A comparison of native and non-native small mammals in adjacent karst and volcanic soil regions in NE Luzon, Philippines

Abstract

Small mammal communities that occur in habitats on volcanic soil substrates have been extensively studied on Luzon Island, but those that occur in forest over limestone are poorly known and have not been directly compared to those over volcanic soils. We conducted field surveys of small mammals in forest over limestone from ca. 100 m to 590 m elevation in the vicinity of Callao Cave, and in adjacent lowland dipterocarp forest over volcanic soil from 490 m to 900 m, near the location of prior surveys from 1300 m to1550 m on Mt. Cetaceo, an extinct volcanic peak in the northern Sierra Madre range. Despite moderately heavy disturbance to the habitats over karst (limestone) and moderate disturbance to forest over volcanic soils, we found native small mammals overall at levels of species richness and abundance similar to what we have documented elsewhere on Luzon over the same elevational range. Non-native mammals were present at all localities in the karstic habitat but were absent in all types of forest over volcanic soils, even in areas recovering from prior disturbance. Although non-natives were moderately common in karstic areas, they rarely were more common than native species, and non-native species were no more successful at invading the disturbed karstic habitat than the native species were at persisting there. The most abundant small mammal in dipterocarp forest over volcanic soil (Apomys sierra) was absent in karstic localities, despite occurring in adjacent areas at overlapping elevation. Overall, the difference between small mammals in karst and lowland dipterocarp forest was mainly due to species composition rather than total abundance. Comparisons with data from a prior study on the upper slopes of Mt. Cetaceo showed that total native species abundance was highest in montane and mossy forest, typically about three times higher than in lowland dipterocarp forest. We confirmed the current presence of one species, Apomys microdon, reported as a fossil from Callao Cave, but the apparent absence of one other, Batomys sp.; both were from deposits dated as ca. 65,000 BP. We also summarize information about large mammals in the study areas. Further study of mammals in the distinctive forest over limestone is clearly needed. KEYWORDS: biodiversity, biogeography, Cagayan Valley, disturbed forest, elevation, fossils, Muridae, Sierra Madre, Soricidae