Fluctuations in food supply drive recruitment variation in a marine fish

Reproductive rates and survival of young in animal populations figure centrally in generating management and conservation strategies. Model systems suggest that food supply can drive these often highly variable properties, yet for many wild species, quantifying such effects and assessing their implications have been challenging. We used spatially explicit time series of a well-studied marine reef fish (black surfperch Embiotoca jacksoni) and its known prey resources to evaluate the extent to which fluctuations in food supply influenced production of young by adults and survival of young to subadulthood. Our analyses reveal: (i) variable food available to both adults and to their offspring directly produced an order of magnitude variation in the number of young-of-year (YOY) produced per adult and (ii) food available to YOY produced a similar magnitude of variation in their subsequent survival. We also show that such large natural variation in vital rates can significantly alter decision thresholds (biological reference points) important for precautionary management. These findings reveal how knowledge of food resources can improve understanding of population dynamics and reduce risk of overharvest by more accurately identifying periods of low recruitment.     

Drought survival is a threshold function of habitat size and population density in a fish metapopulation

Because smaller habitats dry more frequently and severely during droughts, habitat size is likely a key driver of survival in populations during climate change and associated increased extreme drought frequency. Here, we show that survival in populations during droughts is a threshold function of habitat size driven by an interaction with population density in metapopulations of the forest pool dwelling fish, Neochanna apoda. A mark–recapture study involving 830 N. apoda individuals during a one‐in‐seventy‐year extreme drought revealed that survival during droughts was high for populations occupying pools deeper than 139 mm, but declined steeply in shallower pools. This threshold was caused by an interaction between increasing population density and drought magnitude associated with decreasing habitat size, which acted synergistically to increase physiological stress and mortality. This confirmed two long‐held hypotheses, firstly concerning the interactive role of population density and physiological stress, herein driven by habitat size, and secondly, the occurrence of drought survival thresholds. Our results demonstrate how survival in populations during droughts will depend strongly on habitat size and highlight that minimum habitat size thresholds will likely be required to maximize survival as the frequency and intensity of droughts are projected to increase as a result of global climate change.     

Interacting effects of age,density, and weather on survival and current reproduction for a large mammal

Individual‐based study of natural populations allows for accurate and precise estimation of fitness components and the extent to which they might vary with ecological conditions. By tracking the fates of all 701 horses known to have lived on Sable Island, Canada, from 2009 to 2013 (where there is no predation, human interference, or interspecific competition for food), we present a detailed analysis of structured population dynamics with focus on interacting effects of intraspecific competition and weather on reproduction and survival. Annual survival of adult females (0.866 ± 0.107 [ ± SE]) was lower than that of 3‐year‐olds (0.955 ± 0.051), although annual fecundity (producing a foal in a year that was observed during our census) was higher in adults (0.616 ± 0.023) compared to 3‐year‐olds (0.402 ± 0.054). Milder winters and lower densities during gestation increased fecundity. Density negatively impacted survival for all age and sex categories; however, highest adult female survival was observed during high‐density years coupled with a harsh winter, the result expected if pregnancy loss during winter or loss of foals in spring improved survival. Three‐year‐old females, which reproduced at lower rates, experienced higher survival than adults. Our results contrast with a previous study of feral horses that suggested recently feral ungulates might be artificially selected to reproduce even when costs to survival are high. In part, this may be because of the comparably long history of feralization (250 years; at least 25 generations) for Sable Island horses.     

Effects of density dependent sex allocation on the dynamics of a simultaneous hermaphroditic population: Modelling and analysis

In this work we present a mathematical model describing the dynamics of a population where sex allocation remains flexible throughout adult life and so can be adjusted to current environmental conditions. We consider that the fractions of immature individuals acquiring male and female sexual roles are density dependent through nonlinear functions of a weighted total population size. The main goal of this work is to understand the role of life-history parameters on the stabilization or destabilization of the population dynamics.The model turns out to be a nonlinear discrete model which is analysed by studying the existence of fixed points as well as their stability conditions in terms of model parameters. The existence of more complex asymptotic behaviours of system solutions is shown by means of numerical simulations.Females have larger fertility rate than males. On the other hand, increasing population density favours immature individuals adopting the male role. A positive equilibrium of the system exists whenever fertility and survival rates of one of the sexual roles, if shared by all adults, allow population growing while the opposite happens with the other sexual role. In terms of the female inherent net reproductive number, η_F, it is shown that the positive equilibria are stable when η_F is larger and closed to 1 while for larger values of η_F a certain asymptotic assumption on the investment rate in the female function implies that the population density is permanent. Depending on the other parameters values, the asymptotic behaviour of solutions becomes more complex, even chaotic. In this setting the stabilization/destabilization effects of the abruptness rate in density dependence, of the survival rates and of the competition coefficients are analysed.     

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.     

Growth and annual survival estimates to examine the ecology of larval lamprey and the implications of ageing error in fitting models

This study used existing western brook lamprey Lampetra richardsoni age information to fit three different growth models (i.e. von Bertalanffy, Gompertz and logistic) with and without error in age estimates. Among these growth models, there was greater support for the logistic and Gompertz models than the von Bertalanffy model, regardless of ageing error assumptions. The von Bertalanffy model, however, appeared to fit the data well enough to permit survival estimates; using length‐based estimators, annual survival varied between 0·64 (95% credibility interval: 0·44–0·79) and 0·81 (0·79–0·83) depending on ageing and growth process error structure. These estimates are applicable to conservation and management of L. richardsoni and other western lampreys (e.g. Pacific lamprey Entosphenus tridentatus) and can potentially be used in the development of life‐cycle models for these species. These results also suggest that estimators derived from von Bertalanffy growth models should be interpreted with caution if there is high uncertainty in age estimates.     

Stochastic density dependence in population size of a benthic clonal invertebrate: the regulating role of fission

The influence of environmental variation on the demography of clonal organisms has been poorly studied. I utilise a matrix model of the population dynamics of the intertidal zoanthid Palythoa caesia to examine how density dependence and temporal variation in demographic rates interact in regulating population size. The model produces realistic simulations of population size, with erratic fluctuations between soft lower and upper boundaries of approximately 55 and 90% cover. Cover never exceeds the maximum possible of 100%, and the population never goes to extinction. A sensitivity analysis indicates that the model’s behaviour is driven by density dependence in the fission of large colonies to produce intermediate sized colonies. Importantly, there is no density-dependent mortality in the model, and density dependence in recruitment, while present, is unimportant. Thus it appears that the main demographic processes which are considered to regulate population size in aclonal organisms may not be important for clonal species. Received: 18 August 1999 / Accepted: 29 October 1999     

Alternation of bottom-up and top-down regulation in a natural population of an agromyzid leafminer,Chromatomyia suikazurae

The population dynamics and the relative importance of bottom-up and top-down effects in a plant-leafminer-multiparasitoid interaction was studied between 1981 and 1990 in a natural forest in Kyoto, Japan. The leafminer, Chromatomyia suikazurae (Agromyzidae, Diptera), passed two generations (G1 and G2) on Lonicera gracilipes (Caprifoliaceae). The G1 population in February was free from parasitoid attack, and the mortality in G1 was mainly caused by resource limitation. Intraspecific competition for resources occurred at the larval stage in G1, and the larval mortality was density-dependent. The G1 adult density was resource-limited (the number of newly opened leaves), and its variability was lower than that of G2. The G2 population in April was not resource-limited but subject to intense attack by a species-rich parasitoid complex, and thus total mortality was much larger than that in G1. Significant density dependence was detected not in larval but in pupal mortalities, which were mainly caused by parasitism by parasitoids that attacked the pupa. The host population alternately experienced bottom-up effects during the larval stage in G1 and top-down effects during the pupal stage in G2. Overall population fluctuation was non-cyclic and mainly due to climatically-induced fluctuation of available plant resources in G1.     

Woodland caribou calf mortality in Newfoundland: insights into the role of climate,predation and population density over three decades of study

The rates and causes of juvenile mortality are central features of the dynamics and conservation of large mammals, like woodland caribou (Rangifer tarandus caribou (Gmelin, 1788)), but intrinsic and extrinsic factors may be modified by variations in animal abundance. We tested the influences of population size, climate, calf weight and sex on survival to 6 months of age of 1241 radio-collared caribou calves over three decades, spanning periods of population growth (1979–1997) and decline (2003–2012) in Newfoundland, Canada. Daily survival rates were higher and rose more quickly with calf age during the population growth period compared to the decline. Population size (negatively) and calf weight (positively) affected survival during the decline but neither had a detectable influence during the growth phase. Sex, climate and plant productivity (the latter two derived from the North Atlantic Oscillation and Normalized Difference Vegetation Index, respectively) exerted minimal influence during either phase. Predation was the dominant source of mortality. The mean percentage of calves killed by predators was 30 % higher during the decline compared to the growth phase. Black bears (Ursus americanus) and lynx (Lynx canadensis) were the major predators during the population increase but this changed during the decrease to black bears and coyotes (Canis latrans). Our findings are consistent with the hypothesis that Newfoundland caribou experienced phase-dependent survival mediated proximally by predation and competition for food.     

Intraspecific competition and density dependence of food consumption and growth in Arctic charr

1. Intraspecific competition for restricted food resources is considered to play a fundamental part in density dependence of somatic growth and other population characteristics, but studies simultaneously addressing the interrelationships between population density, food acquisition and somatic growth have been missing. 2. We explored the food consumption and individual growth rates of Arctic charr Salvelinus alpinus in a long-term survey following a large-scale density manipulation experiment in a subarctic lake. 3. Prior to the initiation of the experiment, the population density was high and the somatic growth rates low, revealing a severely overcrowded and stunted fish population. 4. During the 6-year period of stock depletion the population density of Arctic charr was reduced with about 75%, resulting in an almost twofold increase in food consumption rates and enhanced individual growth rates of the fish. 5. Over the decade following the density manipulation experiment, the population density gradually rose to intermediate levels, accompanied by corresponding reductions in food consumption and somatic growth rates. 6. The study revealed negative relationships with population density for both food consumption and individual growth rates, reflecting a strong positive correlation between quantitative food intake and somatic growth rates. 7. Both the growth and consumption rate relationships with population density were well described by negative power curves, suggesting that large density perturbations are necessary to induce improved feeding conditions and growth rates in stunted fish populations. 8. The findings demonstrate that quantitative food consumption represents the connective link between population density and individual growth rates, apparently being highly influenced by intraspecific competition for limited resources.     

Climate and demography of the planktivorous Cassin's auklet Ptychoramphus aleuticus off northern California: implications for population change

1. We performed demographic analyses on Cassin's auklet Ptychoramphus aleuticus, a zooplanktivorous seabird inhabiting the variable California Current System, to understand how temporal environmental variability influences population dynamics. 2. We used capture-recapture data from 1986 to 2002 to rank models of interannual variation in survival, breeding propensity, breeding success, and recruitment. 3. All demographic parameters exhibited temporal variability. Interannual variation in survival was best modelled as a nonlinear function of the winter Southern Oscillation Index (SOI). Breeding propensity was best modelled as a threshold function of local sea surface temperature. Breeding success and recruitment were best modelled with year-dependent annual variation. 4. Changes in the SOI force El Ni?o/La Ni?a events, which in turn alter prey availability to seabirds in this system. Demographic responses varied during El Ni?os/La Ni?as. Survival diminished substantially during the 1997-98 El Ni?o event, while breeding propensity was affected during both the 1992 and 1998 El Ni?os. Breeding success was reduced during the 1992, 1993, and 1998 El Ni?os, but was unusually high in 2002. Recruitment was higher at the beginning and end of this time-series. 5. While demographic responses varied interannually, parameter values covaried in a positive fashion, a situation conducive to rapid population change. During the 11 years study period, the Farallon auklet breeding population declined at 6.05 +/- 0.80% (SE) per year, a cumulative decline of 49.7%. This study demonstrates how climate variability has influenced key demographic processes for this diminished marine bird population.     

Bottom-up and top-down effects in a tritrophic system: the population dynamics of Plutella xylostella (L.)–Cotesia plutellae (Kurdjumov) on different host plants

Abstract.  1. The effects of host-plant resistance on the population dynamics of the Diamondback moth, Plutella xylostella L., and its solitary parasitoid, Cotesia plutellae (Kurdjumov), were studied in replicated time-series experiments.
2. Host-plant resistance did not affect the equilibrial abundance of the Diamondback moth, but it affected the dynamics of Diamondback moth populations.
3. The mean population size of Diamondback moth showed no significant difference between Brassica rapa (a susceptible host plant) and Brassica napus (a partially resistant host plant) either in the presence or absence of the parasitoid.
4. Time-series analysis suggests that the dynamics of Diamondback moth on B. rapa were underpinned by delayed density-dependent processes. In contrast, the dynamics of the moth on B. napus were influenced by a direct density-dependent process.
5. Although measures of short-term parasitism showed a significantly higher rate of parasitism by C. plutellae on Diamondback moth feeding on B. napus compared with B. rapa , this individual performance does not translate into differences in the population dynamics. Analysis shows no significant difference in the persistence time of the population-level interaction between the host and parasitoid on the two different host plants.     

Genetic panmixia and demographic dependence across the North Atlantic in the deep-sea fish,blue hake (Antimora rostrata)

The efficient investment of resources and effort into conservation strategies depends on the accurate identification of management units. At the same time, understanding the processes by which population structure evolves requires an understanding of the conditions under which panmixia may exist. Here, we study a species with an unusual, apparently sex-biased pattern of distribution, and test the hypothesis that distribution processes associated with this pattern (for example, congregating at a single dominant spawning site or periodic mixing during reproduction) could lead to panmixia over a large geographic range. Using 13 microsatellite markers, we compared 393 blue hake (Antimora rostrata) from 11 sample sites across a geographic range of over 3000 km, and found no evidence of population structure. We estimated current effective population size and found it to be large (∼15 000) across the sampled area. In addition, we used simulation models to test expectations about demographic correlation among populations and our ability to detect relevant levels of gene flow. All data were consistent with the interpretation of long-range panmixia.     

Multi-scale recruitment patterns and effects on local population size of a temperate reef fish

Recruitment of the temperate reef fish Coris julis was studied across the Azores Archipelago (central North Atlantic), over four consecutive recruitment seasons and at three spatial scales: between islands (separated by 100s of km), sites within islands (separated by 10s of km) and transects within sites (separated by 10s of m). At the largest scale ( i.e . between islands) spatial recruitment patterns were highly variable, suggesting the influence of stochastic processes. Recruitment was spatially consistent within islands, even though magnitude was unpredictable between years, indicating that processes at meso-scales are probably more deterministic. Recruits settled randomly at the transect scale, probably reflecting habitat homogeneity. It was proposed that large and island-scale patterns reflect larval availability, driven by physical and biological processes occurring in the plankton. No evidence was found for a density-dependent relationship between newly settled and 2 week settled C. julis nor between cumulative recruitment and young-of-the-year. It appears that adult density is limited by larval supply (pre-settlement regulation) at low recruitment sites, and determined by post-settlement, density-dependent processes at high recruitment sites. This work is one of few to investigate multiple spatial and temporal scales of recruitment for a coastal fish species inhabiting isolated, temperate oceanic islands and hence, provides a novel comparison to the many studies of recruitment on coral reefs and other, more connected systems.