What factors are responsible for differences in the abundance of species?
This question—perhaps the most fundamental in ecology, and of critical importance for conservation—was addressed by Ryan Phillips, Matthew Barrett, Kingsley Dixon and Steve Hopper in a recent paper in the Journal , for which Ryan (as lead author) was awarded the Harper Prize.
In the study, Ryan and his co-authors examined the causes of differences in abundance in common versus rare species of Drakaea, a genus of orchids found in southwestern Australia.
Noting that all of the ~ 26,000 orchid species found in nature require a mycorrhizal partner, the researchers hypothesized that mycorrhizae would influence adundance of Drakaea. They tested this possibility by examining five species of Drakaea, and their mycorrhizae.
Ryan explained that he was surprised when they found little difference between the three common and two rare Drakaea species in their interactions with mycorrhizae, suggesting that the symbiotic fungi may have little to do with differences in abundance between orchid species.
Interestingly, however, all the Drakaea species in the study were found to be associated with mycorrhizal fungi within a narrow range of the Tulasnella clade – technically the same mycorrhizal species – suggesting that the fungi have differentiated little among their Drakaea partner species.
As Ryan explained, this specialization on a particular mycorrhizal fungal partner does affect abundance of Drakaea in one particular way.
“While this is a very slow-growing fungus that gets outcompeted in productive environments, [the fungus] might be advantageous in these low-carbon environments [where Drakaea lives],” said Ryan.
So by associating with Tulsanella, Drakaea is largely limited to open microhabitats. This has an interesting – and potentially very important – side effect for the orchids.
“By using a fungus that occurs in more open microhabitats…it puts [Drakaea] in a position where it can be located by the pollinator,” Ryan observed.
Drakaea has coevolved with its pollinators – thynnine wasps. In fact, Drakaea are colloquially known as “hammer orchids” because the “head of the hammer” relies on the momentum of a male wasp to swing around to pollinate the stigma, like a hammer swings to hit a nail.
Ryan and his colleagues eliminated another potential factor that could explain differences in abundance among species by showing that abundant and rare Drakaea species did not differ in germination rates.
So what does limit abundance in Drakaea?
Drakaea occurs in arid habitat – more arid than other orchids – which likely limits their abundance in the genus overall.
“A big part of the rarity in the genus may actually be that there are very few sites with suitable microhabitat in that semi-arid area to support Drakaea,” Ryan commented.
This gets at the ultimate causes of rarity in the genus, but does not explain variation in abundance among Drakaea species.
The answer may lie in part in finer scale microhabitat variation. Seed germination in seed packets used in the field varied within seed packets, suggesting that very fine scale variation in habitat conditions contribute to rarity in Drakaea species at the germination stage.
Ryan suggested another possibility – that the rarity of Drakaea hybrids could be maintained by chemical floral cues that vary among Drakaea species, each of which is pollinated by a single wasp species. Interestingly, the wasp species differ in size, despite Drakaea having similarly sized flowers. So far this idea remains untested.
What about antagonists? Above-ground, Drakaea have thumbnail sized heart-shaped leaves that rarely experience herbivory. However, there is one exception.
“…in some populations, particularly ones in fragmented environments where you get a lot of introduced herbivores in the form of rabbits and a higher population of kangaroos, then you tend to start getting high herbivory rates in some populations,” Ryan remarked.
Another potential antagonist might be below-ground bacteria known to associate with roots, but little is known of their interaction with Drakaea.
The research wasn’t without its challenges. Drakaea produce very few seeds, so it took Ryan a few years to collect enough seeds to do the study. The experiment nearly turned deadly when he almost stepped on a copulating pair of tiger snakes.
Ryan was very excited to win the Harper Prize, and was so surprised, in fact, that he thought the email saying he had won the prize was SPAM!
He hopes this research will spur people involved in conservation
“to establish what’s limiting populations in the wild, and I think with orchids those issues haven’t been particularly well addressed,” said Ryan.
 Phillips, R.D., Barrett, M.D., Dixon, K.W. & Hopper, S.D. (2011) Do mycorrhizal symbioses cause rarity in orchids? Journal of Ecology, 99, 858–869.