Rota, Guam show similar reduced seed dispersal

The CNMI’s forests and intact bird population can provide direct point of comparison for “how it should be” in Guam, where crucial seed dispersers have been devastated by the brown tree snake, according to Nadya Muchoney, who works with the Ecology of Bird Loss Project, and spoke at this year’s Asia Pacific Academy of Science, Education and Environmental Management conference.

Up to 80 percent of forest tree species in the Marianas have seeds that are supposed to be dispersed by birds, according to Muchoney. She said fewer than 10 percent of these trees are pollinated by birds.

Because the forest ecosystems in the Marianas rely on these frugivorous (“fruit-eating”) birds as seed dispersers, their loss is important.

EBL recently analyzed seedbanks in forest gaps on Saipan, Guam, and Rota, expecting that the CNMI would compare differently but found that a “similar pattern” of reduced richness of seed dispersal was emerging on Rota, according to Muchoney.

Guam had “uneven” and “very high percentage of areas without seed dispersal,” according to Muchoney, but this was expected. Rota’s results were “interesting,” according to Muchoney, because of its still intact avian population.

Future research could look to see if a recent decline in Rota’s avian population was related to this, she said.

In particular, EBL tested three hypotheses with its seedbank analysis. Guam would have “patchy” seedbanks or more empty samples in its forest gaps compared to Saipan and Rota; Guam would have majority of its seed within 2 meters of its parent tree; and Guam would have lower species richness in its seedbanks than on Saipan and Rota.

Rota had a high level of empty samples just like Guam did, according to Muchoney, while Saipan’s results were as expected.

In Guam, over 90 percent of seed species were found near a parent tree, while Rota and Saipan had a much lower percent, she said. Both Rota and Guam showed lower species richness than Saipan, according to Muchoney.

An audience member at the conference suggested Rota’s forests might be in an “apex situation” to explain Rota poorer results. Muchoney said this is “something to consider.”

EBL is also continuing to look at forest gap regeneration. Muchoney said they are looking if forest gaps in Guam would close more slowly than on Saipan and Rota, and if diversity in these gaps will be less or more than Saipan and Rota.

She said if gaps closed more slowly, this could because of a “feedback loop” where slower regeneration leaves gaps empty longer. This exposure might cause mychorrhizal fungi to die off so that when seeds do enter, there is less fungi to help them grow, she said.