Testing the Effects of an Introduced Palm on a Riparian Invertebrate Community in Southern California

Abstract

Despite the iconic association of palms with semi-arid regions, most are introduced and can invade natural areas. Along the San Diego River (San Diego, California, USA), the introduced Canary Island date palm (Phoenix canariensis) forms dense patches among native riparian shrubs like arroyo willow (Salix lasiolepis). The structural differences between the palm and native shrubs are visually obvious, but little is known about palm’s effects on the ecosystem. We tested for the effects of the palm on a riparian invertebrate community in June 2011 by comparing the faunal and environmental variables associated with palm and willow canopies, trunks and ground beneath each species. The palm invertebrate community had lower abundance and diversity, fewer taxa feeding on the host (e.g., specialized hemipterans), and more taxa likely using only the plant’s physical structure (e.g., web-builders, oak moths, willow hemipterans). There were no observed effects on the ground-dwelling fauna. Faunal differences were due to the physical and trophic changes associated with palm presence, namely increased canopy density, unpalatable leaves, trunk rugosity, and litter accumulations. Palm presence and resulting community shifts may have further ecosystem-level effects through alteration of physical properties, food, and structural resources. These results were consistent with a recent study of invasive palm effects on desert spring arthropods, illustrating that effects may be relatively generalizable. Since spread of the palm is largely localized, but effects are dramatic where it does occur, we recommend combining our results with several further investigations in order to prioritize management decisions.

Impacts of an Invasive Non-Native Annual Weed, Impatiens glandulifera, on Above- and Below-Ground Invertebrate Communities in the United Kingdom

“…At least some members of these groups would use the structure of I. glandulifera as resting sites without feeding on the plant. When studying the impact of an introduced palm species ( Phoenix canariensis Chabaud) along the San Diego River in California, USA, Talley et al [22] showed that there was no impact of the palm on the ground-dwelling invertebrate community when compared to native stands of willow ( Salix lasiolepis Benth.). What our study and that of Talley et al [22] highlights is that in contrast to the foliage invertebrate community, ground-dwelling invertebrates are not solely reliant on live plants for food and development and so might not be affected by changes in vegetation composition caused by an invasive plant.…”

“…Below- and ground-dwelling herbivores are more generalist plant feeders compared to foliage herbivores that specialise on one or a number of closely related species [21]. The varying dominance of different functional feeding groups between trophic levels may account for the more pronounced negative response of above-ground invertebrates to the occurrence of a non-native plant species [22], [23].…”

Effects of Invasive Plants on Arthropods

“…Changes in vegetation composition may lead to changes in structural characteristics, including cover, plant height, and concomitant changes in bare ground (e.g., Toft et al 2001;Standish 2004;Spyreas et al 2010). Structural changes may affect arthropod movement and modify temperature, light intensity, and soil moisture, which are important determinants of the distribution and reproductive success of certain arthropod taxa (Wolkovich et al 2009;Schirmel et al 2011;Talley et al 2012).…”

“…Invasive plants also can alter characteristics of the litter and soil (Standish 2004;Kappes et al 2007;Wolkovich et al 2009). Increases in dominance by invasive plants often are associated with increases in the amount and depth of litter and changes in chemical composition of litter, characteristics important for detritivores (Lambrinos 2000;Talley et al 2012;Alerding & Hunter 2013). Invasive plants may contribute to changes in soil nutrients, moisture, salinity, and pH, and these changes may affect reproduction and composition of arthropods belowground (Witkowski 1991;Gratton & Denno 2005).…”

Invasive goldenrods affect abundance and diversity of grassland ant communities (Hymenoptera: Formicidae)

“…However, the effect of displacement of native plants by goldenrod on aphids is unknown. Second, the decline in ant diversity and abundance could be due to changes in microclimatic conditions (temperature, moisture, light intensity) caused by modifications in structural characteristics (increasing cover, plant height) as well as in litter accumulation and in soil characteristics (nutrients, salinity, pH) (Standish 2004;Wolkovich et al 2009;Talley et al 2012), which may be important especially for thermophilic species such as M. scabrinodis.…”