Mesopredators constrain a top predator: competitive release of ravens after culling crows

Although predator control programmes rarely consider complex competitive interactions among predators, it is becoming clear that removal of larger ‘superior’ competitors often releases the ‘inferior’ ones and can precipitate trophic cascades. In contrast, our study indicates that culling hooded crows Corvus cornix appears to release a larger competitor, the common raven Corvus corax. Ravens ranged more widely, and the predation of artificial nests was significantly faster (although total predation was similar), after the removal of crows. Our study provides evidence of a novel reversal of competitive release where a larger species was freed from constraints imposed on its distribution and behaviour by a smaller species, and emphasizes the importance of considering community and ecosystem effects of predator manipulations when undertaken for conservation or game management.     

Processes organizing open-nesting bird assemblages: competition or nest predation?

Summary The addition of nest predation as a major process to current theories of space utilization and coexistence of open-nesting bird species adds predictive power to hypotheses of resource partitioning and organization of species assemblages. Nest predation can influence the organization of assemblages if predators respond to nests in a density-dependent manner and if predators specialize on nest types. Evidence shows that nest predation is commonly density-dependent and that predators can specialize on nest types. Consequently, nest predation can select for coexistence of bird species that nest at different heights and in different microhabitats (i.e. partitioning of nesting space) to minimize density-dependent responses of predators to the accumulating densities of species within similar nest sites. I establish a series of predicted patterns (1) to test whether predation is operating to influence partitioning of space and coexistence of species, (2) to distinguish effects of nest predation from competition, and (3) to determine the mechanism by which nest predation acts to organize assemblages. Using published and unpublished data to test the predictions, nest predation is seen as a process that we can no longer afford to ignore.     

Can nest predation and predator type explain variation in dispersal of adult birds during the breeding season ?

Many types of predators depredate bird nests and thus potentiallyinfluence the spatial distribution of their prey. We used asimulation model of a double-brooded songbird's nesting seasonto test three predictions about the selective advantage ofdispersing different distances after nest predation by predatorswith varying home range sizes. Our results supported the predictions that (1) dispersing birds had higher success than nondispersingbirds after predation of the first nest, (2) dispersing beyondthe home range of the nest predator increased the success ofthe second nest, and (3) birds whose first nests were depredatedearly in the nesting cycle did better by dispersing fartherthan birds whose nests were depredated later in the nestingcycle. Our results provide evidence that predation and predatorcharacteristics may cause variation in adult dispersal distancesduring the breeding season. However, we did not find an advantagefor long-distance dispersal when predators with small- or medium-sizedhome ranges were responsible for the predation event. The criticaldecisions of dispersal and dispersal distance made by adultbirds are complex, but our model demonstrates that predationevents can create a selective advantage to disperse.     

Identification of 24 polymorphic microsatellite markers for the double-crested cormorant (Phalacrocorax auritus)

Twenty-four polymorphic microsatellite markers were developed for the double-crested cormorant (Phalacrocorax auritus). The number of alleles ranged from two to 13 and observed heterozygosities ranged from 0.032 to 0.871. The use of these loci should enable researchers and biologists to learn more about the population structure and ecology of this species.     

Interference competition between coyotes and raccoons: a test of the mesopredator release hypothesis

Some predator species appear to conform to the mesopredatorrelease hypothesis (MRH), in which larger predators help limitpopulations of smaller predators. This hypothesis has been usedto explain the possible relationship between coyotes, mesopredators,and resultant cascades involving nonpredators. However, relationshipsbetween coyotes and noncanid mesopredators are poorly understood,and predictions from the MRH have rarely been rigorously tested.We monitored sympatric raccoon and coyote populations to assess2 predictions derived from the MRH: coyote predation is an importantcause of mortality in raccoon populations or raccoons avoidareas used by coyotes. Between March 2000 and September 2001,we recorded 3553 locations for 27 radio-collared raccoons and1393 locations for 13 coyotes captured on the Max McGraw WildlifeFoundation in Illinois, USA. No raccoon mortality from coyotepredation was observed during the study, and raccoon survivalwas >0.7 each season. All raccoon 95% home ranges exhibitedoverlap with 95% coyote home ranges in each season. The meanproportion of raccoon locations within 95% coyote home rangesdid not vary by sex but did vary by season. Raccoon overlapof coyote core areas varied considerably among individuals withinseasons, ranging from 0% to 83%. However, 45% of raccoons had<10% overlap with coyote core areas, whereas only 14% ofraccoons exhibited >50% overlap. Mean overlap with core areasdid not vary by season or sex. For those raccoons with homeranges overlapping coyote core areas, mean proportion of observedraccoon locations within coyote core areas was generally greaterthan the mean proportion of random locations. Scent-stationexperiments failed to document raccoon avoidance of specificsites that had been marked with coyote urine. We did not findsupport for a mortality prediction or avoidance prediction tosupport MRH with regard to raccoons and coyotes. These resultssuggest that relationships among mammalian predators may notbe simply dictated by body size, particularly for species outsidethe Canidae.     

Does lethal control of top‐predators release mesopredators? A re‐evaluation of three Australian case studies

Top‐predators can sometimes be important for structuring fauna assemblages in terrestrial ecosystems. Through a complex trophic cascade, the lethal control of top‐predators has been predicted to elicit positive population responses from mesopredators that may in turn increase predation pressure on prey species of concern. In support of this hypothesis, many relevant research papers, opinion pieces and literature reviews identify three particular case studies as supporting evidence for top‐predator control‐induced release of mesopredators in Australia. However, many fundamental details essential for supporting this hypothesis are missing from these case studies, which were each designed to investigate alternative aims. Here, we re‐evaluate the strength of evidence for top‐predator control‐induced mesopredator release from these three studies after comprehensive analyses of associated unpublished correlative and experimental data. Circumstantial evidence alluded to mesopredator releases of either the European Red Fox (Vulpes vulpes) or feral Cat (Felis catus) coinciding with Dingo (Canis lupus dingo) control in each case. Importantly, however, substantial limitations in predator population sampling techniques and/or experimental designs preclude strong assertions about the effect of lethal control on mesopredator populations from these studies. In all cases, multiple confounding factors and plausible alternative explanations for observed changes in predator populations exist. In accord with several critical reviews and a growing body of demonstrated experimental evidence on the subject, we conclude that there is an absence of reliable evidence for top‐predator control‐induced mesopredator release from these three case studies. Well‐designed and executed studies are critical for investigating potential top‐predator control‐induced mesopredator release.