Little fern provides cryptic proof of ongoing New Zealand to Chatham Islands plant dispersal

Hooker's spleenwort ([i]Asplenium hookerianum[/i]) plants growing amongst hoho ([i]Pseudopanax chathamicus[/i]) root plate with two other ferns - maidenhair fern ([i]Adiantum cunninghamii[/i]) and [i]Rumohra adiantiformis[/i] - Waipaua Stream - Rangiauria (Pitt Island). The plants in this image represents both the finely divided frond race sometimes known as Colenso's spleenwort (A. colensoi or A. hookerianum var. colensoi) and the more common less divided frond race of Hooker's spleenwort (A. hookerianum or A. hookerianum var. hookerianum).   Image: P. J. de LangeHooker's spleenwort (Asplenium hookerianum) plants growing amongst hoho (Pseudopanax chathamicus) root plate with two other ferns - maidenhair fern (Adiantum cunninghamii) and Rumohra adiantiformis - Waipaua Stream - Rangiauria (Pitt Island). The plants in this image represents both the finely divided frond race sometimes known as Colenso's spleenwort (A. colensoi or A. hookerianum var. colensoi) and the more common less divided frond race of Hooker's spleenwort (A. hookerianum or A. hookerianum var. hookerianum). Image: P. J. de LangeHooker's spleenwort (Asplenium hookerianum) is not a particularly common fern on the Chatham Islands.  By far the largest population the Department of Conservation knows about is one that was discovered in November 2008 along the banks of the Waipaua Stream, Pitt Island.

Hooker's spleenwort looks superficially like a smaller version of Hen & Chickens fern or pikopiko (Asplenium bulbiferum and the allied A. gracillimum), which is abundant on both Chatham and Pitt Islands. Aside from its size Hooker's spleenwort differs by the absence of "chickens" - small plantlets that develop on the frond, and which if dislodged are capable of growing, stalked pinnules - and depending on which form of Hooker's spleenwort you have, fronds with fewer, less-divided pinnules or fronds with more finely divided pinnules. Both forms have been called different species at one time - Aspleniumhookerianum for the less divided frond race and A. colensoi for the more finely divided type - but nowadays many botanists prefer to treat them as varieties or even as the one species A. hookerianum. As in New Zealand proper, on the Chathams both forms occur and usually grow side by side. In 2007 fern researchers at Te Papa Tongarewa Museum of New Zealand asked if it might be possible to collect some fresh fronds of Hooker's spleenwort from the Chatham Islands. In a previous study (Shepherd et al. 2007) the researchers had found that Hooker's spleenwort samples spanning the North and South Islands had 25 distinct haplotypes (meaning DNA polymorphisms that are inherited as a unit).  What, they wondered, would be the situation on the Chathams Islands?

As a result of their request two fronds were collected in September 2007 by Peter de Lange (Ecosystems and Species Unit, Department of Conservation) and Peter Heenan (Allan Herbarium, Landcare Research), one each from the Tuku-a-Tamatea Nature Reserve, Chatham Island and from the Ellen Elizabeth Preece Conservation Covenant (Caravan Bush), Pitt Island. Following DNA analysis it was found that the frond from the Tuku-a-Tamatea represented a unique "Chatham Island" haplotype that was related to those largely confined to the central and eastern North Island.  The frond from Pitt Island matched haplotype Q of Shepherd et al. (2007) the most common and widespread haplotype in the rest of New Zealand proper. These results have just been published in the international Journal of Biogeography (Shepherd et al. 2009). 

The haplotypes provide clear evidence that Hooker's spleenwort has colonised the Chatham Islands at least twice (since the 2007 gatherings two new populations have been discovered and these have not yet been analysed). While on some levels this is hardly earth shattering news - it is after all common knowledge that plants and animals have naturally reached New Zealand from Australia, and also the Chatham Islands from New Zealand, e.g., spur winged plover and welcome swallow -  very few published studies provide some insight into the frequency of long distance dispersal (LDD). For the Chatham Islands we now have published evidence for within-species LDD in both Hooker's spleenwort (Shepherd et al. 2007) and shield fern (Polystichum vestitium) (Perrie et al. 2003), and both de Lange & Heenan are working on a revision of Cook's scurvy grass (Lepidium oleraceum) in which they have evidence of multiple New Zealand to Chatham Island colonisations, as well as subsequent speciation into local endemics. For the Chathams we know that possibly as many as 50 vascular plants are unique (endemic) to the islands, which is c.12% of a total indigenous Chatham flora of c.410 taxa. While this figure reveals the importance of the islands as a global hot spot of diversity, one feature that is frequently overlooked is the status of the remaining 360 or so taxa which are shared with New Zealand and Australia. Many of these plants, like Hooker's spleenwort are very uncommon on the islands. Is this because they were formerly more widespread and have declined as a result of habitat loss, or is it because they have only recently colonised from New Zealand or both? Currently we just don't know. It may not seem important either but what of plant and animal pests that are capable of LDD? The Chatham Islands may seem remote by world standards but if indigenous plants and animals are capable of naturally colonising it, so too can "problem children" that Chatham Islanders don't want. Studies to ascertain the levels of natural LDD colonisation using many of the plants shared between New Zealand and the Chatham Islands (including weed species) would provide a very useful guide as to the levels of LDD and subsequent genetic divergence of both native and introduced plants.

References:

  • Perrie, L.R.; Brownsey, P.J.; Lockhart, P.J.; Large, M.F. 2003: Morphological and genetic diversity in the New Zealand fern Polystichum vestitum (Dryopteridaceae), with special reference to the Chatham Islands. New Zealand Journal of Botany 41: 581-602.
  • Shepherd, L.D.; Perrie, L.R.; Brownsey, P.J. 2007: Fire and ice: volcanic and glacial impacts on the phylogeography of the New Zealand forest fern Asplenium hookerianum.  Molecular Ecology 16: 4536-4549.
  • Shepherd, L.D.; de Lange, P.J.; Perrie, L.R. 2009: Multiple colonizations of a remote oceanic archipelago by one species: how common is long distance dispersal? Journal of Biogeography 36: 1972-1977.