Climate gradients, and patterns of biodiversity and biotic homogenization in urban residential yards

Results Characterization of snail faunas Snails were found in all 120 yards included in the survey. The total survey count was 18,079 live snails and shells that comprised 49 taxa (S2 Table). Twenty taxa were not native, compris- ing 9 extralimital species (native elsewhere in the country) and 11 alien species (not native to the United States). Snail abundance, as the combined count of shells and live snails in the 10 yards per town, averaged 1508 snails (SE = 216.9) per town, or 150.8 snails/yard. Towns varied in snail abun- dance (Fig 3A) and ranged between catches of 568 snails in Abilene and 3290 snails in Altus. Abundance by collection type (S1 Fig) averaged 46.8% (SE = 3.53%) for visually collected shells, 27.6% (SE = 4.8%) for counted and released live snails and 25.6% (SE = 4.4%) for shells in the soil sample (note: both the shells and soil sample included some live microsnails that were not included in the live snail count). Live snails were relatively more abundant in the east- ern tier of towns and microsnails in the soil samples were relatively more abundant in the western tier of towns (S1 Fig). Like snail abundance, taxonomic richness varied among towns (Fig 3B). Richness averaged 21.7 taxa (SE = 1.7) per town. Snail taxonomic richness ranged from a low of 13 taxa in Hays to a high of 26 taxa in Ardmore. Taxonomic richness was not correlated with town population size (regression: F1,10 = 0.376; p = 0.553; R2 = 0.04). Non-native species were present in all 12 towns and in 111 of 120 yards (versus native spe-

cies, which were found in all towns and 119 yards). Non-native snail abundance (Fig 3A) aver- aged 504.5 snails per town (SE = 72.6) and ranged from 137 snails in Elk City to 922 snails in Norman. Percent abundance of non-native to all identified snails ranged from 8.9% in Elk City to 66.3% in Sallisaw. Two other towns had non-native snails comprising over 50% abun- dance: Hays (57.6%) and Norman (51.7%). Taxonomic richness of non-native snails across towns averaged 5.8 (SE = 0.7) species (Fig 3B) and ranged from one taxon, Vallonia spp. (V. pulchella and V. excentrica, which were both present) in Hays to 9 species in both Altus and Ardmore. Spatial patterns of snail abundance and taxonomic richness Snail abundance was relatively low across towns in the northern tier (Hays, Abilene, Law- rence), the eastern tier (Lawrence, Miami, Sallisaw, Idabel) and the south-central town of Ard- more, and was highest in southwestern Oklahoma (Fig 4A). The distribution of live snail abundance (snails counted and released during surveys) was similar to the pattern of all snails and shells (compare Fig 4A and S2 Fig), with relatively low abundance across the northern and eastern tiers, except for the higher relative abundance of live snails in Lawrence (the NE-most town). Centrally located Norman also had a relatively high abundance of live snails compared to all snails. Both live snails and all snails had the highest abundance in the SW-most town of Altus. The regression of annual rainfall and low winter temperature with snail abundance was sig-

nificant (F2,9 = 7.253; p = 0.013). Snail abundance was negatively associated with increasing rainfall (Fig 5A; R2 = 0.26) and positively but weakly associated with higher temperatures dur- ing the winter (Fig 5B; R2 = 0.03). Snail species richness displayed a spatially complex pattern (Fig 4B). Richness was low in the northwest and southeast portions of the survey and high in the southwest, which is similar to the pattern of total snail abundance (Fig 4B versus 4A). Snail richness showed little associa- tion with either rainfall or low winter temperatures (Fig 5C and 5D; F2,9 = 0.515; p = 0.62). PLOS ONE Diversity patterns of yard-dwelling snails across climate gradients PLOS ONE | https://doi.org/10.1371/journal.pone.0234830 August 28, 2020 6 / 18