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.
Highlights
Images of many semi-arid and/or semi-tropical regions would not be complete without picturesque palms, but in areas such as California these palms are mostly non-native
Unassisted large-scale spread is relatively slow, especially compared to more aggressive invaders e.g., the giant reed grass, Arrrndo donrzx!b, ut localized spread and dense patch formation are common along Southern California rivers due to the high propagule pressure of horticultural sources, the high fecundity of established adults [2], [11 12] and moderate salt tolerance [13 14] allowing it accessto estuarine river reaches [15]
The main goal of this study was to test the effects of the invasion of Canary Island palm on a riparian invertebrate community
Summary
Images of many semi-arid and/or semi-tropical regions would not be complete without picturesque palms, but in areas such as California these palms are mostly non-native Tropical icons, such as the fan palms Ij/ashingtoniaspp.j and the date palms Phoenix spp.j, are commonly used in landscaping [I] and are making their way throughout relatively moist natural areas [2], such as desert springs in California P] and riparian corridors of California and Florida [4 5]. Outcompete native vegetation for sunlight, space, and water; and thick layers of wide, fibrous palm frond litter prevent new plants from growing P], [6 7] These desert invaders were associated with lower abundances and diversity of arthropods, and fewer herbivores than on native plant species P]. Unassisted large-scale spread is relatively slow, especially compared to more aggressive invaders e.g., the giant reed grass, Arrrndo donrzx!b, ut localized spread and dense patch formation are common along Southern California rivers due to the high propagule pressure of horticultural sources, the high fecundity of established adults [2], [11 12] and moderate salt tolerance [13 14] allowing it accessto estuarine river reaches [15]
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