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Biological Sciences Student Research Showcase 2010
Ecology, Behavior & Evolution Abstracts


Photophysiological acclimation of Thalassiosira pseudonana under light limitaton

Elliot Weiss
Dr. Greg Mitchell

Models determining primary productivity and pigment composition for remote sensing instruments such as satellites, moorings, and aircrafts rely on an improved understanding of the photophysiological dynamics that exist within the phytoplankton cell, such as photosynthesis, light absorption, and pigment composition. Hence, such relationships have been investigated in a number of various phytoplankton species. Cultures of Thalassiosira pseudonana, a widespread diatom, were grown under six different irradiance levels ranging from 31.4 to 1332 μmol quanta m-2 sec-1 at 20°C. The amount of chlorophyll per cell decreased from 0.4 to 0.06 pg Chl a cell-1 with increasing light. The chlorophyll-specific absorption (a*ph)of Thalassiosira at 436 nm increased from 0.018 to 0.049 m2 mg Chl a-1 under an irradiance range from lowest to highest, respectively. The variation present in the chlorophyll-specific absorption at 436 nm is attributed to the pigment packaging effect. The values of a*ph (676 nm) ranged from 0.013 to 0.015, a magnitude of nearly 2-fold. This range of absorption at 676 is demonstrative of significant pigment packaging, due in part to cell size and pigment composition. In order to improve upon current models for remote sensing instruments, such variables must be taken into consideration when employing optical systems.


Spatial Analysis of Cactus Plants and Rots in El Saguaral, Mexico

Douglas Hooton
Dr. Therese Markow

This project involves the analysis of a long term data set collected by Dr. Therese Markow and Dr. Sergio Castrezana on the location, size, and presence of rots and flies for over 500 cacti in a small section of the Sonoran Desert near El Saguaral, Mexico.  The analysis performed looked for evidence of clustering in the distributions of the three species of cactus, as well as clustering of each species with regards to the others.  The clustering patterns produced from that analysis was further scrutinized looking for possible correlation with cactus size  or altitude.  Finally, the data on the presence of rots on the cacti was checked for correlation with the size of the individual cacti as well as the spatial density of cacti.


Variation in Water Use Efficiency and Soil Moisture Conditions of Eight Coastal Sage Scrub and Chaparral Species in Relation to Water and Nitrogen Treatments

Marina LaForgia
Dr. Elsa Cleland

Plants communities of southern California are especially adapted to the seasonality of precipitation and limited nutrient availability, making this ecosystem particularly vulnerable to nitrogen deposition and altered precipitation regimes. We looked at the water use efficiency (WUE) and soil moisture conditions in eight species belonging to the coastal sage scrub and chaparral communities of southern California that were subjected to a high and low water and nitrogen addition treatments. Midway through the experiment, half of the plants of all species were subjected to a change in watering treatment, from high to low and low to high water and were compared to plants that were kept in the constant watering treatment. To measure WUE we used the LI-COR 6400XT Portable Photosynthesis System and soil moisture was measured using the TDR 100 Soil Moisture Meter. We expect plants with low WUE to respond more to high watering, but will have the lowest resilience when moved from a high to low water treatment. We expect species with low water use efficiency to have to have drier soil moisture conditions and high leaf transpiration even under high watering treatment conditions.


Top-Down versus Bottom Up Control on Plant Productivity and Diversity

Amanda Schochet
Dr. Elsa Cleland

Ecologists have long sought to understand the controls on the diversity and productivity of ecosystems.  These controls are often divided into top down forces(such as herbivory) controlling productivity and diversity from higher trophic levels, versus bottom - up forces (particularly nutrient availability).  This experiment sought to evaluate the relative strength of top-down versus bottom-up control in a semi-arid grassland community in the UC Elliott Chaparral Reserve.Replicated plots were fertilized with factorial combinations of nitrogen, phosphorus and/or potassium.  An additional set of plots were fenced to exclude herbivores,these either had no fertilization treatment or had all three nutrients added.  We found that in general productivity increased, and species diversity declined, with fertilization.  This effect was even more apparent when herbivores were excluded.One potential mechanism for the negative relationship between productivity and diversity is light limitation as productivity increases.  Thus by removing biomass,herbivores may alleviate light limitation and help to maintain species diversity,especially in highly productive ecosystems.


Adaptive Evolution of Odorant Receptors in Cactophilic Drosophila

Joel Schumacher
Dr. Luciano Matzkin

Cactophilic Drosophila rely on their senses, particularly smell and taste, to locate necrotic cacti tissue of their particular host species. A particular odorant receptor gene, Or83c2, was amplified by polymerase chain reaction (PCR) and then sequenced in four populations of D. mojavensis and one population of its sister species D. arizonae. Analysis showed that their was a significantly higher number of non synonymous fixed differences in the sequences, indicating positive selection. Polarization of the data showed that the changes had occurred in D. arizonae.