittence, (2) after stream intermittence when transport was restricted, and (3) between substrata offering differential cover from fish predation. Ambient fish densities had little effect on total macrobenthic densities and processes on lower trophic levels before intermittence. Fish modestly affected macroinvertebrate densities after intermittence, when surface exchange of prey was interrupted by sections of dry stream. Among substrata, fish influenced macroinvertebrates on bedrock, but not on stony, coarse substrata. Densities of two taxa were significantly affected by fish, and this significantly altered the relative abundance of functional feeding groups in enclosures on bedrock by increasing the proportion of invertebrate predators in fish treatments. Macroinvertebrate densities in microhabitats on both substrata were not affected by fish presence. Dense growths of the stalked diatom Cymbella generally covered microhabitats and added structural complexity, particularly to bedrock surfaces. Unobstructed, natural migration of prey in the large (40 m2) fencelike enclosures (versus containers), ample refuge space, and low natural densities of fish were important in minimizing fish effects in enclosures.

Selective predation on mature male Byblis japonicus (amphipoda: Gammaridea) by the barface cardinalfish, Apogon semilineatus

In our study conducted in Shijiki Bay, Japan, in 1983 nocturnal barface cardinalfish (Apogon semilineatus) fed almost exclusively on mature males of the infaunal tube-dwelling gammarid Byblis japonicus. This predation pattern cannot be explained in terms of the sex composition of B. japonicus in the sediment, because mature males were the least abundant members of the population. Rather, this phenomenon may be explained by two alternative factors: diel vertical migration and size of prey. At night many gammarids, including B. japonicus, move up to the near-bottom water, and consequently become available to A. semilineatus, an epibenthos and/ or plankton feeder. Among these gammarids, juvenile B. japonicus and other gammarid species were more abundant than mature males, but were too small for A. semilincatus to prey on. Since mature B. japonicus males were the most abundant form of epibenthic and/or planktonic, large bodied prey present, the diet of A. semilineatus was limited almost entirely to these mature males. The sex ratio difference between B. japonicus individuals in the sediment and those in the near-bottom water indicated that vertical migration in this species has two functions: to assure the dispersal of juveniles and to allow mating contact for mature males.

A Field Test of Resource Depression by Predatory Stonefly Larvae

Experiments were conducted in two Rocky Mountain streams (Gunnison County, Colorado) to determine the context within which predatory stonefly larvae locally depress their prey populations. In feeding trials where no migration of prey (Baetis bicaudatus, Ephemeroptera) was allowed from flow-through boxes containing natural substrates, Megarcys signata and Kogotus modestus (Perlodidae) significantly depressed prey densities over 14 and 38 h, respectively. However, increasing predator density had no apparent effect on per capita predation rate. In 1984 enclosure experiments allowing prey but not predator migration, Kogotus significantly reduced total prey density and that of two relatively sedentary prey species (Cinygmula mimus, Ephemeroptera, Heptageniidae, and Zapada haysi, Plecoptera, Nemouridae) during three-day trials

Co-evolutionarily stable community structure in a patchy environment

Co-evolutionarily stable community structures are studied when predators search for several species of prey distributed in a number of patches. Prey may migrate between the patches. In a co-evolutionarily stable community (CSC) with two trophic levels, (1) if a predator species feeds on a prey species in some patch, then the predator should in fact exploit the prey in any patch that contains the prey and are utilized by the predator. If a CSC is at a steady state, then (2) the number of predator species which utilize a particular patch does not exceed the number of prey species in the patch. A simpler model, in which only one species of prey lives in each patch, ignoring prey migration and handling times of predators, is analyzed to investigate dynamical properties. At a steady state in a CSC of this system, (3) all prey species can co-exist. Moreover, (4) no two predator species can attack the same prey species, and (5) the steady state in a CSC is always globally stable. Also, in a case when a prey-predator system does not have a stable steady state, the dynamic structure of a co-evolutionarily stable community is discussed. In a non-equilibrium case, (6) the number of predator species which simultaneously utilize the common two patches does not exceed the sum of the numbers of prey species in the two patches.

Do Stonefly Predators Influence Benthic Distributions in Streams?

Experimental manipulations were conducted within the substrate of a Wisconsin stream and a Colorado stream to measure the effect of stonefly predators on the distribution of benthic invertebrates. Screen cages containing free predators, predators restricted from foraging, or no predators, allowed prey migration but no predator migration over 3—d periods. The presence of Acroneuria lycorias (Perlidae) in the Wisconsin stream significantly depressed the establishment of prey populations within cage microhabitats. Mechanisms for reduction were consumption of prey by the stonefly, and predator—avoidance by prey using contact and non—contact cues. The presence of Mergarcys signata (Perlodidae) reduced prey colonization in the Colorado stream by the same mechanisms, but restricted predators produced less consistent effects. This results could be due to colonization of cages by prey that could not detect predators without contact. Pternarcella badia (Pteronarcidae), a large stonefly detritivore that takes occasional prey, did not affect colonization of Colorado stream cages by prey. This differential response by prey to two moprphologically similar, but functionally different, stonefly species suggests that predator avoidance was not purely tactile. Chemotactile and non—contact chemical cues are possible mechanisms by which prey differentiated these stoneflies. The presence of A. lycorias and M. signata in experimental cages significantly increased the attrition of mayfly prey, compared to that from cages with no stonefly or a restricted stonefly in each stream. This result suggests that predation and avoidance by prey of contact with foraging predators were responsible for the higher disappearance of mayflies from cages. Free P. badia had a similar effect, probably due to tactile avoidance of this large detritivorous stonefly by some prey in the Colorado stream.