The Microdistribution and Feeding Preferences of Peromyscus maniculatus in the Strand at Point Reyes National Seashore, California

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A grid of 100 traps in a 1-ha study plot that included beach and dune was used for 1.5 years. The only rodent captured was Peromyscus maniculatus. Estimated density fluctuated between 15 and 40 ha-' but these estimates may be too high. Stomach analyses showed Peromyscus to be an omnivorous, generalized feeder, taking both immature and adult coleopterans, and seeds and herbage of various plants. Choice experiments showed herbage of Montia perfoliata, Stellaria media and Erysimum concinnum and seeds of Cakile maritima, Lupinus arboreus, Lathyrus littoralis and Amsinckia spectabilis to be preferred. In nature, however, peak animal activity coincided with regions densely covered by Ammophila arenaria or Juncus lesueurii. The microdistribution of Peromyscus was inversely correlated with light intensity. Shaded habitats provide nesting material, a more stable substrate, dampened temperature oscillations and protection from aerial predators. Ammophila depressed plant species richness, including food plants of Peromyscus. INTRODUCTION Recent synecological work on the beach and dune vegetation of California (Barbour et al., 1976; Barbour and Robichaux, 1976; Barbour and Johnson, 1977) has shown that general data on plant-animal interactions in the California strand are lacking. To correct this deficiency, we established a 1-ha permanent study plot along the central California coast in strand dominated by Ammophila. Our objective was to identify herbivores and pollinators and to correlate their distribution and activity patterns with plant distribution and microenvironmental gradients. The work with Peromyscus reported here is a part of that larger study (Pitts, 1976). MATERIAL AND METHODS Study site.--Point Reyes National Seashore is located in Marin, Calif., on Point Reyes Peninsula, approximately 37015'-38015' N lat. Our study site is halfway between Beach North and Abbotts Lagoon. It is situated on a small parcel of private land, owned by American Telephone and Telegraph, but it is surrounded by National Seashore property and its receives very little foot traffic. We located a 1-ha study site, (100 m on a side, in November 1974. It was oriented in such a way as: to include a wide range of habitats, from open beach to densely vegetated dunes. The plot was subdivided into 100 1OM2 tracts, each marked by a numbered stake. Rows 1 through 10 were aligned with an E-W trending orientation (roughly perpendicular to shore). Columns were aligned N-S (roughly parallel to shore) and numbered 1.0-10.1 for column 1, 1.1-10.1 for column 2, etc. (Fig. 1). This region is characterized by a marine-moderated Mediterranean climate. The lighthouse at the Point proper records a yearly mean temperature of 11.7 C, with relatively little seasonal fluctuation. Summer fogs no doubt contribute to the moderate temperatures. Point Reyes is one of the foggiest regions on the Pacific Coast. The lighthouse there had an average of 1500 hr of operation a year, about 1/6 of all possible hours. Fog coincides with the months of least rainfall, July-September, and fog drip may play a role in the biota here as it does in other coastal vegetation types of California. Average rainfall is 30 cm year-', with 75% falling November-March I Present address: Department of Biological Sciences, San Jose State University, San Jose, California 95192.