These desert spiders put sandcastle builders to shame

By Elizabeth PennisiDec. 15, 2017 , 5:05 PM

Anyone who has tried to build castles in dry sand knows the challenges desert digging spiders face. Because the grains don’t stick together, it’s hard to dig a burrow and keep its walls from collapsing. Even so, the "flic-flac" spider, Cebrennus rechenbergi—famous because it can cartwheel quickly across the desert—builds a tunnel 25 centimeters deep and uses it daily for shelter from Morocco's hot sun. Likewise, its neighbor, a newly discovered wolf spider called Evippomma rechenbergi makes tunnels, too. Yet they use different tactics, researchers report this week in the Journal of Arachnology. The flic-flac spider picks the right spot to dig, then pushes sand together and scoops it up in a basket formed by fine bristles that overlap. Lacking these bristles, the wolf spider instead glues sand particles together by connecting the grains with fine silk threads. 

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Desert lizards use body oscillations to dive into sand

June 15, 2017 

In the desert, the sand surface can become extremely hot during the day, up to 70ºC. In order to escape these temperatures, some desert lizards adopt a fascinating strategy: They dive a few centimeters under the sand surface where it's much cooler (around 40º C). This is also a good strategy to hide and to escape from predators. But diving into the sand is a difficult task which requires a large force to overcome sand friction. In order to dive more easily, different genera of lizards like the fringe toed lizards and the horned lizards used rapid lateral undulations of their body in order to fluidize the sand. 

A team of researchers at the University of Santiago has inspected how these lateral undulations ease the penetration into granular media such as sand. They considered a model experiment of a solid finger penetrating different granular media made of small glass beads. Thanks to a tiny eccentric motor, similar to those used in cell phones and joysticks, they imposed mechanical vibrations of small amplitudes (about 10 µm) and low frequencies (100-200 Hz) to the penetrating finder. Results of these experiments are reported in PLOS One.

Thanks to this setup, they showed that mechanical vibrations decrease the resistance to penetrate into a granular medium. "The resistive force can be decreased up to ten times by the presence of these tiny vibrations," explained Baptiste Darbois Texier, a post doc researcher of the SMAT-C laboratory where the study has been led. 

The researchers also sought the conditions under which a drop of the resistive force occurs. They proved that this phenomenon is controlled by the acceleration of the mechanical vibrations. When vibrations have an acceleration higher than gravitational acceleration, contacts between grains surrounding the intruding object are constantly broken. In order to observe the grains' motion, the researchers tracked the displacement of the grains in a two-dimensional version of their experiment. 

"We observed that when the vibrations have a sufficient acceleration, the grains in contact with the intruding finger start to follow a convective motion," said Alejandro Ibarra, a PhD student involved in the project. In such a convection zone, force chains that usually propagate into granular media are broken and the material is fluidized, an effect that finally reduces the force experienced by the intruding object.

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‘Human Activities’ Reduce Desert Environment Diversity

Human activities and disturbances can put a significant amount of stress on local environments and a new research review has shown that the functional diversity in arid, desert environments can be affected by the hand of man.

According the report published in the Journal of Arid Environments, mammalian communities living in dry ecosystems are “drastically changing” as a result of human activities.

“We report for the first time that in drylands, the effect of human-induced disturbances on mammal functional diversity is negative,” said study co-author Veronica Chillo, a biologist in the Functional diversity Research Group at theArgentinian Institute of Arid Lands Research.

“Regardless [of] the characteristic of the disturbance, functional diversity is diminished,” she told BBC News.

The review culled information from 25 studies and included 110 mammalian species were that evaluated for the effects of human-caused disturbances like poaching, logging, grazing, fires, and introduction of invasive species.

Although deserts and arid lands appear to be desolate places and therefore more resilient to manmade disturbances, they often support an intricate and sensitive ecosystem for smaller mammal that live in concert with plant and other animal species. Herbivores play important role by dispersing plant seeds and other mammals enrich the soil by digging tunnels that increase water infiltration, according to the review.

Because living in these ecosystems can be a delicate balance for many mammals, any drastic changes to the local ecosystem can result in a negative impact on their diversity. The researchers found several examples of population decreases and even localized extinctions as a result of human disturbances.

The review also noted that low and unpredictable rainfall could exacerbate damage caused to plant life and the larger ecological cycle.

According to the review, the human impacts that are most damaging to mammals are those that fundamentally remade parts of the local habitat, such as fires or overgrazing that wiped out any desert grasses.

“The most negative effect is seen when disturbances change the conditions of the system [such as something that] modifies the structure or dominant plant species,” Chillo said.

Continued:   http://www.redorbit.com/news/science/1112690144/desert-diversity-human-impact-090912/ 

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