Responses of Desert Bighorn Sheep to Removal of Water Sources

Abstract

ABSTRACT In arid regions of the southwestern United States, water is often considered a primary factor limiting distribution and productivity of desert ungulates, including desert bighorn sheep ( Ovis canadensis mexicana ). Thus, wildlife management agencies and sportsmen's organizations have invested substantial time and resources in the construction and maintenance of water catchments. Although the availability of freestanding water sources is believed to influence many aspects of the ecology of desert bighorn sheep, the efficacy of these water sources has been questioned and has not been examined experimentally. We used a before‐after‐control‐impact study design to determine if removal of water catchments changed diet, characteristics of foraging areas used by female desert bighorn sheep, home‐range size, movement rates, distance to catchments, adult mortality, productivity, or juvenile recruitment in 2 mountain ranges on the Cabeza Prieta National Wildlife Refuge, Arizona, USA. During pretreatment (2002–2003), we ensured that water catchments were available to desert bighorn sheep in both mountain ranges; during posttreatment (2004–2005), we drained all water catchments in the treatment range. We measured diet composition, characteristics of foraging areas, 50% and 95% kernel home ranges, movement rates, and distance to water catchments seasonally from 2002 to 2005. We also estimated adult survival, lamb:female, and yearling:female ratios from 2002 to 2005. We predicted that removal of water catchments would result in 1) increased use of foraging areas with more vegetation cover, more thermal cover, and higher succulent abundance; 2) increased consumption of cacti and other succulents; 3) an increase in home‐range size, movement rates, and distance to nearest catchment; and 4) a decrease in adult survival, productivity, and juvenile recruitment. Removal of water catchments in the treatment range did not result in predicted changes in diet, foraging area selection, home‐range size, movement rates, mortality, productivity, or recruitment. Female desert bighorn sheep did use areas with more thermal cover during the summer after removal of water catchments, but other characteristics of foraging areas used by bighorn sheep and their diet did not change appreciably with removal of water catchments. We did not document changes in home‐range area, movement rates, or distance sheep were from water during hotter months; we only documented changes in home‐range area, movement rates, and distance to water catchments during winter and autumn. There were 10 desert bighorn sheep mortalities in the treatment range and 8 in the control range; 7 mortalities in each mountain range were during pretreatment. Twelve of the 18 total mortalities occurred during summer. Survival rate was lower during pretreatment than posttreatment in both mountain ranges. We did not document increased mortality or a change in lamb:female or yearling:female ratios after removal of water catchments. Home‐range area and movement rates declined with increasing precipitation. Annual survival rates increased with increases in the current year's total precipitation and total precipitation during the previous year; annual survival rates declined with increases in average daily temperature during winter. There was a severe drought during pretreatment and abnormally wet conditions during posttreatment. The increase in precipitation that coincided with removal of water sources improved forage conditions during posttreatment and may have provided adequate water for female desert bighorn sheep. The lack of change in home‐range size, movement rates, and distance to the nearest water catchment during hot, dry seasons after removal of water sources suggests that forage conditions played a greater role in determining home‐range area and movement rates than did the presence of water catchments. Higher mortality rates during the drought of the pretreatment period indicates that during droughts as severe as that of 2002, presence of water catchments was not sufficient to prevent mortalities of desert bighorn sheep and a lack of forage quality and quantity was likely the primary limiting factor of the population during this time. Improving forage conditions during posttreatment, increases in forage moisture content, and availability of naturally occurring sources of free water in the treatment range likely minimized any impact of removing water catchments on survival rates and lamb:female and yearling:female ratios. However, due to the climatic conditions during the study we were unable to assess how the treatment population would have responded to a lack of water sources during a drought period. The influence of anthropogenic water catchments, if any, on desert bighorn sheep populations may be strongest during years with weather conditions that are neither drought nor wet periods. Given the high interannual variability in precipitation in many areas of the arid Southwest, it is important for wildlife managers to determine if and when water is the primary limiting factor for populations of desert bighorn sheep in order to make the most efficient use of agency time and resources. RESEMEN En regiones áridas del suroeste de Estados Unidos, agua es a menudo considerada el primer factor limitante en la distribución y productividad de ungulados, incluyendo el borrego cimarrón del desierto ( Ovis canadensis mexicana ). Por lo tanto, las agencias de manejo de vida silvestre y organizaciones de deportes han invertido mucho tiempo y recursos en la construcción y mantenimiento de sistemas de captura de agua. A pesar que la disposición de fuentes de agua libre se cree que influencia muchos aspectos de la ecología del borrego cimarrón del desierto, la eficacia de estas fuentes de agua ha sido cuestionada y no ha sido evaluada experimentalmente. Nosotros usamos un estudio denominado control‐de‐impacto‐antes‐y‐después diseñado para determinar si la remoción de sistemas de captura de agua generan un cambio en el régimen alimenticio, características de las áreas usadas para el forraje por hembras, y el rango de movimiento local, tasas de movimiento, distancia a los sistemas de captura de agua, mortalidad adulta, productividad, y reclutamiento juvenil en dos rangos de montaña en el Refugio de Vida Silvestre Nacional Cabeza Prieta, Arizona. Durante el periodo de pre‐tratamiento (2002–2003) nos aseguramos que los sistemas de captura de agua estuvieran disponibles a todos los borregos cimarrón en ambos rangos montañosos; durante el periodo de postratamiento nosotros vaciamos todos los sistemas de captura de agua en el rango de tratamiento. Nosotros medimos la composición dietética, características forrajeras en el área, 50% y 95% kernel de rango local, tasas de movimiento, y las distancias a los sistemas de agua de captura temporalmente desde el 2002 al 2005. También estimamos la sobrevivencia de los adultos, y la relación de las hembras con una año cumplido del 2002 al 2005. Nosotros predijimos que la remoción de sistemas de captura de agua resultaría en lo siguiente: 1) incrementa el uso de aras de forraje con mas cobertura vegetal, mas protección a las temperaturas, y mayor abundancia de suculentas; 2) incremento en consumo de cactáceas y otras suculentas; 3) un incremento en el rango de movimiento local, tasas de movimiento, distancia al sistemas de captura de agua más cercano; 4) un decrecimiento en la tasa de sobrevivencia adulta, productividad y reclutamiento juvenil. La remoción de los sistemas de captura de agua en el rango de tratamiento no resulto en la predicción de cambios de composición dietética, selección de áreas de forraje, tamaño del rango local, tasas de movimiento, mortalidad, productividad o reclutamiento. La hembras de borregos cimarrón usaron áreas de mayor cobertura sombreada durante los meses del verano después de remover los sistemas de captura de agua, pero otras características de áreas de forraje usadas por los borregos cimarrones y su dieta no cambio apreciablemente con la remoción de estos sistemas. No documentamos cambios en el incremento del tamaño del rango de movimiento local, tasas de movimiento, o distancia convencional donde se encuentra agua en meses de verano; cambios en el rango local, tasas de movimiento, y distancias a los sistemas de captura de agua fueron documentados solo durante las temporadas de otoño e invierno. Se encontraron 10 mortalidades de borrego cimarrón en el rango de tratamiento y 8 en el rango control; 7 mortalidades en cada rango montañoso donde durante el periodo de pretratamiento. Doce de dieciocho mortalidades ocurrieron durante el verano. Las tasas de movimiento fueron más bajas durante el periodo de pre‐tratamiento que durante el pos‐tratamiento en ambos rango de montaña. No documentamos incrementos en mortalidad o cambio en los rango de relación borrego: hembra o hembras que han cumplido un ano después de haber removido los sistemas de captura de agua. El rango del area local y tasas de movimiento declinaron con la precipitación. Las tasas de sobrevivencia anual se incrementaron con el incremento en la precipitación total al ano y la precipitación total del año previo; las tasa de sobrevivencia declinaron con el incremento de la temperatura promedio diaria durante el invierno. Se presentaron varias sequias durante el periodo de pretratamiento y las condiciones anormalmente húmedas durante el periodo de post‐tratamiento pudo proveer agua adecuada para las hembras del borrego cimarrón. Después de