1. The use of spatial variables is a common procedure in ecological studies. The technique is based on the definition of a connectivity ⁄ distance matrix that conceptually defines the dispersal of organisms. The shortest distance between two points is a straight line. Despite the fact that a straight line may not represent the easiest dispersal path for many kinds of organisms, straight-line distances are often used to detect patterns. We argue that other types of connectivity ⁄ distance matrices will better represent dispersal paths, such as the watercourse distance for aquatic organisms (e.g. fish, shrimps). 2. We used empirical and simulated community data to evaluate the usefulness of spatial variables generated from watercourse and overland (straight-line) distances. 3. Spatial variables based on watercourse distances captured patterns that straight-line distances did not, and provided better representations of the spatial patterns generated by dispersal along a dendritic network.

1. The effects of seasonal inundation on the biology of fishes on floodplains of large Amazonian rivers are well studied. However, the small seasonal changes in headwater streams are generally considered to have little effect on fish assemblages. 2. In this study, we analysed seasonal changes in the species composition and abundance of fish in small Amazonian forest streams. We sampled fish with hand and seine nets in headwater streams in a 10 000 ha terra-firme forest reserve near Manaus, Brazil. Each stream was surveyed at the end of the 2005 dry season, at the beginning of the 2006 rainy season and at the beginning of the 2006 dry season, by means of a standardized sampling effort. 3. The numbers of individuals and species caught were higher in the dry season, but rarefaction analyses indicated that greater species numbers could have been due simply to the larger number of individuals caught. 4. Between the dry and rainy season, the direction of changes in species composition in multivariate space varied among sites, especially for quantitative (abundance) data. However, the observed variation among sites was the less than expected if the directions of change were random. 5. Fish assemblages in the second dry season were more similar to those in the previous dry season than expected if changes in species composition among seasons were random. This indicates that a general seasonal pattern in fish assemblages can be detected, despite the existence of some erratic site-specific changes. 6. Most of the species that showed large seasonal variations in density occupy temporary ponds during the rainy season, when much of the valley is inundated and pond networks form adjacent to streams. Short-duration lateral migrations to these ponds may play an important role in the seasonal fish-assemblage dynamics in Amazonian headwater streams. 7. Our results contrast with previous studies on small Amazonian streams, which have found little seasonal change in fish assemblages, and highlight the importance of the abundance of common species as an indicator of general fish assemblage structure in biological monitoring programmes.

To determine the effect of rivers, environmental conditions, and isolation by distance on the distribution of species in Amazonia. Location: Brazilian Amazonia. Time period: Current. Major taxa studied: Birds, fishes, bats, ants, termites, butterflies, ferns + lycophytes, gingers and palms. We compiled a unique dataset of biotic and abiotic information from 822 plots spread over the Brazilian Amazon. We evaluated the effects of environment, geographic distance and dispersal barriers (rivers) on assemblage composition of animal and plant taxa using multivariate techniques and distance- and raw-data-based regression approaches. Environmental variables (soil/water), geographic distance, and rivers were associated with the distribution of most taxa. The wide and relatively old Amazon River tended to determine differences in community composition for most biological groups. Despite this association, environment and geographic distance were generally more important than rivers in explaining the changes in species composition. The results from multi-taxa comparisons suggest that variation in community composition in Amazonia reflects both dispersal limitation (isolation by distance or by large rivers) and the adaptation of species to local environmental conditions. Larger and older river barriers influenced the distribution of species. However, in general this effect is weaker than the effects of environmental gradients or geographical distance at broad scales in Amazonia, but the relative importance of each of these processes varies among biological groups.

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