Jean-Dominique Lebreton

An analysis of demographic tactics in birds and mammals

Using field data on fecundity, age at first reproduction and adult life expectancy, we reconsider the so-called r-K gradient by analyzing relationships between these three demographic parameters in 80 mammal species and 114 bird species. After the allometric effect of adult body weight is removed, the three variables remain correlated. The existence of demographic tactics which are independent of adult body weight is demonstrated by multivariate analyses of these variables. These analyses confirm the importance of ecological and phylogenetic constraints. The main structure is a time-scale gradient ranking species according to turn-over, both in birds and mammals. A second gradient ranking species according to iteroparity level appears significantly both in birds and mammals. In mammals, this pattern is related to patterns of parental investment.

Early survival in roe deer: causes and consequences of cohort variation in two contrasted populations

Abstract Time- and sex-specific summer survival of roe deer fawns was estimated using capture-mark-recapture methods in two enclosed populations living in contrasting conditions. The population of Trois Fontaines (eastern France) was roughly constant in size throughout the study period, while in Chizé (western France), the population experienced frequent summer droughts and numbers decreased continuously during the study. Early survival of fawns was low and highly variable over the years at both Chizé and Trois Fontaines, and demonstrated marked variations between cohorts that need to be taken into account when modelling roe deer population dynamics. In Trois Fontaines, fawn survival was positively correlated with early body growth and total rainfall in May and June. In Chizé, fawn survival decreased with increasing density and tended to increase with increasing rainfall in May and June and adult female body mass. These factors explained more than 75% of the variability in early survival observed in both populations. Variation between cohorts had different consequences for the two populations. At Trois Fontaines, cohort variation was limited to a numerical effect on early survival. However at Chizé, cohort variation was long-lasting and affected the phenotypic quality of survivors at later ages, and thereby future survival and breeding abilities (both numerical and quality effects). Male and female fawns had similar survival over their first summer in both populations. This result contrasts with the lower survival of young males often observed in ungulates. Two ultimate causes can be proposed to account for the low and variable survival of roe deer fawns over the first summer: the high energy expenditures incurred by does during each breeding attempt and/or the low absolute body size of newborn roe deer fawns.

Chapter 3 Modeling Individual Animal Histories with Multistate Capture–Recapture Models

Summary Many fields of science begin with a phase of exploration and description, followed by investigations of the processes that account for observed patterns. The science of ecology is no exception, and recent decades have seen a focus on understanding key processes underlying the dynamics of ecological systems. In population ecology, emphasis has shifted from the state variable of population size to the demographic processes responsible for changes in this state variable: birth, death, immigration, and emigration. In evolutionary ecology, some of these same demographic processes, rates of birth and death, are also the determinants of fitness. In animal population ecology, the estimation of state variables and their associated vital rates is especially problematic because of the difficulties in sampling such populations and detecting individual animals. Indeed, early capture–recapture models were developed for the purpose of estimating population size, given the reality that all animals are not caught or detected at any sampling occasion. More recently, capture–recapture models for open populations were developed to draw inferences about survival in the face of these same sampling problems. The focus of this paper is on multi‐state mark–recapture models (MSMR), which first appeared in the 1970s but have undergone substantial development in the last 15 years. These models were developed to deal explicitly with biological variation, in that animals in different “states” (classes defined by location, physiology, behavior, reproductive status, etc.) may have different probabilities of survival and detection. Animal transitions between states are also stochastic and themselves of interest. These general models have proven to be extremely useful and provide a way of thinking about a remarkably wide range of important ecological processes. These methods are now at a stage of refinement and sophistication where they can readily be used by biologists to tackle a wide range of important issues in ecology. In this paper, we draw together information on the state of the art in multistate mark–recapture methods, explaining the models and illustrating their use. We provide a modeling philosophy and a series of general principles on how to conduct analyses. We cover key issues and features, and we anticipate the ways in which we expect the models to develop in the years ahead. In particular: – MSMR can now be used in a straightforward fashion by population biologists, thanks to the development of sound goodness‐of‐fit procedures, reliable parameter identifiability diagnostics, and robust user‐friendly computer software.Constrained models and model selection procedures can be used in the ANOVA‐like philosophy commonly used over the last 15 years for survival models, to answer a variety of biological questions. We develop as an example a treatment of meadow vole Microtus pennsylvanicus data. – As in survival models, random effects should be an integral part of this philosophy. Some simple approaches to random effects are illustrated. – States can be defined in a very general way, for example, by combining several criteria, such as sites and reproductive states, and can include nonobservable states. We develop as an example a multisite recruitment model of roseate terns Sterna dougallii . – MSMR models appear as a natural framework for combining different sources of information, viewed as different events that can be organized into mutually exclusive alternatives. – With the available developments, MSMR models are becoming a standard tool in population biology, as shown by a rapid growth of their use in the literature. In particular, given the ease with which a variety of constrained models can now be developed, MSMR models appear as less data hungry than was often feared. – MSMR models make it also possible to unify a large array of methodology, and, as such, are both a step towards further unification in a “mother of all” model, and a sound basis for further generalizations. – Future developments concern a variety of generalizations such as the reverse time approach and population size estimation. “Multievent” models, accounting for uncertainty in state determination, and integrated state–space models are key generalizations already in full development.

MIGRATING BIRDS STOP OVER LONGER THAN USUALLY THOUGHT: AN IMPROVED CAPTURE–RECAPTURE ANALYSIS

Migrating animals often divide their journey into alternating phases of migration bouts and stopping over. For investigating many questions of migration ecology it is crucial (1) to estimate the duration of stopover phases, and (2) to test whether animals of different groups differ in their stopover behavior. Using recent advances in capture–recapture statistics, we show how total stopover duration can be estimated from capture–recapture data. The probabilities of immigration are estimated and modeled by recruitment analysis and are converted into the time the animals spent at the stopover place before capture; the probabilities of emigration are estimated and modeled by survival analysis and are converted into the time the animals spent at the stopover place after capture. The sum of the two parts is the total stopover duration. Tests for differences between groups can be addressed by an appropriate model selection procedure. Two examples of migrating passerine birds at a stopover site in Switzerland illustrate this method. Mean total stopover duration was 12.3 d for Reed Warblers and 7.1 d for Reed Buntings. This was considerably higher than values obtained by the minimum stopover duration estimation (6.0 and 4.4 d, respectively). Because of the fundamental weaknesses of the minimum stopover duration estimation, which has been widely used in migration ecology, many findings obtained by this method need to be reconsidered.

The statistical analysis of survival in animal populations.

Estimating, comparing and modelling survival rates are central to population biology. However, there are many difficulties in measuring these rates in animal populations in the wild. The most relevant information is based on samples of marked individuals, i.e. capture-recapture data. In recent years, a number of new statistical approaches to the analysis of such data have been developed, permitting more sophisticated and precise measurement of survival rates.

Breeding Avifauna of a Mediterranean Succession: The Holm oak and Cork Oak Series in the Eastern Pyrenees, 1. Analysis and Modelling of the Structure Gradient

Within the Mediterranean level of the Alberes chain, 186 samples were taken in different formations from grassland to forest, with presence-absence sampling of the bird and plant species, and estimation of the relative cover of 7 vegetation layers, plus the rock itself. After multidimensional ordination (Correspondence Analysis) of the birds x stations matrix, and multiple correlation of the first factor F1 of C.A. with the 8 cover values, a model results which, (1) proves that F. is the expression of a progressive evolution of the vegetation structure, (2) shows that there is a logarithmic relationship between this structural evolution and bird species turnover, (3) permits a calculation, for any station of the studied region, of optimal indices of avifaunistic gradient and of structure gradient, (4) allows one to predict semi-quantitatively, from the vegetation structure, the probable avifauna of any station, and vice versa. This gradient analysis is a good alternative to the classical model (unidimensional sequence of discrete classes), especially when an ecological succession is reconstructed from the simultaneous comparison of different stations.

THE ESTIMATION OF AGE-SPECIFIC BREEDING PROBABILITIES FROM RECAPTURES OR RESIGHTINGS IN VERTEBRATE POPULATIONS. II: LONGITUDINAL MODELS

We provide a statistical framework to estimate age-specific breeding probabilities in vertebrate populations, from recaptures or resightings of individuals marked as young. We consider data collected at one or possibly several points over time, when individual recapture histories are known, leading to models which we call longitudinal models. An example of resightings of black-headed gulls, Larus ridibundus, is considered.

Identifying functional groups for response to disturbance in an abandoned pasture.

In an abandoned pasture in Brittany, we compared artificial small-scale disturbances to natural disturbances by wild boar and undisturbed vegetation. We developed a multivariate statistical approach which analyses how species biological attributes explain the response of community composition to disturbances. This technique, which reconciles the inductive and deductive approaches for functional classifications, identifies groups of species with similar responses to disturbance and characterizes their biological profiles. After 5 months of recolonization, artificial disturbances had a greater species richness than undisturbed vegetation as a result of recruitment of new species without the exclusion of pre-existing matrix species. Species morphology, described by canopy structure, canopy height and lateral spread, explained a large part (16 %) of community response to disturbance. Regeneration strategies, described by life history, seed mass, dispersal agent, dormancy and the existence of vegetative multiplication, explained a smaller part of community response to disturbance (8 %). Artificial disturbances were characterized by therophyte and compact rosettes with moderately dormant seeds, including a number of Asteraceae and other early successional species. Natural disturbances were colonized by leafy guerrilla species without seed dormancy. Few species were tightly related to undisturbed vegetation and were essentially grasses with a phalanx rosette morphology. The functional classification obtained is consistent with the classification of the community into fugitives, regenerators and persistors. These groups are structured according to Grubbs model for temperate grasslands, with regenerators and persistors in the matrix and fugitives taking advantage of gaps open by small-scale disturbances. The conjunction of functional diversity and species diversity within functional groups is the key to resilience to disturbance, an important ecosystem function.

Generation Time: A Reliable Metric to Measure Life-History Variation among Mammalian Populations

Oli and Dobson proposed that the ratio between the magnitude and the onset of reproduction (F/α ratio) allows one to predict the relative importance of vital rates on population growth rate in mammalian populations and provides a reliable measure of the ranking of mammalian species on the slow‐fast continuum of life‐history tactics. We show that the choice of the ratio F/α is arbitrary and is not grounded in demographic theory. We estimate the position on the slow‐fast continuum using the first axis of a principal components analysis of all life‐history variables studied by Oli and Dobson and show that most individual vital rates perform as well as the F/α ratio. Finally, we find, in agreement with previous studies, that the age of first reproduction is a reliable predictor of the ranking of mammalian populations along the slow‐fast continuum and that both body mass and phylogeny markedly influence the generation time of mammalian species. We conclude that arbitrary ratios such as F/α correlate with life‐history types in mammals simply because life‐history variables are highly correlated in response to allometric, phylogenetic, and environmental influences. We suggest that generation time is a reliable metric to measure life‐history variation among mammalian populations and should be preferred to any arbitrary combination between vital rates.

Functional groups for response to disturbance in Mediterranean old fields

Experimental disturbance and fertilization in two Mediterranean old fields, three and nine years old, respectively, were used to identify functional groups for response to disturbance. Five morphological traits (canopy structure, height, lateral spread, habit and plasticity) and five regeneration traits (life cycle, seed mass, fecundity, dispersal mode and germination seasonality) were used for species classification. Correlation patterns of species composition and species attributes with disturbance treatments were analysed in order to characterize groups of response to disturbance. The classification based on morphological traits was repeatable across fields and reflected natural correlation patterns among attributes. Erect rosettes with low morphological plasticity and moderate lateral spread, mostly grasses, were intolerant of disturbance. Partial rosettes with low morphological plasticity and wide lateral spread, e.g. Asteraceae species, colonized disturbed locations. Flat proto-rosettes with a plastic architecture were indifferent to disturbance in the young plot but required disturbance to establish in the older plot. The classification based on regeneration traits repeatedly identified germination period to be correlated with disturbance response. Species with early germination were intolerant of disturbance while late-germinating species colonized disturbances. These groups were clearly distinct from groups based on natural attribute correlation patterns which related primarily to seed mass, and secondarily to dispersal mode and fecundity. Effects of fertilization were detected only within disturbed quadrats of the old plot. Fertilization favoured the colonization of disturbances by species with a partial rosette and low plasticity and by species with late germination and high fecundity. These results, complemented by direct analyses on individual traits, are mostly consistent with previous descriptions of the ruderal strategy. This study shows that additional understanding can be gained from a deductive approach that examines more specific traits such as detailed descriptors of plant architecture and seasonal germination dynamics. Our results are intended to contribute to the general discussion on the identification and use of functional classifications. In particular, it is argued that communities such as Mediterranean old fields, where the dynamics is already well understood, should serve as testing grounds for new methods and the development of theories to be later applied to less well known vegetation types.