no, its not arid enough for prickly pear to be a native species. but there's other prickly plants, some will even leave a sliver. EDIT: No, I failed to comb every square inch of the park. But you.can feel free to do that since the existence of the Pacific Northwest Prickly Pear is one you seem quite concerned with. I answered your question based on these criterion: I have a bachelor's degree in environmental sciences, I used to live in New Mexico for quite some time, now I live in the Pacific Northwest. You will have better luck finding DB Cooper at Fort Ebey State Park than you will a prickly pear.
What she said
Have you confirmed it yourself by exploring the whole park though? Opuntia fragilis does live in Western Washington in as many as 6 counties (Clallam, Island, Jefferson, San Juan, Skagit and Whatcom). According to what I've read in occurs throughout Whidbey Island. Cacti are not endemic to deserts from warm temperate regions, they grow in virtually all ecosystems in the Americas except temperate rainforest and northern tundra. I was wondering if this park was one of the places in Whidbey where it lives. That other prickly plant you mention, is it thistle?
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