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Journal of Resources and Ecology >

2023 , Vol. 14 >Issue 5: 1092 - 1103

DOI: https://doi.org/10.5814/j.issn.1674-764x.2023.05.020

Animal Ecology

Assessing Vulnerability to Environmental Changes of Freshwater Crab, Thaipotamon chulabhorn in the Dun Lumpun Non-Hunting Area, Thailand

  • Arika BRIDHIKITTI , 1 ,
  • Bidur KHADKA 2 ,
  • Suraj SHARMA , 2, *
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  • 1. Environmental Engineering and Disaster Management Program, Mahidol University Kanchanaburi Campus, Kanchanaburi 71150, Thailand
  • 2. Kathmandu Forestry College, Tribhuvan University, Kathmandu 44600, Nepal
*Suraj SHARMA, E-mail:

Arika BRIDHIKITTI, E-mail:

Received date: 2023-01-29

  Accepted date: 2023-04-30

  Online published: 2023-08-02

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Abstract

Thaipotamon chulabhorn (Mealy Crab) is a freshwater crab, listed as ‘Least Concerned’ in the IUCN Red List of Threatened Species. Nonetheless, this crab is exclusively found in one small area of the Dun Lumpun Forest, Thailand, and currently faces high exposure to environmental changes. This study aims to investigate key dynamic factors that influence vulnerability to environmental changes of the Mealy Crab. The study was conducted between 15 March and 23 October 2016, covering the local summer until the end of the rainy season and being influenced by strong El Niño, thus suggesting drought episodes. This vulnerability assessment was carried out through crab population records from 1997 to 2016, field measurements/surveys, and interviews with experienced park rangers. The results revealed that drought is the major vulnerability factor that is threatening the population of the crabs and their livelihoods (7.96 out of 10), followed by an increased number of their natural enemies (7.41), variations in groundwater level (6.11), changes in groundwater quality (4.63), changes in forest soil (4.63) and human intervention (4.26). Since 1996, human intervention has been found to have a little direct impact on the crab population due to the restricted access to forest resources. The 2016 drought was accompanied by anomalously low rainfall and the early onset of the rainy season, contributing to an earlier and shorter mating period of the crabs. Nonetheless, a consistently increased number of the Mealy Crab population indicated that they are not significantly vulnerable to a larger number of their natural enemies, and even to the changes in groundwater and forest soil. In conclusion, the crab population was increasing along with changes in the habitat and climate and its growth cycle was unusual during the drought.

Key words： Thaipotamon chulabhorn; Dun Lumpun Non-Hunting Area; freshwater crab; vulnerability

Cite this article

Arika BRIDHIKITTI , Bidur KHADKA , Suraj SHARMA . Assessing Vulnerability to Environmental Changes of Freshwater Crab, Thaipotamon chulabhorn in the Dun Lumpun Non-Hunting Area, Thailand[J]. Journal of Resources and Ecology, 2023 , 14(5) : 1092 -1103 . DOI: 10.5814/j.issn.1674-764x.2023.05.020

1 Introduction

Thaipotamon chulabhorn Naiyanetr, 1993 (Mealy Crab) is a kind of freshwater crab recognized for its colorful figure, which can be exclusively found in the Dun Lumpun Non-Hunting Area in Mahasarakham Province, Thailand. Since no evidence of its habitat degradation and unknown long-term threats (IUCN taxonomy, 2018), the crab is recently listed as ‘Least Concerned’ in the IUCN Red List of Threatened Species. Since 1996 after the discovery of the crab, Dun Lumpun Forest has been declared as Dun Lumpun Non-Hunting Area. This crab species has not been found outside this forest and their life cycle could be induced by local climate variation, especially rainfall (Mahasarakham University, 1997). The soil there is quite moist and has high fertility, whereas those in the perimeter are composed of higher sand fractions with relatively low fertility and salt rock appearance on the surface (Maharasakham University, 1997). Ketwong (1998) studied land cover changes in Dun Lumpun Non-Hunting Area from 1992 to 1997 using aerial photographs and found a dramatic increase in areas of healthy forest and water surface. The changes were claimed to be due to less human intervention and the construction of a man-made waterway along the road to the forest (Ketwong, 1998). Furthermore, Sardprom (2012) found that upon the establishment of Dun Lumpun Non-Hunting Area, there is limited public involvement in forest conservation because local people have less chance to conduct cultural practices in the forest area.
Implied from the above findings, the socio-environmental dynamics of the Dun Lumpun Non-Hunting Area are currently developing and could more or less affect the livelihood and the population of the crabs. Nonetheless, there is a limited number of research publications presenting updated information regarding biodiversity and the environment of the forest since 1998. This study attempts to bring up-to-date key dynamic factors potentially affecting the crab vulnerability to environmental changes. The study was conducted between 2015 and 2016 when Southeast Asia was experiencing drought due to strong El Niño development in the Pacific Ocean (NOAA, 2016). The vulnerability assessment involved records of the crab population, field observations, and interviews. The findings revealed some vulnerability of the crab to current environmental changes.

2 Methodology

2.1 Studied site description

Surrounding by rice fields, Dun Lumpun Non-Hunting Area is located in Nachuak Sub-district, Mahasarakham Province. In 1993, the Dun Lumpun community forest was in the limelight due to the significant discovery of Thaipotamon chulabhorn, the world’s newly freshwater crab species. Because of its unique ecosystem that is very suitable for this crab species to live, the Department of Forestry, thus, designated this site as Dun Lumpun Non-Hunting Area in 1996, while restricting human access to the site and preserving it as a natural property of the public (Mahasarakham University, 1997). Protected by law since 1996, soil salinity in Dun Lumpun forest has been significantly declined attributed to man-made water channel nurturing trees in the forest (Ketwong, 1998). Nonetheless, this healthy forest attracts more rodents, which could affect the stability of the crab population (Ketwong, 1998).
As illustrated in Fig. 1, Dun Lumpun Non-Hunting Area (15°46′22″N, 103°01′28″E) is characterized by a mixture of deciduous forest and swamp, covering an area of 54.9 ha. Its dominant perennial and sapling plants include Casearia grewiaefolia Vent, Terminalia alata Heyne ex Roth, Vitex pinnata Linn, Eugenia cumini Druce, Streblus asper Lour. Dominant undergrowth plants are Imperata cylindrical (L.) Beauv, Panicum repens Linn, Pteris sp. and Cyperus leucouphalus Retz (Mahasarakham University, 1997). The soil in the forest exhibits high organic content (5%-8%), characterized by a very dark gray to black color with clay loam texture which is quite well-drained and can be found on shallow groundwater (Mahasarakham University, 1997).
Fig. 1 Sampling sites in Dun Lumpun Non-Hunting Area
The climate of this area is influenced by the Southeast Asian monsoon system, exhibiting rainy season from July to September, winter season from October to February, and summer season from March to June (Mahasarakham University, 1997). Based on the analysis of anomaly sea surface conditions across the Pacific Ocean, strong El Niño events had influenced Thailand’s climate from the local summer 2015 to summer 2016 (NOAA, 2016). Under this regional climate variation, extreme drought was expected to affect Thailand in 2015 and 2016, resulting in lower rainfall magnitude in the rainy season and severe drought in the summer season.

2.2 Acquisitioning long-term crab statistical records

The crab population has been counted once a year in the early rainy season since 1997 by the park rangers. All carb holes are counted and the counted holes are marked with colored bamboo sticks to avoid duplication. This count is assumed that each hole is occupied by one crab.

2.3 Identifying significant vulnerable factors

Upon the in-depth interviews with nine park rangers who have worked at the Dun-Lumpun Non-Hunting Area for 11 to 19 years, six environmental factors were identified to have significant impacts on the crab population. They are 1) human intervention; 2) enemy population; 3) groundwater level; 4) groundwater quality; 5) soil characteristics; and 6) drought. Furthermore, each of the forest officers was asked to rank these six factors. Then, their given scores were averaged, rearranged, and normalized using the 10-based scale—ranging from 1 (the lowest) to 10 (the highest).

2.4 Observations of physical characteristics of the crabs, their behaviors, and habitats

Physical characteristics were assessed from the crabs in the exhibition zone. The measurements and observations dealt with sex, size, weight, and the appearance of eggs on female pleon. In terms of their behavior and habitat, the observations were carried out through several means as described below:
Nighttime observations were conducted using an infrared video recorder (Power Bank Full HD 1920×1080 P model), which captured 5 randomly selected holes in the forest from 7:00 PM to 2:00 AM from 9 April to 24 October 2016.
Daytime observations of 34 crab hiding holes at three studied sites (see Fig. 1) in the forest were conducted once a month from 9 April to 24 October 2016 to determine the hole-closure period, seasonal survival rate, and the dimension of the food-scavenging zone. The longest diameter of the hole and the scavenging zone, the area with less leaf litter (Fig. 2), were measured. The dynamic status of the holes was classified into ‘activated’, ‘abandoned’, and ‘closured’. The active hole was suggested from clean holes with fresh-wet soil balls nearby. The abandoned hole refers to an unoccupied status, probably because that crab is dead already or moved to anywhere else. This hole looks like pretty dirty holes without any fresh-wet soil balls, except a spider net inside the hole. The closured hole is identified as a shallow hole sealed with soil.
Fig. 2 Hole configuration and scavenging zone
Day-to-Night observations of crab behaviors in the exhibition zone were done once a month from 9 April to 24 October 2016 using an infrared video recorder (All-In-One HD IP camera). The exhibition zone is designed for the crabs with imitating natural habitats. It is a 1.0×1.5 m2 acrylic box, containing clay, plastic grass, natural rocks, and groundwater from the well outside the forest. Once every 15 minutes, the number of active crabs was counted for five minutes to study physical responses of the crabs to diurnal change. The crabs’ responses to human (such as tourists) intervention were also remarked from the video recorder.
A field survey was conducted on two transect lines (Fig. 1) with distances of 520.93 m for line A and 418.18 m for line B. Along with the survey, the number of holes (activated, abandoned, and closured) ± 1 meter from the lines were count, and photos of the hole environment were taken to determine proportions of the areas preferably inhabited by the crabs. This survey was done in the daytime on 14−15 August 2016. The results were also used to justify crab population density.
Interviews with the park rangers regarding crab spatial distribution started after the completion of the regular annual crab population survey in August 2016. All park rangers were asked to indicate densely crab populated areas on a grid map.
Measurements of the meteorological condition under the forest canopy were conducted from 9 April to 23 October 2016 using humidity, barometric pressure, temperature, and vapor pressure sensor (VP-4) with EM50 data logger from Decagon Devices, Inc. The sensor was installed at 1.60 m above the ground at Site 2. Furthermore, daily rainfall information was acquired from the Dun Lumpun Non-Hunting Area Rain gauge station, which has data available since January 2002.

2.5 Groundwater quantity and quality

Groundwater depth was measured at five locations (Fig. 1). Wells 1−4 each consisted of dual wells with 2-and 5-m depths. Well 5 has an undefined depth with a large open area of 0.8 m in diameter. In this study, groundwater level from well 5 was compared with those reported in the work of Mahasarakham University (1997). For wells 1−4, groundwater depths were measured once a month from 14 July to 23 October 2016. This measurement was incompatible timeframe with those reported in Mahasarakham University (1997), measuring the groundwater depth from April to May 1997. The results obtained from wells 1-4, therefore, were only used to explain the spatial distribution of the crab population in 2016 in response to the groundwater depth. Groundwater quality was solely measured in well 5, which is pH using digital pH meter (PH-8685 AZ Instrument) and conductivity using digital EC meter (Aquapro AP2 HM Digital). The results were compared with the groundwater quality obtained by Mahasarakham University (1997) and then its temporal changes were discussed.

2.6 Measurements of physical soil characteristics

Hourly soil moisture and soil temperature were collected at 0.5 m from soil surface using soil moisture sensor and soil temperature sensor (EC-5 and RT-1, respectively) with EM50 data logger from Decagon Devices, Inc. The sensors were installed at Site 2 (Fig. 1).
Soil texture and soil color were assessed in the field on 18 September 2016 using the ribbon test and Munsell soil color chart, respectively. Twenty-three samples were taken from the soil surface near the three studied sites and the observed wells. Another 23 samples were taken over fringes of the crab holes, represented subsoil carry over the top by the crabs (Fig. 2), and 9 soil samples were from rice fields close to the forest. A total of 55 samples each weighing about 100 g was used in the laboratory for soil pH assessment.
Six soil cores were collected on 18 September 2016 and they were tested for saturated soil hydraulic conductivity and soil density. Three samples were obtained from three studied sites (Fig. 1): in the forest area with the appearance of many crab holes, in the outskirts of the forest, and the Dry Dipterocarp forest. All the soil cores had the same diameter and height of five cm and it was used to collect the soil at a 15-cm depth from the surface. The soil density was estimated by dividing oven-dry soil weight (at 105 ℃ for 24 hours) by the soil core volume. Soil hydraulic conductivity was later assessed in the laboratory with the soil core samples using Minidisk portable tension infiltrometer (Decagon Devices, Inc.) with known soil texture and two to six suction rate adjustments.
The soil pH of the 55 samples was assessed according to the 1:1 soil: water extraction procedure. The soil was first sieved and weighed to 30.00±0.02 g using a two-digit balance. Then, thirty milliliters of distilled water were added into a plastic beaker containing 30 g of soil. The soil: distilled water solution was shaken for 15 minutes before its pH was measured using calibrated digital pH meter (PH-8685 AZ Instrument).
Biomass deposition rate onto the soil was assessed using the weighting method. The air-dried biomass including leaves, twigs, fruits, and seeds, which were deposited on the sampling areas (0.7×0.7 m2, see Fig. 1) over specified periods, were weighed and the deposited vegetation species corresponding to high contribution into the soil biomass were identified from the collected leaves. The leaves were collected monthly from 15 March to 24 October 2016.

3 Results and discussion

3.1 Significance of factors affecting the crab population based on the interviews

There are six environmental factors considered in this study and they are potentially affecting the number of the crab population in the Dun Lumpun Non-Hunting Area. Based on the scores given by the nine park rangers, the significance of the six factors were ranked from drought (7.96 out of 10), the greatest affecting the crab population: followed by an increase in crab-enemy population (7.41), variation in groundwater level (6.11), changes in groundwater quality (4.63), changes in forest soil (4.63) and human intervention (4.26). Human intervention has had little direct impact on the crab population since 1996 the year in which the forest was officially declared as Dun Lumpun Non-Hunting Area. Local people also believe that there is the Angel Guardian who protects the forest and the Mealy Crab. Furthermore, the taste of the Mealy Crab is not as appealing as that of the typical freshwater crabs found in rice fields. Nonetheless, curious tourists could be responsible for the death of the crabs (for example, improperly catching crabs just for fun). Parts of the crab body, especially its legs, could be defected and the crab could easily be hunted at night.

3.2 Dynamics of Mealy crab behaviors and habitats

3.2.1 Physical characteristics of Mealy Crabs

A total of 48 Mealy Crabs (22 females and 26 males) caught for the exhibition zone by the park rangers were measured and weighed. The crabs were of different sizes and sex without the appearance of an egg on a female pleon. The crabs were kept in the exhibition zone for three to seven days before they were released back to the forest. The exhibition zone is designed to allow insects to freely fly in and they finally become food for the crabs. Table 1 shows the measurement results. The crabs weighed from 12 to 47 g, corresponding to their sizes. The male crabs were often found bigger and heavier (up to 47 g) than the female crabs (up to 37 g).
Table 1 Weight, dimensions and sex distribution of the observed Mealy Crabs
Weight (g) Number Carapace dimension (cm)
Female Male Upper length Lower length Width
10-15 3 3 2.17 2.33
16-20 2 3.5 2.38 2.63
21-25 5 5 3.63 2.44 2.99
26-30 5 4 3.87 2.74 3.09
31-35 8 8 3.95 2.83 3.27
36-40 2 4 4.24 3.15 3.4
41-47 2 4.4 3.15 3.25
The coloration of the Mealy Crabs can be characterized by mangosteen purple carapace, yellowish-orange ambulatory legs, and cheliped palm, and white chelal fingers, as shown in Fig. 3a. These colors became paler when the crabs were socked in hard water, taken from off-forest groundwater for a couple of days due to scaling formation of the carapace (Fig. 3b). Typically, the colors on the crab carapace are usually bright and attractive during the mating period. Though this physical change does not affect their behaviors, it could more or less influence the biological selection of the crabs in nature.
Results from monthly observations in the forest and at the crab exhibition zone suggest growth cycle of the crab in 2016 as described below:
Fig. 3 Colors of the crabs on different dates (a) first day and (b) fifth day in the exhibition zone
On 9 April 2016, two female crabs in the forest were found ovigerous (bearing eggs on their pleon). Typically, baby crabs hatch from eggs between March and May. The mother crabs and their babies usually stay inside the hole until the onset of the rainy season, thus triggering the moist soil condition (Mahasarakham University, 1997).
On 1 July 2016, female crabs in the exhibition site were not ovigerous, whereas, in the forest, several couples were found matting. The findings of the present study are similar to the findings in the work of Mahasarakham University (1997), which revealed that mating was observed in July 1997, but typical mating started in August (Mahasarakham University, 1997). This early mating season in 1997 was claimed to be a result of the early onset of the rainy season after long-term drought from May to June 1997 (Mahasarakham University, 1997).
On 14 August 2016, female crabs in the exhibition zone were not ovigerous and no mating couple was found during the line transect survey in the forest. Approximately 7.2% of the total observed holes along the transect lines were abandoned; whereas a similar proportion of the holes was closed (7.5%). The crabs usually close their hiding holes with clayey soil to protect themselves during the most vulnerable periods. Furthermore many tiny holes were found near the water channel. This could imply the appearance of juvenile crabs.
Figure 4 shows the diurnal percentage of active crabs observed in the exhibition zone with an infrared video recorder. The result shows that the crabs were highly active between 6:00 AM and 7:00 PM. They were inactive mostly from 2:00 to 4:00 PM. The crab activities from 9:00 to 11:00 AM were quite varied due to human intervention. Park rangers and tourists came to see the crabs in the exhibition zone and occasionally touched or picked them up.
Fig. 4 Percentage of active crabs in the exhibition zone
Nighttime observations in the forest were conducted using an infrared video recorder. The video reveals that those crabs usually stayed still at the tip of their holes the whole night and they were highly alert when recognizing any moving objects. They did neither stay far from the holes nor eat anything at this time. The field observations show that the period in which the crabs were usually found having foods like decayed leaves and worms, was between 6 and 8 AM. This is corresponding well with the crab active period identified earlier.
In summary, the growth cycle of the Mealy Crabs from the study in 2016 was similar to the work in 1997 by Mahasarakham University (1997) but slightly different from the typical growing epoch. In 2016, the crabs laid eggs from June to July, mated at the end of June until July, and their new generation hatched and was exposed to the outside world in August.

3.2.2 Crab habitats and environment

The field observations were conducted to monitor 34 Mealy Crab holes in three sites (see Fig. 1). Hole sizes ranged from 2.43 to 8.06 cm in diameter and the scavenging distance (appearing clean space centered at the hole, see Fig. 2) was from 5.50 to 23.67 cm. Ten out of 34 holes had a circular shape, but the rest came up with an elliptical shape. Using a hole shape to identify sex (circular for male and elliptical for female) as suggested by Mahasarakham University (1997) is no longer an effective means. Many holes were found being secured with clay during the wet period, 11.7% in the mid of August and 17.6% at the end of October. The crabs could close their holes during the most vulnerable periods, including egg hatching and molting. Unoccupied holes were found in up to two out of a total of 34 holes in each survey. Nonetheless, most of the empty holes were found occupied in the following month. Flooded holes were only found at Site 2 in early July (two out of 34) and mid of September (two out of 34).
Two transect lines were chosen to assess the diversity of the crab hole environment and crab population density. Photos of 268 holes’ environment were taken along the lines with 2-m width. Fractions of bare soil and vegetation covers around the holes were determined from the photos. Results showed crab holes with dominant non-bamboo leaf cover (40%), followed by undergrowth vegetation (34%), bamboo leaf (11%), bare soil (9%), and grass (6%). Furthermore, 7% to 8% of the settlements were seen near large trees, young trees, or timbers. Crab population densities along the lines were 11.90 crabs per 100 m2 and 19.25 crabs per 100 m2 for lines A and B, respectively.
Table 2 shows biomass deposition rate onto soil and common vegetative species at three studied sites, exhibiting high crab population density. The biomass includes leaves, twigs, flowers, and grains. The results suggest a comparatively low biomass deposition rate of 0.53‒0.68 g m-2 d-1 on Site 1 from local summer through the end of the rainy season; this was highly contributed from Streblus asper Lour., Maytenus marcanii (Craib) Ding Hau, Cassia siamea Lamk., Vitex pinnata Linn. and Diospyros montana Roxb. For Site 2, Streblus asper Lour. and Cassia siamea Lamk were primarily contributing to soil biomass increment with deposition rates highly variable from 0.14‒7.11 g m-2 d-1. Biomass deposition (0.42‒8.53 g m-2 d-1) at Site 3 was largely from Diospyros mollis Griff., Casearia grewiaefolia Vent, Alangium salviifolium (L.f.) Wangerin. subsp. Hexapetalum Wangerin and Eugenia cumini Druce. Bambusa arundinacea Willd.
Table 2 Biomass deposition rate (g m-2 d-1) and percent distribution of species accounting to total biomass (in 2016)
Scientific name Common name Site 1 Site 2 Site 3
Mar 15th- May 20th Jul 2nd- Aug 14th Sep 18th - Oct 23rd Mar 15th - Apr 9th Apr 9th - May 20th Jul 1st - Aug 14th Sep 18th -Oct 23rd Mar 15th -Apr 9th Apr 9th -May 20th Jul 3rd- Aug 14th Sep 18th- Oct 23rd
Biomass deposition rate (g m-2 d-1) 0.684 0.535 0.633 7.111 0.141 3.207 0.701 8.533 1.852 0.893 0.426
Bambusa arundinacea Willd. Phai paa 11.34 0.67 0.74 1.89
Diospyros mollis Griff. Ebony tree 14.43 8.02 21.74
Casearia grewiaefolia Vent Kruai Paa 4.12 2.16 22.64 14.13
Alangium salviifolium (L.f.) Wangerin. subsp. Hexapetalum Wangerin Pruu 11.34 7.69 15.83 0.22 9.63 11.98 9.90 20.28 3.26 36.36 57.89
Terminalia alata Heyne ex Roth. Laurel 7.22 1.92 1.57 5.93 1.98 2.36 5.43 5.26
Spatholobus parviflorus (DC.) Kuntze Thao phan saai 1.03 2.92 7.41 0.99 3.77
Streblus asper Lour. 24.74 89.21 31.11 10.89
Eugenia cumini Druce 3.09 10.37 1.84 1.98 14.62 27.17 54.55 31.58
Maytenus marcanii (Craib) Ding Hau Naam daeng 16.49 21.15 2.70 11.85 0.47 6.52 2.63
Cryptolepis buchananii Roem. & Schult. Thao en on 2.06 9.62 0.72 6.67 1.84 16.83 0.94 1.09 2.63
Olax scandens Roxb. 4.12 2.70
Cassia siamea Lamk. Thai Copper Pod 13.46 26.86 68.20 30.69
Vitex pinnata Linn. Teen nok 3.85 23.02 12.59 9.43 14.13
Uvaria rufa Blume 3.85
Diospyros montana Roxb. Ma Kluea paa 25.00 26.62 3.70 14.29 15.84 6.13
Streptocaulon juventas (Lour.) Merr. 1.92 1.38
Osbecfia chiuensis Linn. En aa noi 0.46 2.97 4.55
Artocarpus lakoocha Roxb. Haat 6.13
Pluchea indica (L.) Less. Indian March Fleabane 3.30 2.17 4.55
Capparis sepiaria Linn. Naam wua sang 2.17
Others 11.54 10.79 7.92 2.17

3.3 Field observations on environmental factors affecting the crab population

3.3.1 Meteorological drought

Drought was the highest rank (7.96 out of 10) among the vulnerable factors to the population and livelihood of the crabs. During drought development, groundwater levels abnormally declined and little water in soil resulting in unsatisfactory habitats of the crabs; this is noticeable from the appearance of the pale color on the crab shell. Furthermore, rainfall had a role to play in the crab growth cycle. Usually, the crabs start mating in August, when the soil is saturated with continuous rainfall (Mahasarakham University, 1997). The changes in the onset of the rainy season and the duration of the rainy period can significantly affect the growth cycle.
Meteorological drought, existing anomaly lower rainfall, had been found in Dun Lumpun Non-Hunting Area during 2014 to 2016. The 2014 drought was characterized by the delay of the rainy season followed by rapidly high rainfall intensity. Similarly, the delay was also observed in 2002. The 2015 and 2016 droughts were characterized by overall lower annual rainfalls which were 955 mm in 2015 and 753 mm in 2016, whereas the 2002‒2016 average annual rainfall was 1143 mm.
Linear correlations between monthly and annual rainfalls and the number of the Mealy Crabs in the forest were analyzed and the results show a significantly (P<0.05) negative correlation (R2=0.39) between annual cumulative rainfall and the crab population (Fig. 5). In 2016, the crab population was 40537, which was higher than that of the previous year’s count. This finding could imply that the drought, characterized by dry soil and dry/warm air, did not significantly contribute to a decreased number of the crab population.
Fig. 5 Time-series crab population and annual rainfall (left) and number of crab population against its corresponding annual rainfall (right)
The crabs usually started mating in July, when soil moisture is saturated with a total saturation period of about 40 days from the end of June until July, whereas the typical mating season starts in August (Mahasarakham University, 1997). In 2016, the moisture declined at the end of July in response to less rain, and no crab mating was found during the survey on 14−15 August. Although there was high rainfall once again in September, crab mating did not reoccur. It could be summarized that the 2016 drought could affect the crab population to some extent due to early and shorter mating periods.

3.3.2 Crab-enemy population

According to the ranking, an increasing number of natural enemies was the second (7.41 out of 10) vulnerability factor to affect the crab population. An increase in the crab-enemy population is evident. Hunting activities mostly occur at night and crab bodies are regularly found along the walking trail or near crab holes the next morning. Many kinds of crab enemies were found in the forest including rodents, Asian water monitor (Varanus salvator), Bengal monitor (Varanus bengalensis), small Asian mongoose (Herpestes javanicus), and python (Python reticulatus). During the studied period, small Asian mongooses and a python were found near artificial cannel outside the forest and the wetland (Nong Dun), whereas the Asian water monitors and Bengal monitors, were likely found along the walking trail inside the forest. Rodents usually live in the rice fields near the forest and they are scarcely seen in the forest during the daytime.

3.4 Groundwater quantity and quality

Changes in groundwater level and its quality were ranked the third (6.11) and the fourth (4.63), respectively, vulnerability factors. Previous surveys found that qualities of soil and groundwater in the forest distinguished from those found in the surrounding areas and they attracted the crabs to live in the forest. Long-term changes in soil and groundwater qualities, however, cannot be proclaimed. The park rangers agree that the changes could not be substantial since the forest ecosystem has a high buffering capacity for environmental changes. Nonetheless, groundwater level affected the crab livelihood to some extent since crab holes need to be down to the groundwater level. Many park rangers viewed that the crabs could adapt to low groundwater levels by either digging the hole deeper or moving their new holes closer to the wetland or waterway.
Figure 6 shows spatial maps of the areas with high crab population in 1997 (Mahasarakham University, 1997), 2003 (Department of National Park, Wildlife and Plant Conservation, 2003), and 2016 (This study). The map suggests that the crab tends to move southeastward closer to Nong Dun wetland and northeastward closer to the man-made waterway lying along the park borderline. Furthermore, the spatial groundwater depth maps in Fig. 7 markedly indicated that groundwater levels in the crab preferable habitat zones (observation wells 1, 5) were shallow or near the surface, as compared to the levels outside the grids (observation wells 2, 3).
Fig. 6 Locations of observation wells and Nong Dun wetland (upper) and crab spatial distribution maps from several studies (lower)
Fig. 7 Groundwater depth taken from 2-m depth wells (left) and groundwater level from the surface taken from 5-m depth wells (right)

Note: * The observation well 5 has no depth information.

Measurements at the observation well 5 (Fig. 8) shows a significant decline of groundwater level during the local summer season in 2016 as compared to that obtained in 1997. The groundwater quality at the well in 2016 was also different from that observed during the local summer of 1997, exhibiting pH of 7.5 to 8.4 and conductivity of 1270 to 1500 μs cm‒1. In 2016, groundwater pH was comparatively lower toward neutral water with the lowest pH (6.5) in the local summer of March and highest (7.8) in the summer-rainy transition of May. The lower pH could have resulted from the degradation of soil organic materials and the long-term leaching of alkaline cations into soils, including Ca2+, Mg2+, K+, and Na+. Groundwater conductivity increased from 1514 in 1997 to 2040 μs cm‒1 in 2016. From these results, it could imply dynamic groundwater ecosystems in the forest and the crabs’ ability to adapt to the changes quite well, as suggested by an increasing crab population since 2002.
Fig. 8 Groundwater quality and quantity observed at well 5

3.5 Physical soil characteristics

Change in the forest soil characteristics was ranked the fourth (4.63) out of six vulnerability factors influencing the crabs’ population and livelihood. As seen from the physical soil characteristics in Table 3, soils in the forest are dominated by clay loamy texture and have a very dark grey to black color, with a density of 0.97 to 1.25 g cm‒3 and hydraulic conductivity of 0.9 to 3.2 μm s‒1. Soils in the rice field, grassland, and the Dry Dipterocarp forest adjacent to the Dun Lumpun Forest are different; dominated by loamy to loamy sand texture, more reddish-tone color, higher soil density (1.80 g cm‒3 for grassland and 1.92 g cm‒3 for the Dry Dipterocarp forest) and higher hydraulic conductivity (10.03 μm s‒1 for grassland and 9.27 μm s‒1 for the Dry Dipterocarp forest).
The chemical characteristics of the soil in the forest were also clearly different from the adjacent areas. The soil in the forest contained neutral to slightly alkaline (pH = 6.4 to 8.35); whereas the loam sandy soil with grass and sparse vegetation adjacent to the forest was composed of stronger alkaline (pH=10.3). Furthermore, this soil pH was also totally different from the loamy sand soil in the near Dry Dipterocarp forest, exhibiting pH=5.5 (see Table 3). The soils in the rice fields were in the neutral range from 7.3 to 8.3 and they exhibited significantly lower organic carbon contents (0-2.25%) as compared to those in the Dun Lumpun Forest soil (1.5%-2.75%). The alkaline soil with grass and sparse vegetation had also very-low carbon content (0-0.25%), which is not suitable for plants to grow.
Table 3 Physical and chemical characteristics of soils from the Dun Lumpun forest and its adjacent areas
Area Site description Depth pH OC
(%)		 Soil texture Soil color Hydraulic conductivity (μm s‒1) Bulk soil density (g cm‒3)
Dun Lumpun Forest Site 1: Forest soil Surface 8.00 2.5-2.75 Silty Clay Loam 7.5YR 2.5/1 Black 1.1‒1.8 1.25
Underground 8.35 1.5-2.25 Clay Loam 7.5 YR 3/1 Very dark gray ‒ ‒
Site 2: Forest soil Surface 8.15 2.25-2.5 Clay Loam 7.5 YR 3/1 Very dark gray 3.2 0.97
Underground 8.13 2.5-2.75 Clay Loam 7.5 YR 3/1 Very dark gray ‒ ‒
Site 3: Forest soil Surface 8.27 2.25-2.5 Silty Clay Loam 7.5YR 2.5/1 Black 0.9 1.07
Underground 8.26 2.25-2.5 Silty Clay Loam 7.5YR 2.5/1 Black ‒ ‒
Forest soil Surface 6.40 2.5-2.75 Clay Loam 7.5 YR 3/1 Very dark gray 2.1 1.00
Soils near the Dun Lumpun Forest Grassland soil Surface 10.30 0-0.25 Loamy Sand 2.5YR 4/6 Red 10.03 1.80
Dry Dipterocarp forest soil Surface 5.50 1.25-1.5 Loamy Sand 2.5YR 4/2 Weak Red 9.27 1.92
Rice field soil Surface 7.60 0.25-0.5 Loam 7.5YR 2.5/2 Very dark brown ‒ ‒
Rice field soil Surface 8.20 0.5-0.75 Loam 7.5YR 2.5/1 Very dark brown ‒ ‒
Rice field soil Surface 7.90 1.25-1.5 Loamy Sand 2.5YR 4/2 Weak red ‒ ‒
Rice field soil Surface 8.30 1.5-2.25 Sandy Clay Loam 7.5 YR 3/1 Very dark gray ‒ ‒
Rice field soil Surface 7.30 0-0.25 Loamy Sand 2.5YR 4/2 Weak red ‒ ‒

Note: ‒ means not measured.

As suggested from the above soil characteristics, the level of soil fertility in the Dun Lumpun Forest is relatively high, containing lots of humus, products of organic matter degradations. This was suggested from its high organic content, dark color, neutral pH, and low bulk density (suggesting high air and water penetrations). Biomass deposition rate on the forest soil varied seasonally and spatially from 0.14 g m‒2 d‒1 at Site 2 in the local summer (9 April to 20 May 2016), to 8.53 g m‒2 d‒1 at Site 3 in the local summer (15 March to 9 April 2016). Since the crab is exclusively found in the Dun Lumpun Forest and the soil in this forest differed markedly from its surrounding areas, it can be concluded that this kind of crab is highly sensitive to soil characteristics and unlikely to adapt to different soils. Nonetheless, current evidence on the dynamics of the forest soils associated with changes in groundwater and vegetation cover (Ketwong, 1998) did not show any effects on the crab population.

4 Conclusions

This study aims to investigate key dynamic factors, potentially affecting the vulnerability of Thaipotamon chulabhorn or Mealy Crab, threaten freshwater crab species exclusively found in the Dun Lumpun Non-Hunting Area, Nachuak Sub-district, Mahasarakham Province, Thailand. The study was conducted from 15 March to 23 October 2016 during strong El Niño, suggesting drought episodes. The vulnerability assessment was based on reviewing the history records of the crab population, field observations/measurements, and interviews with experienced park rangers. Results from the study suggest the vulnerability of the crabs to the dynamic environment. Six potential vulnerability factors considered in this study are: 1) human intervention, 2) crab-enemy population, 3) groundwater level, 4) groundwater quality, 5) soil characteristics, and 6) drought.
The results show that the crabs soaking in groundwater taken outside the Dun Lumpun Forest were more likely to have unappealing color, which could be affecting biological selection. In 2016, the egg hatching period could be expected in April, which is in the typical biological epoch.
Early matting season in June to July 2016 (typically in August) was found and it was probably due to early-onset rainfall. From August to October 2016 (typical rainy season), closure of the crab holes was found together with many tiny holes. This finding suggests it was during the molting period and hatching period. Diurnal observation of those crabs in the exhibition zone reveals that the crabs were highly active at approximately 6:00 AM and 7:00 PM. They are mostly inactive from 2:00 to 4:00 PM. The crab activities from 9:00 to 11:00 AM were highly uncertain due to human intervention. Nighttime video camera recording showed that the crabs were most likely to be exposed to the ground surface, stay still near the hole but rarely ate food. Scavenging for food was frequently seen in the morning.
Crab habitats and environment were observed along the transect lines. The results show the population density of the crabs: 11.90 crabs per 100 m2 for transect line A, and 19.25 crabs per 100 m2 for transect line B. Crab holes were usually dominated by non-bamboo leaf cover (40%), followed by undergrowth vegetation (34%), bamboo leaf (11%), bare soil (9%), and grass (6%).
Based on ranking scores obtained from the park rangers, the highest vulnerability factor influencing the crabs’ population and livelihood is drought (7.96 out of 10), followed by an increase in crab−enemy population (7.41), variation in groundwater level (6.11), changes in groundwater quality (4.63), changes in forest soil (4.63) and human intervention (4.26). These factors are briefly described below:
In 2016 from March (local summer) to October (end of rainy season), there were anomaly low rainfalls and an early onset of intensive rainfall, resulting in an earlier and shorter crab mating period.
Crab-enemies, which were found in this study were rodents, Asian water monitor (Varanus salvator), Bengal monitor (Varanus bengalensis), small Asian mongoose (Herpestes javanicus), and python (Python reticulatus). Their number is expected to be increasing.
As compared to the results obtained from previous studies, significant changes in groundwater levels and qualities were observed. This implies that there is dynamic groundwater in the forest.
Physical and chemical characteristics of the soil in Dun Lumpun Forest were diverse from those observed in the adjacent areas, including the rice fields, grasslands, and Dry Dipterocarp forest. The finding suggests that the crabs cannot adapt to different soil types.
The human intervention had a little direct impact on the crab population as the result of the restriction of human access to the forest since 1996. Furthermore, the forest and the crabs have also been protected by the cultural belief about the Forest’s Angel Guardian.
Consistent increases in the Mealy Crab population suggest that the crab population was not significantly vulnerable to several crab-enemies, changes in groundwater, and unique characteristics of the forest soil.
Animal ethics
This work is under a research project entitled “Adaptive capacities to drought in sensitive areas in Northeast Thailand,” which has been approved by the Institutional Animal Care and Use Committee, Mahasarakham University (IACUC-MSU), Thailand.

Acknowledgment

This research is financially supported by the Office of the Higher Education Commission, Ministry of Education, Thailand in the fiscal year 2016. The author is thankful to the Department of National Park, Wildlife and Plant Conservation, Thailand, and all staff in the Dun Lumpun Non-Hunting Area for allowing researchers to conduct the work and providing kind assistance. Special thanks must be given to Mr. Somsak Bunkum and Mr. Chavee Yodkaew, who provides a great contribution to this research work. Useful comments from anonymous reviewers are thankful and acknowledged. I have also appreciated our tireless research assistants, Miss Thanawan Khlopimai and Miss Nipaporn Tanoo, for their dedication.

References

Publishing order | Descend order by publishing year | Descend order by cited within
[1]
Department of National Park, Wildlife and Plant Conservation, Ministry of Natural Resource and Environment Thailand. 2003. Crab population survey report for the year 2003 during May 15th-23rd, 2003, Dun Lumpun Non-Hunting Area. Ministry of Natural Resource and Environment (Thailand).

[2]
IUCN taxonomy. 2018. Thaipotamon chulabhorn Naiyanetr, 1993. Available at: Viewed on 2018-02-03.

[3]
Ketwong A. 1998. Changes of Doonlumpun Forest areas Amphoe Nachuak Maha Sarakham. Diss., Mahasarakham, Thailand: Mahasarakham University.

[4]
Mahasarakham University. 1997. Study and investigation of biodiversity conservation in natural habitats in Mahasarakham Province. A final report submitted to the Department of Environmental Policy and Planning, Ministry of Science, Technology and Environment, Thailand.

[5]
NOAA National Oceanic and Atmospheric Administration. 2016. Multivariate ENSO Index (MEI). Washington DC: Department of Commerce of USA.

[6]
Sardprom N. 2012. Public participation in the development of the recreational area: A case study of Dun Lumpun Non-Hunting Area, Nachuak District, Mahasarakham Province. Diss., Mahasarakham, Thailand: Mahasarakham University.

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