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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.

(Source: Smith­son­ian Sci­ence, 16.07.2012; Mongabay, 18.07.2012)