Although an ecosystem's response to biodiversity loss depends on the order in which species are lost, the extinction sequences generally used to explore such responses in food webs have been ecologically unrealistic. We investigate how several extinction orders affect the minimum number of secondary extinctions expected within pelagic food webs from 34 temperate freshwater lakes. An ecologically plausible extinction order is derived from the geographically nested pattern of species composition among the lakes and is corroborated by species' pH tolerances. Simulations suggest that lake communities are remarkably robust to this realistic extinction order and highly sensitive to the reverse sequence of species loss. This sensitivity is not well explained by the known sensitivity of networks to the loss of highly connected species but appears to be better explained by our observation that trophic specialists preferentially consume widely distributed species at low risk of extinction. Our results highlight an important aspect of community organization that may help to maintain biodiversity amidst changing environments.

Mixed‐species animal groups (MSGs) are widely acknowledged to increase predator avoidance and foraging efficiency, among other benefits, and thereby increase participants' fitness. Diversity in MSG composition ranges from two to 70 species of very similar or completely different phenotypes. Yet consistency in organization is also observable in that one or a few species usually have disproportionate importance for MSG formation and/or maintenance. We propose a two‐dimensional framework for understanding this diversity and consistency, concentrating on the types of interactions possible between two individuals, usually of different species. One axis represents the similarity of benefit types traded between the individuals, while the second axis expresses asymmetry in the relative amount of benefits/costs accrued. Considering benefit types, one extreme represents the case of single‐species groups wherein all individuals obtain the same supplementary, group‐size‐related benefits, and the other extreme comprises associations of very different, but complementary species (e.g. one partner creates access to food while the other provides vigilance). The relevance of social information and the matching of activities (e.g. speed of movement) are highest for relationships on the supplementary side of this axis, but so is competition; relationships between species will occur at points along this gradient where the benefits outweigh the costs. Considering benefit amounts given or received, extreme asymmetry occurs when one species is exclusively a benefit provider and the other a benefit user. Within this parameter space, some MSG systems are constrained to one kind of interaction, such as shoals of fish of similar species or leader–follower interactions in fish and other taxa. Other MSGs, such as terrestrial bird flocks, can simultaneously include a variety of supplementary and complementary interactions. We review the benefits that species obtain across the diversity of MSG types, and argue that the degree and nature of asymmetry between benefit providers and users should be measured and not just assumed. We then discuss evolutionary shifts in MSG types, focusing on drivers towards similarity in group composition, and selection on benefit providers to enhance the benefits they can receive from other species. Finally, we conclude by considering how individual and collective behaviour in MSGs may influence both the structure and processes of communities.

Excess fishing capacity and the growth in global demand for fishery products have made overfishing ubiquitous in the world's oceans. Here we describe the potential catch losses due to unsustainable fishing in all countries' exclusive economic zones (EEZs) and on the high seas over 1950-2004. To do so, we relied upon catch and price statistics from the Sea Around Us Project as well as an empirical relationship we derived from species stock assessments by the U. S. National Oceanic and Atmospheric Administration. In 2000 alone, estimated global catch losses amounted to 7-36% of the actual tonnage landed that year, resulting in a landed value loss of between $6.4 and 36 billion (in 2004 constant US$). From 1950-2004, 36-53% of commercial species in 55-66% of EEZs may have been overfished. Referring to a species-level database of intrinsic vulnerability (V) based on life-history traits, it appears that susceptible species were depleted quickly and serially, with the average V of potential catch losses declining at a similar rate to that of actual landings. The three continental regions to incur greatest losses by mass were Europe, North America, and Asia-forming a geographic progression in time. But low-income and small island nations, heavily dependent on marine resources for protein, were impacted most profoundly. Our analysis shows that without the inexorable march of overfishing, ~20 million people worldwide could have averted undernourishment in 2000. For the same year, total catch in the waters of low-income food deficit nations might have been up to 17% greater than the tonnage actually landed there. The situation may be worst for Africa, which in our analysis registered losses of about 9-49% of its actual catches by mass in year 2000, thus seriously threatening progress towards the UN Millennium Development Goals

., et al. 2012. Positive relationships between association strength and phenotypic similarity characterize the assembly of mixed-species bird flocks worldwide. American Naturalist 180: 777-90. AuthorsHari Sridhar, Umesh Srinivasan, Robert A. Askins, Julio Cesar Canales-Delgadillo, Chao-Chieh Chen Submitted February 10, 2012; Accepted August 1, 2012; Electronically published MONTH? xx, 2012 Online enhancement: appendix. Dryad data: http://dx.doi.org/10.5061/dryad.th198.abstract: Competition theory predicts that local communities should consist of species that are more dissimilar than expected by chance. We find a strikingly different pattern in a multicontinent data set (55 presence-absence matrixes from 24 locations) on the composition of mixed-species bird flocks, which are important subunits of local bird communities the world over. By using null models and randomization tests followed by meta-analysis, we find the association strengths of species in flocks to be strongly related to similarity in body size and foraging behavior and higher for congeneric compared with noncongeneric species pairs. Given the local spatial scales of our individual analyses, differences in the habitat preferences of species are unlikely to have caused these association patterns; the patterns observed are most likely the outcome of species interactions. Extending group-living and social-information-use theory to a heterospecific context, we discuss potential behavioral mechanisms that lead to positive interactions among similar species in flocks, as well as ways in which competition costs are reduced. Our findings highlight the need to consider positive interactions along with competition when seeking to explain community assembly.

As human impacts to the environment accelerate, disparities in the distribution of damages between rich and poor nations mount. Globally, environmental change is dramatically affecting the flow of ecosystem services, but the distribution of ecological damages and their driving forces has not been estimated. Here, we conservatively estimate the environmental costs of human activities over 1961-2000 in six major categories (climate change, stratospheric ozone depletion, agricultural intensification and expansion, deforestation, overfishing, and mangrove conversion), quantitatively connecting costs borne by poor, middle-income, and rich nations to specific activities by each of these groups. Adjusting impact valuations for different standards of living across the groups as commonly practiced, we find striking imbalances. Climate change and ozone depletion impacts predicted for low-income nations have been overwhelmingly driven by emissions from the other two groups, a pattern also observed for overfishing damages indirectly driven by the consumption of fishery products. Indeed, through disproportionate emissions of greenhouse gases alone, the rich group may have imposed climate damages on the poor group greater than the latter's current foreign debt. Our analysis provides prima facie evidence for an uneven distribution pattern of damages across income groups. Moreover, our estimates of each group's share in various damaging activities are independent from controversies in environmental valuation methods. In a world increasingly connected ecologically and economically, our analysis is thus an early step toward reframing issues of environmental responsibility, development, and globalization in accordance with ecological costs. ecological degradation ͉ ecosystem change ͉ ecosystem services ͉ external cost

Longstanding theory predicts that competitive interactions set species' range limits in relatively aseasonal, species-rich regions, while temperature limits distributions in more seasonal, species-poor areas. More recent theory holds that species evolve narrow physiological tolerances in aseasonal regions, with temperature being an important determining factor in such zones. We tested how abiotic (temperature) and biotic (competition) factors set range limits and structure bird communities along strong, opposing, temperature-seasonality and species-richness gradients in the Himalayas, in two regions separated by 1500 km. By examining the degree to which seasonal elevational migration conserves year-round thermal niches across species, we show that species in the relatively aseasonal and speciose east are more constrained by temperature compared with species in the highly seasonal west. We further show that seasonality has a profound effect on the strength of competition between congeneric species. Competition appears to be stronger in winter, a period of resource scarcity in the Himalayas, in both the east and the west, with similarly sized eastern species more likely to segregate in thermal niche space in winter. Our results indicate that rather than acting in isolation, abiotic and biotic factors mediate each other to structure ecological communities.

Summary1. Animals group for various benefits, and may form either simple single-species groups, or more complex multi-species associations. Multi-species groups are thought to provide anti-predator and foraging benefits to participant individuals. 2. Despite detailed studies on multi-species animal groups, the importance of species in group initiation and maintenance is still rated qualitatively as 'nuclear' (maintaining groups) or 'attendant' (species following nuclear species) based on species-specific traits. This overly simplifies and limits understanding of inherently complex associations, and is biologically unrealistic, because species roles in multi-species groups are: (i) likely to be context-specific and not simply a fixed species property, and (ii) much more variable than this dichotomy indicates. 3. We propose a new view of species importance (measured as number of inter-species associations), along a continuum from 'most nuclear' to 'least nuclear'. Using mixed-species bird flocks from a tropical rainforest in India as an example, we derive inter-species association measures from randomizations on bird species abundance data (which takes into account species 'availability') and data on 86 mixed-species flocks from two different flock types. Our results show that the number and average strength of inter-species associations covary positively, and we argue that species with many, strong associations are the most nuclear. 4. From our data, group size and foraging method are ecological and behavioural traits of species that best explain nuclearity in mixed-species bird flocks. Parallels have been observed in multispecies fish shoals, in which group size and foraging method, as well as diet, have been shown to correlate with nuclearity. Further, the context in which multi-species groups occur, in conjunction with species-specific traits, influences the role played by a species in a multi-species group, and this highlights the importance of extrinsic factors in shaping species importance. 5. Our view of nuclearity provides predictive power in examining species roles in a variety of situations (e.g. predicting leadership in differently composed communities), and can be applied to examine a broad range of ecological and evolutionary questions pertinent to multi-species groups in general.

Non‐trophic interactions (or, inter‐species associations) play a prominent role in determining community structure and function. Mixed‐species bird flocks are networks of non‐trophic associations that confer foraging and anti‐predator benefits to participant species. Yet, the responses of these interspecific associations to anthropogenic environmental changes are poorly understood. Selective logging is pervasive in the tropics, and can affect associations in mixed‐species bird flocks by altering resource availability and/or predation risk, or through the altered abundances of species participating in mixed flocks. Across a gradient of logging intensity, we examined how the number and strength of associations in two different mixed‐species flock types responded to logging intensity, while simultaneously controlling statistically for changes in the abundances of species in response to logging. Across the logging gradient, we used network analyses to: (1) quantify the proportion of potentially realizable associations, and (2) measure the strengths of these realized associations. For both these analyses, we used null models to investigate whether changes in the network properties of mixed flocks were simply abundance‐driven, or congruent with expectations of how flock properties might be modified by selective logging. In understorey flocks, after controlling statistically for changing abundances of participant species, the proportion of realized associations decreased with increasing logging intensity, whereas mean association strength did not show any relation with logging intensity. In midstorey flocks, both the proportion and mean strength of realized associations increased with increasing logging intensity. Synthesis and applications. By statistically separating abundance and behaviour‐mediated effects, we show that interspecific associations in mixed‐species bird flocks respond to potential resource and/or predation changes from logging, but that their functional roles persist in logged forest. We show that logged forests can conserve not only community richness, but also function. Thus, these logged forests must be prevented from ongoing conversion into non‐forest habitats such as agriculture and plantation.