Variability between Scales: Predictors of Nomadism in Birds of an Australian Mediterranean-climate Ecosystem

Nomadism in animals is a response to resource distributions that are highly variable in time and space. Using the avian fauna of the Mediterranean-climate region of southcentral Australia, we tested a number of variables to determine if they predicted nomadism. These variables were species body mass, the distance in body mass terms to the edge of a body mass aggregation, and diet (for example, seeds, invertebrates, nectar, or plants). We utilized two different classifications of the avifauna that diverged in their definition of nomadic to build two different predictive models. Using both classifications, distance to the edge of a body mass aggregation was found to be a significant predictor of nomadism. There was also evidence that nomads tend to feed on nectar and tend to be large. The significance of the variables body mass and diet (nectar) may reflect the greater energy requirements of large birds and the inherent variability of nectar as a food source. The significance of the variable distance to the edge of a body mass aggregation provides further evidence of inherent variability in resources between domains of scale. The edges of body mass aggregations are hypothesized to be areas of increased resource variability that reflect the transition from one scale of landscape pattern to another. Received 14 November 2000; accepted 27 June 2001.     

Predictors of Nomadism in Australian Birds: A Reanalysis of Allen and Saunders (2002)

In recent analysis of factors related to nomadism in birds of semi-arid southern Australia, Allen and Saunders concluded that nomadism was related particularly to proximity to natural breaks in an ordered sequence of body weights across species, with less important relationships with weight per se and diet. That analysis is flawed by incorrect dietary codings for more than half of the species considered. This paper corrects those errors and repeats the analysis. In contrast to the previous findings, nomadism is best predicted by diet, being by far most prevalent among nectarivorous birds and least prevalent among insectivores. Such a finding is consistent with previous studies of nomadism in Australian birds. The variable of proximity to a natural break in the weight distribution of species in this community, considered by Allen and Saunders to be closely associated with nomadism, is not significantly related to nomadism for six of the eight combinations of data considered, and it is far less important than diet for the two combinations in which it does have a weakly significant (P = 0.049, 0.034) relationship with nomadism.     

Predictors of Introduction Success in the South Florida Avifauna

Biological invasions are an increasing global challenge, for which single-species studies and analyses focused on testing single hypotheses of causation in isolation are unlikely to provide much additional insight. Species interact with other species to create communities, which derive from species interactions and from the interactions of species with the scale specific elements of the landscape that provide suitable habitat and exploitable resources. I used logistic regression analysis to sort among potential intrinsic, community and landscape variables that theoretically influence introduction success. I utilized the avian fauna of the Everglades of South Florida, and the variables body mass, distance to nearest neighbor (in terms of body mass), year of introduction, presence of congeners, guild membership, continent of origin, distribution in a body mass aggregation or gap, and distance to body-mass aggregation edge (in terms of body mass). Two variables were significant predictors of introduction success. Introduced avian species whose body mass placed them nearer to a body-mass aggregation edge and further from their neighbor were more likely to become successfully established. This suggests that community interactions, and community level phenomena, may be better understood by explicitly incorporating scale.     

Response to commentary by Woinarski (Critical‐weight‐range marsupials in northern Australia are declining: a commentary on Fisher et al. (2014) ‘The current decline of tropical marsupials in Australia: is history repeating?’)

The recent commentary by Woinarski (2014, Global Ecology and Biogeography, doi: 10.1111/geb.12165) disagreed with our conclusions on the correlates of decline in the marsupials of tropical Australia (Fisher et al., 2014, Global Ecology and Biogeography, 23 , 181–190). We compared traits of species that were associated with range decline in southern and northern Australia. We found that habitat structure, climate and body size were correlated with range decline. In the north, declines of marsupials were most severe in savanna with moderate rainfall. In the south, the ranges of species in open habitat with very low rainfall have declined most. Also, the association between range decline and body mass differed between north and south: this is the main concern of Woinarski, who further disagreed with our choice of the Tropic of Capricorn as a boundary between north and south, our omission of rodents, how to treat timing of extinctions, and our inference that cats are major drivers of decline. We address these concerns in this response.     

A general modeling framework for describing spatially structured population dynamics

Variation in movement across time and space fundamentally shapes the abundance and distribution of populations. Although a variety of approaches model structured population dynamics, they are limited to specific types of spatially structured populations and lack a unifying framework. Here, we propose a unified network‐based framework sufficiently novel in its flexibility to capture a wide variety of spatiotemporal processes including metapopulations and a range of migratory patterns. It can accommodate different kinds of age structures, forms of population growth, dispersal, nomadism and migration, and alternative life‐history strategies. Our objective was to link three general elements common to all spatially structured populations (space, time and movement) under a single mathematical framework. To do this, we adopt a network modeling approach. The spatial structure of a population is represented by a weighted and directed network. Each node and each edge has a set of attributes which vary through time. The dynamics of our network‐based population is modeled with discrete time steps. Using both theoretical and real‐world examples, we show how common elements recur across species with disparate movement strategies and how they can be combined under a unified mathematical framework. We illustrate how metapopulations, various migratory patterns, and nomadism can be represented with this modeling approach. We also apply our network‐based framework to four organisms spanning a wide range of life histories, movement patterns, and carrying capacities. General computer code to implement our framework is provided, which can be applied to almost any spatially structured population. This framework contributes to our theoretical understanding of population dynamics and has practical management applications, including understanding the impact of perturbations on population size, distribution, and movement patterns. By working within a common framework, there is less chance that comparative analyses are colored by model details rather than general principles.     

Inferring the potentially complex genetic architectures of adaptation,sexual dimorphism and genotype by environment interactions by partitioning of mean phenotypes

Genetic architecture fundamentally affects the way that traits evolve. However, the mapping of genotype to phenotype includes complex interactions with the environment or even the sex of an organism that can modulate the expressed phenotype. Line‐cross analysis is a powerful quantitative genetics method to infer genetic architecture by analysing the mean phenotype value of two diverged strains and a series of subsequent crosses and backcrosses. However, it has been difficult to account for complex interactions with the environment or sex within this framework. We have developed extensions to line‐cross analysis that allow for gene by environment and gene by sex interactions. Using extensive simulation studies and reanalysis of empirical data, we show that our approach can account for both unintended environmental variation when crosses cannot be reared in a common garden and can be used to test for the presence of gene by environment or gene by sex interactions. In analyses that fail to account for environmental variation between crosses, we find that line‐cross analysis has low power and high false‐positive rates. However, we illustrate that accounting for environmental variation allows for the inference of adaptive divergence, and that accounting for sex differences in phenotypes allows practitioners to infer the genetic architecture of sexual dimorphism.     

Variation in diet and ranging of black and white colobus monkeys in Kibale National Park,Uganda

Recently, considerable intraspecific variation in the diets and ranging behavior of colobine monkeys has been described, although in most cases this has involved documenting variation between, not within, sites. Some African colobines, such as guerezas (Colobus guereza), are relatively abundant in disturbed habitats that are very heterogeneous, raising the intriguing possibility that even groups with overlapping home ranges may exhibit large behavioral differences. If such differences occur, it will be important to understand what temporal and spatial scales adequately portray a species’ or population’s diet and ranging behavior. This study documents within-site variation in the diet and ranging behavior of guerezas in the habitat types in which they are described to be most successful—forest edge and regenerating forest. We collected data on eight groups of guerezas with overlapping home ranges for 3–5 months each in Kibale National Park, Uganda. The guerezas were highly folivorous, with leaves constituting 78.5–94.0% of the groups’ diets. The percentage of mature leaves and fruit in the diet varied widely among and within groups. We show that differences among groups in the intensity with which they fed on specific tree species were not just related to phenology, but also to differences in the forest compositions of groups’ core areas. Range size estimates varied more than fivefold among groups and the minimum distance from groups’ core areas to eucalyptus forest (which all groups regularly fed in) was a better predictor of range size than was group size. These results reveal considerable variation in the diet and ranging behavior among groups with overlapping ranges and have implications for comparative studies, investigations of within- and between-group feeding competition, and the potential for populations to adapt to anthropogenic or natural environmental change.     

Do second-generation Asian Americans face a systematic disadvantage in occupational attainment? Comment on Tran,Lee and Huang

ABSTRACT

We critically review and discuss several methodological problems with their analysis. Their definition of the occupational hierarchy is unclear, and their inadequate statistical models exacerbate the vagueness of their results. They ignore gender interactions, and the age range of their target population seems too narrow for their research concerns. Their interpretation of their own statistical findings on occupational attainment is fundamentally flawed although they refuse to acknowledge it. Their reported results cannot be replicated based on the information that they are willing to reveal about their analysis which suggests additional, undisclosed errors. In our reanalysis of their data using more appropriate methods, we find that rather than being systematically disadvantaged, the occupational attainment of most second-generation Asian Americans has either achieved parity with whites or has exceeded them after controlling for age, educational level, and survey year.     

Weather, not climate, defines distributions of vagile bird species

Background

Accurate predictions of species distributions are essential for climate change impact assessments. However the standard practice of using long-term climate averages to train species distribution models might mute important temporal patterns of species distribution. The benefit of using temporally explicit weather and distribution data has not been assessed. We hypothesized that short-term weather associated with the time a species was recorded should be superior to long-term climate measures for predicting distributions of mobile species.

Methodology

We tested our hypothesis by generating distribution models for 157 bird species found in Australian tropical savannas (ATS) using modelling algorithm Maxent. The variable weather of the ATS supports a bird assemblage with variable movement patterns and a high incidence of nomadism. We developed “weather” models by relating climatic variables (mean temperature, rainfall, rainfall seasonality and temperature seasonality) from the three month, six month and one year period preceding each bird record over a 58 year period (1950–2008). These weather models were compared against models built using long-term (30 year) averages of the same climatic variables.

Conclusions

Weather models consistently achieved higher model scores than climate models, particularly for wide-ranging, nomadic and desert species. Climate models predicted larger range areas for species, whereas weather models quantified fluctuations in habitat suitability across months, seasons and years. Models based on long-term climate averages over-estimate availability of suitable habitat and species'' climatic tolerances, masking species potential vulnerability to climate change. Our results demonstrate that dynamic approaches to distribution modelling, such as incorporating organism-appropriate temporal scales, improves understanding of species distributions.     

A trade-off between female lifespan and larval diet breadth at the interspecific level in Lepidoptera

A prediction arising from several evolutionary diet breadth models is that, in insect herbivores whose adults practise adaptive host plant selection based on larval performance, female adult lifespan should be negatively correlated with larval diet breadth. In one category of models, female adult lifespan drives evolutionary changes in larval diet breadth; in the other category, larval diet breadth drives evolutionary changes in female adult lifespan. Applying the method of independent contrasts to a biologically and phylogenetically diverse array of Lepidoptera, we ask whether larval diet breadth—as measured by the number of larval food plant species reported in the literature—is negatively correlated with female adult lifespan at the interspecific level. We show that these two life history variables are indeed inversely related. Next, we relax the assumption, common to all of the models, that the female adult is the life stage responsible for the distribution of progeny among different host plants. By introducing into our data set three species whose females are incapable of flight (due to either aptery or brachyptery), and whose larvae are the dispersive stage, the negative correlation between female adult lifespan and larval diet breadth is lost, when using the independent contrasts method. We interpret this effect as supporting the models’ common prediction. Ours is the first reported evidence of a lifespan/diet breadth trade-off at the interspecific level among insects, and it confirms the findings of a previous study in which the degree of habitat specialisation among arthropods was inversely related to proxy measures of the degree of search time constraint. In one of our “diet breadth drives changes in lifespan” models, the females’ type of egg maturation strategy (as measured by the ovigeny index) is predicted to be positively correlated with larval diet breadth, and it mediates a female adult lifespan/larval diet breadth trade-off; however, we found no convincing support for such a role.     

Merged trees in second-growth, fire-origin forests in Patagonia, Chile: positive spatial association patterns and their ecological implications

?Premise of the Study: Negative density-dependent processes have been thought to be the primary cause of shifting spatial patterns of tree populations through time. The existence of adult tree clusters might challenge this classical prediction. Here, we document the prevalence of merged stems (clustering of mature trees leading to stem fusion) in second-growth forests of Nothofagus pumilio and hypothesize that it is nonrandom but predictable in space. ?Methods: We stem-mapped nine sites in second-growth edge and interior forests of fire origin and in mature forests of N. pumilio (>3500 trees) in central Patagonia, Chile. The spatial structure of stand-level and individual-level features was estimated with spatial analyses (pair-correlation function and nearest-neighbor distances). ?Key Results: Multistemmed trees were merged clusters of separate individuals. Merged trees were predominantly found at the edge of the second-growth forests. We found strong clustering (≤5 m) at forest edge sites and none at interior sites. Nearest-neighbor distance distributions were unimodal for unmerged trees and monotonically decreasing for merged trees; interstem distances were much smaller at the edge sites than at the interior sites. ?Conclusions: The occurrence of merged trees at the forest edge, and the resulting high spatial aggregation of stems, is consistent with the hypothesis that establishment was probably aggregated. The spatial pattern found at the forest edge changes the standard spatial pattern sequence through time in temperate forests, altering traditional forest-stand-dynamics models.     

Neophobia and Dietary Conservatism:Two Distinct Processes?

Birds show distinct hesitation when approaching novel food and this has been termed ‘neophobia’. In laboratory-held birds like domestic chicks this effect lasts for a matter of a few minutes at most, but hesitant attack of novel foods can last for weeks or even months in wild birds. This effect, called ‘dietary conservatism’, seems to be a different type of learning process from neophobia as first described and has hitherto been largely overlooked. This paper presents some evidence for the view that the processes may be fundamentally different. We outline results from laboratory chicks that show neophobia to be easily deactivated by experience, which renders it unlikely to be an important force in wild birds. We also report evidence that the process of incorporation of novel food into the diet is not a simple one-stage process but includes at least four steps of assessment. The paper concludes with an outline of the importance of dietary conservatism in our understanding of the evolution of aposematism and the workings of mimicry. This revised version was published online in July 2006 with corrections to the Cover Date.     

How structure determines correlations in neuronal networks

Networks are becoming a ubiquitous metaphor for the understanding of complex biological systems, spanning the range between molecular signalling pathways, neural networks in the brain, and interacting species in a food web. In many models, we face an intricate interplay between the topology of the network and the dynamics of the system, which is generally very hard to disentangle. A dynamical feature that has been subject of intense research in various fields are correlations between the noisy activity of nodes in a network. We consider a class of systems, where discrete signals are sent along the links of the network. Such systems are of particular relevance in neuroscience, because they provide models for networks of neurons that use action potentials for communication. We study correlations in dynamic networks with arbitrary topology, assuming linear pulse coupling. With our novel approach, we are able to understand in detail how specific structural motifs affect pairwise correlations. Based on a power series decomposition of the covariance matrix, we describe the conditions under which very indirect interactions will have a pronounced effect on correlations and population dynamics. In random networks, we find that indirect interactions may lead to a broad distribution of activation levels with low average but highly variable correlations. This phenomenon is even more pronounced in networks with distance dependent connectivity. In contrast, networks with highly connected hubs or patchy connections often exhibit strong average correlations. Our results are particularly relevant in view of new experimental techniques that enable the parallel recording of spiking activity from a large number of neurons, an appropriate interpretation of which is hampered by the currently limited understanding of structure-dynamics relations in complex networks.     

Multiphasic Uptake in Plants II. Mineral Cations, Chloride, and Boric Acid

Uptake of alkali cations, metal ions, chloride, and boric acid is shown by reanalysis of available data to be mediated by single, multiphasic mechanisms which may be fundamentally similar in a variety of plants and tissues.     

A possible mechanism of stochastic resonance in the light of an extra-classical receptive field model of retinal ganglion cells

Traditionally the intensity discontinuities in an image are detected as zero-crossings of the second derivative with the help of a Laplacian of Gaussian (LOG) operator that models the receptive field of retinal Ganglion cells. Such zero-crossings supposedly form a raw primal sketch edge map of the external world in the primary visual cortex of the brain. Based on a new operator which is a linear combination of the LOG and a Dirac-delta function that models the extra-classical receptive field of the ganglion cells, we find that zero-crossing points thus generated, store in presence of noise, apart from the edge information, the shading information of the image in the form of density variation of these points. We have also shown that an optimal image contrast produces best mapping of the shading information to such zero-crossing density variation for a given amount of noise contamination. Furthermore, we have observed that an optimal amount of noise contamination reproduces the minimum optimal contrast and hence gives rise to the best representation of the original image. We show that this phenomenon is similar in nature to that of stochastic resonance phenomenon observed in psychophysical experiments.     

The likelihood of aggregation during protein renaturation can be assessed using the second virial coefficient

Protein aggregation is commonly observed during protein refolding. To better understand this phenomenon, the intermolecular interactions experienced by a protein during unfolding and refolding are inferred from second virial coefficient (SVC) measurements. It is accepted that a negative SVC is indicative of protein-protein interactions that are attractive, whereas a positive SVC indicates net repulsive interactions. Lysozyme denatured and reduced in guanidinium hydrochloride exhibited a decreasing SVC as the denaturant was diluted, and the SVC approached zero at approximately 3 M GdnHCl. Further dilution of denaturant to renaturation conditions (1.25 M GdnHCl) led to a negative SVC, and significant protein aggregation was observed. The inclusion of 500 mM L-arginine in the renaturation buffer shifted the SVC to positive and suppressed aggregation, thereby increasing refolding yield. The formation of mixed disulfides in the denatured state prior to refolding also increased protein solubility and suppressed aggregation, even without the use of L-arginine. Again, the suppression of aggregation was shown to be caused by a shift from attractive to repulsive intermolecular interactions as reflected in a shift from a negative to a positive SVC value. To the best of our knowledge, this is the first time that SVC data have been reported for renaturation studies. We believe this technique will aid in our understanding of how certain conditions promote renaturation and increase protein solubility, thereby suppressing aggregation. SVC measurements provide a useful link, for protein folding and aggregation, between empirical observation and thermodynamics.     

Spatial context influences patch residence time in foraging hierarchies

Understanding responses of organisms to spatial heterogeneity in resources has emerged as a fundamentally important challenge in contemporary ecology. We examined responses of foraging herbivores to multi-scale heterogeneity in plants. We asked the question, “Is the behavior observed at coarse scales in a patch hierarchy the collective outcome of fine scale behaviors or, alternatively, does the spatial context at coarse scales entrain fine scale behavior?” To address this question we created a nested, two-level patch hierarchy. We examined the effects of the spatial context surrounding a patch on the amount of time herbivores resided in the patch. We developed a set of competing models predicting residence time as a function of the mass of plants contained in a patch and the distance between patches and examined the strength of evidence in our observations for these models. Models that included patch mass and inter-patch distance as independent variables successfully predicted observed residence times (bears: r ²=0.67–0.76 and mule deer: r ²=0.33–0.55). Residence times of grizzly bears (Ursus arctos) and mule deer (Odocoileus hemionus) responded to the spatial context surrounding a patch. Evidence ratios of Akaike weights demonstrated that models containing effects of higher levels in the hierarchy on residence time at lower levels received up to 34 times more support in the data than models that failed to consider the higher level context for grizzly bears and up to 48 times more support for mule deer. We conclude that foraging by large herbivores is influenced by more than one level of heterogeneity in patch hierarchies and that simple empirical models offer a viable alternative to optimal foraging models for the prediction of patch residence times.     

Nomadization in Rajasthan, India: Migration, Institutions, and Economy

Despite a global trend toward settlement, the incidence of pastoral nomadism is on the rise in the Marwar region of Rajasthan, India. Typical explanations for this change use models of population pressure; increasing herds and decreasing pasture are held to blame. This explanation, however intuitive, is unsatisfactory. Instead, changing institutional and economic patterns are creating new contexts for strategic movement. Bottlenecks in the yearly resource calendar, caused by the disintegration of obligatory social relationships, force migrations during periods of scarcity. Changes in the volume and pattern of the meat and wool markets have also created opportunities for migrating pastoralists. Producers increase their access to markets and the reproductive rate of their herd through long, annual, migration. While nomadism is a general adaptation to changes in the socioeconomic conditions of the region, differential resource endowments account for the range of strategies; wealthy herders have opportunities not enjoyed by more marginal producers.     

Effects of herbivore species richness on the niche dynamics and distribution of blue sheep in the Trans-Himalaya

Aim To understand the community structure of mountain ungulates by exploring their niche dynamics in response to sympatric species richness. Location Ladakh and Spiti Regions of the Western Indian Trans‐Himalaya. Methods We used the blue sheep Pseudois nayaur, a relatively widely distributed mountain ungulate, as a model species to address the issue. We selected three discrete valleys in three protected areas with similar environmental features but varying wild ungulate species richness, and studied blue sheep’s diet and habitat utilization in them. Habitat variables such as slope angle, distance to cliff and elevation at blue sheep locations were recorded to determine the habitat width of the species. Faecal pellets were collected and microhistological faecal analysis was carried out to determine the diet width of blue sheep in the three areas with different ungulate species richness. Blue sheep’s niche width in terms of habitat and diet was determined using the Shannon’s Index. Results The habitat width of blue sheep had a negative relationship with the number of sympatric species. However, contrary to our expectation, there was a hump‐shaped relationship between blue sheep’s diet width and the sympatric species richness, with the diet width being narrower in areas of allopatry as well as in areas with high herbivore species richness, and the greatest in areas with moderate species richness. Main conclusions We suspect that the narrow diet width in allopatry is out of choice, whereas it is out of necessity in areas with high herbivore species richness because of resource partitioning that enables coexistence. We suggest that interactions with sympatric species lead to niche adjustment of mountain ungulates, implying that competition may play a role in structuring Trans‐Himalayan mountain ungulate assemblages. Given these results, we underscore the importance of including biotic interactions in species distribution models, which have often been neglected.