Identifying potential threats to soil biodiversity

Abstract

A decline in soil biodiversity is generally considered to be the reduction of forms of life living in soils, both in terms of quantity and variety. Where soil biodiversity decline occurs, it can significantly affect the soils' ability to function, respond to perturbations and recover from a disturbance. Several soil threats have been identified as having negative effects on soil biodiversity, including human intensive exploitation, land-use change and soil organic matter decline. In this review we consider what we mean by soil biodiversity, and why it is important to monitor. After a thorough review of the literature identified on a Web of Science search concerning threats to soil biodiversity (topic search: threat* "soil biodiversity"), we compiled a table of biodiversity threats considered in each paper including climate change, land use change, intensive human exploitation, decline in soil health or plastic; followed by detailed listings of threats studied. This we compared to a previously published expert assessment of threats to soil biodiversity. In addition, we identified emerging threats, particularly microplastics, in the 10 years following these knowledge based rankings. We found that many soil biodiversity studies do not focus on biodiversity sensu stricto, rather these studies examined either changes in abundance and/or diversity of individual groups of soil biota, instead of soil biodiversity as a whole, encompassing all levels of the soil food web. This highlights the complexity of soil biodiversity which is often impractical to assess in all but the largest studies. Published global scientific activity was only partially related to the threats identified by the expert panel assessment. The number of threats and the priority given to the threats (by number of publications) were quite different, indicating a disparity between research actions versus perceived threats. The lack of research effort in key areas of high priority in the threats to soil biodiversity are a concerning finding and requires some consideration and debate in the research community.

Keywords: Biodiversity; Ecoystem services; Functional redundancy; Molecular ecology; Mycorrhiza; Soil communities; Soil ecology; Soil fauna; Soil functions; Soil microbiology.

Figure 1. Simple model showing the effect of a perturbation on the resistance and resilience of a soil biological function or property.

Higher biodiversity is thought to correspond to high resistance and resilience. A loss of biodiversity is thought lead to a soil with lower resistance to a perturbation and lower capacity to recover.

Figure 2. Size classification of soil organisms.

As body size increases, abundance decreases. Photos from Global Soil Biodiversity Atlas (Orgiazzi et al. 2016); Credit: B Jakabek, Y Eglit, M Shaw, H Segers, L Galli, A Murray, RR Castro Solar, T Tsunoda, S Franzenburg, D Hope, C Abbe.

Figure 3. Jenga™ analogy for functional diversity.

(A) A stable tower represents the function of the system and each of the blocks represent a species, a number of species in the system contribute to the function; (B) some species can be removed during a disturbance but the overall function of the system can still be maintained (i.e. some bricks can be removed without the tower falling) however (C) some species may have more importance in maintaining the function compared to others, or there may be a critical limit where too many species are removed which will lead to (D) collapse of the systems function.

Figure 4. Findings of expert assessment of threats to soil biodiversity versus published literature on threats to soil biodiversity.

Grey bars represent expert assessment assessment weight (%) in Jeffery & Gardi( 2010) black bars represent actual declarations in the journal literature.