Exercise 2
Question Asked: Are latitudinal differences in dolphin sightings in the San Diego, CA survey area related to El Niño Southern Oscillation (ENSO) index values on a monthly temporal scale?
- My previous question for Exercise 1 was: do the number of dolphin sightings in the San Diego, CA survey region differ latitudinally? I was finally able to answer this question with a histogram of sighting count by latitudinal difference. I defined latitudinal difference as the difference from the highest latitude of dolphin sightings (the Northernmost sighting point along the San Diego transect line) to the other sighting points, in decimal degrees. Therefore it becomes a simple mathematical subtraction in ArcMap. Smaller differences would be the result of a small difference and therefore mean more Northerly sighting, with large differences being from more Southerly areas. I used all sightings in the San Diego region (from 1981 through 2015). As you can see from below, there is an unequal distribution of sightings at different latitudes. Because I had visual confirmation of differences at least when all sightings are binned (in terms of all years from 1981-2015 treated the same), I looked for what process could be affecting these differences in latitude.
Comparing the Latitudes with the frequency of dolphin sightings in San Diego, CA
ENSO is a large-scale climate phenomena where the climate modes periodically fluctuate (Sprogis et al. 2018). The climate variability produced by ENSO affects physical oceanic and coastal conditions that can both directly and indirectly influence ecological and biological processes. ENSO can alter food webs because climate changes may impact animal physiology, specifically metabolism. This creates further trophic impacts on predator-prey dynamics, often because of prey availability (Barber and Chavez 1983). During the surveys of bottlenose dolphins in California, multiple ENSO cycles have caused widespread changes in the California Current Ecosystem (CCE), such as the squid fishery collapse (Nezlin, Hamner, and Zeidberg 2002). With this knowledge, I wanted to see if the frequency of dolphin sightings in different latitudes of the most-consistently studied area was driven by ENSO.
Tool/Approach:
Primarily R Studio, some ArcMap 10.6 and Excel
Step by Step:
- 1.For this portion of the analysis, I exported my table of latitudinal differences within my attribute table for dolphin sightings from ArcMap 10.6. I saved this as a .csv and imported it into R Studio.
- Some of the sighting data needed to be changed because R didn’t recognize the dates as dates, rather as factors. This is important in order to join ENSO data by month and year.
- Meanwhile, I found NOAA data on a publicly-sourced website that had months as the columns and years as the rows for a matching ENSO index value of either: 1, 0, or -1 for each month/year combination. A value of 1 is a positive (warm) year, a value of 0 is a neutral year, and a value of -1 is a negative (cold) year. This is a broad-value, because indices range from 1 to -1. But, to simplify my question this was the most logical first step.
- I had to convert the NOAA data into two-column data with the date in one column by MM/YYYY and then the Index value in the other column. After multiple attempts in R studio, I hand-corrected them in Excel. Then, imported this data into R studio.
- I was then able to tell R to match the sighting date’s month and year to the ENSO data’s month and year, and assign the respective ENSO value. Then I assigned the ENSO values as factors.
- I created a boxplot to visualize if there were differences in distributions of latitudinal differences and ENSO index. (See figure)
Illustrating the number of sightings grouped by ENSO index values (1, 0, and -1). - Then I ran an ANOVA to see if there was a reportable, strong difference in sighting latitudinal difference and ENSO index value.
Results:
From the boxplot, it appears that in warm years (ENSO index level of “1”), the dolphins are sighted more frequently in lower latitudes, closer to Mexican waters when compared to the neutral (“0”) and cold years (“-1”). This result is intriguing because I would have expected dolphins to move northerly during warm months to maintain similar body temperatures in the same water temperatures. However, warm ENSO years could shift prey availability or nutrients southerly, which is why there are more sightings further south. The result of the ANOVA, was a p-value of <2e-16, providing very strong evidence to reject the null of hypothesis of no difference. I followed up with a Tukey HSD and found that there is strong evidence for differences between both the 0 and -1, -1 and 1, and 1 and 0 values. Therefore, the different ENSO indices on a monthly scale are significantly contributing to the differences in sighting latitudes in the San Diego study area.
Tukey HSD output:
diff lwr upr p adj
0–1 0.01161047 0.004250827 0.01897011 0.0006422
1–1 0.04101170 0.030844193 0.05117920 0.0000000
1-0 02940123 0.020689737 0.03811272 0.0000000
Critique of the Method(s):
These methods worked very well for visualization and finally solidifying that there was a difference on sighting latitude related to ENSO index value on a broad level. Data transformation and clean-up was challenging in R, and took much longer than I’d expected.
References:
Barber, Richard T., and Francisco P. Chavez. 1983. “Biological Consequences of El Niño.” Science 222 (4629): 1203–10.
Sprogis, Kate R., Fredrik Christiansen, Moritz Wandres, and Lars Bejder. 2018. “El Niño Southern Oscillation Influences the Abundance and Movements of a Marine Top Predator in Coastal Waters.” Global Change Biology 24 (3): 1085–96. https://doi.org/10.1111/gcb.13892.