Density-dependent Recruitment Research Articles

Density-dependent demography in a Japanese temperate broad-leaved forest

A method of detecting the density-dependent dynamics of a size-structured population is developed. The method is applied to a Japanese broad-leaved forest and the density-dependent and-independent projection matrix models are constructed based on the data of the forest. Then, the difference between the density-independent and density-dependent dynamics is compared in terms of several statistical quantities obtained from the matrices. Three kinds of sensitivity matrices are proposed for the density-dependent matrix model. At earlier stages, the sensitivity when the population density is low are higher than at the equilibrium. On the other hand, the inverse result is obtained at later stages. Moreover, the responses of the forest is analyzed to a decrease in survival rate and to an increase in the probability of gap formation. The decrease in survival rate leads to an extreme decrease in the density of the forest. However, the decrease in recruitment rate gives little effect on it because the effect of density-dependence in recruitment of new individuals is strong. The forest has the optimal rate of gap formation such that leads to the maximum population density.

The Ecology of Wildlife Disease Control: Simple Models Revisited

Simple disease/host models are used to identify the ecological criteria governing the likely relative effectiveness of culling, sterilization and vaccination as methods of wildlife disease control. The models include non-linear host death or birth rate/density relationships, and non-linear disease contact rate/density relationships. The following conclusions are based on these simple homogeneous-mixing models, and may be modified by certain spatial effects such as territoriality and density-dependent settlement of dispersing hosts. The threshold density of susceptible hosts for elimination of a disease, relative to the disease-free carrying capacity, is largely determined by characteristics of the disease. For any given threshold, relative to the carrying capacity, the degree of control required to maintain the host density at this threshold is solely determined by ecological characteristics of the host. The sustained rate of control required to eliminate disease can be expressed, exactly or to a good approximation, in terms of the basic reproductive rate of the disease, R 0 the host density-independent birth and death rates, and the parameters describing non-linearities in the host density dependence and disease contact rate/density functions. In terms of the sustained rates of control (e.g. percentage per year) necessary to eliminate disease, sterilization is as effective as culling (i.e. it requires the same level of control effort). Culling is likely to be more effective than vaccination (i.e. requires a lower rate of control) when R 0 is high (> 3) or host density dependence acts on mortality rather than recruitment. When R 0 is low (< 3) and density dependence in the host acts on recruitment rather than overall mortality, then either culling or vaccination can be more effective depending on the ratio of the maximum host death rate to its intrinsic rate of increase, and the non-linearity in the host density dependence and/or the disease contact rate/density relationship; vaccination is favoured by a low death rate relative to r, the host intrinsic rate of increase, and by non-linear density relationships. For a combined strategy of culling and vaccination, the total rate of control required to eliminate disease exceeds that for at least one of the strategies in isolation; to this extent the combined control is less effective. Using the above criterion for comparison, for all six published case studies considered as examples, culling is theoretically the most effective control strategy given the authors' assumptions in the original models. More recent data for possum density dependence suggests that vaccination could more closely match culling in effectiveness. The extent to which vaccination appears more effective in practice, in cases like fox rabies in Europe, probably reflects the spatial behaviour of the host not considered in these simple models. Comparing strategies on the basis of the relative rates at which they reduce endemic disease or the density of susceptible hosts, gives different results. In particular, sterilization is less effective than vaccination, and very much less effective than culling. Non-linearities in the density relationships greatly increase the effectiveness of vaccination relative to cutting ana sterilization, but the distinction between host density-dependent mortality or recruitment is no longer important. The biological reasons for these conclusions are discussed.

A stochastic model of selection on selfing rates in structured populations

SummaryPrevious theoretical studies of the evolution of the selfing rate have shown that mixed mating systems are not evolutionary stable states. Such models have, however, not included the effects of population structure and thus biparental inbreeding together with the evolution of selfing rates and inbreeding depression. In order to examine selection on selfing rates in structured populations, a stochastic model simulating a finite population with partial selfing and restricted pollen and seed dispersal has been developed. Selection on the mating system was followed by introducing modifiers affecting the selfing rate. The major result was that, with density dependent recruitment, a process which maintains the population structure necessary for biparental inbreeding to occur, a mixed mating system could be maintained. This result was associated with an increase of the mutation load with high selfing rates, and the selected selfing rate depended on the degree of population structure rather than on the initial selfing rate. With low dominance of deleterious alleles, complete allogamy can be selected for. Further studies showed that the more general condition of spatial heterogeneity of recruitment can lead to similar results, the most important condition being the maintenance of genetic structure within populations. A brief survey of the empirical literature shows that a positive relationship between the magnitude of inbreeding depression and the inbreeding coefficient within populations has been observed, in support of the present model.

Lottery coexistence models extended to plants with disjoint generations

Abstract. Neither conventional niche theory nor current lottery models offer a satisfactory theoretical scope for modelling coexistence of species with disjoint generations. South‐African fynbos and Australian kwongan include many species which are killed by, and recruit only after, fire. We propose a density‐dependent lottery model which accommodates the unusual demographics of these species. We show that coexistence requires density dependence in recruitment. The result applies to a wider class of populations than the one considered here. It is applied to non‐resprouting species in fynbos and kwongan. We show that the lottery assumption of recruitment in proportion to propagules is often satisfied, while the production of such propagules is often density‐dependent, and we discuss some evidence of mechanisms whereby this may occur.

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.

Nonlinear Feedback Dynamics in Fisheries: Analysis of the Deriso–Schnute Model

Fishery models of the Deriso–Schnute form are based on the dynamics of an age-structured population, together with a nonlinear stock–recruitment relationship. Cyclical dynamics are commonly observed in fisheries and have been attributed to feedback between stock and recruitment. In this paper, we do four things. First, we present analytical results on sustained oscillations driven by nonlinear recruitment. These results show explicitly how density dependence near equilibrium determines the character of sustained population oscillations. Second, we briefly characterize the dynamics of the Deriso–Schnute model when the density-dependent response becomes very strong. We find that the Deriso–Schnute model displays sustained, complex (probably chaotic) variability of large magnitude, but only typically when reproduction is concentrated at very few ages. Third, we dissect the nature of density dependence in recruitment, contrasting a "local" view that uses information about response to small variations in stock with a "global" view that uses a function such as Schnute's over the entire range of stock levels. Finally, we argue that the global approach leads to practical and theoretical difficulties and that a local view may be more biologically realistic.

An Age-Structured Open-Access Fishery Model

A dynamic model for open-access fisheries is presented. In addition to density dependence in recruitment and fishing effort changing in proportion to the level of profit fishermen earn which characterizes previous open-access models, it incorporates full age structure for the fish stock, lognormal environmental recruitment variability, and gear selectivity. The predator–prey cycling solution of the original Schaefer dynamic model, and subsequent open-access models, persists for these model extensions. Density dependence in recruitment induces greater global stability. Environmental recruitment variability, common in marine populations, is destabilizing in the neighborhood of the open-access equilibrium. These two influences, combined in the open-access fishery model, generate robust long-lasting irregular cycles of stock and effort. Volterra proved for the original Lotka–Volterra model that the time averages of the variables over one cycle were exactly equal to their equilibrium steady states. This is shown to extend as a good approximation for the model presented here. Approximating model steady states of effort and catch by the corresponding averages from data time series underlies a new algorithm of parameter evaluation, applied here to an open-access model of the Georges Bank sea scallop (Placopecten magellanicus) fishery.

Density‐Dependent Effects and the Regulation of Crucian Carp Populations in Single‐Species Ponds

Crucian carp (Carassius carassius) is often the only fish species present in small ponds of northern Europe. Such populations are typically at high densities and consist primarily of small, relatively slender, and short—lived fish; populations in multispecies assemblages in lakes display opposite traits. Field and laboratory experiments have implicated predation as an important mechanism structuring crucian populations in multispecies assemblages. To investigate the role(s) of density dependence in structuring pond populations, we manipulated densities of crucians stocked into four sections of a subdivided natural pond in Finland. Mortality of stocked fish during the 3—mo experiment was low (0—11%) and unrelated to density. Growth rates of crucians were both size and density dependent. Larger fish did not noticeably grow in either high— or low—density sections, but growth of smaller size classes was 22% greater in low—density sections. Lower densities and smaller sizes of zooplankton, especially of inshore cladocerans that are an important food resource for smaller crucians, indicated that resource limitation caused by exploitation competition was an important density—dependent process restricting growth of small fish in high—density sections. Crucians rely on reserves of glycogen, stored in the liver and muscle, to fuel the over winter anerobic metabolism that allows them to maintain populations in ponds where winter hypoxia eliminates other fishes. Liver size was significantly larger and glycogen levels were 82% higher in our low—density pond sections than in high—density sections. At higher, but not lower, densities, relative liver size and glycogen levels decreased with body size; combined with the lack of growth of large fish, this indicates that density—dependent energy limitations contribute to the short life—spans and rarity of large fish that characterize pond populations. An increase in relative body depth can be induced in crucians by the presence of piscivorous fish, reducing vulnerability to predation. It has been proposed that increased body depth incurs a substantial energy cost during swimming and should be restricted to specific size classes in populations sympatric with predators. However, in our experiment, crucians of all sizes became significantly deeper bodied, which contributed to higher condition factors in low—density vs. high—density sections. This morphological change, occurring in the absence of piscivores, suggests that energy benefits (accumulation of overwintering reserves) can override any proposed costs for this sluggish, but metabolically unique, fish. Crucian carp successfully reproduced in all sections during the experiment, but growth and recruitment of 0 + fish (young of year) were strongly and negatively related to the numbers of older fish present. The observed density—dependent recruitment was likely caused by starvation—induced mortality and cannibalism; reduced growth observed in 0 + fish would also be expected to cause higher fist—winter mortality, reducing further recruitment at higher densities. Density dependent has a variety of regulatory effects on pond populations of crucian carp that contribute to their presence and resilence.

Maximum sustainable yield with continuous age structure and density-dependent recruitment

The problem addressed is that of finding the harvesting policy that will maximize the yield and maintain a given population in a steady state. It is assumed that the equations describing the evolution of the population and the yield are linear but that the equation describing reproduction is nonlinear (density dependence). The optimal strategy consists in harvesting two age classes only: a younger age class is partially harvested, and an older age class is completely harvested as in the linear case. This problem is a particular case of a general abstract maximization problem in which, for example, nonlinear mortality or nonlinear value of the individuals can also be introduced. The appropriate compactness for this problem is obtained from the fact that maximizing sequence are decreasing.

The influence of spatially and temporally varying oceanographic conditions on meroplanktonic metapopulations

Abstract We synthesize the results of several modelling studies that address the influence of variability in larval transport and survival on the dynamics of marine metapopulations distributed along a coast. Two important benthic invertebrates in the California Current System (CCS), the Dungeness crab and the red sea urchin, are used as examples of the way in which physical oceanographic conditions can influence stability, synchrony and persistence of meroplanktonic metapopulations. We first explore population dynamics of subpopulations and metapopulations. Even without environmental forcing, isolated local subpopulations with density-dependence can vary on time scales roughly twice the generation time at high adult survival, shifting to annual time scales at low survivals. The high frequency behavior is not seen in models of the Dungeness crab, because of their high adult survival rates. Metapopulations with density-dependent recruitment and deterministic larval dispersal fluctuate in an asynchronous fashion. Along the coast, abundance varies on spatial scales which increase with dispersal distance. Coastwide, synchronous, random environmental variability tends to synchronize these metapopulations. Climate change could cause a long-term increase or decrease in mean larval survival, which in this model leads to greater synchrony or extinction respectively. Spatially managed metapopulations of red sea urchins go extinct when distances between harvest refugia become greater than the scale of larval dispersal. All assessments of population dynamics indicate that metapopulation behavior in general dependes critically on the temporal and spatial nature of larval dispersal, which is largely determined by physical oceanographic conditions. We therfore explore physical influences on larval dispersal patterns. Observed trends in temperature and salinity applied to laboratory-determined responses indicate that natural variability in temperature and salinity can lead to variability in larval development period on interannual (50%), intra-annual (20%) and latitudinal (200%) scales. Variability in development period significantly influences larval survival and, thus, net transport. Larval drifters that undertake diel vertical migration in a primitive equation model of coastal circulation (SPEM) demonstrate the importance of vertical migration in determining horizontal transport. Empirically derived estimates of the effects of wind forcing on larval transport of vertically migrating larvae (wind drift when near the surface and Ekman transport below the surface) match cross-shelf distributions in 4 years of existing larval data. We use a one-dimensional advection-diffusion model, which includes intra-annual timing of cross-shelf flows in the CCS, to explore the combined effects on settlement: (1) temperature- and salinity-dependent development and survival rates and (2) possible horizontal transport due to vertical migration of crab larvae. Natural variability in temperature, wind forcing, and the timing of the spring transition can cause the observed variability in recruitment. We conclude that understanding the dynamics of coastally distributed metapopulations in response to physically-induced variability in larval dispersal will be a critical step in assessing the effects of climate change on marine populations.

Control of Endemic Bovine Tb in New Zealand Possum Populations: Results From a Simple Model

(1) Detailed formulae, and approximate ones based on disease reproductive rates, are derived for the rates of harvesting, sterilization and vaccination required to eliminate Tb from a possum (Trichosurus vulpecula) population, assuming either densitydependent recruitment or density-dependent mortality, asymmetric population growth and pseudo-vertical disease transmission. (2) The threshold density of susceptible possums for disease elimination is about half the uncontrolled population density, achieved by harvesting or sterilizing an annual proportion of the population approximately equal to r (0-2-0.3) or immunizing an approximate annual proportion 3r/2. The required immunization rate for disease elimination is highly sensitive to the nature of host density dependence. (3) Simulation of realistic control policies over time, and including the costs of implementation, suggests that the most cost-effective options currently available are widespread poisoning with 1080 every 6 years, a single intensive poisoning operation followed by less intense ground control, or a poisoning operation followed by subsidized and carefully targeted trapping effort. (4) Controlled areas must be large (>50km2) to prevent breakdown of control through immigration from surrounding diseased populations. Such immigration may preclude the possibility of total disease eradication in many areas but not its reduction to extremely low levels, given indefinitely repeated control. (5) The model suggests the need for research into control of reproduction, preferably using microbiological agents, as the best option for longer-term management both of the disease and of the possum as a pest of New Zealand's native forests. (6) The results of the model form the basis for current control policies, already implemented in response to an urgent need. At the same time specific recommended regimes are being rigorously tested in the field.

Evolution of yields from populations with age-specific cropping

Age-specific exploitation of a natural population acts as a selective force on genetic variation in life history traits. Evolution arising from this selection may bring about evolutionary changes in the total yield which the population is able to sustain. An analysis of this process is given for a harvested population with densitydependent recruitment, in which selection of life history traits by cropping is independent of density and frequency. Evolution of the total yield depends on an interplay between the yield from an individual over the course of its life and recruitment; whether the total yield increases or decreases depends on the properties of particular populations. Evolution brought about by harvesting does not, in general, lead to the maximization of the total yield. Nonetheless, by appropriate choice of an age-specific harvest pattern, it is possible in principle to select the life history which gives the maximum total yield following evolution; this harvest pattern is called the ‘evolutionarily stable optimal harvesting strategy’ (ESOHS). Results of the analysis are illustrated with data on the Arcto-Norwegian cod.

Dynamics of an age-structured prey with density- and predation-dependent recruitment: The Dungeness crab and a nemertean egg predator worm

We analyzed stability of a discrete-time model of an age-structured egg predator and an age- and sex-structured prey, then used the results to evaluate the contribution of an egg predator nemertean worm to cycles in Dungeness crab abundance in northern California. Dynamics of the egg predator-prey system were found to be similar to an intraspecific, density-dependent recruitment mechanism, except that the relative importance of different age classes depended on the equilibrium egg mortality rate. Stable limit cycles occurred when the normalized slope of the egg survival function was sufficiently negative. The frequency of these cycles depended on equilibrium level. From comparisons of (1) egg survival data to the derived stability condition and (2) the frequency of observed cycles to those of the model, we concluded that the worm is not the sole cause of the cycles in the northern California crab catch. Changes in the model, survival of the worm past one year, and density-dependent efficiency of prey conversion were shown to be destabilizing, but they did not change the conclusion that the worm was not the sole cause. A model with both the egg predator and density-dependent recruitment in the prey indicated that these mechanisms in combination could produce the observed cycles, even when each by itself could not.

An assessment of several of the historically most influential theoretical models used in ecology and of the data provided in their support

Certain theoretical models (the logistic, Lotka-Volterra and density-dependent stock recruitment models in fisheries) and their mathematical representations are used pervasively in the teaching of ecology and in the application of ecology to various applied problems. Although few claim that these models will or should give accurate predictions of populations in nature, it nevertheless is claimed routinely that they have great heuristic value and are ‘a good place to start’ when making more complex and, presumably, more accurate formulations. Yet if these equations are a good place to start, there should be some data to show that they are good predictors of populations in nature under at least some conditions. Considerable data do exist in the literature that have been used to support the use of these equations. I examine these data and find that in fact none of it support the use of these equations. I for which they are offered as support. Some implications of the transfer of these unvalidated models to management are discussed with particular reference to Pacific salmon.

The relation between the number of parasites/host and host age: population dynamic causes and maximum likelihood estimation.

We examined dynamical factors that shape the distribution of the number of parasites/host in constant or temporally varying environments, and with or without host-age dependent variation in host susceptibility and parasite mortality. We predict properties of the parasite distribution in the absence of density-dependent factors such as density-dependent mortality of recruitment and parasite-induced host mortality. These properties provide a criterion for the detection of density dependence in temporally variable systems with host-age dependent interactions. We have then introduced methods to estimate and statistically evaluate the effects of host age or size on the distribution of parasites/host. The methods are based on a maximum likelihood protocol for linear and non-linear regression when data are negatively binomially distributed. We have illustrated the use of the theoretical results and statistical methods by re-analysing the data of Halvorsen & Andersen (1984) on cestode infections in Norwegian arctic charr and by analysing new data on nematode infections in Caribbean Anolis lizards.

Recruitment of Yellow Perch ( Perca flavescens) Affected by Stock Size and Water Temperature in Lakes Erie and St. Clair, 1965–85

Two influences on recruitment of yellow perch ( Perca flavescens) were considered: parental stock size and spring water temperatures. The data for this analysis were obtained from two sources. First, from samples of trawl catches (near bottom) from April to October (1965–85) in the western, central, and eastern basins of Lake Erie, and from Lake St. Clair and second, from gillnet catches from April to July in the three basins of Lake Erie (1978–86). The relative abundances (CPUE) of stocks in these study areas were as follows: western > St. Clair > eastern > central basin. Degree-days and mean water temperatures of the basins were ranked as follows: St. Clair > western > central > eastern basin. Rates of warming of water were greatest in the western basin, followed by St. Clair, central basin, and eastern basin. The rates of increase in water temperature (April to June) were positively related to average water temperatures and total degree-days (above 0°C. Only in the western basin of Lake Erie could the variations in recruitment to age 1+ be apportioned to the effect of parental stock size (12%) and water warming in the spring (51%). Recruitment at age 0+ (young-of-year(YOY)) was explained also by both warming rate (17%) and parental stock size (12%) for the samples from the western basin. The pooled data obtained from samples of catches in the gillnets indicated that only parental stock size (19%) determined the abundance of perch at age 1+ yrs-old. The relative abundance of yearlings was related to the abundance of YOY perch in the previous year in only the central and western basins (trawl sample). The dependence of recruitment on the density of the parental stock was evident from the regression of the ratio of recruits (age 1 +) to spawners on parental stock size in the combined trawl samples from Lakes Erie and St. Clair, and in the separate samples from the eastern and central basins of Lake Erie and Lake St. Clair. Similarly, recruitment at age 0+ (YOY) was also shown to be density-dependent in all but the western basin of Lake Erie. The coefficients obtained by regressing recruitment (0 +, 1 + years) on stock size were significant and less than 1 in the pooled sample from the western basin and St. Clair, indicating density-dependent recruitment.

Recruitment Dynamics of Spring-Spawning Herring in the Northwest Atlantic

Recruitment patterns of the seven major spring-spawning herring populations in the Northwest Atlantic were found to be determined largely by annual variations in overwintering temperatures and salinities associated with the Labrador Current, and the patterns were therefore synchronous. Removal of the environmental component of recruitment variability revealed dome-shaped stock–recruit curves with sharply ascending left limbs and relatively high degrees of compensatory density dependence in recruitment as measured by the Ricker parameter, β. The degree of such density dependence was positively correlated with recruitment variability which was itself negatively associated with mean stock size, i.e. large herring stocks tended to have less recruitment variability but lower levels of density dependence in recruitment than smaller stocks. In addition, environmental variability was positively associated with recruitment variability, and for stocks of the same size, the more northern populations of spring-spawning herring, inhabiting a more variable environment, have a higher level of density dependence on recruitment. Simulation studies showed that larger herring stocks could sustain higher levels of fishing mortality than smaller herring stocks, due mainly to reduced recruitment variability. We suggest that the most appropriate fishing strategy for herring is to maintain fishing mortality at a level of F = 0.20 with the option of closure when parent stock size falls below the critical level. A surprising finding from our study was the high level of density dependence of recruitment in herring stocks which we attributed to several features: (1) a large egg size and the ability of herring populations to adapt to a wide range of spawning conditions, (2) a density-dependent mortality associated with egg hatching success, and (3) a strong homing tendency ensuring that changes in spawning population sizes are spread evenly throughout the spawning area, thereby optimizing larval production.

The effects of parasites on fish populations—theoretical aspects

This brief review has indicated how essential aspects of relevant epidemiological considerations may be included into models of fisheries. The main conclusions to emerge from the crude models outlined above are that the ability of a pathogen to establish itself is dependent upon the relative magnitudes of the threshold host density for parasite establishment, H t, and the level of the host population density at its current level of exploitation H(E). If H t is less than H(E), the parasite will always be able to establish itself. Disease prevalence would also seem to be roughly independent of the level of exploitation of the fishery, providing exploited population density is significantly higher than the threshold density for disease establishment. The complications introduced by the presence of disease will in general further increase the levels of uncertainty that fisheries managers have to contend with (Beddington, 1984). In cases where pathogens are having a serious impact on the fishery it would seem sensible to develop methods to quantify the impact of the parasite on the host (Lester, 1984). The simple models discussed here can, moreover, readily be extended to include other factors which can be important in determining management strategies for fisheries where parasites and disease are an important consideration. Three particularly important such considerations are: inclusion of age-structure and more realistic density-dependent recruitment functions in the host population (May, 1980); consideration of the immune response of the host to the parasite (Anderson & May, 1979); and inclusion of environmental stockasticity (May, Beddington, Horwood & Shepherd 1978; Ludwig & Walters, 1981).

Régulation par le recrutement chez la mouette rieuse Larus ridibundus

The Black-headed Gull population of the Forez (central France), under study since 1976, is remarkably stable. This stability, or slow growth at the very most, implies a change in some demographic parameters of the population. Indeed, large differences in breeding success were found between colonies established on traditional sites, successfully occupied year after year (where breeding success is high), and those established on sites that are occupied discontinuously (where breeding success is low). Such differences suggest that the gull population has reached saturation at the best sites. Most of our work was carried out in one of the traditional colonies (with more than 2 000 breeding pairs). Here, the saturation hypothesis is supported by the strong competition taking place for nest sites, which leads the younger breeders to make use of nest sites low over water, therefore highly vulnerable to wave action, and to variations of water level. A study of mortality and recruitment, using a capture-resighting technique, provided a more direct evidence of population saturation on this colony : full breeding was only reached at five years of age. But the strongest evidence of density-dependence in recruitment is brought forward by the estimation of the proportion of birds locally recruited is each cohort : this proportion varies inversely with the initial fecundity of the cohort. Only a limited number of breeders can therefore be recruited each year in this colony. A density-dependent population model confirms that density-dependence is strong enough to induce a stabilization in numbers of the colony members. Theoretical and practical consequences of this mode of population regulation are discussed.

Effects of Environmental Forcing on Age-Structured Populations: Northern California Dungeness Crab (Cancer magister) as an Example

Evaluation of potential environmental effects on recruitment requires knowing how environmental forcing can affect recruitment and subsequent catch, and how these effects depend on life history. General relationships between a time series of an environmental forcing variable, the resulting time series of recruitment, and the subsequent time series of catch or abundance are described here for a fishery in which environmental forcing affects density-dependent recruitment and the population is fished at a constant rate. For a stable population with overcompensatory, density-dependent recruitment, the recruitment time series will resemble the environmental time series except that frequencies near 1/(2 × mean age in the population) will be emphasized. The amount of selective emphasis will depend on the degree of overcompensation, harvest rate, and width of the cohort size distribution (in a size-selective fishery). For a population harvested at constant rate, the catch time series will resemble the recruitment time series except that lower frequencies will be emphasized. The degree of selective emphasis will depend on harvest rate and width of the cohort size distribution. Application of these results to Dungeness crab (Cancer magister) research has allowed analysis of combined effects of density dependence and environmental forcing. They show (1) that criteria formerly used to evaluate density-dependent recruitment mechanisms were too strict, (2) that female harvest could reduce harvest variability due to environmental forcing, and (3) how a nonlinear influence of spring wind stress in combination with density-dependent recruitment could cause the observed catch record. Implications for other crustacean populations are discussed.