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Between bats and arachnids

Researchers found that caves inhabited by fruit bats were characterized by longer food chains than caves inhabited by insect bats or caves without bats

Barefoot in a cave. Photo: Dr. Efrat Gabish-Regev.
Barefoot in a cave. Photo: Dr. Efrat Gabish-Regev.

What animals stay in caves and how do they exist? Caves are a habitat with unique environmental conditions (such as isolation and darkness) and enable ecological and evolutionary research that examines the processes of differentiation of animal groups, their adaptation to habitat conditions, and the factors that influence the composition of the food web and the collection of species. One of these groups is the arachnid class belonging to the arthropod system. The results of the research in the field enrich the knowledge about arthropods, and especially about the arachnids living in caves in Israel, and every year new species are reported to Israel and to science.

Arthropods are invertebrates with an exoskeleton, whose body and limbs are divided into joints. They belong to several well-known classes of animals, including insects, crustaceans, millipedes and arachnids. Each class includes several series (for example, the arachnid class includes spiders, scorpions and ticks); Each series includes several families (for example orbs - one of the largest families of spiders in the world); Each family includes several types (e.g. widow); And each type includes several species (eg black/white/brown widow).

What is the question? How does the source of food affect the food chains and the composition of arachnids in caves?

Dr. Efrat Gabish-Regev, the scientific director of the collection of arachnids and terrestrial arthropods in the national nature collections of the Hebrew University, studies arachnids, and in recent years has focused on those that live in caves. According to her, "Almost every arthropod that is adapted to life in caves and collected in Israel is a new species to science because almost no research is done in these habitats in our region. In recent years we have found many new species in caves in Israel. I focus on arachnids because this is a very diverse group that has not been studied enough. One of the reasons for this is that most species are not harmful to agriculture or dangerous to humans. The group includes a variety of lifestyles and behaviors and it is fascinating. My background includes my childhood in Arad, where I wandered a lot in the nature, in the wadis, and was exposed to the amazing world of the barefoot. I was fascinated by the variety - the wealth of species - especially of the arachnids."

Since the caves are dark and do not contain plants that carry out photosynthesis, the common source of energy in them is organic matter that comes from outside. In caves where bats live, their droppings (guano) are an important source of energy on which a variety of organisms exist. Microorganisms and arthropods such as isopods break down the dung, as well as bat carcasses and other organic materials in the cave. Arthropods and predators such as weevils and spiders feed on the decomposed materials and thus can thrive and reproduce.

In their latest study, which won a grant from the National Science Foundation, Dr. Gabish-Regev and her team conducted a survey of arachnids in more than 100 caves throughout the country and classified them according to size, location and type (caves inhabited by fruit bats, insectivorous bats or caves without bats). In addition, within each cave, quantitative data on arthropods were collected with a focus on arachnids. They classified all the arachnids found, some still unknown, into series, family, genus and species and compared them according to the types of caves. This is how they discovered that caves with a certain energy source support an array of arachnids characterized by a unique composition.

the bottom of the cave. Photo: Dr. Efrat Gabish-Regev
the bottom of the cave. Photo: Dr. Efrat Gabish-Regev

Following this, the research team made several hypotheses regarding the length of the food chains in the cave depending on the source of energy that entered it: a cave without bats would have fewer available and constant sources of energy, and therefore would have lower levels of nutrition (for example, only arthropods that break down organic matter and primary predators); A cave with fruit bats will have at least three feeding levels: arthropods, primary predators, and secondary predators; And in a cave with insect bats there will be at least four feeding levels: arthropods, primary predators, secondary predators and tertiary predators.

One hypothesis was that in caves with insect bats, the food chain would be longer and include more levels of joints, and more levels of predators than caves with fruit bats or caves without bats.

"One hypothesis was that in caves with insectivorous bats, the food chain would be longer and include more levels of arthropods (both microorganisms and arthropods such as land crabs and millipedes), and more levels of predators (such as spiders that are predators of arthropods and spiders that are predators of predators, spiders and vandals for example) than caves with fruit bats (whose food is rich in sugars) or caves without bats. This is because bats are supposed to have more proteins (because their food is rich in protein), and food rich in protein may support a longer food chain," explains Dr. Gabish-Regev.

To test their hypotheses, the researchers collected bat droppings and the variety of arthropods in the caves, ground and dried them into a uniform substance and sent them to a laboratory in Germany that performs chemical analysis and isotopic composition of nutrients (nutrient analysis). In this analysis it is possible to discover the concentration of the nitrogen 15 isotope, which is one of the building blocks of proteins. According to Dr. Gabish-Regev, "This isotope accumulates along the food chain. As the predator belongs to a higher nutrient level in the food web, we will observe a higher concentration of nitrogen-15 in its body."

According to this analysis, the researchers found that in caves without bats, the food chain is shorter than caves with bats, as they hypothesized, and they include lower levels of nutrition. Correspondingly, the arthropod species assemblage included fewer feeding levels. However, they were wrong about their hypothesis of feeding levels in caves with fruit bats versus insect bats; Fruit bat caves have been found to have higher nitrogen 15 values ​​and are therefore hypothesized to contain higher nutrient levels than insect bat caves.

Dr. Gabish-Regev: "In the next phase of the research, we will examine what leads to this phenomenon. In addition, an experiment is planned to add an energy source rich in sugar or proteins to the caves, and an examination of its effect on the arthropods and their nitrogen 15 level."

Life itself:

Efrat Ghavish-Regev

Dr. Efrat Gabish-Regev, 47, married, lives in the Ben Gurion Midrash, loves to travel and be in nature ("Fortunately, my profession is also the main hobby in my life. Every possible variety fascinates me").

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