OvtT llh' piist KH) years jihiiit imniignints lia\e arrixcd in tin' United Stales in luig<' niinihcrs, the resiills of accidrrilal ami (lelilieraU* iiilroduclltin l»y Iiuinaiis. \rr\ (r\\ inimi^rati (His, Iiowcvcr. resnil in nalurali/aliims; iIk^ rnviroiinunilal Iia/ai(]s for iniini^ranls in a new lan^r are usuall) aeule and ilirotilc. W<-traced llie history of inlrodnetion anion*-tlie nalurali/ed ta\a uilliin a ^rou|> of U.S. regions and stales llial >|)an inueh of llie einirontnental an4)lilude of the United Stat<»s. Despite dillerenees among llii^se n^^ions in ]>hysieal environments and the lenglli of llu'ir histories of plant immigralion. the pro[)orlion of their natnrali/eouri-es of riu'(hcine. Ersluhile t)rnaineiitals are promlui'nt componn for llieir persisler of which non-indigenous sj)iH-i(\s will persist. Tins fa...

Anthemiscotula was introduced to Denmark 500 years ago, and its distribution is presently limited and in decline. A manipulative field experiment was performed to investigate the effects of native plant competitors and native invertebrate herbivores on its performance. Generally, both herbivory and competition treatments had great impact, and when both factors were operating, the effects were additive for all variables except plant height. Although A. cotula showed plasticity in growth, resource allocation and flowering timing, it was unable to adjust to competition and compensate for losses due to herbivory sufficiently to ensure and restore its achene production. This vulnerability, combined with improved cereal cleaning techniques and thus fewer reintroductions of A. cotula seeds, may be the cause of its current decline. A. cotula responded to herbivory by prolonging its flowering period, a "bet-hedging" strategy. In Denmark this strategy is unreliable since risks of sub-optimal conditions are much greater in August-October.

In Denmark Cypripedium calceolus is found in only two small and isolated populations, and its fruit set is suspected to be pollinator‐limited. Collection and observations of flower visitors were carried out between 1995 and 1997, and fruit set were estimated. Forty‐four visitors were collected. The majority belonged to the genera Halictus, Lasioglossum and Andrena. The latter is known as pollinator of C. calceolus in other parts of Europe, but in this study Andrena spp. were both not regular visitors and too large to function as efficient pollinators. Medium‐sized halictine species, especially females of Halictus tumulorum and Lasioglossum calceatum, were the most regular and frequent pollen vectors. Andrena jacobi and Lasioglossum rufitarse are recorded for the first time in Denmark.