Burn Aftermath

by Carl Strang

Mayslake Forest Preserve had much of its acreage burned for management purposes last spring, as described earlier. One result, aided by good amounts of seasonal rains, was a very lush, tall growth of prairie vegetation.

Part of one of Mayslake’s prairies on August 12.

Part of one of Mayslake’s prairies on August 12.

What impact did this have on the prairie insects, in particular the singing insects? I expected the species that lay their eggs in the tops of prairie plants would be impacted the most, but those that lay their eggs in the soil would be relatively unharmed. It was clear, though, that despite the unusual completeness of the burn, small patches of prairie here and there were missed by the fire, as were wetland and woodland edges, and there were portions of the preserve not included in the burn plan. These provided a reservoir from which affected species might spread.

My impression through the season was that the numbers of fall field crickets (a species which lays its eggs in the soil) were down from last year, but the numbers don’t bear this out. Counts on the whole in the various habitats are similar between this year and last. Likewise, the 3 species of common ground crickets are so abundant in all habitats that no quantitative comparison seems necessary.

Greenstriped grasshoppers overwinter as nymphs, and so are more vulnerable. If anything, however, their numbers seemed somewhat larger in all habitats, including burned ones.

Greenstriped grasshopper nymph

Greenstriped grasshopper nymph

Unfortunately, confusion about the species identity of meadow-dwelling tree crickets (described in a post earlier this week) prevented my gathering quantitative data last year. I did record numbers this year, though, and attended their locations through the season. It was clear that the earliest singers in this group were concentrated in unburned areas and around the edges of burned areas, where they might have hatched from eggs in the unburned adjacent habitats. As the season progressed, though, these tree crickets (mainly Forbes’s tree crickets) proved to be very mobile, and spilled into the hearts of the burned areas (where the forage no doubt was richer thanks to the burn, and where there was an advantage to escape the competition). Though numbers overall may have been down a little, there were plenty of these tree crickets to ensure a rapid population recovery.

As for meadow katydids, they all to some extent concentrate in wetlands, which were scorched in places but not thoroughly burned. There again appeared to be plenty of survivors to reproduce and fill the habitat.

Perhaps the most interesting observation relevant to this question this year was a big drop in wasps of the genus Sphex. There were a lot of these last year, crowding into the areas where swamp milkweeds were blooming. The great black wasp and great golden digger specialize in capturing katydids to feed their young, and potentially can influence populations significantly. I saw only a very few of those wasps this year. As they overwinter underground, I doubt the fire had anything to do with their absence. Whatever the cause, their departure further assured a successful reproductive season for the katydids of Mayslake.

Great golden digger

Great golden digger

The upshot of all of this is that the extensive spring burns, while they may have had some minor and spotty effects on singing insect populations (and, by extension, other invertebrates), did not devastate any populations as far as I can tell. This was somewhat surprising, but in retrospect it becomes clear that it would take an extraordinarily complete and extensive burn to have a long-term impact. Refugia within and without the burn area seem likely to carry populations through enough to recover from this disturbance.

Dispersal Ability

by Carl Strang

In order for us to understand insects well enough to know which ones need the most attention in conservation, there are some pieces of information we need: how abundant they are, how broad or narrow their habitat needs, their reproductive potential, and their dispersal ability. The first two items are readily obtained in the course of a regional survey such as I am conducting for singing insects in northeast Illinois and counties in neighboring states. Reproductive potential has been studied to some extent and can be found in the literature for some species. Dispersal ability is a critical point that is not well studied as far as I can tell, and so it is good to take advantage of observations that reveal which species spread easily, and which ones do not.

Over the past two weeks I spent much time in the St. Joseph Hospital in Mishawaka, Indiana, where the medical professionals saved my mother’s life.

The hospital was built 3-4 years ago, and is surrounded by extensive areas planted mainly in native prairie plants.

The hospital was built 3-4 years ago, and is surrounded by extensive areas planted mainly in native prairie plants.

Occasionally I took walks along the paths, or made observations while arriving or departing. The species present in the plantings can be regarded as ones with high dispersal ability. These included field crickets (I cannot be sure which, as this was the cusp between the spring and fall field cricket seasons), striped and Allard’s ground crickets, Carolina grasshoppers, Roesel’s katydids, and a sword-bearing conehead. All of these are regionally abundant, and fairly broad in their habitat (dry to mesic mixes of grasses and forbs). Three have good flying ability (in the case of Roesel’s, there are long-winged individuals as well as medium and short-winged ones). Field crickets and the ground crickets can take advantage of their regional abundance and tendency to hop and walk over land. One limitation here is that I was only able to make observations over a brief portion of the season.

If I had to point to the weediest singing insect in our region, I’d have to say it’s the striped ground cricket, which is the quickest to appear in a new site.

Where are the Galls?

by Carl Strang

In site monitoring we are recording the ongoing story of a place, and change is the currency in which we trade. Some changes are easier to notice than others. When something newly appears on the scene, there is a good chance we will pick it out. A tree is down that was standing the previous day. A bird new to the site starts singing. The mother raccoon’s tracks now are accompanied by her babies’.

Less easy to notice are absences, things that have failed to appear as usual, especially if they are small. Such was the case for me this year at Mayslake Forest Preserve with respect to the goldenrod galls. Last week as I made my way through the prairies and meadows, I realized that there were very few galls, and I started to pay attention. I saw no spindle galls of the moth caterpillar (Gnorimoschema gallaesolidaginis), but usually there are few of those anyway. In the three previous winters there were dozens to hundreds of bunch galls (produced by the midge Rhopalomyia solidaginis ) and especially of ball galls (stimulated by the gall fly Eurosta solidaginis) along the route I was taking.

A goldenrod bunch gall

A goldenrod bunch gall

This year I counted 2 bunch galls and no more than 15 ball galls. If only one species were involved, I would suspect that a specialized disease or parasitoid had caught up with them and caused a population collapse. There were two, however, and furthermore they belong to different families of insects.

A goldenrod ball gall

A goldenrod ball gall

The ball galls mostly were in little bunches, suggesting that each group was the product of a single female. The places they were clustered seemed unremarkable, though possibly the plants were larger in those spots (smaller and thinner in most places). It seems unlikely that parasites of both species would have knocked them down in the same year. It’s tempting to blame the drought for all of this, as it was the oddest aspect of this past year, but it’s also true that the winter was relatively mild, and other factors I haven’t perceived may have come into play. For now this must remain an open question.

Lessons from Travels: Rodent Cycles

by Carl Strang

One of the phenomena of wildlife in the far North is the dramatic cycling of small rodent populations. I had the opportunity to witness this when I was doing my graduate research in western Alaska. I was studying glaucous gulls rather than small mammals, but there was some relevance because the gulls feed heavily on tundra voles early in the season, when the thaw floods the voles into exposed positions.

Tundra vole, enjoying a snack provided by a colleague at the tent frame.

Lemmings were present in small numbers as well, but the only rodent that we saw undergoing violent population fluctuations was the vole. At the low point in the cycle one was hard pressed to find an active runway, and sightings of the voles themselves were few and far between. At the high point the pingos (ice-elevated rounded hills) were riddled with runs.

The voles dug into the soil, chewed a dense maze of runways, and seemed to be everywhere.

I was there in four consecutive summers, and saw one high-density year.

In the peak year they invaded my home.

Now I want to refer back to my recent literature review on food web stability. Species diversity is relatively low in the North, and in general there is a gradient of diminishing diversity from tropics to tundra. Low species diversity is associated with lower stability, and stability clearly is lacking in the vole population. It is not, however, simply a matter of few predators available to exert top-down control. In addition to glaucous gulls there were mew gulls, parasitic and long-tailed jaegers, less common predators like short-eared owls, and foxes. The last were represented by two species.

Red foxes were larger. This one got muddy.

Arctic foxes were the smaller species. This one, which appears to have a tundra vole in its mouth, hasn’t yet molted to its summer pelage.

Furthermore, all of these predators have broad diets and so can switch to focus on the most abundant prey (birds and their eggs being the chief alternative for most of them). Switching, however, isn’t stabilizing the voles. I haven’t followed the literature on this, so I don’t know where the current consensus is, but I think it’s important to point out that for much of the year the voles are protected by a deep layer of snow, and the avian predators all are gone outside the relatively short breeding season. The long winters are depauperate of species indeed, voles can breed in every month, and that surely plays a role in this food web.

While modest cycling of small rodents occurs in Illinois, it doesn’t come close to matching what we saw in Alaska. We have many more kinds of plants, rodents and predators here, and the rodents are vulnerable year round. This seems to be enough to account for the difference in food web stability of the two places.

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.

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