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Scorpions of the Pilannesberg
National Park - an informal study

From the Spider Club of Southern Africa

by Jonathan Leeming


Introduction

In today’s day and age, National Parks not only preserve natural heritage for future generations, but must also be able to sustain themselves financially, simply if it doesn’t pay it doesn’t stay. This attitude has resulted in a great deal of attention being directed at the more ‘crowd pulling’ friendly facets of fauna and flora, good examples being elephants and lions.


Other elements are sadly neglected due to financial constraints and through no fault of themselves.

During the course of this study attempts were made to obtain an official permit to collect inside the Pilanesberg National Park, unfortunately to no avail, hence this is an informal study. Because of access restrictions due to the introduction of dangerous animals, in particular, lion into the park, areas displaying prime habitat remained unexplored, resulting in biased sampling. Using historical biographic data it is possible to some degree of accuracy to predict scorpion fauna for a particular region. Taking specific species requirements it is possible to further refine this prediction to an even finer degree of accuracy.

Few arthropod fauna have enjoyed more attention than scorpions. The Southern African scorpion fauna have enjoyed more attention than most other parts of the world (Hewitt 1918, 1925; Lamoral and Reynders 1975, Lamoral 1979, Lawrence 1955). Several taxonomic reports characterised the substrate on which particular species are found (Lamoral 1979; Williams 1980). In conjunction with such reports, known species habitat requirements, climatic data and the topographical and geological of the Pilanesberg National Park, it was possible to predict scorpion diversity to a fine degree.

Collection records from Southern African Museums and some overseas museums and personal records were consulted and general distributions noted. Physical collection of specimens was sparse and concentrated at specific areas, commonly already disturbed due to development. It was noted that more Opistophthalmus species were collected from disturbed areas than any other species, although this reflects upon their pioneering dispersal abilities rather than their tolerance to a disturbed habitat or vague habitat requirements.

The Friends of the Pilanesberg Society operate within the parks under to Authority of the North West Parks department, Pilanesberg National Park, Borokalalo National Park and more recently, Madikwe. Much of the work involves development of areas for future public access. Records have been kept from April 1991, pertaining to scorpion distributions. Due to the introduction of lions into the Pilanesberg National Park limitation imposed upon FOP resulted in biased sampling methods in direct relation to Friends Of Pilannesberg Society’s (FOPS) activities were incurred. Activities were generally restricted to areas of public access with few exceptions, for example picnic sites, view points, Geological sites, Potokwane and Tambooti Camps, Ecological Educational Zone. Moreover, these areas have, in most cases been disturbed to a lesser or greater degree by development. It is unclear whether this has any effect upon scorpion distributions. Most specimens were collected by overturning rocks and surface debris during the day. Some specimens were dug out of their burrows. No nocturnal collecting was conducted employing ultraviolet lights. Previous records were sparse. No formal study was conducted upon the arachnid fauna of the park. Other areas exhibiting favourable habitats are unexplored due to access limitations, the Northern area in particular remained excluded from sampling. Collection records from the Spider Club of Southern Africa members were noted.

Evolutionary influences of Scorpion distributions in the Pilanesberg National Park

As members of populations, individual organisms are part of societies or communities that, over time evolve to suit the environmental fields in which they are engulfed, providing that these fields stay constant long enough for adaptation to occur. An individual species can live only within a certain tolerance of environmental factors - the effect of too much or too little of any one factor may inhibit growth or even prove fatal. (Huggett 1995)

Many physical factors influence the spatial distribution of scorpions, including temperature, precipitation, soil or rock characteristics, stone or litter cover and environmental physiognomy (Polis 1994). Scorpion distribution also seems to be influenced by edaphic factors (Sam Martin 1961; Smith 1966; Lamoral 1978; 1979; Bradley and Brody 1984; Polis and McCormick 1986). Temperature and precipitation are probably the most important determinant of general geographic range (MacArthur 1972; Kock 1977, 1981; Newlands 1978).

For instance, the type and texture of soil or substrate is crucial to animals that seek refuge in burrows, and those that have modes of locomotion best suited to relatively smooth surfaces. Burrowing species, may be confined to a particular kind of soil (Huggett 1995). In particular scorpions that burrow in soft soils tend to be restricted to a smaller ranges of soil hardness than those burrowing in harder soils( Huggett 1995). Scorpions, like other poikilotherms, exhibit a limited temperature range within which, optimal field activity is maximised (Shulov and Levy 1978; Polis 1980) For example O. latimanus has a preferred thermal zone of 32 C to 38 C (Polis 1994). Hadogenes troglodytes exhibit a skatotaxic affinity to black foyite formations although their distribution within the park is not exclusively determined by it. The Pilanesberg National Park receives on average 650mm of rain annually, although the actual annual rainfall fluctuates greatly between 200mm and 1400mm annually. The genus Parabuthus exhibit a relation to the 600mm isohyte (Newlands 1978), and as a general rule are not found in areas receiving more than 600mm annual rainfall. Areas of complex geomorphology contain a higher scorpion density than the surrounding plains (Prendini 1996), however even though the Pilanesberg rise less than 700m above the surrounding plains, they do have a profound effect upon climatic conditions, probably regulating the exclusion of Parabuthus sp. due to rainfall. The distribution of P. transvaalicus in Southern Zimbabwe indicates that they occur at an altitude less than 1000m above sea level.

Scorpions are not distributed randomly within a habitat, rather particular species are normally associated with specific microhabitats (Polis 1970). Several taxonomic reports characterised the substrate on which particular species are found (Lamoral 1979; Williams 1980). In particular soil hardness and texture determine to distribution of Opistophthalmus sp. in Southern Africa. Lamoral found that each species is restricted to soils within a particular range of hardness rather than a particular soil type. Scorpion response to soil hardness is so defined that as burrows descend they actually track changes in hardness and stay within a preferred range. This pattern is most likely explained by the highly specialised morphological and behavioural adaptations that characterise psammophilic and lithiphilic scorpions. Such ecomorphotypical adaptations facilitate burrowing and locomotion on sand or rock but make these animals inefficient in or on foreign mediums (Polis 1994). The strongly recurved tarsus of Hadogenes sp. enable them to locomote inverted on smooth rocks, their elongated and laterally compressed body and appendages lend themselves towards a trogofilic existence. Indeed, these scorpions are so restricted to distinct rocky outcrops and mountains that even narrow valleys between populations create an insurmountable barrier that prohibits gene flow. (Newlands 1985) Uroplectes planimanus, a semi-psammophilic scorpion shares this affinity but not in such an exacting manner. U. flaviviridis prefer sandy areas displaying typical psammophilous
adaptations for life on sand i.e. sand combs on the tarsus. Uroplectes carinatus demonstrate a very wide habitat tolerance, making them generalists in their habitat selection.

It is unclear whether vegetation influences scorpion distributions. Studies of scorpion-vegetation associations in North America (Gertsch and Alfred 1965; Williams 1970; Polis and McCormick, unpl. data) suggest that most are generalists living in several vegetative zones. Pseudolychas pegleri are distributed in the moister eastern part of the subcontinent. They exhibit a close affection to vegetated zones, this may be due to the high humidity found within vegetated areas in comparison to sparsely vegetated areas. Scorpions such as Hadogenes sp. and Opistophthalmus sp. are protected from desiccation in different ways. Opistophthalmus sp. shelter in burrows, in this way temperature and humidity of the burrow can be regulated, the deeper the burrow the cooler the ambient temperature inside the burrow. Hadogenes sp. shelter under rocks, where the water content of the soil under a rock is considerable higher than that of the soil surrounding a rock.

The extra water found under rocks is the result of one or more of three processes. Firstly, while rain is falling, water flows over the stones and accumulates in the soil matrix between and under the stone. Secondly, evaporation loss is reduced by the stone layer with prevents capillary rise to the soil surface. Thirdly, at sunset, falling temperatures promote condensation in the hollow spaces between the stones.

Animal species are adapted to conditions in their local environments. Many animal species adapt to gradual geographic changes in climate. Such adaptation is often expressed in the phenotype as a measurable change in size, colour, or other trait. The gradation of form along a climatic gradient is called a cline (Huxely 1942). Clines result from local populations developing tolerance to local conditions, including climate, through the process of natural selection. Such local populations constitute an ecological race. Ecological races may display either gradual or abrupt change along an environmental gradient, and these racial gradients are called ecoclimes. Clinal factors, notably temperature and moisture, might be designated climoclines (Hugget 1995). For example, Hadogenes troglodytes display intraspecific colour variants. Populations within the Pilanesberg National Park exhibit a uniform charcoal-black coloration. Populations elsewhere exhibit colour variation effecting their appendages, the largest specimens inhabiting the Northern parts of their range, notably the Soutpansberg Mountains.


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