1. Human Influence Weed Evolution de Wet & Harland (1975; Weeds & domesticates: Evolution in the man-made habitat. Econ. Bot. 29:99-107)de Wet & Harland (1975; Weeds & domesticates: Evolution in the man-made habitat. Econ. Bot. 29:99-107) –Weeds, domesticates, & wild plants plants react in different ways to human disturbancesplants react in different ways to human disturbances –some flourish, some die & are replaced

2. Human Influence Weed Evolution de Wet & Harlandde Wet & Harland –Weeds, domesticates, & wild plants wild plantswild plants –naturally occur outside human disturbed habitats –Some are aggressive colonizers »when habitat not frequently disturbed, successive groups of different species occupy sites (invade) until dynamic, but stable population occurs »small changes

3. Human Influence Weed Evolution de Wet & Harlandde Wet & Harland –Weeds, domesticates, & wild plants weeds & domesticatesweeds & domesticates –occupy human disturbed habitats –neither compete successfully with wild plants »Weeds may invade disturbed habitats, but generally replaced by wild plants if disturbance does not occur again »Invasive & noxious weeds??? –weeds will colonize human disturbed habitats »do not need assistance from humans »domesticates need assistance »domesticates can be weedy

4. Human Influence Weed Evolution de Wet & Harlandde Wet & Harland –weeds evolved 3 ways from wild colonizersfrom wild colonizers –adaptation to & selection for continuous habitat disturbance derivatives of hybridizationderivatives of hybridization –between wild and cultivated races domesticates abandoned domesticatesabandoned domesticates –selection for a decreased relationship with humans

5. Human Influence Weed Evolution Weeds associated with disturbed habitats:Weeds associated with disturbed habitats: –Sen, D.N. 1988. Key factors affecting weed-crop balance in agroecosystems. In M.A. Altieri & M. Liebman, eds. Weed Management in Agroecosystems: Ecological Approaches. CRC Press. three types disturbed habitatsthree types disturbed habitats –degraded –ruderal –cultivated

6. Human Influence Weed Evolution Weeds associated with disturbed habitats: (definitions)Weeds associated with disturbed habitats: (definitions) –degraded habitat where disturbance of original plant community is incomplete and sporadicwhere disturbance of original plant community is incomplete and sporadic –rangeland »grazing pressure not severe –flood plains –fault lines

8. Human Influence Weed Evolution Weeds associated with disturbed habitats: (definitions)Weeds associated with disturbed habitats: (definitions) –ruderal habitat area not being used for economic crop production but where original (& succeeding) plant community repeatedly destroyedarea not being used for economic crop production but where original (& succeeding) plant community repeatedly destroyed –roadsides –industrial areas –developments –other highly disturbed areas

11. Human Influence Weed Evolution Weeds associated with disturbed habitats: (definitions)Weeds associated with disturbed habitats: (definitions) –cultivated habitat area being used for economic crop production (agroecosystems)area being used for economic crop production (agroecosystems) –cultivated fields –pastures »high use –moderately to highly disturbed

12. Human Influence Weed Evolution Weeds associated with disturbed habitats:Weeds associated with disturbed habitats: –all disturbed habitats conducive to weed invasion human manipulation (slight to severe) creates ideal situation for weed invasionhuman manipulation (slight to severe) creates ideal situation for weed invasion –related to weedy characteristics in natural systems weeds classified as invaders, increasers, or pioneersin natural systems weeds classified as invaders, increasers, or pioneers –weeds may be thought of as primary invaders or pioneers of secondary succession –Invasive weeds relatively new classification

13. Human Influence Weed Evolution Weeds associated with disturbed habitats:Weeds associated with disturbed habitats: –weed invasion follows concepts of secondary succession continued disturbance re-initiates secondary successioncontinued disturbance re-initiates secondary succession –cultivated environment an extreme example –human use of environment creates disturbance weed mgmt essential to allow for intended land useweed mgmt essential to allow for intended land use

14. Human Influence Weed Evolution Selection pressures for weed evolution or changes in weed flora:Selection pressures for weed evolution or changes in weed flora: –Froud-Williams, R.J. 1988. Changes in weed flora with different tillage and agronomic management systems. In Altieri & Liebman two types selection pressurestwo types selection pressures –general selection (indirect) –specific selection (direct)

15. Human Influence Weed Evolu tion Selection pressures for weed evolution or changes in weed flora:Selection pressures for weed evolution or changes in weed flora: –general selection or indirect selection from changes in crop production methods or land management methodsfrom changes in crop production methods or land management methods –amending soil conditions –manipulation of crop densities or spatial arrangements –also manipulation of plant communities in natural systems »e.g. seeding efforts »e.g. changes in grazing management

16. Human Influence Weed Evolution Selection pressures for weed evolution or changes in weed flora:Selection pressures for weed evolution or changes in weed flora: –specific selection or direct selection changes in weed control methodschanges in weed control methods –improved seed cleaning –improved physical control (mechanical) –adoption of &/or improved chemical control –adoption of &/or improved biocontrol »including changing grazing mgmt to simulate biocontrol

17. Human Influence Weed Evolution Selection pressures for weed evolution or changes in weed flora:Selection pressures for weed evolution or changes in weed flora: –effects of (changing) land mgmt practices on weed communities are indirect and direct –changes that occur in plant (weed) communities may involve alterations within a population (species) species shift more likely or more importantspecies shift more likely or more important –Changes favor some plant species at the expense of others

18. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Edaphic factors soil pH modifications:soil pH modifications: –liming causes decline red sorrel, knawel, corn spurry

19. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Edaphic factors several spp indicators of lime deficiencyseveral spp indicators of lime deficiency –success on acid soils due to no competition »redtop invades acid soils where barley less vigorous but not in limed soils where barley more competitive chickweed and speedwell depressed by ammonium sulfate that lowered pHchickweed and speedwell depressed by ammonium sulfate that lowered pH –prefer alkaline soils

21. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Edaphic factors Park Grass ExperimentPark Grass Experiment –additions of ammonium sulfate lowered pH »redtop dominated –when limed »tall oatgrass and meadow foxtail dominated

22. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Drainage & irrigation effects: drainage (England) decreased:drainage (England) decreased: –toad rush, field mint, mousetail, marshpepper smartweed flooding in rice cultureflooding in rice culture –may suppress barnyardgrass and nutsedge –although sometimes flooding worsens nutsedge »biotype?

23. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Drainage & irrigation effects: irrigation:irrigation: –can contribute to weed problems thru dispersal »Washington 1975 non-irrigated soil averaged 125,000,000 seeds/ha; 5 yr surface irrigation averaged 500,000,000 seeds/ha »Nebraska 1980 77 weed species found in irrigation water; 48,000 seeds/ha introduced through irrigation water

24. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Soil fertility: Upland rice purple nutsedge, barnyardgrass more competitive under high NUpland rice purple nutsedge, barnyardgrass more competitive under high N N fertilization may discourage some sppN fertilization may discourage some spp –scouring rush (field horsetail) –cutleaf eveningprimrose barnyard manure good source weed seedsbarnyard manure good source weed seeds

26. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Soil fertility: Park Grass Expt (revisited)Park Grass Expt (revisited) –unmanured plots greatest spp diversity »30-40/M 2 –plots receiving ammonium sulfate (acidified) dominated by Agrostis –ammonium sulfate + phosphorous plots dominated by yellow chamomile –addition of potash to highest N plots caused common velvetgrass to dominate

27. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Soil fertility: grazed pastures generally less diverse than non-grazedgrazed pastures generally less diverse than non-grazed –animals, intensity, timing, pattern of dung deposition low fertility areas generally more diverse than high fertility areaslow fertility areas generally more diverse than high fertility areas –high fertility favors those species that can take advantage of increased nutrient resources

29. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Crop rotation: historically, weed control accomplished by crop rotationhistorically, weed control accomplished by crop rotation –some resurgence today to decrease chem use succession of different crops prevented any one weed from becoming dominatesuccession of different crops prevented any one weed from becoming dominate –cereal crops - aka ‘fouling crops’ –rotated to row crops to allow inter-row tillage to control weeds

30. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Crop rotation: reduced rotation modern times due to:reduced rotation modern times due to: –inorganic fertilizers »replaced need for legumes in rotation –decreased available farm labor selection pressure against weeds less variedselection pressure against weeds less varied –Great Britain, winter cereals raised continuously »winter annual weeds now dominate »1960s spring barley primary crop and spring broadleaf weeds dominated. »volunteer cereals also a problem

31. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Crop geometry, density, cultivar: increased winter wheat densityincreased winter wheat density –decreased # of flowering heads of slender foxtail and biomass of wild oats increased density and narrower row widthincreased density and narrower row width –decreased competition from wild oats in spring barley narrower row width favors crop development over weeds several row cropsnarrower row width favors crop development over weeds several row crops –corn, grain sorghum, soybeans, peanuts

32. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Crop geometry, density, cultivar: cultivars with different growth habits, maturity dates, etc. have different competitive abilitiescultivars with different growth habits, maturity dates, etc. have different competitive abilities –semi-dwarf wheat (India) favored littleseed canarygrass (Phalaris minor) »not a problem in tall cultivars –green foxtail (Manitoba) less competitive tall wheat cultivars than semi-dwarf

34. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Tillage: generally tillage done to prep seedbed and control weeds (indirect & direct selection pressure).generally tillage done to prep seedbed and control weeds (indirect & direct selection pressure). Tillage has selected for perennial weeds dependent upon (or at least adapted to) fragmentation for dispersalTillage has selected for perennial weeds dependent upon (or at least adapted to) fragmentation for dispersal –and for annual weeds with long soil seed longevity »subsequent tillage to bring these weeds to surface after burial further selects

35. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Tillage: any change in tillage will alter weed populationsany change in tillage will alter weed populations –reduced tillage to decrease soil erosion »selected for annual and perennial grasses and decreased broadleaves »probably broadleaf herbicide selection pressure »in N.America reduced tillage selected for green foxtail and downy brome in wheat »crabgrass in corn

36. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Tillage: any change in tillage will alter weed populationsany change in tillage will alter weed populations –reduced tillage to decrease soil erosion »perennial weeds such as hemp dogbane, horsenettle, nutsedges increased in corn »quackgrass in reduced tillage small grains »no-till fruit increased field bindweed & scouring rush

37. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Tillage: any change in tillage will alter weed populationsany change in tillage will alter weed populations –reduced tillage to decrease soil erosion »species diversity less in reduced tillage »but individual species occur in greater density because of change in selection pressure

38. Human Influence Weed Evolution General or indirect selection:General or indirect selection: –Crop mimics: flax productionflax production –large seeded falseflax (Camelina sativa) »several ecotypes evolved in close association with flax production »growth & reproduction similar to crop »in open-grown habitats, wide branching growth »in flax, less branched and more closely resembles crop prostrate form yellow foxtailprostrate form yellow foxtail –developed in alfalfa; –mowing stimulated growth habit

42. Human Influence Weed Evolution Specific or direct selection:Specific or direct selection: –Improved seed cleaning or conditioning: corncockle historic problem wheatcorncockle historic problem wheat –short soil seed longevity –improved cleaning eliminated »no longer sow with crop jointed goatgrass in wheatjointed goatgrass in wheat –improved cleaning using gravity table removes cylinders from crop seed –will this decrease or eliminate problem?

44. Human Influence Weed Evolution Specific or direct selection:Specific or direct selection: –Chemical weed control: herbicides relatively new selection pressureherbicides relatively new selection pressure some have speculated that annual herbicide use with similar mode action will cause spp. diversity declinesome have speculated that annual herbicide use with similar mode action will cause spp. diversity decline little evidence to support hypothesislittle evidence to support hypothesis –e.g. wild mustard still primary weed problem in 60% of English farms after 20 to 40 years of phenoxy (2,4-D) use.

45. Human Influence Weed Evolution Specific or direct selection:Specific or direct selection: –Chemical weed control: herbicides relatively new selection pressureherbicides relatively new selection pressure continuous similar herbicide use & decrease spp diversity:continuous similar herbicide use & decrease spp diversity: –data supports decline in susceptible dicots and increase in non-susceptible dicots & monocots »3 yr use dinoseb decreased wild mustard, wild buckwheat, bedstraw with concomitant increase in slender foxtail (Germany) »17 yr phenoxy decreased wild mustard, corn poppy, corn buttercup w/increase buckwheat, chamomile, annual grasses

46. Human Influence Weed Evolution Specific or direct selection:Specific or direct selection: –Chemical weed control: herbicides relatively new selection pressureherbicides relatively new selection pressure –N.America control of grassy weeds in soybeans using trifluralin »increased nightshades (Solanum spp) –repeated use of herbicides with same/similar mechanism of action »selects for populations of highly tolerant to resistant weed biotypes »triazine resistant kochia & pigweed »ALS (sulfonylurea & imidazolinone) resistant prickly lettuce, kochia, Russian thistle

50. Human Influence Weed Evolution Specific or direct selection:Specific or direct selection: –Biological weed control: presumably, host-specific biocontrol agents preying on a particular weed spp. may open habitat to invasion by another weed not control by agentpresumably, host-specific biocontrol agents preying on a particular weed spp. may open habitat to invasion by another weed not control by agent –probably will not see resistance to classical biocontrol »selection pressure not great enough –mycoherbicides or bioherbicides???