Daniel C. Reuman

Identification of 100 fundamental ecological questions

Summary 1. Fundamental ecological research is both intrinsically interesting and provides the basic knowledge required to answer applied questions of importance to the management of the natural world. The 100th anniversary of the British Ecological Society in 2013 is an opportune moment to reflect on the current status of ecology as a science and look forward to high-light priorities for future work.

Global patterns in predator–prey size relationships reveal size dependency of trophic transfer efficiency

Predator-prey body size relationships influence food chain length, trophic structure, transfer efficiency, interaction strength, and the bioaccumulation of contaminants. Improved quantification of these relationships and their response to the environment is needed to parameterize food web models and describe food web structure and function. A compiled data set comprising 29582 records of individual prey eaten at 21 locations by individual predators that spanned 10 orders of magnitude in mass and lived in marine environments ranging from the poles to the tropics was used to investigate the influence of predator size and environment on predator and prey size relationships. Linear mixed effects models demonstrated that predator-prey mass ratios (PPMR) increased with predator mass. The amount of the increase varied among locations and predator species and individuals but was not significantly influenced by temperature, latitude, depth, or primary production. Increases in PPMR with predator mass implied nonlinear relationships between log body mass and trophic level and reductions in transfer efficiency with increasing body size. The results suggest that very general rules determine dominant trends in PPMR in diverse marine ecosystems, leading to the ubiquity of size-based trophic structuring and the consistency of observed relationships between the relative abundance of individuals and their body size.

Ecological Networks in a Changing Climate

Summary Attempts to gauge the biological impacts of climate change have typically focussed on the lower levels of organization (individuals to populations), rather than considering more complex multi-species systems, such as entire ecological networks (food webs, mutualistic and host–parasitoid networks). We evaluate the possibility that a few principal drivers underpin network-level responses to climate change, and that these drivers can be studied to develop a more coherent theoretical framework than is currently provided by phenomenological approaches. For instance, warming will elevate individual ectotherm metabolic rates, and direct and indirect effects of changes in atmospheric conditions are expected to alter the stoichiometry of interactions between primary consumers and basal resources; these effects are general and pervasive, and will permeate through the entire networks that they affect. In addition, changes in the density and viscosity of aqueous media could alter interactions among very small organisms and disrupt the pycnoclines that currently compartmentalize many aquatic networks in time and space. We identify a range of approaches and potential model systems that are particularly well suited to network-level studies within the context of climate change. We also highlight potentially fruitful areas of research with a view to improving our predictive power regarding climate change impacts on networks. We focus throughout on mechanistic approaches rooted in first principles that demonstrate potential for application across a wide range of taxa and systems.

Extinction Debt and Windows of Conservation Opportunity in the Brazilian Amazon

Growing Extinction Debt Predicting, and potentially preventing, extinction is a central goal of conservation biology. Wearn et al. (p. 228; see the Perspective by Rangel) describe a mathematical approach for predicting the time lags in extinction following habitat loss. The model was applied to the highly biodiverse Brazilian Amazon region, and used to reconstruct the spatial and temporal patterns of extinction and the accumulation of extinction debt from 1970 through to the present, and to extrapolate to 2050 under four deforestation scenarios. The Amazon basin sits at a critical point: Few species have been driven extinct to date, but an extinction debt is rapidly accumulating, which could lead to an increasing rate of extinction in the next four decades. Deforestation scenarios predict species extinction rates and identify targets of conservation efforts. Predicting when future species extinctions will occur is necessary for directing conservation investments but has proved difficult. We developed a new method for predicting extinctions over time, accounting for the timing and magnitude of habitat loss. We applied this to the Brazilian Amazon, predicting that local extinctions of forest-dependent vertebrate species have thus far been minimal (1% of species by 2008), with more than 80% of extinctions expected to be incurred from historical habitat loss still to come. Realistic deforestation scenarios suggest that local regions will lose an average of nine vertebrate species and have a further 16 committed to extinction by 2050. There is a window of opportunity to dilute the legacy of historical deforestation by concentrating conservation efforts in areas with greatest debt.

Three allometric relations of population density to body mass: theoretical integration and empirical tests in 149 food webs

Predicting species population density-body mass scaling in community food webs (henceforth webs) is important for conservation and to understand community structure. Very different types of scaling have been studied, based on either individuals or species. The individual size distribution (ISD) describes the distribution of individual-organism body masses regardless of taxonomy, and contains the same information as the abundance spectrum. Focusing instead on species, the local size-density relationship (LSDR) plots population densities vs. mean body masses of species. The distribution of species mean body masses (the species-mean-size distribution, SMSD) is also important but previously little studied in webs. We here combine and formalize theory of several authors to predict: how these three descriptions are related; the forms of the LSDR and ISD; and variation in scaling among webs. We describe empirically the SMSDs of two pelagic, one estuarine, and 146 soil webs by power laws and generalizations. We test theory and find it broadly validated.

Impacts of Warming on the Structure and Functioning of Aquatic Communities : Individual-to Ecosystem-Level Responses

Environmental warming is predicted to rise dramatically over the next century, yet few studies have investigated its effects in natural, multi-species systems. We present data collated over an 8-year period from a catchment of geothermally heated streams in Iceland, which acts as a natural experiment on the effects of warming across different organisational levels and spatiotemporal scales. Body sizes and population biomasses of individual species responded strongly to temperature, with some providing evidence to support temperature size rules. Macroinvertebrate and meiofaunal community composition also changed dramatically across the thermal gradient. Interactions within the warm streams in particular were characterised by food chains linking algae to snails to the apex predator, brown trout These chains were missing from the colder systems, where snails were replaced by much smaller herbivores and invertebrate omnivores were the top predators. Trout were also subsidised by terrestrial invertebrate prey, which could have an effect analogous to apparent competition within the aquatic prey assemblage. Top-down effects by snails on diatoms were stronger in the warmer streams, which could account for a shallowing of mass-abundance slopes across the community. This may indicate reduced energy transfer efficiency from resources to consumers in the warmer systems and/or a change in predator-prey mass ratios. All the ecosystem process rates investigated increased with temperature, but with differing thermal sensitivities, with important implications for overall ecosystem functioning (e.g. creating potential imbalances in elemental fluxes). Ecosystem respiration rose rapidly with temperature, leading to increased heterotrophy. There were also indications that food web stability may be lower in the warmer streams.

Climate change impacts in multispecies systems: drought alters food web size structure in a field experiment

Experimental data from intergenerational field manipulations of entire food webs are scarce, yet such approaches are essential for gauging impacts of environmental change in natural systems. We imposed 2 years of intermittent drought on stream channels in a replicated field trial, to measure food web responses to simulated climate change. Drought triggered widespread losses of species and links, with larger taxa and those that were rare for their size, many of which were predatory, being especially vulnerable. Many network properties, including size–scaling relationships within food chains, changed in response to drought. Other properties, such as connectance, were unaffected. These findings highlight the need for detailed experimental data from different organizational levels, from pairwise links to the entire food web. The loss of not only large species, but also those that were rare for their size, provides a newly refined way to gauge likely impacts that may be applied more generally to other systems and/or impacts.

Chapter 1 Allometry of Body Size and Abundance in 166 Food Webs

Summary The relationship between average body masses ( M ) of individuals within species and densities ( N ) of populations of different species and the mechanisms and consequences of this relationship have been extensively studied. Most published work has focused on collections of data for populations of species from a single broad taxon or trophic level (such as birds or herbivorous mammals), rather than on the populations of all species occurring together in a local food web, a very different ecological context. We here provide a systematic analysis of relationships between M and N in community food webs (hereafter simply webs ), using newly collected, taxonomically detailed data from 166 European and North American pelagic, soil, riparian, benthic, inquiline, and estuarine webs. We investigated three topics. First, we compared log( N )‐versus‐log( M ) scatter plots for webs and the slope b 1 of the ordinary‐least‐squares (OLS) regression line log( N ) = b 1 log( M ) + a 1 to the predictions of two theories (Section V.A). The energetic equivalence hypothesis (EEH) was not originally intended for populations within webs and is used here as a null‐model. The second theory, which extends the EEH to webs by recognizing the inefficiency of the transfer of energy from resources to consumers (a trophic transfer correction, or TTC), was originally proposed for webs aggregated to trophic levels. The EEH predicts approximate linearity of the log( N )‐versus‐log( M ) relationship, with slope −3/4 for all webs. The relationship was approximately linear for most but not all webs studied here. However, for webs that were approximately linear, the slope was not typically −3/4, as slopes varied widely from web to web. Predictions of the EEH with TTC were also largely falsified by our data. The EEH with TTC again predicts linearity with b 1 b 1 >−3/4, indicating that populations of larger taxa absorb more energy than populations of smaller ones. Slopes b 1 > −3/4 can occur without violating the conservation of energy, even in webs that are energetically isolated above trophic level 0 (discussed later). Second, for each web, we compared log–log scatter plots of the M and N values of the populations of each taxon with three alternate linear statistical models (Section V.B). Trophic relationships determined which taxa entered the analysis but played no further role except for the Tuesday Lake and Ythan Estuary webs. The assumptions of the model log( N ) = b 1 log( M ) + a 1 + ɛ 1 (including linearity of the expectation) were widely but not universally supported by our data. We tested and confirmed a hypothesis of Cohen and Carpenter (2005) that the model log( N ) = b 1 log( M ) + a 1 + ɛ 1 describes web scatter plots better, in general, than the model log( M ) = b 2 log( N ) + a 2 + ɛ 2 . The former model is also better than the model of symmetric linear regression. Third, since not all of our log–log scatter plots formed approximately linear patterns, we explored causes of nonlinearity and examined alternative models (Section V.C). We showed that uneven lumping of species to web nodes can cause log( N )‐versus‐log( M ) scatter plots to appear nonlinear. Attributes of the association between N and M depended on the type of ecosystem from which data were gathered. For instance, webs from the soil of organic farms were much less likely to exhibit linear log( N )‐versus‐log( M ) relationships than webs from other systems. Webs with a larger range of measured log( M ) values were more likely to appear linear. Our data rejected the hypothesis that data occupy a polygonal region in log( N )‐versus‐log( M ) space.

International migration beyond gravity: A statistical model for use in population projections

International migration will play an increasing role in the demographic future of most nations if fertility continues to decline globally. We developed an algorithm to project future numbers of international migrants from any country or region to any other. The proposed generalized linear model (GLM) used geographic and demographic independent variables only (the population and area of origins and destinations of migrants, the distance between origin and destination, the calendar year, and indicator variables to quantify nonrandom characteristics of individual countries). The dependent variable, yearly numbers of migrants, was quantified by 43653 reports from 11 countries of migration from 228 origins and to 195 destinations during 1960–2004. The final GLM based on all data was selected by the Bayesian information criterion. The number of migrants per year from origin to destination was proportional to (population of origin)0.86(area of origin)−0.21(population of destination)0.36(distance)−0.97, multiplied by functions of year and country-specific indicator variables. The number of emigrants from an origin depended on both its population and its population density. For a variable initial year and a fixed terminal year 2004, the parameter estimates appeared stable. Multiple R2, the fraction of variation in log numbers of migrants accounted for by the starting model, improved gradually with recentness of the data: R2 = 0.57 for data from 1960 to 2004, R2 = 0.59 for 1985–2004, R2 = 0.61 for 1995–2004, and R2 = 0.64 for 2000–2004. The migration estimates generated by the model may be embedded in deterministic or stochastic population projections.

The relationship between body mass and field metabolic rate among individual birds and mammals.

Summary The power-law dependence of metabolic rate on body mass has major implications at every level of ecological organization. However, the overwhelming majority of studies examining this relationship have used basal or resting metabolic rates, and/or have used data consisting of species-averaged masses and metabolic rates. Field metabolic rates are more ecologically relevant and are probably more directly subject to natural selection than basal rates. Individual rates might be more important than species-average rates in determining the outcome of ecological interactions, and hence selection. We here provide the first comprehensive database of published field metabolic rates and body masses of individual birds and mammals, containing measurements of 1498 animals of 133 species in 28 orders. We used linear mixed-effects models to answer questions about the body mass scaling of metabolic rate and its taxonomic universality/heterogeneity that have become classic areas of controversy. Our statistical approach allows mean scaling exponents and taxonomic heterogeneity in scaling to be analysed in a unified way while simultaneously accounting for nonindependence in the data due to shared evolutionary history of related species. The mean power-law scaling exponents of metabolic rate vs. body mass relationships were 0·71 [95% confidence intervals (CI) 0·625–0·795] for birds and 0·64 (95% CI 0·564–0·716) for mammals. However, these central tendencies obscured meaningful taxonomic heterogeneity in scaling exponents. The primary taxonomic level at which heterogeneity occurred was the order level. Substantial heterogeneity also occurred at the species level, a fact that cannot be revealed by species-averaged data sets used in prior work. Variability in scaling exponents at both order and species levels was comparable to or exceeded the differences 3/4−2/3 = 1/12 and 0·71−0·64. Results are interpreted in the light of a variety of existing theories. In particular, results are consistent with the heat dissipation theory of Speakman & Król (2010) and provided some support for the metabolic levels boundary hypothesis of Glazier (2010). Our analysis provides the first comprehensive empirical analysis of the scaling relationship between field metabolic rate and body mass in individual birds and mammals. Our data set is a valuable contribution to those interested in theories of the allometry of metabolic rates. The authors provide the first comprehensive empirical analysis of the scaling relationship between field metabolic rate and body mass in individual birds and mammals. The analysis reveals the importance of heterogeneity in the scaling exponent, with consequences for biomass and nutrient flow through communities, and the structure and functioning of whole ecosystems.