Macro Land Snail Diversity and Community Assemblage in Selected Forest Fragments
of Leyte Island, Philippines
Fretzeljane O. Pogado1, Ian Kendrich C. Fontanilla2, Keshia N. Tingson3, and Emmanuel Ryan C. de Chavez4
1Department of Biological Sciences, College of Arts and Science, Visayas State University, Baybay City, Leyte 6521-A Philippines
2Institute of Biology, University of the Philippines Diliman, Diliman, Quezon City 1101 Philippines
3Institute of Renewable Natural Resources, College of Forestry and Natural Resources, University of the Philippines Los Baños, Laguna 4030 Philippines
4Animal Biology Division, Institute of Biological Sciences, College of Arts and Sciences, University of the Philippines Los Baños, Laguna 4030 Philippines
The diversity of land snails in Leyte Island has not been examined over the past few years. To address this, a survey of macro land snails (> 5 mm in shell height and diameter) in selected forest fragments of Leyte was conducted. A total of 120 quadrats (10 x 10 m) were set in three sampling sites (Inopacan, Baybay, and Maasin) that varies in area size – with Baybay being the largest fragment, followed by Inopacan and Maasin. From a total of 592 individuals, 22 macro land snail species and five families were identified. The macro land snail assemblage was dominated by endemic eupulmonates (Order Stylommatophora) and caenogastropods (Order Architaenioglossa). Among eupulmonates, Family Camaenidae had the most species (10), followed by Trochomorphidae and Achatinidae (two each), and Helicarionidae (one).
For caenogastropods, all seven species were under Family Cyclophoridae. Cyclophorus appenidiculatus was the most abundant, representing 16.65% of the total number of individuals.
Lissachatina fulica, an invasive land snail, was also documented in all sites. A β-dominated community assemblage was revealed by species accumulation curves with sampling efficiency using a completeness ratio of 1, which indicates a highly efficient sampling method. Detrended correspondence analysis indicated high similarity in species composition among study sites, with two unique species (Subulona cylindracea and Hypselostyla boholensis) recorded in Maasin and two incidental records (Obba bigonia and Helicobulinus sarcinosa). This study yielded 15 new records of macro land snail species during the sampling. There were five species (Hemiglypta semiglobosa, Amphidromus maculiferus, Leptopoma perlucidum, Chloritis leytensis, and Obba bigonia) that were documented on earlier records on the island, but 33 of the previously recorded macro land snail species were not encountered in the study. The community ecology patterns were also documented in the fragmented forests of Leyte, which is very important in local malacofaunal conservation and management.
Keywords: forest fragments, land snails, Leyte Island, Visayas
*Corresponding author: [email protected]
Date Received: 19 Jan 2022
INTRODUCTION
Land snails, although small in body size, play a significant role in the forest by their influence on leaf litter decomposition and contribution to the retention of calcium in the upper soil layer by fixing Ca through intra- and extracellular biomineralization (Astor 2014) and providing food essential to other fauna like birds and small mammals (Douglas et al. 2013). Land snails are highly vulnerable to habitat changes since they require specific habitat requirements (Gotmark et al. 2008; Kappes et al. 2009; Nurinsiyah et al. 2016) like constantly moist litter-rich forest floor (Kappes et al. 2009), basic soil chemistry (seasonal CaCO3 composition of the soil for shell-building), and the forest loss from fragmentation affect a particular species' occupancy (Franklin et al.
2002), especially indigenous biota.
They are found abundant in karst ecosystems (Clements et al. 2006; von Oheimb et al. 2019, Parcon et al. 2020) and rainforests (Barker and Mayhill 1999; Emberton et al.
1997; Schilthuizen and Rutjes 2001), providing the needed elements for growth and reproduction, and characterized by many microclimates that provide an opportunity for the explosion of species. Despite the benefits rainforests provide, they have been constant subjects of human disturbance resulting in forest fragmentation, which is one of the most dramatic anthropogenic changes in rainforests through the years. In Southeast Asia, 73% of forest- covered land in 1973 have been reduced to 51% in 2009 (Hughes 2017), with an average loss of 1% forest cover annually (Miettinen et al. 2011). The reduction of forest area results in habitat fragmentation and various species respond differently to habitat fragmentation depending on species-specific traits such as body size, dispersal ability, mating system, and habitat requirement. The effect of habitat fragmentation on small populations could be greater sensitivity to threats due to reduced genetic variation and species richness per fragment (Gotmark et al. 2008; Stoll et al. 2009).
Land snails and slugs are among the organisms that are considered to be vulnerable to ecological disturbance especially forest fragmentation because of their great dependence on environmental conditions (Bloch and Bloch 2012). Studies also suggest that habitat fragmentation greatly affect specialists and groups with low dispersal rate like land snails (Gotmark et al. 2008;
Stoll et al. 2009).
In line with this, the diversity of macro land snails in an area can indicate the current status of its environment. The malacofauna diversity in the Philippines, specifically in Luzon (de Chavez 2008; de Chavez and de Lara 2011;
Sosa et al. 2014; Parcon et al. 2020; Valdez et al. 2021), have been well-studied, but only a few works have focused
on the Visayas, including Leyte. An earlier study of land snails was conducted on Leyte Island, wherein about 60 species of macro (38) and micro (21) land snails were recorded from the collection of H. Cumming, C. Semper, and O. Moellendorff covering various parts from both northern and southern regions of Leyte (Blum et al. 1893).
However, updated data on the current diversity and the land snail assembly patterns in the fragmented forest of the island’s malacofauna are lacking. Thus, this study sought to determine the macro land snail diversity and the community assemblage in selected forest fragments of Leyte Island.
MATERIALS AND METHODS
Study Site
Leyte (11° 5' 20.76" N, 124° 53' 32.28" E) is the eighth- largest island in the Philippine archipelago, situated in the biogeographic region of Eastern Visayas. It has a total forest cover of 386,638 ha (233,770-ha pristine forest;
152,868-ha degraded forest), with those in Southern Leyte constituting around 12.5% of Leyte’s total land area (Mallari et al. 2013). The Leyte-Samar Region has been a major source of forest products since the 1950s, which have been significantly reduced through logging, deforestation, and intensive agriculture such as slash-and- burn kaingin farming with adverse consequences on the biodiversity and other forest resources (Dargantes and Koch 1994; Navarete and Peque 2018). A 2015 forest land cover map was also obtained from the Visayas State University GIS services. Three sampling areas were selected in this study – namely, Baybay, Inopacan, and Maasin. These study sites were chosen based on the degree of vegetation change in the analysis done by Mallari et al.
(2013) and were strategically located in the researcher’s location. Baybay and Inopacan are in Leyte province, whereas Maasin is in Southern Leyte. Aside from the fact that these areas have undergone a massive vegetation change, these three sampling sites are also classified as
“high conservation value areas” with moderate to severe vegetation change from 2007–2013 (Mallari et al. 2013).
At the same time, no updated survey on the malacofaunal resources in this area is available. The sampling areas in Maasin were Barangay Cambooc, Malapoc Norte and Maasin Forest Park; in Inopacan were Barangay (Brgy.) Hinabay, Linao, and Can-angay; in Baybay were Brgy.
Pangasugan and Visayas State University Nature Park.
All three sites have secondary forests with the presence of large pioneer tree species (Polyscias nodosa, Leucaena leucocephala, etc.) and agroforests (mostly coconut, banana, and cacao) at an altitudinal range of 40–360 masl.
Sampling Protocol
A preliminary survey was conducted prior to actual sampling to estimate the number of quadrats to be set in each forest patch depending on the fragment’s area.
A total of 120 quadrats were established in all sampling sites – 30 were in Maasin, 40 in Inopacan, and 50 in Baybay. The sampling was a one-time sampling technique adapted from Nurinsiyah et al. (2016) using a 10 x 10
m (100 m2) quadrat set in accessible areas at least 75 m away from the forest edge to reduce the edge effect (Kappes et al. 2009). For each sampling site, quadrats were placed at least 10 m apart in the agroforest and the secondary forest with an equal number of quadrats for each forest type. Macro land snails (> 5 mm in shell height and diameter) – including empty shells – were collected by direct visual searching and handpicking focused on
Figure 1. Location of the study sites in Leyte Island with inset map of the Philippines. Generated using ArcMap version 10.2.2.
microhabitats such as deadwood, bark, and underside of palms and trees with a 30-min sampling effort per quadrat to standardize the sampling. Most live snails were photographed and returned to their habitat after sampling, whereas some empty shells in good condition were collected for photo documentation. The shells were stored in the Department of Biological Sciences, Visayas State University for safekeeping. Land snails were identified up to species level using available published references, e.g. Faustino (1930), Springsteen and Leobrera (1986), and Bouchet et al. (2017). Environmental variables were measured prior to sampling macro land snails during each fieldwork. Geographic coordinates and elevation for each quadrat were determined using the Garmin 12 Hand-held Global Positioning System (Garmin International, Inc., USA). The canopy cover was measured using a concave spherical densiometer (Forest Suppliers, Inc., USA). Air temperature and relative humidity were measured by a digital thermometer or a portable thermo hygrometer (Uni-T, Model UT333, China). For leaf litter depth, five points (corners and center of the quadrat) were sampled by penetrating a ruler into the forest floor. Soil pH and soil exchangeable calcium were determined by collecting approximately 100 g of topsoil at random points inside the quadrat. Soil exchangeable calcium and soil pH were analyzed in the Central Analytical Services Laboratory, Philippine Root Crops Research Center, Visayas State University.
Data Analyses
Species richness, abundance, and diversity. To obtain the overall diversity of the sampling sites and increase the sample size and statistical power of the data, live snails and empty shells were pooled together (Cameron and Pokryszko 2004). The total abundance per quadrat was determined by individual counts. Relative abundance was computed by dividing the species abundance over the total abundance multiplied by 100. Diversity was computed using the Shannon diversity index (H’) complimented with Pielou’s index of evenness (J’) and Simpson’s index of dominance (D). All indices were computed using PAST (Paleontological Statistical Tool) v.3.14 (Hammer et al. 2016) and SPSS (Statistical Package for the Social Sciences). A generalized linear mixed model (GLMM) was generated to identify the best model parameter to determine the most significant predictor of species richness and abundance. Selected environmental factors (e.g. exchangeable calcium, canopy cover) were assigned as fixed factors, whereas species richness and abundance were the response variables. The number of quadrats was the random factor used. The GLMM was run in R (v.3.1.3, R Foundation for Statistical Computing) and R Studio with the use of lmer function in the lme4 package, arm package, and MuMln package.
Species accumulation curve (SAC). Individual-based and sample-based SAC were generated to determine richness estimation and diversity pattern in the study site and the efficacy of sampling effort using EstimateS ver.8 (Dove and Cribb 2006) and plotted in SigmaPlot ver.10.1 (Systat Software, Inc.). Sampling efficacy was evaluated using the completeness ratio (CR = estimated number of species/
observed number of species) (Clements et al. 2006).
Ecological association of species to environmental variables. Canonical correspondence analysis (CCA) was used to compare the relationship between the snail diversity, selected environmental variables, and sampling sites. Detrended correspondence analysis (DCA) was done to infer similarity in the sites based on species. All the analyses were performed in PAST version 3.14 (Hammer et al. 2016).
RESULTS
Sampling Efficacy
The SACs revealed a β-dominated community assemblage, whereas sampling efficiency using CR was adequate for all sampling sites (Figure 2). This means regionally enriched species with a heterogeneous habitat for each species.
The overall CR is 1.00 suggesting that the sampling method employed was very efficient. However, additional sampling effort should be done in Maasin (CR = 0.56) and Baybay (0.86), as reflected in its CR values.
Land Snail Diversity and Assembly Patterns
A total of 592 individuals belonging to 22 species and five families were identified in the three sites. This includes both live snails (87%) and empty shells (13%).
Figure 2. Sample and individual-based species accumulation curves of land snails in the three sampling sites in Leyte Island, Philippines.
Table 1. Macro land snails among the three fragmented forests in Leyte.
Family Scientific name CCA code Status Inopacan Baybay Maasin Total Percentage Camaenidae Amphidromus maculiferus (G.B
Sowerby I 1838) Apc End 12 0 3 15 2.53%
Trachystyla cryptica (Broderip 1841) Tcy End 1 0 1 2 0.34%
Helicostyla dubiosa (Pfeiffer 1846) Cdu End 28 4 7 39 6.59%
Calocochlea valenciennii
(Eydoux 1838) Cvy End 14 0 1 15 2.53%
Chloraea puella (Broderip 1841) Cpu End 13 0 2 15 2.53%
Chloritis leytensis (Moellendorff
1890) Cly End 3 0 2 5 0.84%
Helicostyla faunus (Broderip 1841) Hfa End 23 3 1 27 4.56%
Helicobulinus sarcinosa
(Férussac 1821) Has End 1 0 2 3 0.51%
Hypselostyla boholensis (Broderip
1841) Hyb End 1 0 16 17 2.87%
Obba bigonia (Férussac 1823) Obi End 3 0 0 3 0.51%
Trochomorphidae Geophorus sp. (P. Fischer 1885) Geo End 11 0 32 43 7.26%
Videna sp. (H. Adams & A. Adams
1855) Vid End 19 1 5 25 4.22%
Helicarionidae Hemiglypta semiglobosa (L. Pfeiffer
1845) Hse End 13 43 1 57 9.63%
Achatinidae Subulona cyclindracea (Bourguignat
1890) Scy int 1 0 3 4 0.68%
Lissachatina fulica (Bowdich 1822) Lfu int 26 80 4 110 18.58%
Cyclophoridae Leptopoma sericatum (L. Pfeiffer
1853) Lse End 0 3 2 5 0.84%
Leptopoma perlucidum (Grateloupe
1840) Lpe End 6 20 1 27 4.56%
Cyclophorus appendiculatus (L.
Pfeiffer 1854) Cya End 29 31 38 98 16.55%
Cyclophorus linguiferus (G.B. Sow-
erby I 1843) Cyl End 2 16 30 48 8.11%
Cyclophorus sp. 1 (Montfort 1810) End 0 0 8 8 1.35%
Cyclophorus sp. 2 (Montfort 1810) End 0 0 7 7 1.18%
Cyclotus auriculatus (Kobelt 1884) Cau End 9 11 38 58 9.80%
Total 205 212 175 592 100%
Number of species 22 19 10 21
Note: end – endemic; int – introduce
The 22 species of land snails belonged to Families Camaenidae (10 species), Trochomorphidae (two species), Achatinidae (two species), Helicarionidae (one species), and Cyclophoridae (seven species). Macro land snails’
assemblage was dominated by native snail fauna, which includes eupulmonates and caenogastropods (Table 1;
Figure 3). Five species identified in this study were also included in an earlier inventory from the collection of
H. Cumming, C. Semper, and O. Moellendorff covering various parts from both northern and southern regions of Leyte – namely, Hemiglypta semiglobosa, Amphidromus maculiferus, Leptopoma perlucidum, Chloritis leytensis, and Obba bigonia (Blum et al. 1893).
The highest individual counts were recorded in Baybay with 212, followed by Inopacan with 205 and Maasin with 175. Among the native snail species recorded,
Figure 3. Shells of the macro land snail species collected from the three sampling sites. Amphidromus maculiferus (a); Helicobulinus sarcinosa (b); Helicostyla faunus (c); Calocochlea valenciennii (d); Trachystyla cryptica (e); Calocochlea dubiosa (f); Chloraea puella (g); Chloritis leytensis (h); Obba bigonia (i); Lissachatina fulica (j); Hypselostyla boholensis (k); Subulona cylindracea (l). Scale bar = 10 mm.
Cyclophorus appenidiculatus – a caenogastropod – was the most abundant (98 individuals) representing 16.65% of all sites. It was followed by Cyclotus auriculatus (9.80%), Hemiglypta semiglobosa (9.63%), Cyclophorus linguiferus (8.11%), Geophorus sp. (7.26%), and Calocochlia dubiosa (6.59%). All the remaining native species represent less than 5% of the abundance. In addition, Lissachatina fulica (18.58%) – an invasive snail – was observed in all the study sites in high abundance. The species that were only recorded once or twice in the study areas were Trachystyla
cryptica, Hypselostyla boholensis, Subulona cylindracea, Obba bigonia, and Helicobulinus sarcinosa – mostly found in Maasin and Inopacan.
Species, Sites, and Environmental Parameters CCA showed associations of macro land snails to environmental variables in the study sites (Figure 4). All axes generated were significant (p < 0.01) using the Monte Carlo permutation test. In the CCA biplot, species were represented as filled circles with elevation assigned with
Figure 3. Hemiglypta semiglobosa (m); Cyclophorus appendiculatus (n); Cyclophorus linguiferus (o);
Leptopoma sericatum (p); Cyclophorus mp. 1 (q); Leptopoma perlucidum (r); Cyclophorus mp. 2 (s); Cyclotus auriculatus (t); Videna sp. (u); Geophorus sp. (v). Scale bar = 10 mm.
CCA codes reflected in Table 1, soil temperature, soil pH, soil exchangeable calcium, canopy cover, and leaf litter depth as continuous variables represented as vectors (green). The dominant variables in the study sites as indicated by longer vectors were exchangeable calcium (EXC), elevation (ELV), canopy cover (CC), and soil pH (SPH).
Various associations can be seen, which are denoted by the proximity of each species to each variable. Among the snail species, Cyclophorus linguiferus (Cyl), Cyclotus
auriculatus (Cau), and C. appendiculatus (Cya) were closely related to elevation. Most individuals of these species were collected in Maasin, being the site with the highest elevational range (52–360 masl). Macro land snails such as L. sericatum, L. perlucidum, C. leytensis, Videna sp., and Geophorus sp. are unaffected by leaf litter depth. On the other hand, H. faunus, L. fulica, C. dubiosa, and C. valenciennii were species that were mostly found in areas with increasing soil pH and exchangeable calcium.
This clustering of snails could show their requirement for
Figure 4. Canonical correspondence analysis (CCA) of macro land snail species and environmental variables in Leyte study sites.
Large circles denote the groupings observed in the macro land snails and environmental variables. Biplot represents macro land snails (blue circles with image) and continuous environmental variables (green vectors). Monte Carlo permutation test revealed that all CCA axes are significant with p < 0.05. ELV – elevation; LLD – leaf litter depth; SPH – soil pH;
EXC – soil exchangeable calcium; AT – air temperature; ST – soil temperature; RH – relative humidity; CC – canopy cover. Species names are assigned a CCA code (refer to Table 1).
microhabitats; however, a clearer pattern is needed to have a conclusive finding. The macro land snail species between sites are highly similar, as shown in the DCA (Figure 5) because of the habitat similarities with each sampling site having both secondary forests and agroforests. Most live snails (51%) encountered were found on tree trunks and leaves of dipterocarp and mahogany trees, followed by palm (29%), shrubs (including lianas and broad-leaf plants) (13%), and coconut (7%). Of all species, Obba
Figure 5. Detrended correspondence analysis (DCA) of the land snail species in the sampling sites. Land snail communities were organized into three groups marked by convex hulls (red – species found in Maasin only, yellow – species found in Inopacan, and blue – species recorded in the three sites; n = 22).
bigonia (Obi), Hypselostyla boholensis (Hyb), and Helicobulinus sarcinosa (Hsa) situated extremely in the CCA are considered incidental records since only empty shells were found only once or twice.
Generalized Linear Mixed Modeling
Exchangeable calcium (EXC) was the most significant factor in predicting species richness of macro land snails
Table 2. General linear mixed models testing each environmental variable on the species richness, and abundance across the sampling sites.
Parameter Estimate* SE p-value
Species richness
Elevation (E) –0.00 0.00 0.16
Exchangeable calcium
(EXC) 0.14 0.03 0.00 ***
Soil temperature (ST) –0.08 0.06 0.21
Canopy cover (CC) 0.00 0.29 0.47
Abundance
Elevation (ELV) –0.00 0.00 0.04 **
Leaf litter depth (LLD) 0.18 0.00 0.00 **
Soil pH (SPH) –0.09 0.11 0.41
Soil temperature (ST) –0.09 0.07 0.17
Significant levels: 0 *** 0.001 ** 0.01 * 0.05
Table 3. Summary statistics of model averaging for species richness and abundance of macro land snails across sampling sites (n = 120). Models are ranked based on Akaike's information criterion for (AICc), where AICc weights (wAICc) < 0.10 are excluded. Predictor variables: ELV – elevation, LLD – leaf litter depth, AT – air temperature, SPH – soil pH, ST – soil temperature, RH – relative humidity, EXC – soil exchangeable calcium, CC – canopy cover.
Model component k* AICc DAICc wAICc
Species richness
ELV + EXC + SPH 6 390.59 0.00 0.11
CC + ELV + EXC + SPH 7 390.69 0.10 0.11 ELV + EXC + LLD + ST 7 391.27 0.68 0.08
Abundance
ELV + LLD + SPH + ST 7 583.23 0.00 0.27
ELV + LLD + SPH 6 584.08 0.85 0.18
*k – number of parameters
in the study sites among the standardized parameter estimates (E = 0.14, p < 0.001) (Table 2). The estimated values showed a direct relationship between the response variable and the EXC, wherein more species are expected to be found in areas with higher soil pH. This was also supported by the CCA, where exchangeable calcium was one of the dominant variables in the sites. Exchangeable calcium in the study based on the results has a direct relationship with species richness, i.e. more species are expected to be found in areas with higher exchangeable calcium, which agrees with most studies. The combination of elevation (ELV), soil exchangeable calcium (EXC), and soil pH (SPH) was the most parsimonious model
(DAICc = 0.00, wAICc = 0.11) to explain species richness in Leyte. For abundance, elevation (ELV) (E = –0.00, p <
0.001) and leaf litter depth (LLD) (E = 0.18, p < 0.001) are the most significant predictor, wherein as elevation decreases, abundance increases; conversely, as leaf litter depth increases, the response variable also increases.
DISCUSSION
The species richness and diversity of macro land snails in sampled fragmented forests in Leyte Island shows a promising result and adds to the scant literature available for malacofauna data on the island. It harbors numerous native species, as recorded in other studies in the Philippines with similar forest habitats: Mt. Makiling in Laguna has 14 species (de Chavez and de Lara 2011), Mt. Polis in Central Cordillera with 13 species (Baoanan and Obanan 2011), and Marinduque with 23 species, (Sosa et al. 2014). When compared to other islands in the Visayas, the abundance of land snails in selected forests of Leyte Island was relatively low compared to Mt. Lantoy (n = 872) in Cebu, which can be attributed to several factors. First, the presence of limestone hills in Mt. Lantoy is an excellent source of calcium important for shell development. Calcium-rich soils have been proven to be an important factor in snail abundance observed in karst areas (Valdez et al. 2021) and rainforest areas (de Chavez and de Lara 2011). Second is the elevational range difference in Mt. Lantoy (182–691 masl) compared to this study (45–364 masl). Elevational gradient has been associated with snail abundance, specifically influencing microclimatic conditions in mid-elevations and gradually decreases in higher (de Chavez and de Lara 2011; Horáčková et al. 2014; Rosales et al. 2020). Third, Mt. Lantoy is a protected landscape, and the survey was conducted in a permanent biodiversity monitoring plot with minimal anthropogenic disturbance. In contrast, the sampling sites in Leyte were not protected landscapes with evident anthropogenic activities. Lastly, the difference in sampling methodology and environments in the area might have also affected the result. The selected fragmented forests in Leyte have high species richness but with low species distribution and evenness, possibly caused by habitat loss and degree of anthropogenic disturbance like conversion to agricultural areas. The decline of diversity in areas with evident human activities such as agroforestry among the sites agrees with the finding of Belhiouani et al. (2019) in northeast Algeria, wherein areas with anthropogenic disturbance such as a change in vegetation recorded lower land snail abundance and diversity. Similar findings were also observed in Mount Makiling, wherein habitat destruction had an impact on the land snail population (de Chavez and de Lara 2011).
Nurinsiyah et al. (2016) also reported that decreased human impact can enhance the abundance and diversity of pulmonates and prosobranchs.
The β-dominated community assemblage among sampling sites indicated a close association between habitat characteristics and human activities, which can cause significant shifts in local diversity (Socolar et al. 2016).
Although the CR was relatively high (0.91–1.00), the sampling for Maasin and Baybay was not enough as the asymptote graph projected upward. This suggested that more quadrats and individuals were needed to capture species diversity. This also implies that more species may be recorded in these study areas if the number of quadrats or number of individuals is increased. The results of CCA suggested that although particular environmental variables were prominent in each sampling site, i.e. their effect on the abundance across all sites were the same;
however, it was different for species richness. In several studies, macro land snail species richness was shown to be affected by combined effects in their habitat (Horsák et al. 2014; Astor 2014).
Exchangeable calcium was the most significant factor in predicting the species richness of macro land snails in the study sites. As calcium went higher, more species were recorded in the study. The same observation was recorded by Martin and Sommer (2004), Schilthuizen et al. (2002), and Nurinsiyah et al. (2016), where calcium availability and soil moisture are the strongest determinants of species richness and abundance in a forest ecosystem. This was evident in Maasin since it can be categorized as forest over karst, hence the high species counts. It was also recorded in the study that forest fragments with denser vegetation and less anthropogenic disturbance contained higher species richness compared to highly disturbed forests despite the size of the forest fragment. These human-induced habitat alterations leading to forest fragmentation may have longer- lasting effects on population structure due to prolonged ecological pressure (Bloch and Bloch 2012). Species richness inverse relationship with elevation agrees with most studies (de Chavez and de Lara 2011; Schilthuizen et al. 2011; Sosa et al. 2014; Rosales et al. 2020) that as elevation decreases, more species or higher count of individuals are expected to be recorded. This is known as the mid-domain effect (Colwell et al. 2004), which reflects the natural species ranges. Forests can provide shelter and refuge to various species, but regular cyclic disturbances such as typhoons or extreme dry season (Bloch and Bloch 2012) exacerbated by anthropogenic modifications can cause a decline in land snail abundance. Land snails persisting in rainforests with restricted mobility, limited dispersal, and specific microhabitat requirements (Baoanan and Obanan 2011; de Chavez and de Lara 2011; Pfenninger and Posada 2002) are, hence, highly vulnerable to habitat
disturbance that can pose a risk for local extinction. Thus, research on the status of land snails in threatened and fragmented forests should be done to prevent the reduction of endemic species.
CONCLUSION AND RECOMMENDATIONS
The present study demonstrated that macro land snails in forest fragments of Leyte Island are highly diverse despite having low abundance and constant exposure to anthropogenic disturbances. It shows the need to protect the forest fragments of Leyte Island because these fragments support endemic malacofauna and provide other ecological services. This study, however, assessed only a fraction of Leyte Island with a focus on macro land snails. To complete the documentation of malacofauna diversity in Leyte, it is recommended to examine other areas. There is also a need to compare land snail diversity based on biogeography, i.e. across Leyte and from areas separated by mountain ranges. Future studies on gastropod populations are also highly recommended in the same sites to monitor their responses to the long-term effect of environmental and land-use change. Furthermore, genetic connectivity research for forest fragmentation on the island should be incorporated.
ACKNOWLEDGMENTS
The authors would like to thank the Forest Foundation of the Philippines for the financial support under the Small Grant (GA No. 2019-33), as well as the local government units of Baybay, Inopacan, and Maasin for their assistance in providing local guides during the sampling. To DENR Region 8 for granting the gratuitous permit (RO8 2019-10) for sample collection, as well as to the Department of Biological Sciences of Visayas State University for allowing the use of their facilities. Lastly, to the National Research Council of the Philippines for the thesis publication grant.
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