Interspecific patterns of fish life histories were evaluated in relation to several theoretical models of life-history evolution. Data were gathered for 216 North American fish species (57 families) to explore relationships among variables and to ordinate species. Multivariate tests, performed on freshwater, marine, and combined data matrices, repeatedly identified a gradient associating later-maturing fishes with higher fecundity, small eggs, and few bouts of reproduction during a short spawning season and the opposite suite of traits with small fishes. A second strong gradient indicated positive associations between parental care, egg size, and extended breeding seasons. Phylogeny affected each variable, and some higher taxonomic groupings were associated with particular life-history strategies. High-fecundity characteristics tended to be associated with large species ranges in the marine environment. Age at maturation, adult growth rate, life span, and egg size positively correlated with anadromy. Parental care was inversely correlated with median latitude. A trilateral continuum based on essential trade-offs among three demographic variables predicts many of the correlations among life-history traits. This framework has implications for predicting population responses to diverse natural and anthropogenic disturbances and provides a basis for comparing responses of different species to the same disturbance.

Observed properties of natural food webs have both important theoretical and important management implications. Four lowland aquatic food webs were investigated over the course of two years: a large swamp and a small stream in Costa Rica, and a similar swamp and stream in the Venezuelan llanos. Each local ecosystem differed from the three others with respect to environmental changes associated with seasonal rainfall. Phylogenetic composition and diversity of biotas also varied among systems. Volumetric proportional utilization coefficients from fish gut contents were used as estimates of the intensity of predator—prey interactions. An annual and two or more seasonal food webs were constructed for each local community. Aquatic communities were defined operationally using common fish species as consumers, and using the sink subweb associated with the top predator of each system. A computer calculated a variety of food—web statistics and plotted food—web diagrams containing either (a) all observed trophic links (predator—prey interactions), or (b) subsets with weak links eliminated at prescribed thresholds. Individual community food webs contained from 58 (stream, Costa Rica) to 104 (swamp, Venezuela) interactive taxonomic units and from 208 to 1243 total trophic links. Food—web parameters were very sensitive to changes in level of link threshold. Web connectance and related parameters converged near link threshold 0.04 (utilization coefficients <0.04 eliminated) in a variety of inter—web comparisons. Despite large differences in assemblage composition and attributes of the physical environment, distributions of trophic levels calculated according to a trophic continuum algorithm were very similar among study systems. Herbivores, detritivores, and their direct predators formed the largest proportions of fishes in each assemblage, followed by omnivores and secondary carnivores. Fishes that fed at more than one trophic interval were extremely common in all food webs. Analysis of covariance was used to compare structural features of different webs across a range of link thresholds. Extensive among—site variation in food—web parameters was associated with differences in species richness and environmental differences associated with rainfall patterns, physiography, and gross primary production. Seasons generally influenced food—web parameters less than did site differences. Relative importance of detritus, aquatic primary production, and terrestrial production in aquatic food webs varied seasonally in each system. Detritus, derived primarily from aquatic macrophytes, was an important pathway in both tropical swamp ecosystems. Aquatic primary productivity comprised the largest fraction of fish diets during the wet season in the Venezuelan swamp, but it formed the major component of fish diets during the dry season at all other sites. Based on comparisons using 13 webs, two—thirds of the pairings among six food—web parameters used (number of nodes, compartmentation, connectance, average number of prey per node, average numb...

This study investigates the relationships among species diversity, community structure, and convergent evolution among divergent fish faunas. Morphological traits can be used as surrogates for ecological data in the comparative study of community niche relationships. In the present study I examined 30 morphological features related to the ecology of the dominant fish species from lowland stream and backwater habitats in five widely separated geographic regions: nearctic Alaska, temperate North America, Central America, South America, and tropical Africa. The study regions exhibited a general gradient of species richness from a minimum of 6 dominant species at one of the high—latitude sites (65° N) to a maximum of 43 numerically dominant species at a neotropical site (8° N). Fishes from Alaskan sites near the edge of the polar circle tended to be larger than fishes at other sites. Mean values for most morphological characters varied little between regions, indicating similar faunal centroids in morphological space. Morphological diversification within fish assemblages was estimated from species similarities based on Euclidean distances plus species projections on the principal axes from multivariate analysis. The total morphological space encompassed by ichthyofaunas in both stream and backwater sites was generally concordant with the latitudinal and species—richness gradient, with low—diversity nearctic assemblages exhibiting little morphological diversification relative to high—diversity tropical faunas. The Central American assemblages showed a greater range of ecomorphological diversification than African assemblages that contained a few more species, and this pattern may be related to greater seasonal stability at the Central American site. Phenetic patterns of dispersion reflect ecological relationships in which greater numbers of coexisting species are associated with higher levels of niche diversification and ecological specialization, leading to enhanced resource partitioning. Without additional ecological information, a community morphological analysis cannot directly determine whether or not increased ecological specialization is associated with the addition of new resources on the fringes of resource space or with increased subdivision of previously utilized core resources. Based on ecological information gathered concurrently with the fishes used in this analysis, I conclude that close species packing in morphological space is associated with niche generalists rather than with niche compression. With the possible exception of the two high—latitude sites, assemblages that contained more fish species generally did not exhibit tighter packing in niche space than species—poor assemblages, and this result was observed for comparisons both within and between regions. In contrast with several earlier studies, I interpret the lack of correlation between species richness and the average minimum distance between species in assemblage morphospace as being entirely consistent with the observed expansion of morphos...

No abstract

Techniques are developed for the analysis of community organization and bench tested on a set of simple model systems with known structure (i.e., with and without guild structure, with varying degrees of resource partitioning, with and without "core" resources). Proportional utilization coefficients, pi, are positively correlated with the abundance of resources, whereas electivities, ei, correlate negatively with resource abundance. The geometric mean of pi and ei, termed gi is a superior measure of utilization, more nearly independent of biases associated with resource availability than either of its components, and performs better in bench tests. Organization in observed patterns of resource utilization by four desert lizard and four tropical freshwater fish assemblages is critically evaluated via comparisons with results from two randomization algorithms. Randomizations follow a Monte Carlo technique whereby the dimensions of the original m × n resource matrix are preserved during hundreds of independent runs. The first "scrambled zeros" algorithm rearranges observed values for resource utilization by each consumer and retains consumer dietary niche breadths, but destroys guild structure (zero structure) of observed matrices. The second algorithm ("conserved zeros") also retains observed consumer diet breadths, but only rearranges observed resource utilization coefficients among the particular resources actually used by consumers, thus retaining observed guild structure (i.e., matrix zero structure). By plotting average dietary overlap against ordered niche neighbors, we evaluate (1) relative guild structure using the randomization algorithm that scrambles observed matrix zero structure, and (2) consumer resource segregation within guilds using the algorithm that conserves zero structure. Statistically significant guild structure is evident to varying degrees in all but one low—diversity fish assemblage. All four tropical fish assemblages reveal significant partitioning of food resources during both the wet and dry seasons, particularly among intermediate to distant neighbors. The most species—rich assemblage exhibits extremely high levels of resource segregation during the period of desiccation of aquatic habitat and increased fish densities. Diverse Australian desert lizard assemblages show significant partitioning of microhabitats among ecologically similar species, even though only 15 microhabitat categories are recognized. Analysis of dietary resource matrices based on only 19 prey resources shows that Australian lizards are piled up on certain prey types, forming functional dietary guilds of lizards that eat termites, ants, other lizards, etc. However, no dietary segregation is evident when only 19 prey resource states are recognized. In contrast, when the analysis is redone on more refined resource matrices based on more than 200 prey types, guild structure essentially disappears but niche segregation becomes evident. In the less diverse Kalahari desert, lizard assemblages are more variable and do not ap...

No abstract

Tropical rivers and their associated floodplain habitats are dynamic habitat mosaics to which fishes are challenged to respond in an adaptive manner. Migratory fishes create linkages among food webs that are partitioned along a nested hierarchy of spatial scales. Such linkages are examined across a hierarchy of spatio‐temporal scales, ranging from small streams to entire drainage basins, for rivers in South America and Africa. Migratory herbivorous fishes originating from eutrophic, productive ecosystems may subsidize resident predators of oligotrophic river ecosystems, which may result in cascading direct and indirect Effects on other species in local food webs. Successful management of many of the most important stocks of tropical river fishes requires conceptual models of how fish movement influences food web structure and dynamics.

We use stomach contents and stable isotope ratios of predatory fishes, collected over a 10‐year time span from a species‐rich river in Venezuela, to examine potential body‐size–trophic‐position relationships. Mean body size of predator taxa and their prey (determined by stomach content analyses) were significantly correlated, but trophic position of predators (estimated by stable isotope ratios) was not correlated with body size. This reflects no apparent relationship between body size and trophic position among prey taxa. Primary consumer taxa (algivores and detritivores) in this system are characterized by diverse size and morphology, and thus predatory fish of all body sizes and feeding strategies are able to exploit taxa feeding low in the food web. Regardless of relative body size, predators exploit short, productive food chains. For any given food chain within a complex web where predators are larger than their prey, trophic position and body size are necessarily correlated. But in diverse food webs characterized by a broad range of primary consumer body size, apparently there is no relationship between trophic position and body size across all taxa in the web.