Editor: Marcel Cardillo
Past climate and species ecology drive nested species richness patterns along an east-west axis in the Himalaya
Article first published online: 11 JUN 2013
© 2013 John Wiley & Sons Ltd
Global Ecology and Biogeography
Volume 23, Issue 1, pages 52–60, January 2014
How to Cite
Srinivasan, U., Tamma, K. and Ramakrishnan, U. (2014), Past climate and species ecology drive nested species richness patterns along an east-west axis in the Himalaya. Global Ecology and Biogeography, 23: 52–60. doi: 10.1111/geb.12082
- Issue published online: 3 DEC 2013
- Article first published online: 11 JUN 2013
- Altitudinal migration;
- body size;
- climate change;
- dispersal ability;
- murid rodents;
Although global species richness patterns appear consistent across taxa and continents, patterns are elusive at smaller spatial scales. At regional/subcontinental scales, climatic, environmental and taxon-specific contingencies are likely to interact to modify general richness patterns. We develop a biogeographical paradigm for the Himalayan range as representative of regions at similar spatial scales, and where historical climate fluctuations might interact with species ecology to drive species richness patterns.
Himalayan range, Asia
We obtained a cell × species presence–absence matrix for babblers and murid rodents in 1° latitude × 1° longitude cells in the Himalayan range. We investigated nestedness in species richness patterns in these taxa along a distance gradient from the species-rich eastern towards the relatively depauperate west. We also investigated the relationship between species autecology and westward extent along the Himalaya. Climate data were obtained from published sources.
Himalayan babbler and murid assemblages are nested along an east–west axis, with assemblages in westward cells tending to be subsets of assemblages immediately to the east. Distance westward from the eastern Himalaya was related positively to altitudinal mobility of babbler assemblages, while body size increased with distance westward for murid assemblages.
The eastern Himalaya, which was not glaciated over during glacial maxima, was a potential refugium for babbler and murid species. Following glacial retreat, species could have recolonized the Himalaya westwards to different extents based on ecological traits (size, altitudinal migration) determining ability to deal with the more seasonal west. This produces both (1) a nested species richness pattern, and (2) correlations between ‘filtering’ autecological traits and distance. Such patterns should be replicated in other regions with historical climatic refugia; investigating nestedness along distance gradients from refugia would be a powerful tool in mapping biogeographical history, especially in separating historical effects from currently proposed energy-productivity relationships.
Decline in species richness with distance from the tropics is a consistent pattern that has been demonstrated in several taxa across several continents (review in Hawkins et al., 2003). Such macro-scale patterns have fascinated ecologists for several decades, and hypotheses attempting to explain these diversity patterns are abundant (e.g. Gaston, 2000; Hawkins et al., 2003; Ricklefs, 2004; Rohde, 1992). Explanations for global richness patterns point to current environmental conditions, such as energy and climate (Hawkins et al., 2003; Kissling et al., 2007), productivity (or the amount of energy available to heterotrophs; Currie et al., 2004), niche conservatism (or the retention of ancestral traits; Hawkins et al., 2006, 2012), species area relationships (Rosenzweig, 1995), and evolutionary processes, such as speciation and extinction (e.g. Chown & Gaston, 2000). At smaller spatial scales, consistent patterns are elusive, possibly because of regional and taxon-specific contingencies at smaller scales that interact with each other to modify general patterns (see Dobrovolski et al., 2012).
Biogeographical history, for instance, is likely to play a vital role in influencing richness patterns at regional scales, in conjunction with species ecology, geological history and climate (Wiens & Donoghue, 2004; Araujo et al., 2008; Flojgaard et al., 2011; Hawkins et al., 2011). Histories of speciation and extinction are contingent on changes in regional climate, and in turn determine regional diversity patterns (see Buckley & Jetz, 2007, and references therein; Hawkins et al., 2011). Flojgaard et al. (2011), for example, demonstrated the importance of biogeographic history in colonization dynamics following glacial retreat as an important determinant of species richness in European mammals (Flojgaard et al., 2011; see Araujo et al., 2008 for a discussion on amphibians). Further, glacial history appears also to be an important driver of diversity patterns in New World birds, mammals and amphibians (Dobrovolski et al., 2012).
Ecological theory predicts that key life history traits would interact with climatic history to produce predictable patterns of species richness at regional scales. For instance, following glacial retreat, species would therefore be expected to colonize new habitats (or persist in changing ones) to different degrees, depending on physiological or behavioural life history traits (such as temperature tolerance and migratory behaviour), implicating autecology in species richness patterns. Such selective colonization from glacial refugia or selective extinctions from climate change can lead to predictable and nested patterns of species richness (Flojgaard et al., 2011), with differential colonization and extinction driven by species-specific traits, such as dispersal ability and capacity to tolerate temperature extremes. Nested patterns, wherein assemblages at species-poor sites tend to be subsets of assemblages at species-rich sites (Patterson & Brown, 1991), are of late being used to explore the effect of biogeographic history in driving continental richness patterns (e.g. Flojgaard et al., 2011; Dobrovolski et al., 2012).
The interaction between environmental factors, historical effects and autecology in determining species richness patterns remain poorly understood. Such interactions are likely to be important (and hence, become apparent) (1) at relatively smaller spatial scales with limited environmental heterogeneity (Patterson & Brown, 1991), (2) in regions with a history of climate change (e.g. mountainous regions in the tropics) and (3) species-rich areas with sufficient species-level variation in autecological traits. One such region is the Himalaya, which spans over 2500 km and is one of the most species rich regions in the world. The Himalaya are a predominantly east–west mountain range, and one consistent species richness pattern of several taxa in the Himalaya is that the Eastern Himalaya is species rich, with declining richness towards the west (e.g. Ding et al., 2006; Price et al., 2011). Studies of glacial landforms suggest that the Himalayas (except in the east) were covered with glaciers during the last glacial maximum (see Owen et al., 2002; Scherler et al., 2010). We propose that historical glaciation in conjunction with differential colonization or extinction (arising from life history) along the Himalaya predicts richness patterns along the east–west gradient in the Himalaya, and should result in a nested richness pattern. Nestedness in species richness in these taxa along the east–west distance gradient (i.e. progressively western ‘depauperate’ assemblages being subsets of more speciose eastern assemblages) would also indicate that assemblages share similar environments and biogeographical histories (Patterson & Brown, 1991).
A nested pattern might arise from one of two mechanisms: (1) species dispersing from a common ‘source’ in the east to different extents towards the west, or (2) a common species pool in the entire Himalaya subject to differential extinction along the east–west gradient (i.e. greater extinction progressively westward). We use our results to discuss how glacial history in conjunction with species ecology might have influenced regional richness patterns in other parts of the world, as well as issues relating to the larger energy-productivity-richness debate.
We test our predictions by examining the nestedness of babbler (Aves: Timaliidae) and murid (Mammalia: Rodentia, Family: Muridae) assemblages along a distance gradient from east to west. Babblers and murids were chosen to examine the patterns in nestedness for two reasons. Firstly, babblers and murids are extremely species-rich taxa, with 115 and 45 Himalayan species, respectively (Molur et al., 2005; Rasmussen & Anderton, 2005). As with other mountain ranges (especially tropical mountain ranges) in the world, the Himalaya are highly species rich, probably because of the interaction between local climate and relatively large-scale topographical heterogeneity (Rahbek & Graves, 2001; Ruggiero & Hawkins, 2008). Further, all species are sedentary, none are long-distance migrants and most are confined to a specific altitudinal zone (Rasmussen & Anderton, 2005). Large-scale patterns in sedentary babbler or murid species richness are therefore not likely to be confounded by seasonal long-distance movement. Some babbler species show annual altitudinal movement, moving to low elevations in the winter, and returning to the higher elevations to breed during the warmer months (Rasmussen & Anderton, 2005). Comparing patterns for different groups will help elucidate general biogeographic patterns.
We hypothesized that (1) babbler and murid rodent species richness in the Himalayas will be nested, because the Eastern Himalaya is likely to be a source for species that then colonized the Western Himalaya (e.g. Johansson et al., 2007), and (2) given that the Western Himalaya is colder and experiences greater temperature variation than the east (Price et al., 2011), there will be a relationship between distance from the Eastern Himalaya and ecological traits that help species adapt to the marked seasonality of climate in the west. Specifically, we predict that for babblers, the degree of altitudinal movement in response to seasonal changes should be correlated with distance westward from the Eastern Himalaya; for murids, larger body size should be correlated with westward extent, because larger size is expected to be adaptive to colder conditions through its heat conservation mechanism (Bergman, 1847; Blackburn et al., 1999; Issac, 2008).
We overlaid a grid (1° latitude by 1° longitude cells) on the Himalayan range. By overlaying this grid over distribution maps of all Himalayan babbler (BirdLife International & NatureServe, 2012) and murid species (Molur et al., 2005; IUCN Red List of Threatened Species, 2012) in the software QGIS (Quantum GIS Development Team, 2012), we were able to calculate the number of babbler and murid species in each grid cell. The distance between the midpoint of each Himalayan cell from the easternmost Himalayan cell was then calculated to explore patterns in species richness and life history traits along a distance gradient from the Eastern Himalaya. The coordinates of the centre of each cell were used in analysing these patterns statistically by taking spatial autocorrelation into account.
Data on the altitudinal movement of babbler species was obtained from Rasmussen & Anderton (2005). Each species has a winter and summer altitudinal range. For completely sedentary species, these coincide perfectly. We used the difference between the minimum altitude of the range of a species in winter and that in summer to characterize the altitudinal movement of a species. Species that show no altitudinal movement received a score of zero, and higher values indicate greater elevational movement by a species. Many ecological filters operate in community assembly, and we used body size (head–body length) (from Roberts, 2005; Francis, 2008; Smith & Xie, 2008) to summarize the ability of mammal species to overcome these filters; temperature tolerance and dispersal ability are especially relevant in this case.
We plotted the cell-wise species richness as a function of the distance of the cell from easternmost cell and used ordinary least squares (OLS) models to explore this relationship. One of the limitations of OLS models in dealing with spatial data is that they cannot account for spatial autocorrelation. We therefore checked for spatial autocorrelation in the data (babbler and murid species richness, babbler altitudinal movement and murid body size), as well as in the residuals of the OLS models using Moran's I correlograms. Since these correlograms showed spatial structure in the data and the residuals at varying spatial scales, we used generalized least square (GLS) models in preference to OLS in making inferences about the impact of distance from the Eastern Himalaya on species richness and autecological traits.
Specifically, we used GLS models with a linear spatial autocorrelation structure to model the relationship between altitudinal movement (in babblers) and body size (in murids) and distance from the Eastern Himalaya. GLS models that incorporate spatial structure in the residuals are advised in grid-based analyses to avoid potential statistical issues arising from spatial autocorrelation, especially increased chances of Type I error, and bias (Diniz-Filho et al., 2003).
We used the ‘nestedness metric based on overlap and decreasing fill’ (NODF; Almeida-Neto et al., 2008). Although Atmar & Patterson's (1993) nestedness temperature metric has hitherto been the most popularly used to quantify nestedness, it has been criticized for methodological inadequacies (Rodriguez-Girones & Santamaria, 2006; Almeida-Neto et al., 2008). The NODF nestedness metric of Almeida-Neto et al. (2008) resolves a number of these statistical issues, and is much less prone to bias and type I statistical errors when compared with other classical nestedness metrics (Almeida-Neto et al., 2008). All analyses were conducted in the R statistical programming language (R Development Core Team, 2012).
A total of 96 cells, each 1° × 1° in size, were categorized as Himalayan cells. Cell-wise babbler species richness appeared to decline with distance from the Eastern Himalaya (Fig. 1a & b). The Himalayan babbler assemblages are also nested, with any assemblage tending to be, on average, a subset of the assemblage immediately to its east (Fig. 2a; NODF for babblers = 68.04). The data (babbler species richness), and the residuals of the OLS models (of species richness with distance from the Eastern Himalaya) showed spatial autocorrelation at varying distances (Fig. 3). We therefore used GLS models to with a linear spatial autocorrelation structure to examine the relationship between species richness and distance from the Eastern Himalaya (GLS for babbler species richness: t1,94 = −4.88, P < 0.01). When we examined the assemblages in each cell, we found that with increasing distance from the Eastern Himalaya, assemblages appeared to be, on average, composed of more altitudinally mobile species for babblers. (Fig. 4a & b). Once again, because of spatial structure in the data, we used GLS with a linear spatial autocorrelation structure to examine the significance of these patterns (for babbler altitudinal movement, t1,94 = 4.09, P < 0.01). We also tested whether babbler assemblages were, on average, composed of larger species with increasing distance from the Eastern Himalaya using the same analytical framework, but failed to find a significant effect of distance on the average size of the babbler assemblage.
Himalayan murid assemblages are nested, with the western assemblages being subsets of the eastern ones (Fig 2b, NODF for murids = 63.58). Since the data and OLS residuals showed spatial autocorrelation at varying distances (Fig. 3), we used GLS models with a linear spatial autocorrelation structure to examine the relationship between species richness, body size and distance from Eastern Himalaya. Species richness for murids decreased with distance from Eastern Himalaya, though the trend was not significant at this scale (t1,94 = −1.90, P > 0.01). However, the trend of declining species richness of rodents with distance from the Eastern Himalaya was significant at the 2° latitude × 2° longitude scale (t1,31 = −3.69, P < 0.01). Average body size of the cells increased with distance to Eastern Himalaya (Fig. 4c & d; t1,94 = 3.00, P < 0.01).
As with other taxa, babbler and murid species richness in the Himalayan range declines from east to west (Fig. 1). Although the Himalaya span roughly 5° in latitude (Price et al., 2003), and the eastern areas are more southerly (and therefore closer to the tropics) than the west, the rate of decline in species richness is much greater than that expected by latitude alone (Blackburn & Gaston, 2006). More importantly, species richness also shows a nested pattern along an east–west gradient.
The progressive nestedness of Western Himalayan assemblages within more eastern ones (Fig. 2) is indicative of certain underlying processes having shaped present-day species richness patterns. Firstly, babbler and murid assemblages in the Himalaya are highly likely to have a common biogeographic origin (Patterson & Brown, 1991), and might have migrated into the Himalaya from Southeast Asia, as some warbler groups appear to have done (Johansson et al., 2007). This is consistent with the hypothesis that Southeast Asia/Eastern Himalaya is the source of babbler and many murid species, from where they might have subsequently dispersed towards the west. Second, as is clear from distribution patterns of the species we focus on, they share relatively similar habitats (i.e. various types of forests). Taken together, these conditions mean that Himalayan babbler and murid assemblages are drawn from a common species pool, that has been subject to further modification along a distance gradient by differential colonization or extinction (Patterson & Brown, 1991). Differences between species in extent of colonization or extinction probability are very likely dependent on species ecology. Therefore, we suggest that fluctuations in past climate acted along with filters imposed by species ecology to determine present patterns of species richness in the Himalaya.
We hypothesize that at some point in historical time, the Eastern Himalaya was species-rich and the Western Himalaya faunally depauperate, arising from differential climate in the east and west resulting in differential availability of suitable habitats in these two areas. During periods of glacial maxima in the Pleistocene, most of the entire Himalayan range was covered with polar/alpine desert, or was under ice (Adams & Faure, 1997; Field et al., 2007; Petraglia et al., 2009). The Eastern Himalaya, however, was vegetated, probably with warm temperate evergreen forest (Adams & Faure, 1997). The Eastern Himalaya, therefore, would have been a suitable refugium for species during glacial maxima, when the rest of the Himalaya would have been inhospitable for most species. Similar Pleistocene refugia have been described from other parts of the world for a diverse range of taxa (Ursenbacher et al., 2006; Anthony et al., 2007; Schmitt, 2007; Wiseley et al., 2008).
Following glacial retreat, and the return of suitable habitats in the Western Himalaya, species would be expected to disperse westward from the glacial refugium in the east. Dispersal ability is likely to be tightly linked to life history traits of organisms. Towards the west, the Himalaya is more seasonal than in the east, with wider fluctuations in temperature, and also faces colder winters (Price et al., 2011). Under such conditions, animal species might need to adapt to wide variations in annual temperature, through behaviour (altitudinal migration) or physiology (body mass). The concordant importance of climate tracking and physiological constraints in determining differential extents of colonization along the Himalayas has already been recognized for bird species (Price et al., 2011). We found that babbler assemblages are, on average, composed of more altitudinally mobile species in an increasing westward direction (Figs. 4a & b). This lends supports to the idea that from the potential source, i.e. the Eastern Himalaya, the distance of dispersal to the west might depend on how well a species is adapted to seasonal variation.
In mammals, larger body sizes allows better cold tolerance (Blackburn et al., 1999). As predicted, we found that murid assemblages increased in their mean body size with increasing distances from Eastern Himalaya (Fig. 4b). This not only suggests that they are able to survive in the harsher conditions of Western Himalaya, but also possibly that larger body size allows for greater dispersal ability along the east–west gradient. In contrast, we were unable to detect any relationship between the average body size of the babbler assemblage and increasing distance from the Eastern Himalaya. Although both babblers and rodents are endotherms, and should be expected to show similar patterns in terms of body size, we hypothesize that birds, because of their greater mobility, respond to increased seasonality and colder temperatures in the west with altitudinal migration, rather than an increase in body size. In other words, even smaller babblers can potentially ‘escape’ the greater cold in the Western Himalaya through movement, instead of increasing in body size to reduce heat loss.
Our results are also consistent with an alternative hypothesis that patterns of species richness in the Himalaya result from differential extinction of species in the east and west, with a gradient of increasing extinction from east to west. Species that currently occur in the west might be those capable of dealing with more extreme seasonal climatic conditions through elevational movement or greater body size. Under this scenario, genetic data from species along the Himalayan range would indicate that species in the east and west are equally old in ancestry (unlike in the colonization scenario, where western populations would be progressively younger). We consider this scenario equally plausible in generating the patterns we report, but unlikely given that the Western Himalaya was likely to have been glaciated during periods of maximum glaciation, whereas the east still had suitable habitats (Adams & Faure, 1997), and evidence of westward immigration into the Himalaya for other taxa (e.g. Johansson et al., 2007).
Taken together, our results on species richness gradients and published paleoclimatic models (Adams & Faure, 1997; Field et al., 2007; Petraglia et al., 2009) suggest a relatively recent colonization of the Western Himalaya. This provides, for the first time, a testable and potentially generalizable phylogeographic paradigm for this region. In the future, genetic data from a variety of species could be used to test whether (1) for species present in both the Eastern and Western Himalaya, populations in the Western Himalaya are relatively younger than those in the Eastern Himalaya, (2) for species present only in the Western Himalaya, whether these have evolved relatively recently, following the last glacial retreat, and (3) for species present only in the Eastern Himalaya, whether the life history strategies are such that their chances of dispersal are low. This paradigm, though based on patterns of birds and mammals, is likely to be generalizable to amphibians, and other ectothermic species as well; the trends for these groups will probably be stronger as they are ectothermic species, with lower dispersal abilities.
Additionally, our results are consistent with and support the results of Taberlet et al. (1998), Araujo et al. (2008), Flojgaard et al. (2011) and Dobrovolski et al. (2012), among others. Thus, at least for areas impacted by glaciers historically, selective colonization and extinctions (historical factors along with ecological traits) seem to be determining current species distribution patterns. Our findings are also support the Tropical Conservatism Hypothesis, which states that climate interacts with evolutionary traits and the tropical origins of biota to determine species richness patterns (Wiens & Donoghue, 2004).
More generally, we hypothesize that energy, climate and productivity might influence whether an area is able to act as a refugium, and that globally, observed productivity/energy-species richness correlations at continental or subcontinental scales for regions with glacial history might simply reflect differential recolonizations from refugia (see Adams & Faure, 1997 for a map of paleovegetation). Thus, in the Himalaya, and elsewhere, patterns of species richness that correlate with energy or productivity (e.g. Ding et al., 2006) might be historical in origin, with the most productive areas being refugia from which species colonized other areas following the return of suitable conditions or habitats during inter-glacial periods.
Globally, several species rich areas are also likely to be past refugia during glaciations, including in Asia, North and South America and Africa (Adams & Faure, 1997), and species richness declines with distance from these refugia (e.g. amphibian and reptile richness in Florida; Currie, 1991). Glacial retreat might result in increased species numbers from dispersal of species from refugia to new habitats and subsequent speciation in these ‘sink’ areas (see Taberlet et al., 1998; Hortal et al., 2011). Subsequent glaciation might result in higher species richness in the refugium as these ‘new’ species retreat to the refugial area (Hortal et al., 2011; Sandel et al., 2011). Repeated cycles of glaciation and retreat might conceivably result in a large number of species from such a mechanism, as species ranges contract and expand, and speciation occurs in areas that are distant from the refugial ‘source’. Such processes can be multi-layered, and we caution against assuming simplistic single causal explanations for richness patterns even in regions with strong glacial history. Other factors (such as niche conservatism and area; see Rosenzweig, 1995; Wiens & Donoghue, 2004) might influence long-term patterns of species richness, creating a template that is then modified by shorter-term historical factors (such as post-glacial colonization) to influence diversity patterns.
Finally, our results also indicate the importance of the Eastern Himalaya as a reservoir of diversity across time. The Eastern Himalaya is currently classified as a global biodiversity hotspot. However, our results highlight the importance of this region through evolutionary time. Additionally, given that the Eastern Himalaya was potentially a refugium during the last glacial maxima, we might expect it to be more severely impacted by current global warming. The species present in this region might be particularly vulnerable to the impacts of climatic warming. Past, present and future considerations therefore point to the Eastern Himalaya as a significant site with immense potential as a biodiversity hotpot through long time-scales.
We thank S. Vaidyanathan for help with the mammal distribution ranges. U.R. and K.T. were supported by the Ramanujan fellowship. K.T. also acknowledges support through a CSIR-SRF fellowship. U.R. and K.T. acknowledge support from the Department of Biotechnology grant to study biodiversity in the Eastern Himalaya.
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Umesh Srinivasan is interested in community ecology and conservation biology. He is currently investigating demographic and community-level response of birds in the Eastern Himalaya to human-induced habitat modification.
Krishnapriya Tamma is interested in spatial and temporal variation in rodent communities in the Indian subcontinent. She is currently trying to understand biogeographic patterns for small mammals in the Indian subcontinent.
Uma Ramakrishnan is interested in molecular ecology, phylogeography and conservation genetics of mammals and birds. She uses genetic data to understand demographic responses of populations to climatic change in the past, with a view to predict species' response to future changes.