Pollination and seed dispersal
An estimated 90 percent of flowering plants
depend on pollinators such as wasps, birds, bats, and bees, toreproduce. Plants and their pollinators are increasingly
threatened around the world (
Buchmann and Nabhan 1995 ;
Kremen and Ricketts 2000 ).
Pollination is critical to most major crops and virtuallyimpossible to replace. For instance, imagine how costly fruit
would be (and how little would be available) if its naturalpollinators no longer existed and each developing flower had to
be fertilized by hand.
Many animal species are important
dispersers of plant seeds. It has been hypothesized that theloss of a seed disperser could cause a plant to become extinct.
At present, there is no example where this has occurred. Afamous example that has often been cited previously is the case
of the dodo (
Raphus cucullatus ) and the
tambalacoque (
Sideroxylon grandiflorum ). The
dodo, a large flightless bird that inhabited the island ofMauritius in the Indian Ocean, became extinct due to overhunting
in the late seventeenth century. It was once thought that thetambalacoque, a now endangered tree, depended upon the dodo to
germinate its hard-cased seeds (
Temple
1977 ). In the 1970s, only 13 trees remained and it was
thought the tree had not reproduced for 300 years. The seeds ofthe tree have a very hard coat, as an experiment they were fed
to a turkey; after passing through its gizzard the seeds wereviable and germinated. This experiment led scientists to
believe that the extinction of the dodo was coupled to thetambalacoque's inability to reproduce. However, this hypothesis
has not stood up to further scrutiny, as there were severalother species (including three now extinct species, a
large-billed parrot, a giant tortoise, and a giant lizard) thatwere also capable of cracking the seed (
Witmar and Cheke 1991 ;
Catling 2001 ). Thus many factors,
including the loss of the dodo, could have contributed to thedecline of the tambalacoque. (For further details of causes of
extinction see Historical Perspectives on Extinction and theCurrent Biodiversity Crisis). Unfortunately, declines and/or
extinctions of species are often unobserved and thus it isdifficult to tease out the cause of the end result, as multiple
factors are often operating simultaneously. Similar problemsexist today in understanding current population declines. For
example, in a given species, population declines may be causedby loss of habitat, loss in prey species or loss of predators, a
combination of these factors, or possibly some other yet
unidentified cause, such as disease.
In the pine forests of western North
America, corvids (including jays, magpies, and crows),squirrels, and bears play a role in seed dispersal. The Clark's
nutcracker (
Nucifraga columbiana ) is
particularly well adapted to dispersal of whitebark pine(
Pinus albicaulis ) seeds (
Lanner 1996 ). The nutcracker removes the
wingless seeds from the cones, which otherwise would not open ontheir own. Nutcrackers hide the seeds in clumps. When the
uneaten seeds eventually grow, they are clustered, accountingfor the typical distribution pattern of whitebark pine in the
forest.
In tropical areas, large mammals and
frugivorous birds play a key role in dispersing the seeds oftrees and maintaining tree diversity over large areas. For
example, three-wattled bellbirds (
Procnias
tricarunculata ) are important dispersers of tree seeds
of members of the Lauraceae family in Costa Rica. Becausebellbirds return again and again to one or more favorite
perches, they take the fruit and its seeds away from the parenttree, spreading Lauraceae trees throughout the forest (
Wenny and Levy 1998 ).
