Introduction
Coastal sand dunes are a type of coastal terrain formed by wind-blown sand deposited behind sandy beaches or tidal sand flats (Ryu, 2014). Numerous varieties of coastal sand dunes have developed on the western coast of Korea based on the surrounding undulations and coastal currents. In addition, the majority of warm and damp coastal sand dunes are covered with vegetation; this facilitates the deposition and accumulation of sand because the germination and growth of vegetation increase surface irregularities (Kahng, 2006).
Coastal sand dunes and tidal mudflats are iconic coastal ecosystems of major ecological value because they serve as a transition zone between marine and terrestrial ecosystems while also fulfilling a buffering function as an ecotone (Van der Maarel, 2003; Walker et al., 2003). Despite their modest size, coastal sand dunes exhibit higher biodiversity than numerous other ecosystems; moreover, they contain specialized flora and fauna that are distinct from those of inland ecosystems (Kutiel et al., 2000).
Insects are the primary and secondary consumers in food webs within an ecosystem; they constitute the center of an ecosystem (Triplehorn et al., 2005). In addition, insects in coastal sand dunes play an important role as consumers and decomposers of various organisms that wash ashore. However, the rapid expansion of the tourism industry in recent years has reduced the surface area of coastal sand dunes owing to the construction of various infrastructure, such as camping sites and parking lots, in and around coastal sand dunes (Choi et al., 2011). These developments have caused the destruction and disruption of sand dunes and contributed to the decline and loss of insect biodiversity in these areas (Shin & Bae, 2015).
In Korea, research on insects inhabiting coastal sand dunes has primarily focused on emerging species (Han, 2010; Yahiro & Lee, 1995). For example, ecological studies have investigated the behavior and life cycle of Scarabaeus typhon inhabiting the coastal sand dunes of Shindu-ri. Changes in the insect population of the Goraebul coastal sand dunes have also been documented (Min et al., 2018; Shin & Bae, 2015).
The objective of this study was to quantify the population size and analyze the spatial and temporal distributions of three predatory Scarites species (i.e., Scarites aterrimus, Scarites sulcatus, and Scarites terricola pacificus) in the Sohwang coastal sand dunes. These species occur together are endemic to this area and serve as indicators of coastal sand dunes. This phenomenon is rare in Korea. Scarites species are the most voracious sand dune predators, preying on Lepidopteran and Coleopteran larvae as well as earthworms (Larochelle & Larivieve, 2003; Yamazaki & Sugiura, 2006).
Materials and Methods
Study area
The Sohwang coastal sand dunes are situated in Sohwang-ri, Ungcheon-eup, Boryeong-city, and Chungcheongnam-do and cover an area of approximately 121,358 m2. This area has restricted public access because of the nearby military base and is subsequently well preserved. The Sohwang coastal sand dune area was designated an ecology and landscape conservation area by the Ministry of Environment in 2005.
Pinus thunbergii forest serves as a windbreak on the gradually sloping terrain at the rear. The zones between the sand dunes and P. thunbergii accommodate Robinia pseudoacacia communities. In addition, there are other sand dune vegetation types, such as Elymus mollis, Vitex rotundifolia, Carex kobomugi, and Calystegia soldanella present. In this dune vegetation area, a total of 12 families and 23 species of vascular plants have been surveyed, including nine families and 14 species of coastal sand dune vegetation, and three species of naturalized plants (Lee, 2011).
Three sites within the Sohwang sand dune vegetation were selected to investigate the Scarites species. Site A, which was dominated by Calystegia soldanella, contained P. thunbergii, which attracted frequent visitors from the general public. Site B, which was dominated by Vitex rotundifolia, formed a dune hill with Robinia pseudoacacia. Site C was characterized by a large expanse of dune vegetation dominated by Elymus mollis (Table 1).
Survey method
Our surveys were conducted on five consecutive days in June and August 2014 in a phased manner (first phase: June 21-25; second phase: August 30-September 3) while Scarites species imagoes were active. Trapping webs (TWs) were deployed on beaches and dune vegetation areas, and a mark-release-recapture method was used for quantitatively investigating Scarites species. TWs were installed at three different sites on the Sohwang coastal sand dunes, subdivided into Calystegia soldanella, Elymus mollis, and Vitex rotundifolia areas based on vegetation dominance values, in order to assess the insect density contingent on each vegetation type. The TWs encompassed an area of 28×28 m from the shore to the sand dune vegetation with 12 lines extending from a central point, seven circles at 2 m intervals, and 84 pitfall traps (PFTs) at the intersections of the lines and circles (Fig. 1). Regardless of the number of days in question, opportunistic captures were conducted when the daily sample size was minimal. If the population size was sufficient, the number of captured individuals would likely decrease over time.
Estimation of population size
First, the threshold was calculated using the model proposed by Anderson et al. (1983) to estimate ground beetle population density. The threshold was calculated as follows:
where CT represents the area enclosed by the circle beyond which data are truncated, and ni denotes the total number of unmarked insects captured by traps in circle i.
Next, the estimation value was calculated as below.
where f represents an absolute value.
The Fourier-series (F-s) model was adopted to establish the goodness-of-fit of the estimate. The F-s model estimates the expected values of the beetles that can be captured by the PFTs installed in each circle, which are subsequently compared to the outdoor values to assess the fitness level (Anderson et al., 1983).
Results
Captured and estimated populations
In June, our surveys of the Sohwang coastal sand dunes revealed that S. sulcatus was dominant, with 383 individuals captured. This was followed by S. aterrimus with 13 and S. terricola pacificus with five captured individuals. In the August survey, 30 S. sulcatus, 20 S. aterrimus, and no S. terricola pacificus individuals were captured. Consequently, S. sulcatus was the most frequently recorded insect among the three species, whereas S. terricola pacificus was the least frequently recorded. In addition, the largest quantities of S. sulcatus and S. occurred in June and August, but S. terricola pacificus was only observed in June (Table 2). At the three different sites (A, B, and C), 188 and 30 S. sulcatus individuals were recaptured using TWs in June and August, respectively, while neither S. aterrimus nor S. terricola pacificus individuals were recaptured. In terms of different vegetation communities, the largest quantity of S. sulcatus was captured in Vitex rotundifolia trees. In contrast, the largest quantities of S. aterrimus and S. terricola pacificus were found in Calystegia soldanella. This demonstrated distinct differences in population density within various plant communities.
In terms of population, it was estimated that S. sulcatus dominated site B (36.6 individuals in a 10 m2 area) with a higher density than the other two species. In contrast, S. aterrimus dominated site A in August, with a density of 2.9 individuals per 10 m2. Reliability analysis of the estimate showed that S. sulcatus at site B in June had a significantly higher density (43.12 individuals per 10 m2, χ2: 0.005, 20.277) than at the other sites. In addition, S. sulcatus exhibited significantly higher densities at each site in August (χ2: 0.050, 12.591). S. aterrimus was captured in June and August, but only the August result was statistically significant for this species. The number of S. terricola pacificus captured was insufficient for estimation purposes (Table 2).
Daily captured population
S. sulcatus was recorded in the highest numbers at all sites on the first day of both the June and August surveys, after which the captured population gradually decreased. S. aterrimus was captured in the highest amount on the third day, except at site B, where nothing was captured, with no bias in terms of the capture method over the specific period. S. terricola pacificus was captured in modest numbers and thus was captured opportunistically as opposed to at a specific time period (Fig. 2).
Spatial distribution
Concerning the spatial distribution of the three Scarites species, insects were captured in specific regional habitat areas between S. sulcatus and the other two Scarites species. The June and August surveys revealed that S. sulcatus was evenly distributed throughout the sandy beaches of the foreshore, embryo dunes, and dune crests at all three sites. In particular, several of these species were observed in the Vitex rotundifolia community, demonstrating their affinity for hiding places. However, in both the June and August surveys, S. aterrimus was recorded on the beach of the Calystegia soldanella community, whereas S. terricola pacificus was captured in the Calystegia soldanella region, a beach dominated by Elymus mollis, and embryo dunes in the June survey (Fig. 3).
Discussion
The difference between S. aterrimus and S. terricola pacificus habitats and S. sulcatus habitat on the Sohwang coastal sand dune in terms of the spatial distribution of each beetle species could be attributed to the tendency of S. aterrimus and S. terricola pacificus to avoid competition for food and predators by living in the vicinity of the beach. Moreover, surveys on insects inhabiting the coastal sand dunes of the Korean Peninsula demonstrated that S. aterrimus preferred beach topography over vegetation and was primarily found in beach areas as opposed to vegetation areas. This same holds true for S. terricola pacificus (Lee, 2011). Although additional research on the local landscape and vegetation is required, it appears that these two species are greatly affected by sand dune coverage, surrounding vegetation, and predators.
Spatial distribution analysis of all three species indicated that S. aterrimus and S. terricola pacificus, but not S. sulcatus, mainly inhabited the beach dominated by Calystegia soldanella. This area featured low vegetation coverage, a well-developed beach, and a variety of floating matter pushed from the ocean, providing excellent hiding places for insects. As such, vegetation coverage close to the habitat has a considerable effect on the activity and population density of beetles (Eyre et al., 2016), with beetle communities differing markedly depending on the vegetation type (Latty et al., 2006). It is also recognized that altitude above sea level, exposure to sunlight, and canopy density have considerable impacts on the community structure of beetles (Bergmann et al., 2012; Larochelle & Larivieve, 2003). This is also assumed to be associated with the comparatively small population of S. sulcatus, which is ecologically dominant in this area, as well as the relatively small population of Eremias argus, which is a prominent predator in this area. This is why abiotic factors (pathogens) and predators are the most causative factors of ground beetle mortality (Lövei & Sunderland, 1996).
In terms of monthly occurrences, S. terricola pacificus was solely observed in the June survey, whereas S. sulcatus and S. aterrimus were observed in both the June and August surveys. Years of monitoring have confirmed that S. terricola pacificus is primarily observed in May and June; in contrast, the other two species are recorded in June and August, indicating noticeable differences in the appearance periods of S. terricola pacificus and the other two species (ME, 2014).
A recent survey indicated that seasonal factors affected the emergence patterns among different Scarites species. Furthermore, it is likely that various species have distinct habitat preferences, allowing them to avoid competition for food and space. In particular, S. terricola pacificus had the smallest habitat, necessitating more precise beach-focused surveys. The estimated population was calculated using the TW mathematical model, which indicated that the number of captured individuals would likely increase as the target sites advanced from the center point to the outskirts. The exclusion of these edge effects was anticipated to result in more accurate population estimates per unit area. However, in this study, we demonstrated that there was something to be sought beyond the edge effect (Fig. 4). It is presumed that complex landforms of coastal sand dunes, characterized by sandy beaches, embryo dunes, and dune crests, have developed within a small area (Choi et al., 2011). Therefore, additional surveys and analyses are required to apply the recently estimated population sizes to all areas of the Sohwang sand dunes.
This study was based on vegetation types, estimated spatial distributions, and population sizes of three Scarites species recognized as emblematic carnivorous insects of sand dune ecosystems. Consequently, it is anticipated that the findings of this study will contribute to the estimation of the population density of insects on the Korean Peninsula and the development of assessment methods for conducting investigations.
Figures and Tables
Fig. 1
Sites of quantitative survey on sand dune insects and pit-fall traps conducted in Sohwang sand dune. (A) Trapping web installation site. (B) Trapping web design. (C) Trapping web installation. (D) Markings on Scarites sulcatuscaptured in pit-fall traps.
Fig. 2
Populations of three Scarites species captured at each site (A, B, C) on a daily basis.
Fig. 3
Spatial distribution of the three Scarites species (non-border circle, population in June; border circle, population in August).
Fig. 4
Captured population of Scarites species on Sohwang coastal sand dune per each circle number from the center point.
Table 1
Dominant plants and coverage at different sites
| Site | Dominant plant communities | Coverage (%) | Beach length (m) | Remark |
|---|---|---|---|---|
| A | Calystegia soldanella, Carex kobomugi | 50-60 | 13 | A lot of floating matter |
| B | Vitex rotundifolia, Elymus mollis | 90-100 | 8 | A little floating matter |
| C | Elymus mollis, Carex kobomugi | 80-90 | 34 | A lot of floating matter |
Table 2
Mark-release-recaptured estimated population of Scarites sp. in Sohwang coastal sand dune
| Species name | June | August | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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| Number of unmarked insects captured (number of recaptured) |
Population density estimate (10 m2) (G-value) |
Number of unmarked insects captured (number of recaptured) | Population density estimate (10 m2) (G-value) |
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| Site | Site | Site | Site | ||||||||||||
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| A | B | C | A | B | C | A | B | C | A | B | C | ||||
| S. sulcatus | 40 (25) | 242 (123) | 101 (40) | 6.1 (4.27) | 36.6 (43.12*) | 20.8 (4.23) | 8 (1) | 9 (1) | 13 (2) | 0.8 (14.23**) | 1.4 (26.9**) | 0.9 (31.44*) | |||
| S. aterrimus | 12 (0) | - | 1 (0) | 2.3 (24.0*) | - | - | 18 (0) | 1 (0) | 1 (0) | 2.9 (30.8*) | - | - | |||
| S. terricola pacificus |
3 (0) | 1 (0) | 1 (0) | - | - | - | 1 (0) | - | - | - | - | - | |||