Spiders as catalysts for ecosystem development

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. Please visit his work!

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.

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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

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A special thanks to Bryan Reynolds for permission to use his photograph of the dispersing Pisaurid spider.  Please visit his work here.

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Where did all the spiderlings go? A story about egg-sac parasitism in Arctic wolf spiders

This week we are in a deep freeze in the Montreal area, so it seems somewhat fitting to discuss Arctic spiders.  I’ve discussed the life-history of Arctic wolf spiders (Lycosidae) before, specifically in the context of high densities of wolf spiders on the tundra.    Much of this work was done with my former PhD student Joseph Bowden.  The latest paper from his work was published last autumn, and was titled ‘Egg sac parasitism of Arctic wolf spiders (Araneae: Lycosidae) from northwestern North America‘. In this work we document the rates of egg sac parasitism by Ichneumonidae wasps in the genus GelisThese wasps are fascinating, and we have found them to be very common on the tundra.  There are often multiple wasps in a single egg sac, and as is typical with Gelis, they leave nothing behind: all eggs within an egg sac are consumed.  After fully developed, the adult wasps pop out of the egg sac; the Gelis adults we encountered had both winged forms and wingless females, the latter superficially resembling ants.

A Gelis emerging from a wolf spider egg sac. Photo by Crystal Ernst, reproduced here with permission.

A Gelis emerging from a wolf spider egg sac. Photo by Crystal Ernst, reproduced here with permission.

The rates of parasitism of Pardosa egg sacs (by Gelis) were, at some sites, extremely high.  In some cases over 50% of the wolf spider egg sacs were parasitized.  Stated another way,  half of all the females encountered with egg sacs had zero fecundity because the female was  carrying around wasps within the egg sac instead of spider eggs.

It’s quite interesting to think about these wingless Gelis femalesafter emerging from egg sacs, they end up wandering around the tundra in search of hosts.  Spiders with egg sacs must be encountered frequently enough for the wasps to grab on to a passing wolf spider in order to parasitize the egg sac.  Recall, densities of wolf spiders can be very high in the Arctic (4,000 per hectare, at least).  Hmmm…. this is all starting to fit… high densities of wolf spiders support high rates of egg parasitism and these wasps can ‘afford’ to be wingless since their hosts are frequently encountered:  an interesting feedback loop!   We can also speculate about large-scale gradients in diversity – many Ichneudmonidae show high diversity in northern regions.  Within Gelis, it’s a good bet that they will find many suitable spider hosts in these environments.

Looking down the microscope - all those Gelis!

Looking down the microscope – all those Gelis!

So, how extreme are these rates of egg parasitism?  Looking at some of the literature, there are certainly a number of papers about  wasps that parasitize spider egg sacs.  Cobb & Cobb (2004) studied two Pardosa species in Idaho, and recorded a egg parasitism rate of about 15% (by Gelis wasps and wasps in the genus Baeus [Sceleonidae]). Van Baarlen et al (1994) studied egg parasitism in European Linyphiidae spiders and their maximum rates of parasitism were about 30%.   Finch (2005) did a detailed study of four spiders species (non-Lycosidae) and rates of egg parasitism varied between 5% up to as high as 60% in an Agroeca species.

Our documented parasitism rates for Arctic wolf spiders are certainly quite high (for Lycosidae), but not out of the range of other published studies for non-Lycosidae.  I do wonder whether we will continue to find high egg parasitism rates if more species were examined in detail – certainly a fertile area of study.  Related to this, what are the population-level consequences of this interaction?  What is the relationship between spider densities and parasitism rates?  Although Joe and I did try to speculate on this, our data are preliminary – again, a key area for future research.

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In the Arctic context, we will continue to uncover fascinating food-web dynamics.  Our research group has already been thinking seriously about this – Crystal Ernst has written a nice post about the idea of an ‘inverse trophic web’ (i.e., predator-dominated) in the Arctic, and a fair amount of my future research will pursue this avenue of research.

Pique your interest…?  Why not think about graduate school in my lab, and study Arctic arthropod biodiversity?

References:

Bowden, J., & Buddle, C. (2012). Egg sac parasitism of Arctic wolf spiders (Araneae: Lycosidae) from northwestern North America Journal of Arachnology, 40 (3), 348-350 DOI: 10.1636/P11-50.1

Cobb, LM & Cobb VA (2004). Occurrence of parasitoid wasps, Baeus sp and Gelis sp., in the egg sacs of the wolf spiders Pardosa moesta and Pardosa sternalis (Araneae: Lycosidae) in southeastern Idaho. Canadian Field Naturalist 118(1); 122-123.

Baarlen, P., Sunderland, K., & Topping, C. (1994). Eggsac parasitism of money spiders (Araneae, Linyphiidae) in cereals, with a simple method for estimating percentage parasitism of spp. eggsacs by Hymenoptera Journal of Applied Entomology, 118 (1-5), 217-223 DOI: 10.1111/j.1439-0418.1994.tb00797.x

Finch, O. (2005). The parasitoid complex and parasitoid-induced mortality of spiders (Araneae) in a Central European woodland Journal of Natural History, 39 (25), 2339-2354 DOI: 10.1080/00222930500101720

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Natural History: unknown.

I sometimes see this statement in taxonomic papers that describe a new species:

Natural History: Unknown

Think about this… specimens have been collected, somewhere, sometime. Perhaps these specimens sat in an Entomology museum for decades until a MSc student took them out and started a revision. Perhaps the specimen was recently sorted from a bulk malaise trap sample from the Amazon basin, and sent to a taxonomic expert for identification. S/he recognized it was something different and later, while doing a taxonomic revision, included it, measured it, did a line drawing, extracted some DNA, wrote a description, gave it a name. However, when writing what is known about its natural history and biology had to write “unknown“. (by the way, discussions about defining natural history can be found here and here).

An unknown weevil with unknown natural history.

I recognize why nothing is known, but when trying to get some sense of why a particular species might be found in a particular habitat, having no information about natural history and biology can be frustrating. This is especially true for ecologists, whose research might benefit immensely from ANY natural history information. In my own work, after I key out a species of wolf spider, for example, I immediately flip to the description, and scan down to the notes about the biology of the species – these notes can confirm details about the species (hey look, I found it under rocks on a shoreline, and that is where it is reported, also!; or, indeed, it makes sense that I found that egg sac in late summer – that species is known to mate in mid-summer).

Natural history is important, as is so elegantly stated in many papers (e.g., see Greene’s 2005 paper) and the impending extinction of natural history was written about over 10 years ago by Wilcove & Eisner.  The world needs natural history information, and although I recognize that having a name is clearly very important, it is also essential to have some natural history information. Such information can lead to additional research on the species, or allow others to document the species in new locations around the globe. Having some information will help future graduate students figure out when during the growing season they should find specimens, and perhaps what host plants they should look on.

So, I ask these questions, and I look forward to responses, especially from taxonomists:

Should taxonomists wait to describe a species until there are some details known about its natural history? (this will, of course, take more specimens and more time…)

and,

Under what conditions is it acceptable to state “Natural History: unknown”?

Caveats:  I am coming from this question as an ecologist with an appreciation for taxonomy, but not as someone trained in taxonomy.  I am, therefore, biased in my views.  I also recognize that in many cases, taxonomists only have one specimen and a label to work with, and data on the label itself may be lacking, hence the need to state “natural history: unknown”.  My questions are meant to be more general, and I am hoping to gain insights into whether seeking additional natural history information about species (when it is described) is a losing battle… and whether this task should be in the hands of the individuals who describe species.

References

Greene, H.W. (2005). Organisms in nature as a central focus for biology Trends in Ecology and Evolution, 20 (1), 23-27 DOI: 10.1016/j.tree.2004.11.005

Wilcove, D. and Eisner, T. (2000) The impending extinction of natural history. Chron. Higher Ed. Sept. 15, B24. Available here.

Spider cakes!

My graduate students are a very talented bunch – they are intelligent, creative, and have a good sense of humour.  Some of our lab group celebrated birthdays recently, and in honour of this, we had two cakes earlier this week.  The first, made by MSc student Sarah Loboda, is the VERY BEST SPIDER CAKE I have ever seen (or eaten!).  Check this out:

Spider Cake!

Of course, let’s discuss how anatomically correct that cake is!  Two body parts, pedicel, eight legs (coming from the cephalothorax, of course), and a bunch of eyes.

Spider cake! (eyes0

As you may know, most spiders in Canada have eight eyes, but since some do have six, I find it quite acceptable that this spider has six eyes.  Furthermore, not all spider eyes are identical so it is appropriate to have two kinds represented on the cake.  Well done, Sarah.

And in case that STUNNING MASTERPIECE isn’t enough, another student (Dorothy Maguire)  made a cake that is a very good approximation for the female epigynum of wolf spiders in the genus Pardosa.

Pardosa epigynum

And not just any Pardosa:  this is diagnostically similar to one of the species that graduate student Katie Sim is working on!  Incredible!

….want some proof – look at this image, taken from Dondale & Redner’s text on the Lycosidae of Canada.  Enough said.

Pardosa concinna epigynum

Seven-legged spiders walking on water

Many spiders are known to ‘walk on water‘ (including dock spiders): spiders are small enough that many species and life stages can be held by the meniscus of water.  Spiders also have eight legs but they often lose one or more of their legs (in the scientific jargon, this is ‘leg autotomy‘).   In general, this is often part of defensive behaviour, and is common in many animals.  Sacrificing an appendage is a better idea than being eaten by a predator.

So… let’s link these thoughts together- spiders run on land as well as water, and they are often missing a leg.

A wolf spider (Pardoa mackenziana). In this photo, a leg that was previously lost has been ‘re-grown’ (4th leg on right side). The cost of this spider’s lost leg must have been minimal, since it survived and moulted again!

So, next comes the research question:   what is the ‘cost’ of leg autotomy and does this cost vary depending on whether the spider is traveling on the land or on water.  This is an interesting question, and one that was addressed directly by Christopher Brown and Daniel Formanowicz Jr in a recent research paper in the Journal of Arachnology.   These authors used the wolf spider Pardosa valens as their model species, and conducted ‘speed trials’ for male and female spiders on a terrestrial track as well as an aquatic track (i.e., these were constructed in a laboratory setting).  After doing the trials with intact spiders, the authors ‘induced autotomy’ (yes, this sounds somewhat horrific, but autotomy is very common with wolf spiders, and although they lose a little of their hemolymph, they heal quickly) and ran the trials again, with the same spiders (sans legs).

Results?  Well, perhaps not surprisingly, in the first year of their study, the species ran more slowly when they were missing a leg, but in the second year of the study, this affect varied by sex (males were slower, and autotomy only affected the females).   They report some rather complicated results when comparing terrestrial to aquatic trials, but in general, the spiders tended to run more slowly on the aquatic tracks when they were missing legs.  Again, this is perhaps not a surprising result, since having seven instead of eight legs will certainly change the biomechanics when considering how the spiders interacts with the meniscus of water.

Clearly, there are some costs associated with missing legs, but it is important to note that even without legs, these wolf spiders were able to run effectively on land and water, and even if their speed was slower than when they had all eight legs, they can still move an impressive speeds.  The range of speeds in some of the trials was between  20 cm/s and 50 cm/s – this translates to  running speeds between 0.72 and 1.8 km per hour!

Leg autotomy in wolf spiders in natural habitats range from between 8% and 32% as reported by Brueseke et al. in 2001 and by Apontes & Brown in 2005.  In the present study, the authors state that natural populations of P. valens exhibit between 25% and 45% autotomy.    These numbers are in line what what I have observed, as well.  This is pretty amazing – wolf spiders exhibit leg autotomy at a very high frequency, and in some cases, half the spiders in a population are missing a leg.  What can we infer from this?   Although there are some costs associated with leg autotomy (as reported by Brown and Formanowicz), they must not be that high – otherwise, natural selection certainly wouldn’t have favoured autotomy as a means to escape predation.  Brueseke et al., research supports this as they found very few costs associated with autotomy in Pardosa milvina.  In their work, Brueseke et al. studied locomotory behaviour as well as prey capture, and found overall support for the ‘spare leg hypothesis’ (i.e., look at all of my legs!  I can manage without one!).

So, here are the take-home messages:

Wolf spiders can run quite quickly, some species can run across water and land, and they can do so with missing legs.  Although they may be a little slower without their full complement of legs, the costs must be relatively minor given the frequency of leg autotomy in wolf spiders. 

This gives you more reasons to watch spiders – count some legs and see how many individuals are without their full complement of legs.

References:

Apontes, P., & Brown, C.A. (2005). Between-set variation in running speed and a potential cost of leg autotomy in the wolf spider Pirata sedentarius. American Midland Naturalist, 154, 115-125 DOI: 10.1674/0003-0031(2005)154[0115:BVIRSA]2.0.CO;2

Brown, C.M., & Formanowicz Jr, D.R. (2012). The effect of leg autotomy on terrestrial and aquatic locomation in the wolf spider Pardosa valens (Araneae: Lycosidae). Journal of Arachnology, 40, 234-239 DOI: 10.1636/Hill-59.1

Brueseke, M.A., Rypstra, A.L., Walker, S.E., & Persons M.H. (2001). Leg autotomy in the wolf spider Pardosa milvina: a common phenomenon with few apparent costs. America Midland Naturalist, 146, 153-160 DOI: 10.1674/0003-0031(2001)146[0153:LAITWS]2.0.CO;2

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Notes from the field: Yukon wildlife (Part 3)

Here is Part 3 from the “notes from the field” series  – an account of a recent field research trip to the Yukon.  Click here for Part 1 and here for Part 2. 

17 July, 10 AM, Dawson City, Yukon

I am back in the world of electricity, Internet, hotels, and tourists.  The layers of mosquito repellent have finally been washed off after a much-needed shower in the Hotel last night.

Arctic Pardosa wolf spiders… captured.

The big news is that the day after I last wrote, we managed to find and collect Pardosa glacialis! We woke early on July 15 and went up to the high elevation tundra habitats located exactly on the border of the Yukon and NWT (we are not even sure what Territory to write on our collection labels! – the site was, literally, on the border!).  All five of us helped Katie look for wolf spiders, and after a couple of hours of searching and collected, we found dozens of specimens – this was thrilling, as these specimens are very important for Katie’s research and we were getting anxious about not finding any. We also got a little bit lucky – within an hour of that sampling, some rather nasty weather blew in and we were forced back to camp for the afternoon.  In the rain, tundra wolf spiders tend to hunker down deep into the moss and lichens, not to be seen.

I have mixed feelings about being able to catch up on e-mails, and I certainly miss my family.  However, I am also missing the fields of cottongrass on the Arctic tundra, eating cloudberries in high mountain passes, and seeking new localities for the Arctic pseudoscorpionThe Dempster Highway is a biologist’s dream – full of wildlife, stunning vistas, amazing habitats, a unique biogeographical history, and a region that hosts a rather stunning and diverse arthropod fauna.

I will be back up here again.

The Yukon landscape.

Notes from the field: Yukon wildlife (Part 2)

Here is Part 2 from the “notes from the field” series  – an account of a recent field research trip to the Yukon.  Click here for Part 1. 

14 July, 11 PM, Rock River Campground, km 445 (Dempster Highway), Yukon

“Bag of spiders” – a nice haul of wolf spiders!

We have had a busy few days – we finally got some drier weather in Tombstone and Laura and Barb were able to do some collecting, and Crystal set some more traps.  We left Tombstone a couple of days ago to drive north, collecting en route.  We have seen some of the larger wildlife, including arctic fox, moose, and grizzly bears.   However, our sights were really set on the smaller wildlife: Barb was particularly impressed with the diversity of parasitic wasps at a place called “Windy Pass” – this area is known for hosting a lot of rare, Beringian species, and entomologists have collected at this locality for decades. We crossed the Arctic Circle yesterday, and the Rock River campground is nestled in a river valley just north of the Arctic Circle.  We are now officially in the Richardson Mountain range – the tundra habitats about 10 km north of this campground is one of the most beautiful places on the planet.  I feel very lucky and privileged to be here.

Although we had some more rain and cold weather yesterday, today was a perfect summer day at this latitude (i.e., it got just above 20C) – it was also a very windy day, which was bliss since higher winds mean that the incessant hordes of mosquitoes are kept at bay.  Fieldwork in the sub-arctic is quite challenging, in part because of the mosquitoes.

Self-portrait geared up for the biting flies.

We collected well into the NWT, getting all the way to the Peel River (located about 540 km up the Dempster).  Crystal found the most northern locality for Wyochernes asiaticus in the NWT and for that reason I will buy her a beer whenever we get back to civilization!   Unfortunately we have yet to find Katie’s wolf spider species – we have checked a few locations but have come up empty – there are certainly many other species of wolf spiders on the Tundra, but the ones we have collected have not been Pardosa glacialis.  Our team is a little anxious about this, as we only have a few more days at the Richardson Mountains before heading south.

We are now back in camp and it should be time to crawl into the tents.  At this latitude it is pretty difficult to think about going to sleep – it is light 24 hours a day, so it is hard to trick the body into thinking it is time for sleep.   It’s even harder to get to sleep knowing that Pardosa glacialis is out there…somewhere.

Stay tuned for Part 3, coming Friday…