Recruitment and year-to-year variability in a population of Macoma balthica (L.)

Because of methodological problems, macrobenthic studies usually neglect the juvenile stages of invertebrate communities, due to the fact that appearance of recruits in samples is only detected some weeks or even months after their true recruitment. During this period, the temporary meiobenthos undergoes high rates of mortality. From year to year, juvenile survival rate is thus responsible for temporal patterns observed in adult population densities.The results presented here relate to the population dynamics of the tellinid bivalve Macoma balthica (L.). A study of temporary meiobenthos was conducted over two consecutive years in an intertidal Macoma-community located at the mouth of the Gironde Estuary in southwest France. Sampling of juvenile stages required short intervals (2 weeks) between successive samplings and a fine sieving mesh size (63 µm). Other population parameters, such as temporal patterns in density, reproductive cycle, and individual growth, were recorded.Recruitment processes showed a year-to-year variability, with regard to settlement density, settlement period, and survival rate. In 1983, recruitment was moderate and protracted over several months. Only one main recruitment period was detected in 1984, resulting in a high juvenile density. In a previous study (1977), by contrast, recruitment was almost non-existent.This variability is discussed as a function of climatic and sedimentological conditions which prevailed in the estuary throughout the study period. However, none of these physical factors appeared to underlie the recruitment fluctuation in Macoma balthica. It is suggested that biological interactions are of prime importance in regulating population densities in this community.     

Differential survival among life stages contributes to co‐dominance of Abies mariesii and Abies veitchii in a sub‐alpine old‐growth forest

Questions: Are there interspecific differences in mortality and recruitment rates across life stages between two shade‐tolerant dominant trees in a sub‐alpine old‐growth forest? Do such differences in demography contribute to the coexistence and co‐dominance of the two species? Location: Sub‐alpine, old‐growth forest on Mt. Ontake, central Honshu, Japan. Methods: From 1980 to 2005, we recorded DBH and status (alive or dead) of all Abies mariesii and A. veitchii individuals (DBH ≥ 5 cm) in a 0.44‐ha plot. Based on this 25 year census, we quantified mortality and recruitment rates of the two species in three life stages (small tree, 5 cm ≤ DBH < 10 cm; subcanopy tree, 10 cm ≤ DBH < 20 cm; canopy tree, DBH ≥ 20 cm). Results: Significant interspecific differences in mortality and recruitment rates were observed in both the small tree and sub‐canopy tree stages. In this forest, saplings (< 5 cm DBH) are mostly buried by snow‐pack during winter. As a consequence, saplings of A. mariesii, which is snow and shade tolerant, show higher rates of recruitment into the small tree stage than do those of A. veitchii. Above the snow‐pack, trees must tolerate dry, cold temperatures. A. veitchii, which can more readily endure such climate conditions, showed lower mortality rate at the subcanopy stage and a higher recruitment rate into the canopy tree stage. This differential mortality and recruitment among life‐stages determines relative dominance of the two species in the canopy. Conclusion: Differential growth conditions along a vertical gradient in this old forest determine survival of the two species prior to reaching the canopy, and consequently allow co‐dominance at the canopy stage.     

Density‐dependent mortality in adults,but not juveniles,of stream‐resident brown trout (Salmo trutta)

1. This study investigates when and where density dependence operates on the mortality rates of stream‐resident brown trout Salmo trutta. To this aim, I explored populations in habitats of different quality containing high, low or intermediate densities over broad scales of space and time. The study is based on census data of 170 cohorts quantified from recruitment to the total disappearance at 12 sites in four contrasting tributaries of the Rio Esva drainage (north‐western Spain), over the years 1986–2007. 2. Log₁₀‐transformed survivor density over time highlighted a consistent pattern for the 170 cohorts characterised by the occurrence of only two life stages. An early stage starts at recruitment, lasts about half the lifetime and shows no or negligible mortality. A threshold time at 425–620 days after emergence preceded a second stage of continuous and constant mortality until the final disappearance of the cohorts. Consequently, in all scenarios, mortality only occurred in the adult component and no effect of season, year, age‐class and/or reproductive stage was detected. 3. Substantial spatial and temporal variations typified both recruitment (range R = 0.01–1.62 ind m^?2) and adults’ mortality rates (range Z = 0.03–0.38 day^?1). Nested anova s revealed strong effects of site and year on both recruitment and mortality with sites interspersed along the stream gradients where recruitment and mortality were typically high relative to other sites located either nearby in the same stream or distant in another stream, where both recruitment and mortality rates were typically low or intermediate. 4. Adult mortality rates plotted against recruitment for the 170 cohorts pooled revealed a continuous, positive power relationship that explained 45.3% of variation in mortality rates over the whole range of recruitment values. Similarly, highly significant power relationships were elucidated for site‐specific mortality rates averaged across years and for annual‐specific mortality rates averaged across sites against the corresponding mean recruitment averaged across years and sites, respectively. These relationships support the hypothesis that the operation of density dependence is scale independent and context independent but operates in a continuous manner across all scenarios examined. 5. A chronic effect of density dependence on adult losses induces temporally persistent populations maintained by a low number of spawners. Apparently, the operation of density dependence adjusts the number of spawners to the availability of rearing and spawning habitat. This dynamic process may also help to explain the small effective population size (Ne) recently documented by genetic studies of stream‐living brown trout and other salmonids.     

Population viability analysis to identify management priorities for reintroduced Elk in the Cumberland Mountains,Tennessee

We used an individual-based population model to perform a viability analysis to simulate population growth (λ) of 167 elk (Cervus elaphus manitobensis; 71 male and 96 female) released in the Cumberland Mountains, Tennessee, to estimate sustainability (i.e., λ > 1.0) and identify the most appropriate options for managing elk restoration. We transported elk from Elk Island National Park, Alberta, Canada, and from Land Between the Lakes, Kentucky, and reintroduced them beginning in December 2000 and ending in February 2003. We estimated annual survival rates for 156 radio-collared elk from December 2000 until November 2004. We used data from a nearby elk herd in Great Smoky Mountains National Park to simulate pessimistic and optimistic recruitment and performed population viability analyses to evaluate sustainability over a 25-year period. Annual survival averaged 0.799 (Total SE = 0.023). The primary identifiable sources of mortality were poaching, disease from meningeal worm (Parelaphostrongylus tenuis), and accidents (environmental causes and unintentional harvest). Population growth given pessimistic recruitment rates averaged 0.895 over 25 years (0.955 in year 1 to 0.880 in year 25); population growth was not sustainable in 100% of the runs. With the most optimistic estimates of recruitment, mean λ increased to 0.967 (1.038 in year 1 to 0.956 in year 25) with 99.6% of the runs failing to be sustainable. We suggest that further translocation efforts to increase herd size will be ineffective unless survival rates are increased in the Cumberland Mountains. © 2011 The Wildlife Society.     

Simulation of the effects of Acanthaster planci on the population structure of massive corals in the genus Porites: evidence of population resilience?

Scleractinian corals in the genus Porites are slow growing, can live for centuries, and can attain great size. In these respects they differ from the majority of coral species, which grow faster and live for years to decades. The predatory starfish Acanthaster planci L. feeds on a wide range of coral species including Porites spp., and during outbreaks in its populations, causes high coral mortality and injury over much of the affected reefs. Because they are slow growing and because recent outbreaks of the starfish occurred only 15 years apart, it may be argued that the Porites populations on affected reefs will be sent into a period of prolonged decline. The present study uses a size stage model of the transition matrix type to predict effects of starfish outbreaks of various frequencies on Porites populations. A transition matrix characterizing the mortality and injury caused in different Porites size classes at John Brewer Reef during an outbreak year was determined from field data. Transition matrices for non-outbreak years were constructed on the basis of realistic growth rates and postulated survivorship and recruitment schedules. The medium term (100 years) effects of outbreaks were simulated by alternation of a single iteration of the outbreak matrix with many iterations of each non-outbreak matrix. By varying the interval between simulated outbreaks it was possible to define combinations of growth rate, survivorship and recruitment which were viable for various outbreak intervals. Simulations based on estimates of the initial size frequency distribution, recruitment rates and colony growth rates for the John Brewer Reef population predicted that the population would remain viable in the face of outbreaks every 15 years only if juvenile and adult survivorship were high. However, within the range of colony growth rates known to occur throughout the Great Barrier Reef and at recruitment rates of the same order as those estimated in the field population, it appears that a much wider range of survivorship schedules could lead to parity or even sustained growth in the face of outbreaks recurring at intervals of from 1 to 3 decades. It is suggested that because the key measurable parameters (initial size structure, damage characteristics, recruitment rate and growth rate) are likely to be very patchy at the scale of whole reefs, no general statement concerning the prognosis for Porites would be meaningful. However the model provides a tool by which a standardized evaluation of this type of field data may be made on a reef by reef basis.     

Demographics of an Experimentally Released Population of Elk in Great Smoky Mountains National Park

ABSTRACT We assessed the potential for reestablishing elk (Cervus elaphus) in Great Smoky Mountains National Park (GSMNP), USA, by estimating vital rates of experimentally released animals from 2001 to 2006. Annual survival rates for calves ranged from 0.333 to 1.0 and averaged 0.592. Annual survival for subadult and adult elk (i.e., ≥ 1 yr of age) ranged from 0.690 to 0.933, depending on age and sex. We used those and other vital rates to model projected population growth and viability using a stochastic individual-based model. The annual growth rate (λ) of the modeled population over a 25-year period averaged 0.996 and declined from 1.059 the first year to 0.990 at year 25. The modeled population failed to attain a positive 25-year mean growth rate in 46.0% of the projections. Poor calf recruitment was an important determinant of low population growth. Predation by black bears (Ursus americanus) was the dominant calf mortality factor. Most of the variance of growth projections was due to demographic variation resulting from the small population size (n = 61). Management actions such as predator control may help increase calf recruitment, but our projections suggest that the GSMNP elk population may be at risk for some time because of high demographic variation.     

Size-structure dynamics of two conifers in relation to understorey dwarf bamboo: A simulation study

Abstract. Size-structure dynamics and co-existence of the conifers Abies sachalinensis and Picea glehnii in subalpine forests in Japan, in relation to the understorey dwarf bamboo Sasa, were examined using a simulation model. This model explicitly incorporated recruitment rate, size growth rate and mortality. Recruitment and growth rates were assumed to be suppressed by the cumulative basal area of taller trees and the density of Sasa. As for the one-sided crowding effect, two types of model were examined. The total basal area of the two species suppresses the growth and recruitment rates in an additive model, while the basal area of each species has a separate effect included in a specific model. Two types of recruitment process were examined, i.e. open and closed systems; recruitment rates of open and closed systems are independent of, and proportional to, the basal area of the same species within the plot, respectively. Parameters were estimated from data of plot censuses for four years. The parameters of the specific model showed that recruitment and size growth rates were more suppressed by the same species than the other species. Recruitment of A. sachalinensis was more sensitive to suppression by Sasa compared with P. glehnii. The stationary size structure of the two species, generated from both the models with the open system, fitted well to the observed size structures across various Sasa densities. A. sachalinensis and P. glehnii dominated at lower and higher densities of Sasa, respectively. However, the closed-system simulation showed that the stable co-existence of the two species with a good fit to the observed size structure occurred only in the specific model. These results suggest that within-species interference, which is more severe than between-species interference, is important for the co-existence of the two species and that the relative dominance of the two species readily explained the differential responses to Sasa abundance.     

Population structure of the echinoid Heterocentrotus mammillatus (L.) along the littoral zone of south-eastern Sinai

The population structure of the slate-pencil sea urchin Heterocentrotus mammillatus was studied along the southeastern coast of the Sinai peninsula. Evenly-distributed size classes were observed in populations occupying the forereef, while populations of this species from the reef slope and the back reef contained smaller proportions of small and medium-sized individuals. During the five years of study, growth rates and abundances of each size group were found to be stable. Analysis of size frequency distributions show that recruitment of H. mammillatus at the study area was regular and mortality rates were low during the second to fifth year of life and increasing later. This was supported by aging of dead individuals and observations indicating that large individuals were more susceptible to predation than smaller ones.     

Contrasting population characteristics of yellow bass (Morone mississippiensis) in two southern Illinois reservoirs

Two southern Illinois reservoirs were investigated with contrasting size structures of yellow bass (Morone mississippiensis) to compare growth, mortality and recruitment patterns. Yellow bass were collected from Crab Orchard and Little Grassy lakes during April–May 2009 using AC electrofishing. Total length and weight were recorded and sagittal otoliths sectioned and aged by two readers. Increments between otolith annuli were measured and the Weisberg linear growth model was used to assess age and environmental (growth year) effects on individual growth for fish from the two lakes. Von Bertalanffy growth models indicated faster growth and a greater maximum total length for yellow bass in Little Grassy Lake. However, growth of fish in Little Grassy Lake nearly ceased after age 4. The Weisberg model indicated differences in individual growth rate between the two lakes that were consistent across years (age effects were significant, but growth year effects and the age‐growth year interaction were not). Inter‐lake differences in fish growth were present up to age 3. Recruitment was relatively stable in Crab Orchard, with year classes up to age 7 observed. Recruitment was more erratic in Little Grassy, with age 5 being the dominant year class and fish up to age 11 present. Differences in growth and recruitment patterns for yellow bass in the two lakes may be attributed to substantial inter‐lake differences in turbidity, morphoedaphic index, or yellow bass density. Maximum age of yellow bass (age 11) was higher than previously reported for this species, likely due to the use of otoliths to age fish rather than scales. This study provides baseline information on age and growth, mortality, recruitment, and size structure of yellow bass that can be compared to data in future studies to elucidate factors influencing population dynamics of this species.     

Demographic aspects of the soft coral Sinularia flexibilis leading to local dominance on coral reefs

We evaluated the role that demography may play in the formation of local aggregations of Sinularia flexibilis (Quoy & Gaimard, 1833), a soft coral that commonly dominates inshore coral reefs of the Great Barrier Reef (GBR), Australia. Two populations on inshore reefs of the Palm Islands were censused once a year for 3 years, starting 10 mo after the extensive bleaching mortality in early 1998. Larger colonies became more prevalent over time; mean colony size increasing by 35%, from 276 cm² in 1998 to 373 cm² in 2000. Growth rates were size dependent, with smaller colonies growing proportionally faster than larger colonies. Change in size relative to initial size indicated an expected mean annual growth of 128 cm² for a 50-cm² colony. Zero growth was predicted at 532±21cm², with colonies larger than this likely to undergo fission or shrink. Forty-three percent of colonies were undergoing fission at any time at both localities. Most new colonies were produced by fission (70%, n=285), with the remainder produced by the recruitment of sexually produced larvae (19%) or by colony translocation (11%). The sexual and asexual recruitment rates were 0.24 and 1.0 recruits m- ² year−¹, respectively. Opportunistic recruitment and rapid growth following disturbances are commonly assumed to be the mechanisms leading soft corals to dominate locally. In this study, these mechanisms operated more slowly than expected, with no net change in population size.     

Structure and Dynamics of Populations of Japanese Barberry (Berberis Thunbergii DC.) in Deciduous Forests of New Jersey

Japanese barberry, Berberis thunbergii DC., has become a prominent exotic species in deciduous forests throughout the eastern and midwestern US. Populations range from small plants occurring at low densities to dense, impenetrable thickets of plants with up to 40 stems/individual. A study was undertaken at Morristown National Historical Park in New Jersey to document plant densities, plant size, recruitment through vegetative processes of new shoot initiation and clonal spread and recruitment from seedling establishment, and mortality of stems and plants. Nearly 2000 shoots on 370 plants were individually marked and followed for two growing seasons, and over 1000 seedlings were also individually marked and followed. Populations vary much more in total shoots/area than they do in plant individuals/area, or in mean plant size (shoots/plant), as even the sparse populations have a few large individuals. Shoot mortality is less than new shoot initiation, but most plants do not change in size or change by small numbers of stems. However, the number of new shoots per plant increases as plant size increases. Once plants have three stems, they suffer little or no mortality. Seedling establishment is proportional to the density of shoots, so that as plants grow in size, local recruitment from seed increases. Large numbers of seedlings, and a survival rate of 10%, combine to make seedling recruitment a major component of population increase. The combination of multiple forms of vegetative and seed-based population growth, and the very low rates of plant mortality due to the multi-stemmed growth form explains the ability of this invasive species to rapidly produce dense, persistent populations.     

Mediterranean warming triggers seagrass (Posidonia oceanica) shoot mortality

Rapid warming of the Mediterranean Sea threatens marine biodiversity, particularly key ecosystems already stressed by other impacts such as Posidonia oceanica meadows. A 6‐year monitoring of seawater temperature and annual P. oceanica shoot demography at Cabrera Archipelago National Park (Balearic Islands, Western Mediterranean) allowed us to determine if warming influenced shoot mortality and recruitment rates of seagrasses growing in relative pristine environments. The average annual maximum temperature for 2002–2006 was 1 °C above temperatures recorded in 1988–1999 (26.6 °C), two heat waves impacted the region (with seawater warming up to 28.83 °C in 2003 and to 28.54 °C in 2006) and the cumulative temperature anomaly, above the 1988–1999 mean annual maximum temperature, during the growing season (i.e. degree‐days) ranged between 0 °C in 2002 and 70 °C in 2003. Median annual P. oceanica shoot mortality rates varied from 0.067 year^?1 in 2002 to 0.123 year^?1 in 2003, and exceeded recruitment rates in all stations and years except in shallow stations for year 2004. Interannual fluctuations in shoot recruitment were independent of seawater warming (P>0.05). P. oceanica meadows experienced a decline throughout the study period at an average rate of ?0.050±0.020 year^?1. Interannual variability in P. oceanica shoot mortality was coupled (R²>0.40) to seawater warming variability and increasing water depth: shoot mortality rates increased by 0.022 year^?1 (i.e. an additional 2% year^?1) for each additional degree of annual maximum temperature and by 0.001 year^?1 (i.e. 0.1% year^?1) for each accumulated degree water temperature remained above 26.6 °C during the growing season. These results demonstrate that P. oceanica meadows are highly vulnerable to warming, which can induce steep declines in shoot abundance as well indicating that climate change poses a significant threat to this important habitat.     

Tree species richness and turnover throughout New Zealand forests

Abstract. Patterns of mortality, recruitment, and forest turnover were investigated using permanent plot data from temperate forests in 14 localities throughout New Zealand. Tree mortality and recruitment rates were calculated from tagged trees ≥ 10 cm diameter at 1.4 m on individual 400 m² plots, and turnover rates were calculated as the mean of mortality and recruitment rates. Turnover rates (1.4% per year) were very similar to those recorded for tropical forests (i.e. 1.5% per year). As was shown in tropical forests, we also found significant relationships between forest turnover and species richness. In New Zealand forests there was also a decrease in species richness and turnover rates with increasing latitude. Although species richness is well known to decline with latitude, our study provides support for a possible link between seasonality and disturbance with tree turnover and species diversity. While tree mortality and recruitment rates were approximately in balance at some localities, in others there were imbalances between mortality and recruitment rates.     

Biomass turnover in populations of viviparous sphaeriid clams: Comparisons of growth,fecundity, mortality and biomass production

This paper reports studies carried out on natural populations of two species of viviparous clams, Sphaerium striatinum and S. simile, at three localities in two mesotrophic lakes in upstate New York. Sphaeriids are dominant members of the benthic fauna at the second trophic level in these localities, and more generally are cosmopolitan and ubiquitous animals.A growth survey involved monthly collections of regular samples, and these were used in measurements of growth, fecundity, mortality, and biomass production. S. striatinum proved to have an 18 to 24 month lifespan and S. simile one of about 26 months. Differences in reproduction included a maximum brood size of 12 in S. striatinum and of only 6 in S. simile, spat recruitment extending over 8 months in S. striatinum and over 9 months in S. simile, and a mean spat size at birth of 3.6 mm in S. striatinum and 6.4 mm in S. simile. The gross fecundity is thus markedly lower than that in the majority of molluscs. Mortality is to some extent compensating, adult survivorship being greater in S. simile, but mortality rates are remarkably steady and relatively low in both species. Field assessments of density showed that S. striatinum was dominant in one locality, and S. simile in another locality in the same lake, but that S. striatinum occurred at greater densities than any other sphaeriid species.Cage experiments confirmed the steady mortality rates and low fecundity values for these sphaeriids. Individual growth rates in cages are considerably higher than the mean values from population samples, since growth rates there are not obscured by recruitment of spat.A more complete analysis of growth in both species involved a series of over 400 determinations of total nitrogen (by micro-Dumas). These were utilized in computations of productivity. Embryonic production makes up only a small fraction (< 10 per cent) of the biomass turnover in these viviparous sphaeriids. The overall annual productivity (in terms of total nitrogen) is o.8 grams nitrogen per square meter per year for S. striatinum and only 0.28 grams nitrogen per square meter per year for S. simile. In a discussion of these figures they are shown to correspond to about 33 per cent of the second trophic level productivity for S. striatinum and about to per cent for S. simile. Since sphaeriids are long-lived, these percentages reflect a real dominance. In further discussion of the coexistence of these two species, it is pointed out that mortality is higher for the less common species in each locality.This research was supported by an N.S.F. pre-doctoral traineeship to the author and an N.I.H. Grant GM 11693 to Dr. W. D. Russell-Hunter, and represents a portion of a dissertation submitted as partial satisfaction for the Ph.D. at Syracuse University.     

Effects of black bear relocation on elk calf recruitment at Great Smoky Mountains National Park

Previous research from 2001 to 2006 on an experimentally released elk (Cervus elaphus) population at Great Smoky Mountains National Park (GSMNP or Park) indicated that calf recruitment (i.e., calves reaching 1 yr of age per adult female elk) was low (0.306, total SE = 0.090) resulting in low or negative population growth (λ = 0.996, 95% CI = 0.945–1.047). Black bear (Ursus americanus) predation was the primary calf mortality factor. From 2006 to 2008, we trapped and relocated 49 bears (30 of which were radiocollared) from the primary calving areas in the Park and radiomonitored 67 (28 M:39 F) adult elk and 42 calves to compare vital rates and population growth with the earlier study. A model with annual calf recruitment rate correlating with the number of bears relocated each year was supported (ΔAIC_c = 0.000; β = 0.070, 95% CI = 0.028–0.112) and a model with annual calf recruitment differing from before to during bear relocation revealed an increase to 0.544 (total SE = 0.098; β = −1.092, 95% CI = −1.180 to −0.375). Using vital rates and estimates of process standard errors observed during our study, 25-yr simulations maintained a mean positive growth rate in 100% of the stochastic trials with λ averaging 1.118 (95% CI = 1.096–1.140), an increase compared with rates before bear relocation. A life table response experiment revealed that increases in population growth were mostly (67.1%) due to changes in calf recruitment. We speculate that behavioral adaptation of the elk since release also contributed to the observed increases in recruitment and population growth. Our results suggest that managers interested in elk reintroduction within bear range should consider bear relocation as a temporary means of increasing calf recruitment. © 2011 The Wildlife Society.     

Predicting tree death for Fagus sylvatica and Abies alba using permanent plot data

Question: How well can mortality probabilities of deciduous trees(Fagus sylvatica) and conifers (Abies alba) be predicted using permanent plot data that describe growth patterns, tree species, tree size and site conditions? Location: Fagus forests in the montane belt of the Jura folds (Switzerland). Method: Permanent plot data were used to develop and validate logistic regression models predicting survival probabilities of individual trees. Backward model selection led to a reduced model containing the growth‐related variable ‘relative basal area increment’ (growth‐dependent mortality) and variables not directly reflecting growth such as species, size and site (growth‐independent mortality). Results: The growth‐mortality relationship was the same for both species (growth‐dependent mortality). However, species, site and tree size also influenced mortality probabilities (growth‐independent mortality). The predicted survival probabilities of the final model were well calibrated, and the model showed an excellent discriminatory power (area under the receiver operating characteristic curve = 0.896). Conclusion: Mortality probabilities of Fagus sylvatica and Abies alba can be predicted with high discriminatory power using a well calibrated logistic regression model. Extending this case study to a larger number of tree species and sites could provide species‐ and site‐specific tree mortality models that allow for more realistic projections of forest succession.     

Influence of Salinity on the Structure and the State of Bivalve Mytilus galloprovincialisPopulations

The influence of salinity on the structure and status of mussel Mytilus galloprovincialispopulations in the Black Sea was examined. Lowering salinity results in an increase in mortality rate and a decrease in numbers, biomass, juvenile recruitment, growth rates, and annual production. Low salinity produces changes in the size, age, sex, and phenotypic structure of mussel populations, namely, a decrease in the average size and age of mussels and an increase in the proportion of females and heterozygote deficiency.     

Comparative studies on the biology of nerita atramentosa Reeve, bembicium nanum (Lamarck) and cellana tramoserica (Sowerby) (gastropoda: Prosobranchia) in S.E. Australia

Populations of three coexisting intertidal gastropods, Nerita atramentosa Reeve, Bembicium nanum (Lamarck) and Cellana tramoserica (Sowerby), were sampled from a shore in Botany Bay, New South Wales, from July 1972 to September 1973. The recruitment and growth rates of each species were analysed from size frequency distributions. Mortality of each age cohort, and longevity, were estimated from analyses of the densities of Nerita atramentosa and Bembicium nanum.Nerita atramentosa showed no significant mortality during the first two years on the shore, but high mortality (at an instantaneous rate of 0.084 deaths/individual/month) after reaching the age of reproductive maturity, which was 20 months from settlement and at a mean shell-length of 13.5 mm. No growth could be detected after reproductive maturity was reached. Longevity of N. atramentosa was estimated as 3–5.5 years. Bembicium nanum juveniles showed higher mortality (at an instantaneous rate of 0.233 deaths/individual/month) than reproductively mature animals (0.060 deaths/individual/month). Reproductive maturity was reached at a mean shell-breadth of 11.0mm, i.e., about ten months after settlement on the shore. Longevity was estimated as from 4–8 years. Cellana tramoserica showed different growth patterns depending on the time of settlement.The three species showed different patterns of growth and life history relating to variable recruitment (which was demonstrated in all three species) and different rates of mortality of age cohorts.     

Growth facilitation by the octocoral Gorgonia ventalina explains spatial difference in the population size structure of the common demosponge Ircinia felix

In this study, the demography of the common demosponge Ircinia felix was examined at Tamarindo, a coral reef located in the island municipality of Culebra, Puerto Rico. A preliminary study comparing the size structure of two subpopulations within the reef, Tamarindo Norte (TN) and Tamarindo Sur (TS), indicated that sponges at TN are significantly larger than sponges at TS. This result served as a baseline for the present comparative study in which we aimed to determine whether the spatial differences in population size structure can be explained either by a difference in rates of survival, growth, or recruitment, or a combination of these. To accomplish our goal, we followed the growth, survival and recruitment of I. felix at the two localities for one year. Growth was the only demographic parameter that differed significantly between localities. Because the most obvious distinction between the study sites was the absence of the octocoral Gorgonia ventalina at TS, we hypothesized that the faster overall growth rate of sponges at TN was related to the presence of the octocoral. To test this hypothesis, we compared growth rates between sponges associated with the octocoral and those individuals not associated. We found that sponges growing near G. ventalina grew significantly faster than non-associated sponges. This result suggests that the octocoral facilitates the growth of I. felix and therefore may account, at least in part, for the spatial differences in population size structure.     

On the detection of dynamic responses in a drought-perturbed tropical rainforest in Borneo

The dynamics of aseasonal lowland dipterocarp forest in Borneo is influenced by perturbation from droughts. These events might be increasing in frequency and intensity in the future. This paper describes drought-affected dynamics between 1986 and 2001 in Sabah, Malaysia, and considers how it is possible, reliably and accurately, to measure both coarse- and fine-scale responses of the forest. Some fundamental concerns about methodology and data analysis emerge. In two plots forming 8 ha, mortality, recruitment, and stem growth rates of trees ≥10 cm gbh (girth at breast height) were measured in a ‘pre-drought’ period (1986–1996), and in a period (1996–2001) including the 1997–1998 ENSO-drought. For 2.56 ha of subplots, mortality and growth rates of small trees (10–<50 cm gbh) were found also for two sub-periods (1996–1999, 1999–2001). A total of c. 19 K trees were recorded. Mortality rate increased by 25% while both recruitment and relative growth rates increased by 12% for all trees at the coarse scale. For small trees, at the fine scale, mortality increased by 6% and 9% from pre-drought to drought and on to ‘post-drought’ sub-periods. Relative growth rates correspondingly decreased by 38% and increased by 98%. Tree size and topography interacted in a complex manner with between-plot differences. The forest appears to have been sustained by off-setting elevated tree mortality by highly resilient stem growth. This last is seen as the key integrating tree variable which links the external driver (drought causing water stress) and population dynamics recorded as mortality and recruitment. Suitably sound measurements of stem girth, leading to valid growth rates, are needed to understand and model tree dynamic responses to perturbations. The proportion of sound data, however, is in part determined by the drought itself. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.