Birds’ roles in ecosystems

An ecosystem consists of all of the organisms living within an area and the interactions between them and the physical environment.

All ecosystems, whether they are marine, freshwater or located in native bush, involve the transfer of energy. Energy flows into an ecosystem usually via sunlight. This light energy is used in a process called photosynthesis, allowing plant matter (flora) to grow. Flora then becomes a food source for birds, animals and insects. This transfer of energy continues as feeding relationships occur between plants and animals.

Mutualism

Native birds interacting with the flora in our ecosystems have a mutualistic relationship – that is, they both benefit from the relationship. While the bird receives nectar or fruit (in the form of berries) from the tree it visits, the tree benefits by having another organism carry out the process of pollination or seed dispersal. Many native trees cannot perform these processes without the intervention of birds.

Birds are the primary pollinators and seed dispersers in New Zealand native bush.

Pollination

Pollination is the process where pollen is transferred within and between plants enabling fertilisation and reproduction.

Bees, wasps, butterflies and (most importantly in New Zealand) birds act as pollinators. These organisms carry pollen grains from the anther to the receptive part (stigma) of the plant to allow pollination to occur.

Flax (Phormium tenax), kōwhai (Sophora microphylla), northern and southern rātā (Metrosideros robusta and Metrosideros umbellate) and tree fuchsia (Fuchsia excorticata) are New Zealand native trees all pollinated by birds. Our native birds are attracted to the flowers of these trees and carry pollen from flower to flower on their beaks as they seek nectar, pollinating flora as they move.

In New Zealand, birds are important pollinators. The honeyeater family consisting of tūī (Prosthermadera novaeseelandiae), bellbirds (Anthornis melanura) and silvereyes (Zosterops lateralis) perform the majority of pollination by all birds in our native bush. Two native species of mistletoe (Peraxilla spp.) require the finely refined beaks of tūī or bellbirds to tweak open their flowers and allow pollination to occur.

Seed dispersal

Over 70% of plants in our woody forest in New Zealand have fleshy fruit. Many seeds located within these fruits have coats that must be weakened by chemicals as they pass through the digestive system of another organism.

Native plants are dependent on birds for successful seed dispersal and regeneration. Once a seed has passed through the digestive tract of a bird, it will often be dropped far away from the host tree’s location, enabling the tree to potentially colonise a new area. As many of our native birds are now confined to small predator-proof mainland sites or offshore islands, what might the future hold for our native flora, which is so dependent on this process?

In New Zealand forests, only 12 species of bird have been responsible for the majority of tree seed dispersal, but many are now extinct:

• Extinct – piopio (Turnagra capensis), 2 species of moa with small gizzard stones (Euryapteryx spp.), huia (Heteralocha acutirostris)
• Confined to predator-proof mainland sites or offshore islands – saddleback (Philesturnus carunculatus), hihi (Notiomystis cincta), kōkako (Callaeas cinerea), whitehead (Mohoua albicilla)
• Present on the mainland – tūī (Prosthermadera novaeseelandiae), bellbird (Anthornis melanura), kererū (Hemiphaga novaeseelandiae), weka (Gallirallus australis).

The importance of fruit size

Trees that produce the largest fruit (>14mm diameter) almost solely rely on the kererū for seed dispersal. These trees include miro (Prumnopitys ferruginea), pūriri (Vitex lucens), tawa (Beilschmiedia tawa) and taraire (Beilschmiedia tarairi). Within any species, there is variation, and on any given tree, there will be a variety of fruit sizes along with variation in the size and gape of a bird’s beak. This factor will allow the smaller tūī and bellbird to disperse some of the seeds from these trees.

As the weka is a flightless bird, it can only scavenge the fruit that has fallen from the tree and could not cover the same distance that a bird of flight would be able to. Therefore the 3 keystone species for seed dispersal in New Zealand are the kererū, tūī and bellbird.

New Zealand native birds have a key role in the pollination and seed dispersal of our native flora. Birds are essential to ensure the future of our native bush ecosystem in generations to come.