Month: February 2019

Coexistence and Beyond

On the heels of our community structure discussion, these papers seemed like the perfect place to start discussing coexistence theory. How do species that are competing for the same resource coexist, when the principle of competitive exclusion suggests that the dominant species should outcompete all of the others.  This week we discussed two papers that looked at the balance between coexistence and competitive exclusion and how it can affect community assembly.

We started with the Paradox of the Plankton (Hutchinson, 1961) to get a historical perspective first on coexistence theory. Hutchinson explores the concepts of coexistence theory with phytoplankton as his model system. He wonders how in an un-structured environment there so many species could coexist when ultimately one should dominate the system. He poses several hypotheses that fall into two broad categories, either biotic interactions (via facilitation or predation) or environmental conditions (variability in conditions). Our discussions of Hutchinson’s work felt reminiscent of discussing parasite aggregation in hosts, wherein a few number of hosts support the bulk of the parasite community.  Similarly Hutchinson’s plankton seemed to demonstrate that a few number of species made up most of the abundance of plankton. He attributes this distribution to habitat heterogeneity, or temporal fluctuations in the environment, and suggests both as a mechanism driving the observed patterns of plankton coexistence.

In a more recent conceptual review, HilleRisLambers et al used coexistence theory as a tool to view community assembly. They highlight consider the main mechanisms, either stabilizing niche differences or relative fitness differences, both of which apply to between species differences, as well as frequency dependent population growth for within species dynamics. Using this approach, they evaluate the strengths and limitations of different empirical approaches to understanding the role of niche or fitness differences in community assembly, and ultimately advocate for a combination of approaches.  While combining approaches may not always be feasible, HilleRisLambers emphasizes the importance of recognizing and considering the limitations of your own approach and what information could make your conclusions stronger.

While both Hutchinson and HilleRisLambers approach coexistence from the perspective of environment vs biotic interactions, the latter expressly addresses multiple sources of stabilizing mechanisms including resource partitioning or storage effects, that were really formalized through the work of Chesson in 2000.  Hutchinson helped lay the groundwork for some of these ideas by introducing resource variation, differences in predation, and opportunism of different species.

Biodiversity: origins and obstructions

Following John Weins‘ seminar “Understanding the Origin and Future of Biodiversity Patterns” this week, we are well poised to pursue our first theme: biodiversity. His talk centered on the interplay of phylogeny, ecology, and species richness. In particular, he focused on clade diversity addressing if variation across the clades of life is determined by clade-age hypothesis or the diversification-rate hypothesis. We, however, will take a step back in our papers to look a much earlier perspective of biodiversity in Hutchinson and a contemporary look at changes in biodiversity through a field study by Uchida & Ushimaru.

Biodiversity didn’t surface as a term until the 1980s, with Thomas Lovejoy named as the “Father of Biodiversity”. At this time, the term was often synonymous with species diversity. Of course, ecologists now take a view of several axes of biodiversity: taxonomic, phylogenetic, and functional. Also, the early thought of biodiversity limited to primarily animals and plants, with parasites dynamics often unexplored in this context.

Hutchinson’s perspective of biodiversity stems from the hypothesis that resource heterogeneity drive biodiversity patterns. In his “Homage to Santa Rosalia”, he focuses largely on food webs and the mosaic structure of the world. He begins to link life-history traits to diversity patterns, showcasing animal size and habitat range as drivers of diversification (i.e. the more microhabitats an animal population is exposed to the more opportunities for divergence). This agrees with some of the contemporary work Wein’s presented on the importance of microhabitats for clade diversity. That an emergent process (in this case environmental exposures) produce different (more or less diverse) systems. Further, Hutchinson’s talk propels some early though on the yet-unnamed process of autocatalysis in evolution: diversity promotes diversity. This likely leads acts as a natural precursor and inspiration for E.O. Wilson’s theory of biophilia. Ultimately, Hutchinson talk is both grounded in theory as well as promoting the continued exploration of life’s mysteries.

An experimental approach to biodiversity, Uchida and Ushimaru test how agriculture practices in Japan impact plant and insect diversity. Their study supports the intermediate disturbance hypothesis, with the traditionally-managed plots showcasing certain rare species that are lost in both intensively-managed and abandoned plots. Uchida’s study is unique in looking at multiple trophic levels, as well as various relevant agricultural practices in the area. It would be interesting if there are historical records of plant and insect to couple these agriculture practices with pristine areas. With the growing anthropogenic change across the globe, it will become impossible to decouple natural patterns of diversity with the dynamic impacts of change due to agriculture, invasive species, and climate change (to name a few).