Rats, seeds and rainforest trees

Plant-animal interactions: seed predation and plant demography

Forest rats love to eat seeds. Some of them also move seeds and collect them as future food supply. How far are rodents able to carry the seeds? Do rodents always remember to eat everything they store? What happens to the seeds that are forgotten? To find out, we tracked the movement of seeds fallen on forest floor.

  • Forest habitat

  • Some fruits and seeds gathered from the forest floor

  • Rattus sp.

  • Niviventer sp. removing two seeds at a time from experimental plot

  • Brush-tailed porcupine eating seeds inside experimental plot

  • Hoary-bellied squirrels eating seeds inside experimental plot

To eat today or tomorrow?

The interactions among different living organisms fit, much like a jigsaw puzzle, in a complex but structured forest community. We explored the scope and nature of interactions that exists among rainforest trees and a lesser-known group of animals, the terrestrial forest rodents. Rodents seek out plant seeds as an immediate food resource (seeds are eaten as soon as they are discovered) or as a future food resource (seeds are stored or ‘cached’ for later consumption).

We followed seed fates of 10,777 seeds belonging to ten different evergreen tree species by exploring effects of distance from parent tree and density of conspecific seeds. Our results showed that overall seed removal was higher than on-site predation by rodents. There were differences in seed handling behaviour between rodent groups. Murid rodents were more likely to remove and cache seeds while porcupines were more likely to indulge in onsite predation. Interestingly, a true cricket Brachytrupes sp. was also documented caching seeds of three tree species. Hard seeds were more likely to be removed and cached while soft seeds were more likely to be preyed upon.Seeds were moved up to 19m from the parent tree. Approximately 12% of the cached seeds germinated, indicating that seed removal behaviour could play an important role in reducing the competition between conspecific tree species during reproduction.

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Niviventer sp. removing seeds from an experimental plot

People

Funding

  • National Geographic Society (Committee for Research & Exploration), USA

Publications

  • Journal Article
    2015
    Tracking seed fates of tropical tree species: evidence for seed caching in a tropical forest in north-east India
    PLoS ONEDOI: 10.1371/journal.pone.0134658
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    PDF, 1.3 MB

    Rodents affect the post-dispersal fate of seeds by acting either as on-site seed predators or as secondary dispersers when they scatter-hoard seeds. The tropical forests of north-east India harbour a high diversity of little-studied terrestrial murid and hystricid rodents. We examined the role played by these rodents in determining the seed fates of tropical evergreen tree species in a forest site in north-east India. We selected ten tree species (3 mammal-dispersed and 7 bird-dispersed) that varied in seed size and followed the fates of 10,777 tagged seeds. We used camera traps to determine the identity of rodent visitors, visitation rates and their seed-handling behavior. Seeds of all tree species were handled by at least one rodent taxon. Overall rates of seed removal (44.5%) were much higher than direct on-site seed predation (9.9%), but seed-handling behavior differed between the terrestrial rodent groups: two species of murid rodents removed and cached seeds, and two species of porcupines were on-site seed predators. In addition, a true cricket,Brachytrupessp., cached seeds of three species underground. We found 309 caches formed by the rodents and the cricket; most were single-seeded (79%) and seeds were moved up to 19 m. Over 40% of seeds were re-cached from primary cache locations, while about 12% germinated in the primary caches. Seed removal rates varied widely amongst tree species, from 3% inBeilschmiedia assamicato 97% inActinodaphne obovata. Seed predation was observed in nine species.Chisocheton cumingianus(57%) andPrunus ceylanica(25%) had moderate levels of seed predation while the remaining species had less than 10% seed predation. We hypothesized that seed traits that provide information on resource quantity would influence rodent choice of a seed, while traits that determine resource accessibility would influence whether seeds are removed or eaten. Removal rates significantly decreased (p< 0.001) while predation rates increased (p= 0.06) with seed size. Removal rates were significantly lower for soft seeds (p= 0.002), whereas predation rates were significantly higher on soft seeds (p= 0.01). Our results show that murid rodents play a very important role in affecting the seed fates of tropical trees in the Eastern Himalayas. We also found that the different rodent groups differed in their seed handling behavior and responses to changes in seed characteristics.

  • Report
    2013
    Hornbills, rats, seeds and rainforest trees: plant-animal interactions and plant demography
    Final Report submitted to National Geographic Society, June 2013

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