Forest fragmentation relaxes natural nest predation in an Afromontane forest

Nest predation is widely regarded as a major driver underlying the population dynamics of small forest birds. Following forest fragmentation and the subsequent invasion by species from non-forested landscape matrices, shifts in predator communities may increase nest predation near forest edges. However, effects of human-driven habitat change on nest predation have mainly been inferred from studies with artificial nests, despite being regarded as poor surrogates for natural ones. We studied variation in predation rates, and relationships with timing of breeding and characteristics of microhabitats and fragments, on natural white-starred robin Pogonocichla stellata nests during three consecutive breeding seasons (2004–2007) in a Kenyan fragmented cloud forest. More than 70% of all initiated nests were predated during each breeding season. Predation rates nearly quadrupled between the earliest and the latest nests within a single breeding season, increased with distance to the forest edge, and decreased with the edge-to-area ratio of forest fragments. These spatial relationships oppose the traditional perception of edge and fragmentation effects on nest predation, but are in line with results from artificial nest experiments in other East African forests. In case of inverse edge and fragmentation effects on nest predation, such as shown in this study, species that tolerate edges for breeding may be affected positively, rather than negatively, by forest fragmentation, while the opposite can be expected for species restricted to the forest interior. The possibility of inverse edge effects, and its conservation implications, should therefore be taken into account when drafting habitat restoration plans.     

The influence of nest predation on mating strategies under polygyny

The mating status of an advertising male is one important factorfor female assessment. In species where already-mated malestry to attract additional females at some distance from theirmate, mating status may be difficult to assess. Limitationsin the time available to breed and a decrease in reproductivesuccess over the breeding season could make the costs outweighthe benefits of a time-consuming mate assessment process, whichautomatically postpones the onset of breeding. Time and energyconstraints will then limit the behavioral options for femalesin search of breeding opportunities, and females would suffera cost when assessing male mating status. Female choice maythen be better understood by analyzing the behavioral optionsfor females; the success of a strategy in which costly assessmentsare avoided and females choose mates indiscriminately, disregardingmating status ("fast" strategy), could be compared to the successof a strategy in which females carefully try to assess matingstatus ("coy" strategy). Nest predation should be included insuch an analysis because it influences the amount of male assistancefemales of polygynous males receive, as nest predation for onlyone of a polygynous male's two females leads to more male assistancefor the other female. In this study, I compare the outcome ofa "fast" strategy with that of a "coy" strategy under differentrates of nest predation. Although high probabilities of becominga mate of a polygynous male associated with a fitness reductiondecrease the relative success of a "fast" strategy, simulationsshow that high rates of nest predation considerably lower thedifference in the outcome of the two strategies. Therefore,the cost of assessing mating status must be relatively smallto be economical.     

Factors affecting nest predation on forest songbirds in North America

Nest predation is an important factor in the ecology of passerines and can be a large source of mortality for birds. I provide an overview of factors affecting nest predation of passerines in North America with the goal that it may provide some insight into the ecology and management of woodland birds in the United Kingdom. Although several factors influence productivity, nest success is perhaps the most widely measured demographic characteristic of open-cup-nesting birds, and nest predation is usually the largest cause of nest failure. The identity of predator species, and how their importance varies with habitat and landscape factors, must be known for managers and scientists to design effective conservation plans and place research on nest predation in the appropriate context. Recent studies using video surveillance have made significant contributions to our understanding of the relative importance of different predator taxa in North America. Spatial and temporal variation in nest predation can be better understood when landscapes are placed in a biogeographical context and local habitat and nest-site effects are placed in a landscape context. Low productivity resulting from high nest predation is one of several potential causes of bird population declines in North America and the UK. Although the 'forest fragmentation paradigm' from the eastern US may not apply directly to the UK, thinking about avian demographics from a multiscale perspective, and consideration of factors affecting nest predation with knowledge of the dominant predator species, may provide insight into population declines.     

The influence of parent body colouration and nesting habitat on bird nest predation

ABSTRACT

Capsule: Parent body colouration and nesting habitat may influence nest predation risk and, therefore, influence parental decisions on offspring care.

Aims: To investigate the adaptiveness of bird colouration under two different habitat and light conditions.

Methods: We used painted polyurethane dummies to simulate parental presence near nests, where some dummies were red, and others were cryptic brown. We placed artificial nests on trees in open and forested areas, along with two quail eggs and parental dummies.

Results: The results showed major predation on nests of the open area, however, we found no significant effect of parent colour on nest predation.

Conclusion: We suggest that the costs involved with nest site selection may overcome the costs of colouration in species that occur in these environments.     

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     

Frugivorous butterfly species in tropical forest fragments: correlates of vulnerability to extinction

Insects exhibit a variety of population-level responses to forest fragmentation, ranging from population increase to extinction. However, the biological attributes that underlie differences in extinction vulnerability among insects have been little-studied. Using the frugivorous butterfly community of tropical dry forest in Venezuela, we studied body size, population density and colonization ability as attributes that might underlie the range of responses of insects to forest fragmentation. The study was carried out in a set of forest fragments in the reservoir Lago Guri, formed by the damming of the Caroni River in eastern Venezuela. Results show that larger butterfly species were more vulnerable to extinction from habitat fragments than smaller ones. Rarer species were not more vulnerable to extinction, showing that rarity may not be an important correlate of vulnerability to extinction amongst insects. Contrary to expectation, faster-flying species were more and not less vulnerable to extinction from small habitat fragments. We speculate on the possible reasons for the observed patterns in extinction vulnerability using additional observations on behavioural patterns and larval host plant distributions of some of the butterfly species.     

The influence of parental care behaviour on localized nest spacing in smallmouth bass, Micropterus dolomieu

Synopsis I examined the consequence of territorial behaviour on localized nest spacing by male smallmouth bass, Micropterus dolomieu in Jones Bay, Lake Opeongo, Ontario. I compared observed nest spacing to that predicted from (i) territory size and shape measured using sonic tracking procedures; and (ii) 1000 simulated, randomly established nesting stocks. The observed spacing between nests (17.52 ± 1.46 m; mean ± standard error) was not different from that predicted from measurements of territory size and shape (17.60 ± 3.20 m) but was significantly greater than that predicted for randomly established nests (9.53 ± 0.06 m). Parental care-associated territorial behaviour influences the spacing between smallmouth bass nests.     

Artificial nest predation in hedgerows and scrub forest in a human-dominated landscape of central Mexico

Hedgerows as well as other narrow corridors could be valuable habitats for birds in regions of intensive agriculture, however, it is still not clear how successful breeding birds are in different types of hedgerows as compared to birds nesting in their natural habitats. We used artificial nests to examine whether hedgerows were sinks (ecological traps) for birds by comparing rates of predation in two types of hedgerows with different vegetation structure (simple and complex), and in a tract of scrub forest in an agricultural landscape of central Mexico. We determined also the types of predators responsible for egg predation. Ground and elevated nests were baited with one Japanese quail Coturnix japonica egg and one plasticine egg and placed alternately along transects. Significantly, greater predation rates were found in scrub forest and complex hedgerows than in simple hedgerows. Higher predation rates in complex habitats seemed to reflect the higher number of predator types found there. The most important predator types were carnivores followed by rodents, birds, and humans. Carnivores and rodents mainly predated ground nests, whereas birds and humans predated elevated nests. Simple hedgerows in this landscape appeared to offer relatively safe nest sites in terms of predation pressure when compared to more complex habitats (complex hedgerows and scrub forest).     

Changing vulnerability to predation related to season and sex in an African ungulate assemblage

The consequences of predation for prey population dynamics depend on the extent to which this mortality is predisposed by malnutrition or senescence, or additive in the sense that animals that would otherwise not have died at that time were killed. In places lacking effective predators, few adult ungulates die during the summer or wet season months when food is plentifully available. Hence the seasonal distribution of predator kills as well as the age and sex classes of the prey indicates the extent to which malnutrition contributes to mortality as well as other influences on vulnerability. Using records of animal deaths assembled over 35 years in South Africa's Kruger National Park, these patterns were investigated for 12 ungulate species forming the prey of lions, and for three other large predators with respect to one prey species. Buffalo, kudu and giraffe were more strongly represented in kills made during the late dry season, while wildebeest and zebra made relatively greater contributions during the wet season. Impala, waterbuck, warthog and rarer antelope species became more prominent in kills during transitional periods between seasons. Five prey species showed an elevation in representation of males in lion kills during the mating season, as well as impala for all predator species. Females were more prominently represented in kills during the time of late gestation and parturition for three prey species. Hence reproductive activities as well as changing vegetation cover and food resources affected vulnerability to predation. Shifts in susceptibility to predation over the seasonal cycle corresponded with rainfall‐related variation in the annual representation of these ungulate species in lion kills. The availability of vulnerable prey species, age and sex classes at different stages of the seasonal cycle helps maintain a high abundance of lions. These factors contribute to the strong additive impact that predation has had on the abundance of some of these ungulate populations.     

Plant reproductive biology of herbaceous monocots in a Venezuelan tropical cloud forest

The reproductive biology of 29 herbaceous understory monocot species was studied in a tropical cloud forest of northern coastal Venezuela (Henri Pittier National Park). Of the 23 plant species, 6 (26.1%) were self-incompatible and 17 (73.9%) self-compatible, the latter consisting of 12 (52.2%) fully self-compatible and 5 (21.7%) partially self-compatible species. An analysis of sexual systems and temporal variation of sex expression showed that of 29 monocot species, 14 (48.3%) were hermaphrodite and adichogamous, 14 (48.3%) monoecious and dichogamous, and one (3.4%) dioecious. The combination of monoecy and dichogamy prevented autogamy in 11 (91.7%) of the 12 self-compatible and monoecious species; one (8.3%) was autogamous. Of all species, 24 (82.8%) were obligate outbreeders, 2 (6.9%) spontaneously autogamous, and 3 (10.3%) partially autogamous. Ovule and flower abortion were not significantly different between the different groups. Pollen-ovule ratio was significantly different between monoecious and hermaphrodite species. The number of pollen grains and ovules per flower and the pollen-ovule ratio were variable among various pollination types. The average pollen-ovule ratio was higher in beetle-, dipteran-, and wind-pollinated species than in bee-, bird-, and bat-pollinated species. The first three pollination types were restricted to monoecious species, and the second three groups were restricted to hermaphrodite species. Of 33 plant species investigated, 12 (36.4%) displayed vegetative reproduction. The reproductive features of the understory monocot species are discussed in the context of life form, devices of cross-pollination, and geological history of the tropical cloud forest.     

The effect of human observers on the intensity of nest predation

Disused nests containing eggs from aviary birds were used to determine the effect of human observers on the intensity of nest predation. Nest predation was significantly higher for nests visited daily than for nests visited only at the end of a typical incubation period. Care should be taken by nest observers to minimise the number of visits to nests of ground-nesting birds. The average distance of nests from the edge of the supporting plant was greater for unsuccessful than successful nests, implying that the position of nests is an important factor affecting their survival. Predation rates on active nests of White-fronted Chats Ephthianura albifrons visited daily over the same period were similar to those of experimental nests, suggesting that 'artificial' nests may be a useful tool for studying nest predation.     

Brood predation pressure during parental care does not influence parental enzyme activities related to swimming activity in a teleost fish

Predation is considered one of the main costs to reproduction but is rarely examined from a physiological perspective. In particular, little is known about the influence of brood predation pressure on the physiology of parents engaged in care. Brood defense, even when there is no direct threat to the parent, can be costly as it requires constant vigilance and chasing predators to protect the developing brood and maintain parental investment (i.e., fitness). Our goal was to examine the influence of natural variation in nest predation pressure on the physiology of the teleost smallmouth bass Micropterus dolomieu, an animal that provides sole-paternal care for developing offspring. More specifically, we used indicators of anaerobic (lactate dehydrogenase [LDH]) and aerobic capacity (cytochrome c oxidase [CCO] and citrate synthase [CS]) in axial white muscle and pectoral red muscle to test for differences in antipredator performance of nest guarding males across six lakes with natural variation in nest predation pressure. Pectoral red muscle enzyme activities and protein concentrations were highly conserved among populations, while axial white muscle showed differences in LDH activities, CCO activities and protein concentrations. However, there was no evidence for higher metabolic capacities in fish from lakes with increased brood predation pressure. Clearly, factors other than predation pressure have a greater influence on white muscle metabolic capacities. Additional research is needed to clarify the extent to which biotic and abiotic factors influence the enzyme activity and organismal performance in wild animals, particularly at the level of the individual and population.     

Variation in performance of the tree fern Cyathea caracasana (Cyatheaceae) across a successional mosaic in an Andean cloud forest

In Andean forests, Cyathea caracasana grows across a range of successional habitats. This study documents variation in several measures of plant performance (stem growth, leaf production, leaf longevity, and spore production) in C. caracasana growing in open habitat, low-canopy secondary forest, and high-canopy secondary forest, based on 33 mo of observation. In open habitat, C. caracasana displayed significantly higher growth rates, leaf production rates, and leaf turnover than in either of the forested habitats. The highest rates of spore production were also observed in open-habitat individuals, with only one plant in the forest understory producing spores during the study. Despite low growth and no reproduction, I observed no mortality among ferns in the forest understory. These data show that C. caracasana performs best under conditions of full sun but can persist under the closed canopy. This suggests a life history in which periods of rapid growth, spore production, and recruitment in forest gaps alternate with low growth rate and persistence in the understory. A phylogenetic perspective suggests that the habitat flexibility, which might conventionally mark C. caracasana as a habitat generalist, is better interpreted as specialization for the dynamic forest in which it grows.     

The vulnerability of juvenile signal crayfish to perch and eel predation

1. We conducted laboratory experiments to determine how juvenile signal crayfish (Pacifastacus leniusculus) responded to two predators with different foraging behaviours (European eel Anguilla anguilla and European perch Perca fluviatilis), and whether these responses affected their vulnerability. 2. By accelerating a model predator towards crayfish which had been temporarily blinded, we assessed whether crayfish had to see predators in order to escape. Blind crayfish escaped later than did sighted crayfish attacked in the same manner. 3. Both eels and perch elicited similar evasive behaviour when approaching and attacking sighted crayfish. Crayfish usually escaped these initial attacks but perch chased and caught more crayfish than did eels. 4. In a third experiment, perch preyed more rapidly on a population of crayfish than did eels. By the end of the experiment, however, crayfish survival rates were similar in response to eel and perch predation. 5. Our results show that crayfish are well adapted to evade predatory attacks based on rapid responses to both visual and/or mechanical stimuli. The results also suggest that behaviourally, crayfish are more vulnerable to attacks by perch than by eels. We discuss these findings in relation to other factors that affect the vulnerability of crayfish to these predators.     

The effects of soil phosphorus on aboveground biomass are mediated by functional diversity in a tropical cloud forest

Plant and Soil - Soil phosphorus is a key driver of plant biodiversity and aboveground biomass (AGB) in tropical forests. A plant community’s ability to exploit such limiting resources may be...     

Choice of nest site protects army ant colonies from environmental extremes in tropical montane forest

Unlike most social insects, Eciton burchellii army ants cannot thermoregulate through nest construction. Instead, army ants thermoregulate behaviorally by creating a living nest (bivouac), shifting its position and structure, and potentially through nest site selection. We hypothesized that bivouac site selection is critical to E. burchellii colony survival. We predicted elevation above sea level, with associated variation in local abiotic environments, would affect bivouac site selection by E. burchellii colonies. We also expected nest sites to buffer against ambient variation in abiotic conditions. We recorded bivouac site choice by E. burchellii colonies at sites ranging from lowland wet forests to montane forests and reviewed previously published data. We measured microclimatic variables associated with nest sites in high-elevation montane forests: temperature, relative humidity, and light levels. Bivouac site selection varied with elevation: as elevation increased, fewer bivouac sites were exposed, more were underground, and fewer were elevated (in trees). High-elevation bivouac sites moderated diurnal temperature variation and had higher relative humidity levels and lower light levels than ambient conditions. The buffering of ambient temperature and humidity decreased with elevation in montane forests, suggesting that abiotic extremes in bivouac sites at the highest elevations may contribute to the upper elevational range limits of E. burchellii.     

The influence of vigilance on intraguild predation

Theoretical work on intraguild predation suggests that if a top predator and an intermediate predator share prey, the system will be stable only if the intermediate predator is better at exploiting the prey, and the top predator gains significantly from consuming the intermediate predator. In mammalian carnivore systems, however, there are examples of top predator species that attack intermediate predator species, but rarely or never consume the intermediate predator. We suggest that top predators attacking intermediate predators without consuming them may not only reduce competition with the intermediate predators, but may also increase the vigilance of the intermediate predators or alter the vigilance of their shared prey, and that this behavioral response may help to maintain the stability of the system. We examine two models of intraguild predation, one that incorporates prey vigilance, and a second that incorporates intermediate predator vigilance. We find that stable coexistence can occur when the top predator has a very low consumption rate on the intermediate predator, as long as the attack rate on the intermediate predator is relatively large. However, the system is stable when the top predator never consumes the intermediate predator only if the two predators share more than one prey species. If the predators do share two prey species, and those prey are vigilant, increasing top predator attack rates on the intermediate predator reduces competition with the intermediate predator and reduces vigilance by the prey, thereby leading to higher top predator densities. These results suggest that predator and prey behavior may play an important dynamical role in systems with intraguild predation.     

The influence of nest availability on the abundance and diversity of twig-dwelling ants in a Papua New Guinea forest

Tropical ant communities are frequently diverse, but highly patchy in nature. The availability of suitable nest sites may be a regulating force in structuring litter ant communities. Our aim was to examine ant resource utilization in naturally occurring twigs, and to modify the availability of these resources in order to quantify the influence of nest availability on ant communities in a Papua New Guinean forest. First, we compared ant communities that assemble in artificial twigs (drilled, wooden dowels), naturally occurring twigs, and the leaf litter. A total of 55 ant species were captured: 33 from the leaf litter, 29 from naturally occurring twigs, and only 12 from artificial nests. Significantly different communities formed in each of the three nest types. Second, we examined how the density of natural or artificial nest material influenced the ant abundance and species richness. Plots had between 5 and 96 potential nest sites. An average of only 11.2% of these twigs was colonized. Both species richness and the total abundance of adult ants were significantly positively correlated with increasing naturally occurring twig density. Conversely, increasing the availability of artificial nests from 5 to 20 per plot had no significant effect on the proportion of artificial nests colonized, species richness, or the colony size. We observed that ant species richness and abundance increased with natural twig density, at least for naturally occurring communities. But why so many twigs remain vacant and available for ant colonization remains unknown. Other biotic and abiotic factors likely influence the use of nesting habitat in these ant communities.     

Understory spider diversity in two remnants of tropical montane cloud forest in Chiapas, Mexico

We evaluated the spider diversity of a tropical montane cloud forest understory in two nearby sites with different degree of human disturbance at the Biosphere Reserve Volcán Tacaná, Chiapas, Mexico. The study was conducted over a 24 days period distributed in 6 months in 2009, covering dry and rainy seasons. A total of 8,370 spiders (1,208 adults and 7,162 juveniles) were collected. Determined specimens (7,747) represented 112 species and morphospecies, 71 genera and 22 families. The results showed that human disturbance has an influence on spider communities: species richness was significantly higher in the preserved site as regards to the disturbed site. Despite their proximity, the composition of spider communities showed only a moderate similarity between the two sites. No differences in abundance were found among sites when considering the whole sample, but sites differed clearly when seasons were analyzed separately. The rainy season had a negative effect on the abundance of spiders in the preserved site. Although the spider community structure was very similar between sites, there was a trend towards a greater species evenness in the preserved site for the whole sampling period and for the dry season.