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Hunting and Tree Seeds – what is the connection?

by:Annika Keeley.

Trees have different ways of ensuring that their seeds move some distance away from the mother tree and can germinate. In tropical forests, some rely on the wind; some depend on water; some let their seed husks snap to throw seeds some distance; but most count on animals. Birds, bats, and other mammals such as squirrels and raccoons are well known as dispersers of tree seeds. Agoutis (Dasyprocta spp.) store large seeds by burying them. They inevitably forget some of them which can then germinate and grow. Thus agoutis are important seed dispersers, but they are often hunted for their meat. Manuel Guariguata and some of his colleagues wanted to know if hunting noticeably reduces the number of seeds that are dispersed from certain tree species . To find out they selected two study sites. In La Selva, hunting is prohibited and the entire forest is permanently patrolled by armed guards. In Tirimbina, hunting is also prohibited, but because it is not patrolled, poaching is a problem. Consequently, there are more agoutis in the La Selva forest compared to the Tirimbina forest.

In both study areas, the researchers placed two groups of 10 seeds each of a certain tree species in 15 locations in the forest understory. One group was covered with a cage from chicken wire that kept everything but small rodents such as mice out; the other group was not covered. In regular intervals, Manuel monitored how many seeds were left. He then repeated this experiment but tied a piece of fishing line painted with a fluorescent color to each seed in order to know whether the seeds are eaten or actually dispersed with the chance to grow into seedlings. When seeds were gone from the little pile of seeds placed on the forest floor he looked in the surrounding area for the fishing line. He repeated these experiments with seven different kinds of large-seeded trees.

Some results: The seeds of one of the trees, the gavilan tree (Pentaclethra macrolobum), are toxic to most mammals and therefore avoided. Accordingly, in both forests most gavilan seeds stayed where the researchers put them, whether they were caged or not. More seedlings survived though at Tirimbina than at La Selva, probably because there are fewer peccaries at Tirimbina which dig through the soil for food and uproot seedlings in the process. The seeds of the Welfia palm (Welfia regia) are very tasty to many animals and disappeared rapidly from the uncaged seed piles. None were found with the fishing line still attached. The seeds of two trees, the andiroba (Carapa nicaraguensis) and the monkey pod (Lecythis ampla), are mostly dispersed by rodents, such as agoutis. As expected, many more seeds of these trees were removed from the seed piles at La Selva than at Tirimbina. The fishing line experiment showed that at La Selva only some of the seeds that were removed were not eaten right away, but dispersed. Even the seeds of the monkey pod that were placed under the cages disappeared more readily from the La Selva forest. Small rodents that really like those seeds and can get through the wire must be more common in La Selva than in Tirimbina.

hunting and tree seeds 1hunting and tree seeds 2
hunting and tree seeds 3

Seeds of two species, manú (Minquartia guianensis) and tropical nutmeg (Virola koschnyii), are smaller than those of the other tree species studied. They were removed faster from the cages in Tirimbina. A different study showed that small rodents, especially the spiny pocket mouse (Heteromys desmarestianus), were much more abundant in Tirimbina than in La Selva. These little rodents that can enter the seed cages likely removed all those seeds from the Tirimbina experiments.

In general, the researchers concluded dispersal activity goes up with the number of mammal species, but at the same time, more seeds and seedlings are eaten and destroyed. For example, trees that rely on agoutis for seed dispersal will not be dispersed readily in forests without agoutis. Thus in hunted forests, tree species whose seeds are dispersed by hunted mammals will become rare whereas tree species that do not rely on these mammals for dispersal (such as the gavilan tree) may get more and more common. The continued seed dispersal of many trees depends on the conservation of an intact animal community.

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