The Community Assembly and Neutral Theory II covered a diverse range of systems (from microcosms to primates), scales, and methods of study. Lin Jiang presented an experiment examining the relationship between diversity and invasibility, in particular testing whether priority effects reduce the oft-seen negative relationship between diversity and invisibility. Most manipulative experiments "assemble" communities instantaneously rather than continuously and stochastically as in natural systems, and so more realistic assembly may weaken the sampling effect and niche complementarity, which are suggested to drive the negative relationship. Using protist-based microcosms of 5 resident species and one invading species, Jiang examined how more realistic assembly of communities affected the diversity-invasibility relationship. Under these conditions, there was still evidence of a negative invasion diversity effect. His most interesting result however were that in fact the presence of a close relative had the strongest influence on the success of the invasive species, in line with other theoretical and empirical results (although not conclusive given the small number of species).
In the Community Assembly and Neutral Theory II session common topics in this session included the widely-used framework of hierarchical filters (i.e. abiotic, biotic, dispersal limitation) determining local species composition and tests of the predictions of the neutral theory (with a focus on non-SAD predictions). For example Wang et al. looked at the patterns of clade age versus abundance that were predicted by neutral theory, in comparison with empirical data from the BCI dataset. There was a clear divergence between the observed data, which included old clades with high or medium abundances compared to the neutral theory prediction that old species should have low abundances. Wang examined how relaxing the assumption of equal rates of speciation among species affected the age-abundance patterns, but concluded that different rates of speciation among species wouldn't produce the observed pattern without then failing the SAD predictions. However, one astute commenter noted that it might also be important to model the possibility of changing rates of speciation through time.
There were many other interesting talks. For example, in Biodiversity I, Matthew Leibold provided a master class on resilience to human disturbance, focusing on concept of communities and ecosystems as complex sets of coupled oscillators. Finally, Angela Brandt presented the results of a 7-year experiment in California grasslands (which no doubt represents many hours of hard work), in which she examined the relationship between invasion success, resource availability, and disturbance. In particular, she framed the question from a phylogenetic context, and discovered evidence that disturbed communities tended to be both more species-rich and phylogenetically diverse, and also less phylogenetically clustered, compared to non-disturbed communities. However, if communities received nutrient enrichment and disturbance, invasion was greater, and diversity lower, than in communities that received disturbance treatments only.
Looking forward to day 3!
***Addendum by Marc:
There were two additional talks that were particularly interesting. First was Andy Gonzalez’s talk on Evolutionary rescue, which is when a population is in demographic decline, heading towards extinction and adaptation saves it from extinction. This is particularly important in changing environments. He has previously shown this using yeast growing under salt stress. His question now was whether migration in a metapopulation with heterogeneous affects this evolutionary rescue. This is an interesting question because too little dispersal means that genetic variation or beneficial mutations do not get to other patches, and too high means that suboptimal genes are maintained in patches where they are maladapted. Not surprisingly he showed that in a constant environment, dispersal is not very important. In heterogeneous metapopulations, patches at the edge of salt tolerance thresholds increase in yield with dispersal.
The other very interesting talk was from K. Anderson on niche-based sorting in highly diverse palm assemblages. She looked at soil type and resource availability, as well as herbivore damage. She experimentally planted 13 species across a soil gradient with and without herbivore exclosures. In low nutrient soils, the palms invested more in roots while in high resources soils, there was a greater investment in above ground biomass and an increase in photosynthetic rate. Leaf toughness also increased in poor sites, meaning that they were more resistant to herbivory and plants growing in the high resource sites experience more herbivore damage. She mentioned that there were differential responses from the different species, and I am very interested to see more about this neat system.
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