Basin-scale planning is needed to minimize impacts in mega-diverse rivers

Ten traits related to life history theory were measured or estimated for 71 freshwater fish species from two locations in the Venezuelan llanos. Multivariate statistics and cluster analysis revealed three basic endpoint patterns bounding a two-dimensional continuum. A suite of attributes associated with parental care and aseasonal reproduction appeared to correspond to an equilibrium strategy. A second group of small fishes was distinguished by traits associated with rapid colonizing ability: early maturation, continuous reproduction, and small clutches. The third basic pattern was associated with synchronized reproduction during the early wet season, high fecundity, absence of parental care, and breeding migrations. A subset of mostly small fishes exhibiting little or no parental care, small clutches, and two to four month reproductive seasons was intermediate between the opportunistic (rapidly colonizing) and seasonal strategies. All ten life history variables showed significant effects of phylogeny. The cluster of species corresponding to the equilibrium group was dominated by siluriform fishes and perciforms of the Cichlidae. The opportunistic cluster was dominated by cyprinodontiform and characiform fishes, whereas the seasonal cluster contained primarily characiform and siluriform fishes. Seven of nine traits were significantly correlated with body length. The three reproductive patterns are interpreted as being adaptative with respect to relative intensity and predictability of temporal and spatial variation in abiotic environmental parameters, food availability, and predation pressure.

Density‐dependent processes such as growth, survival, reproduction and movement are compensatory if their rates change in response to variation in population density (or numbers) such that they result in a slowed population growth rate at high densities and promote a numerical increase of the population at low densities. Compensatory density dependence is important to fisheries management because it operates to offset the losses of individuals. While the concept of compensation is straightforward, it remains one of the most controversial issues in population dynamics. The difficulties arise when going from general concepts to specific populations. Compensation is usually quantified using some combination of spawner–recruit analysis, long‐term field monitoring or manipulative studies, and computer modelling. Problems arise because there are limitations to each of these approaches, and these limitations generally originate from the high uncertainty associated with field measurements. We offer a hierarchical approach to predicting and understanding compensation that ranges from the very general, using basic life‐history theory, to the highly site‐specific, using detailed population models. We analyse a spawner–recruit database to test the predictions about compensation and compensatory reserve that derive from a three‐endpoint life‐history framework designed for fish. We then summarise field examples of density dependence in specific processes. Selected long‐term field monitoring studies, manipulative studies and computer modelling examples are then highlighted that illustrate how density‐dependent processes led to compensatory responses at the population level. Some theoretical and empirical advances that offer hope for progress in the future on the compensation issue are discussed. We advocate an approach to compensation that involves process‐level understanding of the underlying mechanisms, life‐history theory, careful analysis of field data, and matrix and individual‐based modelling. There will always be debate if the quantification of compensation does not include some degree of understanding of the underlying mechanisms.

Life history theories attempt to explain the evolution of organism traits as adaptations to environmental variation. A model involving three primary life history strategies (endpoints on a triangular surface) describes general patterns of variation more comprehensively than schemes that examine single traits or merely contrast fast versus slow life histories. It provides a general means to predict a priori the types of populations with high or low demographic resilience, production potential, and conformity to density-dependent regulation. Periodic (long-lived, high fecundity, high recruitment variation) and opportunistic (small, short-lived, high reproductive effort, high demographic resilience) strategies should conform poorly to models that assume density-dependent recruitment. Periodic-type species reveal greatest recruitment variation and compensatory reserve, but with poor conformity to stock–recruitment models. Equilibrium-type populations (low fecundity, large egg size, parental care) should conform better to assumptions of density-dependent recruitment, but have lower demographic resilience. The model's predictions are explored relative to sustainable harvest, endangered species conservation, supplemental stocking, and transferability of ecological indices. When detailed information is lacking, species ordination according to the triangular model provides qualitative guidance for management and development of more detailed predictive models.

Aim  To examine the roles of local and regional environmental variables and biotic interactions in determining the structure of local stream fish assemblages, and to compare results derived from analyses based on taxonomic and functional groups. Location  Texas, USA. Methods  Species abundance data were compiled for 157 stream fish assemblages in several river basins across Texas. Species were condensed into functional groups based on trophic and life‐history characteristics. Local and regional environmental variables were either measured at each location or determined from scale maps and public‐access data bases. The original taxonomic and functional group data sets were analysed using similarity indices, null models of co‐occurrence, and direct and indirect ordination techniques. Results derived from taxonomic and functional group data sets are compared. Results  Inferences regarding the relative roles of local and larger‐scale factors in determining stream fish assemblage structure differ dramatically between analyses of taxonomic and functional groups. Taxonomic analyses suggest a prominent role of regional‐scale environmental factors, and local assemblages sorted according to a biogeographic pattern. Functional group analyses suggest almost equal roles of factors representative of local and larger scales, and assemblages were distinguished by a habitat template irrespective of geographic region. Main conclusions  The structure of local stream fish assemblages is determined ultimately by factors representing multiple scales, with the relative importance of each depending on the biological unit employed (species or functional groups). We suggest that analyses using functional groups can more directly infer ecological responses to environmental variation, and therefore may provide a more fruitful avenue for developing and testing ecological theory of community organization across biogeographic scales.

Aim  To identify key research questions and challenges that will, if addressed in a timely manner, significantly advance the field of freshwater fish biogeography and conservation. Location  Globe. Methods  By drawing on expertise from different regions of the world, we integrate an illustrative conspectus of recent scientific advancements in fish biogeography with a prospectus of needed areas of scientific inquiry to identify information gaps and priority research needs to advance the science. Results  We identified the following core challenges: (1) Testing current and forging new theories in biogeography; (2) Advancing a trait‐based biogeography of freshwater fishes; (3) Quantifying extinction risk and loss of fish species in a changing environment; (4) Evaluating the magnitude and geography of extinction debt for freshwater fishes; (5) Elucidating the patterns and drivers of freshwater fish invasions; (6) Forecasting the future geography of freshwater fishes; (7) Understanding the interactive effects of multiple stressors in freshwater ecosystems; (8) Quantifying new features of the biodiversity crisis: fish faunal homogenization and the emergence of novel assemblages; (9) Promoting scientific rigour in emerging freshwater fish conservation strategies and (10) Improving conservation planning strategies for freshwater fish species. Main conclusions  By reflecting on recent scientific progress in fish conservation biogeography, we have identified a set of core challenges and priorities requiring future research investment.

Ecology is often said to lack general theories sufficiently predictive for applications. Here, we examine the concept of a periodic table of niches and feasibility of niche classification schemes from functional trait and performance data. Niche differences and their influence on ecological patterns and processes could be revealed effectively by first performing data reduction/ordination analyses separately on matrices of trait and performance data compiled according to logical associations with five basic niche ‘dimensions’, or aspects: habitat, life history, trophic, defence and metabolic. Resultant patterns then are integrated to produce interpretable niche gradients, ordinations and classifications. Degree of scheme periodicity would depend on degrees of niche conservatism and convergence causing species clustering across multiple niche dimensions. We analysed a sample data set containing trait and performance data to contrast two approaches for producing niche schemes: species ordination within niche gradient space, and niche categorisation according to trait‐value thresholds. Creation of niche schemes useful for advancing ecological knowledge and its applications will depend on research that produces functional trait and performance datasets directly related to niche dimensions along with criteria for data standardisation and quality. As larger databases are compiled, opportunities will emerge to explore new methods for data reduction, ordination and classification.

High species richness and evenness in structurally complex habitats has been hypothesized to be associated with niche partitioning. To test this idea, relationships between habitat structural complexity in river littoral-zone habitats and morphological diversity of tropical fishes were examined in the Cinaruco River, Venezuela. Six habitat attributes were quantified in 45 sites spanning a range of structural complexity. Fishes were collected during day and night to estimate species density and relative abundances at each site. Twenty-two morphological variables were measured for each species. Principal components analysis (PCA) of physical habitat data yielded two axes that modeled >80% of variation across sites. The first two axes from PCA of fish morphological variables modeled >70% of variation. Species density during both day and night was negatively associated with flow velocity and positively associated with habitat complexity. Similarity of day and night samples from the same site was significantly greater for sites with high habitat complexity and low flow. In general, mean local assemblage morphological PC scores were not significantly associated with habitat PC scores. Average, maximum, and standard deviation of morphological Euclidean distances of local assemblages revealed positive associations with structural complexity and negative associations with flow. These relationships held even when the positive relationship of species density was statistically removed from assemblage morphological patterns. Findings suggest that both species niche compression and assemblage niche space increase when habitat complexity is greater and flow velocity is lower in this tropical lowland river.