It is well known that spiders are effective at dispersal and colonization, in part because of their ability to ‘balloon‘ – small spiders (i.e., immature specimens, or adults of species that are small) will release a strand of silk and let the wind pick them up and carry them far distances. This passive ability to disperse has served spiders well, and enabled them to be among the first animals to colonize new habitats. For example, after the eruption of Mount St Helens, the depopulated Pumice Plain was re-colonized over time, and biologists kept an eye on what was dropping from the skies. Not surprising (to me!) was that spiders represented a lot of this ‘aerial plankton‘ – Crawford et al. (1995) reported that spiders represented “23% of windblown arthropod fallout and contributed 105 individuals per square meter“.
A spider about to launch! Photo by Bryan Reynolds, reproduced here with permission.
Many, many people have recognized this amazing ability of spiders to get to places effectively and quickly. During his voyages on the HMS Beagle, Darwin observed and commented on this. He noticed spiders landing on the ship when they were far offshore. Here’s a lovely quote:
These, glittering in the sunshine, might be compared to diverging rays of light; they were not, however, straight, but in undulations like films of silk blown by the wind.
-Charles Darwin, Voyage of the Beagle, 1832
A wonderful paper titled “Distribution of Insects, Spiders, and Mites in the Air” (Glick 1939) also discusses aerial plankton. In this work, Glick reports on how a plane was used to collect arthropods in the skies – this was done by modifying the plane so it had a collection net attached to it. Spiders were among the most commonly collected taxa, and were found up to 15,000 ft in altitude. Glick followed this up with work published in 1957, and spiders were again reported as common aerial plankton.
Convinced? Spiders really are everywhere and can get anywhere – from dominating the tundra, to floating far above as tiny eight-legged aeronauts.
This takes me (finally) to the point of this post, and some reflection about a paper by Hodkinson et al. (2001), titled “What a wonderful web they weave: spiders, nutrient capture and early ecosystem development in the high Arctic – some counter-intuitive ideas on community assembly”. In this work, the authors provide some data about aerial plankton in a series of sites representing different stages of succession in Midtre Lovénbreen – a ‘small valley’ glacier in Spitsbergen (a Norwegian high Arctic Island). This forum paper was meant to present an idea about ecosystem development in the Arctic, with a focus on spiders and other aerial plankton and their relationship to nutrients.
- Spiders are among the first to arrive due to their amazing abilities at dispersal and colonization.
- Many spiders will just die, and their sad, little bodies will decompose and leave behind nutrients.
- Many of the spider species that arrive will build webs, and the silk contains many nutrients. Regardless of whether the silk successfully captures prey, the silk will eventually be a hot-spot of nutrients.
- A lot of other aerial plankton will hit these webs – this will include other arthropods (Hodkinson et al. rightfully point out the importance of Chironomids, or midges, as key prey for spiders in the north) and these prey may or may not be eaten by spiders. The aerial plankton also includes other ‘debris’ that would be floating around (fungal spores, dirt, etc). The webs capture all these goodies, and act as a concentrated area for a growing soup of nutrients.
- The spider webs will collect moisture. In Arctic systems, dry polar-deserts, and many other newly created habitats, the accumulation of moisture is rather essential for continued ecosystem development.
Taken together, Hodkinson et al. (2001) argue that spiders and their webs represent little pockets of concentrated nutrients in landscapes that are void of much other life. These hotspots could be catalysts for ecosystem development in systems that are starting from scratch. I really like this idea – not only does is stir up the imagination (little spiders gently falling from the sky, landing on habitat never before touched by animals, and providing the start of an ecosystem…), it really makes some biological sense. Ecosystem development requires nutrients and substrates – of course, these would both be available without spiders, but our eight-legged friends are helping move things a long a little more quickly.
The paper by Hodkinson et al. has been cited less than I would have expected. Although they don’t provide any experimental data, their ideas are interesting and relevant and should be studied in detail. Recently, a few papers have come out that are taking the ideas to the next level. Konig et al. (2011) studied arthropods of glacier foregrounds in the Alps. They found that although Collembola and other ‘decomposers’ are quite important in early successional stages, overall, generalist predators (including spiders) were dominant and using stable isotope analyses, they showed that these generalist predators often ate each other – an interaction known as intraguild predation.
I often discuss Hodkinson et al.’s (2001) paper in lectures, and invariably I get the question “If spiders are first to arrive, what do they eat?“. I typically answer that spiders eat other spiders, and it’s reassuring to see literature that supports this claim. In turn, intraguild predation itself contributes further to the accumulation of nutrients (more sad, little spider bodies littering the landscape…).
Placing this work in a more general framework, these ideas are pointing to the increased importance of predators in overall nutrient dynamics in ecosystems. I was thrilled to see a paper by Schmitz et al. (2010) that argues “predators can create heterogeneous or homogeneous nutrient distributions across natural landscapes“. Bingo. This is exactly what Hodkinson et al. were arguing – predators, such as spiders, can arrive quickly to an area, and in the context of newly formed ecosystems, may provide a hotspot for nutrients in an otherwise desolate landscape.
Although the Hodkinson et al. paper is over a decade old, it’s still relevant, and quite important. I suspect that if more newly created habitats are studied in detail, spiders will indeed prove to be catalysts for ecosystem development.
References:
Crawford, R., Sugg, P., & Edwards, J. (1995). Spider Arrival and Primary Establishment on Terrain Depopulated by Volcanic Eruption at Mount St. Helens, Washington American Midland Naturalist, 133 (1) DOI: 10.2307/2426348
Hodkinson, I., Coulson, S., Harrison, J., & Webb, N. (2001). What a wonderful web they weave: spiders, nutrient capture and early ecosystem development in the high Arctic – some counter-intuitive ideas on community assembly Oikos, 95 (2), 349-352 DOI: 10.1034/j.1600-0706.2001.950217.x
König, T., Kaufmann, R., & Scheu, S. (2011). The formation of terrestrial food webs in glacier foreland: Evidence for the pivotal role of decomposer prey and intraguild predation Pedobiologia, 54 (2), 147-152 DOI: 10.1016/j.pedobi.2010.12.004
Schmitz, O., Hawlena, D., & Trussell, G. (2010). Predator control of ecosystem nutrient dynamics Ecology Letters, 13 (10), 1199-1209 DOI: 10.1111/j.1461-0248.2010.01511.x
A special thanks to Bryan Reynolds for permission to use his photograph of the dispersing Pisaurid spider. Please visit his work here.
Arthropod Ecology 