Modeling Kelp Forest Health
Sea urchins are natural grazers of kelp. Historically, their numbers have been kept in check by keystone predators in kelp forest ecosystems, namely sea otters and the sunflower sea star. However, contemporary sea otter populations tracked by the Annual California Sea Otter Census puts their numbers at 2,996 individuals, far less than the hundreds of thousands that used to inhabit coastal ecosystems worldwide. The sunflower sea star, on the other hand had not seen dramatic population decline until 2013, when a global pandemic of sea star wasting syndrome reduced the global population by over 90%.
In response to the lack of predatory pressure, sea urchin populations have grown dramatically, leading to the erasure of kelp forest via overgrazing and replacement by urchin barrens.
This project aims to map kelp forest abundance using Landsat satellite imagery and provide a statistical analysis of kelp forest trends before and after the onset of the global sea star wasting syndrome pandemic.
Kelp Area Time-lapse in Selected Areas
Santa Rosa Island
Southern Mendocino County
Statistical Analysis of Kelp Trends
Scatter Plot of Total Kelp Area for Each Quarter Between 1984 and 2013
Scatter Plot of Total Kelp Area for Each Quarter Between 2014 and 2023
Sampling With Replacement
Taking 20,000 samples with replacement from the pre-sea-star-wasting-syndrome data (1984-2013) generated the following distribution of slopes.
Using a left-tailed test comparing the post-wasting data (2014-2023) with the sampling distribution allows us to claim at a p-value of 0.043 that the trend of kelp forest abundance, measured by area in square meters, since 2014 is different than the trend beforehand. While this does not offer definitive proof that the onset of the sea star wasting syndrome pandemic in 2013/2014 is the cause of the decline, more thorough statistical analysis may reveal connections between kelp forest abundance and sunflower sea star abundance in the future.
All statistical analysis performed using StatKey
References In Order of Appearance
Hatfield, B. B., Yee, J. L., Kenner, M. C., & Tomoleoni, J. A. (2019). California sea otter (enhydra lutris nereis) census results, spring 2019. US Geological Survey. https://doi.org/10.3133/ds1118
US Fish and Wildlife Service. (n. d.). Historical and Current Sea Otter Distribution. US Department of the Interior. https://www.fws.gov/sites/default/files/documents/Sea%20Otter%20History%20and%20Distribution%2007282021.pdf
Harvell, C. D., Montecino-Latorre, D., Caldwell, J. M., Burt, J. M., Bosley, K., Keller, A., Heron, S. F., Salomon, A. K., Lee, L., Pontier, L., Pattengill-Semmens, C., & Gaydos, J. K. (2019). Disease epidemic and a marine heat wave are associated with the continental-scale collapse of a pivotal predator (pycnopodia helianthoides). Science Advances 5,eaau7042. https://doi.org/10.1126/sciadv.aau7042
Gravem, S. A., Heady, W. N., Saccomanno, V. R., Alvstad, K. F., Gehman, A. L. M., Frierson, T. N. & Hamilton, S.L. (2021). Pycnopodia helianthoides (amended version of 2020 assessment). The IUCN Red List of Threatened Species 2021: e.T178290276A197818455. https://dx.doi.org/10.2305/IUCN.UK.2021-1.RLTS.T178290276A197818455.en.
Rogers-Bennett, L. & Catton, C. A. (2019). Marine heat wave and multiple stressors tip bull kelp forest to sea urchin barrens. Scientific Reports, 9, Article 15050. https://doi.org/10.1038/s41598-019-51114-y
Harvell, C. D., Montecino-Latorre, D., Caldwell, J. M., Burt, J. M., Bosley, K., Keller, A., Heron, S. F., Salomon, A. K., Lee, L., Pontier, L., Pattengill-Semmens, C., & Gaydos, J. K. (2019). Disease epidemic and a marine heat wave are associated with the continental-scale collapse of a pivotal predator (pycnopodia helianthoides). Science Advances 5,eaau7042. https://doi.org/10.1126/sciadv.aau7042
Bell, T., Cavanaugh, K. & Siegel, D. (2024). SBC LTER: Time series of quarterly NetCDF files of kelp biomass in the canopy from Landsat 5, 7 and 8, since 1984 (ongoing) ver 23. Environmental Data Initiative. https://doi.org/10.6073/pasta/2c1218b7ebe6967da52000adf02f6a8b
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