Different nest predator faunas and nest predation risk on ground and shrub nests at forest ecotones: an experiment and a review

This study examined predator faunas of artificial ground and shrub nests and whether nest predation risk was influenced by nest site, proximity to forest edge, and habitat structure in 38 grassland plots in south-central Sweden. There was a clear separation of predator faunas between shrub and ground nests as identified from marks in plasticine eggs. Corvids accounted for almost all predation on shrub nests whereas mammals mainly depredated ground nests. Nest predation risk was significantly greater for shrub than for ground nests at all distances (i.e. 0, 15 and 30 m) from the forest edge. However, nest predation risk was not significantly related to distance to forest edge, but significantly increased with decreasing distance to the nearest tree. Different corvid species robbed nests at different distances from the forest edge, with jays robbing nests closest to edges. We conclude that the relationship between the predation risk of grassland bird nests and distance to the forest edge mainly depends on the relative importance of different nest predator species and on the structure of the forest edge zone. A review of published articles on artificial shrub and ground nest predation in the temperate zone corroborated the results of our own study, namely that shrub nests experienced higher rates of depredation in open habitats close to the forest edge and that avian predators predominantly robbed shrub nests. Furthermore, the review results showed that predation rates on nests in general are highest <50 m inside the forest and lower in open as well as forest interior habitats (≥50 m from the edge). Received: 16 March 1998 / Accepted: 30 July 1998     

Tree size distributions in an old-growth temperate forest

Despite the wide variation in the structural characteristics in natural forests, tree size distribution show fundamental similarities that suggest general underlying principles. The metabolic ecology theory predicts the number of individual scales as the −2 power of tree diameter. The demographic equilibrium theory predicts tree size distribution starting from the relationship of size distributions with growth and mortality at demographic equilibrium. Several analytic predictions for tree size distributions are derived from the demographic equilibrium theory, based on different growth and mortality functions. In addition, some purely phenomenological functions, such as polynomial function, have been used to describe the tree size distributions. In this paper, we use the metabolic ecology theory, the demographic equilibrium theory and the polynomial function to predict the tree size distribution for both the whole community and each species in an old-growth temperate forest in northeastern China. The results show that metabolic ecology theory predictions for the scaling of tree abundance with diameter were unequivocally rejected in the studied forest. Although these predictions of demographic theory are the best models for most of the species in the temperate forest, the best models for some species ( Tilia amurensis , Quercus mongolica and Fraxinus mandshurica ) are compound curves (i.e. rotated sigmoid curves), best predicted by the polynomial function. Hence, the size distributions of natural forests were unlikely to be invariant and the predictive ability of general models was limited. As a result, developing a more sophisticated theory to predict tree size distributions remains a complex, yet tantalizing, challenge.     

Vertebrate scavenger guild composition and utilization of carrion in an East Asian temperate forest

Scavenging is a common feeding behavior by many species that plays an important role in ecosystem stability and function while also providing ecosystem services. Despite its importance, facultative scavenging on large animal carcasses has generally been overlooked in Asian temperate forest ecosystems. The aim of this study was to determine the composition and feeding behavior of the facultative scavenger guild as it relates to sika deer (Cervus nippon) carcasses in Japanese forests. There are no obligate scavengers or large predators that kill adult ungulates, but humans fill the role of large predators by culling deer for population management. We documented nine vertebrate species scavenging on deer carcasses and found that mammals were more frequent scavengers than birds and also fed for longer durations. This result suggests that there is a facultative scavenger guild composed mainly of mammals in our forest ecosystem and that carcass utilization by birds was restricted to only forest species. Raccoon dogs (Nyctereutes procyonoides) and Asian black bears (Ursus thibetanus) were the most frequent scavenger species and also fed for longer durations than other scavengers. There were significant seasonal differences in scavenging by Asian black bear, Japanese marten (Martes melampus), and mountain hawk‐eagle (Nisaetus nipalensis), suggesting the availability of other food resources may alter facultative scavenging by each species. Our results support that scavenging is widespread in this system and likely has important functions including building links in the food web.     

Juvenile exposure to predator cues induces a larger egg size in fish

When females anticipate a hazardous environment for their offspring, they can increase offspring survival by producing larger young. Early environmental experience determines egg size in different animal taxa. We predicted that a higher perceived predation risk by juveniles would cause an increase in the sizes of eggs that they produce as adults. To test this, we exposed juveniles of the mouthbrooding cichlid Eretmodus cyanostictus in a split-brood experiment either to cues of a natural predator or to a control situation. After maturation, females that had been confronted with predators produced heavier eggs, whereas clutch size itself was not affected by the treatment. This effect cannot be explained by a differential female body size because the predator treatment did not influence growth trajectories. The observed increase of egg mass is likely to be adaptive, as heavier eggs gave rise to larger young and in fish, juvenile predation risk drops sharply with increasing body size. This study provides the first evidence that predator cues perceived by females early in life positively affect egg mass, suggesting that these cues allow her to predict the predation risk for her offspring.     

The predator guild associated withAceria litchii (Acari: Eriophyidae) in Australia and China

Lychees were surveyed in Queensland, Australia and in Guangdong Province and Hainan Island China, for natural enemies of the lychee erinose mite,Aceria litchii (Keifer), one of the most serious pests of lychee in Australia. A guild of seventeen predators, including ten species of phytoseiid mites, was associated with lychee erinose in Queensland. Six other predaceous mite species and a cecidomyiid larva,Arthrocnodax sp. were also part of the complex. Despite the apparent predation of most of the seventeen species recorded in Queensland onA. litchii, the pest continues to cause major problems. In China, whereA. litchii is a relatively minor pest, nine phytoseiid species were collected in lychee orchards. The value of introducing additional predators to Australia, especially from China, is discussed.     

Patterns of predator behaviour and Wood Warbler Phylloscopus sibilatrix nest survival in a primaeval forest

Understanding the foraging behaviour of predators is key to interpreting the role of anti‐predator adaptations of birds in reducing nest losses. Conducting research in primaeval habitats, with a low level of direct human interference, is particularly valuable in the understanding of predator–prey interactions. Using nest cameras, we investigated the identity and behaviour of potential and actual predators appearing at Wood Warbler Phylloscopus sibilatrix nests, and the importance of different predator groups for nest survival, in the primaeval part of Bia?owie?a Forest (Poland). Mammals formed the main predator group (30 of 32 nest depredations), particularly medium‐sized carnivores (24 of 32), which attacked nests more frequently than merely passing by. This contrasted with other species, especially small rodents, which were commonly recorded near nests but rarely attacked them. Most nest attacks (22 of 32) took place at night and nest survival did not depend on nest visibility, indicating a reduced utility of nest concealment in defence against predators using mainly sound or olfaction when hunting. Daily nest survival declined strongly with nest progression (from egg‐laying to fledging of chicks), probably due to increased predator detection of nests containing older and louder chicks, rather than to increasing parental activity at nests during the day. The set of actual nest predators differed from some previous studies in human‐transformed habitats, showing that Wood Warblers may face different threats in modified vs. near‐pristine environments.     

Helpers at the nest compensate for reduced maternal investment in egg size in carrion crows

Life history theory predicts that mothers should trade off current and future reproductive attempts to maximize lifetime fitness. When breeding conditions are favourable, mothers may either increase investment in the eggs to improve the quality of the offspring or save resources for future reproduction as the good raising environment is likely to compensate for a 'bad start'. In cooperatively breeding birds, the presence of helpers improves breeding conditions so that mothers may vary the number, size and quality of the eggs in response to the composition of the group. Here, we show that in cooperatively breeding carrion crows Corvus corone corone, where nonbreeding males are more philopatric and more helpful at the nest than females, breeding females decreased egg size as the number of subordinate males in the group increased. However, despite the smaller investment in egg size, fledglings' weight increased in groups with more male subordinates, improving post-fledging survival and indicating that helpers fully compensated for the initial 'bad start'. These results highlight a 'hidden effect' of helpers that bears profound implications for understanding the ultimate function of helping.     

Spatial heterogeneity in predator activity, nest survivorship, and nest-site selection in two forest thrushes

The ability of prey to find and use predator-free space has far-reaching consequences for their persistence and interactions with their predators. We tested whether nest survivorship of the ground-nesting veery (Catharus fuscescens) and shrub-nesting wood thrush (Hylocichla mustelina) was related to the local absence of a major nest predator, the white-footed mouse (Peromyscus leucopus). Mouse-free space was defined by trap stations that failed to trap a mouse during the avian breeding season (~May through July). In addition, mouse activity was quantified at individual trap stations based on the number of captures during the same period (six 2.25-ha trapping grids, each containing 121 trap stations trapped repeatedly throughout the summer between 1998 and 2002.) Annual mouse-free space was correlated with other measures of mouse activity based on trapping data. Both mouse-free space and activity metrics were significantly related to annual rates of nest predation (i.e., nest daily mortality rate) in veery but not wood thrush. Likewise, mouse-free space and mouse activity within the nest neighborhood (~30×30 m² surrounding each nest) was significantly related to nest survivorship in veery but not wood thrush. More trap stations had consistently greater (hotspots) and lesser (coldspots) mouse activity than expected by chance, and veeries were significantly more likely to nest near stations that had below the grid-average trapping success. Our study thus documented significant spatial variability in predator activity and its relationship to nest predation and nest-site selection in a ground-nesting songbird.     

A null model of guild proportionality,applied to stratification of a New Zealand temperate rain forest

Summary Much ecological theory assumes that the number of species that can coexist (by species packing) is limited, because competitive exclusion occurs when any pair of species within a guild is too similar — species saturation or niche limitation. If such niche limitation occurs, the proportion of species in each guild should be relatively constant — guild proportionality. This concept is applied to the guilds represented by strata in a forest. A method is produced, and used to examine a New Zealand temperate rain forest. Most strata showed no deviation from a null model of no niche limitation, i.e. no tendency to guild proportionality. The proportion of lianes was more variable than in the null model, tending to be inversely related to the proportion of epiphytes, Canopy tree proportion was significantly more constant than in the null model, but this could be interpreted as a limit caused by the size of a canopy tree individual.     

Feeding habits and niche partitioning in a predator guild composed of tigers, leopards and dholes in a temperate ecosystem in central Bhutan

We describe the food habits, niche overlap and prey preferences in a predator guild comprised of tigers Panthera tigris , leopards Panthera pardus and dholes Cuon alpinus in a mountainous region of central Bhutan. Scat analyses revealed that these predators consumed 11 different prey species including livestock and rodents, of which leopards consumed 11. The combined relative occurrence of the three species, sambar Cervus unicolor , muntjac Munticus muntjac and wild pig Sus scrofa , constituted 42.7, 33.7 and 71.1% of the tiger, leopard and dhole diets, respectively, while livestock comprised 44.5, 73.4 and 15.9% of the prey consumed, respectively. Regression equations from earlier feeding trials were used to estimate the relative biomass and the numbers of prey consumed. Results showed that sambar featured more frequently than did muntjac and wild pig in the diets of tiger, leopard and dhole and contributed more relative biomass than did muntjac and wild pig. Sambar, muntjac and wild pig together provided 36.9, 28 and 63.1% of the biomass consumed by tigers, leopards and dholes respectively. All else being equal, there was evidence that all three predators ate livestock less than might have been expected on the basis of the abundance and high biomass of this prey category in the area. There was a high dietary niche overlap between the predators (Pianka's overlap index of 0.58–0.92), with a greater overlap between the two felid species than between the felids and the canid. This study provides evidence of a substantial diet overlap among the three sympatric carnivores, and thus highlights the potential for high intra-guild competition among them, especially given the relatively low density of prey.     

Seed size and seedling emergence in 16 temperate forest herbs and one dwarf-shrub

The impact of seed size (seed mass) on seedling emergence beneath a leaf litter layer and post-dispersal seed predation was investigated in two field experiments including 16 forest herbs and one dwarf-shrub in southeastern Sweden. In the first experiment, I studied the relationship between seed mass of eight forest herbs (0.3–16.7 mg) and seedling emergence after removal of litter and reduction of seed predation (rodents and insects). Removal of litter and reduction of seed predation did not affect seedling emergence. However, regardless of treatment, species with large seeds (> 3 mg) had a higher seedling emergence than those with small seeds (< 1 mg). In the second experiment, I investigated the relationship between seed mass of 12 species (0.007–18.4 mg) and seedling emergence after removal of litter, reduction of seed predation (insects) and seedling herbivory (molluscs). Total emergence over three years was significantly higher in species with large seeds (> 3 mg) than in those with small seeds (< 2 mg). Removal of litter increased total seedling emergence, while application of insecticide and molluscicide had no effect. Similar results were obtained from both a deciduous and a mixed coniferous forest, but seedling emergence was in general higher in the mixed coniferous forest. Seedling emergence in temperate forest herbs and dwarf-shrubs seems to be higher in species with large than in those with small seeds, and it is often enhanced by disturbance.     

Insect guild structure associated with eastern hemlock in the southern Appalachians

An assessment of the insect guild structure associated with immature and mature eastern hemlock, Tsuga canadensis (L.) Carrière, at high and low elevations was made before the invasion by the hemlock woolly adelgid, Adelges tsugae (Annand), and elongate hemlock scale, Fiorinia externa Ferris. Guild dynamics were determined among 243 insect species associated with eastern hemlock in the southern Appalachians. Detritivore, hematophage, herbivore, fungivore, parasitoid, predator, scavenger, and transient guild assignments were made. The herbivore and transient guilds were subdivided into chewers, sapsuckers, flower feeders, pollen feeders, pollen/nectar/sap feeders, and seed feeders. A significant association was found between guilds on immature and mature hemlocks at low elevations, but no significant association was found at high elevations. There was also a strong association of phytophagous insects on immature hemlocks and scavengers on mature hemlocks. The observed transient, scavenger, and predator guilds had a significantly lower co-occurrence among species indicating segregation of the species within these guilds. The herbivore guild had a significantly higher co-occurrence among species than the simulated index indicating aggregation of species within this guild across sites. Detritivore, hematophage, and parasitoid guilds did not differ significantly from the simulated community. These results document a diverse and dynamic insect community on eastern hemlock before invasion by hemlock woolly adelgid and elongate hemlock scale.     

Questioning assumptions of trophic behavior in a broadly ranging marine predator guild

We evaluated whether existing assumptions regarding the trophic ecology of a poorly‐studied predator guild, northwest (NW) Atlantic skates (family: Rajidae), were supported across broad geographic scales. Four hypotheses were tested using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope values as a proxy for foraging behavior: 1) species exhibit ontogenetic shifts in habitat and thus display a shift in ¹³C with differential use of the continental shelf; 2) species exhibit ontogenetic prey shifts (i.e. from smaller to larger prey items) and become enriched in ¹⁵N; 3) individuals acquire energy from spatially confined local resource pools and exhibit limited displacement; and 4) species exhibit similarly sized and highly overlapping trophic niches. We found some evidence for ontogenetic shifts in habitat‐use (δ¹³C) for thorny and little skate and diet (δ¹⁵N) of thorny and winter skate and hypothesize that individuals exhibit gradual trophic niche transition, especially in δ¹⁵N space, rather than a clear and distinct shift in diet throughout ontogeny. Spatial isoscapes generated for little, thorny, and winter skate highlighted distinct spatial patterns in isotopic composition across the coastal shelf. For little and thorny skate, patterns mimicked expected spatial variability in the isotopic composition of phytoplankton/POM, suggesting limited displacement and utilization of spatially confined resource pools. Winter skate, however, exhibited a much narrower range of δ¹³C and δ¹⁵N values, suggesting individuals may use resources from a more confined latitudinal range. Although high total trophic niche overlap was observed between some species (e.g. little and thorny skate), sympatric species (e.g. little and winter skate) exhibited a degree of trophic niche separation. These findings offer new insight into the trophic dynamics of a poorly‐studied, vulnerable group of predators, and highlight a need to re‐examine assumptions pertaining to aspects of their ecology.     

Food use is affected by the experience of nest predation: implications for indirect predator effects on clutch size

Indirect predator effects on prey demography include any effect not attributable to direct killing and can be mediated by perceived predation risk. Though perceived predation risk clearly affects foraging, few studies have yet demonstrated that it can chronically alter food intake to an extent that affects demography. Recent studies have used stable isotopes to gauge such chronic effects. We previously reported an indirect predator effect on the size of subsequent clutches laid by song sparrows (Melospiza melodia). Females that experienced frequent experimental nest predation laid smaller clutches and were in poorer physiological condition compared to females not subject to nest predation. Every female was provided with unlimited supplemental food that had a distinctive ¹³C signature. Here, we report that frequent nest predation females had lower blood δ¹³C values, suggesting that the experience of nest predation caused them to eat less supplemental food. Females that ate less food gained less fat and were in poorer physiological condition, consistent with the effect on food use contributing to the indirect predator effect on clutch size. Tissue δ¹⁵N values corroborated that clutch size was not likely constrained by endogenous resources. Finally, we report that the process of egg production evidently affects egg δ¹³C values, and this may mask the source of nutrients to eggs. Our results indicate that perceived predation risk may impose food limitation on prey even where food is unlimited and such predator-induced food limitation ought to be added to direct killing when considering the total effect of predators on prey numbers.