Diversity predictors
Two macrophyte measures were previously shown to influence
susceptibility to invasion (Capers et al. 2007): diversity andplant cover . We therefore developed three complementary
indicators from the macrophyte PVI data to estimate macrophyte
diversity: native species richness, the Shannon–Wiener diversity index
(henceforth referred to as native Shannon diversity) and native
macrophyte beta diversity. These indicators address key elements of
native macrophyte community structure: the number of species in a lake
(richness), their relative abundances within a lake (Shannon diversity),
and their compositional variation across space and time (beta
diversity). The native macrophyte cover data (%) collected during the
PVI assessments at each sampling point were assigned to native plant
cover (total sum of native macrophyte species cover) and to two
functional growth forms (submerged plant cover or floating plant cover).
Beta diversity was calculated as the compositional variation in the
native macrophyte species’ PVI values (excluding E. canadensis )
across all sampling points as estimated via principal curves analysis
(PC; De’ath 1999). PC analyses were run using the prcurvefunction in the “analogue” package in R (Simpson et al. 2020), using
Canonical Analysis as the starting point on square-root transformed PVI
data. The plant macrofossil data were similarly grouped into five native
diversity predictors: taxon richness, Shannon diversity, native plant
cover, submerged plant cover and floating plant cover.