Life and death in a Westland rainforest: Drought-hit mudfish give insight into surviving climate change
University of Canterbury researchers studying Westland mudfish have witnessed a life and death struggle that sheds new
light on how species may survive extreme conditions, gaining the attention of the Royal Society of London.
Researchers at the UC’s School of Biological Sciences have been studying brown mudfish that, remarkably, live in tree
‘tip-up’ pools in South Westland rainforest. The mudfish colonise the water-filled depressions formed when rimu trees
topple over.
To do this, the mudfish have developed a significant tolerance to harsh conditions, coping with frequent pool drying,
low oxygen levels, high acidity and variable temperatures. The mudfish move through the forest floor, likely at times of
high rainfall, to find pools where they are able to escape competition from other fish.
In the middle of the UC research team’s study, the West Coast was hit by a 1-in-25-year drought. From February to March
2013, it didn’t rain for six weeks – extremely rare on the South Island’s rainy West Coast. Because the pools dried out
for a long time, a large proportion of the brown mudfish died; more than 80 per cent in some pools.
UC doctoral student Richard White had individually marked mudfish in 48 forest pools so he could follow their movements
and record deaths and births very precisely. His study has just been published in the Proceedings of the Royal Society
of London.
Freshwater ecosystems expert UC Professor Angus McIntosh, research team leader and co-author of a 2016 paper on the subject – Drought survival is a threshold function of habitat size and population density in a fish metapopulation – says that, most importantly, Dr White’s research showed that larger mudfish populations actually recovered reasonably
well after the drought.
“This allowed him to test a longstanding theory relating to the underlying ability of populations to withstand big
disturbances, or extreme ‘random’ events, like this sudden West Coast drought.”
With population models, the UC researchers were able to show that, as expected, smaller pools with fewer mudfish were
least resilient to drought. However, in contrast to the theory, even the most extreme events were not enough to drive
the biggest populations to extinction.
This suggests that large populations may be more resilient to natural disturbances than previously thought, whereas the
theory had suggested that even the largest populations would still be vulnerable to extinction given a sufficiently
large random disturbance event, Professor McIntosh says.
“While this might sound like good news, there are some important insights from the study for how populations are managed
in the face of climate warming, which is expected to see the frequency of extreme events like this drought increase in
New Zealand.
“The probability of populations persisting during drought consistently declined with the carrying capacity of the
habitat, so anything that decreases carrying capacity will likely erode the capacity of vulnerable species to recover
from extreme events,” he says.
“In the case of these mudfish, the biggest populations were living in relatively deep pools in native forest, so things
that reduce the quality of that habitat, like logging or drainage, would likely compromise the resilience of the
populations.”
It’s not surprising that Canterbury mudfish, living on the other side of the Alps, are critically endangered, Professor
McIntosh says.
“Moreover, part of the resilience of these forest-dwelling brown mudfish populations came from the connections between
pools; Richard recorded mudfish travelling 120 metres through the forest. This connection between populations allows
larger populations to ‘rescue’ smaller ones, and is very important for recovery. Thus, processes that fragment the
habitat and disrupt that movement will reduce the resilience of vulnerable populations to extreme events.”
Brown mudfish are listed as 'At Risk, Declining' under the New Zealand Threat Classification System.