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Guest Blog: Birds and Berries

Native birds and native trees go together like fish and chips—but why are they so mutually important?

Guest blogger Ellis Nimick (@ellisecologist) explains some of the ways our native species can support each other within Aotearoa's ecosystems.

Over 70% of Aotearoa's plants distribute themselves by producing fruit and utilizing bird-frugivory for seed dispersal. With some fruit, such as karaka berries, the digestive system of the birds will dissolve the skin of the fruit, allowing the seed to germinate more effectively. 

Karaka berries

Many frugivorous manu capable of digesting large fruit have become either nationally or regionally extinct in Aotearoa. All nine moa species are believed to have gone extinct in the 15th century, huia at the start of the 20th century, and piopio in the 1970s. Kākāpō are notably restricted to two islands, and both species of the kōkako have declined, with the last reported sighting of Te Waipounamu's kōkako being in 1967, and only an estimated 1300 Te Ika-a-Māui kōkako remaining.

Consequently, besides a few mainland holdouts of kākā, the kererū has become the sole disperser for several large-fruited native rākau, such as tawa, taraire, karaka, miro, pūriri. As usual, introduced possums and introduced European birds provide little assistance to the ecosystem, with the possums destroying many of the larger seeds as they nibble them, and the exotic birds being too small to digest something as massive as a karaka berry.

Kererū in the Hunua Ranges

The dependency native rākau now have for effective seed dispersal using kererū is a remnant vestige of an ecosystem once teeming with bird life, which only deteriorates further in the absence of kererū. Many of these species will be largely unable to effectively propagate throughout a native habitat (and certainly unable to travel a great distance from the parent tree).

Without kererū, we face a very real risk of these trees disappearing or diminishing within our ngahere. Likewise, without an abundance of native fruit, kererū begin to starve (relying on the few palatable and often invasive exotic plants), which can be seen most evidently in Northland—where habitat loss and invasive species presence is significant.

Regarding invasive species, both the native rākau (leaves and fruit) and the kererū (particularly eggs and chicks) are predated by possums, highlighting the threat that possums pose not just to species as individuals within an ecosystem, but the entire ecosystem as an interacting and interdependent habitat. 

Similarly, the taraire, which is largely restricted to the northland area, is threatened not only by historic habitat loss, intense (and often unchecked) possum browsing, and regional kererū-decline, but also by prolonged droughts and rainy-periods which are becoming more frequent and disastrous due to climate change. Water instability shocks the shallow roots of the canopy-dominant species, often killing the tree (causing greater food scarcity for kererū).

Drought-shocked taraire

All of these pressures combined underscore the dramatic and contemporary decline of our indigenous ngahere due to global and national trends without radical action. If you're a native bird fan, consider donating a native tree: it will help provide habitat and food for our manu. If you're a native tree fan, consider trapping in order to help our kererū thrive and fulfil their vital role as seed dispersers.