Energetic constraints on an early developmental stage: a comparative view

Biologists have long sought a means by which to quantify similarities and differences in embryonic development across species. Here we present a quantitative approach for predicting the timing of developmental events based on principles of allometry and biochemical kinetics. Data from diverse oviparous species support model predictions that most variation in the time required to reach one early developmental stage-the time to first heartbeat-is explained by the body size and temperature dependence of metabolic rate. Furthermore, comparisons of this stage with later developmental stages suggest that, after correcting for size and temperature, the relationship of metabolic rate to the rate of embryogenesis is approximately invariant across taxonomic groups and stages of ontogeny.     

Species–energy relationships and habitat complexity in bird communities

Species–energy theory is a commonly invoked theory predicting a positive relationship between species richness and available energy. The More Individuals Hypothesis (MIH) attempts to explain this pattern, and assumes that areas with greater food resources support more individuals, and that communities with more individuals include more species. Using a large dataset for North American birds, I tested these predictions of the MIH, and also examined the effect of habitat complexity on community structure. I found qualitative support for the relationships predicted by the MIH, however, the MIH alone was inadequate for fully explaining richness patterns. Communities in more productive sites had more individuals, but they also had more even relative abundance distributions such that a given number of individuals yielded a greater number of species. Richness and evenness were also higher in structurally complex forests compared to structurally more simple grasslands when controlling for available energy.     

Connecting species richness, abundance and body size in deep-sea gastropods

Aim This paper examines species richness, abundance, and body size in deep‐sea gastropods and how they vary over depth, which is a strong correlate of nutrient input. Previous studies have documented the empirical relationships among these properties in terrestrial and coastal ecosystems, but a full understanding of how these patterns arise has yet to be obtained. Examining the relationships among macroecological variables is a logical progression in deep‐sea ecology, where patterns of body size, diversity, and abundance have been quantified separately but not linked together. Location 196–5042 m depth in the western North Atlantic. Method Individuals analysed represent all Vetigastropoda and Caenogastropoda (Class Gastropoda) with intact shells, excluding Ptenoglossa, collected by the Woods Hole Benthic Sampling Program (3424 individuals representing 80 species). Biovolume was measured for every individual separately (i.e. allowing the same species to occupy multiple size classes) and divided into log₂ body size bins. Analyses were conducted for all gastropods together and separated into orders and depth regions (representing different nutrient inputs). A kernel smoothing technique, Kolmogorov‐Smirnov test of fit, and OLS and RMA were used to characterize the patterns. Results Overall, the relationship between the number of individuals and species is right skewed. There is also a positive linear relationship between the number of individuals and the number of species, which is independent of body size. Variation among these relationships is seen among the three depth regions. At depths inferred to correspond with intermediate nutrient input levels, species are accumulated faster given the number of individuals and shift from a right‐skewed to a log‐normal distribution. Conclusion A strong link between body size, abundance, and species richness appears to be ubiquitous over a variety of taxa and environments, including the deep sea. However, the nature of these relationships is affected by the productivity regime and scale at which they are examined.     

Effects of metabolic level on the body size scaling of metabolic rate in birds and mammals

Metabolic rate is traditionally assumed to scale with body mass to the 3/4-power, but significant deviations from the '3/4-power law' have been observed for several different taxa of animals and plants, and for different physiological states. The recently proposed 'metabolic-level boundaries hypothesis' represents one of the attempts to explain this variation. It predicts that the power (log-log slope) of metabolic scaling relationships should vary between 2/3 and 1, in a systematic way with metabolic level. Here, this hypothesis is tested using data from birds and mammals. As predicted, in both of these independently evolved endothermic taxa, the scaling slope approaches 1 at the lowest and highest metabolic levels (as observed during torpor and strenuous exercise, respectively), whereas it is near 2/3 at intermediate resting and cold-induced metabolic levels. Remarkably, both taxa show similar, approximately U-shaped relationships between the scaling slope and the metabolic (activity) level. These predictable patterns strongly support the view that variation of the scaling slope is not merely noise obscuring the signal of a universal scaling law, but rather is the result of multiple physical constraints whose relative influence depends on the metabolic state of the organisms being analysed.     

Energetic costs of activity by lizards in the field

1. The available data related to the activity energetics of lizards in the field were collated with respect to three indices of activity energetics: the ecological cost of transport (ECT) expressed as a percentage of the total energy, the proportion of total energy used in all forms of activity (%AR), and the sustained metabolic scope (SusMS), defined as the ratio of the total energy expenditure to the total resting metabolism.
2. The ECT values of lizards ranged from 3 to 36% with five of 11 species having values >20%. The percentage AR ranged from 23 to 80% for lizards during active seasons, with most species having values > 50%. The SusMS ranged from 1·1 to 5·1.
3. Values of ECT are higher for lizards than for mammals, in part because the costs of maintenance metabolism are higher in mammals.
4. The percentage AR and SusMS values of mammals are higher than those of lizards.
5. It follows from the previous two points that the proportion of the total daily energy that is expended in non-locomotory activities is disproportionately higher in mammals compared with lizards.
6. The energy expended in locomotion is a significant portion of the energy budget of lizards. This is generally true for all seasons in which there is activity.     

Host densities as determinants of abundance in parasite communities

Several epidemiological models predict a positive relationship between host population density and abundance of directly transmitted macroparasites. Here, we generalize these, and test the prediction by a comparative study. We used data on communities of gastrointestinal strongylid nematodes from 19 mammalian species, representing examination of 6670 individual hosts. We studied both the average abundance of all strongylid nematodes within a host species, and the two components of abundance, prevalence and intensity. The effects of host body weight, diet, fecundity and age at maturity and parasite body size were controlled for directly, and the phylogenetically independent contrast method was used to control for confounding factors more generally. Host population density and average parasite abundance were strongly positively correlated within mammalian taxa, and across all species when the effects of host body weight were controlled for. Controlling for other variables did not change this. Even when looking at single parasite species occurring in several host species, abundance was highest in the host species with the highest population density. Prevalence and intensity showed similar patterns. These patterns provide the first macroecological evidence consistent with the prediction that transmission rates depend on host population density in natural parasite communities.     

Energy availability, abundance, energy-use and species richness in forest bird communities: a test of the species–energy theory

Aim To test the ‘more individuals hypothesis’ as a mechanism for the positive association between energy availability and species richness. This hypothesis predicts that total density and energy use in communities is linearly related to energy availability, and that species richness is a positive function of increased density. We also evaluate whether similar energy–density patterns apply to different migratory groups (residents, short‐distance migrants and tropical migrants) separately. Location European and North American forest bird communities. Methods We collected published breeding bird census data from Europe and North America (n = 187). From each census data we calculated bird density (pairs 10 ha^?1), energy use by the community (the sum of metabolic needs of individuals, Watts 10 ha^?1) and geographical location with an accuracy of 0.5°. For each bird census data coordinate we extracted the corresponding monthly values of actual evapotranspiration (AET). From these values we calculated corresponding AET values that we expected to explain the density energy use of forest birds: total annual, breeding season (June) and winter AET. We used general linear modelling to analyse these data controlling for the area of census plots, forest type and census method. Results Total density and energy use in European and North American forest bird communities were linear functions of annual productivity, and increased density and energy use then translated into more species. Also resident bird density and energy consumption were positive functions of annual productivity, but the relationship between productivity and density as well as between productivity and energy use was weaker for migrants. Main conclusions Our results are consistent with the more individuals hypothesis that density and energy use in breeding forest bird communities is coupled tightly with the productivity of the environment, and that increased density and energy consumption results in more species. However, not all community members (migratory groups) are limited by productivity on the breeding grounds.     

Energetic demands of larval krill, Euphausia superba, in winter

Metabolic rates of larval and juvenile krill, Euphausia superba, were measured on board ship during three winter cruises west of the Antarctic Peninsula (June-July 1987, June 1993, and June 1994), and also under different temperature regimes and feeding conditions during long-term maintenance in the laboratory (Palmer Station, winter 1993). A mean oxygen consumption and nitrogen excretion ratio of 31.1 measured on board ship at ambient ocean temperatures suggested that larval and juvenile krill from ice-covered waters were primarily herbivorous. Results from both shipboard and laboratory experiments demonstrated that oxygen consumption increased with temperature, but that larvae subjected to acute temperature increases exhibited higher rates. Experiments conducted at near ambient water temperatures for winter were also conducted to test the effect of habitat on the energy requirements of larval and juvenile krill. A comparison of the field and laboratory studies conducted at −1.5 to −1.8 °C showed that larvae from ice-covered waters and fed larvae in the laboratory had oxygen consumption rates significantly higher than those of larvae collected from open, i.e. ice-free, water and those starved in the laboratory. Results of the comparison lend support to the concept that in winter, larval and juvenile krill are better fed in ice-covered waters than in open water, and to the hypothesis that ice biota in the pack ice are an important food resource in winter for larval and juvenile krill.     

Lower body mass and higher metabolic rate enhance winter survival in root voles,Microtus oeconomus

Although the biological significance of individual variation in physiological traits is widely recognized, studies of their association with fitness in wild populations are surprisingly scarce. We investigated the effect of individual phenotypic variation in body mass, resting (RMR) and peak metabolic rates (PMR) on mortality of the root vole Microtus oeconomus. Body mass and metabolic rates varied significantly among consecutive years and were also age dependent, as individuals born in late summer and autumn were characterized by significantly lower body mass and metabolic rates than animals born earlier. At the beginning of winter voles born in spring and early summer exhibited reduced body mass and metabolic rates, whereas animals born later maintained lower body mass and RMR, which may be interpreted as phenotypic plasticity enhancing the probability of survival. Body mass had no significant effect on vole survival during summer. In contrast, smaller individuals were characterized by lower mortality during early winter, whereas higher body mass was positively associated with survival later in the season. High body‐mass‐corrected RMR positively affected survival in both summer and winter. The effect of PMR was apparent only during winter, though its direction (and correlation with RMR) varied among years. Deep snow cover negatively affected the survival of voles in both early and late winter. Ambient temperature was positively associated with winter survival, except for late winter, when rising temperature caused flooding of vole habitat. We conclude that the lack of consistency in the directionality and strength of the effects of body mass and metabolic rates on winter survival does not undermine their importance, but rather demonstrates the ability of individuals to adjust metabolic rate to changing environmental conditions. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 297–309.     

Energy density and its variation in space limit species richness of boreal forest birds

Aim An area’s ability to support species may be dependent not only on the total amount of available energy it contains but also on energy density (i.e. available energy per unit area). Acknowledging these two aspects of energy availability may increase mechanistic understanding of how increased energy availability results in increased species richness. We studied the relationship between energy density, its variation in space and boreal forest bird species richness and investigated two possible mechanisms: (1) metabolic constraints of organisms, and (2) increased resource availability for specialists. Location Protected areas in Finland’s boreal forest. Methods We tested whether bird species richness was best determined by total energy availability in an area or by energy density and its variation within the area, before and after including bird abundance in the models. We evaluated two main explanatory variables: tree growth reflecting the rate of energy production and tree volume as a measure of biomass. In addition, we modelled individual species’ responses to energy density and its variation, and evaluated the prediction of the metabolic constraints hypothesis that small species are limited by energy density whereas large species are limited by total energy availability in the area. Results Energy density and its variation were good predictors of species richness: together with abundance they explained 84% of variation in species richness (compared with 74% for abundance alone). Pure metabolic constraints were unlikely to explain this relationship. Instead, the mechanism probably involved increased habitat heterogeneity benefiting specialist species. Total energy availability was also an important factor determining species richness but its effect was indirect via abundance. Main conclusions Our results corroborate the importance of energy availability as a driver of species richness in forest bird communities, and they indicate that energy density and its variation in the landscape strongly influence species richness even after accounting for abundance.     

Climatic seasonality, resource bottlenecks, and abundance of rainforest birds: implications for global climate change

We demonstrate that within-year climatic variability, particularly rainfall seasonality, is the most significant variable explaining spatial patterns of bird abundance in Australian tropical rainforest. The likely mechanism causing this pattern is a resource bottleneck (insects, nectar, and fruit) during the dry season that limits the population size of many species. The patterns support both the diversity–climatic–stability hypothesis and the species–energy hypothesis but clearly show that seasonality in energy availability may be a more significant factor than annual totals or means. An index of dry season severity is proposed that quantifies the combined effect of the degree of dryness and the duration of the dry season. We suggest that the predicted increases in seasonality due to global climate change could produce significant declines in bird abundance, further exacerbating the impacts of decreased range size, increased fragmentation, and decreased population size likely to occur as a result of increasing temperature. We suggest that increasing climatic seasonality due to global climate change has the potential to have significant negative impacts on tropical biodiversity.     

On the use of Timed Species Counts to estimate avian abundance indices in species-rich communities

Timed Species Counts (TSCs) have been found to provide useful data for assessing the relative abundances of birds in conservation studies, particularly in species‐rich areas, such as many tropical habitats. An index is conventionally based upon the mean TSC from a number of counts. In this paper, we develop a new statistic, λ, which is based upon the assumption that the number of encounters with a species in unit time has a Poisson distribution. Using the same assumptions on detectability that underlie earlier analyses of TSC data, values of λ, unlike the simple mean TSC, are directly proportional to abundance. However, TSC data are affected by detectability, so that comparisons of relative abundance for the same species between similar sites have greater validity than those between different species. Explicit formulae for calculating an estimate of λ and its SE from sample data are described. Although λ can be estimated from either TSC data or the simple proportion of counts in which the species was encountered, for the commonest species a simulation study shows that only data in full TSC form provide accurate estimates. The difference becomes negligible for scarcer species. The use of λ in preference to mean TSC scores is recommended where it is important to compare values of relative abundance.     

Exploring an efficient habitat index for predicting population and abundance of migratory birds in Poyang Lake Wetland,South China

Poyang Lake wetland is an important wintering habitat of migratory birds with a complexity of coupled nature and human systems. Reported studies suggested that habitat indices were mostly developed based on single-class factors and scale-dependent. Therefore the indices might not be sufficient and applicable to predict regional changes for population and abundance of migratory birds (PMB and AMB) in Poyang Lake wetland at watershed and basin scales. In order to explore an efficient habitat index for predicting PMB and AMB, an integrated and scale-independent habitat index was proposed by integration of landscape and environmental temperature variables at basin and watershed scales. The landscape index and environmental temperature index based on single-class dominant factors, were also evaluated by weighted additive method. Multi-source data, including multi-spatial and temporal remote sensing images and field survey data, was used in this study. Results indicated that the average 10-night temperature in July (TN-Jul.) had dominant influence on PMB and the environmental temperature index was a better predictor for predicting PMB; AMB was influenced by both landscape and temperature variables and the proposed integrated habitat index was better for prediction of AMB in Poyang Lake wetland. The proposed integrated habitat index is a supplement of the single-class habitat index, which provides a more scientific technique for the study of biodiversity.     

Mass–abundance scaling in avian communities is maintained after tropical selective logging

1. Selective logging dominates forested landscapes across the tropics. Despite the structural damage incurred, selectively logged forests typically retain more biodiversity than other forest disturbances. Most logging impact studies consider conventional metrics, like species richness, but these can conceal subtle biodiversity impacts. The mass–abundance relationship is an integral feature of ecological communities, describing the negative relationship between body mass and population abundance, where, in a system without anthropogenic influence, larger species are less abundant due to higher energy requirements. Changes in this relationship can indicate community structure and function changes.
2. We investigated the impacts of selective logging on the mass–abundance scaling of avian communities by conducting a meta‐analysis to examine its pantropical trend. We divide our analysis between studies using mist netting, sampling the understory avian community, and point counts, sampling the entire community.
3. Across 19 mist‐netting studies, we found no consistent effects of selective logging on mass–abundance scaling relative to primary forests, except for the omnivore guild where there were fewer larger‐bodied species after logging. In eleven point‐count studies, we found a more negative relationship in the whole community after logging, likely driven by the frugivore guild, showing a similar pattern.
4. Limited effects of logging on mass–abundance scaling may suggest high species turnover in logged communities, with like‐for‐like replacement of lost species with similar‐sized species. The increased negative mass–abundance relationship found in some logged communities could result from resource depletion, density compensation, or increased hunting; potentially indicating downstream impacts on ecosystem functions.
5. Synthesis and applications. Our results suggest that size distributions of avian communities in logged forests are relatively robust to disturbance, potentially maintaining ecosystem processes in these forests, thus underscoring the high conservation value of logged tropical forests, indicating an urgent need to focus on their protection from further degradation and deforestation.

     

Measurement of body size and abundance in tests of macroecological and food web theory

1. Mean body mass (W) and mean numerical (N) or biomass (B) abundance are frequently used as variables to describe populations and species in macroecological and food web studies. 2. We investigate how the use of mean W and mean N or B, rather than other measures of W and/or accounting for the properties of all individuals, can affect the outcome of tests of macroecological and food web theory. 3. Theoretical and empirical analyses demonstrate that mean W, W at maximum biomass (W(mb)), W when energy requirements are greatest (W(me)) and the W when a species uses the greatest proportion of the energy available to all species in a W class (W(mpe)) are not consistently related. 4. For a population at equilibrium, relationships between mean W and W(me) depend on the slope b of the relationship between trophic level and W. For marine fishes, data show that b varies widely among species and thus mean W is an unreliable indicator of the role of a species in the food web. 5. Two different approaches, 'cross-species' and 'all individuals' have been used to estimate slopes of abundance-body mass relationships and to test the energetic equivalence hypothesis and related theory. The approaches, based on relationships between (1) log(10) mean W and log(10) mean N or B, and (2) log(10) W and log(10) N or B of all individuals binned into log(10) W classes (size spectra), give different slopes and confidence intervals with the same data. 6. Our results show that the 'all individuals' approach has the potential to provide more powerful tests of the energetic equivalence hypothesis and role of energy availability in determining slopes, but new theory and empirical analysis are needed to explain distributions of species relative abundance at W. 7. Biases introduced when working with mean W in macroecological and food web studies are greatest when species have indeterminate growth, when relationships between W and trophic level are strong and when the range of species'W is narrow.