Pollution
Of critical concern is the strong impact of pollution on soil
biodiversity. We found that both metals and pesticides had a strong
impact on soil biodiversity. The fact that metals did have such strong
impacts on biodiversity is not necessarily surprising. Typically,
studies focused on metal pollutants were conducted in landscapes with a
long history of pollution associated with mining and smelting activities
. The sustained toxicity of the soil may not only result in direct
mortality but may also prevent recovery of the soil fauna populations ,
thus resulting in large soil biodiversity loss. Additionally, authors
often investigated gradients of metal pollution across a large range of
concentrations , and given that our meta-analysis cases were based on
the most intense comparison, this may also result in the larger effect
size for metal pollutants. Indeed, pesticide studies mostly applied
pesticides at the recommended application rates, that may represent much
less intense levels than metal pollution gradients. In addition,
although metals in soil are ‘press’ stressors, many pesticides can be
degraded over time so their classification as a ‘press’ stressor may not
be correct for all the studies included here. However, as long-term
studies of pesticide degradation times are lacking for all the types of
pesticides covered by this meta-analysis , a dichotomy in the response
of press/pulse stressors may have held true if we were able to classify
pesticides into their most appropriate press/pulse categories.
Additional insight would also be likely if we were able to further
determine or classify the concentration of pollutants or, alternatively,
compare different types of pollutants at equivalent levels. In
traditional dose-effect theory in ecotoxicology, the dose of the
pollutant is typically one of the strongest drivers of biological
response. For instance, focussed their meta-analysis solely on nematodes
and heavy metal pollutants and were able to determine that increasing
concentrations of heavy metals resulted in more pronounced declines in
biodiversity. However, found that across terrestrial and aquatic
systems, decomposers’ community responses to pollutants did not change
with pollutant levels but were generally negative. The community-level
response may be affected by combined direct toxic and indirect effects
mediated by changes in species interactions, yielding not so
straightforward dose-response curves at this ecological scale. Moreover,
primary studies often addressed the combined effects of multiple
pollutants that may have interactive effects causing even further
deviation from classical dose-effect theory .
In general, the impact of pollutants on terrestrial biodiversity is
particularly understudied , and although there are several studies on
the impact of pollutants on soil biodiversity, there are still large
gaps in our knowledge . As there was a limited number of pollutant
stressors that could be analysed in this meta-analysis, we hope that
future studies will address other types of pollutants (such as
microplastics, hydrocarbons, emerging pollutants; ) in order to
understand the role they may play in biodiversity change. Given the
clear negative effects reported here, our results thus call for more
research focusing on the mechanisms that lead to community-level
responses, on the impacts of a broader range of pollutant types, and
more consistent ways of standardising pollutant levels in primary
studies. Our results also reinforce recent calls for coordinated global
monitoring and policy-actions based on scientific evidence and call for
a proper integration of soil biodiversity in such initiatives .