Seeds, Rats, Fire and Forest Regeneration
GEORGE PERRY (University of Auckland)
Invasive mammals are well known for the destruction they have wreaked on New Zealand’s endemic fauna. However, mammals also affect plants and hence regeneration. Mammalian herbivores browse palatable species and have been implicated in the collapse of populations of tree species such as kohekohe (Dysoxylum spectabile) and northern rata (Metrosideros robusta). Rats and other invasive mammals, such as possums, directly and indirectly compromise seed dispersal by predating dispersers and consuming fruit and seeds.
Rats and seeds
Rat gnawed miro seeds in beech forest, Nelson Lakes National Park (Photo: George Perry)
Seed dispersal is a key stage in the plant lifecycle because it is the only chance that plants have to ‘move’. This movement is important as it helps genetic mixing and allows plants to respond to climate change. Plants have developed a range of dispersal adaptations including those for dispersal by animals (‘zoochory’).
In New Zealand prior to human arrival in the mid-thirteenth century, seeds were dispersed by a range of animals including birds (some of which like the moa and huia are extinct), reptiles and potentially wētā. More than 50% of New Zealand’s vertebrates are involved in seed dispersal and about 33% of NZ plant species dispersed by animals(1) . As populations of these dispersers have declined, so too have the seed dispersal services they provided, which may ultimately contribute to regeneration failure due to a lack of seed supply (termed ‘dispersal limitation’).
Another effect of rats, but one that has received less attention than their direct predation effect, is the consumption of seeds In southern beech forests, seed predation is well documented because it triggers trophic cascades. Beech trees seed irregularly with some years seeing almost no seed and occasional years seeing very heavy seedfall (‘mast years’).
During mast years, the abundance of food leads to irruptions of mouse populations, followed by irruptions of mustelids and eventually high levels of predation on native birds and other fauna. This dynamic underpins the Department of Conservation’s ‘battle for the birds’. Mast seeding is less prevalent in northern forests (although some tree species do mast). So what are the effects of rats on seeds in northern ecosystems?
Impact of rats in northern New Zealand ecosystems
A study spanning a number of northern offshore islands(2) showed that kiore (Rattus exulans) significantly reduced the recruitment of 11 of 17 coastal tree species including kohekoke, parapara (Pisonia brunoniana), karo (Pittosporum crassifolium), tawapou (Planchonella costata) and nikau (Rhopalostylis sapida).
“A computer (simulation) model
based on data from Aotea, showed
that interactions between fire and
seedling predation can completely
halt succession. ”
This reduction in recruitment can occur through predation of seeds and/or seedlings. For example, kiore eat the seeds, underground organs and leaves of nikau. Prior to human arrival, northern offshore islands, including Aotea, would have been home to dense colonies of burrowing seabirds. Once rats reduce the densities of these seabirds it appears that a more diverse range of plant species are able to establish driving what has been described(2) as “massive compositional changes and major alteration of ecosystem processes”.
These dynamics have also been described in island ecosystems in other parts of the Pacific (e.g. Hawaii) and for some plant species (e.g. the Hawaiian hau kuahiwi (Hibiscadelphus giffardianus) rat control is a component of the recovery plan (there to increase fruit retention on the plant) even if not enough on its own for the species persistence(3).
Making it happen: the project details
Using an enclosure trial, Campbell and Atkinson(2) estimated the ratio between the expected and observed germination of seedlings of 10 plant species (Figure 1) and clearly demonstrated that a suite of species are adversely affected. Another study(4), left seeds of 11 fleshy-fruited species in piles (simulating what happens after dispersal) with some protected from mammalian predators and others not. Very few (about 10%) of seeds were removed (whether predators had access or not) but more seeds were removed when mammalian predators could access them.
Figure 1: Based on an enclosure trial, Campbell & Atkinson2 estimated the expected and observed proportion of seedlings that successfully germinated for 10 species potentially influenced by rat predation. Points above the dotted line indicate species that germinated more than expected in the presence of rats, and those below, less. The colour of the dots indicates whether the differences were statistically significant.
Seeds and fire
Yellow flowering gorse is a common species on Aotea and is one species favoured by fire (Photo: T Wills (Wikimedia Commons))
Even if it can be a bottleneck, seed dispersal is just one component of the forest regeneration process. In contemporary New Zealand landscapes, fire, seed predation, herbivory and invasive plants interact to hinder regeneration. A computer (simulation) model based on data from Aotea(5) , showed that interactions between fire and seedling predation can completely halt succession. In New Zealand shrublands and forests, young vegetation (e.g. mānuka shrubland) is naturally more flammable than older vegetation.
Thus, any process (e.g. seedling predation by rats, dispersal failure due to a lack of seed dispersers) that slows succession means the landscape spends longer in a vulnerable condition and is also susceptible to invasion by plant species favoured by fire (e.g. gorse). Each time a fire occurs, more forest burns and so the landscape falls into a ‘trap’. There are some areas in this trap on Aotea, including the recent fire scar near the airport at Claris. So, as is usually the case in ecosystems, a number of processes interact through feedbacks to influence ecosystem composition and function (Figure 2).
Figure 2: In New Zealand forests ,young vegetation is naturally more flammable than older vegetation and so there is a window (in red) in which fire risk is high. Over time, as the forest regenerates and composition changes, it becomes less flammable. Human activity has ‘stretched’ this curve so that regeneration is slower (e.g., through seed dispersal limits - ‘vertical stretching’) and so that the young vegetation is more flammable (e.g., through invasive weeds such as gorse – ‘horizontal stretching’).
As an aside…
Finally, despite the deleterious effect that rats have had on New Zealand’s ecosystems, they have had one scientific bonus. In 2008, Janet Wilmshurst and colleagues(5) used radiocarbon dating of kiore gnawed seeds (miro and hinau) to pin down the arrival of Maori in New Zealand! Their answer: c. 1280 AD.
References:
Thorsen, M.J., Dickinson, K.J.M. & Seddon, P.J. 2009. Seed dispersal systems in the New Zealand flora. Perspectives in Plant Ecology, Evolution and Systematics, 11, 285–309.
Campbell, D. J., and I. A. E. Atkinson. 2002. Depression of tree recruitment by the Pacific rat (Rattus exulans Peale) on New Zealand’s northern offshore islands. Biological Conservation 107:19–35.
Moles, A. T., and D. R. Drake. 1999. Post-dispersal seed predation on eleven large-seeded species from the New Zealand flora: a preliminary study in secondary forest. New Zealand Journal of Botany 37:679–685.
Gill, N.S., Yelenik, S., Banko, P., Dixon, C.B., Jaenecke, K. & Peck, R. 2018. Invasive rat control is an efficient, yet insufficient, method for recovery of the critically endangered Hawaiian plant hau kuahiwi (Hibiscadelphus giffardianus). PLOS ONE, 13, e0208106.
Perry, G.L.W., Wilmshurst, J.M., Ogden, J. & Enright, N.J. (2015). Exotic mammals and invasive plants alter fire-related thresholds in southern temperate forested landscapes. Ecosystems, 18, 1290–1305.
Wilmshurst, J.M., Anderson, A.J., Higham, T.G.F. & Worthy, T.H. 2008. Dating the late prehistoric dispersal of Polynesians to New Zealand using the commensal Pacific rat. Proceedings of the National Academy of Sciences, 105, 7676–7680.