The flora of California, a global biodiversity hotspot, includes 2387 endemic plant taxa. With anticipated climate change, we project that up to 66% will experience >80% reductions in range size within a century. These results are comparable with other studies of fewer species or just samples of a region's endemics. Projected reductions depend on the magnitude of future emissions and on the ability of species to disperse from their current locations. California's varied terrain could cause species to move in very different directions, breaking up present-day floras. However, our projections also identify regions where species undergoing severe range reductions may persist. Protecting these potential future refugia and facilitating species dispersal will be essential to maintain biodiversity in the face of climate change.

Copyright: © 2008 Loarie et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Introduction

The California Floristic Province has over 5500 native plant taxa; 40% of them are endemic, that is, their entire native distributions are within the Province [1]. (By taxa, we mean distinct species, subspecies, or varieties, and we use “species” hereafter for simplicity [2].) Models project that California's temperature and rainfall will change considerably in this century [3]. Here, we use observed data on species' distributions and present-day climate to build multiple bioclimatic models. We then apply these models to project changes in endemic species' range sizes, distribution and diversity under future climate scenarios.

Empirical examples of species' range shifts resulting from climate change have been recorded for numerous taxa [4]–[5]. Projecting future changes is a crucial step towards planning for and mitigating the impacts of climate change on biodiversity [6]. Most previous attempts have focused on small subsets of species [7]–[9] or vegetation types [10]–[11]. They incorporated varying degrees of data on physiology and dispersal. A small number of related studies have focused on estimating changes in biodiversity [12]–[13]. As we describe in the methods, biodiversity studies must limit themselves to species subsets restricted to the region – such as endemics [14]. Sparse flora-wide data on physiology and dispersal has meant that studies across floras have included simplified treatments of individual species' biology.

A recent study of two California oak species projected significant range reductions for both species [7]. In southeastern California, a study of Yucca brevifolia that included physiological responses to increased CO 2 levels projected a slight decrease in range size [15]. Analysis of the responses of vegetation types in California to climate change projected decreased coniferous forests in the northwestern part of the state and increases in broadleaf vegetation [16]. In Eastern North America, models for 80 tree species project range expansions for approximately 30 species and an equal number of range contractions. In that study, the centroids of nearly half of the species were projected to move at least 100 km to the north [17].

Outside of North America, regional studies have addressed both range shifts and potential levels of extinction in the face of climate change. Studies of the Proteaceae in the Cape Floristic Province – another Mediterranean hotspot — estimate that this group may lose up to 20% of the species considered [8], [18]–[19]. A study of 975 endemic plant species in southern Africa projected that the Mediterranean climate portion of the study area will lose the highest proportion of species [20], while flora-level studies from Europe have projected that as many as half of the species studied will be threatened [12]–[13], [21].

Currently, there are no published assessments of potential impacts of climate change on regional endemic floras for any part of North America. California is particularly well suited to such a study, as it has high endemic plant diversity and the quality of plant distribution and climate data across the region are excellent. California also provides an interesting case study because of its topographic complexity, extensive urban and agricultural land use, and Mediterranean climate characterized by distinctive rainfall and temperature patterns.

We assess 8 different potential scenarios for the future of the California flora in the face of climate change. These are the combinations of three pairs of possibilities. First, we compared two projections of future emission levels from human activities. One is higher, with global CO 2 emissions reaching almost 30 GtC per year, or 4 times present-day levels, by 2100 (SRES A1FI) while the other emission scenario is lower, with CO 2 emissions rising slightly by mid-century before dropping to below present-day levels by the end of century (SRES B1) [22]. By 2100, global atmospheric CO 2 levels reach 550 and 970 ppm under the lower and higher emissions scenarios, respectively. Second, we compared projections centered 80 years from now (2070–2099) from two global climate models with higher and lower sensitivities to atmospheric greenhouse gas levels. The U.K. Meteorological Centre's Hadley Centre Coupled Model version 3 (HadCM3) model [23]–[24] is moderately sensitive to increases in emissions, while the DOE/NCAR Parallel Climate Model (PCM) is less sensitive [25]. Third, we explored two distinct and widely used dispersal scenarios: one where plants exhibit unrestricted movement to new locations, and one with no movement [26], [13], [18].

Projecting the impacts of climate change to an entire endemic flora is complicated by scarce and variable distribution data. Studies conflict on how many geo-referenced specimens are necessary to obtain robust species projections [27], [28]. Including poorly known species risks biasing projections of biodiversity patterns if the error is directional. In contrast, poorly known species may have smaller ranges, and small ranged species are known to be more vulnerable to extinction [29]. Excluding such species may be equally inappropriate.

A recent study recommends using Maxent and at least 30 non-validation specimens for robust species projections [28]. Following these recommendations, we model and evaluate the 591 out of 2387 California Floristic Province endemic species that have at least 42 specimens using Maxent [30]. Specimen records were obtained from the Consortium of California Herbaria, a centralized portal accessing over 959,000 specimens from 16 herbaria [31]. To address whether poorly known species tend to have small ranges, we compile an independent dataset of range maps for each species to compare with the number of specimens. (We refer to these as TJM1 range maps, see Materials and Methods.)

To assess whether excluding poorly known species biases diversity patterns, we build a multilevel generalized linear model (MLGLM) [32] incorporating all 2069 species with at least 2 specimens. This model simultaneously estimates relationships between the probability of a plant being found in a location, and climatic variables. It does so both at the level of each species as well as the entire flora. The hierarchical structure of this model gives an unbiased predictor of climate influences on presences, and allows poorly known species to draw inferential strength from the flora as a whole [33]. As a result, the model is informed by data from all species, but the influence of poorly known species is properly weighted against the flora. We then compare biodiversity patterns from this hierarchical approach with Maxent projections from the best known 591 species.

To summarize the impacts of climate change on the California flora and to compare the projections with other studies, we ask four questions. First, where will endemic species diversity be most influenced by climate change? Second, if species are permitted to move, where will they go? Third, how do we project range sizes to change? Fourth, where do we expect future refugia — locations where species at risk from climate change will persist under future climates? To date no studies have mapped the locations of such refugia.