Mixed-species flocks are an important component of bird communities, particularly in the Neotropics, where flocks reach their highest diversity. The extent to which mixed-species flocks represent ...unique functional or ecological roles within communities, and how these attributes change over environmental gradients, however, is not well understood. We use a trait-based approach to examine functional aspects of flocking assemblages as they relate to those observed in the larger avian community across a 3000 m elevational gradient. Our results reveal similar ecological strategies among flocking species and the communities in which they occur, at the scale of the regional pool and across elevations. Trait variation in flocking and non-flocking assemblages is structured along two major axes defined by size- and resource-related traits. The trait space occupied by flocking species, however, represents only half (51%) that of the larger community. Similarly, the trait space of flocks across elevations is restricted compared to non-flocking species. The shared trait space across flock types represents small-bodied invertivores foraging in lower forest strata, traits associated with increased vulnerability to predation. The concentration of flocking species in functional trait space suggests high niche packing and either more overlap in ecological strategies or more finely divided niches relative to non-flocking species. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.
Tropical montane species are characterized by narrow elevational distributions. Recent perspectives on mechanisms maintaining these restricted distributions have emphasized abiotic processes, but ...biotic processes may also play a role in their establishment or maintenance. One historically popular hypothesis, especially for birds, is that interspecific competition constrains ranges of closely related species that "replace" each other along elevational gradients. Supporting evidence, however, is based on patterns of occurrence and does not reveal potential mechanisms. We experimentally tested a prediction of this hypothesis in two genera of tropical songbirds,
Catharus
(Turdidae) and
Henicorhina
(Troglodytidae), in which species have nonoverlapping elevational distributions. Using heterospecific playback trials, we found that individuals at replacement zones showed aggressive territorial behavior in response to songs of congeners. As distance from replacement zones increased, aggression toward congener song decreased, suggesting a learned component to interspecific aggression. Additionally, aggressive responses in
Catharus
were asymmetric, indicating interspecific dominance. These results provide experimental evidence consistent with the hypothesis that interspecific competitive interactions restrict ranges of Neotropical birds. Our results also underscore the need to consider biotic processes, such as competition, when predicting how species' ranges will shift with climate change. Asymmetric aggression could be particularly important. For example, if warming in montane landscapes allows upslope range expansion by dominant competitors, then high-elevation subordinate species could be forced into progressively smaller mountaintop habitats, jeopardizing viability of their populations.
A central current debate in community ecology concerns the relative importance of deterministic versus stochastic processes underlying community structure. However, the concept of stochasticity ...presents several profound philosophical, theoretical and empirical challenges, which we address here. The philosophical argument that nothing in nature is truly stochastic can be met with the following operational concept of neutral stochasticity in community ecology: change in the composition of a community (i.e. community dynamics) is neutrally stochastic to the degree that individual demographic events – birth, death, immigration, emigration – which cause such changes occur at random with respect to species identities. Empirical methods for identifying the stochastic component of community dynamics or structure include null models and multivariate statistics on observational species-by-site data (with or without environmental or trait data), and experimental manipulations of 'stochastic' species colonization order or relative densities and frequencies of competing species. We identify the fundamental limitations of each method with respect to its ability to allow inferences about stochastic community processes. Critical future needs include greater precision in articulating the link between results and ecological inferences, a comprehensive theoretical assessment of the interpretation of statistical analyses of observational data, and experiments focusing on community size and on natural variation in species colonization order.
Tropical birds are purported to be longer lived than their temperate counterparts, but it has not been shown whether avian survival rates covary with latitude worldwide. Here, we perform a ...global‐scale meta‐analysis of 949 estimates from 204 studies of avian survival and demonstrate that a latitudinal survival gradient exists in the northern hemisphere, is dampened or absent for southern hemisphere species, and that differences between passerines and nonpasserines largely drive these trends. We also show that while extrinsic factors related to climate were poor predictors of apparent survival compared to latitude alone, the relationship between apparent survival and latitude is strongly mediated by intrinsic traits – large‐bodied species and species with smaller clutch size had the highest apparent survival. Our findings reveal that differences among intrinsic traits and whether species were passerines or nonpasserines surpass latitude and its underlying climatic factors in explaining global patterns of apparent avian survival.
We synthesize data on avian survival rates from around the world to examine the hypothesis of latitudinal gradient in survival and explore whether these patterns can be explained using a combination of environmental climate factors and species intrinsic traits. We find evidence of a negative relationship between survival and latitude, but only in northern hemisphere passerines and birds from South America. These patterns are best accounted for by a considering a combination of species body mass and clutch size.
Tropical mountains contain some of the world's richest animal communities as a result of high turnover of species along elevational gradients. We describe an approach to study the roles of biotic and ...abiotic factors in establishing elevational ranges, and to improve our ability to predict the effects of climate change on these communities. As a framework we use Hutchinson's concept of the fundamental niche (determined by the match between the physical environment and the organism's physiological and biophysical characteristics) and realized niche (the subset of the fundamental niche determined by biotic interactions). Using tropical birds as an example, we propose a method for estimating fundamental niches and discuss five biotic interactions that we expect to influence distributions of tropical montane animals: predation, competition, parasites and pathogens, mutualisms, and habitat associations. The effects of biotic factors on elevational ranges have been studied to some extent, but there is little information on physiological responses of tropical montane animals. It will be necessary to understand all of these ecological constraints in concert to predict current and future elevational ranges and potential threats to montane species. Given the importance of tropical mountains as global biodiversity hotspots, we argue that this area of research requires urgent attention.
A feature of biodiversity is the abundance of curves displayed by organs and organisms. Curvature is a widespread, convergent trait that has important ecological and evolutionary implications. In ...pollination ecology, the curvature of flowers and pollinator mouthparts (e.g., hummingbird bills) along the dorsiventral plane has been associated with specialization, competition, and species coexistence. Six differing methods have historically been used to measure curvature in pollination systems; we provide a solution to this inconsistency by defining curvature using well-established concepts from differential geometry. Intuitively, curvature is the degree to which a line is not straight, but more formally it is the rate at which the tangent of a curve changes direction with respect to arc length. Here, we establish a protocol wherein a line is fitted against landmarks placed on an image of a curved organ or organism, then curvature is computed at many points along the fitted line and the sum taken. The protocol is demonstrated by studying the development of nectar spur curvature in the flowering plant genus Epimedium (Berberidaceae). By clarifying the definition of curvature, our aim is to make the language of comparative morphology more precise and broadly applicable to capture other curved structures in nature.
Summary
Along tropical mountains, species often occupy narrow altitude ranges. Numerous biotic and abiotic factors have been proposed as determinants of altitude occupancy.
We measured several ...aspects of thermal physiology of 215 bird species across a 2·6‐km altitude gradient in the Peruvian Andes. We predicted that highland species would show adaptation to the colder high‐altitude climate and that energy costs of thermoregulation might limit upslope dispersal of lowland natives.
We found reductions in thermal conductance, body temperature and lower critical temperature in highland birds compared with lowland species. These combine to make highland natives more resistant to heat loss.
We did not find convincing evidence that acute thermal limits or energy costs of thermoregulation constrained altitude distributions. Heat‐budget models predicted low‐to‐moderate long‐term costs at native altitudes. Costs increased for lowland natives modelled in the highland climate, but for all but a few species, costs remained within putative expenditure limits.
Although we did not test heat tolerances, we measured all species at temperatures similar to the hottest air temperatures at the lowland site. There was no evidence that high lowland temperatures preclude downslope movements of highland birds.
While thermal tolerances probably do not directly determine altitude occupancy by most species, the additional energy cost of thermoregulation experienced by lowland species moving upslope may trade off against investment in important life‐history components such as breeding, and thereby affect altitude range limits.
A Lay Summary is available for this article.
Lay Summary
Basal metabolism in tropical birds Londoño, Gustavo A.; Chappell, Mark A.; del Rosario Castañeda, María ...
Functional ecology,
March 2015, Letnik:
29, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Summary
Life history varies across latitudes, with the ‘pace of life’ being ‘slower’ in tropical regions. Because life history is coupled to energy metabolism via allocation tradeoffs and links ...between performance capacity and energy use, low metabolic intensity is expected in tropical animals. Low metabolism has been reported for lowland tropical birds, but it is unclear if this is due to ‘slow’ life history or to a warm, stable environment.
We measured basal metabolic rates (BMR) of 253 bird species across a 2·6 km altitude gradient in Peru. We predicted higher BMR at high altitude due to lower temperatures leading to elevated thermoregulatory costs. We also tested for BMR differences between widely separated tropical regions (Peru and Panama), and between tropical‐ and temperate‐breeding birds.
We found no effect of altitude on BMR in Peruvian species and no difference in BMR between Peruvian and Panamanian birds, suggesting that BMR in Neotropical birds is consistent and independent of environmental temperature. In a data set encompassing more than 500 species, tropical birds had significantly lower BMR than temperate‐breeding birds.
In contrast to several recent analyses, we found higher BMR in passerine birds than in non‐passerines, independent of breeding latitude.
Breeding latitude affects BMR, but diversity in avian life history within and between temperate and tropical regions may explain some of the residual variation in BMR after accounting for body mass and breeding latitude. Future studies of links between life history, metabolism and environmental factors might benefit from examining these variables within individual species as well as across broad geographic contrasts.
Lay Summary
Life‐history theory postulates that physiological traits, such as energy metabolism, can be understood in terms of allocation trade‐offs between self‐maintenance and reproduction over an organism's ...life span, and data show that metabolic intensity and survival vary inversely with latitude, with tropical birds exhibiting a “slow” pace of life relative to temperature species. However, tropical regions harbour strong environmental gradients of their own, and it remains to be shown whether similar life‐history trade‐offs between metabolism and longevity are reflected among tropical birds of the same latitude.
We estimated apparent annual survival in 37 species of tropical passerine birds along an elevational gradient (400–3,000 m) in Peru to test whether variation in survival was influenced by basal metabolic rate (BMR; estimated at the same sites), elevation or both factors. We used path analysis to test our prediction that survival would decline as BMR increased, while accounting for the potential direct effects of elevation on survival due to differences in predation pressure or environmental conditions as well as potential indirect effects of elevation on BMR via temperature and the costs of thermoregulation.
Higher BMR in tropical passerine birds predicted lower apparent survival, regardless of the elevation at which species occurred. In addition, elevation had a direct negative effect on apparent survival, perhaps due to harsher abiotic conditions, low site fidelity or both at high elevations.
We provide evidence of a link between metabolic rate and longevity previously undescribed in populations of free‐living birds. Our results illustrate that tropical montane species may be characterized by a unique suite of traits in their pace of life, in which BMR does not differ from lowland birds, but survival does.
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