Mammals of Mt. Pinatubo, Luzon Island, Philippines: Extreme Resilience Following Catastrophic Disturbance

Philippine Journal of Science 150 (S1): 121-133, Special Issue on Biodiversity ISSN 0031 - 7683 Date Received: 24 Sep 2020 Mammals of Mt. Pinatubo, Luzon Island, Philippines: Extreme Resilience Following Catastrophic Disturbance Eric A. Rickart1*, Lawrence R. Heaney2, and Danilo S. Balete2† 1Natural History Museum of Utah, 301 Wakara Way, Salt Lake City, UT 84108 USA 2Field Museum of Natural History, 1400 S Lake Shore Drive, Chicago, IL 60605 USA The catastrophic eruption of Mt. Pinatubo in 1991 destroyed the forests that covered the peak and impacted the surrounding habitat over a broad area of central Luzon. Information on the mammal fauna of Mt. Pinatubo prior to the eruption is limited but documents a variety of native mammals. In 2011 and 2012, we surveyed mammals at localities along an elevational gradient on the eastern slope of the mountain where vegetation had been devastated by pyroclastic flows and subsequent lahars, and habitat reflected early stages of plant succession. We documented eight species of bats (five fruit bats and three insectivorous bats), seven species of small non- volant mammals (two non-native and five native rodents), and two native large mammals. Additional species of bats and non-volant mammals present in the vicinity of Mt. Pinatubo prior to the eruption or recently documented at other localities in the Zambales Mountains may be present in remnant forest habitat elsewhere on Pinatubo. Across five survey localities where habitat was in the early stages of regeneration, native species of small non-volant mammals were more widespread and much more abundant than were non-natives. The most abundant native species, Apomys sacobianus, may be endemic to Mt. Pinatubo. It is an extreme example of a “disturbance specialist” that thrives in a severely disturbed habitat. Results underscore the disturbance tolerance of many native small mammals of Luzon and reveal a resilience that is remarkable for a highly endemic insular fauna. Mt. Pinatubo presents opportunities for further studies on how natural disturbance has shaped the evolution of the Philippine biota and may influence its future conservation. Keywords: bats, biogeography, disturbance tolerance, elevational gradients, endemism, faunal recovery, habitat recovery, rodents, volcanic eruption INTRODUCTION large caldera in its place, thereby reducing the height of the peak by 250 m. Subsequent heavy precipitation from Mt. Pinatubo, a volcanic peak in central Luzon Island, seasonal typhoons and monsoons created a caldera lake Philippines, was long considered dormant until it erupted and triggered destructive lahars that continued for many explosively on 15 Jun 1991 in the second most powerful years following the eruption (Newhall et al. 1996). terrestrial volcanic eruption of the 20th century. The eruption covered the upper slopes of the mountain with Dormant for about 500 years preceding the 1991 eruption, massive amounts of ejecta in the form of falling ash and Mt. Pinatubo and surrounding lowlands had supported pyroclastic flows, destroying the summit and creating a old-growth forest. Although the lowlands of central Luzon were largely deforested during the 20th century (Heaney *Corresponding Author: [email protected] et al. 2016b), remnant mature dipterocarp forest remained †Deceased 01 Jul 2017 121 Philippine Journal of Science Rickart et al.: Mammals of Mt. Pinatubo Vol. 150 No. S1, Special Issue on Biodiversity in portions of Clark Air Base, and the upper portions of 1956. As discussed below, the presence of a potentially Pinatubo were heavily forested prior to the 1991 eruption rare endemic species prior to the eruption raised the (Punongbayan et al. 1996). question of its possible extinction, given the frequent assumption that rare mammals on oceanic islands are In 2006, 15 years after the eruption, an assessment of intrinsically susceptible to extinction following significant plant succession on Mt. Pinatubo revealed a predominance ecological disturbance (Berglund et al. 2009; Walsh et of non-native plants and the influence of elevation on al. 2012). species richness, diversity, and plant cover (Marler and del Moral 2011). Although there were no comprehensive surveys of the mammals of Mt. Pinatubo prior to the eruption, there are specimens housed at the United States STUDY AREA AND METHODS National Museum of Natural History (USNM) from Clark Air Base and nearby localities at low elevations in Tarlac and Pampanga provinces. These include rodents reported Location and Geology from rice croplands (Barbehenn et al. 1973), and bats and Mt. Pinatubo (15.13°N, 120.35°E; current elevation 1,486 rodents obtained by D.H. Johnson and others (records m), is a dacitic-andesitic stratovolcano and caldera located summarized in Heaney et al. 2010). However, there is no in the Zambales Mountains of central Luzon on the tripoint direct information on the mammals that may have been boundary of Pampanga, Tarlac, and Zambales provinces present at higher elevations on Pinatubo prior to its eruption. (MGB 2010). It is one of several volcanoes along the Bataan Lineament formed by the eastward subduction Several previous studies have dealt with the disturbance of the Eurasian Plate beneath the Philippine Mobile ecology of the mammals of Luzon. These have utilized Belt along the Manila Trench (Wolfe and Self 1983). field surveys of small mammals across gradients of The eruptive history of Pinatubo consists of two phases. habitat disturbance ranging from relatively pristine old- “Ancestral” Pinatubo was a massive stratovolcano that growth forests to heavily disturbed human-modified may have had an elevation approaching 2,300 m. It has landscapes. The principal results of these studies can be been dated at around 1.1 Ma and left no evidence that it summarized as follows: although some native species are erupted explosively. In contrast, “modern” Pinatubo had at highly tolerant of disturbance, in general, diversity and least six periods of explosive eruption, each separated by abundance of native species decline with increasing levels longer periods of inactivity. Its earliest eruption occurred of disturbance whereas the opposite is true for non-native more than 30,000 BP and the most recent (prior to 1991) species (Rickart et al. 2011, 2013; Heaney et al. 2016b; ca. 500 BP. The 1991 eruption was estimated to be one Reginaldo and Ong 2020). of the least powerful in the discernable geological record of Pinatubo (Newhall et al. 1996). Here, we present the results of a survey of the mammals of Mt. Pinatubo conducted in 2011 and 2012. This work was undertaken to determine the nature of the pioneering Survey Localities mammal community in early successional habitat two Our survey was conducted in 2011 and 2012 on the eastern decades after the 1991 eruption. Based on disturbance slope of Mt. Pinatubo in Pampanga Province, between gradient surveys elsewhere on Luzon (Rickart et al. 2011, 300 and 1,100 m elevation (Figure 1). Survey localities 2013) we developed the following hypotheses to inform and were selected in consultation with the indigenous Aeta direct the work on Mt. Pinatubo, which we test in this paper: who manage the area as their ancestral domain. In 2011, we surveyed three localities in portions of the rolling hills • Non-native small mammals are numerically (300–700 m) in Mabalacat, Pampanga Province near the dominant in the most severely disturbed habitat. boundary of Bamban, Tarlac Province, on the northeastern • A subset of native species that tolerate severe flank of Mt. Pinatubo north of the Sacobia River (Figure habitat disturbance coexists with non-native 2A). In 2012, we surveyed the upper eastern slope of Mt. species. Pinatubo (680–1,100 m), near the northern tributaries of Pasig-Potrero River in the western-most portion of • Native small mammals are resilient, moving Mabalacat, Pampanga Province. Localities were in areas into areas of regenerating habitat and ultimately of early-stage plant succession with sparse tree cover, as displacing non-native species as native vegetation described below. Because of the steep slopes and loose matures. lahar substrate, erosion was especially severe along riverbanks and gullies (Figure 2B). Of special interest to us was the status of Apomys sacobianus, a native mouse that was known only from the Locality 1: 8.4 km N, 12.3 km E Mt. Pinatubo peak, holotype captured on the lower slopes of Mt. Pinatubo in 365 m elev., 15.20556o N, 120.46278o E; 27 Feb–04 122 Philippine Journal of Science Rickart et al.: Mammals of Mt. Pinatubo Vol. 150 No. S1, Special Issue on Biodiversity Figure 1. Map of Mt. Pinatubo showing the location of numbered survey localities along the eastern slope of the mountain. Contour intervals 400 m. Inset map shows the location of Mt. Pinatubo on Luzon. Figure 2. Habitat features in areas surveyed on Mt. Pinatubo in 2011 and 2012. A) View downslope from a point on the northeastern flank of Mt. Pinatubo north of the Sacobia River illustrating the general nature of the regenerating habitat surrounding localities 1–3 in March 2011. B) View upstream along an upper tributary of the Pasig-Potrero River showing the eroding lahar and nature of the surrounding regenerating vegetation in March 2012; the camp at locality 4 was in the left-center of the image (a small tarp is barely visible) and locality 5 was on the steep slope above and beyond the camp. Mar 2011. This locality, selected as a base camp, was Climbing and erect bamboos (Dinochloa and Bambusa) a nursery established by the Aeta of Sitio Burog in an were common along the steep stream banks. A narrow attempt to reforest the surrounding area. Trees planted strip of riparian habitat along a stream included a variety here included Eucalyptus (ca. 12 m tall), Leucaena, of native palms (Areca, Arenga, Caryota, and Calamus) Swietenia, and particularly Gmelina, along with native and other scattered trees up to 20 m tall (Afzelia, Alstonia, Pterocarpus.

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