Comparison of Pellet-Group Counting Methods to Estimate Population Density of White-Tailed Deer in a Mexican Tropical Dry Forest

Abstract

Abstract:Wildlife species population density estimation is important from both ecological and management perspectives. The pellet-group counting method has been used to evaluate density of the white-tailed deer (Odocoileus virginianus). This species is an important component of human diet in tropical habitats. The objectives of this study were to: 1) compare density estimates using four methods: counts in circular plots (FSC, Fecal Standing Crop and FAR, Fecal Accumulation Rate), and counts in transects (LT, Line Transect and ST, Strip Transect); 2) simulate the effect of increased sampling effort on density and precision, and; 3) evaluate the effort required to detect changes in population density using LT. From 2006 to 2007, we intensively sampled a 1 km 2 quadrant in a Mexican tropical dry forest. The results indicate that all four methods produce similar mean population density estimates. However, estimates of precision were dependent on sample size which in turn was associated with the particular counting method used. In descending order of estimate precision, the methods ranked as: LT, ST, FSC, and FAR. To detect population changes of < 20%, we suggest the establishment of 5 to 22 transects (LT) of 390 m during the dry season. To reduce bias in density estimation, it is important to obtain defecation and pellet decomposition rates in the study site.Key words: fecal counts; sampling methods; density; precision; Odocoileus virginianus. Resumen:Desde una perspectiva ecológica y de manejo es necesario estimar la densidad poblacional de la fauna. El método de conteo de grupos fecales es empleado frecuentemente para estimar la densidad de muchas especies incluido el venado cola blanca (Odocoileus virginianus). Este cérvido es importante en la cacería de subsistencia en regiones tropicales. Los objetivos del estudio fueron: 1) comparar las estimaciones de densidad empleando cuatro métodos: conteos en parcelas circulares (FSC, "Fecal Standing Crop" y FAR, "Fecal Accumulation Rate"), y conteos en transectos (LT, "Line Transect" y ST, "Strip Transect"); 2) simular el efecto de aumentar el esfuerzo de muestreo sobre la densidad y su precisión, y 3) evaluar el esfuerzo requerido para detectar cambios en la densidad empleando LT. De 2006 a 2007, monitoreamos intensivamente un cuadrante de 1 km 2 localizado en un bosque tropical seco del centro de México. Encontramos que los cuatro métodos proporcionan estimaciones poblacionales similares. Sin embargo, la precisión de las estimaciones dependió del tamaño de muestra, el cual a su vez fue determinado por el método de conteo utilizado. Así, LT produjo la estimación de densidad más precisa, seguido por ST, FSC y finalmente FAR. Para detectar cambios poblacionales < 20%, sugerimos poner 5-22 transectos (LT) de 390 m de longitud durante la época seca. Para reducir el sesgo de la estimación es importante obtener las tasas de defecación y de descomposición de los excrementos en el área de estudio.

PELLET: An Excel®-Based Procedure for Estimating Deer Population Density Using the Pellet-Group Counting Method

“…Distance between sampling transects is also considered, in order to avoid counting the same individuals [e.g., 1,3,51, 53]. There has even been the application of techniques such as the line transect method [11,27,46,54,55], in which the program DISTANCE [79] is used to derive estimates of the average density and variation of pellet-groups in the transects.…”

“…The parameter Dpg is estimated differently depending on the sampling method: sampling in circular plots or in strip or line transects (Table 1). PELLET calculates Dpg for the first two methods, but the DISTANCE method [see 11,27] is used for the latter. Table 1.…”

“…It is widely documented that this rate varies, depending mainly on the sex and age of the animals, habitat type, season, and plants consumed [56][57][58][59][60][61][62]. Rain and humidity affect the persistence of pellet-groups, which can also be a source of bias in field counts [27,[63][64][65][66][67]. Depending on the defecation rate used and the deposition time of the feces, different density estimates will be obtained at the same site [50,55].…”

“…It is an index that correlates positively with other population estimates [2,23,45,46]; however, it is not exempt from potential errors that can introduce bias into estimates [47][48][49][50]. As a consequence, different sampling and data analysis schemes have been tested in order to evaluate the efficiency, precision, and accuracy of the method [1,27,46,[51][52][53][54][55].…”

“…In Mexico, the method has been used for the mule deer [13,14] and the white-tailed deer, mainly in temperate [15][16][17][18][19][20][21][22] and tropical dry [23][24][25][26][27][28][29] forests. The method also serves to estimate the population density of various species of leporids [30][31][32][33][34], kangaroos [35][36][37][38][39], elephants [40] and marmots [41].…”