Literature Review: Food Web Stability

by Carl Strang

This week I want to bring together a number of recent papers, combine them with earlier concepts, and summarize them into one current view on food webs: how they are structured, how they work, and especially what keeps them from falling apart.

Introduction. Food webs include all the species in a biological community and the connections between them through which energy and nutrients flow. Food webs are organized in certain ways, apparently following rules that produce stability (resistance to change) in the webs. Over time, food webs lacking such organizing features cannot last, so as the individual species within them evolve interactions which produce those features, food webs retain them and become more stable.

Food webs are composed of food chains. Here is one link: bald eagles with glaucous-winged gull, Adak Island.

Component Communities. One important way in which food webs are organized is through component communities, the groups of consumers associated with each particular plant species (Thébault and Fontaine 2010). This specialization produces stability, because a disturbance associated with a fluctuation in a species largely is confined to that species’ component community. While too close a duplication of ecological roles within a component community detracts from stability (competition threatening to drive some species to local extinction), such duplication in a mutualistic group (e.g., a number of pollinators shared by a group of plant species) contributes to stability (a given plant or pollinator has other species to work with if one is lost; Thébault and Fontaine 2010).

My study of leaf miners in sugar maples focuses on both a component community and a guild.

Switching. A trophic level is a step in the flow of energy and nutrients, with producers (most commonly, green plants) occupying one level, primary consumers (plant eaters) occupying the next level, and so on. Here an important contributor to food web stability is the degree to which it contains generalist consumers (Thompson et al. 2007). If one food becomes scarce, the generalist can switch to another. If one food becomes abundant, the generalists can focus on it. Switching tends to keep populations as well as communities stable, because increasing numbers of an abundant species draw attention that keeps them in check, allowing less common species to recover and, therefore, persist (Neutel et al. 2007).

Raptors like this red-tailed hawk readily switch to take advantage of abundant prey.

“Top Down” Control. The action of predators and parasites, keeping prey in check, also limits the degree to which primary consumers endanger plants (“top down” control of food webs; Estes et al. 2011). At the same time, this limitation on populations provides a check that limits the ability of competitive dominants to drive other species to local extinction. Another, more evolutionary process which limits competition is the development of guilds, groups of ecologically similar species which specialize in such a way that they subdivide a resource.

Wolves are classic top predators.

Diversity and Stability. Food webs become less stable as they become simpler (less diverse), because they do not have enough species to provide such compensatory checks and balances (Anderson and Sukhdeo 2011, Irmis and Whiteside 2011). Low productivity (resulting from a limitation in nutrients, for example) is the most common condition leading to such simpler systems in which food webs are controlled from the production end rather than by consumers (Cebrian et al. 2009).

Some Recent Literature

Anderson TK, and MVK Sukhdeo. 2011. Host Centrality in Food Web Networks Determines Parasite Diversity. PLoS ONE 6(10): e26798. doi:10.1371/journal.pone.0026798

Cebrian J, et al. 2009. Producer Nutritional Quality Controls Ecosystem Trophic Structure. PLoS ONE 4(3): e4929. doi:10.1371/journal.pone.0004929

Estes, James A., et al. 2011. Trophic downgrading of planet Earth. Science 333:301-306.

Irmis, Randall B., and Jessica H. Whiteside. 2011. Delayed recovery of non-marine tetrapods after the end-Permian mass extinction tracks global carbon cycle. Proceedings of the Royal Society B, Published online Oct. 26, 2011; DOI: 10.1098/rspb.2011.1895

Neutel, Anje-Margriet, et al. 2007. Reconciling complexity with stability in naturally assembling food webs. Nature 449: 599-602.

Thébault, Elisa, and Colin Fontaine. 2010. Stability of ecological communities and the architecture of mutualistic and trophic networks. Science 329: 853-856.

Thompson, Ross M., et al. 2007. Trophic levels and trophic tangles: the prevalence of omnivory in real food webs. Ecology 88:612-617.

NOW is it Spring?

by Carl Strang

There seems to be some impatience in the Eurasian component of our flora. Earlier I documented dandelions blooming in the first week of January. Yesterday I ran across this bit of green at Mayslake Forest Preserve.

Light was poor, so I wasn’t able to get a sharp photo. These are newly expanding leaves in a little clump of tartarian honeysuckle.

All the buds on these 2-foot plants were green. I am guessing that this little tuft of stems has doomed itself. If only all their local congeners would follow suit! There was nothing special I could see about this shrub’s location. It was in a low-quality woodland, and surrounded by other honeysuckles more sensible about the season.

The Friary Juncos

by Carl Strang

This has been a very slow winter for birds at Mayslake Forest Preserve. The main exception has been a flock of dark-eyed juncos that has taken residence at the former friary site. The area grew a lot of annuals in the late summer and fall, including a number of weedy species, as well as oats.

The juncos most often are to be found around the edge of the site, especially here where the garden used to be.

The seeds from those annuals are keeping these gray sparrows going through the winter. There are enough of them that I doubt that my counts have ever been complete. The high count, in December, was 19. That is many more juncos than I have found on this preserve in previous years.

This week, when I took this photo, I counted 17.

Juncos tend to establish winter home ranges and stick with them, so I expect to see this flock for several weeks, yet.

Literature Review: Climate Change and Range Shifts

by Carl Strang

Global climate change, like evolution, is a matter of debate among politicians, but not among scientists. At this point it’s a routine matter of measurement and documentation. One example from last year was a study published in Science (Chen, I-Ching, et al. 2011. Rapid range shifts of species associated with high levels of climate warming. Science 333:1024-1026).

They reviewed a variety of terrestrial organisms, including invertebrates, vertebrates and plants, and found that species distributions “have recently shifted to higher elevations at a median rate of 11.0 meters per decade, and to higher latitudes at a median rate of 16.9 kilometers per decade…The distances moved by species are greatest in studies showing the highest levels of warming,…However, individual species vary greatly in their rates of change.” While their data show most species shifting poleward, a few shifted in the opposite direction.

The round-tipped conehead is one of several singing insect species I have been finding with significant northward range shifts in the past 70 years.

The Chen et al. paper will allow me to compare their measurements with rates of expansion I can estimate from the literature and my observations of at least 4 species of crickets, katydids and cicadas. I’m pretty sure the movements of these species will prove to be consistent with their findings.

Lessons from Travels: Clouds from Above

by Carl Strang

One of the delights of air travel is the opportunity to view clouds from above. To be sure, there is much beauty to be enjoyed from the play of light on clouds we see from the ground.

Winter sunset

The new perspective from a jet liner adds a different perspective. We’ll follow the common summer sequence, beginning with simple cumulus.

Cumulus clouds are the common puffs we all know and draw as children.

These are simple enough that they look much the same from above.

The mosaic of the ground provides a visually interesting backdrop.

As a summer day passes, solar energy causes an increase in the rising air columns and their moisture content. The cumulus clouds swell as that moisture hits cooler air above and condenses.

They become bigger and more complex in shape.

Now there is more to be seen from the air, as well.

The clouds seem to form a landscape of rounded mountains and valleys.

If conditions are extreme enough, the clouds coalesce, and build high into the atmosphere.

Those who know something of the process, and see this, begin to assess where shelter is to be found.

A full appreciation can be gained from the higher-altitude perspective.

The anvil shape of a cumulonimbus, the cloud of the thunderstorm.

A final lesson comes when one notices that at a typical cruising altitude of only 5 miles, nearly all the clouds are below.  The realization begins to sink in that the breathable, active atmosphere is thin indeed, the barest skin’s thickness when compared to the size of the Earth as a whole. Yes, it is a small enough volume easily to be changed by the actions of billions of technologically leveraged human beings.

Literature Review: Savanna Ecology

by Carl Strang

A pair of papers in Science last year described global surveys of savannas and the ecological conditions that produce them. Their results were not surprising, but valuable in further documenting the ecology of these woodlands with their trees scattered more widely than in forests. They defined forests as woodlands with tree cover greater than 50-60%. Above 2500mm of annual rainfall, a region will be forested. Between 1000 and 2500mm, an area can be forest or savanna, with fire tipping the balance. In savannas, low tree cover promotes the spread of fire, and fire limits tree cover. Lengthy dry seasons and lower rainfall amounts tend to produce savanna.

By their standards, this woodland at Mayslake Forest Preserve probably would be called forest rather than savanna.

When actual tree cover is measured, forests characteristically have 80%, and savannas 20%. There are few woodlands that measure close to the cutoffs separating these categories, supporting the idea that forest and savanna represent alternative stable states, or “attractors.” All of this is relevant to northeast Illinois. We fall in the range of rainfall where forest would cover the ground, but the rain is in the lower end of that range, and fire historically produced a prairie landscape with patches of savanna and, where rivers or topographic breaks produced little fire shadows, bits of forest. Here are the references:

Staver, A. Carla, Sally Archibald, and Simon A. Levin. 2011. The global extent and determinants of savanna and forest as alternative stable states. Science 334: 230-232.

Hirota, Marina, et al. 2011. Global resilience of tropical forest and savanna to critical transitions. Science 334:232-235.

West Bluffs Walk: 2

by Carl Strang

Yesterday I shared the tracking highlights of my recent walk through south Waterfall Glen Forest Preserve. Today, some winter botany. These are not unusual plants, but such a large area provides a lot of good examples to choose from for photos. The first species is one I haven’t found at Mayslake Forest Preserve, my main area for botanical study.

Someone familiar with this species will recognize it from this photo.

A closer view of the distinctive fruiting stalk reveals it to be lopseed.

Another woodland herbaceous plant, and one of our most common, is the wood avens, also known as white avens.

Again, if you are familiar with this one, this photo is enough.

Close up, the seeds in their loose ball project the hook-like extensions that latch onto fur or clothing for dispersal.

In this case I have a flower photo to show.

Wood avens is in the rose family.

One more common woodland plant, this time beginning with the seed array:

Again, little hooks serve to aid dispersal.

Here is the entire plant, a woodland knotweed.

I’ll close with a weedy plant from the Old World. It grows in the open, and belongs to one of two species. I do not know how to tell them apart without the flowers.

The sprawling, spindly plant form is rather nondescript in winter.

The seeds, many of which have been knocked off at this point in the season, have a vanilla flavor if chewed. I don’t recommend chewing on unfamiliar plants, however.

When blooming it looked either like this:

White sweet clover

Or this:

Yellow sweet clover

All in all, this was a satisfactory walk even without the spice of bobcat tracks.

West Bluffs Walk: 1

by Carl Strang

Once or twice a winter I like to get into the south part of Waterfall Glen Forest Preserve. Apart from the general attraction of being our most biologically and topographically diverse area, and scenically beautiful, it also is where a bobcat is most likely to show up. I didn’t find any bobcat signs this time, but the warm (40F) conditions made for some interesting tracking.

Footprints were deteriorating rapidly. Here is a comparison of one of my tracks going in against a fresh one as I came out less than 3 hours later.

Most trails were composed of blob tracks.

A typical coyote trail that day.

When I found sharply defined footprints, I knew they had to be fresh.

This coyote trail couldn’t be more than half an hour old.

Another fresh coyote track on deeper snow.

Blob tracking does allow some species identification.

Small round footprints in a diagonal walk, within half a mile of a residential neighborhood, point to domestic cat.

Sheltered from the sun and close to the base of a tree, the next trail had held up better.

The odd, irregular footprints of an opossum.

It wasn’t all tracks, though. Away from the trails I encountered several deer.

This young buck allowed a relatively close approach.

A track of a different sort was this old concrete and rock structure. I don’t remember encountering it before.

It appears to be one of the old Lincoln Park Nursery cribs, but it is away from the main trail, and distant from the others.

I also got some winter plant photos, which I’ll share next time.

Lessons from Travels: Aliens Like Deserts

by Carl Strang

This one’s for fun. Have you ever wondered why Aliens like deserts so much? It certainly seems to me that a disproportionate number of UFO sightings are reported from deserts. I have seen consequences of this on two continents. In Australia I took a bus tour to travel from Alice Springs to Darwin, rather than renting a car. This was the better choice, as the driver/guide knew the best stops. One of these was a roadside restaurant with an Aliens theme.

Little green men emerge from an angular transport. No doubt this sculpture commemorates an actual event.

The picnic area was lined with murals documenting the Aliens’ visit. This one’s my favorite:

Hold tight, mate!

In North America we have our own deserts, and our own Aliens. One of the more infamous associations is Area 51 in a remote corner of Nevada. When I did my driving tour of that state some years ago I just had to include the Extraterrestrial Highway in my itinerary.

As you can see, this is an official copyrighted designation.

This was the first road I ever encountered that literally vanished in the distance.

Roads can be made quite straight in deserts.

As I drove by this little herd, I could tell they were watching me intently.

Were they really cows? Or something else in disguise…

Midway along the road is the little town of Rachel, with its own roadside stop.

A slightly lower production value than its Australian counterpart, but charming and getting much less traffic. If I were an Alien I would stop here.

I also liked the rounder forms of the Nevada Aliens’ vehicles.

Everyone needs a tow now and then.

So, why do Aliens prefer deserts? Perhaps the more appropriate question is, why do desert humans attract Aliens?

Winter Arrives at Mayslake

by Carl Strang

It took until the middle of January, but at last we got a snowstorm worthy of the name. The 4-6 inches of windblown white stuff provided the conditions needed for a crew to begin burning the year’s worth of accumulated brush piles from the restoration program.

The sterilized circles create little spots for invasion or seeding. Hm, I should consider mapping these and documenting the succession that occurs on them.

The white backdrop also allows me to resume collecting photos of plants in winter. On Friday I made a start with white vervain.

Overall, white vervain has a thin and spindly look.

Up close, the tiny bumps where flowers were located give the plant a grainy texture.

A peek back at the plant when it was blooming makes sense of its winter form.

Here you can see where those little bumps come from. Some of the nettle-like leaves remain attached in winter, but they are dark brown and so curled up that their shape is difficult to discern.

I had taken photos of the compass plant flowering stalk, but needed one of the leaves.

Compass plant leaves are large, and stiff enough to hold their shape for easy identification in winter.

Plenty of plants remain at Mayslake to be so documented.

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