Bottom-up control of consumers leads to top-down indirect facilitation of invasive annual herbs in semiarid Chile

Abstract

Abstract. The abundance of exotic plants is thought to be limited by competition with resident species (including plants and generalist herbivores). In contrast, observations in semiarid Chile suggest that a native generalist rodent, the degu (Octodon degus), may be facilitating the expansion of exotic annual plants. We tested this hypothesis with a 20-year data set from a World Biosphere Reserve in mediterranean Chile. In this semiarid environment, rainfall varies annually and dramatically influences cover by both native and exotic annual plants; degu population density affects the composition and cover of exotic and native annual plants. In low-rainfall years, cover of both native and exotic herbs is extremely low. Higher levels of precipitation result in proportional increases in cover of all annual plants (exotic and native species), leading in turn to increases in degu population densities, at which point they impact native herbs in proportion to their greater cover, indirectly favoring the expansion of exotic plants. We propose that bottom-up control of consumers at our site results in top-down indirect facilitation of invasive annual herbs, and that this pattern may be general to other semiarid ecosystems.

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“…Degus are a main prey species in central Chile (Jaksic 1986). Through ecosystem engineering in the form of runways, these rodents also interact with plants, invertebrates, and birds (Madrigal et al 2011;Root-Bernstein 2012). In addition, degu burrows have been observed to provide shelter or nesting sites to a range of commensal species (Table 2).…”

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“…ENSO-related rainfall in western South America leads to a complex set of biotic responses involving bottom-up and top-down processes [89]. Bottom-up regulation involves vegetation and seed blooms, followed by small rodent increases, which lead to predator increases [89,[92][93][94]. Following an ENSO event between 2000 and 2002, the density of the herbivorous rodent, Octodon degus, increased dramatically [24,94].…”

“…Bottom-up regulation involves vegetation and seed blooms, followed by small rodent increases, which lead to predator increases [89,[92][93][94]. Following an ENSO event between 2000 and 2002, the density of the herbivorous rodent, Octodon degus, increased dramatically [24,94]. In turn, high degu densities have direct top-down effect on native herbs and indirect effects on invasive annual herbs [89,94].…”

“…Following an ENSO event between 2000 and 2002, the density of the herbivorous rodent, Octodon degus, increased dramatically [24,94]. In turn, high degu densities have direct top-down effect on native herbs and indirect effects on invasive annual herbs [89,94]. The Meserve et al study is unique in that the investigators have monitored the response of small mammal communities to experimental manipulations of competitor density (Octodon degus) and predation risk for >25 years (1989-present).…”

“…First, degu herbivory and behavior has complex effects on plant diversity including negative impacts on perennial plant diversity through degu herbivory [24]. In contrast, degu runway development and digging is thought to have a positive effect on the density of annual plant seed densities [94,101]. In this way, the abundance of degus has indirect effects on other small mammals by increasing or decreasing the availability of food and cover (critical for shade, safety).…”