Literature Review: Some Finer Points of Species Interactions

by Carl Strang

This week’s literature focus is on three papers that looked at complex interactions among species.

Ben-Ari M, Inbar M (2013) When Herbivores Eat Predators: Predatory Insects Effectively Avoid Incidental Ingestion by Mammalian Herbivores. PLoS ONE 8(2): e56748. doi:10.1371/journal.pone.0056748 They found that lady beetle adults and larvae of three species responded to the humidity and warmth of mammalian breath by dropping to the ground beneath their aphid plants. The adults dropped instead of flying.

A colony of aphids occupies a goldenrod top. If a deer were to munch that top the aphids would add some protein, but ladybugs preying on the colony might get away.

A colony of aphids occupies a goldenrod top. If a deer were to munch that top the aphids would add some protein, but ladybugs preying on the colony might get away.

Mouillot D, Bellwood DR, Baraloto C, Chave J, Galzin R, et al. (2013) Rare Species Support Vulnerable Functions in High-Diversity Ecosystems. PLoS Biol 11(5): e1001569. doi:10.1371/journal.pbio.1001569 They looked at tropical fish, alpine plants and tropical trees, considering rarity vs. ecosystem function. Rare species often fill ecological roles that are not covered by common species, and so their loss could cause significant ecological damage.

Dunne JA, Lafferty KD, Dobson AP, Hechinger RF, Kuris AM, et al. (2013) Parasites Affect Food Web Structure Primarily through Increased Diversity and Complexity. PLoS Biol 11(6): e1001579. doi:10.1371/journal.pbio.1001579 They compared food web structure with and without parasites, and found that for the most part the parasites’ influence was an increase in community diversity and complexity rather than adding new features to web structure. The exceptions were the incidental consumption of parasites with their hosts by predators, and the odd connections that result when some parasites have more than one host during their life cycle.

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Literature Review: Permian and Triassic

by Carl Strang

The Permian Period that ended the Paleozoic Era, and the Triassic Period that began the Mesozoic Era, continue to attract researchers’ attention. This was a time of dramatic geological activity as the continents ground together to form the supercontinent of Pangaea, a time of two mass extinctions, and a time when the first dinosaurs evolved. Today I share notes from some of last year’s literature on this time period.

Part of a mural depicting a prosauropod (sauropodomorph), one of the early dinosaurs. Field Museum of Natural History exhibit.

Stephen E. Grasby, Hamed Sanei, Benoit Beauchamp. Catastrophic dispersion of coal fly ash into oceans during the latest Permian extinction. Nature Geoscience, 2011; DOI: 10.1038/ngeo1069     Deposits of coal ash in Canada support the view that the Siberian traps volcanic eruption involved coal beds, burning huge volumes of coal and releasing significant greenhouse gases while the toxic ash itself may have been a significant ocean contaminant. The consensus now is that this massive eruption, probably connected to the collision of continents, produced atmospheric changes that led to the greatest mass extinction in the history of multicellular life.

K.M. Meyer, M. Yu, A.B. Jost, B.M. Kelley, J.L. Payne. δ13C evidence that high primary productivity delayed recovery from end-Permian mass extinction. Earth and Planetary Science Letters, 2011; 302 (3-4): 378 DOI: 10.1016/j.epsl.2010.12.033     As described in ScienceDaily article. They found evidence that high carbon dioxide concentrations from the end-Permian volcanic eruptions caused an extended period of erosion that enriched the oceans, resulting in blooms of algae and bacteria that depleted ocean oxygen levels for as long as 5 million years and delayed the recovery of marine diversity.

J. H. Whiteside, P. D. Ward. Ammonoid diversity and disparity track episodes of chaotic carbon cycling during the early Mesozoic. Geology, 2011; 39 (2): 99 DOI: 10.1130/G31401.1     They looked at the relationship between ammonoid (squid-like shelled animals) diversity and ecosystem stability as measured by carbon isotopes, in relation to the end-Permian and end-Triassic mass extinctions. They found that swimming ammonoids (regarded as top predators, along with some fish) became extinct at these times, while some more passively floating species survived. Carbon isotope ratios fluctuated chaotically in association with the mass extinctions, implying extreme food web instability, and did not regain their stability until the evolution of new groups of swimming ammonoids some 10 million years later. They mention the importance of redundancy in top predator niches, with significant overlap in ecological space occupied by diverse species groups. The authors regard these instances as cautionary tales for the current threats to top marine predators (cod, sharks, tuna, etc.).

Randall B. Irmis, Jessica H. Whiteside. 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     As described in a ScienceDaily article. They found that on land, in parallel with their earlier marine study, there was an extended period of ecological instability apparently resulting from low species diversity and food web connectivity. Only a few species survived the end-Permian extinction, and these had practically no competition. The resulting boom and bust population cycles prevented communities from developing stabilizing checks and balances, extending the depauperate period 8 million years into the Triassic. Dominant vertebrates included the dicynodont Lystrosaurus and procolophonids, both of which had been minor players in the Permian. Pleuromeia was a lycopod or club moss, tree or bush sized, that likewise became dominant. They did a count of individual fossils to establish this pattern, finding that 78% of terrestrial vertebrate genera went extinct. The carbon cycle did not become stable until the community structure stabilized.

Martinez, Ricardo N., et al. 2011. A basal dinosaur from the dawn of the dinosaur era in southwestern Pangaea. Science 331:206-210. They describe a new fossil, Eodromaeus murphi, from NW Argentina, and classify it as a basal theropod (predatory dinosaur).  Its contemporary, Eoraptor, they move out of the theropods, reclassifying it as a basal sauropodomorph (early predecessor of the giant, 4-legged, long-necked herbivores that were to be so diverse and abundant in the Jurassic Period). They conclude that both of those groups and the ornithiscians (the remaining dinosaur group) were established by the end of the Triassic. Their analysis of contemporary fossils also supports the idea that dinosaurs rose through a process of opportunistic replacement rather than competitive displacement, filling gaps as other groups went extinct rather than pushing them aside.

Ruhl, Micha, et al. 2011. Atmospheric carbon injection linked to end-Triassic mass extinction. Science 333:430-434. They found changes in the isotopic ratios of carbon at that time that support the idea that massive volcanic eruptions released huge amount of methane, which among other things would have a climate change effect.

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 Elaborations

by Carl Strang

Last week I posted an overview of recent research that casts light on food web and ecosystem function. Things are much more complicated than that relatively simple summary when one digs into the details. This week I want to provide a couple disparate examples. Toward the end of the overview I cited some results suggesting that ecosystems with fewer species tend to be less stable, and that such ecosystems often are marginal, for instance because of low productivity. Evolution is a creative force, however, that produces adaptations allowing organisms to persist in such marginal ecosystems. This increases diversity, improving the chance that the associated food webs will persist.

One example pertains to birds (Jetz, Walter, Dustin R. Rubenstein. Environmental Uncertainty and the Global Biogeography of Cooperative Breeding in Birds. Current Biology, 2010; DOI: 10.1016/j.cub.2010.11.075 ). They reviewed the world’s bird species and found that cooperative breeding patterns such as helpers at the nest and other communal reproductive behaviors are more common in places with inconsistent climate patterns, particularly in rainfall.

One illustrative example is the grey-crowned babbler.

The grey-crowned babbler is on the short list of my favorite Australian birds. They often forage in groups, they have amazing silly sounding vocalizations, and they nest communally.

This babbler nest is huge, perhaps 3 feet long.

The cooperative breeding trait allows this species to persist in a difficult desert environment. It’s not the only strategy, as there are plenty of birds in that community which do not nest communally, but again, evolution is a creative force that can find many solutions to survival problems.

Another evolutionary force, leading to stability in a class of mutualistic relationships, was highlighted in a study last year (Kiers, E. Toby, et al. 2011. Reciprocal rewards stabilize cooperation in the mycorrhizal symbiosis. Science 333:880-882). This research looked at mycorrhizae, partnerships in which fungi channel nutrients from the soil into plant roots, and roots provide a medium (sloughing off bark, for instance) in which the fungi can grow.

Many mushrooms are the spore-producing structures of mycorrhizal fungi.

In laboratory experiments Kiers and company found “that plants can detect, discriminate, and reward the best fungal partners with more carbohydrates. In turn, their fungal partners enforce cooperation by increasing nutrient transfer only to those roots providing more carbohydrates…we conclude that, unlike many other mutualisms, the symbiont cannot be ‘enslaved.’ Rather, the mutualism is evolutionarily stable because control is bidirectional, and partners offering the best rate of exchange are rewarded.”

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.

Literature Review: Food Web Stability

by Carl Strang

Though my annual scientific literature review focuses on the current year’s publications, sometimes I have to backtrack because I learn of a significant paper I missed in a previous year. My most recent time in the Northern Illinois University library included the search for such a reference. I learned of it through a review or news article in Science, which I count on to keep me informed about significant papers in the journal Nature. I don’t have the time to follow both.

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

Random models of communities predict that complexity will lead to instability. If such models were correct, there would be fewer species in wild communities than we observe. This study looked at soil communities in which increasing primary productivity correlated with increasing biodiversity. Critical to stability were interactions involving omnivores and diet switching. If a significant predator became too abundant, threatening food web stability, its numbers were reduced when its own predators switched their diet to concentrate on it. An example involved bacteria, a bacteria-feeding nematode, and another nematode that could feed on either of the others.

Diet switching is a common behavior in animals. Gulls, like the glaucous gulls in the photo, have a very broad diet. In my graduate study of these birds in Alaska, I found them flexibly switching among such diverse foods as fish, marine invertebrates, small rodents, bird eggs, young birds, carrion and berries as these different foods became available in different seasons and different places. Gulls have predators of their own, as I observed on Adak Island.

The young eagle caught the glaucous-winged gull in flight, but shortly after I took the photo the youngster was rewarded for its effort by the adult eagle driving it away from its catch. Neutel et al. point to diet switching as a mechanism for maintaining biodiversity. I also have seen an example of what happens when systems lack such switching. In earlier posts I have described my study of the trailing strawberry bush and ermine moth at Meacham Grove Forest Preserve. 

The ermine moth caterpillars have only one food, the trailing strawberry bush, in this forest, and apparently their own specialist parasites lag behind them. There is no capacity for switching, and the result has been boom-and-bust population dynamics.

Sketching Mayslake’s Trophic Structure

by Carl Strang

 

 

As I familiarize myself with an area, there are several big conceptual nets with which I try to comprehend that place. These lack fine detail, but provide first approximations that can be filled in as I learn more. One conceptual frame for an area is its general topography and drainage pattern. Another is the general human history and influence on the land. A third is the mosaic of vegetation communities. A fourth is the geology. Today I want to focus on yet another frame, the trophic structure. This is a broad-brush first stage in constructing a food web. The trophic levels of an ecosystem are the steps through which energy “flows” (the quotes are a field ecologist’s recognition that abstract food pyramid diagrams, and the arrows that connect food web elements on the page, cover up a certain amount of desperate flight-and-chase, crunching, and screaming. All our lives continue at a cost, but I digress).

 

In earlier posts I have documented some of Mayslake’s animal life. The top predators mentioned already include coyotes and mink. A day seldom goes by without me seeing a red-tailed hawk or two. I also have seen tracks of great horned owls, and the Mayslake staff have observed these birds at that preserve for years.

 

Great horned owl tracks, Mayslake

Great horned owl tracks, Mayslake

 

 

Note that in this photo, the great horned owl tracks are farther apart than in the photo I took on the Christmas bird count. This is the more typical spacing, but again there is that odd asymmetry in the toes.

 

 

Mayslake’s part-time predators include raccoons and skunks, among others.

 

Skunk tracks (far right), Mayslake

Skunk tracks (far right), Mayslake

 

On a smaller scale, I have seen tracks of short-tailed and masked shrews, which are predators of small animals including invertebrates but also mice.

 

Common plant-eating animals at Mayslake include deer, mice, voles, squirrels, chipmunks and cottontail rabbits. There are also the wandering winter birds eating seeds (as in the paper birch account).

 

Rabbit trail, Mayslake

Rabbit trail, Mayslake

 

 

Over 50 kinds of trees, shrubs and vines, along with uncounted herbaceous plants, are the preserve’s primary producers, solar energy harvesters that form the foundation for the whole trophic shebang. In the warm months, each kind of plant hosts several insect consumers that in turn are food for predators including an influx of migrant nesting birds.

 

And let’s not forget the scavengers. Some of the animals listed already, the coyote and skunk for instance, along with opossums such as the young one whose tracks I saw in the southern part of the preserve recently, are happy to clean up the odd dead carcass they encounter.

 

Mayslake is not a huge property as forest preserves go, but it’s big enough and diverse enough in vegetative structure to support a complex community for which I have here provided only the broadest of introductory sketches. More detail to come.

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