AsianScientist (Aug. 14, 2017) – Cockroaches can aid in the dispersal of seeds on the forest floor, according to a study published in the journal Botanical Journal of the Linnean Society. These findings suggest that cockroaches could be important for seed dispersal in the plant kingdom.
Monotropastrum humile is forest floor-dwelling herb belonging to the azalea family. M. humile is mycoheterotrophic, which means that it completely relies on the Russulaceae family of fungi for nutrients.
Its fleshy fruit contains hundreds of seeds that are very small, approximately 0.3 mm long and 0.2 mm wide, and have a hard seed coat. As the fruit ripens, they drop to the ground or the whole plant stalk falls off, making the pulp available at ground level. The ripe fruit has no distinctive scent, is dull white in color and is inconspicuous against the litter layer of the shady forest floor. The slightly juicy pulp is not sweet to the human palate.
In this study, a team led by Associate Professor Naoto Sugiura at the Kumamoto University, Japan, noticed that the fruit of M. humile attracted various invertebrates, but birds and mammals seemed uninterested. Of the invertebrates, only the adult forest cockroach Blattella nipponica consistently visited and consumed the seed-embedded pulp at night.
Adult B. nipponica excreted feces about three to ten hours after their collection from the field, and each fecal pellet contained an average of 3.1 M. humile seeds.
Importantly, seeds in the feces remained intact and viable. A chemical test revealed that about half of the cockroach-ingested seeds remained viable and that their viability (52.0 percent) was not significantly different from that of seeds directly removed from the pulp (49.3 percent). This indicates that B. nipponica cockroaches can function as a legitimate seed disperser for M. humile.
The researchers report that M. humile plants have traits that may reflect cockroach-mediated seed dispersal. First, their fruit ripening period largely coincides with the time the adult forest cockroach emerges from its pupal case, in May and June. Second, its fruits are found at ground level which is inhabited by the forest cockroach. Third, the small seeds with a hard seed coat enables ingested seeds to remain viable.
On the other hand, B. nipponica adults have intrinsic traits that may facilitate their actions as dispersal agents for M. humile. They are abundant in the plants habitat and are ground-dwelling. Moreover, the time that M. humile seeds spend in the gut of the forest cockroach allows occasional long-distance seed dispersal. Consequently, the findings strongly suggest that the fleshy fruit-bearing M. humile is specialized for seed dispersal by adult cockroaches, enabling the plant to steadily disperse minute seeds across the forest floor.
“Given that about 4,600 cockroach species have been identified to date, cockroach-mediated seed dispersal may be a more pervasive but an as yet undocumented mechanism, rather than a unique exploit by a particular plant species,” said Sugiura. “It is hoped that the findings reported here will encourage people to pay more attention to plant-cockroach interactions and seek out further examples of cockroach-mediated seed dispersal.”
The article can be found at: Uehara & Sugiura (2017) Cockroach-Mediated Seed Dispersal in Monotropastrum humile (Ericaceae): A New Mutualistic Mechanism.
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Source: Kumamoto University.
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