Freshwater mollusc diversity in the Kruger National Park : a comparison between a period of prolonged drought and a period of exceptionally high rainfall

Most of the previous records of the freshwater molluscs from the Kruger National Park date back to 1966 and earlier. On account of several droughts between 1966 and 1995, a survey was done in 1995 to evaluate the effect of these droughts on the mollusc population. As a result of extensive rainfall between 1995 and 2000 another survey was conducted to establish the effect of a period of exceptionally high rainfall on the species’ diversity of the mollusc population. With the exception of three habitats, an increase in mollusc diversity was recorded for all the other habitats previously surveyed. One of the invader freshwater snail species, Aplexa marmorata, collected in only one habitat in 1995, was found in seven habitats located mainly in the south during the 2001 survey. Another interesting finding was that juvenile specimens of Lanistes ovum, of which large specimens were present prior to 1966 but none found in 1995, were present in the Sirheni Dam in 2001. From this study the positive effect of the high rainfall on the species’ diversity is highly evident.


Introduction
The importance of regular, normal rainfall to maintain the bio-diversity in conservation areas in South Africa cannot be over-emphasized, especially for those aquatic organisms which lack the ability to migrate to other habitats during periods of serious drought.
This inability to migrate leads to the situation where re-population of isolated habitats, such as dams and pans, is largely dependent on time and passive dispersal of biota.Certain freshwater invertebrates, however, have the ability to aestivate, a strategy enabling them to survive prolonged periods of drought.Apart from these freshwater snail species, among others, Bulinus reticulatus Mandahl-Barth, B. forskalii (Ehrenberg, 1831), B. tropicus (Krauss, 1848) and the B.
ISSN 0075-6458 I africanus group of species (Brown 1994), it is mostly the eggs of some freshwater crus- taceans such as Triops, Eocyzicus and Branchipodopszs which can survive periods of drought (Davies & Day 1997).With regard to the freshwater molluscs, it is known that the largest bio-diversity is to be found in the tropical and sub-tropical areas o{ the world.In South Africa it is mainly the Limpopo, Mpumalanga and KwaZulu-Natal provinces that represent the tropical and subtropical regions with the Kruger National Park situated in both the first two mentioned provinces.A large variety of freshwater molluscs collected up to 1966 were reported by Oberholzer & Van Eeden (1967) for the Krueer National Park.Koedoe 45/2 (2002)   The distribution of freshwater molluscs is not a static process and is mainly influ- enced by the availability of suitable water habitats.The most recent account of the diversity of freshwater molluscs in the park, prior to the 2001 survey, dates back to 1995 and is a report by De Kock & Wolmarans (1998) on a survey done after the drought of 199l-1992.Between 1966 and 1995, the park was struck by several droughts, namely those in 1982-1983 and  1986-1987-the former also being the severest-prior to the third drought of 199l-1992.According to Zambatis & Biggs (1995) several of the rainfall monitoring stations recorded the lowest rainfall in history.Between 1995 and 2001, how- ever, average to above normal high rainfall figures were recorded for most of the rainfall monitoring stations.This situation prompted the idea to do a follow-up of the 1995 survey to evaluate the effect of the availability of more permanent water habi- tats on the freshwater mollusc diversity, and to compare results with the 1995 sur- vey.

Methods and materials
During the present investigation we concentrated mainly on water bodies that had been surveyed for snails during the 1995 collection.We also concentrated on several other water bodies that were dry during the 1995 survey, or seemed suitable for supporting molluscs but of which there were no previous records in the National Freshwater Snail Collection rNFSC).A total of 43 habitats were suveyed during 2001.In each suruey, all the habi- tat ty?es were screened for molluscs by means of aluminium mollusc scoops.The collected molluscs were identified by the senior author who had mainly been responsible for the identification of the molluscs in the NFSC since 1973.The mol- luscs were subsequently incorporated into the National Freshwater Snail Collection.
Habitats without surface water were screened for mollusc shells.Rainfall records, for the periods 1982-1983. 1985-1987. 199l-1994  (Table 1) clearly shows a substantial increase in the diversify in most of the dams and rivers since 1995.A decrease in the species diversity during the 2001 survey is only evident for the Sabie River, the Nwaswitsontso River and the Orpen Dam (which forms part of the Nwaswitsontso River drainage system).During 1995, molluscs were found in only eight of the 24 sites surveyed during both years, while mollucs were present in all of these habitats in 2001.Taking into account the number of habitats sampled during both surveys, it is evident thatB.forskaliiwaspresent in 16 of the 24 habitats surveyed in 2001, while it was recovered from only four of the habitats during the 1995 survey (Table l).Furthermore, this snail was recorded from nine of the 18 habitats surveyed for the first time in 2001.With regard to its distribution in 1995, it is clear that it was only found in the southern part of the area, while it was more or less evenly distributed throughout the whole area in 2001 (Fig. 2).In contrast to lhrs, Physa acutaDraparnaud, 1805, which is well known for its invading potential, in spite of exceptionally high mean rainfall for the last five years (Table 4), was present in only fwo of the habitats, while it was found only once during the 1995 survey (Fig. 3).

Discussion
From the results of this investigation it is clear that the high rainfall average recorded for 1995-2001 period had a significant influence on the number of habitats from which molluscs were recorded as compared to the 1995 survey.For the majority of the habitats the mean rainfall for the 2001 period was nearly double the figures recorded for the 1995 period.The decrease in snail bio-diversity in the Sabie and Nwaswitsiontso rivers and Olpen Dam (which forms part of the Nwaswitsontso River drainage system, as previously mentioned) could probably be ascribed to heavy floods in these habitats prior to the present survey.
In the same localities investigaled, B.
forskalii was found only four times in 1995 as compared to 16 times in 2001 and was, furthermore, present in nine of the 18 habitats surveyed for the first time in 2001.This snail is able to exploit a wide variety of natural and artificial habitats, including the mar- gins of lakes and pernanent swamps, but is most abundant in small water bodies and was also collected in alluvial pools associated with rice cultivation in Gambia (Brown 1994).The fact that this snail was found in many more habitats during the 2001 survey may be ascribed to its ability to aestivate through long periods of drought.However, the possibility that some of these habitats  (Dana 1999) 22" 57'32" 5; 3 1' 1 5'0 1 "E 22" 56' 57 " 5; 3 l" 14' 02"E 25'06'57"S; 31" 54' 4l"E  (1967) and was collected in only three additional localities during the 2001 survey.In contrast to this,,4.marmorata was collected in six additional localities since the 1995 survey.This increase in collection sites over a period of only six years seems to suggest that A. marmorata could become just as successful as Z.
during the 2001 survey are depicted in Fig. l.The results of this investigation are presented in Tables 1,2 & 3 and Figs. 2 5.A comparison of the species diversity recorded during the 1995 survey with that of the present survey

Table
Habitats of the Kruger National Park suweyed in both 1995 and 2001 No molluscs collected only once during the 1995 survey, was present in seven of the localities in 2001, which was investigated during both surveys.Although it was collected several times during the past 15 years in KwaZulu-Natal, it was recorded only once from the Limpopo Province(Dana 1999) at the time ofthe present investigation.The fact that this species was found in a number of habitats during the 2001 survey seems to suggest that it could be in the process of becoming well established in the southern parts of the species

Table 2
, it was present in several rivers and dams in the Kruser National Park In contrast to this, P. acuta, which has a worldwide distribution and which is established in many African rivers and lakes from South Africa to Morocco, and is considered the second most successful invader snail species in South Africa (De Kock et al.Molluscs found in waterbodies of the Kruger National Park surteyed for the first time in 2007 Appleton 1993).It is now found in various arlificial and natural habitat types and taking into account that this species is not highly specific with regard to its habitat preferences it is quite surprising that it has a relatively limited distribution in the Kruger National Park.However, it occurs commonly in pol- luted water and has therefore been nick- named the sewage snail and seems to have difficulties in establishing itself far from human works or habitats(Brown 1978;   Brackenbury & Appleton 1993).This could be one of the reasons for its limited occur- rence in the Kruger National Park, especial- ly with regard to the dams and rivers, which originate in or near the park.Although the drought of 1995 may also serve as an expla- nation for this situation, it must be kept in mind that it had been recovered only three times from the Kruger National Park prior to Brown 1967) was recovered from five habitats during the 2001 survey and from only one during the 1995 survey.Scrutiny of its distribution in the whole of the RSA (De Kock et al. 1989) shows clear- ly why it has earned the title as the most suc- cessful invader snail species, because it has successfully established itself in vast areas of the country since its introduction in the early 1940s.Before 1966 ft was found in 132loci, and in 225 additional loci from 1966 up to the present (De Kock et al. 1989).This species was first reported from the Kruger National Park in 1961 from three localities by Oberholzer & Van Eeden 8RSA shows concentrations of collections around major urban areas(Brackenbury &