Rarity types

We assigned distance decay functions to target plants a priori based on our natural history knowledge of target plants, and their pattern of occurrence within the region. To do this, we considered occupancy as a fourth dimension of rarity, following Crisfield et al. (2024) in their adaptation of Rabinowitz rarity types (Kruckeburg & Rabinowitz, 1985).

In the Rabinowitz typology, species rarity is evaluated based on geographic range (large/small), habitat specificity (wide/narrow) and population size (dominant/non-dominant); occupancy is added by Crisfield et al. (2024) as a species’ frequency of occurrence in habitat patches within its range. We considered ‘geographic range’ regionally, as limited to within the Gulf and San Juan islands, ‘habitat specificity’ as limited to woodland and coastal meadow habitats, ‘population size’ as abundance within habitat patches, and occupancy as the species’ overall frequency of occurrence across habitat patches in the region.

Ratings, and their corresponding kernels, were assigned based on our experience observing target plants in the field and by reviewing their known regional distribution using herbarium and iNaturalist records. We assigned steep kernels to low occupancy species and shallow kernels to higher occupancy species, considering the other dimensions of rarity as well in this approach. The kernels assigned to undetected species were then compared with those inferred for detected species using our second approach.

Species	Rarity / Habitat Notes
Crassula connata	Very low occupancy. Typically found in localised patches within a habitat patch; absent from most islands; widely dispersed across the larger islands of Vancouver and San Juan.
Meconella oregana	Very low occupancy. Absent from most islands; moderately distributed on the southern tip of Vancouver Island but only known from single localities on other islands where it is highly local within a patch when found.
Perideridia montana	Low occupancy. Widely distributed on the southern tip of Vancouver Island, but idiosyncratic distribution on smaller islands; often locally abundant and widely distributed within habitat patches when found; sometimes restricted to a narrower microhabitat (swales) in drier sites.
Trifolium dichotomum	Low occupancy. Found on a few islands where records are highly clustered; locally abundant to common in patches where found, not widely distributed within patches when found.
Aquilegia formosa	Low occupancy. Mostly found on larger islands where populations are spatially clustered, though widely distributed; preference as deer forage may mask distribution.
Castilleja attenuata	Low occupancy. Sparsely distributed on smaller islands but widely distributed on the southern tip of Vancouver Island.
Plagiobothrys tenellus	Low occupancy. Sparsely distributed on larger islands; often locally abundant and widely distributed within habitat patches when found.
Platanthera unalascensis	Moderate occupancy. Sparsely distributed on larger islands but widely distributed on the southern tip of Vancouver Island.
Lepidium virginicum	Moderate occupancy. Fairly broadly distributed across islands and habitat patches within islands, though infrequently occurring.
Primula pauciflora	Moderate occupancy. Found on a few widely scattered islands; localities highly clustered within islands, where it is often locally common and widely distributed within patches when found.

For example, Crassula connata is known only from the Gulf and San Juan Islands in the region, where it is disjunct from its more geographically widespread range in California. Second, it has narrow habitat specificity within coastal meadow and woodland communities, occurring in compacted vernal pools in coastal rock outcrops and slopes. These microhabitats are typically localized to relatively small patches that are infrequently occurring in coastal meadow habitats. Third, given the species’ small size, they are often clustered in dense patches a few meters in diameter, so typically have extremely limited distribution within a larger coastal meadow patch. Thus, even while many hundreds of plants may be present, the species is typically non-dominant within the larger coastal meadow flora. Finally, it has a highly infrequent pattern of occupancy in the region. Given its infrequent, random distribution and the infrequent occurrence of its habitat—both within meadow habitats and across islands—we gave it a very low occupancy probability and assigned it a very steep kernel as our prior.