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Finding Nemoscolus

By Joh Henschel1 and Yael Lubin2
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The web and retreat of Nemoscolus

Nemoscolus showed that it was unafraid to place itself in the way of Ovis, and so Ovis whisked it off.

Thereupon we undertook to find Nemoscolus, the spider, who was taken by Ovis, the sheep, near Tierberg, the long-term ecological research (LTER) site, far across the vast sea of dwarf shrubs of the Karoo.

At tortoise speed we braved the heights of heuweltjies, negotiated the abyss of aardvark, got pointers from a bunch of scorpions, and noted tell-tale signs until we finally found Nemoscolus. Not just one, but 558 of them, of which two out of every three occurred in the sanctuary of Tierberg, there where Ovis dare not tread.

Stone-nest spiders and their webs

Nemoscolus tubicola, or stone-nest spiders, build their webs like an inverse trapeze some 15-20 cm above the ground, straddling 20-60 cm gaps between shrubs. The spider’s retreat towers upwards above the web, placing the spider in the gap between shrubs, exactly where sheep are bound to step and tear the web.

Of ten Nemoscolus where we observed their nest being torn, seven rebuilt webs within three days. The sheep’s step cost them time and energy; silk is a high-quality protein – wonder thread one should add – that is expensive and could cost productivity.

The other three Nemoscolus did not fare well. One disappeared, the second was perhaps snatched by a bird seen on the ground nearby, and all that remained of the third was a pile of frantic ants. Which goes to show that sheep are, indeed, dangerous.

Really?

Besides kicking spiders, sheep eat shrubs, changing vegetation structure and density across the veld, which in turn affects whether, where and how web spiders can place their webs and forage optimally. By affecting plant phenology (growth, flowering and seed production), sheep can also reduce insect populations, particularly pollinators and herbivorous insects such as grasshoppers, which Nemoscolus loves to eat. These and several other ecological impacts of sheep change with stocking density and type of sheep, as was demonstrated in other studies.

Nemoscolus is not the only spider impacted by sheep. In our search for Nemoscolus along five-metre-wide transects of 15-metre length each – 92 transects in total – we recorded that the next most common spiders, Agelena (grass funnel-web spiders), was also over twice as abundant inside the Tierberg sheep-exclosure than outside it. Latrodectus (Karoo button spiders) had a far stronger aversion to sheep, with 92% occurring inside the sanctuary.

Sheep are not the only culprits kicking spiders about the Karoo. Cattle and game animals would do that too.

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Nemoscolus tubicola and its retreat (removed from web) at Tierberg, where we also found another species, N. vigintipunctatus, 100 km beyond its previously recorded range

Karoo button spiders (Latrodectus karooensis) appear to be highly vulnerable to sheep

 

It can be argued that web spiders occasionally losing their webs from such kicks is the natural state. What we recorded adjacent to Tierberg were the consequences of frequent high stocking densities of resident grazers, effects that go beyond the impact on plants alone, suggesting that grazers have far-reaching ecological impacts across trophic levels.

Sanctuary

We could not have recognised this if it had not been for the Tierberg LTER sanctuary, SAEON’s research terrain from which sheep, cattle and larger game are excluded by a fence surrounding the property. This makes the fence an important component of the research being conducted at Tierberg, as it controls disturbance to plants, small animals, experiments and equipment.

The sanctuary of Tierberg may be deemed to be unnatural – the exclusion of most large game species and domestic grazers is not the normal condition in these Karoo rangelands – but it serves as a window through which to recognise the many effects of fencing and impacts of grazers on karoo shrublands.

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Ground-living spiders were less affected by sheep, but face other dangers. When Microhodotermes viator termites from a heuweltjie attacked a female baboon spider and her creche of spiderlings from below inside the spider burrow (inset), a fountain of frenzied spiders gushed out, and the female finally limped off, carrying termites clamped to her foot and belly, ouch!

Finding Nemoscolus has taught us something new about a place we thought scientists already knew so well. There is sure to be a lot else scientists will continue to learn at Tierberg.

When you come to visit be sure to also look out for Nemoscolus with its remarkable, neat web topped with an exclamation mark… or was that a question mark? See for yourself, find Nemoscolus.

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Visiting scientist Yael Lubin from Israel introduces Wolwekraal intern Yondela Nqadala to spiders

1 SAEON Arid Lands Node, Kimberley, South Africa

2 Ben Gurion University, Sde Boqer Campus, Negev, Israel

Since we conducted our fieldwork in January 2017, ownership and land use has changed on Tierberg’s neighbouring farms. Sheep were removed and a large game ranch without internal fences is being established. Fenced-in Tierberg is well-positioned in the middle of the changing landscape and geared to continue observing and conducting critical tests with experiments that help reveal drivers of change.

Exclusion plots are recognised as a powerful tool in rangelands worldwide. The Kenya Long-Term Exclosure Experiments (KLEE) is rated as the most productive field experiments ever carried out on the continent of Africa. By comparison, Tierberg LTER is much smaller in scale and not as complex, but it, too, can boast an impressive output of publications, over a hundred, and many collaborators over the last three decades. Many scientists from South Africa and abroad – in this case from Israel – have made use of Tierberg to learn about the Karoo, about rangelands in arid lands, and about ecology in a changing world.

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