| Advisor : | ELSA CLELAND | ||
|---|---|---|---|
| Abstract Title : | The Effects of Competing Species on the Timing of Germination and Fitness of Stipa pulchra | ||
| Abstract : | Invasions by exotic species can affect community dynamics by changing resource availability, which can result in shifting community compositions. In Mediterranean climate regions, exotic species often exhibit phenological traits that are distinct from native species present within the community. In these systems, exotics generally germinate earlier than natives, which allows them to take advantage of their temporal niche by utilizing resources that otherwise would not be available later in the season. Even small differences in germination timing can have effects on overall plant fitness and competitive dynamics, and may vary based on species identity. To address how these factors affect one focal species, we performed a selection experiment in the UCSD greenhouse on a native perennial grass when grown with a native or exotic competing species that also varied in functional group (grass or forb). Specifically, we recorded the timing of germination of the native focal species, Stipa pulchra, when grown in conjunction with one of eight competing species ? for a total of eight treatments. Height measurements, used as a proxy for fitness, were taken one month after recording germination times. We hypothesized that earlier germinants would be larger than their later germinating counterparts, and that the benefit of early germination varies according to the identity of the competing species. Results from the experiment shows that earlier germination has a positive effect on height of S. pulchra (p<0.001), though competitor identity does not have a significant effect on height. Also, although not significant, there was a trend in which the S. pulchra individuals grown with competing grasses were smaller than those that were grown with forbs, which may indicate that within a functional group, resource uptake is similar, and therefore there are stronger competitive dynamics. As exotic competitors continue to displace S. pulchra populations throughout California grasslands, in order to survive changing climate and disturbance conditions, S. pulchra will likely need to exhibit a phenological plasticity on par with its exotic invasive competitors. | ||
| Advisor : | ELSA CLELAND | ||
|---|---|---|---|
| Abstract Title : | Carbon Storage Along a Natural Precipitation Gradient Under Shrub and Grass-dominated Communities | ||
| Abstract : | Soils store large amounts of terrestrial carbon, and pool size may be altered as a result of climate change. In California, shifting precipitation regimes often precede shifts in aboveground community composition from shrub-dominated to exotic grass-dominated systems; both are factors that may influence soil carbon pools. I investigated how carbon pools are impacted by precipitation and community composition by collecting soil cores from eight University of California Natural Reserves along a naturally occurring precipitation gradient from San Diego, CA to Santa Cruz, CA. At each site, soil cores from both grass and shrub-dominated plots were collected. The soil cores were analyzed for belowground productivity and soil carbon. Aboveground productivity measurements were also quantified at each site. These measurements can be used to calculate carbon storage, which can then be compared across the gradient in order to extrapolate the effect of precipitation on storage capacity in two dominant community types. I hypothesize that the drier, more southern sites will have smaller soil carbon pools as a result of decreased productivity. Additionally, I postulate that the shrub-dominated plots will have greater soil carbon pools than the grass-dominated plots at each reserve. Through an analysis of organic carbon content in the soils of reserves along the coast, I hope to establish a better understanding of the role of precipitation and community composition on soil carbon pools. | ||
| Advisor : | STANLEY LO | ||
|---|---|---|---|
| Abstract Title : | Functional and genetic analysis of soil microbiomes of native vs. invasive plants at Scripps Coastal Reserve. | ||
| Abstract : | Soil microbiomes are closely tied to the plants with which they coexist. Plants provide resources to soil microbes, while microbial communities synthesize materials necessary for plant survival in a symbiotic relationship. Introduction of invasive plants, however, can disrupt this relationship between plants and microbial communities. We conducted a series of experiments to determine whether there were significant functional and genetic differences between the microbial communities in invasive plant soil versus native plant soil at Scripps Coastal Reserve, located in the California Floristic Province biodiversity hotspot. To test for functional biodiversity, we used Biolog Ecoplate to determine what types of carbon sources the microbial communities used. To determine genetic diversity, we isolated genomic DNA from soil samples and amplified 16S rDNA sequences by polymerase chain reaction (PCR). Amplified sequences were ligated into pGEM-T Easy vector plasmids and transformed into E. coli for direct colony sequencing to identify the operational taxonomic units (OTUs) of microbial communities for both invasive and native plant soil. In addition, we measured moisture content and pH in our soil samples. Invasive plant soil tended to be about 10 times more basic (about 1 pH unit) and have lower moisture content than native plant soil. We also found that functional differences (carbon source utilization) are minimal between soil microbiomes associated with invasive and native plants. For genetic biodiversity, DNA sequencing identified a wide variety and difference of phyla over the course of both seasons and soil types, and our data indicate that soil communities associated with our native plant species tended to have a higher evenness (based on Shannon Evenness index) compared to that of our invasive plant species. These results can shed insight into how microbes interact with their ecoystems and broaden our understanding of the relationship between plants and microbial communities. ADDITIONAL PRESENTERS: Brian T. Chao, Na Kyung (Sally) Cho, Jonathan T. Pham, Patrick C. Chang, and Christopher L. Steinke | ||
| Advisor : | KERRI SEGER, M.S., AARON THODE, PHD | ||
|---|---|---|---|
| Abstract Title : | How stereotyped is the call of a humpback whale? An investigation using acoustic tags from Los Cabos, Mexico | ||
| Abstract : | Brooke Hawkins1 Kerri Seger, M.S.1 Aaron Thode, PhD1 Acoustic data containing humpback whale social calls were collected during the 2014 and 2015 breeding seasons off Los Cabos, Mexico, by researchers at the Scripps Institute of Oceanography and the Universidad Autónoma de Baja California Sur. They attached noninvasive suction-cup Acousonde and B-probe tags built by Greeneridge Sciences, Inc., to the backs of several humpback whales. Custom-built MATLAB software from the Marine Physical Laboratory of Dr. Aaron Thode was used to hand-annotate multiple call types in the acoustic datasets recorded on these tags. In this study, I analyzed the variance of acoustic features in four humpback whale vocalizations ? ascending moan, descending moan, short moan, and descending shriek ? from an Acousonde recording of a mother whale and her nearby calf on May 7, 2014. Of the several acoustic features measured, the most variable for all four social calls was the maximum frequency (ascending moan: mean 2.463 kHz ± 1.153 kHz; descending moan: mean 1.882 kHz ± 1.130 kHz; short moan: mean 0.668 kHz ± 0.234 kHz; descending shriek: mean 2.269 kHz ± 0.788 kHz). Interquartile range and median values for the minimum frequency, signal-to-noise ratio, and call duration were also investigated to measure the amount of variation with which a pair of whales produces these four particular call types. This research contributes to a larger project to catalog the vocalizations of the humpback whale breeding population in Los Cabos, Mexico. Having baseline information about the expected amount of variation of a call type used by a mother-calf pair in one breeding population will aid in comparing the structure of call types among larger groups of humpback whales in the same location or even between groups from different breeding locations. | ||
| Advisor : | DR ELSA CLELAND | ||
|---|---|---|---|
| Abstract Title : | Interactive effects of altered precipitation and community composition regimes on seedling establishment in a coastal sage scrub ecosystem | ||
| Abstract : | Coastal sage scrub ecosystems face a number of challenges in the coming decades such as altered precipitation due to climate change and increasing rates of invasion by exotic annual grasses. Shrub seedling establishment is a critical timeframe that can be strongly affected by both changing precipitation and invasion regimes, yet remains poorly understood in the face of climate change. We recorded seedling height, diameter, and survival data over the course of four months for two native shrub species. This was done within a rainfall manipulation experiment with paired plots dominated either by native shrubs or exotic herbaceous species, subjected to treatments of 50%, 100%, or 150% of ambient rainfall. The study site was located in a coastal sage scrub ecosystem at the Santa Margarita Ecological Reserve (Fallbrook, CA). We found that seedling growth for the focal species Artemisia californica is significantly affected by rainfall treatment (p<0.001) with seedlings under the 50% treatment performing worse than those under the other two treatments (Tukey HSD p=0.001 and p<0.001, respectively). However, seedling performance between shrub or grass dominated communities varied according to species; California sagebrush (A. californica) seedlings consistently performed better in shrub dominated plots at an ecologically relevant level (p=0.064), while black sage?s (Salvia mellifera) performance was primarily determined by rainfall regime (p=0.031). Neither species exhibited positive growth under drought conditions. In combination, these results show while that climate-change induced drought may reduce invasion pressure, longer and more extreme droughts will be harmful to natives as well, particularly through the effect on seedling establishment and since fast growing invasive populations may recover faster during wet years. | ||
| Advisor : | ELSA CLELAND | ||
|---|---|---|---|
| Abstract Title : | Effects of Altered Precipitation on Native and Exotic Biomass Production in a Coastal Sage Scrub Ecosystem | ||
| Abstract : | Within California, interannual precipitation variability is expected to increase in coming years. Altered precipitation regimes can greatly influence plant community composition in ecosystems, such as coastal sage scrub systems, and in particular, can affect the abundance of native and exotic plants. Invasion pressure from exotic annual species into historically shrub-dominated systems tends to increase in abundance during high rainfall scenarios, however these dynamics are still poorly understood under various precipitation conditions. To better understand ecosystem level responses resulting from interactions between altered rainfall and dominant community composition, we performed a rainfall manipulation experiment at the Santa Margarita Ecological Reserve (Fallbrook, CA). At this site, there were a total of 30 plots receiving one of the three rainfall treatments (50%,100%, or 150% of ambient rainfall) and belonging to one of two dominant community composition scenarios (shrub or annual grass dominated). Within each of these plots herbaceous biomass from an area of 50 by 20 centimeters was harvested, dried, sorted as belonging to native or exotic origin or litter, and weighed. We found that biomass of native species was only affected by rainfall treatment (p = 0.038) with the 50% treatments having less native biomass than either the 100% or 150% treatments. Litter weight was affected by community composition (p < 0.001), being greater under shrub dominated plots, and there was also a significant interaction between community composition and rainfall treatment (p=.039) with litter biomass generally increasing with increasing rainfall in shrub dominated plots, but decreasing with rainfall in grass dominated plots. We also found that species of exotic origin excel within high rainfall treatments (p = 0.011), yet are also affected by community composition, producing less biomass under shrub dominated plots (p = 0.002). In conjunction, these results suggest that herbaceous species, regardless of origin, are equally likely to be affected by shifting precipitation, and that natives are less likely to be impacted by dominant community composition than are exotics. | ||