This rarely seen lichen crab spider, aka running crab spider (Philodromus margaritatus), was spotted recently on a bench near Loch Ness. It lives on trunks of trees or even electricity pylons covered with lichens. It has also been found on garden apple trees. If you can imagine it standing on a patch of lichen, it will be obvious why it is rarely seen: it would almost disappear.
Philodromus margaritatus © Rachel Taylor.
While running crab spiders are relatively rare, “true” crab spiders or flower spiders (Family Thomisidae) are common garden inhabitants. Many of them show some degree of crypsis, which is the ability to avoid detection by other animals by blending in with their environment (whereas mimicry is disguising by resemblance to another organism). We can just say that running crab spiders and crab spiders are very good at camouflage.
You may wonder what spiders are doing in a blog about pollinators. Well, they and other predators are an important and often overlooked aspect of pollination ecology. Flowers are predictable food sources for pollinators, and so too for predators such as wasps, bugs and spiders. So many unsuspecting flower visitors are seized by opportunistic ambush predators, who sit and wait for lunch to fly in.
Interestingly, flies are less susceptible to spider predation than bees, possibly because they have better vision and can avoid or dodge attackers. Bumble bees are also less likely to become prey than solitary bees and honey bees, just because they are larger and bulkier, so more difficult to capture. It has been suggested that the long proboscis and the swing-hovering flying pattern of some moths have evolved as predator avoidance mechanisms: the further from the flower and less static, the better chance of escaping a lurking predator.
A crab spider (Thomisus onustus) capturing a bee © Alves Gaspar, Wikipedia Creative Commons.
But it’s not only through killing that predators disrupt pollination: in some cases, their mere presence scares pollinators away. There are fewer flower visitations and the time pollinators spend on flowers is reduced. As a result, pollination rates and therefore seed production may be lower.
Predation has enormous influence on the balance of ecological communities. It is beneficial to biodiversity by preventing a few species from taking over, and it helps stabilize ecosystems by keeping prey abundance in check. It has been estimated that the global spider community kills the equivalent of 400 to 800 million metric tons of prey annually. More than 90% of this biomass comprises insects and springtails, including a vast number of agricultural pests. For comparison, the annual food consumption of all the world’s seabirds is estimated at 70 million tons.
We don’t have a clear understanding of the impact of spiders on pollinators and plants. But it is quite possible that the characteristics of our gardens, crops and wider flora are to some degree shaped by these itsy bitsy predators.
Scottish pollinators 