Community structure can be measured by the variety and distribution of individuals among taxa. Taxa richness, or the number of distinct taxa, reflects the diversity within a sample of an assemblage. Taxonomic richness also is recommended as critical information in assays of natural phytoplankton assemblages Schelske, Taxa richness usually is species level but also can be evaluated at designated groupings of taxa, often at higher taxonomic levels that is, genus, family, order in assessments of invertebrate assemblages.
Relative abundance of taxa refers to the number of individuals of one taxon compared with that within the entire sample. Dominance, which is measured as percent composition of the dominant taxon Barbour and others, , is an indicator of community balance or lack thereof. Dominance is an important indicator when the most significant taxa are eliminated from the assemblage or if the food source is altered.
Dominants-in-common Shackleford, is a comparison with reference conditions to evaluate the extent to which dominance may reflect human influence. Taxonomic composition can be characterized by several classes of information, such as identity and sensitivity. Identity is the knowledge of individual taxa and associated ecological principles and environmental requirements.
Key taxa, which are those of special interest or are ecologically important, provide information that is important to the identity of the targeted assemblages. The presence of exotics or nuisance species may be an important aspect of biotic interactions that relates to identity and sensitivity. Sensitivity refers to the numbers of pollutant-tolerant and pollutant-intolerant species in the sample. A similar metric for fish assemblages is included in the IBI table 1.
Recognition of rare, endangered, or important taxa provides additional legal support for remediation activities or recommendations. Species status for response guilds of bird assemblages for example, whether they are threatened or endangered, native or introduced, or of some commercial or recreational value also relates to the composition class of metrics Brooks and others, Individual condition metrics are those that refer to the degradation of physical or physiological health of individual organisms.
This type of metric is not commonly used for benthic macroinvertebrates; examples of fish metrics for individual condition are "percent individuals diseased" and "percent individuals with fin rot. The functional aspects of biological processes can be divided into several categories as potential metrics.
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Trophic dynamics encompass functional feeding groups and relate to the energy source for the system, the identity of the herbivores and carnivores, the presence of detritivores in the system, and the relative representation of the functional groups. Abundance estimates are surrogate measures of standing crop and density that can relate to contaminant and enrichment problems. Inferences on the biological condition can often be drawn from a knowledge of the capacity of the system to support the survival and propagation of the top carnivore.
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This attribute can be a surrogate measure for predation rate. Without stable food dynamics, populations of the top carnivore reflect stressed conditions. Likewise, if production at a site is considered to be high on the basis of organism abundance or biomass and if high production is natural for the habitat type under study as per reference conditions , then biological conditions would be considered to be good.
Fitness is the capacity of an individual or population to maximize reproductive success by the production of viable offspring Price, and figures significantly in recruitment rate.
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Life cycle success, therefore, should include age-specific birth and death rates. Process metrics have been developed for a number of different assemblages. For example, table 1 indicates at least seven IBI metrics that deal with trophic status or feeding behavior in fish, which focuses on insectivores, omnivores, or herbivores. Also, the number or density of individuals of fish in a sample or an estimate of standing crop is a measure of production and, thus, in the function class of metrics.
Additional information is gained from density measures when they are considered to be relative to size or age distribution. Three RBP metrics for benthic macroinvertebrates focus on functional feeding groups table 2; Plafkin and others, ; Barbour and others, Brooks and others used trophic level as one category for rating avian assemblages. It may not be necessary to establish metrics for every attribute of the targeted assemblage.
However, the integration of information from several attributes, especially a grouping of metrics representative of the four major classes of attributes fig. Calibration of Metrics Pilot studies or small-scale research may be needed to define, evaluate, and calibrate metrics.
Metrics can be calibrated by using controlled prospective studies Jongman and others, ; that is, by evaluating the response of metric values to varying levels of stressors. Sites must be carefully selected for controlled prospective studies so that a wide range of suspected stressors on the stream ecosystems can be included. In general, impaired sites are selected because single and combined stressors have impacted them.
The selected impaired sites and the reference sites are the basis for the development of an empirical model of metric response to stressors. Metrics can be evaluated following model development. Candidate metrics that do not respond to any of the stressors expected in a region may be eliminated. Metrics also are evaluated for variability with respect to responsiveness; those with high variability compared with the range of response should be used with caution.
In this case, even lakes indirectly associated with the same river may have a higher connectivity with respect to individual exchanges, than lakes associated with different rivers. In fact, connectivity has been considered an important aspect to explain similarities of different groups of organisms in floodplain aquatic habitats Thomaz et al. This could also explain the similar species composition and richness among habitats of the same river. The most frequent species recorded in the lakes surveyed are also very common in the Pantanal Mato-grossense wetland Pott et al. The proximity of these floodplains and their similarities in water-level fluctuation are probable reasons for this similarity.
Pott et al. Of all the species recorded, two exotics with high invasive potential deserve special attention: Urochloa subquadripara and Hydrilla verticillata.
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Souza, unpublished. There is a lack of information about the invasiveness of H. Data from other ecosystems indicate that it may outcompete native species such as Egeria densa Mony et al. With regard to the temporal variation of community attributes, species richness per lake and species composition represented by the scores of the first DCA axis oscillated erratically during the six years, making it difficult to assess the effects of two atypical floods recorded in January and This confounding response is expected, because richness does not account for dominance or equitability, i.
The species composition as measured by DCA is also not sensitive, because it was estimated only with a presence-absence matrix in our study. Although we did not measure the effects of floods on plant densities in this study, they are widely recognized in floodplain lakes Junk and Piedade, ; Bini, ; Camargo and Esteves, For instance, available data indicate an increase in the occurrence of submersed species after Thomaz et al.
The increase of rooted submersed species including the exotic H. Despite the effects of reservoir operation which presumably affects almost all the lakes simultaneously , directly connected and isolated lakes differ according to species richness, composition, and degree of stability. Higher species richness in directly connected lakes may be accounted for by the constant and direct input of propagules brought by rivers into these habitats.
Ward and Tockner proposed that the alpha diversity of several groups reaches a peak in habitats with an intermediate degree of connectivity. However, because our data only deal with connected versus isolated lakes, it is difficult to infer if our connected lakes really represent intermediate degrees of connectivity. The connected lakes are usually shallower and experience greater water-level fluctuations because of their direct connection with the river main channel Santos and Thomaz, The variation in water level probably represents a selective force favoring certain species, which will dominate over time, and, together with the constant entrance of propagules carried in by river water, may explain the greater stability of these habitats.
In fact, intermediate-level disturbances such as floods in wetland systems may cause selective mortality, which would contribute to maintaining diversity or species richness Connell, ; Petraitis et al. On the other hand, isolated lakes usually follow a succession independent of each other leading to different communities over time , and some of them may occasionally dry completely. Thus, wider temporal variations in species richness temporal beta diversity and composition are expected in these habitats, which is consistent with the greater dissimilarity observed over time.
In summary, our results indicated that the degree of connectivity of the lakes with the main river is an important determinant of macrophyte richness and composition. We also found that, together with species richness, species composition varies more between lakes associated with different rivers, than between lakes associated with the same river.
Long-term trends of macrophyte species richness were not noted, but an increase in the richness and abundance of submersed species was recorded in the last ten years, as a response to increased water transparency. An understanding of the changes in community composition through time has important implications for applied ecology, especially for the application of predictive models Milner et al. Our results improved understanding of the importance of abiotic factors on aquatic macrophyte assemblages over a long time span.
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