Literature Review: Fungal Evolution

by Carl Strang

Fungi work behind the scenes for the most part, but they have played, and continue to play, essential roles in Earth’s ecosystems. That important work began a long time ago.

We most often notice fungi at the brief times when they grow spore-dispersing structures.

We most often notice fungi at the brief times when they grow spore-dispersing structures.

Floudas, Dimitrios, et al. 2012. The Paleozoic origin of enzymatic lignin decomposition reconstructed from 31 fungal genomes. Science 336:1715-1719.

Comparative genomic and molecular clock analyses in fungi “suggest that the origin of lignin degradation might have coincided with the sharp decrease in the rate of organic carbon burial around the end of the Carboniferous Period.” In other words, the immense volumes of Paleozoic coal accumulated because fungi had not yet evolved the ability to decompose wood down into its nutritious chemical components. After they did so, much less wood survived to become coal.

Wolfe BE, Tulloss RE, Pringle A (2012) The Irreversible Loss of a Decomposition Pathway Marks the Single Origin of an Ectomycorrhizal Symbiosis. PLoS ONE 7(7): e39597. doi:10.1371/journal.pone.0039597

They looked at the genetics of nutrition in Amanita fungi, and found that their mutualistic partnerships with vascular plants are obligate. The fungi have lost two genes that once allowed them to decompose organic matter in the soil, so that they now depend upon their mutualistic partners for carbon. This study refers to another very important ecological role many fungi play. They form partnerships with many green plants, channeling in soil minerals in exchange for other goodies. As the authors point out, this trade no longer is an option: the partners can’t survive without it.

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.

More Mayslake Fruits

by Carl Strang

Earlier I featured several plants at Mayslake Forest Preserve that produce fruits timed to coincide with the fall migration of berry-eating birds. This mutualistic interaction for the most part benefits the birds, through nutritional provisioning, while the plants get their seeds dispersed. Today I want to feature some outliers to this pattern. Let’s start with Solomon’s plume, also known as false Solomon’s seal.

Solomon's plume fruit b

Like many fall fruits, these advertise themselves to birds with a bright red color. When analyzed, however, the berries proved to be junk food, or perhaps are more accurately described as food mimics (White and Stiles 1985, Ecology 66:303-307). The plants save their energy, investing no nutritional value in these fruits. The ruse works, apparently, by exploiting the naïve instinctive response of first-time autumn migrants, the young of the year. A little different from this is the offering of the European highbush cranberry.

European highbush cranberry fruit b

Another study (Witmer 2001, Ecology 82:3120-3130) showed that the nutritional value of these berries becomes available only when they are consumed along with a significant protein source. I was impressed to learn that, like the waxwings native to the shrub’s European home, our North American cedar waxwings ignore these tempting berries until spring, when cottonwoods or other poplars are flowering. Then the birds consume the berries along with cottonwood catkins, protein in the pollen providing access to the berries’ nutritional value.

Common buckthorn fruit b

These black berries are common buckthorn fruits. They generally are ignored by birds until late winter when, apparently, the better quality foods have been depleted. Then, robins and waxwings consume them, unfortunately dispersing the seeds throughout our woodlands. Buckthorns leaf out early and lose their leaves late, casting a shade so dense that no other plants can grow beneath them. This is why these Eurasian shrubs must be removed at the beginning of woodland restoration projects. A final fruit is of no interest to birds.

Buckeye fruit 2b

Ohio buckeyes in fact are largely ignored by animals generally. This opens the possibility that, like other trees I discussed earlier, buckeyes may have been dispersed by now-extinct mastodons and other large herbivores.

%d bloggers like this: