Ecology Letters

Competitive interactions between forest trees are driven by species' trait hierarchy, not phylogenetic or functional similarity: implications for forest community assembly

2012-05-24T20:32:56.2203-05:00

Abstract

The relative importance of competition vs. environmental filtering in the assembly of communities is commonly inferred from their functional and phylogenetic structure, on the grounds that similar species compete most strongly for resources and are therefore less likely to coexist locally. This approach ignores the possibility that competitive effects can be determined by relative positions of species on a hierarchy of competitive ability. Using growth data, we estimated 275 interaction coefficients between tree species in the French mountains. We show that interaction strengths are mainly driven by trait hierarchy and not by functional or phylogenetic similarity. On the basis of this result, we thus propose that functional and phylogenetic convergence in local tree community might be due to competition-sorting species with different competitive abilities and not only environmental filtering as commonly assumed. We then show a functional and phylogenetic convergence of forest structure with increasing plot age, which supports this view.

Legacy effects of aboveground–belowground interactions

2012-05-17T20:29:38.104594-05:00

Abstract

Root herbivory can greatly affect the performance of aboveground insects via changes in plant chemistry. These interactions have been studied extensively in experiments where aboveground and belowground insects were feeding on the same plant. However, little is known about how aboveground and belowground organisms interact when they feed on plant individuals that grow after each other in the same soil. We show that feeding by aboveground and belowground insect herbivores on ragwort (Jacobaea vulgaris) plants exert unique soil legacy effects, via herbivore-induced changes in the composition of soil fungi. These changes in the soil biota induced by aboveground and belowground herbivores of preceding plants greatly influenced the pyrrolizidine alkaloid content, biomass and aboveground multitrophic interactions of succeeding plants. We conclude that plant-mediated interactions between aboveground and belowground insects are also important when they do not feed simultaneously on the same plant.

The effects of translocation-induced isolation and fragmentation on the cultural evolution of bird song

Kevin A. Parker, Marti J. Anderson, Peter F. Jenkins, Dianne H. Brunton — 2012-05-17T01:53:59.839098-05:00

Abstract

Understanding the divergence of behavioural signals in isolated populations is critical to knowing how certain barriers to gene flow can develop. For many bird species, songs are essential for conspecific recognition and mate choice. Measuring the rate of song divergence in natural populations is difficult, but translocations of endangered birds to isolated islands for conservation purposes can yield insights, as the age and source of founder populations are completely known. We found significant and rapid evolution in the structure and diversity of bird song in North Island saddlebacks, Philesturnus rufusater, in New Zealand, with two distinct lineages evolving in < 50 years. The strong environmental filters of serial translocations resulted in cultural bottlenecks that generated drift and reduced song variability within islands. This rapid divergence coupled with loss of song diversity has important implications for the behavioural evolution of this species, demonstrating previously unrecognised biological consequences of conservation management.

Fungicide-induced declines of freshwater biodiversity modify ecosystem functions and services

2012-05-15T20:12:30.493814-05:00

Abstract

Although studies on biodiversity and ecosystem function are often framed within the context of anthropogenic change, a central question that remains is how important are direct vs. indirect (via changes in biodiversity) effects of anthropogenic stressors on ecosystem functions in multitrophic-level communities. Here, we quantify the effects of the fungicide chlorothalonil on 34 species-, 2 community- and 11 ecosystem-level responses in a multitrophic-level system. At ecologically relevant concentrations, chlorothalonil increased mortality of amphibians, gastropods, zooplankton, algae and a macrophyte (reducing taxonomic richness), reduced decomposition and water clarity and elevated dissolved oxygen and net primary productivity. These ecosystem effects were indirect and predictable based on changes in taxonomic richness. A path analysis suggests that chlorothalonil-induced reductions in biodiversity and top-down and bottom-up effects facilitated algal blooms that shifted ecosystem functions. This work emphasises the need to re-evaluate the safety of chlorothalonil and to further link anthropogenic-induced changes in biodiversity to altered ecosystem functions.

The evolution of salinity tolerance in Daphnia: a functional genomics approach

Leigh C. Latta, Lawrence J. Weider, John K. Colbourne, Michael E. Pfrender — 2012-05-15T01:03:03.174721-05:00

Abstract

One route to genetic adaptation in a novel environment is the evolution of ecological generalisation. Yet, identifying the cost that a generalist pays for the increased breadth of tolerance has proven elusive. We integrate phenotypic assays with functional genomics to understand how tolerance to a salinity gradient evolves, and we test the relationship between the fitness cost of this generalisation and the cost of transcription that arises from evolved differences in patterns of gene expression. Our results suggest that a salt-tolerant genotype of Daphnia is characterised by constitutively expressed genes, which does not incur a loss of fitness or a cost of transcription relative to a salt-intolerant genotype in low saline environments. We find that many genes whose expression pattern evolved in response to salinity are also involved in the response to predators, suggesting that the cost of generalisation may be due to trade-offs along other environmental axes.

Climate envelope modelling reveals intraspecific relationships among flowering phenology, niche breadth and potential range size in Arabidopsis thaliana

2012-05-15T01:02:26.444373-05:00

Abstract

Species often harbour large amounts of phenotypic variation in ecologically important traits, and some of this variation is genetically based. Understanding how this genetic variation is spatially structured can help to understand species’ ecological tolerances and range limits. We modelled the climate envelopes of Arabidopsis thaliana genotypes, ranging from early- to late-flowering, as a function of several climatic variables. We found that genotypes with contrasting alleles at individual flowering time loci differed significantly in potential range size and niche breadth. We also found that later flowering genotypes had more restricted range potentials and narrower niche breadths than earlier flowering genotypes, indicating that local selection on flowering can constrain or enhance the ability of populations to colonise other areas. Our study demonstrates how climate envelope models that incorporate ecologically important genetic variation can provide insights into the macroecology of a species, which is important to understand its responses to changing environments.

Nematomorph parasites indirectly alter the food web and ecosystem function of streams through behavioural manipulation of their cricket hosts

2012-05-15T01:02:04.653329-05:00

Abstract

Nematomorph parasites manipulate crickets to enter streams where the parasites reproduce. These manipulated crickets become a substantial food subsidy for stream fishes. We used a field experiment to investigate how this subsidy affects the stream community and ecosystem function. When crickets were available, predatory fish ate fewer benthic invertebrates. The resulting release of the benthic invertebrate community from fish predation indirectly decreased the biomass of benthic algae and slightly increased leaf break-down rate. This is the first experimental demonstration that host manipulation by a parasite can reorganise a community and alter ecosystem function. Nematomorphs are common, and many other parasites have dramatic effects on host phenotypes, suggesting that similar effects of parasites on ecosystems might be widespread.

Divergent selection on locally adapted major histocompatibility complex immune genes experimentally proven in the field

Christophe Eizaguirre, Tobias L. Lenz, Martin Kalbe, Manfred Milinski — 2012-05-15T01:00:25.885272-05:00

Abstract

Although crucial for the understanding of adaptive evolution, genetically resolved examples of local adaptation are rare. To maximize survival and reproduction in their local environment, hosts should resist their local parasites and pathogens. The major histocompatibility complex (MHC) with its key function in parasite resistance represents an ideal candidate to investigate parasite-mediated local adaptation. Using replicated field mesocosms, stocked with second-generation lab-bred three-spined stickleback hybrids of a lake and a river population, we show local adaptation of MHC genotypes to population-specific parasites, independently of the genetic background. Increased allele divergence of lake MHC genotypes allows lake fish to fight the broad range of lake parasites, whereas more specific river genotypes confer selective advantages against the less diverse river parasites. Hybrids with local MHC genotype gained more body weight and thus higher fitness than those with foreign MHC in either habitat, suggesting the evolutionary significance of locally adapted MHC genotypes.

Phylogenetic diversity and the functioning of ecosystems

2012-05-15T00:58:06.517974-05:00

Abstract

Phylogenetic diversity (PD) describes the total amount of phylogenetic distance among species in a community. Although there has been substantial research on the factors that determine community PD, exploration of the consequences of PD for ecosystem functioning is just beginning. We argue that PD may be useful in predicting ecosystem functions in a range of communities, from single-trophic to complex networks. Many traits show a phylogenetic signal, suggesting that PD can estimate the functional trait space of a community, and thus ecosystem functioning. Phylogeny also determines interactions among species, and so could help predict how extinctions cascade through ecological networks and thus impact ecosystem functions. Although the initial evidence available suggests patterns consistent with these predictions, we caution that the utility of PD depends critically on the strength of phylogenetic signals to both traits and interactions. We advocate for a synthetic approach that incorporates a deeper understanding of how traits and interactions are shaped by evolution, and outline key areas for future research. If these complexities can be incorporated into future studies, relationships between PD and ecosystem function bear promise in conceptually unifying evolutionary biology with ecosystem ecology.

Body size and dispersal mode as key traits determining metacommunity structure of aquatic organisms

2012-05-15T00:58:02.390545-05:00

Abstract

Relationships between traits of organisms and the structure of their metacommunities have so far mainly been explored with meta-analyses. We compared metacommunities of a wide variety of aquatic organism groups (12 groups, ranging from bacteria to fish) in the same set of 99 ponds to minimise biases inherent to meta-analyses. In the category of passive dispersers, large-bodied groups showed stronger spatial patterning than small-bodied groups suggesting an increasing impact of dispersal limitation with increasing body size. Metacommunities of organisms with the ability to fly (i.e. insect groups) showed a weaker imprint of dispersal limitation than passive dispersers with similar body size. In contrast, dispersal movements of vertebrate groups (fish and amphibians) seemed to be mainly confined to local connectivity patterns. Our results reveal that body size and dispersal mode are important drivers of metacommunity structure and these traits should therefore be considered when developing a predictive framework for metacommunity dynamics.

Precipitation manipulation experiments – challenges and recommendations for the future

2012-05-04T03:12:29.169027-05:00

Abstract

Climatic changes, including altered precipitation regimes, will affect key ecosystem processes, such as plant productivity and biodiversity for many terrestrial ecosystems. Past and ongoing precipitation experiments have been conducted to quantify these potential changes. An analysis of these experiments indicates that they have provided important information on how water regulates ecosystem processes. However, they do not adequately represent global biomes nor forecasted precipitation scenarios and their potential contribution to advance our understanding of ecosystem responses to precipitation changes is therefore limited, as is their potential value for the development and testing of ecosystem models. This highlights the need for new precipitation experiments in biomes and ambient climatic conditions hitherto poorly studied applying relevant complex scenarios including changes in precipitation frequency and amplitude, seasonality, extremity and interactions with other global change drivers. A systematic and holistic approach to investigate how soil and plant community characteristics change with altered precipitation regimes and the consequent effects on ecosystem processes and functioning within these experiments will greatly increase their value to the climate change and ecosystem research communities. Experiments should specifically test how changes in precipitation leading to exceedance of biological thresholds affect ecosystem resilience and acclimation.

Ecological thresholds at the savanna-forest boundary: how plant traits, resources and fire govern the distribution of tropical biomes

2012-05-04T02:59:41.627467-05:00

Abstract

Fire shapes the distribution of savanna and forest through complex interactions involving climate, resources and species traits. Based on data from central Brazil, we propose that these interactions are governed by two critical thresholds. The fire-resistance threshold is reached when individual trees have accumulated sufficient bark to avoid stem death, whereas the fire-suppression threshold is reached when an ecosystem has sufficient canopy cover to suppress fire by excluding grasses. Surpassing either threshold is dependent upon long fire-free intervals, which are rare in mesic savanna. On high-resource sites, the thresholds are reached quickly, increasing the probability that savanna switches to forest, whereas low-resource sites are likely to remain as savanna even if fire is infrequent. Species traits influence both thresholds; saplings of savanna trees accumulate bark thickness more quickly than forest trees, and are more likely to become fire resistant during fire-free intervals. Forest trees accumulate leaf area more rapidly than savanna trees, thereby accelerating the transition to forest. Thus, multiple factors interact with fire to determine the distribution of savanna and forest by influencing the time needed to reach these thresholds. Future work should decipher multiple environmental controls over the rates of tree growth and canopy closure in savanna.

Plant traits affecting herbivory on tree recruits in highly diverse subtropical forests

2012-05-02T05:23:46.419547-05:00

Abstract

Differences in herbivory among woody species can greatly affect the functioning of forest ecosystems, particularly in species-rich (sub)tropical regions. However, the relative importance of the different plant traits which determine herbivore damage remains unclear. Defence traits can have strong effects on herbivory, but rarely studied geographical range characteristics could complement these effects through evolutionary associations with herbivores. Herein, we use a large number of morphological, chemical, phylogenetic and biogeographical characteristics to analyse interspecific differences in herbivory on tree saplings in subtropical China. Unexpectedly, we found no significant effects of chemical defence traits. Rather, herbivory was related to the plants' leaf morphology, local abundance and climatic niche characteristics, which together explained 70% of the interspecific variation in herbivory in phylogenetic regression. Our study indicates that besides defence traits and apparency to herbivores, previously neglected measures of large-scale geographical host distribution are important factors influencing local herbivory patterns among plant species.

Phylogenetic community ecology and the role of social dominance in sponge-dwelling shrimp

Kristin M. Hultgren, J. Emmett Duffy — 2012-05-02T05:12:14.774549-05:00

Abstract

When functional traits are evolutionarily conserved, phylogenetic relatedness can serve as a proxy for ecological similarity to examine whether functional differences among species mediate community assembly. Using phylogenetic- and trait-based analyses, we demonstrate that sponge-dwelling shrimp (Synalpheus) assemblages are structured by size-based habitat filtering, interacting with competitive exclusion mediated by social system. Most shrimp communities were more closely related and/or more similar in size than randomized communities, consistent with habitat filtering facilitated by phylogenetically conserved body size. Those sponges with greater space heterogeneity hosted shrimp communities with greater size diversity, corroborating the importance of size in niche use. However, communities containing eusocial shrimp – which cooperatively defend territories – were less phylogenetically related and less similar in size, suggesting that eusociality enhances competitive ability and drives competitive exclusion. Our analyses demonstrate that community assembly in this diverse system occurs via traits mediating niche use and differential competitive ability.

Origin matters: widely distributed native and non-native species benefit from different functional traits

Sonja Knapp, Ingolf Kühn — 2012-04-23T02:26:55.284817-05:00

Abstract

Recently, ecologists debated whether distinguishing native from non-native species is sensible or not. One argument is that widespread and less widespread species are functionally different, whether or not they are native. An opposing statement points out ecologically relevant differences between native and non-native species. We studied the functional traits that drive native and non-native vascular plant species frequency in Germany by explaining species grid-cell frequency using traits and their interaction with status. Native and non-native species frequency was equally driven by life span, ploidy type and self-compatibility. Non-native species frequency rose with later flowering cessation date, whereas this relationship was absent for native species. Native and non-native species differed in storage organs and in the number of environmental conditions they tolerate. We infer that environmental filters drive trait convergence of native and non-native species, whereas competition drives trait divergence. Meanwhile, introduction pathways functionally bias the frequency of non-native species.

Predicting present and future intra-specific genetic structure through niche hindcasting across 24 millennia

2012-04-20T00:00:46.831217-05:00

Ecology Letters (2012)

Abstract

Paleoclimatic reconstructions coupled with species distribution models and identification of extant spatial genetic structure have the potential to provide insights into the demographic events that shape the distribution of intra-specific genetic variation across time. Using the globeflower Trollius europaeus as a case-study, we combined (1) Amplified Fragment Length Polymorphisms, (2) suites of 1000-years stepwise hindcasted species distributions and (3) a model of diffusion through time over the last 24,000 years, to trace the spatial dynamics that most likely fits the species’ current genetic structure. We show that the globeflower comprises four gene pools in Europe which, from the dry period preceding the Last Glacial Maximum, dispersed while tracking the conditions fitting its climatic niche. Among these four gene pools, two are predicted to experience drastic range retraction in the near future. Our interdisciplinary approach, applicable to virtually any taxon, is an advance in inferring how climate change impacts species’ genetic structures.

Top-down determinants of niche structure and adaptation among African Acacias

A. Carla Staver, William J. Bond, Michael D. Cramer, Julia L. Wakeling — 2012-04-17T01:52:44.279108-05:00

Ecology Letters (2012)

Abstract

The role of top-down factors like herbivory and fire in structuring species’ niches, even in disturbance-dependent biomes like savanna, remains poorly understood. Interactions between herbivory and fire may set up a potential tradeoff axis, along which unique adaptations contribute to structuring communities and determining species distributions. We examine the role of herbivory and fire in structuring distributions of Acacia saplings in Hluhluwe iMfolozi Park in South Africa, and the relationship of species’ niche structure to traits that help them survive herbivory or fire. Results suggest that (1) fire and herbivory form a single trade-off axis, (2) Acacia sapling distributions are constrained by fire and herbivory, and (3) Acacia saplings have adaptations that are structured by the tradeoff axis. Herbivory-adapted species tend to have ‘cage’-like architecture, thicker bark, and less starch storage, while fire-adapted species tend to have ‘pole’-like architecture, relatively thinner bark, and more starch storage.

Climate-driven diversity dynamics in plants and plant-feeding insects

Tommi Nyman, Hans Peter Linder, Carlos Peña, Tobias Malm, Niklas Wahlberg — 2012-04-17T01:46:00.136325-05:00

Ecology Letters (2012)

Abstract

The origin of species-rich insect–plant food webs has traditionally been explained by diversifying antagonistic coevolution between plant defences and herbivore counter-defences. However, recent studies combining paleoclimatic reconstructions with time-calibrated phylogenies suggest that variation in global climate determines the distribution, abundance and diversity of plant clades and, hence, indirectly influences the balance between speciation and extinction in associated herbivore groups. Extant insect communities tend to be richest on common plant species that have many close relatives. This could be explained either by climate-driven diffuse cospeciation between plants and insects, or by elevated speciation and reduced extinction in herbivore lineages associated with expanding host taxa (resources). Progress in paleovegetation reconstructions in combination with the rapidly increasing availability of fossil-calibrated phylogenies provide means to discern between these alternative hypotheses. In particular, the ‘Diffuse cospeciation’ scenario predicts closely matching main diversification periods in plants and in the insects that feed upon them, while the ‘Resource abundance-dependent diversification’ hypothesis predicts that both positive and negative responses of insect diversity are lagged in relation to host-plant availability. The dramatic Cenozoic changes in global climate provide multiple possibilities for studying the mechanisms by which climatic shifts may drive diversity dynamics in plants and insect herbivores.

Predicting plant responses to mycorrhizae: integrating evolutionary history and plant traits

Kurt O. Reinhart, Gail W. T. Wilson, Matthew J. Rinella — 2012-04-17T01:40:03.016609-05:00

Abstract

We assessed whether (1) arbuscular mycorrhizal colonization of roots (RC) and/or plant responses to arbuscular mycorrhizae (MR) vary with plant phylogeny and (2) MR and RC can be more accurately predicted with a phylogenetic predictor relative to a null model and models with plant trait and taxonomic predictors. In a previous study, MR and RC of 95 grassland species were measured. We constructed a phylogeny for these species and found it explained variation in MR and RC. Next, we used multiple regressions to identify the models that most accurately predicted plant MR. Models including either phylogenetic or phenotypic and taxonomic information similarly improved our ability to predict MR relative to a null model. Our study illustrates the complex evolutionary associations among species and constraints of using phylogenetic information, relative to plant traits, to predict how a plant species will interact with AMF.

Temporal coincidence of environmental stress events modulates predation rates

Sylvain Pincebourde, Eric Sanford, Jérôme Casas, Brian Helmuth — 2012-04-12T01:13:50.908252-05:00

Ecology Letters (2012)

Abstract

Climate warming experiments generally test the ecological effects of constant treatments while neglecting the influence of more realistic patterns of environmental fluctuations. Thus, little is known regarding how the temporal interaction between multiple episodes of thermal stress influences biotic interactions. We measured the sensitivity of predation rate in an intertidal sea star to changing levels of temporal coincidence of underwater and aerial thermal stress events. In laboratory trials, we controlled for intensity, variance and temporal patterning of both underwater and aerial body temperature. Predation rate decreased as underwater and aerial thermal stress episodes became temporally non-coincident, despite a similar intensity and variance among treatments. Experiments under constant conditions were a poor predictor of more complex environmental scenarios because of these strong temporal interactions. Such temporal interactions may be widespread in various ecosystems, suggesting a strong need for empirical studies and models that link environmental complexity, physiology, behaviour and species interactions.

The maximum height of grasses is determined by roots

Kun-Fang Cao, Shi-Jian Yang, Yong-Jiang Zhang, Tim J. Brodribb — 2012-04-11T00:59:48.962018-05:00

Ecology Letters (2012)

Abstract

Grasses such as bamboos can produce upright stems more than 30 m tall, yet the processes that constrain plant height in this important group have never been investigated. Air embolisms form commonly in the water transport system of grasses and we hypothesised that root pressure-dependent refilling these embolisms should limit the maximum height of grass species to the magnitude of their root pressure. Confirming this hypothesis, we show that in 59 species of bamboo grown in two common gardens, the maximum heights of culms of 67 clones are closely predicted by the maximum measured root pressure overnight. Furthermore, we demonstrate that water transport in these bamboo species is dependent on root pressure to repair hydraulic dysfunction sustained during normal diurnal gas exchange. Our results established the critical importance of root pressure in the tallest grass species and provide a new basis for understanding the limits for plant growth.

Does increasing mortality change the response of fish populations to environmental fluctuations?

Tristan Rouyer, Alexander Sadykov, Jan Ohlberger, Nils Chr. Stenseth — 2012-04-10T01:51:24.752651-05:00

Ecology Letters

Abstract

Fluctuations of fish populations abundances are shaped by the interplay between population dynamics and the stochastic forcing of the environment. Age-structured populations behave as a filter of the environment. This filter is characterised by the species-specific life cycle and life-history traits. An increased mortality of mature individuals alters these characteristics and may therefore induce changes in the variability of populations. The response of a generic age-structured model was analysed to investigate the expected changes in the fluctuations of fish populations in response to decreased adult survival. These expectations were then tested on an extensive dataset. In accordance with theory, the analyses revealed that decreased adult survival and mean age of spawners were linked to an increase in the relative importance of short-term fluctuations. It suggests that intensive exploitation can lead to a change in the variability of fish populations, an issue of central interest from both conservation and management perspectives.

What controls the distribution of tropical forest and savanna?

Brett P. Murphy, David M.J.S. Bowman — 2012-03-27T05:25:02.125643-05:00

Ecology Letters (2012)

Abstract

Forest and savanna biomes dominate the tropics, yet factors controlling their distribution remain poorly understood. Climate is clearly important, but extensive savannas in some high rainfall areas suggest a decoupling of climate and vegetation. In some situations edaphic factors are important, with forest often associated with high nutrient availability. Fire also plays a key role in limiting forest, with fire exclusion often causing a switch from savanna to forest. These observations can be captured by a broad conceptual model with two components: (1) forest and savanna are alternative stable states, maintained by tree cover-fire feedbacks, (2) the interaction between tree growth rates and fire frequency limits forest development; any factor that increases growth (e.g. elevated availability of water, nutrients, CO₂), or decreases fire frequency, will favour canopy closure. This model is consistent with the range of environmental variables correlated with forest distribution, and with the current trend of forest expansion, likely driven by increasing CO₂ concentrations. Resolving the drivers of forest and savanna distribution has moved beyond simple correlative studies that are unlikely to establish ultimate causation. Experiments using Dynamic Global Vegetation Models, parameterised with measurements from each continent, provide an important tool for understanding the controls of these systems.

The value of coordinated management of interacting ecosystem services

Crow White, Christopher Costello, Bruce E. Kendall, Christopher J. Brown — 2012-04-10T01:55:16.070854-05:00

Ecology Letters (2012)

Abstract

Coordinating decisions and actions among interacting sectors is a critical component of ecosystem-based management, but uncertainty about coordinated management’s effects is compromising its perceived value and use. We constructed an analytical framework for explicitly calculating how coordination affects management decisions, ecosystem state and the provision of ecosystem services in relation to ecosystem dynamics and socio-economic objectives. The central insight is that the appropriate comparison strategy to optimal coordinated management is optimal uncoordinated management, which can be identified at the game theoretic Nash equilibrium. Using this insight we can calculate coordination’s effects in relation to uncoordinated management and other reference scenarios. To illustrate how this framework can help identify ecosystem and socio-economic conditions under which coordination is most influential and valuable, we applied it to a heuristic case study and a simulation model for the California Current Marine Ecosystem. Results indicate that coordinated management can more than double an ecosystem’s societal value, especially when sectors can effectively manipulate resources that interact strongly. However, societal gains from coordination will need to be reconciled with observations that it also leads to strategic simplification of the ecological food web, and generates both positive and negative impacts on individual sectors and non-target species.

Fertile forests produce biomass more efficiently

2012-04-04T02:47:49.729768-05:00

Ecology Letters (2012)

Abstract

Trees with sufficient nutrition are known to allocate carbon preferentially to aboveground plant parts. Our global study of 49 forests revealed an even more fundamental carbon allocation response to nutrient availability: forests with high-nutrient availability use 58 ± 3% (mean ± SE; 17 forests) of their photosynthates for plant biomass production (BP), while forests with low-nutrient availability only convert 42 ± 2% (mean ± SE; 19 forests) of annual photosynthates to biomass. This nutrient effect largely overshadows previously observed differences in carbon allocation patterns among climate zones, forest types and age classes. If forests with low-nutrient availability use 16 ± 4% less of their photosynthates for plant growth, what are these used for? Current knowledge suggests that lower BP per unit photosynthesis in forests with low- versus forests with high-nutrient availability reflects not merely an increase in plant respiration, but likely results from reduced carbon allocation to unaccounted components of net primary production, particularly root symbionts.

Carbon dioxide supersaturation promotes primary production in lakes

Mats Jansson, Jan Karlsson, Anders Jonsson — 2012-03-16T02:50:50.319335-05:00

Ecology Letters (2012)

Abstract

A majority of the world’s lakes are supersaturated with respect to carbon dioxide (CO₂). By experimental manipulation of the CO₂ concentration in supersaturated boreal lakes, we demonstrate that phytoplankton primary production was up to 10 times higher in supersaturated lake water in comparison with water with CO₂ at equilibrium concentrations and that CO₂, together with nutrients, explained most of the variation in pelagic primary production and phytoplankton biomass over a wide variety of unproductive lakes. These results suggest that phytoplankton can be co-limited by CO₂ and nutrients in unproductive lakes. As import of terrestrial organic carbon and its subsequent microbial mineralisation in lakes is a driving force of CO₂-supersaturation our results suggest that lake productivity and carbon cycling may respond to variations in terrestrial organic carbon export, (e.g. caused by land use or climate change) in ways not described before.

Climate change impacts on tree ranges: model intercomparison facilitates understanding and quantification of uncertainty

2012-03-20T05:48:05.869298-05:00

Ecology Letters (2012)

Abstract

Model-based projections of shifts in tree species range due to climate change are becoming an important decision support tool for forest management. However, poorly evaluated sources of uncertainty require more scrutiny before relying heavily on models for decision-making. We evaluated uncertainty arising from differences in model formulations of tree response to climate change based on a rigorous intercomparison of projections of tree distributions in France. We compared eight models ranging from niche-based to process-based models. On average, models project large range contractions of temperate tree species in lowlands due to climate change. There was substantial disagreement between models for temperate broadleaf deciduous tree species, but differences in the capacity of models to account for rising CO₂ impacts explained much of the disagreement. There was good quantitative agreement among models concerning the range contractions for Scots pine. For the dominant Mediterranean tree species, Holm oak, all models foresee substantial range expansion.

Forecasting phenology: from species variability to community patterns

2012-03-20T23:45:55.192101-05:00

Ecology Letters (2012)

Abstract

Shifts in species’ phenology in response to climate change have wide-ranging consequences for ecological systems. However, significant variability in species’ responses, together with limited data, frustrates efforts to forecast the consequences of ongoing phenological changes. Herein, we use a case study of three North American plant communities to explore the implications of variability across levels of organisation (within and among species, and among communities) for forecasting responses to climate change. We show how despite significant variation among species in sensitivities to climate, comparable patterns emerge at the community level once regional climate drivers are accounted for. However, communities differ with respect to projected patterns of divergence and overlap among their species’ phenological distributions in response to climate change. These analyses and a review of hypotheses suggest how explicit consideration of spatial scale and levels of biological organisation may help to understand and forecast phenological responses to climate change.

A curve of thresholds governs plague epizootics in Central Asia

2012-03-26T17:14:10.815806-05:00

Ecology Letters (2012)

Abstract

A core concept of infectious disease epidemiology is the abundance threshold, below which an infection is unable to invade or persist. There have been contrasting theoretical predictions regarding the nature of this threshold for vector-borne diseases, but for infections with an invertebrate vector, it is common to assume a threshold defined by the ratio of vector and host abundances. Here, we show in contrast, both from field data and model simulations, that for plague (Yersinia pestis) in Kazakhstan, the invasion threshold quantity is based on the product of its host (Rhombomys opimus) and vector (mainly Xenopsylla spp.) abundances, resulting in a combined threshold curve with hyperbolic shape. This shape implies compensation between host and vector abundances in permitting infection, which has important implications for disease control. Realistic joint thresholds, supported by data, should promote improved understanding, prediction and management of disease occurrence in this and other vector-borne disease systems.

Anthropogenic mortality on coral reefs in Caribbean Panama predates coral disease and bleaching

2012-03-30T02:39:33.185193-05:00

Ecology Letters (2012)

Abstract

Caribbean reef corals have declined precipitously since the 1980s due to regional episodes of bleaching, disease and algal overgrowth, but the extent of earlier degradation due to localised historical disturbances such as land clearing and overfishing remains unresolved. We analysed coral and molluscan fossil assemblages from reefs near Bocas del Toro, Panama to construct a timeline of ecological change from the 19th century—present. We report large changes before 1960 in coastal lagoons coincident with extensive deforestation, and after 1960 on offshore reefs. Striking changes include the demise of previously dominant staghorn coral Acropora cervicornis and oyster Dendrostrea frons that lives attached to gorgonians and staghorn corals. Reductions in bivalve size and simplification of gastropod trophic structure further implicate increasing environmental stress on reefs. Our paleoecological data strongly support the hypothesis, from extensive qualitative data, that Caribbean reef degradation predates coral bleaching and disease outbreaks linked to anthropogenic climate change.

Spatial scale and divergent patterns of variation in adapted traits in the ocean

Lyndie A. Hice, Tara A. Duffy, Stephan B. Munch, David O. Conover — 2012-03-28T23:41:21.270272-05:00

Ecology Letters (2012)

Abstract

The geography of adaptive genetic variation is crucial to species conservation yet poorly understood in marine systems. We analyse the spatial scale of genetic variation in traits that broadly display adaptation throughout the range of a highly dispersive marine species. We conducted common garden experiments on the Atlantic silverside, Menidia menidia, from 39 locations along its 3 000 km range thereby mapping genetic variation for growth rate, vertebral number and sex determination. Each trait displayed unique clinal patterns, with significant differences (adaptive or not) occurring over very small distances. Breakpoints in the cline differed among traits, corresponding only partially with presumed eco-geographical boundaries. Because clinal patterns are unique to each selected character, neutral genes or those coding for a single character cannot serve as proxies for the genetic structure as a whole. Conservation plans designed to protect essential genetic subunits of a species will need to account for such complex spatial structures.

Density-dependent investment in costly anti-predator defences: an explanation for the weak survival benefit of group living

Derek Daly, A. D. Higginson, Dong Chen, G. D. Ruxton, M. P. Speed — 2012-04-04T15:20:03.783928-05:00

Ecology Letters (2012)

Abstract

A central explanation for group living across animal taxa is the reduced rate of attack by predators. However, many field observations show a weak or non-existent effect of group size on per capita mortality rates. Herein we resolve this apparent paradox. We found that Pieris brassicae larvae defended themselves less readily when in groups than when alone, in that they were more reluctant to regurgitate in response to simulated attacks and produced less regurgitant. Furthermore, a simple model demonstrates that this reluctance was sufficient to cancel out the benefit from being in a group. This conditional strategy can be understood in terms of the costs and benefits of defences. For grouped individuals, defence is less often required because attack rates are lower and the costs of defence may be higher due to competition for resources. These phenomena are likely to be widespread in facultatively gregarious species that utilise anti-predator defences.

Accounting for dispersal and biotic interactions to disentangle the drivers of species distributions and their abundances

Isabelle Boulangeat, Dominique Gravel, Wilfried Thuiller — 2012-03-30T02:39:56.299301-05:00

Ecology Letters (2012)

Abstract

Although abiotic factors, together with dispersal and biotic interactions, are often suggested to explain the distribution of species and their abundances, species distribution models usually focus on abiotic factors only. We propose an integrative framework linking ecological theory, empirical data and statistical models to understand the distribution of species and their abundances together with the underlying community assembly dynamics. We illustrate our approach with 21 plant species in the French Alps. We show that a spatially nested modelling framework significantly improves the model’s performance and that the spatial variations of species presence–absence and abundances are predominantly explained by different factors. We also show that incorporating abiotic, dispersal and biotic factors into the same model bring new insights to our understanding of community assembly. This approach, at the crossroads between community ecology and biogeography, is a promising avenue for a better understanding of species co-existence and biodiversity distribution.

Mechanistic theory and modelling of complex food-web dynamics in Lake Constance

Alice Boit, Neo D. Martinez, Richard J. Williams, Ursula Gaedke — 2012-04-18T21:15:56.123282-05:00

Abstract

Mechanistic understanding of consumer-resource dynamics is critical to predicting the effects of global change on ecosystem structure, function and services. Such understanding is severely limited by mechanistic models’ inability to reproduce the dynamics of multiple populations interacting in the field. We surpass this limitation here by extending general consumer-resource network theory to the complex dynamics of a specific ecosystem comprised by the seasonal biomass and production patterns in a pelagic food web of a large, well-studied lake. We parameterised our allometric trophic network model of 24 guilds and 107 feeding relationships using the lake’s food web structure, initial spring biomasses and body-masses. Adding activity respiration, the detrital loop, minimal abiotic forcing, prey resistance and several empirically observed rates substantially increased the model's fit to the observed seasonal dynamics and the size-abundance distribution. This process illuminates a promising approach towards improving food-web theory and dynamic models of specific habitats.

An inherited virus influences the coexistence of parasitoid species through behaviour manipulation

Sabine Patot, Roland Allemand, Frédéric Fleury, Julien Varaldi — 2012-04-04T16:47:55.794904-05:00

Ecology Letters (2012)

Abstract

The potential role of pathogens or parasites in maintaining species coexistence is well documented. However, the impact of vertically transmitted symbionts, that can markedly modify their host’s biology, is largely unknown. Some females of the Drosophila parasitoid Leptopilina boulardi are infected with an inherited virus (LbFV). The virus forces females to lay supernumerary eggs in already parasitised hosts, thus allowing its horizontal transmission. Using two independent experimental procedures, we found that LbFV impacts inter-specific competition between L. boulardi and the related L. heterotoma. While L. boulardi rapidly outcompetes L. heterotoma in the absence of the virus, L. heterotoma was able to maintain or even to eliminate L. boulardi in the presence of LbFV. By forcing females to superparasitise, LbFV induced egg wastage in L. boulardi thus explaining its impact on the competition outcome. We conclude that this symbiont whose transmission is L. boulardi-density-dependant may affect the coexistence of Leptopilina species.

Environment-dependent selection on mate choice in a natural population of birds

Matthew R. Robinson, G. Sander van Doorn, Lars Gustafsson, Anna Qvarnström — 2012-04-10T00:18:56.905865-05:00

Abstract

Female mate choice acts as an important evolutionary force, yet the influence of the environment on both its expression and the selective pressures acting upon it remains unknown. We found consistent heritable differences between females in their choice of mate based on ornament size during a 25-year study of a population of collared flycatchers. However, the fitness consequences of mate choice were dependent on environmental conditions experienced whilst breeding. Females breeding with highly ornamented males experienced high relative fitness during dry summer conditions, but low relative fitness during wetter years. Our results imply that sexual selection within a population can be highly variable and dependent upon the prevailing weather conditions experienced by individuals.

Diversity-dependent stability under mowing and nutrient addition: evidence from a 7-year grassland experiment

Haijun Yang, Lin Jiang, Linghao Li, Ang Li, Mingyu Wu, Shiqiang Wan — 2012-04-10T00:25:29.16962-05:00

Ecology Letters (2012)

Abstract

Anthropogenic perturbations may affect biodiversity and ecological stability as well as their relationships. However, diversity–stability patterns and associated mechanisms under human disturbances have rarely been explored. We conducted a 7-year field experiment examining the effects of mowing and nutrient addition on the diversity and temporal stability of herbaceous plant communities in a temperate steppe in northern China. Mowing increased population and community stability, whereas nutrient addition had the opposite effects. Stability exhibited positive relationships with species richness at population, functional group and community levels. Treatments did not alter these positive diversity–stability relationships, which were associated with the stabilising effect of species richness on component populations, species asynchrony and portfolio effects. Despite the difficulty of pinpointing causal mechanisms of diversity–stability patterns observed in nature, our results suggest that diversity may still be a useful predictor of the stability of ecosystems confronted with anthropogenic disturbances.

Does phylogeny matter? Assessing the impact of phylogenetic information in ecological meta-analysis

2012-04-10T01:47:16.256727-05:00

Abstract

Meta-analysis is increasingly used in ecology and evolutionary biology. Yet, in these fields this technique has an important limitation: phylogenetic non-independence exists among taxa, violating the statistical assumptions underlying traditional meta-analytic models. Recently, meta-analytical techniques incorporating phylogenetic information have been developed to address this issue. However, no syntheses have evaluated how often including phylogenetic information changes meta-analytic results. To address this gap, we built phylogenies for and re-analysed 30 published meta-analyses, comparing results for traditional vs. phylogenetic approaches and assessing which characteristics of phylogenies best explained changes in meta-analytic results and relative model fit. Accounting for phylogeny significantly changed estimates of the overall pooled effect size in 47% of datasets for fixed-effects analyses and 7% of datasets for random-effects analyses. Accounting for phylogeny also changed whether those effect sizes were significantly different from zero in 23 and 40% of our datasets (for fixed- and random-effects models, respectively). Across datasets, decreases in pooled effect size magnitudes after incorporating phylogenetic information were associated with larger phylogenies and those with stronger phylogenetic signal. We conclude that incorporating phylogenetic information in ecological meta-analyses is important, and we provide practical recommendations for doing so.