Small mammal seed preference in coastal sage scrub communities: New technologies, new insights

Background/Question/Methods

Many rodents in southern California shrublands are granivores (seed eaters) and can affect the distribution of seeds after seed dispersal through selective seed predation. Rodent seed preferences are hypothesized to affect plant community composition, as rodents tend to prefer exotic grass seeds over native shrub seeds. Braswell (2008) conducted an experiment at Rancho Jamul Ecological Reserve (RJER; San Diego County, California) to assess selective seed predation by the granivores in a coastal sage scrub community. She presented native and exotic seed mixtures in partitioned petri dishes across sixteen sites at RJER. At each station, one seed dish was accessible to all granivores, and one dish was only accessible to rodents (caged in with a bent PVC tube entrance). In conjunction with the seed choice experiment, she live trapped rodents in order to estimate densities at each site. In Braswell’s analysis, seed predation was correlated with small mammal densities, assuming that all members of the small mammal community remove seeds in direct proportion to density.

I am re-testing the hypothesis that small mammals are selective seed predators using direct observation. I present native and exotic seed mixtures following Braswell’s protocols. I record all seed removal using a custom built, infrared digital camera and digital video recorder system. The objectives of this study are:  (1) Can direct observation of granivory provide new insight into the role granivores play in this system? (2) Are granivores selective in seed choice (native versus exotic)? and (3) If so, does selectivity vary with animal guild (genus, family)?

Results/Conclusions

Preliminary results show that direct observation provides much more detailed and nuanced information on seed tray visitation and preferential seed predation. For example, the PVC tube entrance to the rodent-only exclosure was assumed to be a benign component of the station and equally used by all rodent species; however, my study is showing tube avoidance behavior by most of the rodent species that visit the stations. My field cameras also allow me to quantify which species are removing seeds, allowing me to correctly understand causality. My results show that while some species appear to prefer exotic seeds, others show no preference.

As technology improves, it is important to consider new and/or alternative methods that can improve our understanding of ecological interactions through direct, detailed observation.