In this post Associate Editor Paul Kardol discusses a paper he recently handled by Yuanhu Shao and colleagues ‘Subordinate plants sustain the complexity and stability of soil micro-food webs in natural bamboo forest ecosystems’
Is it the dominant plant species that rule the system? Some theories suggest so. But, the idea that only the dominants are important is too simplistic and there is increasing evidence that less abundant subordinate species are more important than one would expect. In fact, subordinate species can actually be significant contributors to the functioning of terrestrial ecosystems. This has been shown under stable environmental conditions, but the role of subordinate species might become even more important if environmental or climatic factors change.
Subordinate species often show characteristics different from dominant species. Subordinates are generally slow-growing, long-lived and well-defended against herbivores. For example, for grasslands, it has been shown that subordinate plant species can buffer against disturbances and shifts in the environment such as drought and elevated levels of atmospheric CO2, and help maintain plant community stability and key ecosystem functions like productivity and nutrient cycling. Pierre Mariotte has recently described this phenomenon as the “subordinate insurance hypothesis”. However, ecologists have only just begun exploring the belowground ramifications of subordinate plant species with some studies suggesting that subordinate species exert disproportionately strong effects on soil properties and that the buffering effects of subordinate species can also affect the soil food web.
In their recent paper in the Journal of Applied Ecology, Shao et al. (2015) tested the role of subordinate tree species in determining the complexity and stability of soil food webs in natural bamboo forests in southeast China. Bamboo is an economically valuable species in China and other countries in Asia, as well as some parts of Africa and South America. These bamboo forests often harbour a rich community of subordinate plant species including herbs, shrubs, and shade trees. As a yield-promoting management strategy, shrub removal and selective cutting of subordinate trees is common practice in high-economic value bamboo forests.
Using a plant removal experiment, the authors assessed the effects of removing subordinate species (shrubs, arbor trees, or their combination) on two important groups of soil organisms in the soil food web: microbes and nematodes. Microbes, such as bacteria and fungi, are well known for their role in breaking down organic matter and making nutrients available for use by plants and other organisms. Nematodes might be less well known, but are an important group of soil organisms including microbial-feeding and plant-parasitic taxa as well as omnivores and carnivores, all of them playing an important role in the soil food web. Nematodes are very abundant in the soil and are often used as indicators for soil ecosystem health because of their sensitivity to disturbance and environmental change. Shao et al. (2015) used the microbial and nematode data to calculate diversity and indices of resistance and resilience as indicators for soil food web complexity and stability.
Microbial communities responded quickly to the removal of subordinate plants, but effects on microbes were short-lived. Conversely, nematode communities were affected for a longer period of time. Removal of both shrubs and subordinate trees promoted the dominance of certain nematode taxa, including plant-parasitic nematodes. Perhaps the most interesting finding was that nematodes at the “top of the food chain” (omnivores, predators) were most strongly and negatively affected, and the soil food web complexity and stability was reduced when subordinate species were removed. It should be noted, however, that the removal experiment of Shao et al. (2015) did not distinguish between short-term effects of disturbance (inherent to any removal practice) and the long-term effects of the absence of subordinate species. Longer term evaluation would be needed to tease them apart.
Surprisingly, the study from Shao et al. (2015) indicated that the removal of subordinate plants may negatively affect bamboo productivity. It is not known yet whether or not this effect would continue over longer time frames, but in addition to the short- or long-term effects on soil food web stability, this further argues against the removal of subordinate plant species as a management strategy in bamboo forests.
Although not all questions have been answered yet, this study sheds new light on the potentially important role of subordinate plant species in maintaining soil ecosystem functions, such as the decomposition that soil microbes and nematodes provide. On that account, we should look beyond the obvious, and embrace the lesser (subordinate) and unseen (belowground) organisms!