INTRODUCTION
Understanding the variation in species composition of ecological
communities, commonly known as β-diversity, is a major challenge
confronting ecologists (Sepkoski 1988; Tuomisto & Ruokolainen 2006;
Anderson et al. 2011). Such researches can help to reduce the constant
threat to biodiversity loss by contributing to the question of what
factors control the distributions and abundance of organisms (Socolar et
al. 2016). However, despite substantial efforts to disentangle the
drivers of community assemblages (Legendre et al. 2005; Ferrier et al.
2007; Tuomisto 2010), scientists begin to realize that different
ecological processes may result in very similar patterns of species
composition (Myers et al. 2013).
The realization that the ecological processes are scale-dependent has
assisted in uncovering essential features of the community assembly
(McGill 2000; Barton et al. 2013; Chase et al. 2018). Processes of
community assemblage are assumed to work as constraints with potentially
varying strengths at different scales, which hierarchically determine
the local community structures (Whittaker et al. 2001; Ricklefs & He
2016). For example, macroclimate (i.e. the overall climate of a large
geographical area) is effective at large scales, while microclimate
(i.e. the essentially uniform local climate of a small site or habitat)
at small scales, (McGill 2000), and biotic interactions within
neighborhoods are important at small scales (Huston 1999). Community
structures are thus the result of hierarchical constraints at multiple
scales (Scherrer et al. 2019).
Among the various ecological processes, environmental filtering and
within-habitat spatial aggregation are considered to be the two major
mechanisms invoking the spatial variations in species compositions
(Plotkin et al. 2000; Seidler & Plotkin 2006; Laliberté et al. 2014;
Kraft et al. 2015). Environmental filtering processes tend to support
the survival of specific species in some habitats in preference to other
species. Within-habitat spatial aggregations are clustering processes
resulting from dispersal limitations, patchy extinction, or interactions
among species within a given type of habitat (Kretzschmar & Adler 1993;
Öhman et al. 1998; Cornell et al. 2007; Shen et al. 2009). One
hypothesis regarding community assembly suggests that environmental
filtering and spatial aggregation limit the survival and organization of
potentially coexisting species (Myers et al. 2013; Xu et al. 2015).
Specifically, environmental filtering processes progressively select the
species which are best adapted to local conditions from the regional
pool, first by large-scale environmental factors, and subsequently by
fine-scale environmental factors (Diaz & Casanoves 1998; De Bello et
al. 2013). Then spatial aggregation processes ultimately determine the
spatial organization of the selected species (Cornell et al. 2007; Shen
et al. 2009). However, to our knowledge, no research has been done to
quantify the effects of environmental filtering and spatial aggregation
on variations in species compositions across spatial scales.
According to Cornell et al. (2007), specific null models involving
habitat features can effectively separate the effects of environmental
filtering and spatial aggregation processes at a given spatial scale. On
this basis, we divided the study region into different habitats at
different spatial scales i.e. the region-zone, zone-area, area-district,
plot-district and within-plot scales, ranging from the broad to fine, to
examine the ecological processes. Specific null models were then applied
by shuffling individuals among and within the various habitats at each
spatial scale. The effects of environmental filtering which
progressively select species from the species pool at the region to zone
level, zone to area level, area to district level, and district to plot
level, as well as the effects of spatial aggregation which ultimately
shape the spatial organization of the selected species in the observed
communities were hierarchically partitioned, as detailed in Figure 1.
With the exception of measuring the magnitudes of the ecological effects
at each spatial scale, the focus was on the determination of how the
effects of environmental filtering and spatial aggregation on
β-diversity had vary along the latitudes. In addition, potential factors
related to the environmental filtering processes were explored.
According to the division criteria of the vegetation regionalization map
of China (Zhang 2007), at the zone level, the species compositions were
determined to vary mainly due to the changes in heat patterns from south
to north, or along the elevational gradient; at the area level, the
causes were mainly the changes in water-heat patterns with the medium
geomorphic types; at the district level, local geomorphology was found
to be the main reason. We thus related the effects of environmental
filtering to a group of climate and plot variables.
Overall, the following hypotheses are formulated:H1 , the effects of the environmental filtering
processes are stronger at the broader scales due to an increasing
environmental heterogeneity with increasing spatial scale (Leboucher et
al. 2019). H2 , environmental filtering at the
broader scales increases with increasing latitudes where fewer species
are often observed (Hillebrand 2004), which indicates stronger filtering
effects from the regional species pool (Chu et al. 2019). Environmental
filtering at the fine scales decreases with increasing latitude since
the habitat heterogeneity at the lower latitudes is greater, which
supports greater variation in species compositions (Xing & He 2019).H3 , the effect of the spatial aggregation at
the within-plot scale decreases with increasing latitude. This is due to
clustering caused by dispersal limitations and biotic competition which
is assumed to be more dominant in species-rich communities when compared
with species-poor communities (Myers et al. 2013).
In this study, we first quantified the hierarchical effects of
environmental filtering and spatial aggregation on β-diversity patterns
across five spatial scales. We also proposed one hypothesis to examine
the relative importance of environmental filtering across scales, and
two hypotheses to explain the variation of environmental filtering and
spatial aggregation along latitudes at each scale. We showed that the
pattern of β-diversity was mainly dominated by environmental filtering
at the region-zone, district-plot scales, and spatial aggregation at
within-plot scale. The environmental filtering at the broader scales had
stronger effects at higher latitudes, while at the finer scales only at
lower latitudes. The effects of spatial aggregation were more prominent
at lower latitudes.