Relative Importance Of Recruitment Research Articles

Relationships between vital rates and ecological traits in an avian community.

Comparative studies about the relationships between vital rates and ecological traits at the community level are conspicuously lacking for most taxa because estimating vital rates requires detailed demographic data. Identifying relationships between vital rates and ecological traits could help to better understand ecological and evolutionary demographic mechanisms that lead to interspecific differences in vital rates. We use novel dynamic N-mixture models for counts to achieve this for a whole avian community comprising 53 passerine species, while simultaneously accounting for density dependence and environmental stochasticity in recruitment and survival and, importantly, correcting our inferences for imperfect detection. Demographic stochasticity is taken into account in the form of the binomial and Poisson distributions describing survival events and number of recruits. We then explore relationships between estimated demographic parameters (i.e., vital rates) and ecological traits related to migration patterns, diet, habitat and nesting location of each species. The relative importance of recruitment and adult survival as contributors to population growth varied greatly among species, and interspecific differences in vital rates partly reflected differences in ecological traits. Migratory mode was associated with interspecific differences in population growth and density dependence. Resident species had higher population growth rates than long- and short-distance migrants. We found no relationships between diet and population growth rate. Habitat differences were associated with different growth rates: alpine, wetland and farmland species had lower population growth rates than forest species. Differences in population growth rates among nesting locations showed that breeding habitat is essential for population dynamics. Our study reveals relationships between ecological traits and contributions of vital rates to population growth and suggests ways in which patterns of population growth fluctuations in a community might be determined by life history.

Relative importance of recruitment and post-settlement processes in the maintenance of coral assemblages in an insular, fragmented reef system

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 473:149-162 (2013) - DOI: https://doi.org/10.3354/meps10101 Relative importance of recruitment and post-settlement processes in the maintenance of coral assemblages in an insular, fragmented reef system Lucie Penin1,2,3,*, Mehdi Adjeroud3,4 1ECOMAR, Laboratoire d’Écologie Marine. Faculté des Sciences et Technologies, Université de La Réunion, BP 7151, 97715 Saint-Denis Cedex 09, Reunion Island, France 2USR 3278 CNRS EPHE CRIOBE Centre de Recherches Insulaires et Observatoire de l’Environnement, Université de Perpignan, 66860 Perpignan Cedex, France 3Laboratoire d’Excellence ‘CORAIL’ 4Institut de Recherche pour le Développement, U 227 COREUS 2, BP A5, 98848 Nouméa Cedex, New Caledonia *Email: lucie.penin@univ-reunion.fr ABSTRACT: We compared relationships among distribution patterns of recruits, juveniles and adults of 3 different coral families with distinct life history traits at multiple spatial scales in an insular, fragmented reef system characterized by recurrent disturbances to better understand recruitment and maintenance processes of reef corals. The goal of our study was to detect at what scale most abundance variation occurred, and if this was consistent among life cycle stages and life histories. Results demonstrate a very high spatial variability of recruitment rates at the regional, insular and local hierarchical levels, with some locations displaying much higher recruitment rates than others, in 2 successive seasons. Juvenile and adult abundances were less variable, and most of the variation occurred at a local level for these life cycle stages. Recruit assemblage composition differed from juvenile and adult ones, which were much more similar. Moreover, abundance variation of recruits was unrelated to variation of juveniles and adults, but juvenile abundance was significantly correlated with adult abundance across all levels. These results have important implications for conservation planning, showing that some locations have better recovery abilities, and revealing the paramount importance of events occurring during the first weeks and months after settlement on the distribution of adults at regional, island and local levels, thus confirming the significance of local processes in shaping coral assemblages distribution and maintenance. KEY WORDS: Scleractinian corals · Recruit · Juvenile · Adult · Spatial scale · Life-cycle · French Polynesia Full text in pdf format PreviousNextCite this article as: Penin L, Adjeroud M (2013) Relative importance of recruitment and post-settlement processes in the maintenance of coral assemblages in an insular, fragmented reef system. Mar Ecol Prog Ser 473:149-162. https://doi.org/10.3354/meps10101 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 473. Online publication date: January 21, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Community Regulation: The Relative Importance of Recruitment and Predation Intensity of an Intertidal Community Dominant in a Seascape Context

Predicting the strength and context-dependency of species interactions across multiple scales is a core area in ecology. This is especially challenging in the marine environment, where populations of most predators and prey are generally open, because of their pelagic larval phase, and recruitment of both is highly variable. In this study we use a comparative-experimental approach on small and large spatial scales to test the relationship between predation intensity and prey recruitment and their relative importance in shaping populations of a dominant rocky intertidal space occupier, mussels, in the context of seascape (availability of nearby subtidal reef habitat). Predation intensity on transplanted mussels was tested inside and outside cages and recruitment was measured with standard larval settlement collectors. We found that on intertidal rocky benches with contiguous subtidal reefs in New Zealand, mussel larval recruitment is usually low but predation on recruits by subtidal consumers (fish, crabs) is intense during high tide. On nearby intertidal rocky benches with adjacent sandy subtidal habitats, larval recruitment is usually greater but subtidal predators are typically rare and predation is weaker. Multiple regression analysis showed that predation intensity accounts for most of the variability in the abundance of adult mussels compared to recruitment. This seascape-dependent, predation-recruitment relationship could scale up to explain regional community variability. We argue that community ecology models should include seascape context-dependency and its effects on recruitment and species interactions for better predictions of coastal community dynamics and structure.

New stably transfected bioluminescent cells expressing FLAG epitope-tagged estrogen receptors to study their chromatin recruitment.

BackgroundBiological actions of estrogens are mediated by the two specific estrogen receptors ERα and ERβ. However, due to the absence of adequate cellular models, their respective transcriptional activities are still poorly understood. For instance, the evaluation of such differing properties on the transcription of responsive genes using ChIP experiments was hindered by the deficiency of cells exhibiting the same genotypic background and properties but expressing only one of the ERs. We describe here the generation of such cells, using an estrogen receptor negative HELN cell line that was derived from HeLa cells stably transfected with an ERE-driven luciferase plasmid. These HELN-Fα and HELN-Fβ cell lines stably express either the alpha or beta (full length) estrogen receptor tagged with the FLAG epitope. The use of antibodies directed against the FLAG epitope allowed a direct comparative evaluation of the respective actions of both ERs using ChIP.ResultsHELN-Fα and HELN-Fβ cell lines were found to express comparable levels of their corresponding tagged receptors with a Kd for estradiol binding of 0.03 and 0.27 nM respectively. The presence of a stably transfected ERE-driven luciferase plasmid in these cells allowed the direct evaluation of the transcriptional activity of both tagged receptors, using natural or synthetic estrogens. FLAG-ERα and FLAG-ERβ were found to exhibit similar transcriptional activity, as indicated by a kinetic evaluation of the transcriptional activation of the luciferase gene during 10 hrs of treatment with estradiol. The validity of these model cells was further confirmed by the predictable transcriptional regulations measured upon treatments with ERα or ERβ specific ligands. The similar immunoprecipitation efficiency of both tagged receptors by an anti-FLAG antibody allowed the assessment of their kinetic recruitment on the synthetic luciferase promoter (containing an estrogen response element) by ChIP assays during 8 hours. A biphasic curve was obtained for both FLAG-ERα and FLAG-ERβ, with a peak occurring either at 2 hr or at 1 hr, respectively, and a second one following 4 hr of E2 stimulation in both cases. In MCF-7 cells, the recruitment of ERα also exhibited a biphasic behaviour; with the second peak however not so important than in the HeLa cell lines.ConclusionIn HELN derived cell lines, no fundamental differences between kinetics were observed during 8 hours for FLAG-ERα and FLAG-ERβ, as well as for polymerase II recruitment. However, the relative importance of recruitment between 1 hr and 4 hr was found to be different in HeLa cell line expressing exogenous tagged ERα and in MCF-7 cell line expressing endogenous ER.

Models of open populations with space-limited recruitment in stochastic environments: relative importance of recruitment and survival in populations of Semibalanus balanoides

The relative importance of recruitment and post-recruitment processes in determining adult density among marine species varies considerably between species and populations. This study aimed to investigate the relative effects of variation in recruitment and survival rates on the density dynamics of 3 spatially separated natural populations of the barnacle Semibalanus balanoides (Isle of Man, SW Ireland and west Sweden). We analysed 2 different data-based stochastic matrix models of open populations with space-limited recruitment. Assumption testing supported the application of matrix-model theory for studying and comparing population variables of open populations. Recruit- ment was found to be partly dependent on free space, and mortality was size-specific and indepen- dent of other vital rates. Model simulations showed that the relative importance of recruitment and survival in shaping adult density depends on the existing variation of these vital rates at each loca- tion. In the Isle of Man the variation in density was primarily caused by variability in recruitment, while in SW Ireland the variability in survival induced the most variation. In west Sweden variation was high due to variability in both recruitment and survival. A life-cycle elasticity analysis across locations indicated that the population dynamics were generally more sensitive to changes in sur- vival than to changes in recruitment. Also, locations with high variability in vital rates seemed better able to buffer sudden changes in vital rates. In addition, differences in survival and growth produced population structures that varied between populations. In the Isle of Man and west Sweden the populations consisted of mainly Size Category 1 individuals, while the population in SW Ireland had a linear relationship between size category and the proportion of individuals.

Effect of Variable Recruitment and Post-Recruitment Herbivory on Local Abundance of a Marine Alga

For marine algae, the relative importance of recruitment and post-recruitment processes in determining population abundance is not well understood. We documented patterns of recruitment and post-recruitment mortality for a common, clonally reproducing subtidal red alga from southeastern Australia, Delisea pulchra, and used a combination of empirical analyses and population simulations to assess the relative importance of the two processes to local population abundance. Using a hierarchical sampling design consisting of two depths and two spatial scales (quadrats and sites) within each depth, we followed monthly cohorts from recruitment to adulthood (6 mo of age). Recruitment of D. pulchra varied significantly in space and time and was generally decoupled from local adult abundance. Mortality was consistently high, particularly in the deep sites where only 1 of 291 plants survived >5 mo, and in the shallow sites when high densities of grazing sea urchins were present. Both early (1 mo) and late (6 mo) post-recruitment mortality of monthly cohorts were density independent at both spatial scales examined. Despite this pattern of mortality, variation in recruitment explained a significant amount of the subsequent variation in adult abundance in only 3 out of 10 cohorts at both spatial scales. This result indicates that high density-independent mortality was often important in determining adult abundance for individual cohorts. However, cumulative (across cohorts) recruitment into sites and quadrats over the entire study period explained a significant amount of the variation in the total number of plants that survived to reproduce there. Simulation models based on the measured demography of D. pulchra examined how the relative importance of recruitment and survivorship to population growth changed under two categories of sea urchin density: low (<80 urchins/m2) and high (>80 urchins/m2). These two categories represented the densities of sea urchins above and below a transition point where the mortality of D. pulchra went from being relatively low (17–33%/mo) to very high (63%/mo). When the effects of high densities of sea urchins were simulated, they caused a 95% decline in the population abundance of D. pulchra, compared to only a 31% decline when there were low densities of urchins. Significantly, the relative importance of recruitment and survivorship varied with sea urchin density: when there were high densities of sea urchins, population growth was more sensitive to increases in recruitment, whereas when there were low densities of urchins, survivorship was more important. Our findings demonstrate that the local population abundance of D. pulchra is determined by a combination of recruitment and density-independent mortality following recruitment, but it appears that the relative importance of the two processes varies in time and space.

EFFECT OF VARIABLE RECRUITMENT AND POST-RECRUITMENT HERBIVORY ON LOCAL ABUNDANCE OF A MARINE ALGA

For marine algae, the relative importance of recruitment and post-recruitment processes in determining population abundance is not well understood. We documented patterns of recruitment and post-recruitment mortality for a common, clonally reproducing subtidal red alga from southeastern Australia, Delisea pulchra, and used a combination of empirical analyses and population simulations to assess the relative importance of the two processes to local population abundance. Using a hierarchical sampling design consisting of two depths and two spatial scales (quadrats and sites) within each depth, we followed monthly cohorts from recruitment to adulthood (6 mo of age). Recruitment of D. pulchra varied significantly in space and time and was generally decoupled from local adult abun- dance. Mortality was consistently high, particularly in the deep sites where only 1 of 291 plants survived >5 mo, and in the shallow sites when high densities of grazing sea urchins were present. Both early (1 mo) and late (6 mo) post-recruitment mortality of monthly cohorts were density independent at both spatial scales examined. Despite this pattern of mortality, variation in recruitment explained a significant amount of the subsequent variation in adult abundance in only 3 out of 10 cohorts at both spatial scales. This result indicates that high density-independent mortality was often important in determining adult abundance for individual cohorts. However, cumulative (across cohorts) recruitment into sites and quadrats over the entire study period explained a significant amount of the variation in the total number of plants that survived to reproduce there. Simulation models based on the measured demography of D. pulchra examined how the relative importance of recruitment and survivorship to population growth changed under two categories of sea urchin density: low ( 80 urchins/M2). These two categories represented the densities of sea urchins above and below a transition point where the mortality of D. pulchra went from being relatively low (17-33%/mo) to very high (63%/mo). When the effects of high densities of sea urchins were simulated, they caused a 95% decline in the population abundance of D. pulchra, compared to only a 31% decline when there were low densities of urchins. Significantly, the relative importance of recruitment and survivorship varied with sea urchin density: when there were high densities of sea urchins, population growth was more sensitive to increases in recruitment, whereas when there were low densities of urchins, survivorship was more important. Our findings demonstrate that the local popu- lation abundance of D. pulchra is determined by a combination of recruitment and density- independent mortality following recruitment, but it appears that the relative importance of the two processes varies in time and space.

RECRUITMENT VS. POSTRECRUITMENT PROCESSES AS DETERMINANTS OF BARNACLE POPULATION ABUNDANCE

Determining the relative contributions of recruitment vs. postrecruitment processes to adult populations is an unresolved issue. The “recruit–adult” hypothesis suggests that the density of recruits is a good predictor of adult density when low but not when high. That is, the relative importance of recruitment vs. postrecruitment factors varies inversely with increasing density of recruits. In a rocky intertidal habitat at two Oregon coastal sites, a field experiment was done using two barnacle species to test this hypothesis. The relative impacts of these factors on adult barnacle abundance was determined using a reciprocal transplant design to manipulate both the density of barnacles established by recruitment and the postrecruitment conditions (tidal height, wave exposure) in which they lived. The relative contribution of recruitment to adult densities was strongly context dependent and species specific. While density of recruits clearly influenced density of adults for both species in most combinations...

Recruitment vs. Postrecruitment Processes as Determinants of Barnacle Population Abundance

Determining the relative contributions of recruitment vs. postrecruitment processes to adult populations is an unresolved issue. The “recruit–adult” hypothesis suggests that the density of recruits is a good predictor of adult density when low but not when high. That is, the relative importance of recruitment vs. postrecruitment factors varies inversely with increasing density of recruits. In a rocky intertidal habitat at two Oregon coastal sites, a field experiment was done using two barnacle species to test this hypothesis. The relative impacts of these factors on adult barnacle abundance was determined using a reciprocal transplant design to manipulate both the density of barnacles established by recruitment and the postrecruitment conditions (tidal height, wave exposure) in which they lived. The relative contribution of recruitment to adult densities was strongly context dependent and species specific. While density of recruits clearly influenced density of adults for both species in most combinations of site, zone, and exposure, the effects of physical and biotic factors ranged from strong to weak. For Chthamalus, recruitment generally had a stronger impact on density of adults than did postrecruitment processes, while for Balanus, postrecruitment factors tended to have stronger effects than did recruitment. These differences were an apparent consequence of differential susceptibilities to postrecruitment processes. Chthamalus was more tolerant of both biotic and abiotic forces than was Balanus and usually had high survival during periods of high Balanus mortality. Heat and desiccation were identified as the primary postrecruitment mortality factors in the high zone, whereas biotic interactions (competition and predation) were the most likely postrecruitment mortality processes in the mid-zone. Mortality was generally density dependent at the site with the strongest effects of postrecruitment processes, and generally density independent at the site with the strongest effects of recruitment. To determine whether trends were more consistent with the recruit–adult hypothesis at larger scales, data for each species were pooled across exposures (zone scale) and across exposures and zones (site scale) and also were compared to literature data from sites around the world (global scale). Each analysis led to the conclusion that recruitment can be a strong determinant of density of adults, but that the magnitude of this relationship depends on context, varies with species, and can be strongly modified by postrecruitment processes. Even when density of recruits is low, postrecruitment factors can be important in determining density of adults. Thus recruitment is a necessary but sometimes insufficient determinant of adult population density. These results support earlier suggestions that predictive models must incorporate both recruitment and postrecruitment factors and will thus depend on understanding the coupling between benthic and pelagic processes.

Early Post-Settlement Mortality in a Coral Reef Fish and Its Effect on Local Population Size

The relative importance of recruitment and post-recruitment factors in controlling population fluctuations and determining population sizes in open marine systems is still poorly understood. To address this issue, I conducted an experiment investigating whether very early post-recruitment mortality of a coral reef fish, Thalassoma bifasciatum, is dependent on recruitment density. To quantify the extent to which local populations are likely to be controlled primarily by recruitment or by post-settlement density-dependent mortality, I also measured natural mortality rates and recruitment variation at sites around the island of St. Croix, U.S. Virgin Islands. I manipulated densities of 1–3-d post-settlement T. bifasciatum on large natural patch reefs in Tague Bay, St. Croix, U.S. Virgin Islands. Manipulated recruits were followed for ∼30 d. The loss rate over the initial 24-h period following placement of recruits on the reef was positively related to the density at the beginning of that period. After this period, initial recruit density had no effects on mortality. Natural monthly mortality rates approximating the monthly mortality monitored during the experiment were measured for naturally recruited fish at sites around the island. Mortality was positively correlated with recruitment density. The effect was strong enough that across sites the relationship between recruit density and juvenile density the following month was not proportional; that is, juvenile density tended to reach an asymptote with increasing recruitment. The relationship between total recruitment per season at a site and adult abundance at the end of the season was similarly asymptotic, indicating the persistence of a density-dependent effect at the scale of the island. Mortality rates around the island were extremely variable and density independent at densities below ∼1 recruit/m2. Above that level, mortality rates were higher and tended to increase with further increases in recruitment density. Around St. Croix, three out of eight sites consistently received recruitment above this “threshold,” while five sites never experienced recruitment above that level during the study. Because spatial patterns of recruitment around St. Croix were consistent over time, some local populations may be controlled mainly by recruitment and density-independent mortality at all times, while others may be chronically affected by post-settlement density dependence. This study gives support to the idea that recruitment limitation and density dependence are not mutually exclusive. Rather, local populations must be characterized by the degree of recruitment, the resultant population density, and the subsequent degree of density-dependent regulation.

EARLY POST-SETTLEMENT MORTALITY IN A CORAL REEF FISH AND ITS EFFECT ON LOCAL POPULATION SIZE

The relative importance of recruitment and post-recruitment factors in controlling population fluctuations and determining population sizes in open marine systems is still poorly understood. To address this issue, I conducted an experiment investigating whether very early post-recruitment mortality of a coral reef fish, Thalassoma bifasciatum, is dependent on recruitment density. To quantify the extent to which local populations are likely to be controlled primarily by recruitment or by post-settlement density-dependent mortality, I also measured natural mortality rates and recruitment variation at sites around the island of St. Croix, U.S. Virgin Islands. I manipulated densities of 1–3-d post-settlement T. bifasciatum on large natural patch reefs in Tague Bay, St. Croix, U.S. Virgin Islands. Manipulated recruits were followed for ∼30 d. The loss rate over the initial 24-h period following placement of recruits on the reef was positively related to the density at the beginning of that period. After this period, initial recruit density had no effects on mortality. Natural monthly mortality rates approximating the monthly mortality monitored during the experiment were measured for naturally recruited fish at sites around the island. Mortality was positively correlated with recruitment density. The effect was strong enough that across sites the relationship between recruit density and juvenile density the following month was not proportional; that is, juvenile density tended to reach an asymptote with increasing recruitment. The relationship between total recruitment per season at a site and adult abundance at the end of the season was similarly asymptotic, indicating the persistence of a density-dependent effect at the scale of the island. Mortality rates around the island were extremely variable and density independent at densities below ∼1 recruit/m2. Above that level, mortality rates were higher and tended to increase with further increases in recruitment density. Around St. Croix, three out of eight sites consistently received recruitment above this “threshold,” while five sites never experienced recruitment above that level during the study. Because spatial patterns of recruitment around St. Croix were consistent over time, some local populations may be controlled mainly by recruitment and density-independent mortality at all times, while others may be chronically affected by post-settlement density dependence. This study gives support to the idea that recruitment limitation and density dependence are not mutually exclusive. Rather, local populations must be characterized by the degree of recruitment, the resultant population density, and the subsequent degree of density-dependent regulation.

Interpretation of Elasticity Matrices as an Aid to the Management of Plant Populations for Conservation

Population projection (Lefkovitch) matrices are now a standard method for quantifying and analyzing the demography and population dynamics of plants. Elasticity analysis of such a matrix indicates the relative effect on the population growth rate (λ) of small changes to matrix elements representing different transitions in the life cycle. In a comparison of elasticity matrices for 84 species of plants we show that the relative importance of recruitment from seed (measured by composite elasticity F), stasis (measured by composite elasticity L), and growth (measured by composite elasticity G) varies systematically between the groups semelparous herbs, iteroparous herbs of open habitats, forest herbs, shrubs, and trees. We discuss how this result might be used to construct rules of thumb useful in plant conservation. By looking in detail at how λ covaries with the composite elasticities F, L, and G among 16 populations of the semelparous herb Cirsium vulgare and among 15 populations of the rare iteroparous herb Pedicularis furbishiae, we show that a naive interpretation of elasticities can give a misleading prescription for management. Instead, we show that elasticity analysis supports management prescriptions based upon the successional status of a species or upon its response to disturbance.

The influence of habitat complexity on postrecruitment processes in a temperate reef fish population

In north-eastern New Zealand, adults of the blennioid fish Forsterygion varium Bloch et Schneider exhibited extreme differences in density among habitats of low, intermediate and high physical complexity. The age composition also differed among habitats, with complex sites supporting a greater proportion of older fish (> 1 + yr). The relative importance of recruitment and postrecruitment processes in determining these patterns was evaluated by monitoring the abundance, survival, growth and movement of two separate cohorts of juveniles in habitats of different complexity. The demographic data were based on records of individually recognised fish monitored from time of recruitment in permanent quadrats. Despite high temporal and spatial variability in the intensity and distribution of recruitment between years, adult F. varium were highly dependent on early postrecruitment mortality. Greater rates of early juvenile mortality were experienced within habitats of low complexity. The greatest impact of mortality III structuring spatial patterns occurred within the first week after settlement. Seasonal peaks in movement may have partially contributed to changes in patterns of distribution between habitats. The effects of habitat on growth were less clear, although there was a trend for growth to be better within habitats of high topographic complexity. The effect of topographic complexity on survivorship of juveniles was examined using a manipulative experiment which increased the complexity of simple habitats and reduced the complexity of heterogeneous habitats. Recruitment occurred roughly evenly across all treatments. Low complexity treatments suffered 100% mortality compared to only 13% in high complexity treatments. The demography of natural populations and the subsequent experiment suggest that differences in the abundance of adult F. varium between habitats can be attributed essentially to the early failure of juveniles to survive in large numbers in low complexity habitats.

Relative importance of recruitment and other causes of variation in rocky intertidal community structure

Recruitment limitation has been advocated as a major cause of community structure on rocky shores. Earlier work was criticized for failing to assess this possible source of variation. To evaluate this suggestion in relation to factors already known to be important in such communities, I incorporated estimates of recruitment with data from prior experiments in New England and Panama and reanalyzed the results using multiple regression. Rates of increase of prey abundance in predator exclusion experiments in New England were at least an order of magnitude greater than in Panama (e.g., ti took 4–6 mth vs. 60–72 mth to reach 100% cover, respectively). Recruitment densities of sessile invertebrates and algae were variable in space and time at both sites, but were lower by at least an order of magnitude and less synchronous in Panama than in New England. The analyses indicated that, although predation, competition, recruitment, and level on the shore explained significant amounts of variation in community structure at both places, the proportionate contributions of these factors differed. In New England, recruitment explained at most 11% while predation and competition explained 50% to 78% of variation in sessile invertebrate abundance. In Panama, recruitment explained 39% to 87% while predation and competition explained 8 to 10% of variation in sessile invertebrate adundance. Hence, when low, recruitment density appears important in influencing the structure of these communities.

Effects of Patch Size on the Abundance of Sessile Marine Invertebrates

The effects of patch size on the abundance of sessile invertebrates were investigated for two kinds of patchiness in a subtidal habitat in southern Australia: discrete pieces of substratum that were surrounded by water (isolated patches), and areas that were cleared within a background of other sessile organisms (nonisolated patches). They species composition of nonisolated patches following reoccupation was independent of patch size. Vegetative growth of organisms adjacent to the patches was the major means of reoccupation. Most larvae that recruited into the patches were overgrown, so that the species composition of the patches reflected the outcomes of interference competition among residents of patches. As a result, sponges and colonial tunicates were the most common taxa, with bryozoans and serpulid polychaetes being excluded. In contrast, the occupation of isolated patches was strongly size—dependent. On small substrata, species' abundances reflected their ability to colonize substrata by planktonic larvae, with few interactions among residents of patches. Bryozoans were the most abundant taxon, with a small number of patches containing tunicates, which have the potential to monopolize such patches. Larger substrata had a higher probability of being colonized by tunicates and sponges, in addition to bryozoans. Species compositions and abundances in these large patches were influenced most strongly by interference competition, rather than by recruitment. Bryozoans recruited preferentially onto small substrata (i.e., more recruits per square centimetre), while tunicate recruits occurred disproportionately on large substrata. Previously noted differences between the abundances of sessile invertebrate taxa on pier pilings and on the shells of the bivalve Pinna bicolor at Edithburgh can be ascribed to: (1) Differences in substratum size, which, when combined with the life histories of sessile invertebrates, result in variations in the relative importance of recruitment and interference competition for space. (2) Differential recruitment of bryozoans and tunicates that may be interpreted as evolved responses to patch—size—dependent rates of mortality (bryozoans) or size—dependent mean fitness (tunicates). (3) Predation by monacanthid fish, which reduces the incidence of tunicates (and thus potential competition) on Pinna shells and reduces the potential role of recruitment on pier pilings. (4) Differences between the total recruitment rates of some taxa on Pinna beds and beneath piers.

Changes in firing rate of human motor units during linearly changing voluntary contractions.

1. Human subjects generated approximately linearly increasing or decreasing voluntary, isometric contractions using the first dorsal interosseus muscle of the hand.2. Single motor units began firing at 8.4+/-1.3 impulses/sec (mean +/- S.D. of an observation) and increased their firing rate 1.4+/-0.6 impulses/sec for each change of 100 g in voluntary force. These values were independent of the threshold force for recruiting motor units.3. At intermediate rates of increasing and decreasing voluntary force (one complete cycle every 10 sec) the firing rate of single motor units varied linearly with force over the entire range of forces studied. However, during slow increases in voluntary force, the firing rate tended to reach a plateau, while during rapid increases an initial train of impulses at a roughly constant rate was observed.4. The relative importance of recruitment and increased firing rate, as mechanisms for increasing the force of voluntary contraction, was determined. Only at low levels of force is recuitment the major mechanism. Increased firing rate becomes the more important mechanism at intermediate force levels and contributes the large majority of force if the entire physiological range is considered.