ECR feature: Adriana Uscanga on the origins of tropical sky-islands arthropod communities

Adriana Uscanga is a PhD candidate at the University of Oregon in the US. She is an ecologist interested in unveiling the evolutionary drivers shaping biodiversity distribution in tropical mountains. Here, Adriana shares her recent work on evaluating colonization routes of newly emerged high-elevation habitats.

Adriana Uscanga is a PhD candidate at the University of Oregon.

Personal links. Twitter | GitHub | Orcid

Institute. University of Oregon

Academic life stage. PhD candidate

Major research themes. Tropical Mountain Ecosystems; Landscape Ecology; Geospatial Science; Agroecology; Climate Change; Cartography and Data Visualization

Here, I am collecting pitfall traps in an alpine grassland at the top of the mountain Ajusco, Mexico.

Current study system. My research focuses on the study of tropical landscapes, especially mountains. Tropical mountains exhibit outstanding climatic and ecosystem diversity in short distances, making them fascinating study systems. I am interested in understanding what shapes biodiversity and ecosystem functions in these landscapes. Currently, I am studying the ecological and socioeconomic processes and consequences of agriculture expansion and intensification in Mexican tropical montane forests. In so doing, I use theories and methods from Landscape and Forest Ecology, Geospatial Science, and Political Ecology to understand the complex relationships between forest ecosystem functions and services, food production, and climate change in these beautiful landscapes.

Recent JBI paper. Uscanga, A., López, H., Piñero, D., Emerson, B.C., Mastretta-Yanes, A. (2021) Evaluating species origins within tropical sky-islands arthropod communities. Journal of Biogeography, 48(9): 2199-2210

Alpine grasslands at sunset. Fir forest is visible further below. The difference in elevation between these two ecosystems in the Transmexican Volcanic Belt ranges between 600 to 800 m.

Motivation behind this paper. This study sought to understand how biodiversity is structured along the Transmexican Volcanic Belt, a long tropical mountain range that crosses Mexico from West to East. Previous studies had suggested that animal and plant communities in the Eastern (E) mountains differed from those distributed in the Western (W) mountains, as if biodiversity was structured in two groups, divided in the middle of the mountain range. Although this pattern came repeatedly in the literature, studies that would explicitly test it were lacking. Our main motivation was to test this pattern with a spatially explicit study and to assess whether this pattern varied in ecosystems at different elevations. This last objective would provide information about how new habitats are colonized.

Fir forest viewed from the understory.

Key methodologies. To test whether biodiversity along the Transmexican Volcanic Belt is structured in two groups (a Western and an Eastern group), we sampled arthropods in two ecosystems, alpine grasslands (at around 3,900 m in elevation) and fir forests (at around 3,200 m in elevation), at seven mountains using pitfall traps. We selected all spiders and beetles we found in the traps, classified them in morphotypes, and sequenced a region of the mtDNA COI gene from each individual. In this way, we were able to use classic population genetics methods for studying whole communities. Besides assessing richness and diversity within and among mountains, we used a neutral model to test the W-E groupings. We were also interested in addressing whether the W-E spatial pattern was present in different ecosystems. Thus, we compared grassland and forest communities.  

Unexpected challenges. We faced several challenges in this research. First, we realized that lists or inventories of arthropod communities at these sites are rare. In many cases, species have not been described yet. Because of this, we had to use morphotypes and operational taxonomic units instead of species. Second, doing fieldwork was fun but very challenging. Alpine grasslands are at very high elevations, and we had to hike all the way up the top of these mountains to set up the traps and then again to collect them a week after (no wonder why so many species have not been described yet!). An unexpected outcome was the striking difference we found between grasslands and forests communities within single mountains. Throughout the project, we were mainly focused on the W-E structure story, and the difference between ecosystems turned up to be just as − or even more − interesting in the end.

Alpine grassland and fir forest in the Western mountains of the Transmexican Volcanic Belt.

Major results. Two main results were drawn from this research. First, the difference between W and E communities is larger than expected if only present-day distance among mountains is considered. This pattern is probably related to the geologic history of this mountain range. Second, we found that communities from different mountains, but the same ecosystem, share more species among them than those within a single mountain but different ecosystems. This pattern was more common among lower elevation ecosystems (forests) than higher elevations (grasslands). Our results suggest that the long-distance colonization from similar ecosystems may be a more common pattern than the colonization of new habitats from lower elevations within the same mountain. Also, geologic history and ancient communities leave a strong trace on diversity structure despite recent processes of migration and connectivity. Our results contribute to broadening the understanding of how different factors like geology and climate shape biodiversity.

Next steps for this research. The next step in this research, led by Dr. Mastretta-Yanes, is to explore entire arthropod communities using metabarcoding data to sequence thousands of specimens from hundreds of pitfall traps and examine diversity patterns in multiple hierarchical levels (haplotype, putative species and supra-specific levels). With this high-resolution data, we are examining the role of neutral processes in promoting differentiation not only among mountains but also within a single mountain. With this analytical framework, we will assess if dispersal limitations within and between mountains can act as a source of local-scale genetic differentiation in tropical mountains, which may translate to species-level diversification over time.

Pitfall trap in alpine grassland (left), specimen from the family Curculionidae (middle) and specimen from the family Linyphiidae (right). We made our own traps using plastic cups, brown paint, alcohol, and glycerine. We selected all beetles and spiders that fell in the traps and took photos of every specimen we classified and sequenced. Sequences and photos are available through BoldSystems.

If you could study any organism on Earth, what would it be? I love elephants, octopuses, ferns, and magnolia trees. Actually, I’m fascinated by many organisms, but I always find myself studying processes and interactions more than single units or organisms. I really feel passionate about studying interactions among living beings and between these and their environment.

Anything else to add? I was first involved in this research because of my interest in learning population genetics and phylogeography. Tropical mountains happened to be the system model, but they were not my main focus. With time, as I developed my research further, I got more interested in tropical mountains as a study system. They are so fascinating! They are also under a lot of stress due to deforestation and climate change. Now, I keep working on tropical mountains but with a different focus. I study patterns of forest loss, their effects on ecosystems and people, and their interactions with climate change. Science and curiosity can take you on so many different paths!

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