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Unformatted text preview: eas dominated by another knapweed (spotted knapweed, C.
maculosa , or C. biebersteinii ) had 47% less forage for elk than did
areas where the weed was controlled. Heyer winter Yellow star thistle ( Centaurea solstitialis) — “knapweed ” family
Native to Mediterranean Europe/N. Africa. Probably arrived in California in
contaminated alfalfa seed.
Becomes dominant and appears to displace native bunchgrasses in western
grasslands. Spiny & toxic to most grazers/browsers. Changed vegetation structure • Comparison of the fibrous, soil-holding roots of the native wheatgrass
and the thin taproot of the spotted knapweed. 13 Community Ecology Changed vegetation structure Changed vegetation structure
English Ivy Ivy plants escape from
artificial landscapes to
overgrow forests in
California & Oregon
• Knapweed-dominated habitats have higher runoff than native
bunchgrass communities. Runoff increases depletion of the soil and
sediment load in local streams. Escalating invasion cycle: 1. Pink passionflower vines overgrow Hawai ’ian forests
2. Pink passionfruits attract and feed invasive feral pigs.
3. Pigs uproot native vegetation.
4. Pigs spread pink passionflower seeds.
5. Back to 1. – “light parasites ”
– Trees become top-heavy
and topple more easily A...
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