Assemblage Time Series Reveal Biodiversity Change but Not Systematic Loss
Dornelas et al. 2014. Science 344: 296-299
Blog by Willow
Authors
Maria Dornelas
Reader at U. St. Andrews, UK.
My research focuses on quantifying biodiversity and understanding the processes that shape it. I often work on tropical systems and specifically coral reefs, but I also work with tropical freshwater fish, mangrove crabs and plants for example, as I am more question-driven that organism-driven. I like to combine ecological theory, synthesis of existing data, and fieldwork in my research, and most of the research questions I’m interested in fall under the disciplines of community ecology, macroecology and biogeography. I tend to work on intermediate spatio-temporal scales (that is communities and networks of communities over time-scales of years to tens of years).
Nicholas J. Gotelli
Professor at U. Vermont
My research addresses basic questions about the organization of animal and plant communities. What are the forces that determine the species composition and abundance of natural assemblages? How do competition and predation affect local community structure? What are the biotic and abiotic factors that control population growth and the risk of extinction?
Brian McGill
Professor at U. Maine
I seek to understand the patterns and processes controlling the distribution and abundance of organisms at medium to large scales to lead to more predictive theories of how distribution and abundance will change under anthropogenic global change (especially climate change and landcover change).
Intro
There are increasing numbers of threatened species across the globe, but most of this data comes from local analyses.
Goal: to quantify and assess global patterns of temporal change in species diversity
Background
α diversity- diversity at a site
β diversity- diversity overlap between sites
Temporal β diversity- here is a measure of diversity overlap between the same site at different times. Treating site 1 at time A as different than site 1 at time B.
Methods
100 time series (1 time series= collections of data made at one site in different years= a time progression of community change)
Measure α diversity at each time slice and find the slopes of the lines for each community
Measure temporal β diversity with similarity indices
Results/Discussion
α diversity- there are variations within sites, but no global pattern of change in α diversity
This means that sites around the globe generally have the same number of species in them across time even though they vary locally
-there is no significant difference between marine and terrestrial species
-When sorted by taxonomic group, plants have a slightly positive slope indicating they have, on average, more species in a site today
-When sorted by climatic zone, temperate sites have a positive trend. Tropical sites were nonsignificant and global trends were significantly negative.
This means that temperate sites have more species in them today even though globally there are fewer species.
temporal β diversity- communities exhibit a long-term negative trend in similarity
This means that a community sampled in 1850 has significantly different species in it than the same community sampled today.
Although turnover is expected, this rate of turnover is much more pronounced than expected.
-Globally, β diversity measures are increasing regardless of sorting
This means that the same species are now found in more sites around the globe =homogenization!
Conclusions
Local and regional assemblages are not losing numbers of species, but numbers of different species. This happens through substitution of species at a site. Communities are becoming more homogenous.
“Assemblages are undergoing biodiversity change, but not systematic biodiversity loss”
Thoughts
I thought this was a really neat way to look at this- treating a community at different times like a completely separate community was an interesting way to examine β diversity. I like that homogenization was kind of a surprise result. I got looking at the trends and puzzling over why, then they hit me with the homogenization hypothesis. Wow! I do think they could have improved their climatic analysis, though. Looking at their map, most series fall in the temperatre-tropical and few fall as just tropical and I think that could have influenced how they viewed some of their trends, but not the results as a whole.
I was very surprised by the results of this paper although assemblage composition is definitely changing, I expected slight biodiversity loss on a global scale. They observed the data by organizing it into biomes which addresses some questions but I would like to see it organized by continent. I think culture, population size and government policies regarding the environment and conservation would play a big role in the change in assemblages. For this reason, I would be interested to see if there would be a pattern.
ReplyDeleteI read the Gonzalez et al. paper first and it was interesting to keep their criticisms in mind while reading the Dornelas et al. 2014 paper. It surprised me that the authors didn't mention any sort of potential bias in their dataset. They simply said, here are our results, and yes, they're robust. I'm glad their final paragraph emphasizes that "key habitats and ecosystems are under grave threat". I also appreciate their point that the change in species composition of ecosystems could completely change ecosystem services.
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DeleteJust as Willow mentions, I found interesting the analyses done in time series. I also found confusing the division in regions: in the figure the say the Tropical and Temperate together, but then they have different results for each, how did they do it?
ReplyDeleteI also found interesting what the point out at the end: even if species are replacing each other, they are not being replaced by "equal" species and this still might lead to major changes in the structure of the ecosystems.
About the regions in Laura's comment, I think they have temperate and tropical classified as separate regions, and then have a third region as tropical-temperate.
ReplyDeleteI also liked how they l looked at turnover rather than species lost. I think this is more accurate than the papers that simply focus on species lost.
I was also surprised by these results. Nonetheless, I´m not convinced if the results are as general as the authors claim. For example, tropical forests, which the authors claim are biodiversity hotspots are underrepresented in the sampling localities (Fig 1) as well as the terrestrial realm in general.
ReplyDeleteI guess this huge dataset will be further used to answer more questions. It would be very interesting to see which species are locally extinct and which ones replaces them and what environmental or species-related factors influence this turnover.
Figure 1 and 2 look really cool. Like Laura, I thought it was really interesting that changes in species do not result in substitution of like with like. I thought this was a slightly hard to read paper, because I'm not really familiar with some of the concepts. So hopefully the discussion will clear some things up.
ReplyDeleteSpecies has been used as the smallest unit of a community. They talk about function and productivity. I wonder if someone has actually used another unit to measure productivity in a single community. This is a great mathematical representation of large-scale diversity analysis. I wonder even if exotic species would take over a community (leads homogenization), would species still speciate and make up the community? The paper makes me little relief. And if the community shift is the true case, then humanity still has to face more challenges to adopting the new sets of the assemblages. The paper is great: short and good visualization.
ReplyDeleteI loved looking at figure 1. Also Lam brought up a good point about some of the concepts that were used since we didn't really talk about them along with some of the acronyms that were used such as MCMC... what is that?
ReplyDeleteLove the figures in this paper, each was unique to me. Going off of Maria, it seems a trend that scientists forget or disregard biases in their research. Do you think that they omit a bias discussion to make their paper look better? Is it something that's gotten better or that people have a keener eye for it?
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