Short-Term Effects of Annual Flooding on a Population of Peromyscus Leucopus in a Rio Grande Riparian Forest of Central New Mexico
Ellis, Lisa M., Molles, Manuel C., Jr., Crawford, Clifford S., The American Midland Naturalist
LISA M. ELLIS,1 MANUEL C. MOLLES, JR. AND CLIFFORD S. CRAWFORD
ABSTRACT.-We studied the short-term effects of experimental flooding on a population of Peromyscus leucopus in a Rio Grande riparian forest that had not flooded for over 50 y. We monitored populations at two sites for 2 yr before flooding and then for 3 yr during which we flooded one site between mid-May and mid-June each year. Considerable interannual variation in density was detected at both sites, with no clear effect of flooding on density. Some mice stayed within the forest during flooding and used trees as refugia. Survivorship decreased slightly at the flood site after the first flood but did not differ from the reference site 2 mo after flooding that year, nor in the final year. There was no difference between sites in recruitment of new individuals.
Disturbance plays an integral role in establishing and maintaining southwestern riparian ecosystems, and flooding is the most common natural disturbance in these systems (Szaro, 1991). Although large river floodplains have been considered unstable or temporary habitats for small mammal populations due to the influence of such disturbances (Batzli,1977), Peromyscus spp. are common in floodplain forests throughout the United States, especially where flooding occurs (e.g., Stickel, 1948; Wetzel, 1958; Ruffer, 1961; Blem and Blem,1975). Several studies have reported no negative effects of flooding on Peromyscus populations (Stickel, 1948; Wetzel, 1958; Blem and Blem, 1975), and both the climbing (Horner, 1954; Kaufman et al., 1985) and swimming (Teeters, 1945; Ruffer, 1961; Sheppe, 1965) skills of this species are well-documented. Other studies, however, report decreased population densities after extensive flooding (Blair, 1939; McCarley, 1959; Turner, 1966).
Peromyscus leucopus is the most common rodent in cottonwood forests along the Middle Rio Grande in central New Mexico (Hink and Ohmart, 1984; Ellis et al., 1997). Historically the Rio Grande often flooded its riparian forest, but dam construction in the upper basin has prevented most flooding in recent decades (Crawford et al., 1996), and thus current populations of P leucopus have not experienced flooding for many generations. While P leucopus occurs throughout a variety of habitat types in New Mexico (Findley et al., 1975), in the riparian forest it provides an important food source for predaceous vertebrates and thus its persistence there is vital to energy flow in the riparian ecosystem.
The elimination of flooding, combined with other water management practices, has greatly altered the ecology of the Rio Grande floodplain, including considerable loss of riparian vegetation (Crawford et al., 1996). Therefore, in 1991 we began a long-term study to assess ecosystem- and population-level responses to reintroducing annual flooding to a riparian forest that had remained unflooded for over 50 yr. Our objectives in this paper are to determine short-term effects of reintroducing flooding on a population of P leucopus, selected as a key consumer group in the riparian ecosystem. Specifically, we compare (1) density, (2) survivorship, and (3) recruitment of P leucopus in flooded and nonflooded sites. Such population data will help determine the feasibility of using this as a management technique for riparian forest restoration.
Study sites.-Two 3.1-ha study sites were established in the summer of 1991 as part of a long-term ecosystem study at the Bosque del Apache National Wildlife Refuge, located ca. 5 km S of San Antonio, Socorro County, New Mexico. Sites were in mixed cottonwood forest and included a canopy dominated by medium-to-large Rio Grande cottonwood [Populus deltoides var. wislizenii (Wats.) Eckenwalder], with a subcanopy of Goodding willow (Salix gooddingii Ball.) and saltcedar (Tamarix ramosissima Ledeb.). The understory included seepwillow (Baccharis glutinosa Pers.), New Mexico olive (Forestiera neomexicana Gray), low densities of other shrubs, and a patchy herbaceous layer. …