Which relatively small country is home to six percent of all known animal and plant species in the world?

Colombia. The South American country ranks second of the most biodiverse countries in the world, trailing only Brazil.

With 1826 species, Colombia has more species of birds than any country on Earth. Colombia's biological richness is a product of its variety of ecosystems, including tropical forests in the Amazon and Choco, mountain habitats like the Sierra Nevada and Andes, the grasslands of the llanos and páramos, and islands like Gorgona in the Pacific and San Martin in the Caribbean.

https://news.mongabay.com/2016/05/top-10-biodiverse-countries/

In addition to the more than 1800 species of birds, the country is home to 623 species of amphibians, 467 species of mammals, 518 species of reptiles, and 3,200 species of fish. About 18 percent of these species are endemic to the country. Colombia is also home to 51,220 species of plants, of which nearly 30 percent are endemic.

Costa Rica

Costa Rica accounts for only 0.03 percent of the earth's surface (has only 51.100km2). However, it contains nearly 6 percent of the world's biodiversity. Its natural wealth, both in species and ecosystems, is partly explained by its geographical position. Situated between the North and South American continents, Costa Rica has served as a bridge of countless species of animals and plants for thousands of years. Another factor to consider is the broad expanse of both coasts: a Caribbean coast (255kms) and facing the Pacific Ocean (1103 kms). A mountain range also provides numerous microclimates and a territorial sea of 589.000km2.

http://www.costarica-embassy.org/index.php?q=node/12#:~:text=Costa%20Rica%20accounts%20for%20only,percent%20of%20the%20world's%20biodiversity.