PARENT SESSION
Contributed Oral Session 53: Trophic Communities and Modeling
Tuesday, August 9, 8:00 AM - 11:30 AM, Meeting Room 524 B, Level 5, Palais des congrès de Montréal

Food vs Predators: what drives clutch size patterns in coastal sage scrub avian communities?

Walters, Eric^*,1, Bolger, Douglas¹, Patten, Michael², ¹ Dartmouth College, Hanover, NH² Sutton Avian Research Center, Bartlesville, OK

ABSTRACT- We studied the effect of fragmentation and edge on avian productivity in threatened coastal sage scrub habitat in southern California. In 2001 and 2004, we measured clutch size in focal pairs of four representative avian species. We compared two shrub-nesting species (California Towhee and Wrentit) with two ground-nesting species (Rufous-crowned Sparrow and Spotted Towhee). Four study plots (2-4 ha) were established in each of four treatment types: interior regions (> 600 m from edge) of protected areas (> 2,300 ha), edge regions of protected areas, large fragments (37-80 ha) in the urban matrix, and small fragments (5-17 ha) in the urban matrix. California Towhees (N=137, p= 0.001), Rufous-crowned Sparrows (N=43, p< 0.001), Spotted Towhees (N=50, p=0.072), and Wrentits (N=80, p<0.001) all had larger clutch sizes in 2001 (a typical rainfall year, 21.87 cm) compared with 2004 (a low rainfall year, 13.49 cm). California Towhees (p=0.003) and Wrentits (p=0.028) had significantly smaller clutch sizes in fragmented habitat compared with ground-nesting species. Using Mayfield daily survival probabilities for egg stage, we did not see a corresponding decrease in survival probability for shrub nesters in fragmented habitat. Larger arthropods (a preferred diet item) have been shown to be absent from fragmented sites. Our conclusion is that the reduced clutch size in these fragmented sites is due to bottom-up constraints related to a decrease in arthropod availability and water resources needed for laying, rather than by an increase in egg predators.

Key words: bottom-up, top-down, predation, fragmentation