Habitat fragmentation and altered fire regime create trade-offs for an obligate seeding shrub.

Habitat loss is widely considered the greatest threat to biodiversity. However, habitat loss brings with it myriad other threats that exacerbate impacts to biodiversity. For instance, altered fire regime is associated with habitat loss and fragmentation with unknown consequences to biodiversity. Plant functional groups that rely on fire to complete their life cycle may be adversely affected by disruptions to the natural fire regime, particularly when coupled with population declines due to habitat loss. We used a spatially explicit stochastic population model linked with fire hazard functions to investigate the cumulative effects of habitat loss, fragmentation, and altered fire regime on the expected minimum abundance of a long-lived obligate-seeding shrub, Ceanothus greggii var. perplexans. This species is endemic to the California Floristic Province, a biodiversity hotspot, and is representative of a functional group of plants found in many fire-prone ecosystems. We tested the impact of a range of different fire frequencies under three different combinations of fuel accumulation and weather. The best average fire return interval for population abundance was consistently in the range of 30-50 years. However, observed average fire return intervals in highly fragmented areas can be approximately 20 years or less, and model results show this to be detrimental to C. greggii populations. Results also show that if fires are uncorrelated across habitat fragments then the impact of altered fire regime on populations is worse than the impact of habitat fragmentation because of spatial and temporal decoupling of fire events across the landscape. However, the negative impacts of altered fire regime are outweighed by habitat loss as fragmentation increases. Our results show that large unplanned fires, operating under an altered fire regime, are ultimately detrimental to perennial obligate-seeding shrubs in fragmented landscapes.

Quantifying plant population persistence in human-dominated landscapes.

We assessed population performance of rare plants across a gradient from rural to urban landscapes and evaluated 2 hypotheses central to strategic conservation planning: (1) population performance declines with increasing human dominance and (2) small populations perform poorly relative to larger ones. Assessing these hypotheses is critical to strategic conservation planning. The current conservation paradigm adheres to the well-established ecology theory that small isolated populations, particularly those in human-dominated landscapes, are the least likely to succeed over the long term. Consequently, conservation planning has strongly favored large, remote targets for protection. This shift in conservation toward ecosystem-based programs and protection of populations within large, remote systems has been at the expense of protection of the rarest of the rare species, the dominant paradigm for conservation driven by the endangered species act. Yet, avoiding conservation of small populations appears to be based more on theoretical understanding and expert opinion than empiricism. We used Natural Heritage data from California in an assessment of population performance of rare plants across a landscape with an urban-rural gradient. Population performance did not decrease in urban settings or for populations that were initially small. Our results are consistent with a pattern of few species extinctions within these landscapes over the past several decades. We conclude that these populations within compromised landscapes can contribute to overall biodiversity conservation. We further argue that conservation planning for biodiversity preservation should allocate relatively more resources to protecting urban-associated plant taxa because they may provide conservation benefit beyond simply protecting isolated populations; they may be useful in building social interest in conservation.