Photo by Ude licensed under CC BY-SA 3.0

A new form of seed dispersal has been described. It involves birds but not in the sense we traditionally think. Everyone understands how effectively birds disperse seeds contained in small fruits such as berries, or as barbs attached to their feathers. It took finding an out-of-place patch of Japanese stiltgrass (Microstegium vimineum) for lead author Dr. Robert Warren to start looking at bird dispersal in a different light.

While working in his yard, he noticed a patch of Japanese stiltgrass growing out of a window planter some 6 feet off the ground. Japanese stiltgrass can be highly invasive but its seeds aren't adapted for vertical dispersal. However, it does employ a mixed mating system composed of outcrossing flowers at the tips of the spikes along with cleistogamous flowers whose seeds remain on the stem. Taking out a ladder, Warren discovered that the grass was growing out of a bird nest. It would appear that stiltgrass stems containing seeds were incorporated into the nest as building material and then germinated the following year. Thus began a deeper investigation into the realm of nest seeds.

Teaming up with researchers at Yale and the United States Forest Service, they set out to determine how often seeds are contained within bird nests. They collected nests from 23 different bird species and spread them over seed trays. After ruling out seeds from potential contamination sources (feces, wind, etc.), they irrigated the nests to see what would germinate. The results are quite remarkable to say the least.

Over 2,000 plants, hailing from 37 plant families successfully germinated. In total, 144 different plant species grew from these germination trials. The seeds appeared to be coming in from the various plant materials as well as the mud used to build these nests. What's more, nearly half of the seeds they found came from cleistogamous sources. Birds whose nests contained the highest amounts of seeds were the American robbin (Turdus migratorius) and the eastern bluebird (Sialia sialis). These results have led the authors to coin the term "caliochory," 'calio' being Greek for nest and 'chory' being Greek for spread.

It has long been assumed that cleistogamous reproduction kept seeds in the immediate area of the parent plant. This evidence suggests that it might actually be farther reaching than we presumed. What's more, these numbers certainly hint that this otherwise unreported method of seed dispersal may be far more common than we ever realized. Whether or not plants have evolved in response to such dispersal methods remains to be tested. Still, considering the diversity of birds, their nesting habits, and the availability of various plant materials, these findings are quite remarkable!

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Spiders, toads, and grass. These three organisms seem kind of strange placed together in a single sentence. It would seem that the presence of each would have, if anything, marginal effects on the other. In healthy forests full of native species, this is the case. However, when new players enter the game, things are bound to change. As John Muir once said, “When we try to pick out anything by itself, we find it hitched to everything else in the universe.” As we grow to understand the natural world that we live in, the reality of this statement only becomes more apparent.

The new player in this case is a grass. Microstegium vimineum, commonly referred to as Japanese stiltgrass, was introduced to the US sometime around 1919. Since then it has spread to over 16 states and is especially abundant in the southeast. It invades disturbed habitats and forms dense mats, which can completely displace native vegetation. It quickly rises to monoculture status and it is on the move. It is only a matter of time before it spreads well into the north.

It’s not just vegetation that gets displaced either, most native insects don’t feed on M. vimineum. A monoculture of this grass is almost devoid of an insect community. However, there is one group of creepy crawlies that seems to have benefited from M. vimineum invasions. Wolf spiders are voracious predators. They eat a wide variety of insects and are certainly not above cannibalism. The dense carpets formed by M. vimineum offer security for wolf spiders. They can avoid one another and thus rise to abundance wherever this invasive grass grows. What few insects live in these stands quickly get gobbled up by the spiders. This is bad for yet another member of the forest community, the American toad.

It has been noticed that, in forests where M. vimineum dominates, toads are on the decline. It was long thought that the lack of prey insects was the cause but recent research has pointed to a different culprit, the wolf spiders themselves. Aside from eating what little food can be found in the carpet of grass, they are also dining on young toads. Spider depredation on toads seems to be rather routine among the grass, so much so that toad survival decreased by 65% in these areas. To make matters worse, the effects of the invasive grass seem to be at their worst in areas that were once the best forests for toad survival.

These findings are startling but by no means unique. The researchers are now going to look to see if this is happening to other amphibian species as well. At face value, it is not apparent how an invasive grass could affect toads but it is likely that instances like this are far more common than we even realize.

Photo Credit: JAYNA L. DEVORE