The Arthropod Community on Corn Lily, Veratrum californica,in Carmen Valley, CA

 

BIOL 315 - Field Methods in Ecology Class Summer 1999

(Gail Friedlander, Ellen Hamingson, Val Martino,

David Seay, Pat Stadillo, Ed Connor (instructor))

Carmen Valley is a high elevation meadow (circa 1700 m) on the east slope of the Sierra Nevada range. The construction of a railroad spur line for logging in the 1920's resulted in stream capture by the borrow ditch and subsequent head-cutting of the stream channel. The head-cut stream channel now extends some six miles upstream from the point of stream capture. One consequence of stream capture and head-cutting has been that sections of Carmen Valley dry out early in the summer, while areas away from the head-cut stream channel remain wet for longer periods. An attempt to restore water flow to the natural stream channel and floodplain will occur in the summer of the year 2000. In order to determine the impact of stream-channel restoration on the biota of Carmen Valley, monitoring projects for birds and willow feeding insects are currently in progress.

To provide additional information on arthropod communities in Carmen Valley prior to stream restoration, we examined the community of arthropods occurring on Corn lily, Veratrum californica (Liliaceae) in Carmen Valley on June 22, 1999. We divided Carmen Valley into four zones from the western valley edge, which remains wet, to either side of the Valley's central willow clumps to the dry eastern edge along the head-cut stream channel. Within each of these zones we randomly selected five patches of Corn lily of varying sizes and degrees of isolation. For each patch we measured: 1) patch area (number of stems), 2) patch isolation, 3) plant height, 4) plant transpiration rate, 5) plant leaf temperature, 6) relative humidity, 7) the number of arthropod species per stem, 8) the number of individual arthropods per stem, and 9) the number of defoliating caterpillars per stem.

image227.gif (5247 bytes)Examination of data on arthropod species richness, by ANCOVA (analysis of covariance) indicate that after adjusting for patch isolation, significantly fewer species per stem occur in the eastern, drier areas of Carmen Valley than in the wetter western portion (F3,15 = 3.893, p = 0.031, and Figure 1). Arthropod density (number per stem) was significantly higher in the drier eastern regions after adjusting for patch isolation (F3,15 = 3.279, p = 0.05 and Figure 2). The increased density of larvae of one species of defoliating Lepidoptera accounts for the higher density of arthropods on Corn lily in the eastern regions of Carmen Valley (F3,15 = 4.983, p = 0.014).

Although we surmise that the higher abundance of defoliating caterpillars on Corn Lily in the eastern regions of Carmen Valley may be related to the apparently drier conditions on the eastern side of the Valley, we found no difference in the transpiration rates of Corn Lily plants between the four sample zones (F3,16 = 2.514, p = 0.095). While lower transpiration rates on plants in the eastern zone would be consistent with the drier conditions, surprisingly, the average rates of transpiration were marginally higher on plants in the drier, eastern zones.

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To better understand the causes of the patterns in arthropod species richness and abundance on Corn Lily that we observed in Carmen Valley several lines of inquiry should be followed this summer:

  1. the biology and natural history of Corn Lily should be examined,
  2. the identity and life history of the defoliating lepidopteran should be studied,
  3. data on soil moisture across Carmen Valley should be collected, and
  4. observations on Corn Lily defoliation should be made in other high elevation meadows.

Depending on the information collected on Corn Lily and caterpillar biology, a series of experiments aimed at teasing apart the causes of high caterpillar abundance on Corn Lily in the eastern regions of Carmen Valley should be considered for next year. These experiments include:

    1. avian exclusion experiments to test the effect of bird predation on caterpillars in the western areas of Carmen Valley,
    2. caterpillar exclusion experiments to permit assessment of the water-status of Corn Lily plants across Carmen Valley that are free of the confounding influence of caterpillar defoliation, and
    3. pupal survival and adult emergence experiments across Carmen Valley to determine if caterpillar survival is reduced in areas of high soil moisture.

The observations and experiments described above could also be supplemented with controlled laboratory and field experiments on adult oviposition preference, larval survival, and developmental rates on plants grown in moist soils and plants grown under water deficits.