Structures of intransitive competition network affect functional
attributes of plant community under nitrogen enrichment
Abstract
Atmospheric nitrogen (N) deposition is a potential danger factor for
grassland ecology, and will cause unpredictable consequences to plant
communities. However, how plant species interactions response to N
enrichment and then affect ecological functions are not fully known. We
investigated how intransitive competition network was related to the
functional attributes of plant community under a 13-years N-deposition
experiment. Results showed that intransitive competition network was not
a single structure, but a complexly interwoven structure of various
simple structures. Nested work was more common, accounting for 76.96%,
and gained new species at a higher colonization rate than short network
did. The network had a long-term mechanism to maintain the small-scale
Alpha diversity, and a significant lag effect on the large-scale Gamma
diversity. Under the conditions of N ≥ 2 g N·m-2·year-1, without mowing
and under high fertilization frequency, the increase of network
complexity significantly decreased plot biomass gradually. The
relationship between biomass and network complexity is quadratic curves,
also between abundancy and the complexity, but with the opposite bending
directions, which indicated that biomass and abundance were
complementary to each other, which may be a mechanism of maintaining the
relative balance of species competition. In addition, the decrease of
species asynchronism changing with the increase of N-enrichment
gradually destroyed ecosystem stability. However, at medium N
enrichment, intransitive network counteracted the negative effects of N
enrichment and maintained or even improved the biomass ecosystem
stability. Our results suggested that intransitive competition network
is an internal mechanism of self-restoration of a grassland ecosystem.
Under nitrogen enrichment conditions, competitive networks complexity is
reduced, leading to a reduction in species diversity. These analyses
emphasize the important role of intransitive network structure to
stabilize grassland ecosystem. In order to achieve sustainable
development of grassland, it is indispensable to control nitrogen
addition rate.