Researchers measure extent of mouse menace
Photo: Wendy Ruscoe fitting a singularly unimpressed rodent with an ear tag. Photo: Keven Drew
Scientists researching the threat mice pose to beech forests have found the damage may be less to beech than to other
native species.
More than 70% of New Zealand's indigenous forest is either dominated by or otherwise contains beech trees. These trees
may be particularly susceptible to seed consumption by mice due to "masting", the production of large quantities of seed
on an irregular 2*4 year basis. Many believe that beech trees use masting to swamp the capacity of seedeaters to eat all
seeds before germination can occur. But our beech trees evolved in the absence of mice.
To determine whether mice could regularly consume all beech seed produced in a masting year, Landcare Research
scientists headed to beech forests in Canterbury, and in Fiordland, where they battled dramatic extremes of weather,
fleeing rising floodwaters and measuring mice in traps under a foot of snow! They monitored how many seeds mice could
find and eat, and the rates of increase and decrease in mouse abundance as seed availability rose and fell.
Landcare Research scientists Dr David Choquenot and Dr Wendy Ruscoe initiated the project five years ago. Dr Ruscoe says
that regardless of whether alternative food sources are available to mice, they continue to search for and consume beech
seeds until (local) availability reaches zero.
"This means they are theoretically able to eliminate seed reserves in beech forests, whether or not alternative food
such as native moth larvae is available.
"The rate of change in mouse abundance is closely related to seed availability, and abundance erupts in years of high
seedfall. However, this rise is substantially reduced when mouse density is already high.
"The reason for this is unknown. It may be territorial behaviour, or the effects of disease."
Dr Ruscoe says that when combined and built into an especially developed mathematical model, these factors indicate that
mice would rarely be able to consume all beech seed produced during moderate to heavy masting years.
"Therefore, mice are not a likely threat to the continuous presence of beech trees.
"However, seed availability appears to be important to mouse dynamics. The regular eruption of mouse abundance in high
seedfall years has important flow-on effects on the numbers of mouse predators, notably stoats; and the prey they
subsequently consume, notably our forest birds."
"Our research provides a more reliable basis for predicting when mouse abundance is likely to erupt, and will help
refine our ideas on how to target control measures for them."
The research also highlighted a lack of understanding of the role mice play in forest ecosystems.
"The results raise critical questions about the impacts mice may have through consumption of other native plants,
insects and fungi. By consuming these species as secondary food items, mice may be having a more insidious effect on
forest biodiversity than their consumption of beech seeds alone may suggest.
"Given the pervasive nature of mice in beech forests, and difficulties associated with their control, there will be
further research on understanding and managing their effects."
ENDS