by Jeff Stehm
Halloween is approaching and it’s time to consider that ubiquitous symbol of the haunted house – the spider’s web. We often see spiders as scary or a nuisance, and their webs as something that must be brushed away, but in fact spiders and the webs they weave are one of the complex wonders of nature.
Dating back almost 400 million years ago, spiders are among the most diverse of terrestrial predators. At least 48,200 spider species, and 120 spider families have been recorded by taxonomists. While we typically associate spiders with webs, not all spiders spin webs (see Wolf Spiders) or use the silk they produce for webs (see Jumping Spiders). Species that produce silk, but not webs, may use silk in several ways: as wrappers for sperm and for fertilized eggs; as a "safety rope"; for nest-building; and as "parachutes" by the young of some species.
But webs are what we notice, so let’s learn a bit about web materials, web structure, web functions, and the evolution of webs.
Attached is a paper titled "Biodiversity Loss - The Decline of the North American Avifauna" authored by scientists from Cornell Ornithology Lab, SCBI, and others on the loss of North American birds. It not only documents the extraordinary loss of birds in North America, but also shows important citizen science has been in conducting such research.
Paper Summary: Species extinctions have defined the global biodiversity crisis, but extinction begins with loss in abundance of individuals that can result in compositional and functional changes of ecosystems. Using multiple and independent monitoring networks, the article reports population losses across much of the North American avifauna over 48 years, including once-common species and from most biomes. Integration of range-wide population trajectories and size estimates indicates a net loss approaching 3 billion birds, or 29% of 1970 abundance. A continent-wide weather radar network also reveals a similarly steep decline in biomass passage of migrating birds over a recent 10-year period. This loss of bird abundance signals an urgent need to address threats to avert future avifaunal collapse and associated loss of ecosystem integrity, function, and services.
Link to Science Magazine article
What Can ORMN Members Do?
Cornell Ornithology Lab is encouraging citizen scientists in the month of October to use the eBird application to record bird observations. In particular, October 19th has been designated as the Global Big Day where citizen scientists are asked to use eBird over 24 hours to note the birds observed at their favorite park/county/state/province country/continent (https://ebird.org/octoberbigday). The record to beat is last year’s total of 6,331 species on a single October day.
Have any of you been seeing swarms of dragonflies in your yard and pastures the past few weeks? This is the first time my wife and I have experienced this phenomenon of nature. I’ve discovered that dragonflies do have a swarming behavior, although scientists aren’t sure why. Two types of swarms exist. The migratory swarm where large masses of dragonflies migrate, flying at higher altitudes. Some of these migratory swarms have been dense enough to show up on weather radar.
The other type of swarm that we experienced is a static feeding swarm, where several hundred dragonflies swoop low over a lawn or pasture in figure eight patterns catching bugs. The numbers were thick enough in our yard that all I had to do was swing my butterfly net and bam! I had a dragonfly! She was kind enough to pose for the photo below before flying off to rejoin the group.
The iNaturalist group identified her as a Common Green Darner (Anax junius) which is one of the largest dragonflies; males grow up to 3 inches in length and have a 3-inch wingspan. They also migrate great distances from the northern US to south into Texas and Mexico. Neighbors around us have been observing similar swarms.
If you interested in further information on this phenomena, I found a great website https://thedragonflywoman.com run by an aquatic entomologist, Christine Goforth from the University of Arizona. Her website contains a wealth of information on dragonfly swarming as well as a page to report a swarm siting as part of a citizen science project she is running.
Her descriptions of static swarming matches what I’ve observed – the dragonflies typically appear near dawn or dusk (dusk in my case) because it is thought that dragonflies can see their prey better when the sun is low on the horizon. They appeared very suddenly, fly figure eight patterns over our yard for about an hour, and then are quickly gone. The swarms we have witnessed often start around 5 pm and go until about 6 pm when the sun dips below the trees. Dr. Goforth indicates that dragonflies are attracted to large groups of prey organisms. Once the prey numbers drop or they become less active (e.g. as it gets darker), the dragonflies move on. If the prey returns the next day, the dragonflies likely will too. So far we’ve had about 3-4 evenings of dragonfly visits to our yard.
Dr. Goforth says that the swarming behavior is fairly common in many different species of dragonflies, but the chances of a single person seeing more than one or two swarms in their lifetime in a single area can be quite low. The conditions have to be just right for swarms to occur, perfect for both a large number of prey insects and a large number of dragonflies to exist in the same area at the same time.
I count myself lucky to have had a once-in-a-lifetime experience. What marvelous creatures.
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