3.3 Niche breadths and niche overlap of phyla in degraded
grasslands
To explain the responses of different phyla to grassland degradation, we
further assessed the niche widths of each phylum across the different
degradation levels, as well as the relationships between taxon niche
width and environmental variables.
First, the mean niche width at community level in each degraded region
was calculated, and then the corresponding OTUs in each phylum (top
eight dominant phyla) were calculated based on the Z-score-transformed
abundances of all OTUs. The niche widths of all phyla in each degraded
region were obtained from the average values of corresponding
Z-score-transformed OTUs, and were compared among different regions.
Proteobacteria and Bacteroidetes could be distinguished from other phyla
because of their broader niche widths that were significantly and
negatively correlated with nutrient contents (Figure 3A, 3C and
S6, P < 0.05) . On the contrary, the other major phyla
(Acidobacteria, Actinobacteria, Planctomycetes, Chloroflexi, and
Gemmatimonadetes) exhibited decreasing niche widths with an increase in
soil nutrient contents (Figure 3A, 3C and S6, P< 0.05) . Species with broader niche breadths were distributed
widely and evenly, suggesting greater stability in response to
environmental disturbance (Pandit et al., 2009; Li et al., 2019).
Therefore, the greater niche breadths of Proteobacteria and
Bacteroidetes indicated their high adaptability and positive responses
when compared to other phyla under grassland degradation.
To evaluate variation in niche overlap within each phylum across the
different grassland degradation conditions, we Z-score transformed the
niche overlaps of each comparison (OTU–OTU) and compared the mean
values within each phylum with an increase in degree of degradation. In
the two grasslands, as degradation intensified, the mean values
(Z-score) of niche overlap within Acidobacteria, Actinobacteria,
Planctomycetes, and Gemmatimonadetes decreased, while the average niche
overlaps of Proteobacteria and Bacteroidetes increased (Figure
3B and 3D) , which may be partly due to changes in their species
diversity, which would lead to greater species competition(Table 1) .
The results suggest that phyla with great survivability and others with
weak survivability adopted different strategies under grassland
degradation. With an increase in degree of degradation, Proteobacteria
and Bacteroidetes exhibited higher alpha diversity, and niche width and
niche overlap; on the contrary, other phyla (mainly Actinobacteria,
Acidobacteria, Gemmatimonadetes, and Planctomycetes) exhibited decreased
alpha diversity, and niche width and niche overlap (Table 1;
Figure 3) , which may be attributed to the elimination of certain
species with functional redundancy, which can facilitate better
adaptation to grassland degradation in bacterial communities.