PARENT SESSION
Oral Session # 36: Rare, Threatened, and Endangered Species I: Plants.
Presiding: H Balbach
Tuesday, August 5. 1:30 PM to 5:00 PM, SITCC Meeting Room 204.
Causes of rarity in the vernal pool endemic Lasthenia conjugens (Asteraceae).
Gerhardt, Fritz*,1, Collinge, Sharon1, 1 University of Colorado, Boulder, Colorado
ABSTRACT- Rare species are usually defined by limited distributions and narrow habitat specificities, yet the mechanisms causing rarity remain unclear. Narrow habitat specificity can arise because species are adapted to a particular set of environmental conditions or because these habitats serve as refuges for species that are poor competitors elsewhere. In this paper, we report on field surveys and greenhouse experiments examining rarity in the vernal pool endemic Lasthenia conjugens (Asteraceae, Contra Costa goldfields). We examined the effects of abiotic conditions, especially water depth and duration of inundation, and interactions with other native and exotic plants on the performance of L. conjugens. Field surveys showed that L. conjugens was most abundant in natural pools and in pools that burned in a wildfire in June 2000. Furthermore, L. conjugens abundance increased with increasing water depth. In a greenhouse experiment examining the effects of water depth and duration of inundation, L. conjugens had low overall survival (47%), but height, number of leaves, total biomass, and number of inflorescences all decreased with increasing water depth. Duration of inundation had no significant effects on the performance of L. conjugens. In a greenhouse experiment examining the effects of biotic interactions, the biomass of L. conjugens decreased sharply as the biomass of neighboring plants increased. Collectively, these results show that, although L. conjugens occurs at greater water depths in the field, this species actually grows larger and reproduces more when not inundated. However, competition with native and exotic plants appears to limit L. conjugens to more stressful, inundated sites. Thus, we conclude that L. conjugens and possibly many other rare species are not narrow habitat specialists but rather are refugees restricted to more stressful habitats by competition in other, more favorable environments.
Key words: Lasthenia conjugens, rarity, habitat specialist, competition