Density-dependent Recruitment Research Articles

Recruitment dynamics of juvenile salmonids: Comparisons among populations and with classic case studies.

Understanding recruitment, the process by which individuals are added to a population or to a fishery, is critical for understanding population dynamics and facilitating sustainable fisheries management. Important variation in recruitment dynamics is observed among populations, wherein some populations exhibit asymptotic productivity and others exhibit overcompensation (i.e., compensatory density-dependence in recruitment). Our ability to understand this interpopulation variability in recruitment patterns is limited by a poor understanding of the underlying mechanisms, such as the complex interactions between density dependence, recruitment, and environment. Furthermore, most studies on recruitment are conducted using an observational design with long time series that are seldom replicated across populations in an experimentally controlled fashion. Without proper replication, extrapolations between populations are tenuous, and the underlying environmental trends are challenging to quantify. To address these issues, we conducted a field experiment manipulating stocking densities of juvenile brook trout Salvelinus fontinalis in three wild populations to show that these neighboring populations-which exhibit divergent patterns of density dependence due to environmental conditions-also have important differences in recruitment dynamics. Testing against four stock-recruitment models (density independent, linear, Beverton-Holt, and Ricker), populations exhibited ~twofold variation in asymptotic productivity, with no overcompensation following a Beverton-Holt model. Although environmental variables (e.g., temperature, pH, depth, substrate) correlated with population differences in recruitment, they did not improve the predictive power in individual populations. Comparing our patterns of recruitment with classic salmonid case studies revealed that despite differences in the shape and parameters of the curves (i.e., Ricker vs. Beverton-Holt), a maximum stocking density of about five YOY fish/m2 emerged. Higher densities resulted in very marginal increases in recruitment (Beverton-Holt) or reduced recruitment due to overcompensation (Ricker).

Distance- and density-dependent recruitment of common ragwort is not driven by plant-soil feedbacks

Janzen-Connell effects state that the accumulation of host-specific natural enemies near parent plants can negatively affect their offspring. Negative plant-soil feedbacks can produce patterns of seedling performance predicted by Janzen-Connell effects and influence plant populations, but their relevance in field conditions remains unclear. Here, using spatial point-pattern analysis, we examine the spatial distribution of Jacobaea vulgaris to assess whether distance- and density-dependent predictions of Janzen-Connell effects are evident in the field. We established 27 replicated 64 m2 plots at two grassland sites and mapped positions of rosette-bearing and flowering J. vulgaris plants within each plot. To investigate temporal distribution patterns, we tracked plant positions repeatedly in three plots during a single season. Additionally, we tested whether these patterns are soil-mediated. Soil samples were collected underneath flowering plants and at a distance of 0.5 meter, and used to compare seed germination, seedling survival, and growth under controlled conditions. Furthermore, we measured J. vulgaris growth in soil from patches with high J. vulgaris densities and in soil from areas outside these patches. The density of rosette-bearing plants was lower at close distances from flowering plants than expected from null models, suggesting negative distance-dependent plant recruitment. The degree of clustering decreased over time from rosette-bearing to flowering plants, indicating density-dependent self-thinning. Seed germination was higher in soil further away from flowering J. vulgaris plants than in soil underneath plants at one site, but soil distance was not an overall significant factor in explaining seed germination. However, seedling mortality and biomass did not differ between soils collected at the two distances, and plants produced similar biomass in soil collected from inside and outside J. vulgaris patches. Our study demonstrates conspecific distance- and density-dependent plant recruitment in J. vulgaris in the field, but we found no evidence this depends on belowground natural enemies.

Marine reserves promote cycles in fish populations on ecological and evolutionary time scales.

Marine reserves are considered essential for sustainable fisheries, although their effectiveness compared to traditional fisheries management is debated. The effect of marine reserves is mostly studied on short ecological time scales, whereas fisheries-induced evolution is a well-established consequence of harvesting. Using a size-structured population model for an exploited fish population of which individuals spend their early life stages in a nursery habitat, we show that marine reserves will shift the mode of population regulation from low size-selective survival late in life to low, early-life survival due to strong resource competition. This shift promotes the occurrence of rapid ecological cycles driven by density-dependent recruitment as well as much slower evolutionary cycles driven by selection for the optimal body to leave the nursery grounds, especially with larger marine reserves. The evolutionary changes increase harvesting yields in terms of total biomass but cause disproportionately large decreases in yields of larger, adult fish. Our findings highlight the importance of carefully considering the size of marine reserves and the individual life history of fish when managing eco-evolutionary marine systems to ensure both population persistence as well as stable fisheries yields.

Pulse disturbances in age-structured populations: Life history predicts initial impact and recovery time.

Understanding population responses to discrete 'pulsed' environmental disturbances is essential to conservation and adaptive management. Populations of concern can be driven to low levels by disturbance, and understanding interspecific differences in recovery trajectories is necessary for evaluating management options. We analysed single-species models to investigate the demographic and management factors determining the two components of population 'resilience': the magnitude of initial impact on population abundance, and duration of the recovery time. We simulated age-structured populations with density-dependent recruitment, subjected to a pulse disturbance consisting of a period of increased mortality of either the juvenile age class or all age classes, and calculated both impact and return time. For illustration, we used demographic parameters from a suite of 16 fish species. We formulated the model as a renewal equation, allowing us to describe disturbance impacts mathematically as a convolution. We also included nonlinear dynamics, representing populations that recover to a steady state; this is more realistic (in most cases) than prior analyses of resilience in linear models without density-dependence. When the disturbance affected only one or a few young age-classes, longevity was the major life-history determinant of impact and recovery time. Shorter-lived species endured greater impacts when disturbed because each age class is a greater proportion of the population. However, shorter-lived species also had faster recovery times, for the same reason. When disturbance affected adult age-classes, the impact was more immediate and no longer affected by species' longevity, though the effect of longevity on recovery time remained. These results improve our understanding of interspecific differences in resilience and increase our ability to make predictions for adaptive management. Additionally, formulating the problem as a renewal equation and using mathematical convolutions allows us to quantify how disturbances with different time courses (not just an immediate, constant level of disturbance but gradually increasing or decreasing levels of disturbance) would have different effects on population resilience: delayed responses for species in which biomass is concentrated in older age classes, and for disturbances that become progressively more severe.

Mathematical Modeling to Guide Experimental Design: T Cell Clustering as a Case Study.

Mathematical modeling provides a rigorous way to quantify immunological processes and discriminate between alternative mechanisms driving specific biological phenomena. It is typical that mathematical models of immunological phenomena are developed by modelers to explain specific sets of experimental data after the data have been collected by experimental collaborators. Whether the available data are sufficient to accurately estimate model parameters or to discriminate between alternative models is not typically investigated. While previously collected data may be sufficient to guide development of alternative models and help estimating model parameters, such data often do not allow to discriminate between alternative models. As a case study, we develop a series of power analyses to determine optimal sample sizes that allow for accurate estimation of model parameters and for discrimination between alternative models describing clustering of CD8 T cells around Plasmodium liver stages. In our typical experiments, mice are infected intravenously with Plasmodium sporozoites that invade hepatocytes (liver cells), and then activated CD8 T cells are transferred into the infected mice. The number of T cells found in the vicinity of individual infected hepatocytes at different times after T cell transfer is counted using intravital microscopy. We previously developed a series of mathematical models aimed to explain highly variable number of T cells per parasite; one of such models, the density-dependent recruitment (DDR) model, fitted the data from preliminary experiments better than the alternative models, such as the density-independent exit (DIE) model. Here, we show that the ability to discriminate between these alternative models depends on the number of parasites imaged in the analysis; analysis of about [Formula: see text] parasites at 2, 4, and 8h after T cell transfer will allow for over 95% probability to select the correct model. The type of data collected also has an impact; following T cell clustering around individual parasites over time (called as longitudinal (LT) data) allows for a more precise and less biased estimates of the parameters of the DDR model than that generated from a more traditional way of imaging individual parasites in different liver areas/mice (cross-sectional (CS) data). However, LT imaging comes at a cost of a need to keep the mice alive under the microscope for hours which may be ethically unacceptable. We finally show that the number of time points at which the measurements are taken also impacts the precision of estimation of DDR model parameters; in particular, measuring T cell clustering at one time point does not allow accurately estimating all parameters of the DDR model. Using our case study, we propose a general framework on how mathematical modeling can be used to guide experimental designs and power analyses of complex biological processes.

Strength and consistency of density dependence in marine fish productivity

AbstractThe correct prediction of the shape and strength of density dependence in productivity is key to predicting future stock development and providing the best possible long‐term fisheries management advice. Here, we identify unbiased estimators of the relationship between somatic growth, recruitment and density, and apply these to 80 stocks in the Northeast Atlantic. The analyses revealed density‐dependent recruitment in 68% of the stocks. Excluding pelagic stocks exhibiting significant trends in spawning stock biomass, the probability of significant density dependence was even higher at 78%. The relationships demonstrated that at the commonly used biomass limit of 0.2 times maximum spawning stock size, only 32% of the stocks attained three quarters of their maximum recruitment. This leaves 68% of the stocks with less than three quarters of their maximum recruitment at this biomass limit. Significantly lower recruitment at high stock size than at intermediate stock size was seen in 38% of the stocks. Density dependence in late growth occurred in 54% of the stocks, whereas early growth was generally density‐independent. Pelagic stocks were less likely to exhibit density dependence in recruitment than demersal and benthic stocks. We recommend that both the degree to which productivity is related to density and the degree to which the relationship changes over time should be investigated. Both of these aspects should be considered in evaluations of whether sustainability and yield can be improved by including density dependence in forecasts of the effects of different management actions.

Equilibrium reference point calculations for the next generation of spatial assessments

Fish populations with spatial structure inherently violate the assumption of a single well-mixed stock, necessitating the use of spatially-structured population dynamics models. Accounting for spatial structure accurately and easily is a major goal for the next generation of stock assessment software development. Reference points (e.g., limit or target harvest rates and their associated biomass) are inherent to stock assessments and are often calculated under equilibrium conditions. However, the calculation process can be challenging for spatially-structured population dynamics models. We outline how to calculate equilibrium quantities within spatially-structured models where density-dependence in recruitment is local. We compare those values to equivalent situations when density-dependence in recruitment is global, thereby extending the set of population dynamics models on which spatially-structured stock assessments could be based. Results suggest that our method for calculating reference points under the assumption of local density-dependence can be performed using a straightforward optimization routine, and provide clearer understanding of the effects of fishing on a spatially-structured population. Finally, we address areas of research and development needed to integrate spatially-structured population dynamics models within existing management systems.

A class of fast–slow models for adaptive resistance evolution

Resistance to insecticide is considered nowadays one of the major threats to insect control, as its occurrence reduces drastically the efficiency of chemical control campaigns, and may also perturb the application of other control methods, like biological and genetic control. In order to account for the emergence and spread of such phenomenon as an effect of exposition to larvicide and/or adulticide, we develop in this paper a general time-continuous population model with two life phases, subsequently simplified through slow manifold theory. The derived models present density-dependent recruitment and mortality rates in a non-conventional way. We show that in absence of selection, they evolve in compliance with Hardy–Weinberg law; while in presence of selection and in the dominant or codominant cases, convergence to the fittest genotype occurs. The proposed mathematical models should allow for the study of several issues of importance related to the use of insecticides and other adaptive phenomena.

Post-dispersal factors influence recruitment patterns but do not override the importance of seed limitation in populations of a native thistle.

Whether plant populations are limited by seed or microsite availability is a long-standing debate. However, since both can be important, increasing emphasis is placed on disentangling their relative importance and how they vary through space and time. Although uncommon, seed addition studies that include multiple levels of seed augmentation, and follow plants through to the adult stage, are critical to achieving this goal. Such data are also vital to understanding when biotic pressures, such as herbivory, influence plant abundance. In this study, we experimentally added seeds of a native thistle, Cirsium canescens, at four augmentation densities to plots at two long-term study sites and quantified densities of seedlings and reproductive adults over 9years. Recruitment to both seedling and adult stages was strongly seed-limited at both sites; however, the relative strength of seed limitation decreased with plant age. Fitting alternative recruitment functions to our data indicated that post-dispersal mortality factors were important as well. Strong density-dependent mortality limited recruitment at one site, while density-independent limitation predominated at the other. Overall, our experimental seed addition demonstrates that the environment at these sites remains suitable for C. canescens survival to reproduction and that seed availability limits adult densities. The results thus provide support for the hypothesis that seed losses due to the invasive weevil, Rhinocyllus conicus, rather than shifting microsite conditions, are driving C. canescens population declines. Shifts in the importance of density-dependent recruitment limitation between sites highlights that alternate strategies may be necessary to recover plant populations at different locations.

Conflicting perceptions on the life-history covariates of density-dependent recruitment based on information-generating equations

Abstract The stock–recruit steepness (h) and compensation ratio are alternative measures of the degree of density-dependent recruitment. They relate similarly to the longevity-related life-history traits (LRLHTs), but studies using information-generating equations and describing such relationships convey three conflicting perceptions. Perception 1 is that h and the compensation ratio decrease as longevity increases and are highest in short-lived, high-productivity species. This perception, usually believed to be inherent in the definition of steepness, is not substantiated by the very definition and, hence, theoretically is wrong. Perception 2 is that h and the compensation ratio increase with longevity and are highest in long-lived, low-productivity species; this perception may occur when the relationships between natural mortality and other LRLHTs are strong, and is most likely correct. Perception 3 is that h and the compensation ratio are independent of LRLHTs; this perception may be caused by the lack of the relationships between natural mortality and other LRLHTs, and is a distortion of perception 2.

Density-dependent processes fluctuate over 50years in an ecotone forest.

Spatial patterns can inform us of forest recruitment, mortality, and tree interactions through time and disturbance. Specifically, successional trajectories of self-thinning and heterospecific negative density dependence can be interpreted from the spatial arrangement of forest stems. We conducted a 50-year spatial analysis of a forest undergoing succession at the ecotone of the southwestern Canadian boreal forest. The forest progressed from early to late sere and experienced repeated severe droughts, forest tent caterpillar outbreaks (Malacosoma disstria), as well as the outbreak of bark beetles. Cumulatively, the forest lost 70% of stems due to natural succession and a combination of disturbance events. Here, we describe spatial patterns displaying signals of successional self-thinning, responses to disturbance, and changes in patterns of density dependence across 50years. Forest succession and disturbance events resulted in fluctuating patterns of density-dependent mortality and recruitment that persisted into late seral stages. The combined effects of conspecific and heterospecific density-dependent effects on mortality and recruitment resulted in near-spatial equilibrium over the study period. However, the strength and direction of these demographic and spatial processes varied in response with time and disturbance severity. The outbreak of forest tent caterpillar, pronounced drought, and bark beetles combined to reduce stand aggregation and promote a spatial equilibrium. Density-dependent processes of competition and facilitation changed in strength and direction with succession of the plot and in combination with disturbance. Together these results reinforce the importance of successional stage and disturbance to spatial patterns.

Evolutionary and demographic consequences of phenological mismatches.

Climate change has often led to unequal shifts in the seasonal timing (phenology) of interacting species, such as consumers and their resource, leading to phenological ‘mismatches’. Mismatches occur when the time where resource demands of the consumer species are high does not match with the period when this resource is abundant. Here, we review the evolutionary and population consequences of such mismatches and how these depend on other ecological factors, as, for example, additional drivers of selection or density-dependent recruitment. This review puts the research on phenological mismatches into a conceptual framework, applies this framework beyond consumer-resource interactions, and illustrates this framework using examples drawn from the vast body of literature on mismatches. Finally, we point out priority questions for research on this key impact of climate change.

Frequency content of environmental variability and extinction risk of age-structured populations: Chinook salmon (Oncorhynchus tschawytscha) as an example

Investigations into the influence of environmental variability on population extinction have assessed the role of noise “color” (e.g., “white” or “reddened”), but the influence of spectral sensitivity of age-structured populations on extinction risk is not well understood. Recent findings indicate that age-structured populations are more sensitive to environmental variability that occurs on time scales near their mean age of spawning and on long time scales, particularly when populations are at low abundance. This raises the question of how variability at those sensitive time scales would influence extinction risk. We used an age-structured population model with density-dependent recruitment based on the life history of spring-run Chinook salmon, as an example, to investigate how the spectrum of environmental variability influences (1) variability of spawning female abundance, (2) the probability of quasi-extinction, and (3) the distribution of times to quasi-extinction. Population simulations were run with different spectra to investigate the influence of: (a) low frequencies, (b) frequencies equal to the inverse of the mean age of spawning (generational time scales), (c) both of these at once, and (d) “reddened” white noise. Results showed that even though variability on generational time scales causes greater population variability than other time scales, by itself, it does not increase probability of quasi-extinction. However, environmental variability at the other peak in sensitivity at low frequencies increased extinction in a way similar to the reddened signal. Environmental variability distributed over both sensitive frequency bands (i.e., low frequency and cohort frequencies) actually had less effect on extinction than low frequency variability alone.

Density-dependent adult recruitment in a low-density tropical tree

The Janzen-Connell hypothesis is a well-known explanation for why tropical forests have large numbers of tree species. A fundamental prediction of the hypothesis is that the probability of adult recruitment is less in regions of high conspecific adult density, a pattern mediated by density-dependent mortality in juvenile life stages. Although there is strong evidence in many tree species that seeds, seedlings, and saplings suffer conspecific density-dependent mortality, no study has shown that adult tree recruitment is negatively density dependent. Density-dependent adult recruitment is necessary for the Janzen-Connell mechanism to regulate tree populations. Here, we report density-dependent adult recruitment in the population of Handroanthus guayacan, a wind-dispersed Neotropical canopy tree species. We use data from high-resolution remote sensing to track individual trees with proven capacity to flower in a lowland moist forest landscape in Panama and analyze these data in a Bayesian framework similar to capture-recapture analysis. We independently quantify probabilities of adult tree recruitment and detection and show that adult recruitment is negatively density dependent. The annualized probability of adult recruitment was 3.03% ⋅ year-1 Despite the detection of negative density dependence in adult recruitment, it was insufficient to stabilize the adult population of H. guayacan, which increased significantly in size over the decade of observation.

Regulation dynamics of exploited and protected populations of Haliotis roei, and their response to a marine heatwave

Abstract Exploited and protected populations of Haliotis roei in Western Australia were significantly perturbed by a marine heatwave of 2010/11. The immediate effects were a decline in large animals and recruitment, followed by a lagged decline in the spawning biomass. The data describing these effects were fitted to models of the population dynamic transitions. Depensatory density dependence in recruitment is offset by compensatory density dependence in during replacement and together they create the population equilibrium points. Equilibrium densities were highly influenced by the environmental perturbation, with observed patterns suggesting a temporary ‘regime shift’ had occurred. The main implication for management is to maximise spawning biomass as recruitment is positively correlated with it with some evidence of an Allee effect at low spawning densities, but no evidence of compensatory density dependence at high densities. Development of forecasting models that utilised pre-recruit densities, adjusted for density-dependent replacement, and incorporated an environmental effect, assisted early-management intervention to stock declines. Such an approach is an alternative to traditional assessments which recommend using biological reference points that assume population regulation occurs in the recruitment transition. This assumption cannot easily explain why the high density unfished stock showed the least resilience to the environmental perturbation.

Unequal density dependence between survival and recruitment affects harvesting effectivness

ABSTRACTRecruitment and adult‐mortality may respond differently to increased population density. Such unequal density dependence (UDD) is evident for large mammals showing density dependence mainly in recruitment (reproduction rate or juvenile survival), whereas adult survival is high and varies little with density. Mechanistically, UDD is likely influenced by an unequal allocation of resources toward reproduction versus survival. Unequal density dependence is expected to affect compensation abilities, and hence may have serious implications with regard to population harvesting. We modeled UDD by linking population size to reproduction and survival through per capita resource availability and consumption. We integrated UDD into 2 population models (continuous vs. periodic recruitment) and investigated how it affected the compensatory dynamics of populations. For a population with continuous recruitment, our model predicted that although UDD stabilizes the population dynamics, it makes populations more vulnerable to harvesting as a consequence of lower compensatory ability, regardless of whether more resources are allocated to reproduction or to survival. For populations with periodic recruitment, on the other hand, an asymmetric effect of UDD is predicted. Strong density dependence in recruitment relative to adult survival yields a strong compensatory response, whereas equal or stronger density dependence in adult survival relative to recruitment yields weaker compensatory responses. Stronger density dependence in recruitment also causes the periodic fluctuations in population size to be amplified rather than dampened under harvesting. These results are robust to alternative model formulations (e.g., Ricker and Beverton‐Holt equations). Using an empirical example for coyotes (Canis latrans), wild boars (Sus scrofa), and white‐tailed deer (Odocoileus virginianus), we demonstrated the implications of our results for harvest management of medium‐size mammalian species that are periodic breeders and exhibit density‐dependent recruitment. Accounting for UDD in the population model of coyotes and wild boars, which are characterized by large litters, predicts compensation that is stronger than the response obtained without UDD. For white‐tailed deer, which are characterized by substantially smaller litters, accounting for UDD does not change the outcome. Harvest‐based control attempts of medium‐size mammals may hence be much less effective than predicted when the population models used to estimate them ignore UDD, suggesting the necessity of integrating UDD into these models. © 2018 The Wildlife Society.

Unexpected dietary preferences of Eurasian Spoonbills in the Dutch Wadden Sea: spoonbills mainly feed on small fish not shrimp

After an historical absence, over the last decades Eurasian Spoonbills Platalea leucorodia leucorodia have returned to breed on the barrier islands of the Wadden Sea. The area offers an abundance of predator-free nesting habitat, low degrees of disturbance, and an extensive intertidal feeding area with increasing stocks of brown shrimp Crangon crangon, the assumed main prey of P. leucorodia leucorodia. Nevertheless, newly established and expanding colonies of spoonbills have surprisingly quickly reached plateau levels. Here we tested the often stated assertion that spoonbills mainly rely on brown shrimp as food, by quantifying the diet of chicks on the basis of regurgitates and by analysis of blood isotopes using stable isotope Bayesian mixing models. Both methods showed that, rather than brown shrimp being the staple food of spoonbill chicks, small flatfish (especially plaice Pleuronectes platessa) and gobies (Pomatoschistus spp.) were their main prey. Unlike shrimp, small flatfish have been reported to be rather scarce in the Wadden Sea in recent years, which may explain the rapid saturation of colony size due to food-related density-dependent recruitment declines of growing colonies. By way of their diet and colony growth characteristics, spoonbills may thus indicate the availability of small fish in the Wadden Sea. We predict that the recovery to former densities of young flatfish and other juvenile/small fish in the Wadden Sea will be tracked by changing diets (more fish) and an increase in the size of Eurasian Spoonbill colonies across the Wadden Sea.

Density regulation in Northeast Atlantic fish populations: Density dependence is stronger in recruitment than in somatic growth.

Population regulation is a central concept in ecology, yet in many cases its presence and the underlying mechanisms are difficult to demonstrate. The current paradigm maintains that marine fish populations are predominantly regulated by density-dependent recruitment. While it is known that density-dependent somatic growth can be present too, its general importance remains unknown and most practical applications neglect it. This study aimed to close this gap by for the first time quantifying and comparing density dependence in growth and recruitment over a large set of fish populations. We fitted density-dependent models to time-series data on population size, recruitment and age-specific weight from commercially exploited fish populations in the Northeast Atlantic Ocean and the Baltic Sea. Data were standardized to enable a direct comparison within and among populations, and estimated parameters were used to quantify the impact of density regulation on population biomass. Statistically significant density dependence in recruitment was detected in a large proportion of populations (70%), whereas for density dependence in somatic growth the prevalence of density dependence depended heavily on the method (26% and 69%). Despite age-dependent variability, the density dependence in recruitment was consistently stronger among age groups and between alternative approaches that use weight-at-age or weight increments to assess growth. Estimates of density-dependent reduction in biomass underlined these results: 97% of populations with statistically significant parameters for growth and recruitment showed a larger impact of density-dependent recruitment on population biomass. The results reaffirm the importance of density-dependent recruitment in marine fishes, yet they also show that density dependence in somatic growth is not uncommon. Furthermore, the results are important from an applied perspective because density dependence in somatic growth affects productivity and catch composition, and therefore the benefits of maintaining fish populations at specific densities.

Conspecific negative density dependence in a long-lived conifer, yew Taxus baccata L.

Conspecific negative density dependence in tree species can have major consequences for structuring of communities, yet in temperate forests this phenomenon remains largely unexplored. We investigated density-dependent recruitment in the long-lived conifer yew (Taxus baccata L.), for which low levels of regeneration have been linked with reduced light availability beneath conspecific canopies and speculative autotoxic effects. We combined in situ assessment of yew regeneration in a temperate forest in southwest Ireland with ex situ pot experiments to assess whether light and/or beneath conspecific canopy conditions inhibit the germination and early growth of yew. In field experiments, recruitment was related to distance from adult conspecifics, with higher levels of regeneration found in areas not beneath yew canopies. However, reduced light availability beneath the dense yew canopy was not responsible for this pattern, with abundant seedlings found in areas of equivalent or even lower light availability. Pot experiments showed that seedling survival was highest in deep shade treatments. Experiments also demonstrated that the addition of yew needles negatively affected seedling growth, but not seed germination. Together, our results suggest that under natural forest conditions, the absence of regeneration beneath mature yew canopies, often attributed to low light availability, may at least be partly related to substances in yew canopy or leaf leachates.

Conspecific plants are better 'nurses' than rocks: consistent results revealing intraspecific facilitation as a process that promotes establishment in a hyper-arid environment.

Harsh environmental conditions in arid ecosystems limit seedling recruitment to microhabitats under nurse structures, such as shrubs or rocks. These structures, however, do not necessarily afford the same benefits to plants because nurse rocks provide only physical nurse effects, whereas nurse plants can provide both physical and biological nurse effects. Nevertheless, if the nurse plant is a conspecific, the benefits it provides may be outweighed by higher mortality due to negative density-dependent processes; consequently, negative density-dependence is expected to limit plants from acting as nurses to their own seedlings. The degree to which an abiotic nurse may be more beneficial than a conspecific one remains largely unexplored. Here, we examine the role and elucidate the mechanisms by which conspecific plants and rocks promote plant establishment in a hyper-arid desert. For 4 years, we examined establishment patterns of Myrcianthes coquimbensis (Myrtaceae), a threatened desert shrub that recruits solely in rock cavities and under conspecific shrubs. Specifically, we characterized these microhabitats, as well as open interspaces for comparison, and conducted germination, seed removal and seedling survival experiments. Our results revealed that conspecific shrubs and nurse rocks modified environmental conditions in similar ways; soil and air temperatures were lower, and water availability was higher than in open interspaces. We found no evidence on negative density-dependent recruitment: seed removal was lowest and seedling emergence highest under conspecific plants, moreover seedling survival probabilities were similar in rock cavities and under conspecific plants. We conclude that the probability of establishment was highest under conspecific plants than in other microhabitats, contrasting what is expected under the Janzen-Connell recruitment model. We suggest that for species living in stressful environments, population regulation may be a function of positive density-dependence and intraspecific facilitation may be a process that promotes the persistence of some plant species within a community.