Introduction
The species range size is a fundamental unit in macroecology (Böhm et
al., 2017). Understanding variation of species range size along spatial
gradients is of primary importance in study of climate change,
biodiversity pattern, gene flow and extinct mechanism. A well-known
theory about spatial variation in species range size is the Rapoport’s
rule, which proposed that species range size correlates positively with
latitude and elevation (Stevens, 1989). However, despite of multiple
early evidences from the Northern Hemisphere (e.g., Letcher & Harvey,
1994; Blackburn & Gaston, 1996; Arita et al., 2005), further study from
other regions have found complex and partial support for the rule (e.g.,
Hawkins et al., 2006; Whitton et al., 2011; Feng et al., 2016),
suggesting that the rule might be a regional phenomenon hinge on local
environment (Whitton et al., 2011). Therefore, recent attention has
shifted from simply documenting variation in range size to exploring
drivers for the variation.
Various studies have been conducted to understand the association
between variation in range size and environment factors such as climate
(e.g., Whitton et al., 2011; Sheldon & Tewksbury, 2014), disturbance
(e.g., Lozada et al., 2008; Borkowski et al., 2016), competition (e.g.,
Glazier & Eckert, 2002; Grigione et al., 2010), and the mid-domain
effect (e.g., Luo et al., 2011). Climate factor is the most widely
supported driver in terms of both latitudinal and elevational variation
in range size. Several hypotheses have been proposed to explain the
climate-range size relationship, among which the climate variability
hypothesis is most commonly accepted (Whitton et al., 2011; Pintor et
al., 2015). This hypothesis was first proposed by Stevens at 1989, and
was believed to be the underlying mechanism of Rapoport’s rule (Stevens,
1989,1992). Stevens stated that climate specifically temperature is more
variable at higher latitudes and elevations. Such greater climate
variability favor species with wider tolerances and larger ranges, and
thus leading to a positive relationship between range size and latitude
and elevation. The mean climate condition hypothesis is another
prominent explanation for climate-range size relationship, which are
supposed to cooperate with climate variability to generate increasing
trends of range size along latitude and elevation (Luo et al., 2011).
The mean climate condition hypothesis proposes that species living at
higher latitude or elevation are not only subject to greater climate
variation but also lower mean climate condition, and thus tend to be
geographically widely distributed (Luo et al., 2011; Jiang & Ma, 2014).
Besides contemporary climate, the historical climate such as Quaternary
climate has also been proposed as explanation for range size variations
based on the premise that historical climate oscillations select for
species with wider physiological tolerance and adaption (Jansson, 2003;
Araújo et al., 2008).
Apart from climate factors, disturbance and competition are also
considered to have impacts on species range size. The disturbance
hypothesis proposes that anthropogenic threats might constrains species
distribution due to the consequent population declines and extinctions
(Whitton et al., 2011). The competition hypothesis proposes that species
in rich communities would face intense competitive pressure which might
limit their range size (Stevens, 1996; Gaston et al., 1998). In
addition, patterns of range size variation might be subjected to the
mid-domain effect (MDE), as large range species necessarily overlap in
domain center due to the limit of geometric constraints on species
geographical ranges, leading to a mid-domain peak in species range size
regardless of ecological factors (Colwell et al., 2004; Moreno et al.,
2008).
In addition to environment factors, variations in species range size
might also be associated with life-form and biogeographical affinities.
This is because, species’ life form and biogeographical affinities
reflect their ecophysiological traits and evolutionary history, and
hence affect their response to environmental variation. For example,
compared with herbaceous plants, woody plants tend to have narrower
adaptability for their longer reproductive cycles and slower
accumulation rate of genetic changes (Smith & Beaulieu, 2009), and thus
might more sensitive to the environmental gradient. Similarly, tropical
taxa, which experienced more stable climatic environment in their
evolutionary history, may be hence more susceptible to climate variation
and were prone to increase their range size to adapt to increasing
latitude and elevation (McCain, 2009). However, little work has been
done to examine variations in species range size in terms of the
influence of life-form and biogeographical affinities (but see Fend et
al., 2016 and Zhou et al., 2019).
As one of the world’s 34 biodiversity hotspots, the Himalayas contain a
diverse range of eco-climate zones, and have been receiving much
attention from various ecological and biogeographical studies.
Especially in the central Himalayas, where the towering mountains block
the moisture from the Indian Ocean, a series of north-south valleys
contain rich biodiversity and conspicuous elevational environmental
gradient in small spatial scale, making them ideal place for uncovering
the underlying mechanisms of the spatial variation in species range size
and examining the validity of Rapoport’s rule. However, while a number
of studies exploring the elevational variation and its drivers for
species richness in the Himalayas (e.g., Acharya et al., 2011; Manish et
al., 2017; Kluge et al., 2017; Yang et al., 2018; Sun et al., 2020),
corresponding studies for species range size are limited. Since
understanding range size variations are a prelude to effective
biodiversity conservation (Luo et al., 2011), filling up such a research
gap will not only help address the theoretical issue but also contribute
to the conservation practice in this high-profile region.
Since long, vascular plants have been considered as an excellent matter
of study for the spatial variation in range size, because of their wide
distribution and easy observation. In this study, we aim to examine
elevational variations in vascular plants range size for different life
form and biogeographical affinities, and to explore the role of climate,
disturbance, competition and the mid-domain effect on above variations,
based on a detailed field survey in the Gyirong Valley, the longest
valley in China’s central Himalayas. Since species range size is
considered to be closely associated with species richness (Stevens,
1992), and climate has been found to be the primary determinant for
species richness in the Himalayas (Bhattarai and Vetaas 2003; Manish et
al., 2017; Sun et al., 2020; Liang et al., 2020), we expect that climate
factors are also played a greater role than other factors in explaining
the elevational variations in vascular plants range size in the Gyirong
Valley. If that is the case, considering Rapoport’s rule gets supported
in regions where the influence of climate is most pronounced (Pintor et
al., 2015), we also expect the vascular plants range size increases with
elevation as the rule predicted, especially for woody and tropical
species which are supposed to be more sensitive to climate variation.