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.”

Mast Year

by Carl Strang

Mast is a collective term referring to nuts and acorns. Trees do not produce these in the same amounts each year. In some years very few nuts or acorns develop in a given species, and in other years prodigious numbers appear. High production seasons are called mast years. 2009 is proving to be a mast year for bur oaks and white oaks at Mayslake Forest Preserve, where the trails in places are littered with the fallen acorns. Here is an example for bur oak.

Bur oak mast b

Here, white oak acorns abound.

White oak mast b

Though elsewhere I am seeing lots of walnuts, this does not seem true for that species at Mayslake, which also is having an unremarkable year for hickory nuts. Nearby, at Fullersburg Woods Forest Preserve, I noted in 2007 that walnuts, hickories and red oaks had a mast year. It is common for members of the white oak group and red oak group of species to be decoupled from one another in their mast years.

Fox squirrel 1b

As you might imagine, animals such as tree squirrels are impacted by mast years. Mayslake’s gray and fox squirrels will have an easy winter with so much food available. They help their cause by biting acorns before burying them in an effort to kill them. The acorns, in a countermeasure, are quick to sprout when they fall to the ground. A study published in 2006 in Science (314:1928) found that red squirrels (which live north and south of us, but not in DuPage County) themselves reproduce more heavily in mast years (perhaps responding to an increase in flowering or other advance cue). Such adaptive interactions between species are referred to as coevolution. The phenomenon of the mast year itself likely is, at least in part, an evolutionary tactic by the trees. By coordinating their mast production they can limit their seed-predators’ survival in some years, overwhelm them in others. Such an episodic mass reproduction is reminiscent of the periodical cicadas.

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