Semester Overview and Summation

12/16/11

During the course of this semester I undertook a Directed Readings Seminar with two members of the faculty at the University of North Carolina at Chapel Hill, Dr. James Umbanhowar and Dr. Charles E. Mitchell. Under their guidance and direction I critically examined approximately two sources of literature from the field ecology each week, wrote a combination objective and subjective blog about each, and then met with one of my advisors to discuss the work and its context within the larger field of ecology. The blogs for these papers are given as a transcript below, however the original sources can be found at the website address given in the title of this report.

The range of topics covered stayed mainly within the fields of invasive species ecology and disease ecology, from which both theoretical and experimental studies were analyzed. Generally, the method of article choice for the following week was based on intriguing topics that were embedded in the current weeks’ discussion. We began with an overview of the models used to map the spread of invasive species over time and space. From here we were able to expand into a variety of different types of models used in the ecological sciences, including traveling waves, stratified diffusion, and even the classic SIR models (Susceptible-Infected-Recovered). Several case studies were analyzed for classic epidemiological cases, including measles, Dutch Elm Disease, conjunctivitis of birds in the northeastern United States, influenza, and cholera.

From the experimentally-based literature sources we covered a variety of system choices and pathogen types. Marine diversity of pathogens and their relevance to the resource cycling of primary producers was looked at, along with the interaction of different kinds of pathogens within a single host, whether that host population was buffalo herds from Africa, or bacteria in culture.  We also spent some time investigating the role of lifestyle choices in viruses, whether a lytic (kill first) or lysogenic (incorporate into host genome) lifestyle could be favored in certain environments over others. We ended the semester with an exploration of the role of certain kinds of nutrients on pathogen dynamics.

In summation, a broad range of topics was covered during the course of this seminar. The subject material was chosen from both subfields of invasion and disease ecology. For the most part however, the string of literature sources was woven together from week to week to allow flexibility in topic choice, but also to allow growth and exploration of the more difficult questions presented within the ecological sciences.

Elser, J.J. et al. Ecology Letters. (2007).

"Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine, and terrestrial ecosystems".

Reviewed: 12/02/11

Past research has demonstrated the importance of nitrogen and phosphorus as limiting nutrients across a wide variety of habitats. The relative merits of each one as the primary nutrient of limitation however has been debated across systems. Does the biological demand for N and P depend on the system of study, namely terrestrial, freshwater and marine, or is the biological machinery necessary for photosynthesis and autotroph-ism similarly affected by nutrient additions or limitations across the globe? A meta-analysis comprised of a little more than 1000 separate experiments was the basis for this study. Experiments were drawn from a wide variety of habitat types across each system, and from multiple latitudes. Above- and below-ground experiments were also used for the terrestrial systems. The results of each study was converted into a log-transformed response ratio that looked at the effect of N, P, or N+P addition on some metric of community production or biomass. Only studies that manipulated both N and P within the experimental design were included in the meta-analysis.
Additions of nitrogen and phosphorus were found to induce a significant, non-zero, positive response across all system types. The community response to P additions, averaged across all habitat subtypes, did not significantly differ across the systems, though the response to N alone, as well as to N+P, did differ. Marine systems received a particularly large boost from additions of N relative to P. A synergistic effect was noted from the dual addition of N and P across all systems, whereby the addition of both limiting nutrients induced a larger response than the sum of each nutrient effect alone. The reason for this is unclear and may depend on the habitat of study. Say if for example bacterial autotrophs are better able to cope with reductions in N as long as they are not experiencing a simultaneous decrease in P.
Terrestrial and Freshwater systems were both shown to be equally limited by N and P, though marine was not as previously stated. Within systems however, the equivalence of N and P as limiting nutrients was not always true at the habitat subtype level. Grasslands were shown to be equally limited by both nutrients, while forests responded more readily to phosphorus additions.
The similarities in nutrient limitation across systems highlights the need for researchers to consider both major nutrients in their assessments of ecosystem dynamics, while the variation seen within habitat leads to the notion that every system of study should be assessed independently if possible in order for adequate conclusions to be drawn. The source, cycling, and use of nutrients within each system was not assessed in this article. Though these three dynamics may be of more relative importance within a system depending on the biological demand for a certain quantity, and quality, of nutrients. There are global scale differences in how nutrients are turned over within systems and where they come from. The merits of this study comes from its ability to draw large scale conclusions about community response to key nutrient additions, however a similar broad scale look at the internal dynamics of these nutrients within communities would also be beneficial to future assessments, particularly as they relate to global change.

Cronin, J.P., et al. Ecology Letters. (2010).

"Host physiological phenotype explains pathogen reservoir potential".

Reviewed: 12/02/11

The framework for approaching disease should not always be focused at the level of the individual or population of hosts only. The capacity any given individual has for disease directly affects how that disease might be transferred to other potential hosts within its community. The authors of this study set out to assess the reservoir potential of 6 different grass species, important to California grasslands, as they relate to the the viral pathogen Barley Yellow Dwarf Virus-PAV (BYDV-PAV) and the aphid vector Rhopalosiphum padi. Reservoir potential has been linked to three measurable epidemiological traits: the susceptibility of any given host to transfer of virus by the vector, the competence of the host to pass on the virus to a feeding aphid after the disease had become internally systemic, and lastly the host ability to support vector populations.
During the first Greenhouse experiment physiological traits were measured for the 6 different grasses, independent of viral infection, under conditions of low and high nitrogen. Principal Component Analysis (PCA) was able to condense the results of all five traits down to two primary principal component axes (PC1 and PC2). PC1 explained 50% of the variance in the results and was linked with a beforehand hypothesized continuum of 'Quick-Return' and 'Slow-Return' phenotypes, QR and SR respectively. The QR-SR continuum is a classification continuum that posits that quick growing host species will be more poorly defended but faster growing, while slow return species will allocate less overall to rapid, short-term growth but will have high defenses to enemies. The link of PC1 with this continuum was important for the researchers as it cohesively combined a complex array of traits into one definable axis of study.
Under the two nitrogen regimes, those grasses grown under higher nitrogen were consistently shifted more toward a QR phenotype, except for a single grass host.
The second part of the experiment was designed to test whether the conglomerate of physiological traits was sufficient to explain the three epidemiological parameters previously outlined, or whether host lifespan, phylogeny, and provenance (native vs. exotic) were also required. Nested models were used to test this theory by systematically dropping explanatory terms from the model, and using AIC for comparison. Host physiological traits, in the form of PC1, were consistently necessary to explain host reservoir potential. QR type hosts are more efficient as reservoir hosts, meaning that can support high levels of aphid vectros, become infected more easily, and transfer pathogens more easily. The authors propose host physiology as primary in importance for predicting epidemiological parameters, however the phylogenetic history and lifespan/provenance of a host is still relevant to the discussion, as shown by their significant inclusion in two of the paramters: vector population size and host susceptibility.
This study looked at five traits and ran them through a PCA, however future studies should expand beyond the nitrogen centered framework used here. Multiple nutrients are responsible for the dynamics seen in terrestrial systems, (eg Nitrogen and Phosphorus, as wel as other micronutrients). In this experiment nitrogen was shown to shift grasses towards being a better host. If phosphorus addition or faster growth do to rising CO2 levels do the same, then this will have important implications for global change analysts and the disease emergence in the future. Inclusion of more broad-scope traits would impove the results of this analysis.