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INTRODUCTION
Host diversity has been postulated to influence transmission risk of infectious diseases (Johnson & Thieltges, 2010; Keesing et al. 2010, 2006). In theory, increase in host diversity in communities can either amplify or reduce pathogen transmission through mechanisms such as regulating the abundance of competent hosts or altering the contact rates among competent hosts/vectors (Keesing et al. 2006). The dilution effect (DE hereafter) hypothesis, in which high host species diversity can reduce disease risk, has been reported in a wide range of infectious disease systems (Allan et al. 2009; Johnson et al. 2009; Pongsiri et al. 2009; Suzán et al. 2009; Keesing et al. 2010; Ostfeld & Keesing, 2012; Huang et al. 2013a ; Johnson et al. 2013; Huang et al. 2016). This negative diversity-disease relationship, representing an exciting convergence of conservation and public health interests (Randolph & Dobson, 2012; Young et al. 2013; Wood et al. 2014), has attracted much attention in the context of global biodiversity decline and increasing disease emergence (Ostfeld & Keesing, 2012; Randolph & Dobson, 2012; Wood et al. 2014; Huang et al. 2016). However, recent studies started criticizing the generality of DE and considered that the DE is idiosyncratic and only occurs under certain conditions (Randolph & Dobson, 2012; Salkeld et al. 2013; Wood & Lafferty, 2013; Young et al. 2013; Wood et al. 2014). Better understanding the generality of DE will be critical for predicting future disease outbreaks especially in the condition of ongoing biodiversity decline (Keesing et al. 2006; Ostfeld & Keesing, 2012).
When retrospecting the recent critical studies (Cardinale et al. 2012; Randolph & Dobson, 2012; Salkeld et al. 2013; Wood & Lafferty, 2013; Young et al. 2013; Wood et al. 2014), we found an interesting phenomenon that almost all current criticisms of DE are related to animal diseases, while those for plant diseases are conspicuously rare. A recent meta-analysis study has presented evidence for the DE in various functional groups of parasites, and intended to put a closure to the current debate (Civitello et al. 2015). However, their conclusion remains provisional without addressing the important distinction in plant vs animal diseases. Addressing the question whether the DE is generally weaker in animal diseases than in plant diseases will be fundamental to...