Diagnostic Characteristics
Downingia pusilla is readily distinguished from other members of the genus by its tiny, 2-4 mm (approx 1/8 inch) flowers. Co-occurring species generally have corollas >7 mm (>1/4 inch) in diameter.
In fruit, Downingia pusilla may be identified by the rather short (20-27 mm, ±1 inch), un- twisted ovary which is dehiscent along translucent lines. The seeds are spirally striate, appearing twisted. Co-occuring species generally have much longer, twisted ovaries which are tardily dehiscent or indehiscent and have no apparent translucent lines.
Habitat
Downingia pusilla is restricted to vernal pools and similar seasonal wetlands, including mesic grassland and the margins of small lakes or stock ponds. It is most commonly found in areas of low vegetative cover within the individual pools; whether this is due to reduced competition or increased chances of detection is highly speculative. However, occurrence of plants along the edges of compacted sheep trails in mesic grassland suggests that Downingia pusilla is most suitable to areas of reduced competition.
Downingia pusilla has been reported from all of the vernal pool type classifications in northern California cited by Holland (1986) and refined by Sawyer and Keeler-Wolf (1995). Most of the geomorphic surfaces which support the vernal pool landscape contain at least one known population. The species also occurs over a broad range of soil types. Very often other species of limited distribution may be found co-occurring. These rare plants include Gratiola heterosepala, Legenere limosa, Blennosperma bakeri, Navarretia leucocephala bakeri, Castilleja campestris succulentus, Calycadenia hooveri, Astragalus tener tener, Lasthenia conjugens, Lasthenia burkei, Limnanthes vinculans, and Sagittaria sanfordii; the rare animals include Branchinecta conservatio, Branchenecta lynchii, Branchenecta sp. nov. "midvalley fairy shrimp", Lepidurus packardii, and Linderiella occidentalis.
Ecology
Early spring flowering (March-April) annual plant. Seeds germinate in the standing water of the vernal pools. The plants grow to near full-size while the pools are still inundated. Flowering usually occurs just after the water has evaporated and before the surface soils are too dry. In deeper soils, the plants often produce numerous (up to 50) flowers while in shallow soils the plants general only produce 1-3 flowers. The tiny flowers are cleistomagous which is unusual for members of this genus which are usually pollinated by solitary ground- nesting Andrenid bees. Seed dispersal mechanisms are unknown.