Forget all the arguments about the animals of the Book of Mormon. Take the book's word at face value. And what you have left is a sad commentary on the ecology of the peoples of the Book of Mormon.
Due to the flood of Noah and the division of the land in the days of Peleg, before the dispersal of the people throughout all the earth, the Americas where left with only the migrating wild animals. These wild animals would have by nature found their way back to the lands of America, as they had migrated from the landing of site of Noah's ark. But domestic animals, ones whose livelihood is reliant upon the hand of man for food and protection, would not have survived such a journey even when the land was all one land.
Since the Jaredites where 'scattered' back to the Americas by the hand of the Lord in relation to the time of the Tower of Babel, they had to bring their own domestic animals back with them. Though not discussed in most of the common literature, the fact that the Jaredites had to use submersible barges is an unmistakable reflection of the nature of the oceans of that era. There was no calm 'pacific' ocean surface that would allow even a ark's safe passage, little alone a sailing ship. The tectonic plate movements of a earth divided, where still well in commotion causing the light cork like barges to be taken deep into the depths of the ocean by the mountainous waves of the sea on various occasions. This plate tectonic land motion relates well to the catastrophic continental collision disasters in the Americas recorded in the Book of Mormon at the death of Christ despite the fact that the oceans had calmed by 600 BC enough to allow a sailing vessel. But that's another story for another writing.
In their eight barges, the Jaredites brought with them their domestic animals, the horse, the cow, the ox, the sheep, and various others. The variety of domestic animals which the Jaredites had were likely greater in number than those of the Lehi and Mulek groups. Lehi brought no animals to speak of with their party, just seeds of every kind. The domestic animals where already here. The Jaredites had brought them. But even then, poor ecological management of a nation bent on self annihilation and sweeping the earth clean in a 'scorched earth' maneuver by Shiz, more than likely brought an end to some of the species of domestic and wild animals to be had in the Americas. Only those which found their way south passed the narrow neck of land, wandering there for the want of food, where to be had by the colonies of Lehi and Mulek.
Now the remaining descendants of Lehi upon the America's where no more ecological minded than the suicidal Jaredites. Many species again have disappeared and this despite the later 'Indian' appreciation for the land. The horse, the cow, the sheep, and etceteras, where all pushed into extinction. And this was done by the nature of the Lamanite. True, the Book of Mormon portrays some Lamanites as maintaining herds of domestic animals such as those by King Lamoni. But the more basic mode of operation of the Lamanite was to hunt, scavenge, plunder and steel their livelihood.
This Lamanite method seemed to be to hunt for their food, and what that source could not provide, they would extract and scavenge from the Nephites or in the later years of no Nephites, from nearby neighboring tribes. This was reflected in how the Indian tribes where operating amongst themselves when they where 'discovered' in the Americas. Only a very rudimentary level of farming remained with almost all of the various produce plant varieties, which Lehi had brought, no longer remaining.
Thus the lack of the plant, seed, and animal varieties that are mentioned in the Book of Mormon which no longer graced the land when the Indians where found by the Europeans, was due to 'extinction.' And this extinction was caused by poor ecological management upon the part of a people who were supposed to be the ideal back to nature image of man living in harmony with his environment.
Science says, 'Where is the evidence?' Well for the most part it is just all gone the way of the earth. Only under just right conditions are past evidences preserved to begin with. Take for example the story of the Huns' horses reported in a FARMS Journal article.
"The Huns of Central Asia and Eastern Europe were a nomadic people for whom horses represented both a major form of wealth and the basis of their military power. Estimates are that each Hun warrior may have had as many as ten horses. Nonetheless, "To quote S. Bokonyi, a foremost authority on the subject, 'We know very little of the Huns' horses. It is interesting that not a single usable horse bone has been found in the territory of the whole empire of the Huns.' " During the two centuries of their domination of the western steppe, the Huns must have had hundreds of thousands of horses. If Hunnic horse bones are so rare despite their vast herds, why should we expect extensive evidence of the use of horses in Nephite Mesoamerica, especially considering the limited references to horses in the Book of Mormon text?" (Basic Methodological Problems with the Anti-Mormon Approach To the Geography and Archaeology of the Book of Mormon William J. Hamblin, FARMS Journal of Book of Mormon Studies, vol. 2, no. 1 (Spring 1993), 194.)
Like the phrase 'slim to none,' the evidences of the Book of Mormon past plants and animals are just gone. Many perhaps totally eradicated by one people bent on completely destroy any evidence of another people having ever existed. Such was the strong hatred, resentment, and jealousy of the Lamanite toward the Nephite. There are a few scarce evidences of such things as horses and elephants but hardly enough for a welling acceptance of a world bent on disbelief.