Big seeds produced by many tropical trees were probably once ingested and then defecated whole by huge mammals called gomphotheres that dispersed the seeds over large distances.
But gomphotheres were mostly hunted to extinction more than 10,000 years ago. So why aren’t large-seeded plants also extinct? A new Smithsonian report to be published in the early online edition of Proceedings of the National Academy of Sciences on July 16 suggests that rodents may have taken over the seed-dispersal role of gomphotheres.
By attaching tiny radio transmitters to more than 400 seeds, Patrick Jansen, scientist at the Smithsonian Tropical Research Institute and Wageningen University, and his colleagues found that 85 percent of the seeds were buried in caches by agoutis, common, house cat-sized rodents of tropical lowlands. Agoutis carry seeds around in their mouths and bury them for times when food is scarce.
Radio tracking revealed a surprising finding: when the rodents dig up the seeds, they usually do not eat them, but instead move them to a new site and bury them, often many times. One seed in the study was moved 36 times, traveling a total distance of 749 meters and ending up 280 meters from its starting point. It was ultimately retrieved and eaten by an agouti 209 days after initial dispersal.
Researchers used remote cameras to catch the animals digging up cached seeds. They discovered that frequent seed movement was primarily caused by animals stealing seeds from one another. Ultimately, 35 percent of the seeds ended up more than 100 meters from their origin.
This video shows the relative movement paths of 224 radio-tagged palm seeds handled by rodents on Barro Colorado Island, Panama. Colored dots mark locations of 129 seeds that were found eaten (orange), 86 seeds last seen before they lost their tags (gray), and 9 seeds that were still alive, cached, and being monitored after a year (pink). Smithsonian Tropical Research Institute, Panama.
“Previously, researchers had observed seeds being moved and buried up to five times, but in this system it seems that this re-caching behavior was on steroids,” said Ben Hirsch, who ran the fieldwork as a post-doctoral fellow at the Smithsonian Tropical Research Institute in Panama. “By radio-tagging the seeds, we were able to track them as they were moved by agoutis, to find out if they were taken up into trees by squirrels, and to discover seeds inside spiny rat burrows. This resolution allowed us to gain a much better understanding of how each rodent species affects seed dispersal and survival.”
By taking over the role of large Pleistocene mammals in dispersing these large seeds, thieving, scatter-hoarding agoutis may have saved these tree species from extinction.
This research filled an unknown gap, compared to the gaps identified in a review (Biological Conservation) of research published since 2000, conducted by Kim McConkey and colleagues, including researchers from India, Singapore, Malaysia, and Spain. They try to answer questions about what we do know and what we don’t know about seed dispersal.
Although biodiversity conservation is a huge field, at its heart we find something very small: the seed. From seeds come the plants we need and food for the animals we hope to conserve as well. Knowledge of seed dispersal, or how seeds are generated and move through the landscape, is essential if we are to understand the influence of human activity on biodiversity.
McConkey and colleagues sought to determine what advice should be given to conservationists. At present, advice that might help isn’t often shared in a way that affects outcomes, so they performed this review to widen channels of communication.
Their review looked at the four causes of changing biodiversity and what is known about how seeds interact with each of them. They are (a) fragmenting plant and animal habitat, (b) overharvesting of plants and animals, © invasive species crowding out native plants and animals, and (d) changing climate. They concluded that seed dispersal research needs to be done at larger scales and in more varied landscapes that reflect real terrain, since research often encompasses only a small, homogenous space. Research should also account for all four key drivers of biodiversity change. It is important, the review stated, for scientists to know how those drivers affect seed dispersal and what happens when more than one is present. As scientists develop more comprehensive methods, current research offers insight for both future research questions and conservation decisions.
A summary of that insight:
- Habitat fragmentation: When humans develop landscapes, they break up habitat that is home to native plants and animals. To assist flora and fauna populations to continue to exist there, it is necessary to enable them to move between the newly isolated remnants of habitat. To help species persist, they recommend linking habitat fragments with strips that will support the species. At the very least, habitat “stepping-stones” should be established.
- Overharvesting: Though animals harvest seeds, they also help spread them. Thus, overhunting animals can be just as detrimental to seed dispersal as harvesting the seeds directly. Plant and animal populations should both be monitored to maintain seed biodiversity. For those species essential for seed dispersal, the authors suggest managing for “a population size that maintains their ecological function, rather than merely the minimum viable population.” In lay terms, don’t keep a population alive just so humans know what it looks like, enable its survival at a level that makes it an important part of an ecosystem.
- Invasive species: Invasive plants compete with native flora for habitat and also to make use of local animals as seed dispersers. Conversely, invasive animals compete with native fauna for habitat and, as seed dispersers, may or may not spread the seeds of native plants. It is difficult to generalize about how to control invasive species but the review suggests that a good first step is a comprehensive assessment of the traits of plants and animals as they relate to seed dispersal.
- Climate change: A changing climate means the habitat where a plant now exists may not remain suitable in the near future. Animals are following changing temperature boundaries, but plants, unless they hitch a ride with animals, as some do, are not keeping up. Keeping increasingly isolated habitats connected, as noted, will be important if seeds are to be enabled to relocate as the climate around them changes. For some plant species, conservationists should be prepared to assist their migration.
The above news item is reprinted from materials available at Smithsonian Science and Mongabay. Original text may be edited for content and length.