Discussion
Our results reveal that changes in the composition of insect herbivore
communities track changes in plant species community composition and
plant phylogenetic turnover at multiple spatial scales within the Yunnan
province. Three lines of evidence suggest that the positive association
of plant and insect turnover partly results from insect host
specialization. First, from Fig. 2-I we can see that both plant and
insect communities exhibit high levels of β-diversity across remarkably
short spatial scales (i.e., β1: 40-100 m). This is not a
pattern that is expected if compositional co-variation of these groups
results from shared bio-geographical histories or parallel responses to
climatic gradients, but instead points to host specificity as the
underlying process, suggesting this is the scale where biotic
interaction plays an important role in the community assemblage through
ecological processes (also see Chesson, 2000). Second, we can see that
pure plant species composition as a general control on wood boring
longhorn beetle community composition accounted for 10% percent of the
explained variation (Fig. 4-II). This explanation rate means that after
removing the effect of geographic distance and environment, the insects
and plants are still positively correlated, indicating that host plant
specificity of Cerambycidae is one of the driving factors of the
presented pattern. Finally, from Table 2 we can see that beetles
standardized Shanoon diversity is significantly positively correlated
with standardized plant Chao1 and Shannon diversity.
To further confirm the relationship between plants and insects, it is
important to improve the estimation of host specificity and species
richness through correction for plant phylogenetic relatedness (Ødegaardet al. , 2005). Our study demonstrates that the phylogenetic
diversity of plant communities had a strong positive influence on the
species richness of Cerambycidae communities, and the pure effect of
this on the Cerambycidae distribution was 6% percent (Fig. 4-I). The
strong influence of plant phylogenetic composition on insect community
composition suggests that insects tend to either be specialized on plant
hosts at a supra-specific level (Novotny et al. , 2002), or that
insects are selecting host plants on the basis of phylogenetically
conserved traits (Ødegaard, 2006; Whitfield et al. , 2012). There
is growing appreciation that long-term co-evolutionary interaction
across trophic levels plays a major role in driving the composition and
structure of communities (Futuyma & Agrawal 2009; Dinnage et
al. , 2012). Evidence of these reciprocal evolutionary pathways (Wheatet al. , 2007; Fordyce 2010; Wagner 2000) has supported the view
that phylogenetic information is embedded in the way plant–insect
interactions shape community structure and evolution (Agrawal, 2007).
Our study provides the first evidence in a natural system that plant
phylogenetic β-diversity is highly correlated with Cerambycidae
β-diversity, suggesting that the turnover in host-plant lineage
structures Cerambycidae assembly in space.
In addition to the plant species composition and phylogeny determining
the community assemblage of wood boring longhorn beetles, the
environment also played an important role in influencing wood boring
longhorn beetle community assemblage. From Fig. 2-I we can see that the
relationship between plant and insect β-diversity holds at both regional
(i.e., δ1: 250-300 km, δ2: 500 km) and
local (i.e., β1: 40-100 m) scales. The local scale
co-occurrence pattern is not likely resulted from the environmental
gradients cause it varied slightly, however, with the increase of the
spatial extent, the matched pattern gradually disappeared but recurrent
at macro-scale, which lurking with the mechanism that the effects of
biotic interaction is gradually averaged out with the scale aggregation,
however, the effect of macroclimatic gradient become more and more
obvious, which revealed that the highly associated pattern at
coarse-scale might be the result of parallel responses of insects and
plants to macroclimatic gradients. Second, from the NMDS analysis (Fig.
3), ELE, MTCM, and AMT together explained 45% of the variation in
beetle species composition. All of these suggested that climate
gradients impose constraints on Cerambycidae species richness and
abundance. Third, the isolated effect of environment to Cerambycidea
distribution accounted for 5%, which had a distinct advantage compared
with spatial distance and the latter only accounted for 1% as the pure
effect (Fig. 4). Finally, the best linear mixed effect model showed that
beetles standardized Shanoon diversity is significantly positively
correlated with standardized AHR, AMH, MTCM and MTWM. But negatively
correlated with standardized ATR (Table 2).
From the above analysis, we can conclude that the dominating mechanisms
of insects and tree distribution patterns are related differently at
different spatial scales. On the macro/regional scale, the environmental
factors, including AMT and MTCM, induced by elevation and latitudinal
gradient are the major driving forces on longhorn beetle β-diversity.
However, on the micro/local scale, tree diversity and phylogenetic
relationship affect the β-diversity much more. Considering the huge
possibility of β-diversity scaling relationships varying widely across
disparate organisms with different dispersal ability (Soininen et
al. , 2007), spatial scales are the most important prerequisites to
quantify the relative effects of environmental factors on longhorn
beetle species distribution in the present study. As the results showed
(Fig. 2-II & III), β-diversity had no significant differences for trees
between the tropics and subtropics, and for beetles between the
subtropics and the temperate regions at the regional scale. When
focusing on local scale comparisons, a completely different pattern
emerged. Mechanistically, β-diversity at local and regional scales is
typically ascribed to differing processes operating at different spatial
scales. These mechanisms can help inform our a priori expectations of
how β-diversity might vary among sampling units drawn at each of these
scales. Usually, environmental filtering should dominate over dispersal
limitation at larger spatial scales, and the local assemblages are
mainly influenced by ecological processes such as species interactions,
stochastic occupancy, resource specificity, and niche requirements
(Whittaker et al ., 2001; Ricklefs, 2004; Hortal et al .,
2010). The climate gradients along the latitude are known to be
generally unvarying and result in stabilized patterns, whereas the
different ecological processes interact with each other at a local scale
leading to the β-diversity of various organism exhibiting divergent
distribution patterns. Secondly, the results showed that the dispersal
capacity would affect the ability of organisms to colonize suitable
environments. Trees, compared with insects, showed that low average
rates of dispersal can be expected to show lower average geographic
range sizes and higher rates of local endemism, resulting in higher
rates of species turnover from local to regional scales (Qian, 2009;
Baselga et al. , 2012). Compared with the highest β-diversity of
tree organisms that disperse via seeds (Soininen et al. , 2007),
short-lived insect assemblages, including wood boring beetles, would not
be affected by dispersal limitation to some extent (Novotny et
al. , 2007; Hulcr et al. , 2008).
In conclusion, we compared the β-diversity in wood boring longhorn
beetles and plant communities from the tropics to the cold temperate
regions. The prerequisites of insect and plant interaction for
determining the wood boring longhorn beetles existence at any spatial
scales are definite and clear. Irrespective of whether from a
micro/local or macro/regional spatial scale, β-diversity of longhorn
beetles was related to plant richness and phylogenetic diversity to some
extent, while at micro/local scale, longhorn beetles exhibits its
strongest biotic niche relations (in affecting /or being affected by
plant species), but with the increase of spatial resolution, their
relationship are expected to be averaged out at landscape scale, and
macroclimatic heterogeneity will dominate the community assemblage
processes. Besides, the relative lower β-diversity of beetles than that
of the plant communities from the tropics to temperate regions was
controlled by their stronger dispersal ability.