and compared either in percentages or numbers of species. It is worth remembering that percentages are more meaningful the greater the diversity as well as the higher the taxonomic rank of the group being examined. Thus 50 percent Californian endemism in the grasshopper family Acrididae (with 186 species in the state) means more for the state’s biotic uniqueness than 50 percent endemism in the grasshopper families Eumastacidae and Tanaoceridae (4 and 2 species in the state), or even than 86 percent endemism in the 21 species of Timema (a genus of walking stick insects). Throughout this book, endemism is expressed in both numbers and percentages, in the belief that they provide complementary information.
Comparative information from other geographic regions is essential to characterizing and explaining Californian endemism. Acridid grasshoppers are one of the most endemic-rich groups in California, but they may be equally so in other parts of the mountainous western United States (Knowles and Otte 2000). Whether or not it is remarkable that 5 of 23 kangaroo rats (Dipodomys) or 21 of 22 slender salamanders (Batrachoseps) are endemic to California depends on whether ecologically similar groups are just as diverse in neighboring regions. It is challenging to find, for almost any group, either comparative data or interpretive analyses that place endemism in California in a larger context. This book relies on comparisons with other states and the other four mediterranean climate regions to provide a context for Californian endemism.
LARGE-SCALE PATTERNS IN SPECIES RICHNESS AND ENDEMISM
One of the best predictors of species richness at a global scale is plant productivity, which is determined at large scales by the abundance of water and solar energy. At low latitudes water exerts stronger control, whereas at high latitudes solar energy is a stronger limitation. There are consistently more species of plants and animals in the warm and wet parts of the world than the colder or drier ones, regardless of whether the latitude is tropical or nontropical (Figure 5a; Hawkins et al. 2003). Within the United States as a whole, plant and vertebrate animal diversity is higher in the warmer southerly states (Stein et al. 2000). Within California, in contrast, the diversities of plants, birds, mammals, and amphibians (although not reptiles) are highest in the rainier north (CDFW 2003). However, this is a case where the exception proves the rule, because California is a sunny but arid region in which water is the limiting factor governing plant productivity. Plant diversity in California is positively related to a remotely sensed index of productivity, which in turn is strongly related to rainfall but not to temperature (Figure 5b; Harrison et al. 2006).
Levels of endemism may follow geographic patterns different from total species diversity. Isolated islands, for example, are often high in endemism but low in total richness. Endemism on continents is harder to explain, but one recent analysis suggests that global patterns in endemism are best explained by climatic stability. Sandel et al. (2011) defined the climate change velocity of any given location as the ratio of climatic change over time at that location since the last glacial maximum, 22,000 years ago, to the average change in climate over space at the same location at present. This velocity represents how fast an organism has had to shift its distribution to keep pace with postglacial warming. It is slow, for example, in mild maritime climates that have undergone less change in temperature over time and in rugged regions where present-day temperatures vary sharply over short distances (e.g., from low to high elevations). Globally, animal endemism is higher where climate change velocity is lower, and this effect is stronger for sedentary amphibians than for mobile mammals and birds, suggesting that stable climates have promoted the persistence of sedentary species with small geographic ranges (Figure 6; Sandel et al. 2011).
FIGURE 5. Large-scale species diversity as a function of two different measures of plant productivity.
(a) Global relationship between bird species richness and actual evapotranspiration (AET), an index that combines growing season temperature and moisture (data from B. A. Hawkins unpublished; see also Hawkins and Porter 2003).
(b) Californian plant species richness versus the normalized difference vegetation index (NDVI), a remotely sensed metric of “greenness,” or the red/far red reflectance ratio that correlates strongly with mean annual rainfall in California (data from Harrison et al. 2006).
FIGURE 6. Global relationship between endemism and climate change velocity, defined as the rate at which species must migrate to remain in a constant temperature since the last glacial maximum. Climate change velocity is calculated by dividing the rate of temperature change through time by the local rate of change over space. (Data from B. Sandel unpublished)
Endemism has been an important concept in conservation, as manifested by efforts to identify hotspots of high numbers of species found nowhere else. In the most famous such analysis, Myers et al. (2000) defined global hotspots as regions with more than 1,500 endemic plant species and more than 75 percent loss of primary vegetation cover. The 25 hotspots thus identified make up only 1.5 percent of the world’s terrestrial land surface but hold over one-third of the world’s vertebrates in four major groups, as well as one-half to two-thirds of the plants and vertebrates on the IUCN Red List of Threatened Species. Nearly all hotspots are in the moist tropics or subtropics, contributing to their high overall diversity, and in the tropics they tend to be on islands or in islandlike mountain ranges, contributing to high endemism. Almost the only nontropical hotspots are found in the five mediterranean climate regions of the world, including the California Floristic Province. These five regions are also lower in vertebrate diversity than the other hotspots.
Global analyses of animal endemism generally find similar results to those of Myers et al. (2000), except where the mediterranean climate regions are concerned. For example, Rodrigues et al. (2004) used distributional maps for mammals, amphibians, turtles, tortoises, and endangered birds to identify the global regions with highest “irreplaceability,” or numbers of species not found or protected anywhere else. The results highlighted many of the same tropical islands and mountain ranges identified by Myers et al. (2000) but not the five mediterranean climate regions. Likewise, Lamoreux et al. (2006) found that hotspots of amphibian, bird, mammal, and reptile endemism tended to coincide, but none of these concentrations occurred in the mediterranean regions. Nor did the mediterranean regions score as globally significant for total, endemic, or endangered birds (Orme et al. 2005)
Within the United States, including California, hotspots of endemism have been identified using a metric called rarity-weighted richness (Stein et al. 2000; CDFW 2003). A region is divided into equal-area polygons, within each of which the rarity-weighted richness is the sum of each species present in the polygon divided by the number of polygons occupied by that species. The output is a map showing high concentrations of narrowly distributed species (Figures 7, 8). The input data are often coarse and incomplete; in these examples, only Heritage Network–listed species are included, and their distributions are less than fully known. Also, the results may sometimes be dominated by small numbers of imperiled species with very tiny ranges; there is no single “correct” way to balance the contributions of number of species and range sizes in this type of analysis. Nonetheless, it provides a synoptic view of biodiversity that emphasizes endemism.
FIGURE 7. Hotspots of rarity in the United States. The rarity-weighted richness (RWR) analysis of critically imperiled and imperiled species shows concentrations of limited-range species, thus highlighting locations with species composition different from adjacent areas. The analysis points to locations that are essentially “irreplaceable” and present conservation opportunities found in few other places. (Source: NatureServe and its Natural Heritage member programs, July 2008. Produced by National Geographic Maps and NatureServe, December 2008.)
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