COMMUNITY CHANGE (SUCCESSION) Krebs cpt. 21; pages ; 431

Connell and Slatyer 1977 Connell and Slatyer 1977 Succession driven by interactions between species:  FACILITATION  TOLERANCE (COMPETITION)  INHIBITION

Facilitation SUCCESSION PATHWAYS ABCD AB C D Tolerance & Inhibition

Succession models (Krebs 5 th Fig. 21.4) o o o Species interactions + - 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 ) FIELD ABANDONED IN FALL 1. INITIAL INVASION: Horseweed Horseweed a winter annual a winter annual produces abundant seed produces abundant seed self-allelopathic self-allelopathic Conyza canadensis

2. NEXT SEASON: AstersAsters More susceptible to decaying roots of horseweed, than horseweedMore susceptible to decaying roots of horseweed, than horseweed Tolerant of dry conditionsTolerant of dry conditions

3. SECOND AND THIRD SEASONS 3. SECOND AND THIRD SEASONS: BluestemBluestem Seedlings invading since initial abandonmentSeedlings invading since initial abandonment BroomsedgeBroomsedge Seedlings invading since initial abandonmentSeedlings invading since initial abandonment More tolerant of dry conditions than AstersMore tolerant of dry conditions than Asters Decaying roots of Horseweed promote growthDecaying roots of Horseweed promote growth

Bluestem Broomsedge

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 SUMMARY

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

Red Maple, Acer rubrum Beech, Fagus sylvatica

YELLOW BIRCH SUGAR MAPLE BEECH r K

SPECIES Density (#/m 2 ) (d min /d max ) x 100 % plots with at least 1 seedling Mass/ind (mg) Yellow birch Sugar maple Beech Many small offspring (r-type) Fewer large offspring (K-type) Variable density and high dispersal More even density and lower dispersal

Number of cohort alive at beginning of interval Age interval Yellow birch Sugar maple Beech <

CANOPYSEEDLINGSSAPLINGS < 0.5m tall0.5 – 2 m tall Yellow birch Sugar maple Beech Sugar mapleYellow birch Sugar maple Beech BeechYellow birch Sugar maple Beech Sprouts + (seeds -) + = significantly present 0 = no relation - = significantly absent

CATASTROPHIC DISTURBANCE Yellow birch Sugar Maple Beech (seed) Beech (sprouts) MINOR DISTURBANCE Successional communities