by Julia, Olivia, and Thomas
Bishop O’Dowd High School
Emerita analoga, or the common Pacific sand crab, is an small animal with a colossal impact on beach ecosystems. We will examine the population trend in sand crabs over time at ocean beach, and examine
possible environmental factors that could have caused this, specifically El Nino. The sand crab is an indicator species, so examining the health of their population will allow us to examine the health of the sandy beach ecosystem as a whole.
Sand crabs are herbivores and an important food source for other beach organisms, specifically shorebirds. We assumed that if the sand crab population is thriving on a beach, then the shorebird population should be doing well too. We looked at data of sand crabs and shorebirds along the north-central California coast over the course of a year. Our original hypothesis was that if the population of sand crabs was high, then naturally the population of shorebirds would also be high. But, contrary to our hypothesis, a flourishing sand crab population means there are less shorebirds… More
In November of 2007, the container ship, Cosco Busan collided with a Bay Bridge support tower. The ship was fatally wounded, oozing 53,569 gallons of oil into the San Francisco Bay. The aftermath of this incident may have accounted for a steep drop in the sand crab population on San Francisco’s Ocean Beach in the years to follow. The oil spill may have affected the number of surviving recruits, or young sand crabs, especially since the mating season for sand crabs occurs in spring and summer, thereby affecting the number of recruits in 2008.
Our Marine Biology class participated in the LiMPETS sandy beach monitoring program on September 20th, 2014 at Ocean Beach. To survey the sand crabs living on the beach, we took 50 random samples of sand from the swash zone, or the zone of wave action. In each of these 50 samples, we observed any sand crabs that were found. We recorded their gender and assessed whether they were a recruit (young crab) or adult.
In general, human activities can positively and negatively influence the environment in which organisms inhabit. Every organism on this earth plays an important role in maintaining the balance of life no matter the size. Scientists may research the study of life on a small scale in order for us to better understand how small organisms can influence the environment in big ways. If in a community a small factor is harmed there might be consequences for those who heavily depend on it.
Our marine biology class participated in the LiMPETS sandy beach monitoring program on September 20th, 2014 at Ocean Beach located in San Francisco. We took 50 cores from the beach and recorded how many sand crabs were found per core. We did this so we could analyze and compare the sand crab population in the 11 years that data were collected. This is necessary in order to have a complete picture of any changes in the population.
According to Beyonce, girls run the world, but from Bishop O’Dowd’s sandy beach monitoring data, it’s female sand crabs with eggs the run Ocean Beach. Sand crabs are important animals on California beaches because they can indicate the health of the area of the beach in which they live. Also, they are an important species in the food chain. They are the primary consumers on the beach and many other species rely on their existence.
Over the past ten years, the amount of female crabs with eggs has fluctuated greatly. The crabs had fruitful mating seasons in 2009 and 2011, as shown by the graph below. Following the pinnacle of pregnant crab abundance in 2009, Ocean Beach’s sand crab population boomed (see year 2010 in the graph). Most of these crabs were juveniles and possibly offspring of the pregnant females from the year before. This population boom may be due to climate change; recent studies have shown our planet faces challenges that may be devastating for the species that make their home, reproduce, and live in sandy beaches.
Everybody loves going to a nice sandy beach. We set out to learn more about the health of beaches by analyzing the abundance of mole crabs at beaches near cities and beaches in more remote areas.
Although they are no bigger than the size of your thumb and seemingly unimportant creatures, mole crabs are very significant animals on sandy beaches. Their numbers are also indicative of the general health of the entire beach because they are in the middle of the food chain. This means that they are affected by both a scarcity in their prey or predators.
So, how do mole crab populations vary from urban beaches to remote beaches?
We collected data about sand crabs from Ocean Beach. From the data, we came up with a question about the sand crabs, and supported it with the data we collected. We compared our findings to that of other scientific resources, like the “Pacific Mole Crab” article. We were able to see if sand crabs changed from the time the articles were made to now. When we looked at the findings, we could come up with possible causes. We gathered the information and began to put it into a blog. This process helped us become more confident in science and in writing. It’s amazing that our research is contributing to the scientific world.