Browsing by Author "Brazeiro, A"

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    Community patterns in sandy beaches of Chile: richness, composition, distribution and abundance of species
    (1999) Brazeiro, A
    The search for patterns, i.e., statistical arrangement in the data, is a key step to understand the way in which natural systems change in space and time. In order to detect community patterns I analyzed in this study some community traits of forty-five local assemblages of sandy beach invertebrates of Chile. There were identified three main community patterns. (1) Species richness tended to decrease with beach slope (morphodynamic state index), fitting a power model. The variability non-explained by beach slope (about 50%) was independent of grain size and latitude, but a little fraction (9%) was explained by sampling effort. Beach length seemed also to be related with this unexplained variability, but in a non-linear way. (2) The species composition of the local assemblages tended to be a non-random nested subgroup of the species composition of the: richer assemblages, i.e., the species composition fitted a nested pattern. (3) The species abundance and distribution were positively correlated, which indicate that wide distributed species tended also to be locally abundant. This is the first time that the "nested species composition pattern" (N degrees 2) and the "interspecific abundance-distribution pattern" (N degrees 3) are documented for sandy beach fauna. These three patterns were statistically evident at both regional (similar to 2 x 10(2) km) and geographical (similar to 2 x 10(3) km) scales, suggesting that they are scale-independent along the evaluated range. Much work is still needed to explain how sandy beach communities are structured, in the present study only the first step has been taken, i.e., the detection of patterns, leaving open the question about the underline processes and mechanisms.
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    Detection of non-random zonation patterns in sandy intertidal communities
    (1999) Brazeiro, A
    The issue of zonation patterns has historically received special attention from ecologists of rocky and sandy intertidal systems. Nowadays, the question of whether generalizable zonation patterns exist is still a focus of controversy, especially among sandy beach ecologists. Many procedures have been employed to detect zonation patterns, from graphical (kite diagram) to multivariate methods. These traditional techniques are not suitable, since neither of them allow an objective test of the non-randomness of the patterns detected. Determining whether the pattern observed differs from what is expected by chance is very important from the ecological point of view, since it would imply the existence of any underlying deterministic process. In this paper, I advocate the use of two techniques to analyze zonation patterns, randomized cluster analysis and gradient analysis, which solve the objectivity problem. These methods provide the risk (probability) of detecting spurious patterns, i.e., those that do not differ from what is expected by chance. In other words, the statistical significance of the pattern could be tested. I describe the procedures using data from a sandy intertidal system, and discuss their attributes and limitations.
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    Effects of harvesting and density dependence on the demography of sandy beach populations
    (1999) Brazeiro, A; Defeo, O
    Sandy beach ecologists concur that natural populations living in these harsh environments are controlled almost exclusively by physical factors, biotic factors being largely irrelevant. This paper provides evidence that human-induced perturbations as well as biotic, density-dependent processes also influence sandy beach populations. Results were obtained from a long-term study (8 yr) of the artisanally harvested yellow clam Mesodesma mactroides on a Uruguayan exposed sandy beach. The study included an experimental manipulation of fishing effort based on the closure of the clam fishery for 32 consecutive months. Fine-scale demographic parameters, such as age-specific survival probability and fertility, as well as coarse-scale demographic parameters, such as age composition, elasticity to demographic parameters, adult clam density, and population growth rate, were significantly affected by fishing effort and/or adult density. These results yield useful information from the management and conservation points of view, such as the threshold range of parent stock densities and fishing mortality levels capable of supporting a sustainable yield over time. Critical demographic parameters for population growth rate are also highlighted.
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    Is sandy beach macrofauna only physically controlled? Role of substrate and competition in isopods
    (1997) Defeo, O; Brazeiro, A; deAlava, A; Riestra, G
    Exposed sandy beaches have been defined as physically stressful environments, so that benthic populations living there are thought to be regulated mainly by physical factors, biological interactions being minimal. However, recent long-term studies indicate that potential intra-and interspecific interactions should also play a role in structuring populations and communities. This paper evaluates the role of sediment characteristics and potential interactions in determining the abundance and distribution patterns of the cirolanid isopods Excirolana armata and Excirolana braziliensis in sandy beaches of Uruguay. Results from concurrent field sampling and laboratory experiments showed that: (1) at a macroscale (between beaches), E. armata occurred only in beaches with fine sands, whereas E. braziliensis was observed in both fine and coarse sand beaches, reaching its highest density in the latter; (2) at a mesoscale (within beaches) and in sympatry (fine sands), both cirolanids showed maximum densities at different tidal heights, with E. braziliensis restricted to the upper beach levels; (3) both isopods showed a clear preference for fine sands, when tested in isolation or combined; (4) survivorship of E. armata was higher when tested in the preferred sediment under co-occurrence with E. braziliensis, which in turn presented higher survivorship in coarse sand, either in isolation or combined with E. armata; and (5) individual mean length of both species was consistently higher in allopatry, and similar body lengths were observed in sympatric populations. A geographical analysis of the abundance of E. brazilienis along Pan-American beaches showed that this isopod is most abundant in fine sands; this overall pattern supports conclusions derived from sediment preference experiments, implicating a greater niche breadth than that observed in Uruguayan beaches. It was concluded that E. armata could be defined as a high substrate-specific species in which intraspecific interactions would be of utmost importance in population regulation. However, distribution patterns off. braziliensis could not be explained by a simple animal-sediment relationship, and correlational evidence suggests that it is displaced by E. armata towards coarse sands and upper beach levels. Thus, potential biotic interactions cannot be discarded as a structuring force in sandy beach communities. (C) 1997 Academic Press Limited.
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    Population dynamics of the yellow clam Mesodesma mactroides: recruitment variability, density-dependence and stochastic processes
    (2000) Lima, M; Brazeiro, A; Defeo, O
    Recruitment of benthic marine invertebrates varies greatly at different spatio-temporal scales. For instance, population dynamics of the yellow clam Mesodesma mactroides at Uruguay exhibit large temporal fluctuations, mainly associated with recruitment. We used data from an 8 yr long-term study to develop an age-structured model to show that density-dependent and density-independent forces acting together can jointly explain the population fluctuations in a sandy-beach bivalve population of the yellow clam Mesodesma mactroides. The pure density-dependent deterministic model parameterised with empirical values estimated during the 8 yr study predicted stable dynamics. The dynamics of the deterministic skeleton was markedly influenced by the addition of a relatively small amount of stochastic variability to fertility rates. The yellow clam population dynamics seem to be driven by the combined forces of density-dependent and density-independent factors operating together. A combination of (uncorrelated) stochasticity in reproductive rates and asymmetric intercohort interactions (density-dependent recruitment and density-dependent survival rates) seems to be the key process generating large variability in recruitment.