L’analyse des peuplements d’oiseaux, éléments d’un diagnostic écologique I. la méthode des échantillonnages fréquentiels progressifs (E.F.P.)

This paper describes a new sampling method for bird populations making use of the frequency of occurrence of each species per unit-time of 20 min (Echantillonnage Fréquentiel Progressif, E. F. P.). This technique is derived from the I.P.A. method (Blondel et al., 1970) but is not intended to replace the latter ; the two methods are designed to solve different theoretical and practical problems in ecology. 1. In an introductory chapter, it is argued that the « bird- model » allows one to put forward an ecological diagnosis of ecosystems and that bird communities can be used to test the level of complexity of ecosystems. Furthermore, from a practical point of view, we may make use of bird communities for environmental planning and monitoring. The frequency-sampling method is interesting because it permits one to gather rapidly quantitative and reliable data. 2. The material consists of 279 I.P.A. censuses or 558 frequency units, since 1 I.P.A. = 2 E.F.P., collected in 23 study areas of French Mediterranean Biome. These data were used to deter mine the composition and structure of avian communities by means of densities (I.P.A.) and frequencies (E.F.P.). Comparisons of the two sets of data allows one to determine for each species the relationship between its density and its frequency. The composition of the community involves two parameters : A) richness, that is to say the number of species. A distinction is made between total richness S or absolute number of species noted at least once in a set of N censuses, and mean richness s or mean number of species per count ; B) abundance of individuals in the communities. This parameter can be expressed either as an index of specific abundance (I.P.A. 8) which can be converted to density dt if one knows the coefficient by which to multiply I.P.A. in order to obtain (dt = I.P.A.t X c,), or by mean of fre quencies FC. The specific frequency is shown to be correlated to the logarithm of the density of the species (r between logdt and FCi = 0,84 ; P < 0,001) ; the lower the frequency of a given species, the better the correlation. Thus, frequencies of occurrence can be used as an objective measure of the number of individuals included in the community. The structure of a community accounts for its organization, that is the distribution of individuals among the different species. We recognize three ways of expressing community structure : A) the law of distribution ; the species-abundance and the species- frequency distributions are shown to fit the Galton log-normal model at a high level of statistical significance (P < 0,001). If bird communities are actually organized according to this model, censusing methods give a reliable sampling of communities. Furthermore, this adjustement allows us to check the level of homogeneity of a community ; B) the diversity of a community ; we used here Shannon’s measure of diversity : H’ = — 2 log2 p». i = 1 In all cases frequencies-diversity H’F is higher than abundance- diversity H\, which means a better equitability of individuals in the frequential method of censusing. But the two indices are highly correlated with r = 0,99. Of the two components of diver sity — richness and equitability or evenness — richness is the most important in the determination of diversity as is shown by the high correlation beween S and H’ (r = 0,93) ; C) equitability J’ ; this index shows how far the maximal potential diversity H’max is actually realized by a community of S species. Thus J’ = H’/H’max = log2 S. In most samples, J’ is very high, which suggests that breeding bird communities are relatively constant with respect to the evenness component of diversity except in uncertain environments. 3. In the third part of the paper, the practical problems of bird censuses with the frequential method are discussed. The sampling of plots must be achieved by probabilistic methods in order to have statistical interpretations of the results. Several examples are given. Advantages and disavantages of various ways of sampling are discussed, chiefly in respect with stratified sampling which results in a classification of patches of environ ment according to the ecological criteria, the importance of which one wishes to measure in bird communities. As far as bird censuses are concerned, some practical indi cations are given, concerning especially the differences between the frequential and the abundance methods (E.F.P. versus I.P.A.). The E.F.P. method is highly standardized, which is very useful for a rigorous statistical interpretation of data. This standardi zation together with the simplicity of the actual recording of data («yes» or « no » for presence or absence of each species per 20 min.) allows for an extension of the diagnosis of communities in space and time. This means that we have the possibility to undertake a semi-quantitative areography of birds at the species and at the community levels. 4. In the last chapter three concrete examples are given ; they emphasize the many ways to work out original data. A) the list of 84 breeding bird species from Mont Ventoux was obtained from 310 censuses, the sampling method is explained. Some distributional maps of birds were drawn on a geographical grid. Relative frequencies of the species were indicated in each square of the grid (5 classes of frequencies, cf. Fig. 7) ; B) ecological profiles and niche breadth were calculated for each species. Niche breadth was worked out for altitude (8 clas ses of 200 m each). The formula used is eH’x (see text). Selected examples from the case of Mont Ventoux (Fig. 8) show that species with narrow niche breadth occur at the foothill of the mountain (Sylvia melanocephala) or at its top (Anthus spinoletta) and that other species with large niche breadth (Fringilla coelebs or Sylvia atricapilla ) inhabit almost all altitude classes (excellent equita bility in each of them). Niche breadths of the six species of the genus Sylvia are shown and discussed. C) the structure of bird communities was analyzed according to the structure of vegetation. A stratified sampling led to the diagnosis of communities in ten natural and reforested areas of Mont Ventoux. For each community the following parameters are given (Fig. 9) : total richness S, mean richness s, theoretical- information diversity H’, equitability and the level of fit to Gallon’s log-normal model. The influence of reforestation on bird communities is discussed. In many instances, conifer plan tations, especially Cedar, seem successful in the rebuilding of communities. In conclusion, the standardized and time saving method discussed in this paper seems very well adapted to solve problems of theoretical and applied ecology at the community level, and can be used fruitfully for environmental monitoring

Compte rendu ornithologique pour les années 1964 et 1965

Blondel Jacques. Compte rendu ornithologique pour les années 1964 et 1965 . In: La Terre et La Vie, Revue d'Histoire naturelle, tome 20, n°3, 1966. pp. 237-254

Le problème du contrôle des effectifs du Goéland argenté {Larus argentatus michahellis Naumann) en Camargue

Blondel Jacques. Le problème du contrôle des effectifs du goéland argenté (Larus argentatus michahellis Naumann) en Camargue . In: La Terre et La Vie, Revue d'Histoire naturelle, tome 17, n°3, 1963. pp. 301-315

Etude des populations d’oiseaux dans une garrigue méditerranéenne : description du milieu, de la méthode de travail et expose des premiers résultats obtenus a la période de reproduction

Le présent travail est l’exposé des premiers résultats de l’étude de la communauté avienne d’une garrigue méditerranéenne. Le milieu d’étude, longuement décrit, peut être considéré comme carac téristique de ce qui reste aujourd’hui des milieux méditerranéens après les multiples dégradations dont ils ont été l’objet. La méthode de travail est basée sur le recensement des mâles chanteurs sur une parcelle de 28 Ha, quadrillée par un réseau de sentes se coupant à angle droit et distantes de 50 à 70 m. les unes des autres. La densité de peuplement obtenue par cette méthode est de 21 couples pour 10 Ha (3,6 couples de Rossignol Luscinia megarhynchos, 5 couples de Fauvette mélanocéphale Sylvia melanocephala, 12 couples de Fau vette passerinette Sylvia cantillans et 0,4 couple de Fauvette pitchou Sylvia undata) . On a cherché à évaluer le « rendement » des dénombrements, c’est-à-dire la chance que l’observateur a de noter chaque couple chaque fois qu’il passe à proximité de son territoire. Ce rendement est de 54,2 % pour l’ensemble de la population mais il oscille entre 50,3 % pour la Fauvette mélanocéphale et 62 % pour le Rossignol. La valeur du rendement indique le nombre minimum de dénombre ments qu’il faut faire pour être sûr d’avoir détecté au moins 95 % de la population. Un deuxième test qu’on a fait subir aux dénombrements est le test de « validité » qui consiste à chercher dans quelle mesure chaque couple est bien distinct de ses voisins. Ce test, surtout utile quand la densité de l’espèce est grande, permet de mettre en doute certains cantons non nettement différenciés de leurs voisins. Enfin, sont données quelques indications sur les caractéristiques de la population étudiée : sa pauvreté tant en espèces qu’en individus qu’on a comparée avec des résultats obtenus dans d’autres milieux tempérés de la région paléarctique et la spécialisation poussée du genre Sylvia qui, presque seul, a pu coloniser ce milieu aux condi tions estivales sévères.The present paper is an account of the results so far obtained on a community study of birds in a mediterranean scrub. The area studied may be considered as characteristic of what remains today of a Mediterranean habitat subjected to multiple degradations in the past. The census method is based on counts of singing males in a area of 28 hectares divided up into a network of squares by foot paths 50 to 70 meters apart. The population density obtained by this method is 21 pairs to 10 Ha (3,6 pairs of Nightingales Luscinia megarhynchos, 5 pairs of Sardinian warblers Sylvia melanocephala, 12 pairs of Subalpine warbler Sylvia cantillans and 0,4 pair of Dartford warbler Sylvia undata) . I have tried to estimate the « Effectivity » of the census, that is to say the chance that the observer has of noting each pair each time that he passes the proximity of their territory. This effectivity is 54,2 % for the whole population, but varies between 50,3 % for the Sardinian warbler and 62 % for the Nightingale. The value of the effectivity indicates the minimum number of counts that it is necessary to make to be sure of having detected at least 95 % of the population. A second test carried out for the census is one of « validity ». This consists of finding out to what extent each pair is distinct from its neighbours. It is particularly useful when there is a high den sity of a species, enabling the observer to doubt certain territories not clearly defined from their meighbours. Finally, some indications are given on the characteristics of the population studied : the scarcity of species, as of individuals, compared with results obtained elsewhere in other habitats in the temperate Palearctic Region and the specialisation of the Genus Sylvia which, almost alone, has been able to colonise this region having severe summer conditions

La niche écologique, mythe ou réalité ?

The niche concept in ecology can be approached either empi rically or theoretically. The emphasis is laid in this paper on the empirical approach, and the authors make use of their per- sonnal field experience to illustrate the points discussed. The usefullness of the Hutchinsonian concept of a multi- dimensonial niche is first emphasized, together with that of the distinction between the fundamental and the realized niche. Examples are given which will help to define and quantify a niche dimension in field conditions. Taking the spectrum of seed size eaten by four sympatric species of finches as an example, a number of concepts are discussed, such as the ecological profile of a species, the barycenter of its niche, niche breadth and niche overlap. Niche packing and the number of niches in a given ecological universe are then discussed, together with the concepts of alpha-beta- and gamma-diversity. Niche regulation is also discussed at some length. The notion of a “ vacant niche ” is shown to be a non-problem, and this is illustrated by some examples. It is pointed out that the mecha nisms of niche regulation cannot be understood without reference to the concept of adaptive strategy, whether it be at the individual or demographic level. It is hard to understand how a species has developped its ecological niche without considering the na ture of the selection pressures operating in more or less changing environments. To sum up, it is suggested that although it will always remain difficult (if not impossible) to identify and quantify all the n-di- mensions of the niche of a given species, the concept remains most useful to stimulate working hypotheses, some of which at least can be tested in field conditions

Marine observations with a harmonic single-beam echo-sounder

International audienceTo characterise the seabed or water-column targets with acoustics, it is common to use multiple frequencies and therefore several sonar transducers or echo-sounders. The single beam echo-sounder we present here is able, thanks to non-linearity of the sea water, to generate more than three harmonics above its fundamental transmitted frequency, in effect producing four distinct frequencies with a single echo-sounder. In addition, all transmitted signals are perfectly in phase because they are carried by the same pulse, which has obvious benefits for further processing of the echoes. In this presentation, after a short review of the entire system, its application to seabed characterisation using the reflectivity level (acoustic backscattering strength from the seafloor) will be exposed. Further developments of plans to use this echo-sounder for fishery acoustics will then be highlighted, based on datasets acquired in the Bay of Brest (France). (Project funded by ANR and DGA / ANR-14-ASTR-0022-00)

Comparison of methods employed to extract information contained in seafloor backscatter

International audienceSeabed maps are based on quantities extracted from measurements of the seafloor‘s acoustic response by sonar systems such as single-beam echo-sounders (SBES), multibeam echo-sounders (MBES) or sidescan sonars (SSS). In this paper, a comparison of various strategies to estimate the backscattering strength (BS) from recorded time-series, i.e. seabed echoes extracted from pings, is presented. The work hypotheses are based on processed data from a SBES designed to be tilted mechanically. Ideal survey conditions are taken into account and the seafloor is supposed to be rough so that BS is assumed to be equivalent to the Rayleigh probability density function parameter. Classical methods such as averaging corrected (sonar equation) backscattered single values over a set of pings to estimate BS are compared to other methods exploiting several time-samples being part of pings. Simulated data is considered to estimate BS in different situations (several estimators, natural/squared values, number of samples and pings). The best estimator to reach a 0.1dB uncertainty is proposed, and a formula governing the number of time-samples and pings needed to reach an accurate BS estimation according to the measurement conditions is derived