INTRODUCTION
Describing the pattern and
variations in spatial pattern of biodiversity and revealing its
underlying mechanisms remain a central focus in ecology. With the
popularization of the concept of biodiversity (e.g., species richness),
related research is developing
rapidly
(Wilson & Ehrlich, 1991). But less attention has been paid to the
ultimate cause of species
richness: species range size (e.g., geographic distribution, latitudinal
range, elevational range). The degree of overlap of the range size forms
the species richness pattern we have observed, and the higher the degree
of overlap of species range in a certain area, the higher the species
richness. Moreover, species range size can reflect the niche of species
(Pfenninger, Nowak, & Magnin, 2007). It is also correlated with the
formulation in species protection strategies (Herzog, Oswaldo, Embert,
Caballero, & Quiroga, 2012), especially the conservation of faunas
(Kevin J Gaston, 1996) and the assessment of the probability of
extinction (Price, Helbig, & Richman, 1997). However, the underlying
mechanism shaping species range size is unclear (Kevin J Gaston, 1998;
Webb & Gaston, 2003).
Most studies on species range size focused on examining the positive
relationship between communities range size (species mean range size)
and elevation (or latitude), which is called Rapoport’s
rule (Rapoport, 1975, 1982;
Stevens, 1989, 1992; Böhm et al.,
2017; Colwell & Hurtt, 1994; Kevin J Gaston, 1999; K J Gaston,
Blackburn, & Spicer, 1998; Hausdorf, 2006; Mccain & Knight, 2013;
Rohde, Heap, & Heap, 1993; Zhou et al., 2019). However, few studies
have explored the drivers of species range size (but see J.-Y. Kim et
al., 2019; Letcher & Harvey, 1994; Luo et al., 2011; Stevens, 1989,
1992). Species range size could be related to different factors such as
climate, ambient energy, and habitat heterogeneity which referred to the
climate variability hypothesis, the ambient energy hypothesis, and the
habitat heterogeneity hypothesis. The climate variability hypothesis
states that species living in areas with larger variations in climatic
conditions (e.g., temperature) tend to have wider tolerances; that is,
climate stability is negatively correlated with species range size
(Stevens, 1989, 1992). The
ambient
energy hypothesis predicts that environmental energy (e.g.,
productivity) determines the degree of overlap of species range size
(Kerr & Packer, 1997). For example, species’ interaction is more
frequent in the tropical regions, as the tropical regions receive more
environmental energy than the temperate regions (environmental energy is
negatively related to latitude), which can support finer niche
separation and the coexist of more species, leading to smaller species
range size (Brown, 1981). The
habitat
heterogeneity hypothesis, which is an extension or supplement of the
ambient energy hypothesis, states that the greater heterogeneity of the
spatial and topographical habitat structure allows for finer subdivision
of restricted energy, suggesting that the higher the habitat
heterogeneity, the smaller the species
range size could be (species range
size, to some extent, is a surrogate of species niche) (J.-Y. Kim et
al., 2019). However, the influence of habitat heterogeneity on the
species range size may be closely related to the combination of habitat
types (Hu et al., 2017). The fit of these hypotheses above to different
taxonomic groups and gradients is still uncertain and needs to be
further tested in more biogeographical regions.
Local species richness can be
strongly impacted by the proximity of the species’ range margins of
potentially interacting species. The higher the degree of the proximity
of the species’ range margins of potentially interacting species (which
can be defined as inflow intensity, J.-Y. Kim et al., 2019) can increase
the species richness. This phenomenon was called rescue effect (Stevens,
1992). Moreover, different shapes of richness patterns (e.g., monotone
decreasing and unimodal pattern) may be influenced by kinds of rescue
effects (i.e., Steven’s rescue effect, alternative rescue effect, and
non-directional rescue effect). The premise of Steven’s rescue effect
(or Rapoport’s rescue effect) is that if species in high elevation (or
latitude) region have a larger range size (the distribution of species
range size follows the Rapoport’s rule), species overlap in low
elevation (or latitude) region will increases (species from higher
elevation or latitude tend to inflow to low elevation or latitude)
(Stevens, 1992). While alternative
rescue effect proposes that decreasing species richness along elevation
or latitude could result from differential upper limits of species with
source populations below mid-point of the gradient even if the species
have a larger range size in the low elevation or latitude (Almeida-Neto,
Machado, Pinto-da-Rocha, & Giaretta, 2006). Besides, non-directional
rescue effect (Almeida-Neto et al., 2006) argued that species from both
ends of a gradient (if species from both high and low elevation have
larger range sizes) would lead to an increase of species richness in the
middle elevation and resulting in a unimodal species richness pattern.
Mountain ecosystems are hot spots in biological diversity (Lomolino,
2001). The environmental conditions (e.g., climate, habitat, and
productivity) vary remarkably along elevations over small spatial
extents, making elevational gradient an outstanding natural laboratory
for studies in ecology and biogeography. Here, we used a comprehensive
dataset of breeding birds collected from 2018 to 2019 along an
elevational gradient in the Eastern Himalayas of China. Our objectives
were to a) assess how species mean
elevational range size changes along elevation; b) explore the drivers
influencing species mean elevational range size; c) explain the
relationship between species range size and species richness (rescue
effect) to expand our understanding of underlying mechanisms of
community structure and species richness pattern.