Data, Preconceived Notions and Methods: The Case of Population Sizes of Common Breeding Birds in Spain

Carrascal, Luis M.

doi:10.13157/arla.58.2.2011.371

Cited by 6 publications

(5 citation statements)

References 43 publications

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“…Aspectos relativos al método de censo Dos de los principales limitantes que pueden condicionar la calidad de las estimas y el valor de las comparaciones a través del tiempo son las asunciones de que todos los individuos son detectados durante los inventarios y que las estimas son precisas y muestran poco grado de incertidumbre o variabilidad en torno al valor medio (Buckland et al, 2001;Carrascal, 2011). Desde 1994 se llevan realizando sobre una serie de localidades concretas, con delimitación y extensión bien definidas, una serie de inventarios utilizando el método de las batidas mediante transectos separados entre si 200 m (Martín et al, 1997).…”

Section: Discussionunclassified

“…Los intervalos de confianza son una necesidad en los programas de censo en los cuales se pretenda obtener inferencias acerca de cambios en los tamaños de población (p. ej., Carrascal, 2011), y un programa de censo deficientemente replicado puede conducir a estimas imprecisas sin potencia para poder discernir aumentos o disminuciones tan grandes como un 50% (p. ej., Freilich et al, 2005).…”

Section: Discussionunclassified

“…Table 5. Results of the ANCOVA analyzing the changes in houbara density from 2004/2006-2011 a Lanzarote (p. ej., unas 125.000 y 36.400 cabras respectivamente en 2010; Gobierno de Canarias http://www.gobcan.es/agricultura/).…”

Section: Discussionunclassified

“…Entre los aspectos que más limitan el valor de los datos históricos están la no consideración del fenómeno de la detectabilidad (i.e., se asume que los conteos absolutos observan todos los individuos existentes), la ausencia de un programa de censo correctamente replicado, y no proporcionar medidas de precisión -intervalos de confianza de las estimas (p. ej., Norvell et al, 2003;Lurz et al, 2008;Carrascal, 2011;Archaux et al, 2012).…”

Section: Introductionunclassified

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Tendencias poblacionales recientes de la avutarda hubara en las Islas Canarias: análisis metodológico y estado de conservación

Schuster¹,

Iglesias-Lebrija²,

Carrascal³

2012

Anim. Biodiv. Conserv.

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Recent population trends of the houbara bustard in the Canary Islands. Methods and conservation status Determining conservation status requires rigorous and reliable data about population sizes and trends, especially if they have to be applied to islands where the species have small populations. The Canary bustard houbara (Chlamydotis undulata fuertaventurae) is catalogued as ‘in danger’ by the Red Book of the Birds of Spain. This work analyzes the value of previously published information on the species using the method of adjacent linear transects separated by 200 m, as a baseline for establishing robust population trends in 30 important areas (ranging from 1.3 to 12.8 km2) for the houbara in the islands of Lanzarote and Fuerteventura (Canary Islands). Censuses were repeated on the same dates (from November to December) and localities as those carried out in 1994, 2004 and 2006. The detection probability of the houbara was estimated by means of distance sampling, being 0.42 up to 250 m from the observer, and 0.82 in the main census belt of 100 m on either side of the line transect. The method of adjacent linear transects —counting the maximum number of hubaras detected— provides accurate figures of population densities (detection of 95.2% of the birds). The previous estimations of houbara densities can therefore be considered highly trustworthy, with a probable average underestimation of only 5 %. Nevertheless, the confidence intervals of density estimations using only one census were very large. Therefore, with only one census per sampling area and year it is not possible to obtain precise estimates of houbara densities with small variation around the average value. This raises concern when trying to obtain solid evidence about the increases–decreases of houbara populations comparing different dates or study areas. The density of the Canary Island hubara bustard decreased significantly from 2004/2006 to 2011 in eight areas of Fuerteventura (Vega Vieja, Los Alares–Pocetas, Matas Blancas, Lorenzo– Diviso, Corralejo, Lajares and Fimapaire) and in two areas of Lanzarote (Argana and La Santa). Conversely, the density of the houbara significantly increased in three areas of Lanzarote (Zonzamas, Guatiza and Llano de las Maretas). As a whole, average population density did not differ significantly between 2004/2006 and 2011 in Fuerteventura (slight decrease of 29% in 2011), but there was a significant increase in Lanzarote (increase of 60% from 2004/2006 to 2011). The change in density from 2004/2006 to 2011 tended to be an increase in areas further from paved roads, and a decrease in locations predominantly covered by loose sandy soils.

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Section: Discussionunclassified

Section: Introductionunclassified

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Tendencias poblacionales recientes de la avutarda hubara en las Islas Canarias: análisis metodológico y estado de conservación

Schuster¹,

Iglesias-Lebrija²,

Carrascal³

2012

Anim. Biodiv. Conserv.

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“…Large discrepancies between population estimates may promote debate on the most adequate planning strategies and targets of monitoring programmes, on the comparability of the different approaches and on the likely methodological influences behind these differences (Newson et al 2008, Brotons & Herrando 2011, Carrascal 2011, Murgui Pérez 2011. While intense constructive debate plays a key role in the advancement of science, it may also be intentionally discredited as lacking rigor by non-scientific but socially and politically influential advocacy groups, such as game managers and hunters, with economic interests in management policy (Woodroffe et al 2005, Dickman 2010), especially of those species that are more abundant or difficult to monitor but prone to hunting.…”

Section: Methods (Date)mentioning

confidence: 99%