Links in the food chain
Small critters are everywhere…if we take the time to look for them. We need to observe carefully, and hopefully learn how they fit into nature’s life cycles. What follows is just a minute amount of information about arachnids (spiders). And of course you should know that spiders are not insects. Spiders have eight legs, insects have six.
Just in the United States and Canada, at least 4,000 known spider species live out their lives and do their duty as a predator of other spiders and to a great extent insects. Iowa has only 39 species of spiders living in every diverse habitat type imaginable in grasslands, forests, wetlands, crop fields and even in our gardens and homes. It is important to note that spiders are not bound by the territorial lines defined by humans, therefore their distribution is subject to change. And quite often, to an extent we may never know, is the fact that some species move intentionally or accidentally by transportation by humans in cars, luggage, shipped goods including foods from far away places.
The female garden spider body is big, has lots of bright yellow patches on a black torso, and is about one inch long excluding its legs. Males are only about one-fourth inch long. Spiders have eight eyes and for this species, the median eyes are grouped together in a trapezoid shape and its lateral eyes are more distant on its head. Hanging onto thread-like silt web strands, the spider is the epitome of patience. It waits for an unsuspecting insect to crawl onto or fly into the sticky stands of the web. Once caught, the spider quickly goes to work wrapping more silk around the prey. And of course a bite injected venom on the insect renders it helpless. Food for the spider is secured.
Garden spiders build a web beginning with several radial lines between four or five anchor points up to three feet across. The radial lines meet at a central point. From this frame, more radials are spun, anchored and then the center is filled with spiral rings delicately laid every three-eights of inch apart. A characteristic web design for this critter is a zig-zag dense pattern of silk in the center. That unique pattern is called the stabilimenta.
This Argiope species breeds twice per year. An egg sac is spun to hold the new to be offspring. Each bundle may contain up to 1,000 eggs! And she will make up to four bundles hanging from a portion of her web. She will protect the eggs diligently but as cooler weather of fall arrives she will grow more frail. At the time of first frost, she will die. Young garden spiders stay inside the silken bundle over winter and emerge next spring. Each is the size of a speck of dust. Some will stay close to the area, but others will emit a single strand of silk that can be caught up in the wind and transported long distances from its birthplace.
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Small critter include insects, some we like and others we humans detest. They represent the most diverse groups of organisms in the world. Over 900 thousand different kinds are known. What is unknown are all the species not yet documented or discovered by scientists and researchers. The speculative number of species ranges from a low of 2 million to as high as 30 million. This estimate is just that, an estimate. Insect species representation is approximately 80 percent of the world’s total animal life. At any one time, living insects are collectively the largest biomass of any life forms, on the order of 10 quintillion (that is 10 followed by 18 zeros).
THE United States alone had about 91,000 described insects. These are the known bugs. Unknown insects yet to be scrutinized is over 73,000. Those 73,000 are contained just within four insect Orders: Beetles 23,700, Flies 19,600, Ants, bees and wasps at 17,500 and Moths and butterflies at 11,500. There is a lot to learned by studying insects, both the beneficial kinds and not so nice critters, from a human perspective. However, a word of caution is in order: Until we learn how all the cogs in the wheel function together in an ecosystem, it is wise not to condemn them all.
How many insects can live on one acre of land? One study in North Carolina, in just the first five inches of topsoil, yielded 124 million animals per acre! Of this 90 million were mites, 28 million springtails, and 4.5 million other insects. At a Pennsylvania study site 425 million animals per acre. In this case 209 million were mites, 119 springtails and 11 million other arthropds.
Insects have a long history of survival. Geological history show them to be survivors in part due to their small size, capacity for flight, their ability to store fertile eggs, and adaptive capabilities. One African species of termite, the queen in this case, was able to lay an egg every two seconds. That amounts to 43,000 eggs each day. Another example is the common housefly. Consider this scenario … the descendants of one pair, provided they all survived during a five-month season, would total 190 quintillion individuals. That is enough to make lots of frogs and birds happy.
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Nature has solutions to what we may call too many spiders and too many insects. One critter that eats all the above are frogs. We have lots of species in the world and our North American list of frogs is lengthy with about 90 species. Just one is the Northern Leopard Frog whose diet includes worms and insects of just about any size they can get into their mouth. It takes a lot of frogs to eat hundreds of insects, or thousands, in order to sustain its own life. It is a good thing frogs exist so we are not overrun with insects. It should be noted that science has recorded the loss of 125 amphibian species since the 1980’s due to habitat destruction primarily. Guess what they ate?
Birds play a huge role in insect control. That is very good. And some insects prey on other insects, only to become food for something else bigger and further along the food chain. This interplay of eat or be eaten, the food chain, or another way of putting it is to call it energy transfer. What is eaten becomes energy to allow the next higher level to live. It in turn is food for the next level above it, and so on until the top of the food chain is obtained.
The next time you see any spider or frog species, give them a friendly greeting, understand the role they play in nature’s on-going saga of survival, and be thankful these little critters exist as links in the food chain.
Garry Brandenburg is a graduate of Iowa State University with BS degree in Fish & Wildlife Biology. He is the retired director of the Marshall County Conservation Board. Contact him at P.O. Box 96, Albion, IA 50005.