The species richness pattern
Within few decades, awareness of species richness patterns is increased. Spatial, temporal, and organizational aspects are considered for species richness (Levin, 1992). The factors of delimiting grades of species richness along elevational gradient have been a provocative issue in ecological research. Few studies suggested that the greater species richness seems at the mid-elevational regions (Sanders, 2002; McCain, 2004), and some indicated the decreases species richness monotonically with declining elevation (Stevens, 1992; Ohsawa, 1995). Several factors influenced relationship between species richness and elevations. The area for growth is one of the most influential factors for determining species richness. As species richness increases as increase in the functional area (He et al., 1996; Rahbek, 1997). Generally, heterogeneity and diversity increase as the area increases as compare to smaller area, which facilitates species for their coexistence.
Grain size (sample size or individual quadrat size), the extent of size (geographical distance covered by the study), and several numbers samples units or intensity) are three components that determine the spatial scales of species richness (Palmer, 1994). A strong relationship was established between grain size and study area by several studies (Baniya et al., 2010; Bhattarai & Vetaas, 2003; Mittelbach et al., 2007). Greater grain size is used to detect variations at a regional scale and small grain size is used to detect variations at a local scale. Species richness pattern is also governed by plant succession, development of community, and tenacity of species (Levin, 1992).
Several studies advocate that the MDE, or linear constraints, is effectively explaining elevational species richness pattern (Kluge et al., 2006; McCain, 2009). A review (Rahbek, 2005) showed that about half of the studies showed unimodal hump-shaped richness patterns, whereas about one fourth showed monotonic declines. The present study also confirmed the presence of 57% of studies showed hump-shaped species richness patterns and 26% monotonic decline. Remaining 17% of studies showed either monotonic incline (6%) or reverse hump shape (3%) or without any distinct patterns (8.5%). The hump-shaped curves may reach 80% when incompletely sampled gradients, with unstandardized sampling plots, and differences in regional areal size were taken (McCain, 2009). This study revealed the hump shaped nature of species richness pattern along elevational gradient largely depends on the midpoint of study area and elevational range. Greater the elevation range and higher midpoint favors the hump shaped pattern of species richness.
Most studies were found concentrated in both hemispheres at latitudes between 20°N and 40°N (Alps, Himalayas, Rocky Mountains). As compare to Northern Hemisphere (0°-65°), Southern hemisphere has shorter latitudinal extent (0°-40°). Despite taller mountain ranges and wider latitudinal extent in the Northern Hemisphere, both Hemisphere shows similar patterns of species richness. The most observed pattern of species richness is hump-shaped is not usual for all taxonomic groups in all elevation ranges and areas. The reason behind the hump-shaped species richness pattern is a higher diversity per area in mountains compared to flatland. At higher altitudes, the area of habitat decreases (Körner, 2000) consequently falling the possibility of exitance of species richness. Quicker climatic gradients at higher latitude (harsher condition like cold temperate) may limit elevational bulges in diversity. Anthropogenic activities also abridged species diversity mainly at lower altitudes and in flatland (Nogués-Bravo et al., 2008, Chawla et al., 2008). The overlapping spreading rate species from lower and higher altitude, consequential in higher populations at mid-elevations with lower on extremes (Kessler, 2009).
Peaks of species diversity
In most of the studies, species richness gets higher point at a specific middle elevation; which may depend on the geographical locations or aspect on the mountain. Hump shape may be unimodal, bimodal or multimodal in several studies for different taxa. Sometimes monotonic decline or incline or reverse hump shape or even none specific patterns of specie richness were also reported in several studies. Dispersal restriction (MDE) is usually considered as the main cause of hump shaped distribution pattern of species richness and the monotonic decrease may be due to reduction in the potential area with increasing elevation (Grytnes, 2003; Theurillat et al., 2011). Maximum precipitation at the mid-altitudes (Barry, 1992; McCain, 2007) provides best conditions for heat and rainfall for plant development, and creating niches for coexistence of several species (McCain, 2007).
The increasingly harsher conditions toward upper elevation leads to decreasing species diversity which is supported by less available area, lower productivity and nested distribution of species. Another possible explanation could be the difference in soil temperature in alpine regions with decreasing numbers of pathogens, pollinators, herbivores and agents of seed dispersal by slowing metabolism and the rate of decomposition and formation of species’ fundamental niches (Waldock et al., 2018).
The studies in several elevational ranges produce hump shaped pattern of specie richness with dispersed peaks of maximum diversity. Lazarina et al. (2019) found diversity peaked for vascular plants at mid elevations (800-1200) in the Crete, Greece as did Yang et al. (2014), Lee & Chun (2016) in Baekdudaegan Mountains of South Korea, Saikia et al (2017) in Eastern Himalayas, Aureo et al. (2021) in Balinsasayao National Park plants Philippines, and Grytnes & Beaman (2006) for vascular epiphytes in Mount Kinabalu, Borneo. The vascular plants also showed peak of diversity differently in different geographical ranges as 1500-2500 m asl (for non-endemic vascular plants), 3600-4200 (for endemic vascular plants) (Grytnes & Vetaas 2002), and 2000-2500 (Lai & Feng 2019) in Nepal Himalayas. Western Himalayas showed different peak of diversity at 1500-3500 (Oommen & Shanker 2005), 2500-3000 Chawla et al. (2008) and 3500 (Thakur & Chawla 2019) in independent studies. Highest diversity was reported at 1600-2200 m asl in Eastern Himalayas of China (Sun et al 2021), and 1900-2800 in Eastern Himalayas of India (Manish et al 2017).
Pteridophytes, Bryophytes and Lichen also showed humps shaped species richness pattern along elevation. Pteridophytes showed maximum diversity peak at 900-1000 in Mount Kinabalu, Borneo (Grytnes & Beaman 2006) and Costa Rica (Watkins et al. 2006), 1500-2100 in Mexico (Hernández-Rojas et al. 2018; Kessler et al. 2011), 1800-2100 in Andes (Kessler 2001), Costa Rica (Syfert et al. 2018), Mt. Kilimanjaro (Hemp 2002), and Andean tropical forests (Salazar et al. 2013). Bryophytes showed maximum diversity peak at 100-300 in Scotland (Virtanen & Crawley 2010), 500-700 in Australia (Sanger & Kirkpatrik 2015), 1500-2500 in Nepal Himalayas (Grau et al 2007), and 1150-1350 in Madagascar (Ah-Peng et al. 2012). Lichen showed different peaks of diversity ranging from 400 (Bruun et al. 2006, Henrik et al. 2006) in European country, 2800-2900 (Rai et al. 2015) in Western Himalayas and 5500m asl (Baniya et al 2010) in Nepal. Highest species diversity was reported in higher altitude by Baniya et al (2012) at 5350 m asl for vascular plants and 5400 m asl for cryptogams in Buddha Mountains, Central Tibet.
The hump-shaped species richness in eastern Himalayas has an cumulative drift up to 1500 m, although, decreased additional up to 3800 m asl (Acharya et al., 2011). With a peak of species richness at a specific elevation, according to Colwell & Hurtt (1994) and Colwell et al. (2004), allows an estimate of the diversity based on elevation (Kluge et al. 2006).
Sang (2009) found peak of species diversity of vascular plant in the mainland part of northwestern China at around 1500 m asl, where the grassland turns into the forest, and a another peak at 2700-3300 m asl in alpine meadows above the tree line. Klimes (2003) reported species richness of vascular plant on northwest Himalaya between 4180 - 6622 m asl, with the maximum peak of plant species richness between 4500 - 4750 m asl.
Several studies also focused on vascular plant growth forms as well. They found different diversity peaks for trees (1900-2800), shrubs (2600-3600) and herbs (2600-3500) and 500-1800 (trees), 2050-2150 (shrubs and herbs) in Eastern Himalayas (Manish et al. 2017).
Some studies have discovered two or more peaks of diversity along an elevation gradient. Two peak of diversity (0-500 and 2500-3000) for vascular plants in Peru (Werff et al. 2004), 600-1000 and 160-1800 for tree species in Eastern Himalayas (Behera & Kushwaha 2007), 500-700 and 900-1100 for angiosperms in Australian subtropical forest (Sanger & Kirkpatrik 2015), 750-1500 and 2500-3500 for vascular and understory plants in Nepal Himalayas (Carpentor 2005), 1300-1400 and 1900-2000 for vascular epiphytes Mt. Kalatungan, Philippines (Betanio & Buenavista 2018). Multiple diversity peaks were also noticed by Li & Feng (2015) for vascular plants at genus level in Nepal.
Reversed hump shape species diversity was also observed for angiosperms in Eastern Himalayas (Pandey & Rai, 2018), Egypt (Coals et al. 2018), Slovakia (Hrivnak et al. 2014) and Korea (Park et al. 2020).
However, few studies of monotonic decline pattern of species richness were also reported. The greater tree species richness at lower altitude in temperate forest (1850-2800 m asl) of western Himalaya (Sharma et al., 2018), decline up to 4000 m altitude in the central Himalaya (Bhattarai & Vetaas, 2006), decrease from 700-1100 m Mindoro Island, Philippines (Villanueva & Inocencio 2018). The vascular plant species richness does not change up to 900-1000 m asl, at which it then starts to decrease (Odland & Birks 1999) in western Norway. Monotonic decline in species diversity was noticed from the elevational range of 600-800 m in South Eastern Brazil for vascular plants (Pinto-Junior et al. 2020), Norway and Finland for moss (Bruun et al. 2006), Panama for vascular epiphytes (Ortiz et al. 2019), Mauritius for bryophytes (Al-Peng et al. 2007) and 1500 m for bryophytes in eastern Nepal (Bhattarai & Vetaas 2003).
No distinct patterns of species diversity were reported in studies on angiosperms (Doran et al. 2003, Erschbamer et al. 2006, Lovett et al. 2006, Zhang et al. 2008, Sharma et al. 2009, Kromer et al. 2013, Kluge et al. 2017, Abutaha et al. 2018, Mujawamariya et al., 2018), Pteridophytes (Kluge et al. 2006), Bryophytes (Bergamini et al. 2001), Lichen (Nanda et al. 2021) and Fungi (Rojas & Stephenson 2008, Fisher & Fule 2004) in different geographical ranges.
Constrains for species richness
The disparity in species richness along elevational and environmental gradient may fluctuate according to plant life forms due to different topological, physiographic and climatic variation of the regions. Productivity is recurrently cited as one vital factor of species richness (Chalcraft, Williams, Smith, & Willig, 2004; Loreau et al., 2001). Waide et al. (1999) revised the relationship between productivity–species richness and recognized four types of association: positive, negative, unimodal, and no relationship.
Several studies suggest that only a single variable may not sufficient for the explanation of patterns of species richness along elevation gradients among taxonomic groups of organisms (Lee et al., 2012; Wang et al., 2007; Watkins et al., 2006). Although grain area, climatic variables, MDE, and productivity are commonly painstaking in studies to determine the species richness in mountain regions.
Climate, topography, and disturbance are noteworthy aspects of species richness in the western Himalayas (Panda, Behera, Roy, & Biradar, 2017). The climatic variables like temperature and precipitation, may interlinked; both spatially and temporally. Other variables might also become overlapped with the effects of anthropogenic attributes (Thuiller, 2007), or nitrogen emission (Sala et al., 2000).
However, a negative connection between environmental heterogeneity and species richness has also been described (Douda et al., 2014; Lundholm, 2009). The accidental process may control species coexistence that weakens the implication of niche segmentation and ecological heterogeneity (Hubbell, 2005).
Nonlinear association with temperature and area in tropical species were observed by Manish et al. (2017) in Eastern Himalaya. The influences of geographic area, land use pattern, soil types, topography, slope, aspects, precipitation, temperature, evapotranspiration, solar energy, soil water and disturbance predominate were emphasized to associate species richness. The ecotone effect maximizes vascular plant richness (Behera & Kushwaha, 2007; Grytnes, Heegaard, & Ihlen, 2006; Oommen & Shanker, 2005) in eastern Himalayas as well. The relationship between species and area are closely linked and greatly influenced by anthropogenic activities in mountain environments. The topological condition has an influential part in defining the land use inconsistent pattern by the resident communities at lower altitudes in eastern Himalayan region of Sikkim (Kanade & John, 2018; Tambe, Arrawatia, & Sharma, 2011).
The location of climate zones has been substantial effort to envisage the forthcoming dispersal of species and ecosystems (Jensen et al., 2008; Parmesan, 2006). Sang (2009) considered, in a continental water availability may be pertinent at lower altitudes, and the temperature at higher altitudes. The moistness gradient displays a decrease in humidity from east to the west direction on its longitude (Carpenter, 2005; Behera & Roy, 2019). This study agreed that ecological variables should be scrutinized synchronously to establish the species distribution along elevational gradients with other environmental patterns (Lomolino, 2001).
Future prospective of gradient analysis
A large number of underlying causes of elevational diversity gradient were proposed in several studies of different taxa. Among these studies, no result was found to be decisive for determining elevational gradient in different species. No single mechanism was found responsible for all elevational gradient in both hemisphere and all elevational ranges. For future studies, we should emphasize to delimit the underlying causes with different levels of taxa and their habitat ecology. We need several studied which will play key role in shaping the proper mechanism of variation of species along elevational gradient.