A comparison of strip transects and fixed-radius point counts for a bird community in a semiarid chaco forest

Evaluamos el desempeño de las transectas de faja (transectas) y de los puntos de radio fijo (puntos) respecto de su precisión para estimar las abundancias y los atributos comunitarios (diversidad, riqueza y equitatividad) de las aves en un bosque semiárido santiagueño (Argentina). La evaluación se realizó en un bosque secundario y en un sector del mismo bosque al que se le aplicó un desarbustado manual. Se dispusieron 4 transectas y 4 puntos en cada hábitat. Los resultados indicaron: 1) menor riqueza estimada por los puntos que por las transectas (P < 0,05), aunque no se observaron diferencias en diversidad o equitatividad (P= 0,25 Y P= 0,56, respectivamente), 2) correlación positiva entre las abundancias estimadas por puntos y por transectas para las especies más constantes en cada hábitat (P < O,OI), y 3) congruencia entre los métodos respecto de la precisión con que estimaron las abundancias de las especies, medida a través de los coeficientes de variación porcentual (CV%). Consideramos que, si bien ambos métodos se desempeñaron en forma similar, para estos tipos particulares de hábitat con un estrato arbustivo denso, la técnica de puntos reúne una serie de ventajas sobre la de transecta, ya que son más fáciles de ubicar en el terreno y carecen de la dificultad operativa de las transectas de transitar a paso constante por un terreno accidentado. La menor riqueza detectada por los puntos respecto de las transectas puede solucionarse aumentando el número de réplicas o incluyendo en los conteos a las especies detectadas por fuera del radio fijo.We assessed the performance of strip transects and fixed-radius point counts with respect to their accuracy to estimate bird abundances and commuunity attributes (diversity, richness, eveness) in the semiarid forests of Santiago del Estero province, Argentina. The study was carried out in a secondary forest and in a sector of the same forest where shrubs were removed. We instaled 4 transects and 4 points in each habitat. Our results showed that: 1) estimations of species richness by the point method were lower than by the transect method (P < 0.05), even though there were not significant differences in either diversity or evenness (P=0.25 and P=0.56, respectively); 2) there was a positive correlation between bird abundances estimated by both methods in each habitat (P < O.OI), and 3) there were not significant differences between methods in their accuracy to estimate bird abundances, recorded by means of the coefficient of variation (CV%). We conclude that, even though both methods showed similar performances, the point method has in our case some advantages over the transects, since they are less difficult to allocate in the study area and do not demand to keep a constant pace in a rugged and scrubby terrain, as the transect method does. The lower species richness detected by the point method could be sol ved by increasing replicates, as well as by counting birds which are out of the fixed radius

Habitat use by armadillos in agroecosystems of central Argentina: does plot identity matter?

In agroecosystems, the use of cultivated plots by mammals depends on habitat structure as well as on the degree of species specialization. In the Pampas of Argentina, during the last 2-3 decades, there has been a continuing expansion of cropland. The aim of this study was to analyze how the identity and characteristics of agricultural plots may have affected the activity of 2 species of armadillos (Chaetophractus villosus and Dasypus hybridus) in a rural landscape. We carried out 4 sampling sessions between December 2011 and June 2013, surveying 175 plots during spring-summer and 194 during fall at 25 different sites in the Pampas of Buenos Aires province, Argentina. In each plot, we surveyed for signs (burrows and holes) along a 600×6 m transect in order to assess armadillo activity and measured 4 habitat structural variables. As expected, there were structural differences between types of agricultural plots. Across all plots, a total of 6,654 armadillo signs were found (C. villosus = 5,009, D. hybridus = 1,645). Generalized linear mixed models revealed that during both seasons, the type of plot explained much of the variation in armadillo activity, except during spring-summer for D. hybridus. C. villosus had higher use of plots with a longer history of non-tillage (e.g., soybean), while there was more evidence of D. hybridus activity in plots with less human intervention (e.g., grasslands), especially during fall. Overall, D. hybridus and C. villosus were more sensitive to the particular structural features of plots rather than to the kind of land use practiced (crop field or rangeland). These results have important implications for future management decisions in the Pampas region because during the last decades, farming of soybeans has expanded at the expense of grasslands, and this change appears to have differentially affected these 2 species of armadillos.Centro de Estudios Parasitológicos y de VectoresFacultad de Ciencias Naturales y Muse

Patrones de abundancia relativa en ensambles de pequeños roedores de la región pampeana

Se estudió la composición específica de 8 ensambles de pequeños roedores de la región pampeana en dos tipos de habitat: bordes de cultivo y pastizales. Akodon azarae fue la especie numéricamente dominante en 7 casos. Los ensambles muestreados en los bordes de cultivo contaron con mayores índices de diversidad y de equitatividad que los de pastizales. Los bordes presentaron una mayor representación de Mus y Calomys, en detrimento de Akodon y de Oxymycterus. Se ensayaron pruebas de bondad de ajuste entre los patrones de abundancia relativa de los ensambles y los modelos de “barra partida” (BP), expresión biológica de una distribución idealmente uniforme, y de la “serie geométrica”, que caracteriza a ensambles con estructuras jerarquizadas. Los ajustes fueron rechazados en todos los casos. En 7 de los 8 casos los ensambles presentaron una distribución más jerarquizada que la predicha por el modelo BP. Se proponen diversas interpretaciones de las causas de los desajustes observados: 1) una combinación de los factores de competencia y de variabilidad temporal en el tamaño de los nichos de las especies, 2) el número de recursos por los que las especies compiten, 3) el papel de la interferencia de las especies socialmente dominantes sobre la apropiación de los recursos, y 4) el reclutamiento en los bordes de cultivo de especies procedentes de los campos de cultivo (Calomys, Mus). La representación actual de Calomys en los agroecosistemas superaría a la que habrían tenido originalmente en los pastizales de la llanura pampeana.We studied the composition of 8 small-rodent communities in the Pampean region of Central Argentina in two different habitats: crop field borders and grasslands. Akodon azarae was the most abundant species in 7 communities. Species diversity and evenness were higher infield borders than in grasslands. Borders had higher proportion of Mus and Calomys and lower proportion of Akodon and Oxymycterus than grasslands. We fit the data to species-abundance patterns expected according to the “broken stick” model (BS), which is the biological expression of an ideally uniform distribution, and to the “geometric series “ model, which is related to communities with hierarchical structures. The goodness-of-fit tests were rejected in all cases. Moreover, in 7 of 8 cases, BS was rejected because the communities had a stronger hierarchical structure than predicted by the model. Wepropose several interpretations for this: 1) a combination of both competition and temporal variations of niche breadths in determining the species- abundance curves, 2) the number of resources for which the species are competing, 3) the role that interference competition may play in confering the possesion of resources to socially dominant species, and 4) the recruitment of species (Calomys, Mus) from crop areas to field borders. The present proportion of Calomys spp. in agroecosysterns probably exceeds its original proportion in Pampean grasslands

Habitat use by armadillos in agroecosystems of central Argentina: does plot identity matter?

In agroecosystems, the use of cultivated plots by mammals depends on habitat structure as well as on the degree of species specialization. In the Pampas of Argentina, during the last 2-3 decades, there has been a continuing expansion of cropland. The aim of this study was to analyze how the identity and characteristics of agricultural plots may have affected the activity of 2 species of armadillos (Chaetophractus villosus and Dasypus hybridus) in a rural landscape. We carried out 4 sampling sessions between December 2011 and June 2013, surveying 175 plots during spring-summer and 194 during fall at 25 different sites in the Pampas of Buenos Aires province, Argentina. In each plot, we surveyed for signs (burrows and holes) along a 600×6 m transect in order to assess armadillo activity and measured 4 habitat structural variables. As expected, there were structural differences between types of agricultural plots. Across all plots, a total of 6,654 armadillo signs were found (C. villosus = 5,009, D. hybridus = 1,645). Generalized linear mixed models revealed that during both seasons, the type of plot explained much of the variation in armadillo activity, except during spring-summer for D. hybridus. C. villosus had higher use of plots with a longer history of non-tillage (e.g., soybean), while there was more evidence of D. hybridus activity in plots with less human intervention (e.g., grasslands), especially during fall. Overall, D. hybridus and C. villosus were more sensitive to the particular structural features of plots rather than to the kind of land use practiced (crop field or rangeland). These results have important implications for future management decisions in the Pampas region because during the last decades, farming of soybeans has expanded at the expense of grasslands, and this change appears to have differentially affected these 2 species of armadillos.Centro de Estudios Parasitológicos y de VectoresFacultad de Ciencias Naturales y Muse

Nuevos aportes a la historia natural de la mulita pampeana Dasypus hybridus (Mammalia, Dasypodidae)

En este trabajo se presentan nuevos aportes sobre la historia natural de la mulita pampeana Dasypus hybridus (Desmarest, 1804) (Mammalia, Xenarthra, Dasypodidae). Los estudios de campo fueron llevados a cabo en 100 ha de cuatro establecimientos agropecuarios de la provincia de Buenos Aires, Argentina. Durante tres años se realizó un muestreo estacional de armadillos por captura y liberación. Se obtuvieron datos de hábitos alimentarios, uso del espacio y del tiempo, comportamiento, termorregulación, datos poblacionales y morfológicos. Se realizaron 71 capturas. En la dieta el ítem principal registrado fue material vegetal, seguido por hormigas e insectos coleópteros; no se observó una diferencia estacional en los hábitos alimentarios. La actividad de las mulitas se concentra durante el día, existió una baja en la frecuencia de observación durante las estaciones frías (otoño e invierno). La mulita pampeana prefiere suelos húmicos, terrenos altos y pastizales densos y altos; asimismo seleccionan los montes para refugiarse. Son individuos asociales. La temperatura rectal mostró correlaciones positivas con la temperatura ambiente. La proporción de sexos fue cercana a uno y no se observó dimorfismo sexual. Los resultados obtenidos concuerdan parcialmente con lo observado para otras especies del género, destacando las tendencias observadas en los hábitos alimentarios y en la estrategia termorregulatoria. Este trabajo representa un aporte en varios aspectos de una especie poco estudiada en una zona bajo importantes presiones de uso y modificación de hábitat

Variación estacional de un ensamble de aves en un bosque subtropical semiárido del Chaco Argentino

Fil: Codesido, Mariano. División Ornitología; Facultad de Ciencias Naturales y Museo; Universidad Nacional de La PlataFil: Bilenca, David N.. Departamento de Ecología, Genética y Evolución; Facultad de Ciencias Exactas y Naturales; Universidad de Buenos Aire

Influencia De La Actividad De Cavia Aperea Sobre La Estructura Del Habitat Y La Distribucion De Akodon Azarae Y Oryzomys Flavescens Pampeana (Argentina)

Volume: 79Start Page: 67End Page: 7