A Brief Overview of Kiwi Species Diversity

The kiwi (Apteryx), also known as the apterygid birds, is a group of flightless birds found only on the North and South Islands of New Zealand. Although current taxonomy generally recognizes five species— the great spotted kiwi (Apteryx haastii), the little spotted kiwi (Apteryx owenii), the Okarito brown kiwi (Apteryx rowi), the southern brown kiwi (Apteryx australis), and the North Island brown kiwi (Apteryx mantelli)—phylogenetic analyses indicate that the diversity within kiwi lineages is considerably greater than previously assumed. Genetic studies reveal that at least eleven evolutionary lineages of kiwi are still living today, and when fossil evidence from before human arrival in New Zealand is included, the number of kiwi lineages that once existed rises to approximately sixteen or seventeen.

Five currently recognized species of kiwi（Image source：Auckland War Memorial Museum， CC BY 4.0 ；Image source：Kimberley Collins、West Coast Wildlife Centre、Glen Fergus、Daderot， CC BY-SA 4.0/CC BY-SA 2.5）

Most of the diversification events among these kiwi lineages occurred during the middle to late Pleistocene, roughly between 780,000 and 12,000 years ago. During this period, glacial activity in New Zealand intensified. Analyses indicate that about eighty percent of kiwi diversification events are associated with this interval of repeated glacial expansion. At that time, glaciers in the Southern Alps of the South Island expanded extensively. These glaciers fragmented what had previously been continuous habitats, dividing them into a series of isolated regions. Such geographic isolation provided the primary conditions that promoted population divergence and lineage formation among kiwi populations on the South Island.

A different environmental force contributed to diversification on the North Island. Over the past several hundred thousand years, the Taupo Volcano erupted repeatedly. Volcanic ash from these eruptions accumulated across large areas of the island, altering soil composition and reducing humus content. Because kiwi forage by probing deeply into soil in search of insects and other invertebrates, these changes created unfavorable conditions for feeding. As a result, previously continuous habitats were fragmented into a series of small, isolated patches. This environmental disruption likely promoted geographic isolation among kiwi populations on the North Island and ultimately contributed to the high level of species diversity observed there today.

Together, these geological and climatic processes—glacial fragmentation in the South Island and volcanic disturbance in the North Island—played an important role in shaping the evolutionary history of kiwi. Rather than representing only five clearly defined species, kiwi appear to consist of numerous genetically distinct lineages that originated through repeated episodes of environmental change and geographic isolation across New Zealand.

Phylogenetic tree of kiwi analyzed by Weir JT; detailed information can be found in the study cited below（Image source：Weir JT et al， CC BY-NC-ND 4.0 ）

Author: Shui-Ye You

Reference:

Weir JT et al. (2016). Explosive ice age diversification of kiwi. PNAS.