Updating pyrodiversity-biodiversity theory
Fire dramatically shapes vegetation community composition and pattern, creating heterogeneity in habitat types and successional stages across space and time (Turner 2010). Landscape heterogeneity and associated diversity of ecological niches are often tightly tied to greater levels of biodiversity (Turner & Gardner 2015). These well-established associations are the basis for the proposal that pyrodiversity begets biodiversity (Martin & Sapsis 1992). In the three decades since Martin and Sapsis (1992) first articulated the hypothesis, an increasing number of studies have provided evidence to support their theory (Ponisioet al. 2016; Tingley et al. 2016; Brown & York 2017; Beale et al. 2018; Steel et al. 2019), while others have found the relationship to be weak or non-existent (Parr et al.2004; Davies et al. 2012; Kelly et al. 2012). These occasionally conflicting findings as well as our results showing high variation in pyrodiversity across ecosystems indicate the functional relationship between pyrodiversity and biodiversity may not be absolute but rather is limited or context dependent. For example, we observed a maximum pyrodiversity among watersheds with an approximate 53-year fire rotation. This rate of fire activity and pyrodiversity is unlikely to optimize biodiversity across all ecosystems with highly varied historical relationships with wildfire.
We propose constraints to the pyrodiversity-biodiversity relationship are related to an ecosystem’s historic fire regime and that on average biodiversity may be maximized at levels of pyrodiversity characteristic of the conditions under which ecological communities assembled. This updated hypothesis leads to expected and testable functional forms under different conditions. In fire regimes characterized by relatively frequent fire and variable high-severity patch sizes, such as those found in the semi-dry forests of North America, the peak in biodiversity may occur at moderate to high levels of pyrodiversity (Fig. 5a). In less active fire regimes such as wet temperate forests, the biodiversity peak may occur at lower levels of pyrodiversity either because fire-adapted species have been filtered from the regional species pool and/or fire-adaptive traits have not evolved in situ (Miller & Safford In Press). Ecosystems with little variation in burn severity such as savannas may see an analogous mid-pyrodiversity peak (Davies et al. 2018), above which more severe fires threaten to convert the system to grassland (Fig. 5b). The threat of tipping points or type-conversions may be especially acute in ecosystems like tropical rainforest which have little to no history of lightning wildfire and to which native species are poorly adapted (Silveira et al. 2016). Where fire activity and pyrodiversity increase in these ecosystems the biodiversity response may be predominantly negative (Fig. 5c). The theoretical dependence of the pyrodiversity-biodiversity relationship on historic fire regimes is supported by Miller and Safford (In Press) who provide evidence that plant biodiversity is maximized where burn severities match the predominant historical disturbance regime of an ecosystem. Alternatively, He et al. (2019) proposed the association is constrained by species:area relationships and that at very high levels of pyrodiversity declining patch sizes limit the number of species present. This hypothesis predicts a biodiversity peak at moderate to high levels of pyrodiversity similar to Fig. 5c.
In addition to uncertainties surrounding the mechanisms of the pyrodiversity-biodiversity relationship, perceiving the full pyrodiversity-biodiversity functional form is dependent on the range of pyrodiversity observed. Partially observed relationships could be attributed to limited sampling effort or modern shifts in fire regimes away from historic conditions. For example, where fire activity has been artificially reduced, pyrodiversity may be lower than the biodiversity optimum across a study region and biodiversity would appear to increase with pyrodiversity absolutely (Steel et al. 2019; Fig. 5i). Indeed, Martin and Sapsis (1992) developed their original theory in the context of extensive fire-suppression in the mixed-conifer forests of California, where the detrimental effects of an uncharacteristic lack of pyrodiversity was perhaps most apparent.