Density-independent Factors Research Articles

Density dependence in the camelid Vicugna vicugna: the recovery of a protected population in Chile

The vicuña Vicugna vicugna is a wild South American camelid. Following over-exploitation, which brought the species to the brink of extinction in Chile in the 1960s, the population was protected. Since 1975 the population has been censused annually, generating one of the most extensive long-term census databases for any South American mammal. In this paper we use these data, and measures of environmental parameters, to describe the population growth trend of the species and to estimate carrying capacity. Our results indicate that the vicuña has been protected successfully in northern Chile. The census data reveal that, following protection, the population displayed logistic growth between 1975 and 1992. Population growth rate declined linearly with population size, which indicates a degree of density dependence. Density independent factors, such as rainfall, may also have been important. The principal density dependent effect observed was that birth rate declined in those family groups with the most breeding females. The carrying capacity of the study area was estimated from the census data and from models based on precipitation and local primary productivity. Using the census data, an estimation of carrying capacity as the asymptote of the fitted logistic curve suggested that the vicuña population should reach approximately 26,000 vicuñas, whereas estimation when the population growth rate was equated to zero gave a carrying capacity of c. 22,000. Coe's method based on local precipitation predicted 31,000 vicuña, whereas Lavenroth's method based on local primary productivity predicted 26,000 vicuña. In reality, the census data showed that the population peaked at 22,463 vicuñas in 1990. The results are discussed in relation to the need for better census techniques and the implications of density dependent effects for the management of the vicuña in Chile.

Subsistence strategies and early human population history: An evolutionary ecological perspective

One of the keystones of the evolutionary ecological approach is the concept of energy budget, in which time and energy allocation is conceptually divided into somatic effort (growth, development and maintenance, and includes subsistence activities) and reproductive effort (which is further divided into mating effort and parental effort). Time and energy allocated to one component must be traded off against allocation to another. Using this energy budget approach in conjunction with some of the general implications of foraging theory, this article will explore the relationship between population dynamics and subsistence intensification. My discussion will revolve around two basic propositions regarding long-term human population history: 1) the near-zero growth rates that have prevailed through much of prehistory are likely due to long-term averaging across periods of relatively rapid local population growth interrupted by infrequent crashes caused by density-dependent and density-independent factors; and 2) broad changes in population growth rates across subsistence modes in prehistory are probably best explained in terms of changes in mortality due to the dampening or buffering of crashes rather than significant increases in fertility.

Black Bullhead Growth in South Dakota Waters: Limnological and Community Influences

ABSTRACT Growth was determined for 35 black bullhead (Ameiurus melas) populations in eastern South Dakota lakes. Black bullhead growth was highly variable among populations. Average mean lengths at ages 1–6 for all populations were 101, 155, 203, 232, 262, and 289 mm, respectively. Few populations contained fish older than age 6. Growth was negatively associated with black bullhead abundance, and positively associated with black bullhead size structure, indicating that density dependence likely is a regulating factor. Growth increased with lake depth (maximum and mean), and decreased with increasing productivity (total dissolved solids and morphoedaphic index). The black bullhead appears more likely to overpopulate in shallow, productive lakes, resulting in slower growth. In waters with lower densities of black bullheads, density independent factors may ultimately determine growth. Growth was inversely related to the size structure of predators, suggesting some predation effect. Relationships with possible competitors indicated little evidence of competition.

Population dynamics ofClethrionomys glareolus andApodemus flavicollis: seasonal components of density dependence and density independence

Using long-term data on two forest rodent species [the bank voleClethrionomys glareolus (Schreber, 1780) and the yellow-necked mouseApodemus flavicollis (Melchior, 1834)] from the Białowieża Primeval Forest (E Poland), we decompose the annual density-dependent and density-independent structures into their seasonal components. For this purpose we adopt a state-space modelling approach explicitly incorporating sampling stochasticity. As density-independent factors we use the North Atlantic Oscillation (NAO) — a proxy variable for the overall climatic condition — and data on annual seed production. We find a weak effect of the NAO in the annual models for both species as well as during the winter in the seasonal model forC. glareolus. The effect of the NAO disappears, however, when seed-crops are incorporated into the models (for both the annual and the seasonal — suggesting that NAO primarily affects seed production). Seed production enters the models with a positive effect during the winter only, suggesting that the among-year variation in rodent density is primarily accounted for by differences in seed-production, particularly oak seeds. ForA. flavicollis, a slightly positive effect of hornbeam also appears in the summer dynamics. The obtained results are discussed on the basis of earlier studies on the same populations, on the same species studied elsewhere as well as on the basis of general ecological insight.

Growth, survival and fluctuating asymmetry of Iceland scallops in a test of density-dependent growth in a natural bed

We report an experiment designed to test for overpopulation in an Iceland scallop, Chlamys islandica (O.F. Müller), slow-growing bed. Scallops were installed in pearl nets at the site of the scallop bed and in a nearby unpopulated site, near the bottom and far from the bottom. With this set-up, all groups, except that one near the bottom inside the bed, were free from potential overpopulation effects. In addition, there were two stocking densities to test for containment effects. Shell growth was higher at 2.0 m above the bottom, outside the scallop bed, than at the three other site and height combinations, which were not significantly different from each other. There was a borderline effect of site on soft tissue growth, which was probably slower at the donor site. We conclude that there was no evidence of overpopulation in the scallop bed. On both sites, soft tissue growth was slowest near the bottom, irrespective of group size, intermediate at 2.0 m height, high population density, and fastest at 2.0 m height, low population density. Survivorship was lower near the bottom, but was independent of site and of group size. Available data showed no sustained vertical patterns in phytoplankton concentration in the water column. These results suggest that food depletion occurred in the pearl nets, but that density-independent factors dominated density-dependent factors in the pearl nets near the bottom. Likely explanations involve the interaction between containment effects and current speed variations in the benthic boundary layer. Survivorship decreased with fluctuating asymmetry of the ears of the shells. Fluctuating asymmetry, therefore, may provide a means of including individual effects in growth experiments and forecasting the ability of spat to resist density-independent mortality agents.

Contributions of forage competition, harvest, and climate fluctuation to changes in population growth of northern white-tailed deer.

Recently there has been considerable interest in determining the relative roles of endogenous (density-dependent) and exogenous (density-independent) factors in driving the population dynamics of free-ranging ungulates. We used time-series analysis to estimate the relative contributions of density-dependent forage competition, climatic fluctuation, and harvesting on the population dynamics of white-tailed deer (Odocoileus virginianus) in Nova Scotia, Canada, from 1983 to 2000. A model incorporating the population density 2years previous, an interaction term for the harvest of females and population density 2years previous, and the total snowfall during the previous 2 winters explained 80% of the variation in inter-annual population growth rate. Natality of adult females was negatively related to deer density during the present winter, whereas that of yearlings may have been correlated with the snowfall of three winters previous. Natality of fawns was related to deer density and total snowfall during the previous winter. Coyotes (Canis latrans) prey extensively on deer fawns in northeastern North America and the annual harvest of snowshoe hares (Lepus americanus), the major alternate prey of coyotes, explained 48% of the inter-annual variation in fawn recruitment. The proportions of fawn, yearling, and adult deer suffering from severe malnutrition during late winter were all correlated with deer density during the present winter. We conclude that the limiting effects of winter weather on over-winter survival of deer may be cumulative over two consecutive winters. During the late 1980s, density dependence and winter severity acted in concert to effect substantial declines in deer population growth both by effecting winter losses directly and by exacerbating predation by coyotes. During this period liberal harvesting did not relieve density-dependent forage competition and probably accelerated the decline.

Potential of Winter Cover Crops to Increase Abundance ofSolenopsis invicta(Hymenoptera: Formicidae) and Other Arthropods in Sugarcane: Table 1

A 3-yr study was conducted in Louisiana sugarcane field plots to determine the potential of vegetation management and winter cover crops to enhance abundance of the fire ant, Solenopsis invicta Buren, other arthropods, gastropods, and spring sugarcane density. Treatments included pea, clover, and vetch cultivars, a weed-free herbicide treatment, a vetch with herbicide on the row tops; and a mixed weeds treatment arranged in a randomized complete block design. Compared with similar studies conducted during the summer months, spring collections of arthropods in pitfall traps were very low and few differences in arthropod densities occurred. Neither cover crop cultivar nor biomass substantially influenced arthropod density or cane stand density. Slugs (Limacidae) and earwigs (Dermaptera) were most abundant in mixed weed plots. The highest numbers of carabids in 1994 were found in vetch plots, which tended to have higher biomass than other treatments. Soybean oil-soaked bait cards attracted more ants in clover plots than in the plots with vetch plus herbicide. Compared with previous summer studies, we feel that harsher winter weather and other density independent mortality factors during this study period superseded effects of cover crops, vegetation management and quantity of biomass on arthropod densities during the winter. Although positive impacts of winter cover crops were not detected for the variables measured during the study period these data should not be used to suggest that cover crops do not provide agronomic benefit to farmers.

ASSESSING DENSITY DEPENDENCE USING INTRA-ANNUAL COUNTS

If causes of population fluctuation vary temporally, then tests that search for density dependence in long time series cannot distinguish, at any given time, how much population change is due to density-dependent or density-independent factors. For some species' dynamics, however, it is possible to determine over a restricted period whether density-dependent factors are involved in reducing population densities. I use a new Return Trajectory Likelihood Ratio Test (RTLRT) to determine whether a population perturbed to high density (e.g., by periodic recruitment) exhibits density dependence by returning to a positive asymptote (or leveling density, L) or is density independent and, through proportional hazards, approaches zero exponentially. In a series of simulations, declines after recruitment were mimicked by creating sequences that changed due to a combination of density-dependent and density-independent factors. Until the sequence dropped below L, losses due to each mortality type were combined; after that point the density-dependent component was discontinued. The power of the test was assessed over a range of decline rates (e−b, where b = 0, 0.1, 0.25, 0.5, 0.75, 0.9, 1) to zero and to L, initial density values (2, 4, and 6 times higher than L), and observation and process errors (eσ where σ = 0.1, 0.25, and 0.5). Test size was assessed by generating trajectories with no density-dependent decline. These simulations identified four critical parameters that influence the likelihood of detecting density-dependent decline. (1) Detection rates change with the ratio of initial recruitment peak to leveling density: high peaks generally increase detection, although they may decrease detection of slow density-dependent declines. (2) High rates of density-dependent decline relative to density-independent decline also improve detection, except that populations rapidly dropping below L will appear density independent regardless of dynamics governing change prior to that time. (3) Error reduces detection of density dependence, except when declines are shallow and process error is assumed. (4) Slow rates of density-independent decline (to <10% of original value) cause the RTLRT to detect density dependence when it is not present (i.e., size is excessive for slow declines). As an example of how the RTLRT may be used, I apply it to four years of monthly counts of an intertidal isopod, Idotea wosnesenskii, which recruits annually to high densities under boulders. Based on the RTLRT, subsequent declines were density dependent in some years, although detection differed with the assumption of process or observation error, and with the use of separate or averaged samples. In addition to density dependence, selective mortality of one class of individuals or mortality balanced by continuing low immigration could also cause steep declines to L. For isopods, however, densities were higher under boulders with small depressions than under smooth controls, suggesting that declines may occur when densities exceed the number of safe sites available. The postrecruitment decline of I. wosnesenskii appears to be at least partially density dependent.

Population dynamics of two marine polychaetes: the relative role of density dependence, predation, and winter conditions

Van der Meer, J., Beukema, J. J., and Dekker, R. 2000. Population dynamics of two marine polychaetes: the relative role of density dependence, predation, and winter conditions. – ICES Journal of Marine Science, 57: 1488–1494. We modelled the population dynamics of two polychaete species, a prey (Scoloplos armiger) and its predator (Nephtys hombergii) by simple linear and non-linear time–series models using a 29 year data series of population abundance and winter conditions. The log reproductive rate (i.e., the difference in log density between two succeeding years) of the prey population was density dependent and negatively related to the density of the predator. The log reproductive rate of the predator was also density dependent, but was to a greater extent determined by winter temperature, a density-independent factor. Log reproductive rate of the predator was not related to the density of the prey, nor could delayed density dependence be detected. Apparently, the feedback mechanism between predator and prey was weak, perhaps because the predator is a generalist. Predator density can therefore be categorized as a densityindependent factor with respect to the dynamics of the prey species. In neither of the two species was density dependence strong enough to generate complex or chaotic behaviour, as indicated by the weakly negative slope of the reproduction curve at the equilibrium, the apparent lack of a non-linear component in the reproduction curve, and the negative Lyapunov exponent.

Delayed density-dependent and rainfall effects on reproductive parameters of an irruptive rodent in semiarid Chile

We examined the influence of density-dependent and density-independent factors on reproductive processes of leaf-eared mouse Phyllotis darwini (Waterhouse, 1837) in a semiarid region of Chile subjected to El Nino-driven precipitation. This species undergoes periodic irruptions apparently triggered by unusually high precipitation. The effects of density and precipitation were analyzed statistically regarding the following reproductive parameters: fraction of reproductive females, reproductive number (juveniles + reproductive females), per capita reproductive rates, and reproduction index based on long-term data (10 years). The fraction of reproductive females was affected positively by precipitation during the preceding winter and negatively by population density one year before. The reproduction index and the reproductive rate was positively affected by population density the previous year. In addition, the reproductive number was positively correlated with precipitation levels. The P. darwini population studied was affected by delayed density-dependent and density-independent factors in reproductive parameters. We propose that both destabilizing effects on reproductive rates of P. darwini may be major factors underlying the frequent outbreaks of this mouse observed in semiarid regions of Chile.

The 1996 Flatfish Symposium; a summary report

A summary report of the 1996 Flatfish Symposium is presented. Three topics of the Symposium are discussed: taxonomy and systematics, recruitment processes, and management issues. It is strongly recommended that a taxonomic advisory committee should be formed that will facilitate the transfer of information between different scientific disciplines focused on increasing our knowledge about flatfishes. Recruitment variation in flatfish appears to be dominated by density-independent factors operating at a local scale on the eggs and larvae. Variability in hydrodynamic circulation is suggested to be an important factor. Density-dependent processes are likely to be restricted to the smaller size classes which are not affected by fishing. The debate on the species range hypothesis is continuing. The patterns in latitudinal variation observed in fish did not correspond with the expectations. However, the species range hypothesis could neither be accepted nor rejected, because other sources of variation such as water temperature, sampling strategy, age composition of catches, inshore–offshore gradients and shifts in species ranges are biasing and maybe overruling the patterns in recruitment. Evaluations of the management of different flatfish stocks stressed the importance of the interactions between management measures and flatfish population structures. This field should be strengthened in the future.

Effects of density and weather on survival of bighorn sheep lambs (Ovis canadensis)

Density-dependence in juvenile survival may be difficult to detect if survival is also affected by density-independent factors. We investigated the relationships among weather parameters, population density, and lamb survival of bighorn sheep with long-term data from a marked population where we manipulated population density. We distinguished neonatal survival and winter survival. Density interacted with weather variables to affect neonatal survival; spring and winter temperatures had a positive effect on neonatal survival only when population density was high. Neonatal survival was positively affected by spring precipitation independently of population density. Winter survival was positively correlated with temperature and precipitation during the previous spring, negatively correlated with density, and independent of winter temperature or snowfall. The effect of weather on lamb winter survival did not vary with density. Bighorn lambs are well adapted to harsh winter weather, but spring weather influenced survival of lambs at birth and during the subsequent winter, possibly through its effects on forage availability. Our study clearly demonstrates density-dependence in lamb survival. Some of the effects of weather on lamb survival are density-independent, others are mediated by an interaction with population density.

Modeling biotic and abiotic influences on population size in small mammals.

Most previous work in population ecology has modeled density-dependent effects in isolation. In this paper, we concurrently modeled the effect of density-dependent and density-independent factors on the rate of population change (R t ) in Peromyscus leucopus (white-footed mouse), using a Ricker equation expanded to include weather and seasonality. From 1973 to 1996, we live-trapped P. leucopus monthly in a 2-ha Ohio woodlot. Population peaks (July to August) varied from 27 to 181 individuals, while troughs (December to March) varied from 4 to 46 individuals. We used time-delayed densities to act as surrogates for unobserved density-dependent factors, and principal components to represent 12 highly collinear weather variables. We identified time-delayed correlations by season between R t and the independent variables (i.e., previous densities and weather principal components) using transfer function analysis. In summer, when P. leucopus densities were above the seasonal mean for the month, R t was lower in the following 2 months; however, in winter, R t was greater in the first but lower in the second succeeding month. R t also correlated positively in autumn with contemporaneous precipitation, and was negatively correlated with `extreme' weather in summer with 2- and 3-month lags and in winter with a 3-month lag. We hypothesize that precipitation affected juveniles by influencing food resources and that `extreme' weather affected breeding. Our model explained 65% of the variability in R t , and density-dependent and density-independent factors explained an equal percentage of that variability. This model created good forecasts of population density up to 12 months in the future.

Temperature change and complex dynamics

Density-dependent factors, such as population growth rate and migration, influence dynamic behaviour in ecological models. Temperature, an abiotic and density-independent factor, is also an important determinant of insect population growth. We investigated the endogenous dynamics of a density-dependent response-surface model that included temperature, based on time series for two aphid species. We investigated the effects of temperature and random noise on the model dynamics. In most cases, an increase in temperature resulted in a higher predicted equilibrium density; it could induce complex dynamics. Noise at the level of the natural variation in temperature resulted in extinctions in some models. Our results from these models indicate that aphid populations might become more abundant, and less stable in some circumstances, if there is climate warming.

A comparative analysis of recruitment variability in North Atlantic flatfishes — testing the species range hypothesis

The hypothesis that recruitment variation in flatfishes should be most variable at the northern edge of the species range, least near the centre of the range, and intermediate near the southern limit was tested using stock and recruitment data generated from sequential population analysis for several different flatfish stocks involving four species (plaice Pleuronectes platessa, sole Solea vulgaris from the eastern Atlantic, American plaice Hippoglossoides platessoides, and yellowtail flounder Limanda ferruginae from the western Atlantic). Several groundfish species have been found to conform to this so-called species range hypothesis with the suggestion that density-independent processes predominate at the edges of the distributional range and density-dependent processes dominate in the centre of the range. Our results were generally inconsistent with the hypothesis: the coefficient of variation (CV) of recruitment for plaice in the eastern Atlantic was independent of latitude, the CV of recruitment for sole exhibited a dome-shaped relationship with latitude with the highest CVs occurring at the mid-point of the range, and the CV of recruitment for the western Atlantic stocks exhibited a monotonic decrease with latitude. We extended our latitudinal analyses by assessing both the degree of dependency of recruitment on spawning stock biomass and the spatial and temporal scales of variability in recruitment and pre-recruit survival for the eastern Atlantic stocks. In general our analysis revealed no evidence of a strong stock and recruitment relationship for any of the stocks examined, and previously published analyses revealed no such patterns with latitude. Analysis of both de-trended recruitment and pre-recruit survival time series over the species ranges of sole and plaice revealed strong positive correlations among adjacent stocks and inverse correlations among stocks at the extremes of the range. Recruitment variation in the flatfish stocks examined appears to be dominated by density-independent factors, operating at a local scale, on the egg and larval stages.

Tests for density dependence.

Several statistical tests for 'density dependence' have been proposed in the literature, and so in any practical case the question poses itself which one of these tests to choose. This paper offers a few remarks additional to those made by Fox and Ridsill-Smith (1995) and others. Parametric statistical tested are based on a fully specified mathematical model. Examples of such tests are Bulmer's (1975) first test, and the test of Dennis and Taper (1994). Distribution-free tests are based on far less stringent assumptions. An example of such a test is the one proposed by Pollard et al. (1987). The choice between parametric tests can best be made by considering which one of the underlying mathematical models ist most plausible. If all models are almost equally plausible, considerations of computational requirement and ease of application may be important. Strong doubts concerning the plausibility of mathematical models may lead one to prefer a distribution-free test. An important feature of any test is its power, i.e. the probability of its rejecting the null hypothesis when this hypothesis is not true. Other things being equal, tests are preferable when they have superior powers. But power of a test depends on 'the true state of nature', and the only way to study power quantitatively is by assuming some mathematical model as approximately representing this 'true state'. As any mathematical model can at best only be an approximation to the situation in nature, a mathematical model and the statistical tests based on it should be robust against small deviations from model assumptions. Solow (1990) showed that Bulmer's test is not robust with respect to the assumption that the residuals in the underlying autoregression model be stochastically independent. Contrary to what was suggested by Fox and Ridsill-Smith (1995), who misinterpreted some statements in Reddingius (1990), the present author thinks this is a serious shortcoming of this test since an ecologist cannot assume a priori that important density-independent ecological factors are not somehow serially correlated. Moreover, he is rather sceptical about the usefulness of statistical tests for density dependence. They have contributed more to misunderstandings than to a significant increase in ecological insight. In any case, statistical tests are designed to test hypotheses that are stated before data are collected, and the question which test to use also has to be answered before the data have been collected. Designing and using statistical tests a posteriori to 'detect' things in data mainly leads to confusion and controversy.

Dynamic Behavior of Douglas-Fir Tussock Moth Populations in the Pacific Northwest

Abstract Larval densities of the Douglas-fir tussock moth (Orgyia pseudotsugata [McDunnough] [Lepidoptera:Lymantriidae]) were monitored annually to construct population time series in five different forest ecosystems. The time series were generated in parts of five national forests in three geographical provinces: the northern Blue Mountains of northeastern Oregon, the southern Cascade Range of south-central Oregon, and the northern Cascade Range of north-central Washington. The five series ranged in length from 12 to 23 yr and represented the broad numerical behavior of tussock moth populations in those provinces. Time series with the most variable fluctuations were recorded in the Blue Mountains, a region with a long history of regular tussock moth outbreaks. The least variable series was in the southern Cascades, an area with no previously documented outbreaks. Characteristics of series in the Blue Mountains and the northern Cascades were significantly correlated with one another, but not with those of the series in the southern Cascades. The density-dependent structure of each time Series was examined by autocorrelation and partial autocorrelation functions, which indicated that fluctuations were quasi-periodic, even at low densities, and that the most significant autoregressive process was a second-order feedback. Each time series was fit with a linear second- order autoregressive model and a discrete nonlinear logistic model with a single time delay. The models, based on density-dependent processes associated with preceding insect generations, accounted for an average of 44.6% of the variation in the time series. The remaining 55.4% of variation was due to unknown density-independent factors related to different years in the series. The models showed that the principal dynamics of tussock moth populations are generated by direct and delayed density-dependent processes. In outbreak-prone ecosystems, like those in the Blue Mountains and the northern Cascades, the primary regulation of insect numbers is by delayed feedback processes that result in populations sometimes having extreme fluctuations in density. Conversely, in nonoutbreak ecosystems, as in the southern Cascades, feedback is more balanced between both direct and delayed processes that stabilize populations at lower densities. External factors, like weather, also may influence the numerical behavior of populations to some extent, but their specific contribution to tussock moth dynamics is largely unknown. For. Sci. 42(2):182-191.

Synchrony in population dynamics

Our data are long-term population dynamics of a set of species in different localities in Finland. There is considerable level of species-specific synchrony in population fluctuations among the localities. The degree of synchrony levels off with increasing distance among the populations compared. Climatic perturbations and dispersal have been proposed as pace-making factors for synchrony. According to Moran's theorem, local populations sharing a common structure of density dependence should become synchronized under the influence of a spatially correlated density-independent factor. This predicts synchrony to decay slower with increasing distance between local populations than if the synchrony is caused by dispersal. To explore the significance of the Moran effect and dispersal in explaining the observed regional synchrony, we used a metapopulation system. The Moran effect and dispersal are both capable to synchronize alone local population fluctuations. However, with dispersal the level of synchrony decreases with distance among the populations. Adding Moran's effect does not greatly affect the level of synchrony, nor the negative correlation between synchrony and distance. This finding makes it difficult to tell apart whether an observed negative correlation between the level of synchrony and distance among the compared populations is caused by dispersal alone, or both dispersal and Moran's effect acting together.

Recruitment mechanisms in flatfish: What did we learn and where do we go?

This paper summarizes the developments in flatfish recruitment studies over the last decade with emphasis on the general patterns that have emerged from the contributions to the two Flatfish Symposia of 1990 and 1993. Recruitment variability is largely generated by density-independent factors acting during the pelagic egg and larval phases. Effects of variability generating processes tend to be amplified towards the edges of the distribution range and appear to be related to abiotic conditions. Density-dependent feedback processes occur in the demersal juvenile phase, when flatfish become highly specialized benthic feeding fish. There is evidence that density-dependent feedback processes may also occur during the adult phase in not or lightly exploited populations. Areas of future research that emerge from this symposium are: 1. the classification of flatfish populations in ‘ecological equivalents’, including the habitat requirements of the successive life history stages; 2. comparative studies among species and populations of recruitment processes; 3. analysis of environmental factors determining the survival of pelagic eggs and larvae; 4. density-dependent habitat selection of demersal juveniles in relation with growth and mortality; 5. analysis of the habitat characteristics of species for pelagic eggs and larvae, demersal juveniles and adults, including drift of eggs and larvae, transport from spawning towards nursery areas and adult migration; and 6. simulation studies of relevant population dynamical processes to explore quantitatively the importance of the various processes and the necessary precision with which these should be known.

Effects of changes in agricultural landscape on a breeding population of linnets Acanthis cannabina L. living in adjacent heathland

The effects of a land consolidation project were studied in an agricultural landscape in Brittany on the foraging habitats, breeding parameters and factors causing breeding failure of the linnet Acanthis cannabina L. The study covered five breeding seasons before land consolidation and another five afterwards. The project involved the disappearance of more than 50% of the hedgerows, and about 30% of the meadows and fallow lands were replaced by cultivation. Before land consolidation, breeding success was mainly affected by density-dependent factors, whereas afterwards density-independent factors such as weather or disease played an important role. The preferred feeding sites remained the same during the whole study period; fallow lands, rape fields and meadows were selected positively but cereal and potato fields were avoided. This characteristic became more apparent after land consolidation, because it involved a reduction in the feeding areas of the linnet. Numbers of eggs per clutch, hatchlings and fledglings were unaffected but fledging and breeding success of the first brood increased significantly during this period. Predation, which was the main cause of egg loss and nestling mortality, decreased significantly after land consolidation, through loss of hedges used as movement corridors by some specialist predators. The increase in the distance between breeding site and foraging area probably reduced the time spent by adults on their nests, leaving nestlings unprotected.