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Unfriendly Fronds - Conservation

Unfriendly Fronds

California’s iconic palms could be hurting its ecosystems. Along the San Diego River, these popular plants appear to be reducing the diversity and abundance of invertebrate species.

“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,” the study authors point out in PLoS ONE. Fan palms and date palms are spreading to rivers and desert springs, and they can hog light and water.

The team compared the Canary Island date palm to native arroyo willows along the San Diego River and noted some obvious differences. The palms had bigger, tougher leaves, which formed a denser canopy overhead than the willows did. The palms’ trunks were also much more creased and tended to trap debris.

To find out how the palms might affect invertebrates, the team studied 15 pairs of palms and willows in the area. The palms’ canopies were 24 percent bigger, and they let 7 percent less light through. Invertebrates were about 60 percent as abundant around the palms as the willows, and species diversity was only about 70 to 80 percent as high.

The willows might support more species partly because their leaves are soft and easier to eat, the authors speculate. Birds such as the Least Bell’s Vireo might also have trouble foraging for insects or nesting on palms because of the “impenetrable, detritus-covered, tough (downright sharp!) structure of palm canopies, trunks and litter,” the team writes.

The palms did have one advantage: They were less friendly to invasive Argentine ants, which preferred the willow’s moist undergrowth. Roberta Kwok | 10 August 2012

Source: Talley, T.S., K.-C. Nguyen, and A. Nguyen. 2012. Testing the effects of an introduced palm on a riparian invertebrate community in southern California. PLoS ONE doi: 10.1371/journal.pone.0042460.

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