After a significantly long absence I have decided to get back into updating whoever ‘googles’ my name on how my PhD research is traveling along! It’s almost been a year since I posted anything so keep an eye on this space over the next few months as I fill the interwebs world with musings of my most recent adventures…..
Snapshot of the last year…
Since September last year I spent a couple of months organising my first wet season field trip to Christmas Island, spent 5 months living and working on the island (mid Nov – mid Apr) and have now spent the last 3 months getting my shit back together and getting stuck into my data back in Melbourne. I use the word shit as I ended up leaving Christmas Island earlier than expected due to an unfortunate accident that resulted in the tendons of my middle finger on my right hand being severed. This has put me behind somewhat in that I returned from the island with incomplete data sets. On the plus side, after two surgeries and 10 weeks in a splint my finger may be stiff and not have full flexion but it is pretty much completely functional. I’ll give you more details on that later.
Just like my last post (Sept 11), I have been motivated to ‘blog’ again as I have another Progress Panel coming up next week. I’m now at the 18 month point of this project and am feeling happy about my preliminary results in some aspects of my research but less happy about others. After a couple of fairly non-productive months while my hand was incapacitated and I was busy with therapy, I am now engaged in a heavy game of ‘playing catchup’. Again just like my other post, I have recently finished identifying and counting the snails collected as a wet season comparison to my initial dry season collection. Interesting preliminary results as follows:
Mutualism between primary invaders influences land snail community composition and alters invasion success of alien species in rainforest on an oceanic island
The main objective of the current study was to demonstrate whether invader-invader mutualism resulted in secondary invasions. Using the invasion of rainforest on Christmas Island as the study system, it was hypothesized that an ant-scale insect mutualism would advantage the land snail community through the release of habitat and resources and the creation of enemy-free space. This study specifically asked: i) how does land snail abundance, species richness and composition change across different forest states that have arisen from the ant-scale insect invasion? ii) are exotic species only present where the ant-scale mutualism has altered the forest, therefore representing a secondary invasion? iii) how do native land snails respond to community level changes as a result of the ant-scale mutualism? and iv) can any changes in the land snail assemblage be attributed to changes in habitat understorey complexity, litter or the abundance of the native red land crab?
Across all twenty sites, the dry season sampling period recorded 7,628 individuals representing 20 species of land snail whereas the wet season sampling period recorded 10,407 individuals representing 21 species. Both collections were similarly represented highly by the common species Japonia wallacei and Georissa sp. The following results are simply the initial exploration into the data looking at differences in land snail abundance, species richness and composition across the four forest states (see previous posts) for each season.
For the dry season data, snail abundance showed a strong trend to greater numbers where the ant-scale mutualism had altered the forest, although a significant difference was not detected (X2 = 6.74 p = 0.08; Fig. 1a). However, pairwise comparisons demonstrated that Ghosted forest contained significantly more individuals than Intact forest (Z = -2.19, p = 0.03) and that Supercolony and Intact sites strongly displayed the same relationship (Z = -1.98, p = 0.06). No difference in species density between the treatments was present (F = 0.79, p =0.52; Fig. 1b). Alternatively, the wet season data showed a much stronger trend whereby there was significantly greater abundances of land snails where the ant-scale mutualism had altered the forest (F = 7.94, p = 0.002; Fig 1a). This result was driven by there being significantly greater number of snails in Ghosted and Supercolony forest compared to the Intact state. Similarly, no difference was in species density was observed (F = 1.01, p = 0.41). These results indicate that the snail community is responding positively to the ant-scale invasion through significant increases in population size.
For the dry season data, an overall difference between forest state and land snail species composition was highly significant (Global R = 0.242, p = 0.008; Fig. 2) with post-hoc analysis confirming that Intact sites differed to that at all other forest states. SIMPER analysis revealed that generally it was the presence of the exotic Elasmaias manilensis and the absence of GALSat all Intact sites that contributed most highly to the dissimilarity between treatments. The complete absence of the exotic Subulina octona in Intact forest also contributed strongly to the dissimilarity, most evident between Intact and Supercolony forest states. These results was both expected and surprising. We had assumed the land snail community would be responding positively to the deletion of the Red Land Crab, and subsequent release of resources, and that that would be reversed following the re-introduction of the crabs (Baited/Recovered). The fact that the composition of the Baited/Recovered sites were also dissimilar to the Intact state suggests that species that were able to enter following the removal of the crab were then able to persist in it’s presence.
This pattern was somewhat supported when the land snail species composition for the wet season was analysed. When plotting abundance data (Fig. 3), another highly significant difference between composition was observed (GlobalR = 0.217, p = 0.008). Post hoc analysis again revealed that Intact sites supported a significantly different composition to Supercolony and Ghosted sites, however Baited/Recovered sites were now not significantly dissimilar to any other forest state. This result is closer to what we expected to be going on as the deletion of the Red Land Crab results in a significant shift in community composition as new species move in, and the return of the crabs to sites where they were previously absent results in the community shifting back to an intact state. However, when the same composition data is analysed just using presence/absence of a species (Fig. 4) no difference between any forest state was observed (Global R = -0.025, p = 0.592). Unlike during the dry season, the complete absence of a few invasive species (the Giant African Landsnail and Subulina octona) did not differentiate the Intact forest state from the others.
I’ll leave the discussion of these findings to only that for now because, as previously mentioned, these are only preliminary results and I actually still have the snails from 8 more sites to identify and count (during the latest wet season I upped my replication from 5 to 7).
Stay tuned for more exciting adventures on the life and times of my PhD research looking at the effects of primary invaders on potential secondary invaders on Christmas Island.