Molles: Ecology 2 nd Ed. Chapter Concepts What is succession? What are the causes of succession? How do community composition / diversity, and ecosystem energy flow / nutrient cycling change during succession? How do ecologists know this stuff? What are the implications in a human – dominated world?
Molles: Ecology 2 nd Ed. What does it take to keep this lawn looking like this? What will happen to this lawn over time with no human intervention? What will happen to this lawn over time with no human intervention?
Molles: Ecology 2 nd Ed. Primary Succession Establishment of a biological community on new substrates where no community had previously existed. Examples On land exposed by retreating glaciers On new substrates created by volcanic lava flows
Molles: Ecology 2 nd Ed. Primary Succession – Glacier Bay, Alaska Reiners et.al. (1971) Changes in plant diversity during succession
Molles: Ecology 2 nd Ed. Secondary Succession Re-building of a biological community after a previously existing community is destroyed by a disturbance, but the soil remains. Examples: Recovery of forests after fire, wind storm, insect pest outbreak, logging Re-growth of natural vegetation after farmland abandonment
Molles: Ecology 2 nd Ed. Old Field Succession 1 Year After Abandonment Grasses and Weeds Dominate
Molles: Ecology 2 nd Ed. Old Field Succession 5 Years After Abandonment Goldenrod and Other Perennial Weeds Dominate
Molles: Ecology 2 nd Ed. Old Field Succession 10 Years After Abandonment Tree saplings and shrubs begin to establish
Molles: Ecology 2 nd Ed. Old Field Succession 20 Years After Abandonment Trees and shrubs begin to replace herbaceous plant species
Molles: Ecology 2 nd Ed. Old Field Succession 28 Years After Abandonment Where Did the Grasses and Goldenrod Go ?
Molles: Ecology 2 nd Ed. Definitions Climax Community – Late successional community Remains stable until disrupted by disturbance
Primary vs. Secondary Succession Primary succession slower than secondary succession. With no pre-existing seeds or root systems in the soil, establishment of organisms requires migration into the area from other locations. Establishment of many plant species is delayed until soil development has occurred (sand, silt, clay, organic matter)
Causes of Succession Species differ in dispersal (migration) ability. Species differ in their environmental tolerances / requirements. Species differ in growth rate. Species differ in life span. Species differ in competitive ability.
Pioneer (r-selected) Species Produce large numbers of small, easily dispersed young. Are usually first species to arrive after disturbance. Grow fast in high resource environment. Tolerate harsh physical environments. Reach sexual maturity fast. Short life spans (require replacement via reproduction to remain on the site) Poor competitive ability.
Late-Succession (K-selected) Species Produce fewer, larger young that often have limited dispersal from parent. Usually arrive at disturbed site later (may require pioneers moderate environment first). Have slower growth rate. Longer time to sexual maturity. Long-lived (hold onto space / resources) Good competitive ability (able to displace pioneer species)
Community and Ecosystem Changes During Succession Species Diversity increases (but may decrease during late stages due to competitive exclusion). Net Primary Production increases during early stages, but declines during late stages due to increased respiration. Nutrient Cycling / Retention greatest during middle stages.
Species Richness During Primary Succession at Glacier Bay Study Plots Fig 20.2
Change of Plant Growth Forms During Succession Fig 20.3
Molles: Ecology 2 nd Ed. Woody Plant Species Richness During Secondary Forest Succession (Eastern USA) Piedmont Plateau
Woody Plant Species Richness During Secondary Forest Succession (Eastern USA) Fig 20.4
Breeding Bird Species Richness During Secondary Forest Succession (Eastern USA) Fig 20.5
Species Richness of Macroinvertebrate and Macroalgae Species During Secondary Succession on Intertidal Boulders Fig 20.7
Algal species Diversity During Secondary Succession in Sycamore Creek After Flooding Fig 20.8
How Do We Study Succession ? (Long-Term Research) 1. Establish permanently marked plots in an area recently affected by disturbance. 2. Record community / ecosystem variables Species relative abundance / Species diversity Biomass / Net Primary Production Nutrient Pools and Input / Output Fluxes
Molles: Ecology 2 nd Ed. How Do We Study Succession ? (Long-Term Research) 3. Re-measure variables in exactly the same plots at different times (often intervals of decades). 4. Change in community / ecosystem variables over time attributed to succession.
How Do We Study Succession ? (Space-for-Time Comparative Studies) 1. Record community / ecosystem variables in plots established in different areas that were disturbed at different times in the past. Species relative abundance / Species diversity Biomass / Net Primary Production Nutrient Pools and Input / Output Fluxes
Molles: Ecology 2 nd Ed. 2. Compare community / ecosystem variables between these different areas. Differences are attributed to succession. 3. Valid only if the following conditions are true: Different areas were all disturbed in exactly the same way Different areas have similar environmental conditions How Do We Study Succession ? (Space-for-Time Comparative Studies)
How Do Disturbance Characteristics Influence Succession? Short Return Time (High Frequency) Limits opportunity for immigration of species and re-growth of populations. K-selected species eliminated. Limits opportunity for re-building biomass. Nutrient losses due to disturbance not fully replenished before next disturbance. Community dominated by r-selected species, with low standing crop biomass and low nutrient availability.
How Do Disturbance Characteristics Influence Succession? Large Magnitude + High Intensity Re-establishment of species populations requires long- distance migration from undisturbed areas. Succession is retarded by slow immigration rate. NPP recovers slowly Biomass pool recover slowly Nutrient losses greater during slow recovery Community dominated by r-selected species, with rapid dispersal ability for a longer period.
How Do Disturbance Characteristics Influence Succession? Small Magnitude or Low Intensity Re-establishment of species populations from surviving individuals within / near disturbed area. Succession proceeds quickly. NPP recovers rapidly. Biomass pool recover rapidly. Nutrient losses quickly stop. Community dominated by species that are best able survive disturbance and compete for limited growing space (K-selected).
Implications for Human Activities Human populations typically increase the frequency, magnitude, and intensity of disturbance. Entire landscapes dominated by r-selected species b/c succession never finishes before next disturbance. Depletion of biomass, soil organic matter, and nutrient pools.
Implications for Human Activities Humans sometimes suppress disturbances (e.g. fire), increasing return time, but often with increase in magnitude and intensity. Entire landscapes become dominated by K- selected species, and accumulate biomass. Communities may become more susceptible to catastrophic disturbance (e.g., recent fires in western U.S.)
Effects of Fire Suppression Eastern Hardwood Forest Fire Suppression Begins ▲ Logging Number of trees
Effects of Fire Suppression Western Conifer Forests Frequent Low- Intensity Fires ▼▼▼▼▼▼▼▼ Fire Suppression Begins Fuel build-up leads to catastrophic crown fire Number of trees
Implications for Human Activities Humans depend on rapid growing, r-selected species for agriculture and forestry. Humans use disturbance to prevent succession and maintain open environments required by these pioneer species. Frequent human disturbance to maintain crop species can deplete soil organic matter and nutrient pools.