Kechang Niu, Nanjing University in China, discusses his article: Plant species loss reduces rare soil microbes through diversity effects amplified by multitrophic interactions
Background
A key reason for the flourishing of life on Earth is that diverse groups of organisms mutually thrive in changing environments. However, increasing evidence shows that we are suffering the sixth great extinction of Earth’s biodiversity, likely due to human dominance. Ecologists have well illustrated that plant species diversity promotes various ecosystem functions through the selection (mass) effect of dominant species and complementary effects of diverse members in artificially assembled plant communities. This suggests a potential decline in ecosystem functions as plant diversity decreases.
Yet, whether plant diversity loss in natural communities ultimately erodes ecosystem functions in the ways predicted by experimental studies remains uncertain. For instance, studies that directly remove key species or functional groups often emphasize the significance of species’ functional identity rather than species richness, particularly for rare species. Although rare in abundance, these species constitute a major portion of the community and may play a crucial role in maintaining community stability and functional resistance. Moreover, previous studies primarily focused on the relationship between plant diversity and productivity, while little is known about how ecosystem multifunctionality is regulated by the unseen biodiversity belowground.
Hypotheses
We conducted a 12-year long experiment in a species-rich alpine meadow on the eastern Tibetan Plateau to provide a robust evaluation of the consequences and significance of plant diversity loss on biodiversity — both aboveground and belowground — and ecosystem multifunctionality. The experimental design considered not only classical concerns about the importance of species richness for ecosystem function but also different perspectives on species loss: (i) functional loss based on Grime’s CSR strategies, i.e., through the removal of legumes, grasses, and forbs species; and (ii) ecological loss according to community assembly theories, i.e., through the removal of dominant, common, and rare plant species.
We hypothesized that the mass effect of plant species loss on soil biodiversity and ecosystem functions would be more important in the early years of the experiment, while the diversity effect would strengthen over time. For instance, the decline in soil carbon and nutrients following biomass loss due to the removal of dominant species would initially reduce the abundance of all soil organisms. However, over time, the diversity effect would become more pronounced, as the loss of many rare plant species would lead to a reduction in unique resources essential for supporting diverse soil organisms, ultimately resulting in the loss of rare soil biota that regulate soil multifunctionality. Furthermore, we predicted that changes in the abundance and diversity of soil organisms following plant species loss would trigger shifts in multitrophic interactions and food web structures, thereby shaping the dynamics of soil multifunctionality.
Key findings
Over the 12-year experiment, we obtained many important results that deserve reporting, though presenting them in an engaging way remains challenging. For instance, we found that almost all measured variables (e.g., biomass) during the first five years were affected by the artificial removal of target plants, providing limited information for isolating and distinguishing the functional consequences of plant species loss. After ten years of species removal, aboveground biomass did not decline significantly in most removal plots as expected. This was likely due to compensatory growth within and among plant functional groups, reflecting the functional adaptation of plant phenotypes upon soil food web structure. We are now conducting an in-depth analysis of these intriguing yet unexplained results.
In the published paper, we focused on reporting how and why the ecological loss of plant community members influences the diversity of soil microbes. Briefly, we found that the loss of rare plant species, rather than dominant or common species, reduced the richness of soil bacteria and fungi by decreasing rare microbial taxa. This decline in richness of rare microbes was attributable not only to the decreased plant species richness, i.e., direct effect of plant diversity, but also to belowground multitrophic interactions. Specifically, the decline in rare microbial taxa was associated with an increased abundance of fungivorous and omnivorous nematodes following the removal of rare plant species. The loss of rare microbes altered the composition and dissimilarity of soil microbial communities, leading to shifts in food web structure, belowground multitrophic interactions, and ultimately, soil multifunctionality.
Implications
Implications? Yes, there are many… Over the course of this long-term experiment, we have encountered so many insights that summarizing them concisely is challenging. Perhaps the most significant realization is that despite increasing research on biodiversity loss and its underlying mechanisms — often inferred from artificially assembled communities, comprehensive investigation, and complex models — there is still little known about its real effects in natural ecosystems. For instance, the loss of rare plant species is a key driver of plant diversity loss in most global change scenarios. Yet, surprisingly, very few field experiments directly manipulate and evaluate the effects of rare plant loss. Thus, a primary implication of our work is the urgent need for more in-depth field experiments. Despite the rise of ecological data science, ecology still requires a new generation of researchers to return to the field to observe, listen, feel, and learn from nature. Fieldwork remains essential for testing hypotheses, developing theories, and pushing the boundaries of ecological knowledge.
Regarding the specific implications of this study, we revealed that the loss of ecologically rare plants — i.e., reduced species richness — can trigger complex dynamics in soil biodiversity and belowground multitrophic interactions, especially through the loss of rare soil microbes. This decline in rare microbial taxa is primarily driven by resource shortages following plant species loss, leading to a thinning process in which many individuals disappear randomly due to the loss of diverse plant resources. This is further exacerbated by increased nematode abundance, which engage in non-selective feeding. Thus, the disappearance of tiny, rare plants may set off a positive feedback loop, where the loss of rare organisms weakens ecosystem resilience in a changing environment. This, in turn, could ultimately erode ecosystem multifunctionality and the services ecosystems provide to humanity.
Being rare is fundamental to the coexistence of diverse organisms. We are just beginning to explore how the loss of rare nodes within multilayer ecological networks influences community dynamics and ecosystem multifunctionality.
