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COMMUNITY CHANGE (SUCCESSION) Krebs cpt. 21; pages 403-424; 431 Connell and Slatyer 1977 Succession driven by interactions between species: FACILITATION TOLERANCE (COMPETITION) INHIBITION SUCCESSION PATHWAYS Facilitation A B C Tolerance & Inhibition A B D C D Succession models (Krebs 5th Fig. 21.4) Species interactions +-o + + + + + - + - o o o FACILITATION …only certain pioneer species can establish after disturbance. Early species enhance the ability of later species to establish. INHIBITION …any species can invade. Once established, species resist further invasion and are displaced only by death or further disturbance. TOLERANCE …any species can invade. Early species have little or no effect on abilities of later species to invade. Later species can displace early species by reducing resources to levels below those tolerated by earlier occupants. A study from abandoned farmland illustrates some aspects of Facilitation, Tolerance and Inhibition (see Krebs pp 419-422) FIELD ABANDONED IN FALL 1. INITIAL INVASION: • Horseweed • a winter annual • produces abundant seed • self-allelopathic Conyza canadensis 2. NEXT SEASON: • Asters • More susceptible to decaying roots of horseweed, than horseweed • Tolerant of dry conditions 3. SECOND AND THIRD SEASONS: • Bluestem • Seedlings invading since initial abandonment • Broomsedge • Seedlings invading since initial abandonment • More tolerant of dry conditions than Asters • Decaying roots of Horseweed promote growth Bluestem Broomsedge SUMMARY FACILITATION: • Broomsedge is promoted by decaying roots of horseweed & aster TOLERANCE: • Broomsedge displaces aster through competition for water INHIBITION: • Horseweed litter prevents aster from persisting FORCIER 1975 Succession driven by interaction of disturbance & life history traits of species Studied trees in a New Hampshire forest • Trees with dbh >2 cm (400 plots) • 90% of canopy: • Sugar maple • American beech • Yellow birch Eastern deciduous hardwood forests Several tree species Grey Birch, Betula populifera Black Gum, Nyssa sylvatica Beech, Fagus sylvatica Red Maple, Acer rubrum YELLOW BIRCH r SUGAR MAPLE BEECH K Many small offspring (r-type) Density (#/m2) (dmin/dmax ) x 100 % plots with at least 1 seedling Mass/ind (mg) Yellow birch 21.3 2 92 8 Sugar maple 10 Beech 3.9 SPECIES Variable density and high dispersal 57 73 268 More even density and lower dispersal 66 55 Fewer large offspring (K-type) 569 Number of cohort alive at beginning of interval Age interval Yellow birch Sugar maple Beech 0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 1000 70 <30 1000 583 317 225 166 118 84 54 48 42 1000 410 262 188 138 121 110 92 87 83 CANOPY SEEDLINGS < 0.5m tall SAPLINGS 0.5 – 2 m tall Yellow birch Yellow birch Sugar maple Beech + + 0 + - Sugar maple Yellow birch Sugar maple Beech Beech Yellow birch Sugar maple Beech 0 + + 0 0 + 0 0 ++ 0 0 Sprouts + (seeds -) + = significantly present 0 = no relation - = significantly absent Beech (sprouts) Beech (seed) MINOR DISTURBANCE Sugar Maple Yellow birch Successional communities CATASTROPHIC DISTURBANCE