The New Zealand Plant Conservation Network

General question for people - but probably one that Mike Thorsen can answer best. What are the dispersal mechanisms for Sonchus kirkii and Wahlenbergia vernicosa, and are they also dispersed by seabirds? I appreciate that S. kirkii is primarily wind and gravity dispersed, but I ask because both species are spreading in the Kermadecs. I figure this is mostly due to sea birds moving between island groups that had hitherto been rodent and cat infested - wind may also be a factor, as may cyclones (less likely) but I feel that birds probably are the main reason. This is especially so because both species on those islands (and for the Wahlenbergia in New Zealand) are often closely associated with sea bird roosts and nesting grounds. Again I appreciate that disturbance and nutrient levels are also a key factor influencing the occurrence of these plants within sea bird haunts.

I am asking also because I cannot find on the NZPCN website any mention of their dispersal mechanisms.

Hi Peter, in 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 I considered Wahlenbergia to be primarily ballistically dispersed using a censer mechanism (a swaying capsule that discharges small seeds apically) and secondarily by wind based on the paper Benson, D., McDougall, L. 1995. Ecology of Sydney plant species part 3: Dicotyledon families Cabombaceae to Eupomatiaceae. Cunninghamia 4, 217-431. Though I think wind dispersal is less frequent in NZ the seed size (c. 0.5mm) is in the range of species that can be dispersed by seed but it lack the distinctive testa ornamentation of other wind dispersed species. i don't think there are any differences between teh Wahlenbergia species in how they are dispersed, but seed viability and availability of suitable germination sites will strongly influence spread (as you know).

to be continued .....

Yes I read this - but its not noted in the NZPCN and I would argue that the occurrences of species like W. vernicosa in highly disjunct sea bird colonies suggests some role being played in dispersal. Also the seed is fine and can easily get lodged in feathers and dirt stuck to feathers (though of course it can wash off). Interesting anyway - and what of Sonchus?

Sonchus kirkii has all the attributes of a wind dispersed species. i didn't consider it to have any special gravity dispersal attribute beyond the seed fall under a parent plant that all species have (from memory, I only considered tawapou to be gravity dispersed). Like most Sonchus species, the capitula remains tightly closed unless very dry.

Jesse has been manually loading the seed dispersal data (my sympathies!) which is why the information not available for some species.

To be continued ....

Thanks - for Sonchus then wind dispersal probably hastened by Cyclones. After all something had to get it from New Zealand 1000 km NNE to the Kermadecs. Wahlenbergia remains to me an enigma.

Explanations for those species turning up on new islands/ i would first suggest the possibility that rat predation was keeping the species at low numbers and it is now visible now they are not browsed.

Seabirds are definitely a possibility, but neither species shows any special adaption for attaching to birds. I could only use the existing information (and lots of "educated' guess work) in assigning dispersal mechanisms. As you say it is possible for small seeds to attach to birds, but I really only considered this for species that are frequently found in association with bird colonies. Darwin hypothesised seeds in mud on birds, but i have not been able to find any confirmation this actually occurs. Seed adherence to birds could be tested using sticky tape applied to feathers and feet (something for the bird botherers to have fun with!).

If these species do regularly turn up in new bird colonies then I'm happy to consider adding attachment as a potential dispersal mechanism

to

I've been surprised how few times people have challenged the dispersal attributes I assigned. You and Colin Ogle are the only two to date. I'd love to hear more suggested alternatives - or even better someone going out and emperically testing my suggestions. Unless we know how a plant moves around the landscape other branches of plant sciences such as ecology and biogeography are only giving part of the picture

yeah Cyclones are what I categorised as "non-normal" dispersal! Severe weather events do have a role to play. I'd posit the dispersal of Nothofagus has something to do with wind blown branches and I was thinking about including a catapault dispersal mechanism with wind-loaded branches flinging fruit further away than it would normally fall.

Wahlenbergia vernicosa is a good contender for birds - not worried about the others Mike. As a species it is virtually restricted to sea bird colonies and nesting sites. Whilst I believe this is partly a disturbance / nutrient issue, the association is so close (and commented on when this species was considered to be W. gracilis and also by Petterson as W. vernicosa using my notes) that I suspect its likely this plant is moved about by birds. I don't believe its seeds are being picked up and dispersed a 1000 km routinely by wind, but hey its guess work (as you say)

See: Norton DA, de Lange PJ, Garnock-Jones PJ, Given DA 1997. The role of seabirds and seals in the survival of coastal plants: lessons from New Zealand Lepidium (Brassicaceae). Biodiversity and Conservation 6: 765-785.

Petterson JA. 1997. Revision of the genus Wahlenbergia (Campanulaceae) in New Zealand. New Zealand Journal of Botany 35: 9-54.

Thanks Peter, there is a lot to be said for seeing species in a range of natural situations. I don't know W. vernicosa much at all (I would see it occasionally on 3 kings and in Northland) but I didn't have the chance to look closely at seabird colonies. Both those papers I cite in my paper (in the supplementary material). You should also check out my paper "Faunal influences on NZ seed dispersal" Evol Ecol (2011) 25:1397-1426. This type of disperser relationship is treated in "flighted attachment" which is over-represented in coastal and strand environments