Geoff Rogers gives a science brief on plant monitoring
One of the key components of the MDM eradication project is to comprehensively report on the benefits to the whole Antipodes Islands’ ecosystem of eradicating the mice. Ecosystems include birds (both land and sea), insects, plants and vegetation communities, and the functional interactions of all those components like pollination and seed dispersal. My role was to try and predict and then design a plant monitoring scheme sensitive to any likely vegetation changes from removing the mice. The mice are functionally both herbivores and carnivores. The task would be no mean feat when there is little existing science reporting on mice impacts on plants, particularly on islands at extreme southern latitudes. It was suggested a visit to the island might help with the scheming, and I didn’t object!
And, so, Hannah Edmonds of DOC Te Anau and I accompanied the hut building team on the yacht Evohe in August 2014 to the islands to look further at the status of vulnerable land birds and potentially palatable plants. But the reputation for foul weather that the Antipodes enjoys came to pass and we cruised about unable to land for over a week at sea playing yacht tourists.
In terms of background research, I had the benefit of some published plant science from subantarctic Gough and Marion islands showing mice severely threatened one or two plant species whose seed they prefer. More specifically, a mice diet study on Antipodes Island showed mice rely heavily on plant seeds, at least seasonally, particularly those of Carex appressa, an abundant sedge of the coastal zone. But my field impression at the Antipodes was that few plants might be artificially restricted by seed predation. Just Stilbocarpa polaris (a megaherb), Senecio radiolatus var. antipodus (a megaherb daisy), and Lepidium oligodontum (an endemic coastal cress) seemed unusually restricted in terms of their habitat preferences.
The other intriguing dimension to plants is that those whose biologies are tied to the guarno-based nutrient subsidies from nesting seabirds might be artificially restricted if mice as carnivores limited chick survival and therefore nutrient subsidies by the adults as they do on Gough Island. Finally, it is possible on Antipodes Island that diet switches by mice caused severe decline (or even extinctions) to similar guilds of plants to those heavily impacted by mice on Marion and Gough islands before plant distributions and densities were first studied in the late 1950s…that’s potentially 150-years of unknown vegetation modification.
From that background, our recommendation was to invest in plant monitoring that emphasised the status of the high nutrient-demanding guild. That’s no easy undertaking when plant population studies are resource and time hungry (but not as demanding as animal population studies), quite apart from the challenge of remoteness. It is, indeed, possible that reproductive output of ground-nesting birds might boom with the mice removal, thus benefitting the nutrient-dependant herbs listed above. On our mid-winter visit, Hannah and I were struck by the frequency of dead albatross chicks (whose skeletons were scavenged clean of flesh by marauding skua) and also large numbers of dead petrels. However, the cause of these deaths mystified us.
It was Brian Rance’s task on a subsequent voyage to implement our foundation advice on plant ecosystems by setting up monitoring that might detect changes in plant species abundance with the removal of mice. Whilst targeting the nutrient-dependant herbs, he also had an eye for investing monitoring effort in several other plant communities just in case ecosystem change (including mice removal) solicits plant composition change not directly predictable at present. For instance, the Antipodes are one of the best subantarctic islands for detecting compositional changes from climate change because the vegetation is not fire-modified, unlike the Auckland and Campbell groups.
Some of the more compelling ecological impressions the islands left on me are:
- In terms of limited sunshine hours, persistently high humidity and total wind-run, the Antipodes have an even less appealing climate than our other subantarctic islands. Thus, peat accumulation rates probably eclipse those of the more southern Auckland and Campbell islands because peat is a function of cold, wet climates on poor-drainage-promoting topography.
- The comparative unimportance of woody plant communities was intriguing. Few shrubs and no tree species have established there, probably because they are uncompetitive against the vigour of sedges and grasses on the ubiquitous, poorly-drained, acidic and nutritionally-impoverished peat. In contrast, the Snares, Auckland and Campbell island floras and vegetation have extensive forest and scrub and diverse woody floras in comparison to the Antipodes.
- The frequency, spatial extent, and scale of disturbance on vegetation communities of peat slipping are far greater on the Antipodes than on Auckland and Campbell islands. This strongly influences vegetation patterns by exposing as a new soil surface the thoroughly impoverished older peat below the surface veneer of peat and plants that slip away. Consequently, vegetation colonisation and rates of succession on the slip faces are very slow. The large patches of conspicuous white vegetation above coastal cliffs confronting ship-born visitors at sea are lichen-covered slip faces – see the photo of this community at the beginning of the Brian Rance blog. Why do just the uppermost peat layers of a mostly 5-m-deep peat profile slip? It appears that the vegetation veneer, especially its root zone becomes super-saturated in cyclonic rain events and the giant sponge that the surface layer represents slumps away. Rivers of fluidised peat debris can travel a kilometre or more across the upland plateau down the shallowly-incised stream channels at remarkably gently gradients to eventually canyon into the sea down the coastal sea-cliffs. It would have been quite a sight from a boat, preferably at a safe distance.
- The island group has suffered very little if no impact of fire. In contrast, the Auckland and Campbell island groups have vegetation extensively modified by fires lit by sealers and whalers in the 19th century and farmers in the 20th century that have caused a retreat of woody plants at the expense of tussocks. No doubt, Antipodes islands’ sealers and possibly castaways of the two 19th century ship wrecks would have attempted to burn the vegetation but the weather and sodden conditions thwarted their attempts.
- There is a big contrast in the luxuriousness and composition of the coastal vegetation receiving the high nutrient supplements from onshore salt winds, seals and birds, and rotting vegetation from peat slumping with the impoverished rolling tops of low-growing grasses and forbs surrounding peat bogs. Just the river channels and hill-slope gullies with prickly shield fern and the odd Coprosma shrubland respond to relief from the ubiquitous poor drainage on the plateau.
- The flora is quite limited compared to our other subantarctic islands (Brian Rance’s blog). I don’t think this has much to do with limits on long-distance dispersal of plant propagules. Rather, the chances of establishment of spores and seed dispersed from afar are constrained by the habitat uniformity of the peat and the competition imposed by the tightly-interlaced resident plant cover. After all, an Antipodes Island pollen diagram shows the pollen rain has a high proportion of tree pollen derived from the NZ mainland so stuff must continually arrive.
Geoff Rogers will be helping with more plant monitoring post mouse eradication.