Natural settlement dynamics of a young population of Turbinaria ornata and phenological comparisons with older populations

Our study was done during 16 months on two sites at Moruroa to (i) compare Moruroan populations (young) with Tahitian populations (old) of Turbinaria ornata and (ii) follow the natural dynamic of a young population at Moruroa. Moruroan and Tahitian populations have maximal density and maximal reproductive characteristics during the cool and hot seasons, respectively. Recruits are present throughout the year at all sites. Within the reference field site, individuals were initially distributed regularly and then became clumped through time, recruits appeared around adults. Recruits mortality followed a similar pattern, whereas mortality of adults and juveniles was even throughout the monitoring period. Populations initially had similar proportions of recruits, juveniles and adults (28, 40 and 32%, respectively), but at the end, recruits represent 92% of the population. A linear relationship between the number of individuals present within the station and the colonized substrate area was determined (R² = 0.96), suggesting a limiting effect of the substrate against the settlement of T. ornata with time. Moreover, the growth of recruits in the close vicinity of adults was arrested in the young populations: recruitment may therefore, significantly buffer high rates of adult mortality. It may contribute to the invasion and, more interesting to the persistence of T. ornata in new colonized area.     

Intraspecific interactions between adults and juveniles of the subtidal limpet,Patelloida mufria

Summary Regular monitoring of populations of the subtidal limpet Patelloida mufria (Hedley), which were at naturally large densities (>1000/m²), showed that at the times when there were peaks in juvenile recruitment, the mortality of adults increased greatly. This produced a cyclical pattern in the density of adults, suggesting the juveniles play a partial regulatory role. Experimental manipulations were done to test the influence of overall density and the density of juveniles on the growth and survival of adults. These indicated that an increase in overall density reduced the rate of growth of individuals and increased the rate of adult mortality during the peaks in juvenile recruitment. Similarly, removing these juveniles as they recruited averted this decline in adult abundance. The proposed mechanism to explain these results is that the juveniles have a competitive advantage over adults as they are much smaller and therefore require less energy to survive. It is hypothesisied, therefore, that there will be a threshold of adult size below which the recruits will lose their competitive advantage. Furthermore, if their density became too great, the cover of crustose algae (which is the main food of this species) would be removed resulting in a rapid decline in the numbers of P. mufria. Such an occurrence could be the ultimate regulatory agent for this species.     

Prerequisites for recruitment of Kittiwakes Rissa tridactyla

Observations at the North Shields Kittiwake colony between 1982 and 1984 showed over 100 potential recruits (birds of breeding age) associated with the colony, only about 40% of which were actually recruited. In addition, there were younger birds which visited the colony. The origins, age structure and intercolony movements of members of the pool of prospectors have been examined. Porter & Coulson (1987) demonstrated that recruits are heavier than prospectors, implying that they are in better body condition. Three further prerequisites for breeding in Kittiwakes are recognized: the age of recruits is at least three years; all birds which recruit are present at the colony at least one year prior to breeding; and recruits show a high degree of attachment to colony as illustrated by their colony and nest site tenacity, attendance and early arrival. The results suggest that there is considerable selection of individuals at the time of recruitment; the mechanism for this has yet to be examined.     

Is the small clutch size of a Corsican blue tit population optimal?

In an attempt to test predictions of the optimisation hypothesis of life history traits in birds, we estimated fitness consequences of brood size manipulations. Experiments were carried out over a period of 4 years in a Mediterranean population of blue tits Parus caeruleus which is confronted with a particular set of environmental constraints. Effects of brood size manipulation were investigated in relation to year-to-year variation in environmental conditions, especially caterpillar abundance. There was a strong variation in the effects of brood size manipulation depending on year. Most effects were on offspring quality (fledging mass, tarsus length). The absolute number of recruits did not significantly differ among categories (reduced, control, enlarged broods) but varied considerably among years. Females recruited from enlarged broods were of lower quality, started to breed later and laid fewer eggs than those recruited from control and reduced broods. Neither parental survival nor reproductive performances of adults in year n + 1 was affected by brood size manipulation in year n. Thus there was no evidence for a cost of reproduction in this population. Since the number of recruits did not depend on brood size manipulation (recruitment rates were higher in reduced broods), but recruits from reduced broods were of better quality compared with other groups, we conclude that adults lay a clutch that is larger than that which is predicted by the optimisation hypothesis. Producing more young could incur some penalties because offspring from large broods are of lower quality and less likely to recruit in the population. Two possible reasons why decision rules in this population seem to be suboptimal are discussed. Received: 10 March 1998 / Accepted: 1 July 1998     

Strong density-dependent survival and recruitment regulate the abundance of a coral reef fish

Debate on the control of population dynamics in reef fishes has centred on whether patterns in abundance are determined by the supply of planktonic recruits, or by post-recruitment processes. Recruitment limitation implies little or no regulation of the reef-associated population, and is supported by several experimental studies that failed to detect density dependence. Previous manipulations of population density have, however, focused on juveniles, and there have been no tests for density-dependent interactions among adult reef fishes. I tested for population regulation in Coryphopterus glaucofraenum, a small, short-lived goby that is common in the Caribbean. Adult density was manipulated on artificial reefs and adults were also monitored on reefs where they varied in density naturally. Survival of adult gobies showed a strong inverse relationship with their initial density across a realistic range of densities. Individually marked gobies, however, grew at similar rates across all densities, suggesting that density-dependent survival was not associated with depressed growth, and so may result from predation or parasitism rather than from food shortage. Like adult survival, the accumulation of new recruits on reefs was also much lower at high adult densities than at low densities. Suppression of recruitment by adults may occur because adults cause either reduced larval settlement or reduced early post-settlement survival. In summary, this study has documented a previously unrecorded regulatory mechanism for reef fish populations (density-dependent adult mortality) and provided a particularly strong example of a well-established mechanism (density-dependent recruitment). In combination, these two compensatory mechanisms have the potential to strongly regulate the abundance of this species, and rule out the control of abundance by the supply of recruits.     

Evidence of stable genetic structure across a remote island archipelago through self‐recruitment in a widely dispersed coral reef fish

We used microsatellite markers to assess the population genetic structure of the scribbled rabbitfish Siganus spinus in the western Pacific. This species is a culturally important food fish in the Mariana Archipelago and subject to high fishing pressure. Our primary hypothesis was to test whether the individuals resident in the southern Mariana Island chain were genetically distinct and hence should be managed as discrete stocks. In addition to spatial sampling of adults, newly‐settled individuals were sampled on Guam over four recruitment events to assess the temporal stability of the observed spatial patterns, and evidence of self‐recruitment. We found significant genetic structure in S. spinus across the western Pacific, with Bayesian analyses revealing three genetically distinct clusters: the southern Mariana Islands, east Micronesia, and the west Pacific; with the southern Mariana Islands being more strongly differentiated from the rest of the region. Analyses of temporal samples from Guam indicated the southern Mariana cluster was stable over time, with no genetic differentiation between adults versus recruits, or between samples collected across four separate recruitment events spanning 11 months. Subsequent assignment tests indicated seven recruits had self‐recruited from within the Southern Mariana Islands population. Our results confirm the relative isolation of the southern Mariana Islands population and highlight how local processes can act to isolate populations that, by virtue of their broad‐scale distribution, have been subject to traditionally high gene flows. Our results add to a growing consensus that self‐recruitment is a highly significant influence on the population dynamics of tropical reef fish.     

Disappearance of a spring cohort in a population of the dragonet,Repomucenus valenciennei, with spring and autumn spawning peaks in Tokyo Bay, Japan

The occurrence of eggs, pelagic larvae and juveniles, and settled juveniles of the dragonetRepomucenus valenciennei in Tokyo Bay, Japan, were investigated by plankton net and bottom trawl surveys between September 1990 and September 1991. Eggs, and pelagic larvae and juveniles appeared from April to November (spring to autumn), peaking in both spring and autumn. From the temporal pattern of egg and pelagic fish occurrence, and pelagic duration reported elsewhere (ca. one month), settlement could be predicted as occurring from late spring to autumn. However, settled juveniles appeared from August to December, with an abrupt peak in November. Aging from daily increments in the otoliths of settled recruits in 1990 indicated that the latter comprised individuals which had hatched between mid-September and early November (i.e. autumn cohort), implying that individuals which had hatched in spring to summer (April to August) were not recruited. Benthic hypoxia occurs widely in Tokyo Bay, from June to October each year, particularly in the northern part, which is the main nursery area ofR. valenciennei. The timing of dissolved oxygen recovery, and appearance of settled fish coincided closely (i.e. November), indicating that summer hypoxic conditions prevented the settlement of the spring cohort and hence recruitment to the population.     

Paving the way for the new generations: Astarte borealis population dynamics in the White Sea

Populations of marine bivalves are known to be unstable but it is unclear whether this instability has internal or external causes. In this paper we summarize results from long-term (1978–2007) monitoring of an Astarte borealis (Astartidae) population in the White Sea. We sought significant long-term changes among a population of subtidal clams existing under relatively stable conditions. The studied population was initially (in 1978–1995) formed exclusively by adult clams, with no recruitment occurring until a mass inflow of unidentified astartid’s juveniles in the late 1990s. We assessed the suitability of periostracum microsculpture for identification of astartid juveniles and identified the juveniles recruited the population as A. borealis. Thus significant changes in population structure were detected. Our study suggests that A. borealis may have a very long life span (>20 years) and a prolonged recruitment failure could be associated with scarcity of hard substrates needed for egg attachment in a silted environment. As old clams died, their valves provided a hard substrate on the bottom surface, facilitating recruitment. We propose a new conceptual model of subtidal clams’ population dynamics predicting long-term cyclic changes driven by internal mechanisms associated with an unusual type of positive influence of adults upon recruits.     

Weights and growth rates in the Mountain hare Lepus timidus scoticus

Mountain hares were weighed during live trapping on a study area near Dufftown, Banffshire, from June 1958 to August 1966. Hares shot or killed by other means on the study area and elsewhere were also weighed. Females were heavier than males throughout the year and this difference became apparent in juveniles by August of their year of birth. Male hares lost weight during the breeding season (January to June) but regained it in late summer. Young hares gained weight initially at about 14 g per day, then at about 6 g per day to adult weight. Small juveniles, or those born late in the season, tended to become small adults, large or early juveniles to become large adults. Small hares moulted less completely andjbegan to breed later in the season than large hares. The effects of disease, starvation, severe weather and injury on hare weights were considered. Female weights were greater when the population was small, due to more late pregnancies or more embryos per female. Weight could not be used to distinguish between adult and juvenile hares above 2.1 kg, nor between young and older adults.     

Influences of multilocus heterozygosity on size during early life

Genetic diversity has been hypothesized to promote fitness of individuals and populations, but few studies have examined how genetic diversity varies with ontogeny. We examined patterns in population and individual genetic diversity and the effect of genetic diversity on individual fitness among life stages (adults and juveniles) and populations of captive yellow perch (Perca flavescens) stocked into two ponds and allowed to spawn naturally. Significant genetic structure developed between adults and offspring in a single generation, even as heterozygosity and allelic richness remained relatively constant. Heterozygosity had no effect on adult growth or survival, but was significantly and consistently positively related to offspring length throughout the first year of life in one pond but not the other. The largest individuals in the pond exhibiting this positive relationship were more outbred than averaged size individuals and also more closely related to one another than they were to average‐sized individuals, suggesting potential heritability of body size or spawn timing effects. These results indicate that the influence of heterozygosity may be mediated through an interaction, likely viability selection, between ontogeny and environment that is most important during early life. In addition, populations may experience significant genetic change within a single generation in captive environments, even when allowed to reproduce naturally. Accounting for the dynamic influences of genetic diversity on early life fitness could lead to improved understanding of recruitment and population dynamics in both wild and captive populations.     

Coral reef fish larvae settle close to home

Population connectivity through larval dispersal is an essential parameter in models of marine population dynamics and the optimal size and spacing of marine reserves. However, there are remarkably few direct estimates of larval dispersal for marine organisms, and the actual birth sites of successful recruits have never been located. Here, we solve the mystery of the natal origin of clownfish (Amphiprion polymnus) juveniles by mass-marking via tetracycline immersion all larvae produced in a population. In addition, we established parentage by DNA genotyping all potential adults and all new recruits arriving in the population. Although no individuals settled into the same anemone as their parents, many settled remarkably close to home. Even though this species has a 9-12 day larval duration, one-third of settled juveniles had returned to a 2 hectare natal area, with many settling <100 m from their birth site. This represents the smallest scale of dispersal known for any marine fish species with a pelagic larval phase. The degree of local retention indicates that marine reserves can provide recruitment benefits not only beyond but also within their boundaries.     

Genetic maternity and paternity in a local population of armadillos assessed by microsatellite DNA markers and field data

Genetic data from polymorphic microsatellite loci were employed to estimate paternity and maternity in a local population of nine-banded armadillos (Dasypus novemcinctus) in northern Florida. The parentage assessments took advantage of maximum likelihood procedures developed expressly for situations when individuals of neither gender can be excluded a priori as candidate parents. The molecular data for 290 individuals, interpreted alone and in conjunction with detailed biological and spatial information for the population, demonstrate high exclusion probabilities and reasonably strong likelihoods of genetic parentage assignment in many cases; low mean probabilities of successful reproductive contribution to the local population by individual armadillo adults in a given year; and statistically significant microspatial associations of parents and their offspring. Results suggest that molecular assays of highly polymorphic genetic systems can add considerable power to assessments of biological parentage in natural populations even when neither parent is otherwise known.     

Genetic and demographic variation in new recruits of Naso unicornis

Demographic data showed that larvae of Naso unicornis colonizing the reefs of Moorea, French Polynesia, on the same night within a restricted area originated from several spawning events that occurred 67 to 94 days previously. Based on the demographic structure of larvae of N. unicornis colonizing the reef, it cannot be entirely dismissed that siblings colonize together because five spawning dates grouped c . 55% of the captured larvae. Relatedness analysis reinforced these observations and also confirmed that larvae colonizing together were not all siblings. Larvae of the same spawning dates, however, appeared related in some age-classes (72, 74 and 77 days old), suggesting that siblings recruited together. In addition, the larvae appeared genetically different from adults ( P =0·002) and juveniles ( P <0·001) while juveniles and adults were similar ( P =0·100). The variations in allelic and genotypic frequencies in the larvae probably reflect a family-structure that is diluted once larvae are recruited into the lagoon because of high mortality and active movement of fish.     

The population dynamics and reproductive cycle of Septifer virgatus (Bivalvia: Mytilidae) on an exposed rocky shore in Hong Kong

Exposed rocky shores in Hong Kong are dominated at mid-tidal levels by a broad band of the mytilid Septifer virgatus. A 48-month study of a population inhabiting the shores at Cape d'Aguilar has been undertaken. The species matures at a shell length of ≅ 15 mm, about one year after recruitment. Septifer virgatus is dioecious with a slight, but insignificant, female bias overall. There is, however, a significant juvenile male bias. Generally, most adult individuals remain mature year round, although spawning is limited to two periods in spring (February to March) and autumn (September to December) with subsequent recruitment into the adult population. The species lives for ≅ 4–5 years, although older individuals, possibly up to 12 years of age and with a maximum recorded shell length of 65 mm, occur as solitary individuals lower down the shore. Mortality in winter mainly affects newly recruited juveniles. A heavy mortality of adults in summer is thought to be related to high rock temperatures at midday (≅ 50°C), concurrent with low spring tides.
The life-history tactics and sexual strategy of Septifer virgatus can be correlated with seasonal changes in hydrography. There are a number of similarities with other local mytilids, notably the dioecious condition and the bimodal pattern of spawning and recruitment. Such similarities are discussed in relation to the habitats occupied by these species.     

Population biology of the rare military orchid (Orchis militaris L.) at an established site in Suffolk, England

In Britain, where it reaches the north-westerly limit of its European distribution, Orchis militaris L. is extremely rare. Well-established and persistent populations of O. militaris are known to exist at only two sites. The largest extant population of O. militaris occurs in a disused chalk pit in Suffolk. A preliminary demographic analysis of this population, covering the period 1975 to 1991, along with estimates of key life stage transition probabilities are presented here. From 1975 to 1986 the number of separate identifiable plants in the population declined substantially. Until 1986 recruitment of rosettes was poor. The largest cohort of new plants, recorded in 1976, was 35. Approximately 48% of new individuals recruited between 1976 and 1985 failed to flower. Of those flowering, approximately 55% flowered during their first year above ground. Of the original population recorded in 1975, 67.8% flowered at least once during the study. The reproductive performance, i.e. the frequency of flowering and the period between episodes of flowering, varied considerably between individuals. Some plants flowered every year while others only flowered once during the study. Few plants remained below ground for more than one year, while several apparently persisted below ground for more than 6 successive years. Although the number of plants that can be identified as separate individuals has declined, the total number of rosettes in the population has, from 1986, increased dramatically. Because of the dense clumping of these recruits it is not possible to determine whether they are derived from seed or vegetative propagation. When post 1986 recruitment is combined with the number of plants that established before 1986 and survive, the apparent number of plants present at the site has more than doubled between 1975 and1991.     

Yearly variation in factors associated with local recruitment of Tree Swallows

Identifying factors that influence the chances of fledglings becoming local recruits can inform us about the ecology of population dynamics and factors influencing parental fitness. We studied Tree Swallows (Tachycineta bicolor) nesting in boxes in western Michigan from 1994 to 2005 and found that 145 of 2405 (6.0%) nestlings that fledged from 840 nests became local recruits. Most local recruits returned to breed at the study site the year after they fledged. The sex ratio of local recruits (76 males and 69 females) did not differ from 1:1. Analysis of data from all years combined revealed that the likelihood of fledglings becoming local recruits was not significantly influenced by the year they fledged, clutch initiation date, clutch size, brood size, age class and physical characteristics of female parents, or the physical characteristics of putative male parents. However, analysis of data from each year revealed that some variables had a significant effect on the likelihood of fledglings becoming local recruits in some years, including female age class in two years, female parent right tail fork length in one year, female parent right wing length in one year, clutch initiation date in two years, clutch size in two years, and brood size in one year. The proportion of fledglings produced each year that became local recruits was not correlated with either mean air temperature or total rainfall during June and July, the immediate post-fledgling period. Taken together, these analyses suggest that chance or yearly variation in some other factor(s) with the potential to affect post-fledging survival that we did not measure (e.g., predation post-fledging or climatic conditions during migration or in migratory stopover sites and wintering areas) had an important effect on local recruitment of fledgling Tree Swallows in our study. We did find that local recruits were more likely to be from nests where egg-laying began earlier rather than later in the season, with 137 of 145 (94%) of local recruits coming from clutches initiated between 5 and 30 May. Because swallows that fledged earlier in the season had more time to prepare for migration than those that fledged later, we hypothesize that the “relative age effect” had an important effect on local recruitment of Tree Swallows at our study site.     

Density-associated recruitment mediates coral population dynamics on a coral reef

Theory suggests that density-associated processes can modulate community resilience following declines in population size. Here, we demonstrate density-associated processes in two scleractinian populations on the outer reef of Moorea, French Polynesia, that are rapidly increasing in size following the effects of two catastrophic disturbances. Between 2006 and 2010, predation by the corallivorous crown-of-thorns sea star reduced coral cover by 93 %; in 2010, the dead coral skeletons were removed by a cyclone, and in 2011 and 2012, high coral recruitment initiated population recovery. Coral recruitment was associated with coral cover, but the relationship differed between two coral genera that are almost exclusively broadcast spawners in Moorea. Acroporids recruited at low densities, and the density of recruits was positively associated with cover of Acropora, whereas pocilloporids recruited at high densities, and densities of their recruits were negatively associated with cover of Pocillopora. Together, our results suggest that associations between adult cover and density of both juveniles and recruits can mediate rapid coral community recovery after large disturbances. The difference between taxa in sign of the relationships between recruit density and coral cover indicate that they reflect contrasting mechanisms with the potential to mediate temporal shifts in taxonomic composition of coral communities.     

Life-history characteristics and sexual strategy of Mytilopsis sallei (Bivalvia: Dreissenacea), introduced into Hong Kong

First recorded from Hong Kong in 1980, Mytilopsis sallei now dominates the intertidal pier community within the Government Dockyard, Victoria Harbour. Analysis of monthly samples of M. sallei has demonstrated aspects of the reproductive strategy and life-history characteristics correlated with seasonal changes in local water quality.
Mytilopsis sallei is dioecious, has a high fecundity, matures early (at 8–10 mm shell length), grows fast and the majority of the population is probably semelparous. These are characteristic traits of opportunistic r -strategists. Comparable characteristics are typical of other introduced bivalves. Maximum life span of M. sallei in the Dockyard is probably less than 20 months with, however, most of the population dying before that. Heavy recruitment, particularly during spring-summer, suffocates older individuals. The shells of dead animals similarly last for little more than one year, dissolution and crushing effecting this. A mat of byssal threads serves to bind the mass and anchor it to the substratum.
Mytilopsis sallei exhibits two periods of reproductive activity and settlement per year. These are possibly two elements of a single recruitment divided into minor (autumn-winter) and major (spring-summer) components by either high summer temperatures or low salinities. Summer and winter recruits of M. sallei , respectively, mature to produce the summer and winter juveniles of the following year. Over-stepping generations plus survival of a few individuals into their second year thus ensure continued success of M. sallei , and account for its spread and dominance in its introduced range. Such a bimodal pattern appears typical of many Hong Kong intertidal, estuarine and freshwater bivalves.     

Age bias in the bag of pink-footed geese <Emphasis Type="Italic">Anser brachyrhynchus</Emphasis>: influence of flocking behaviour on vulnerability

In pink-footed goose (Anser brachyrhynchus) wintering in Denmark, The Netherlands and Belgium, the proportion of juveniles in the hunting bag is consistently higher than that observed in the autumn population. Such juvenile bias in the bag is usually ascribed to young geese lacking experience with hunting or disruption of juveniles from families. An alternative explanation may be that flocking behaviour of families make juveniles more vulnerable. Observations of morning flights of pink-footed geese to the feeding grounds from two of the major autumn-staging areas showed that geese were distributed in many small flocks (median flock size = 9). This was not significantly different from the flock size distribution shot at by hunters (median = 8), suggesting that hunters targeted goose flock size in proportion to the general probability of encounter. The rate at which hunters downed geese was independent of flock size. The ratio between juveniles and adults in flocks decreased with flock size and flocks of <60 individuals primarily comprised family groups. The likelihood of being shot at was 2.4 times higher for juveniles and 3.4 times higher for older birds in small flocks (<10 individuals) compared to larger flocks. The observations suggest that both juveniles as well as successful adult breeding birds were more vulnerable than non-breeding/failed breeding birds as a result of flocking behaviour.     

Seasonal recruitment,habitat associations and survival of pomacentrid reef fish in the US Virgin Islands

Patterns of distribution and abundance of coral reef fish depend in part on recruitment of a pelagic larval stage, on subsequent dispersal among habitats, and survival of new recruits. We studied recruitment of five species of Stegastes and two species of Chromis damselfish onto reef habitats of St. Thomas, USVI during one year. The two study sites, Flat Cay and Outer Brass Island, were on the southern and northern sides of St. Thomas, respectively. At both sites, recruitment occurred largely in the summer months, although one species (Stegastes planifrons) showed significant winter recruitment at Flat Cay. The onset of increased summer recruitment in 1992 of other species occurred several weeks later and was shorter in duration at Outer Brass Island than at Flat Cay, perhaps indicating differences in oceanographic conditions (currents etc.) or spawning cycles between sites. The two Chromis species showed lunar periodicity of settlement at Flat Cay. At Flat Cay, recruits of three species (S. leucostictus, S. diencaeus and S. planifrons) were associated with conspecifics possibly due to preferential settlement. Similarly, new recruits were more often found near live coral than coral rubble, and very few occurred on sand habitat. Substratum complexity was a poor predictor of recruitment within a habitat, although larger juveniles of some species were more common on more complex substrate. Contrary to other studies, there were no apparent depth preferences among recruits, although larger juveniles of two Stegastes species were found more often in deeper water. It appears that within habitats, newly arriving larvae may be attracted first to the presence of conspecifics and secondarily take up position adjacent to live coral. Apparent survivorship of some Stegastes species and one Chromis species was higher at Outer Brass Island than at Flat Cay, and may partly compensate for lower recruitment of some species at Outer Brass Island.