The point of collapse

Posted by Ellen Hume

Imagine a clear winding stream flowing from the hills above into a small yet pristine lake. Birdlife is abundant, and the water is teeming with fish including freshwater eels known as native tuna, while macrophytes, aquatic plants, provide habitat and food to the aquatic species. Over time, as people move into the area, the surrounding lush native forest makes way for paddocks of farmland. The local whānau use the waterways for gathering mahinga kai to feed their families and enjoy being connected to the beauty of nature. Eventually the landscape is one of commercial productivity, with a mosaic of cropping and grassland feeding livestock and only small pockets of fenced forest remaining. It is harder to catch fish and tuna but children still splash around in the water in warmer months. Agriculture in the area continues to intensify with further use of fertiliser and irrigation water and more animals on the land. Then one summer the stream dries down to a trickle. The lake is murky and native fish are far and few between. Aquatic plants along the once clear stream and lake beds have been replaced by masses of slimy green growth. No-one goes near the water anymore. The connection with the land, the balance, the kaitiakitanga have been lost.

So what has happened here?

Well, the situation can be explained by the concept of tipping points. A tipping point is the point at which a system changes from one state to another, sometimes quite unexpectedly. In this case, the previously healthy pristine waterway system has reached a tipping point, collapsing into a degraded unhealthy state. This is due to the gradual accumulation of small changes to the local landscape and greater human inputs increasing the nutrient levels in the water to a critical point where a significant change to the whole aquatic community occurs. This process is called eutrophication and can be very difficult to reverse due to the system being stable and resistant to change. In today’s world of change, if we could identify which systems are likely to experience tipping points then we could use management actions and policy to avoid these occurring. Knowledge of how tipping points affect a system is also invaluable when trying to shift a system purposely into another state, for example restoration of the degraded waterway and modified landscape.


Image credit: Troy Baisden

In the case of the waterway system described above, the local community rallied together to take action in claiming back their kaitiakitanga. It is a long journey but through concerted effort and connection to the land they have a strong vision to tip the system back into the healthy, functioning waterway it once was.


Ellen Hume is a PhD student funded by Te Pūnaha Matatini Centre of Research Excellence. Her project is looking at detecting temporal and spatial regime shifts to enable better risk-based decision making, with supervision from Dr Cate Macinnis-Ng and Professor Troy Baisden.



Which tree to plant where? Do we know enough to plant 1 billion trees?

Posted by Cate Ryan, @cate_ryan

Aotearoa was once cloaked in lush temperate rainforests and beech forests. They ensured the land was well looked after by providing ecosystem functions such as habitat, soil protection, carbon storage, fuel, localised shading and cooling effects, and filtration of water.

Today only 43% of Aotearoa is in native vegetation compared to c.80% before human settlement (Norton and Pannell, 2018). Ecosystem functions have degraded, especially water quality and soil protection (erosion) and we must also increase carbon sinks to play our part in tackling climate change.

The current NZ government seeks to significantly increase the number of trees in NZ with the 1 billion trees policy. Species that will be planted include radiata pine, redwood, eucalyptus, Douglas fir, totara and mānuka and these will be planted on private, public and Māori owned land (MPI , 2018). However, which species we plant has a big impact on ecosystem function. Plants trade-off water and carbon during the gas exchange process so that the quicker a tree grows, sinking carbon in the process, the more water it uses, and vice versa. Introduced species like eucalypts grow quickly so are great from a carbon perspective but they use a lot of water. Natives grow more slowly so they don’t accumulate carbon as quickly but they often use less water. We don’t yet have the numbers to properly quantify this in the field so we can’t currently make good decisions on prioritising carbon over water. Moreover, we need to plan ahead for climate change – which is expected to increase dryness and water scarcity.

My PhD research will determine the water use of a range of native compared to plantation exotic tree species in Northland and Canterbury farms, from the leaf to the landscape level – to help inform decisions about what trees to plant where. This is in collaboration with a wider Biological Heritage National Science Challenge programme around ‘Enhancing the ecological function of native biodiversity in agroecosystems’.


Cate Ryan is a PhD student in Biological Sciences at the University of Auckland, supervised by Dr. Cate Macinnis-Ng