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.