Abstract
Contemporary defaunation has profound ecological consequences ranging from local or even global co‐extinctions of interacting species to the loss of ecosystem functions and services critical for humanity. Other components of global change (climate change, introduced pests, land use changes) are also harming ecosystem functioning by augmenting tree mortality worldwide. Defaunation and increased tree mortality often coincide in many human‐altered ecosystems but whether they interact, leading to non‐additive effects on ecosystem functioning, remains largely unknown. However, under some ecological circumstances, the decline or extirpation of one species due to defaunation can be neutralized by increases in the abundance of some functionally similar species (i.e. ‘density compensation’). We combined long‐term field data with individual‐based modelling to investigate the potential interactive effects of seed disperser loss, increased tree mortality and density compensation on seed dispersal in a heterogeneous landscape. Our simulation experiments showed that both stressors markedly limit not only the quantity of seed dispersal but also its quality since the impact on seed dispersal strongly varied among habitat types that differ strikingly in suitability for tree establishment. Density compensation had a marked positive effect on seed dispersal which, however, was largely limited under increased tree mortality. The combined negative effects of defaunation and increased tree mortality on seed dispersal were lower than the expected additive effect. This highlights the need to account for the joint operation of multiple stressors to accurately predict the impacts of global change on the link between biodiversity and ecosystem functioning.
Highlights
Contemporary defaunation has profound ecological consequences ranging from local or even global co-extinctions of interacting species to the loss of ecosystem functions and services critical for humanity (Dirzo et al 2014, Young et al 2016, Emer et al 2019)
Other components of global change are harming ecosystem functioning by promoting tree mortality in many human-altered habitats (Linares et al 2009, Van Mantgem et al 2009, Carnicer et al 2011, Boyd et al 2013)
We aimed to address the following three questions: 1) does fox and badger loss reduce the number of P. bourgaeana dispersed seeds and, if so, is the reduction comparable between disperser species in terms of seed dispersal into different habitat types? 2) Does density compensation counteract the impact of defaunation on seed dispersal and, if so, does such compensation occur in a similar extent across habitats? 3) Do the effects of defaunation and increased tree mortality on seed dispersal interact and, if so, do they amplify or ameliorate each other?
Summary
Contemporary defaunation has profound ecological consequences ranging from local or even global co-extinctions of interacting species to the loss of ecosystem functions and services critical for humanity (Dirzo et al 2014, Young et al 2016, Emer et al 2019). Other components of global change (climate change, introduced pests, wild fires) are harming ecosystem functioning by promoting tree mortality in many human-altered habitats (Linares et al 2009, Van Mantgem et al 2009, Carnicer et al 2011, Boyd et al 2013). Given the importance of biodiversity for the maintenance of ecosystem functions and the services that they underpin (Oliver et al 2015, Schleuning et al 2015), a comprehensive understanding of such potential interactive effects is crucial. Whether and how these two stressors interact and lead to potentially non-additive effects on ecosystem functioning remains largely unknown (but see Granados et al 2018)
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