ELSEWHERE IN THE NEWS 



Cornell Ecologists Study 

 Invasives 



Among 473 of the alien plant spe- 

 cies that have invaded from Europe 

 and become naturalized in the 

 United States as noxious vkfeeds, the 

 "most successful" traveled light — 

 carrying fewer plant diseases from 

 their native habitats — and were 

 more immune to New World plant 

 diseases. 



That is the conclusion of Cornell 

 University ecologists after examining 

 plant-health records on both sides of 

 the Atlantic. The study, reported in 

 the latest issue of the journal Na- 

 ture (Feb. 6, 2003) by Charles E. 

 Mitchell and Alison G. Power as 

 "Release of invasive plants from fun- 

 gal and viral pathogens," is particu- 

 larly significant in that it reconciles 

 two theories, dating back to Charles 

 Darwin in 1859, about successful 

 naturalization of invading species. 



Their findings, the ecologists say, 

 should encourage biological-control 

 strategists to look for weed-control 

 pathogens both in the invading 

 weeds' native and adopted habitats. 

 However, they warn that biological 

 control can negatively impact native 

 species and is no panacea. What 

 most surprised Mitchell and Power 

 was the finding that pathogens can 

 help keep invasive plants in check. 



"We're coming to realize we 

 should be grateful for our native 

 plant pathogens," adds Mitchell, a 

 postdoctoral researcher in Cornell's 

 Department of Ecology and Evolu- 

 tionary Biology. "Invasive plants 

 cost an estimated $33 billion a year 

 to the U.S. economy, but the dam- 

 age would be worse were it not for 

 our native fungi and viruses that 

 control invasive plants to some de- 

 gree." 



A parallel study of invasive ani- 

 mals, ranging from moUusks to 

 mammals, reported in the same is- 

 sue of Nature as "Introduced species 



and their missing parasites" by 

 ecologists at the University of Cali- 

 fornia-Santa Barbara and Princeton 

 University, reached similar conclu- 

 sions about aliens' success in new 

 lands. 



Both the plant study and the ani- 

 mal study sought data for two long- 

 standing and much-debated theories, 

 explains Power, a professor of ecol- 

 ogy and evolutionary biology who 

 also serves as dean of the Cornell 

 University Graduate School. "The 

 enemy-release hypothesis argues that 

 invaders' success results from re- 

 duced attacks by natural enemies 

 from their native habitat, while the 

 biotic-resistance hypothesis says in- 

 vaders' impacts are limited by inter- 

 actions with native species, includ- 

 ing natural enemies, in their new 

 habitat," she notes. " Our study 

 found that both factors — enemy re- 

 lease and biotic resistance — are im- 

 portant in determining whether an 

 invading plant species thrives to be- 

 come a noxious weed or struggles to 

 survive." 



The Cornell plant study began 

 with 4,100 naturalized plant species, 

 regarded by the U.S. Department of 

 Agriculture as invaders surviving in 

 wild populations in the United 

 States without human intervention, 

 and focused on 1,165 randomly 

 chosen species. A further focus on 

 plants from Europe, particularly the 

 Mediterranean region, narrowed the 

 study to 473 species, such as leafy 

 spurge, sulphur knapweed and Rus- 

 sian thistle. The Cornell ecolo- 

 gists then compiled information on 

 viral and fungal infections of the 

 473 species in their native and 

 adoptive habitats. Mitchell credits 

 Cornell undergraduates Jennifer 

 Gardell and Brian Youn for assis- 

 tance in what he calls a "mammoth 

 data-mining task. For the first time, 

 we were able to bring real numbers 

 to the theories." 



The results: Invasive plants in the 

 United States, on average, have 77 

 percent fewer diseases (84 percent 

 fewer fungal diseases and 24 percent 

 fewer viral diseases) compared with 

 the same species in their native Eu- 

 ropean habitats. (Viral diseases are 

 harder for plants to escape because 

 the viruses can travel, systemically, 

 in the plants or in their seeds, the 

 ecologists note.) And invading 

 plants that had acquired the most 

 pathogens in their naturalized ranges 

 were less likely to become wide- 

 spread, noxious weeds that are 

 costly to agriculture. "These results 

 suggest that invasive plants' impacts 

 are a function of both release from 

 and accumulation of natural en- 

 emies, including pathogens," the 

 Cornell ecologists reported in Na- 

 ture. 



A second phase of the study — 

 supported by the National Center 

 for Ecological Analysis and Synthe- 

 sis, the National Science Founda- 

 tion, the U.S. Department of Agri- 

 culture and Cornell University — is 

 being planned to consider the role 

 of herbivorous insects and soil-borne 

 plant diseases in controlling invasive 

 plant species. In the meantime, this 

 confirmation of the enemy-release 

 and biotic-resistance hypotheses 

 should inform efforts to control in- 

 vasive plants, Mitchell says. 



"This is also an answer for those 

 who wonder, what has Mother Na- 

 ture done for us lately? We continue 

 to receive free service from our 

 natural ecosystem in the form of 

 pest control," Mitchell says. "The 

 natural process of pathogen infec- 

 tion is helping to prevent invasive 

 plants from becoming worse pests to 

 humanity — without any subsidy 

 from humans." 



Feb 2003 Cornell University News 

 Service 



SPRING 2003 



15 



