Species richness patterns
A major goal of ecology is to explain patterns of species richness, from global to local scales. Ultimately, explanations for diversity patterns must include the processes that directly change species numbers in a region or community: speciation, extinction (local or global) and dispersal (e.g. Ricklefs 2004). These processes can be viewed as evolutionary or biogeographic. Yet, large-scale patterns of species diversity are often strongly associated with climate, and with other ecological factors at local scales (e.g. soil pH). These two perspectives are sometimes seen as being in conflict (i.e. evolution vs. ecology; Algar et al. 2009). The concept of NC offers a bridge between them (e.g. Wiens & Donoghue 2004). NC may explain why species fail to disperse between different climates and habitats (e.g. a tropical species cannot rapidly adapt to cold winter temperatures, and fails to colonize temperate regions). The tendency for a group to remain in its ancestral environment as it diversifies could lead to higher richness in some climates (e.g. tropical vs. temperate) or habitats (e.g. mesic vs. arid) than in others, even without differences in rates of speciation and extinction in different environments (Fig. 1). The idea that regions or habitats occupied longer will have more species is called the ‘time-for-speciation’ effect (TSE; review in Stephens & Wiens 2003). There is growing evidence that the combination of NC and TSE may help explain many richness patterns, including latitudinal, elevational, and local diversity.
Several studies have supported the role of NC and TSE in generating high tropical species richness. Wiens et al. (2006) showed that high tropical richness in treefrogs (Hylidae) is related to their origin and longer time in tropical regions (TSE), and that expansion of tropical clades into temperate North America is limited by a climatic variable (temperature seasonality), which shows significant phylogenetic signal (consistent with NC). Algar et al. (2009) claimed to refute this, but ignored both time and temperature seasonality, did not explicitly compare tropical and temperate regions, and offered no comparable alternative hypothesis. A similar pattern of ancient tropical origin and recent temperate dispersal occurs in ranid frogs (Wiens et al. 2009), which dominate the Old World tropics (unlike the predominately Neotropical hylids). Local diversity in New World bats also shows a strong latitudinal TSE (Stevens 2006). In birds, tropical regions globally are dominated by more basal clades, a pattern consistent with NC and TSE (Hawkins et al. 2007). A simulation study of South American birds showed that their richness patterns are best explained by strong climatic NC (Rangel et al. 2007). Recent analyses of global distribution patterns in plants showed evidence for NC constraining dispersal between major biomes (Crisp et al. 2009) and for TSE in explaining lowland tropical species richness (Jansson & Davies 2008). Analyses of climate and richness across mammal clades suggest that the mammalian latitudinal diversity gradient may be related to NC and the TSE (Buckley et al. 2010).
Other studies have suggested that tropical richness may be explained instead by higher rates of tropical speciation or temperate extinction (many reviewed in Mittelbach et al. 2007). However, most did not test for a biogeographic TSE at all, making it difficult to evaluate which hypothesis (rates vs. time) is more important in explaining diversity patterns. Furthermore, even if higher rates of tropical diversification (speciation extinction) prove to be more important than the TSE, NC might still be important in generating latitudinal diversity patterns, for example, by limiting dispersal of tropical species into temperate regions (e.g. Allen & Gillooly 2006). Reconciling the relative importance of diversification rates, TSE, and NC in generating the latitudinal diversity gradient is a major challenge for future research, and future studies should consider all of these processes, not just diversification rates.
Niche conservatism-based hypotheses can potentially explain many other diversity patterns beyond high tropical richness. For example, some groups actually have higher richness in temperate regions than in tropical regions. Analyses of predominately temperate clades of frogs and snakes (Smith et al. 2005; Pyron & Burbrink 2009) suggest that TSE and NC (i.e. temperate origins and climatic constraints on dispersal, respectively) explain their unusual diversity patterns. Richness varies elevationally as well as latitudinally, and in many clades and areas, regional richness is highest at mid-elevations (e.g. McCain 2005; Oömmen & Shanker 2005; Smith et al. 2007; Li et al. 2009; Kozak & Wiens 2010). This mid-elevation hump also appears to be caused by the TSE (based on studies in frogs, salamanders and fish; Smith et al. 2007; Wiens et al. 2007; Li et al. 2009; Kozak & Wiens 2010), with major clades seemingly originating in environments presently situated at mid-elevations, followed by dispersal to lower and higher elevations. NC is hypothesized to limit dispersal between elevational climatic zones, although rigorously demonstrating this remains a major challenge (but see Kozak & Wiens 2010). NC may help explain other elevational diversity patterns as well (e.g. decreasing richness at higher elevations).
Perhaps the least explored interface of NC and species richness relates to local-scale diversity. Local and regional species richness patterns are often strongly correlated (review in Harrison & Cornell 2008), and recent analyses demonstrate that effects of NC on regional diversity can trickle down to local communities. For example, Partel (2002) showed that local plant richness increased with increasing soil pH in regions of generally high pH but decreased in regions of low pH, and attributed this difference to the larger pool of species adapted to the prevailing pH level in each region. Harrison & Grace (2007) showed that the positive productivity-richness relationship in the California flora is driven by the large proportion of species regionally with evolutionary affinities to high-productivity conditions (moist, north-temperate environments) and that the consequences of this NC filtered down to affect the richness and composition of local communities (see also Ackerly 2009a).
In addition to abiotic factors (e.g. climate, pH), biotic factors might also be involved in the interplay of NC and TSE in explaining patterns of local diversity. For example, Brown et al. (2000) argued that local species richness of Enallagma damselfly larvae in lakes with fish as top predators (fish lakes) is higher than in lakes where dragonfly larvae are top predators (fishless), because use of fish-lake habitat has been conserved in Enallagma for tens of millions of years. In contrast, fishless lakes (which require special adaptations to cope with predation by dragonflies) represent a habitat that has been colonized much more recently by Enallagma, leaving less time for speciation to build up diversity in these lakes.
In summary, there is now evidence that NC may be relevant to many richness patterns at many scales. Yet, most patterns to date have been addressed with only a handful of studies, and few have explicitly tested for both NC and the TSE.
Recent studies suggest that NC may have important consequences for ecosystem function. Maherali & Klironomos (2007) used experimental communities of mycorrhizal fungi to show that plant productivity (a common index of ecosystem function) was lowest when communities contained only closely related fungal species. This seemingly occurs because two of the fungus families sampled have complementary effects on productivity (one protects plants against pathogens, the other enhances phosphorus uptake), but there is functional redundancy of species within families, such that NC in functional roles reduces the benefits of having confamilial species. Other authors have shown that ecosystem function (e.g. plant productivity) is associated with higher phylogenetic diversity, and that phylogenetic diversity may be a better predictor of ecosystem function than species richness or even functional diversity (e.g. Cadotte et al. 2008, 2009). However, the exact mechanisms by which phylogeny, traits and NC interact to drive higher productivity in these non-fungal systems remain an important area for future research. Presumably, phylogenetically diverse species capture important functional diversity not reflected in the functional traits measured, and NC leads to functional redundancy among close relatives (reducing the importance of species richness alone).
Invasive species are often considered a major threat to biodiversity, especially on islands (Dirzo & Raven 2003). Given climatic NC, the distribution of species in their native ranges may predict where they can successfully invade and subsequently spread (e.g. Peterson 2003). Recent studies have also shown that climatic niches of invasive populations may change significantly relative to the species’ native range (e.g. Broennimann et al. 2007; Beaumont et al. 2009; Rodder & Lotters 2009). However, these counter-examples involved few species, as did the initial studies using SDMs to test for climatic similarity between native and introduced ranges (e.g. Peterson & Vieglais 2001). A study of 29 introduced reptile and amphibian species in North America (Wiens & Graham 2005) found a strong relationship between native and introduced range limits (poleward latitudinal extents). An earlier study of dozens of introduced bird and mammal species (Sax 2001) showed significant (but weaker) correlations between native and introduced latitudinal extents. In summary, there is some evidence for NC based on relationships between native and introduced latitudinal limits across dozens of species, whereas studies using SDMs of fewer species reveal more variable results. What are lacking are large-scale comparisons of climatic niches between native and introduced ranges, utilizing the available data from the hundreds of introduced animal species and thousands of introduced plants. Such studies are urgently needed to assess both short-term NC and the ability of SDMs to predict the spread of invasive species.
Responses to climate change
The threat of global climate change to biodiversity can be viewed from a NC perspective. If the climatic tolerance of a species is not wide enough to encompass the new conditions or acclimatize to them (physiologically or behaviourally), species with strong climatic NC must either migrate or go extinct, whereas more evolutionarily labile species can potentially adapt (Holt 1990). Persistence may depend on several other factors, including the speed of climatic change (e.g. Loarie et al. 2009), the location of suitable habitat to migrate to, dispersal rate, and changes in biotic niche dimensions (e.g. novel predators or competitors, loss of pollinators). Nevertheless, the strength and generality of climatic NC remains a critical issue in determining how species respond to climate change. For example, SDMs are frequently used to predict range shifts and extinction in response to climate change (e.g. Thomas et al. 2004), based on the assumption that climatic niches are conserved.
A review by Parmesan & Yohe (2003) found that hun-dreds of plant and animal species have modified their ranges latitudinally (poleward) and elevationally (upward) as climate has changed, suggesting widespread climatic NC. Subsequent studies have found similar patterns. For example, Tingley et al. (2009) documented the climatic niches of 53 California bird species from recent and historical distributional data and found that 48 tracked their climatic niche (exhibited NC) as climate warmed, leading to distributional shifts.
Responses to climate change can also be studied in a phylogenetic context. Willis et al. (2008) found that declines in abundance (and local extinctions) of plant species in Thoreau’s woods (Concord, Massachusetts, USA) during the last 150 years are related to different responses to flowering times, which show strong phylogenetic signal. Specifically, species with temperature-insensitive flowering times had decreased abundances relative to temperature-sensitive species.