Community Dynamics The changing nature of community structure

Subsets of species can be defined by: • Taxonomic affinity (e. g. , all

Species are not assembled randomly in nature. Physical stress 3 Herbivory 2 1 0

~ 59 seaweed species 20 – 26 common species 2 species 30 cm 17

Community Dynamics § Factors affecting changes in community structure – – – Intermediate Disturbance

Intermediate Disturbance Hypothesis § § Ecological Community stability is an uneasy balance Areas with

Community Instability Disturbances can cause a community to change in ways that persist even

Keystone Species § A species that has a disproportionately large effect on community structure

Copyright © 2009 Pearson Education, Inc. sea star community 15 species coexist - 1

§ Keystone species exert an important regulating effect on other species in a community.

§ If Keystone Species are removed, community structure is greatly affected. Copyright © 2009

Interactions of Multiple Species Trophic cascade: A carnivore eats an herbivore (a direct negative

Interactions of Multiple Species In kelp forests, sea otters feed on sea urchins, which

Copyright © 2009 Pearson Education, Inc. Copyright © 2002 Pearson Education, Inc. , publishing

Species Introductions § Exotic species – A species moves out of its home range

Caulerpa taxifolia • Native to Indian Ocean, used as ornamental in aquaria • Specific

Spread of Caulerpa in the Mediterranean Sea

The introduction of Caulerpa to the Mediterranean dramatically changed native species interactions, and thus

The seagrass Posidonia and Caulerpa have different growth cycles: Posidonia loses blades in the

Caulerpa acts as an ecosystem engineer, accumulating sediments around its roots, which changes the

Connections in Nature: Stopping Invasions Requires Commitment In 2000, Caulerpa was discovered near San

Prickly Pear Cactus in Australia 1916 – introduced to Australia 1925 – 30 million

Kudzu in Georgia § Imported for erosion control § No natural herbivores, pathogens, or

Aquatic Invasives: Unintended Stow-aways § Ballast water has brought many invaders to Bay Area

Community Structure Changes through Time § Community structure varies in time and space –

Copyright © 2009 Pearson Education, Inc.

Ecological Succession § Over time, one array of species is replaced by another Copyright

Community Structure Changes through Time § The sequence of communities from grass to shrub

Types of Succession § Primary succession: new environments – Begins with “foundation species” –

Climax Community § Stable array of species that does not change over time §

Primary Succession Occurs on Newly Exposed Substrates § Primary succession begins on sites that

Primary Succession § Primary succession begins on sites that have never supported a community

Primary Succession § Colonization of sand dunes § The establishment and growth of plant

Sand Dune Succession Copyright © 2009 Pearson Education, Inc.

Primary Succession Occurs on Newly Exposed Substrates § Newly deposited alluvial soil on a

Glacial Succession Copyright © 2009 Pearson Education, Inc.

Most communities are disturbed by events like fire, weather, or human activities Copyright ©

Marine communities are subject to disturbance by tropical storms. Any major changes in the

Volcanic Island Succession Foundation Species Climax Community a relatively stable complex community at the

§ Secondary succession – occurs where an existing community has been cleared, but the

Secondary Succession: Disturbed Habitats Along Roadsides Abandoned Fields Copyright © 2009 Pearson Education, Inc.

Cyclic Replacement Disease Damage – Sudden Oak Death Storm Damage Copyright © 2009 Pearson

Cyclic Replacement: Fire Climax Chaparral – fire adapted plant community Copyright © 2009 Pearson

No fire – no seedlings, growth of white fir Low intensity – allows Giant

Community Structure Changes through Time § The process of succession is common to all

Community Structure Changes through Time § Patterns of succession are not random § Early

Community Structure Changes through Time § Succession in terrestrial plant communities at the Hubbard

Ecological Issues: American Forests § Old-field communities commonly occur in the eastern United States

Ecological Issues: American Forests § When European colonists first arrived on the eastern shores

Ecological Issues: American Forests § The trend was reversed in the Dust Bowl period

Secondary Succession Occurs After Disturbances § Old-field succession in the Piedmont region of North

Secondary Succession Occurs After Disturbances § Studies of physical disturbance in marine environments have

Secondary Succession Occurs After Disturbances § D. Duggins (University of Washington) examined the process

Secondary Succession Occurs After Disturbances § J. Zieman (University of Virginia) described the secondary

The Study of Succession Has a Rich History § E. Warming (Denmark) and H.

The Study of Succession § Monoclimax hypothesis: the community is a highly integrated superorganism

The Study of Succession § Initial floristic composition: the process of succession is individualistic

The Study of Succession § Facilitation model: Early successional species modify the environment so

The Study of Succession § Inhibition model: The first species to arrive hold the

The Study of Succession § Tolerance model: Later successional species are neither inhibited nor

Succession Is Associated with Autogenic Changes in Environmental Conditions § Autogenic environmental change is

Succession Is Associated with Autogenic Changes in Environmental Conditions § Though a general model

Succession Is Associated with Autogenic Changes § Alteration of light environment – In the

Succession Is Associated with Autogenic Changes § Autogenic changes in soil development — stabilization

Species Diversity Changes during Succession § Chronosequences (chronoseres) are groups of sites used to

Species Diversity Changes during Succession § Species colonization and replacement drive succession – Colonization

Species Diversity Changes during Succession § Disturbance can have a similar effect to that

Species Diversity Changes during Succession § Patterns of diversity depend on the frequency of

Succession Involves Heterotrophic Species § Succession also occurs in heterotrophic communities § The decomposition

Succession Involves Heterotrophic Species § As the structure of the plant community changes during

Systematic Changes in Community Structure Are a Result of Allogenic Environmental Change at a

Primary succession – begins in a new or lifeless area where soil has not

Community Structure Changes Over Geologic Time § Earth’s surface has changed profoundly over the

The Concept of Community Revisited § Clements viewed the community as a quasiorganism made

The Concept of Community Revisited § The organismal community is a spatial concept §

The Concept of Community Revisited § The structure of communities is the product of

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Community Dynamics § The changing nature of community structure across the landscape reflects the shifting distribution of populations in response to: – Changing environmental conditions – Interactions among species § Community structure is dynamic – Shifting pattern of species’ dominance and diversity through time Copyright © 2009 Pearson Education, Inc.

Subsets of species can be defined by: • Taxonomic affinity (e. g. , all bird species in a community) • Guild—group of species that use the same resources • Functional group—species that function in similar ways

Species are not assembled randomly in nature. Physical stress 3 Herbivory 2 1 0 Low Mid High Disturbance level (Sousa 1979 Ecology) 15 10 Nitrogen availability 5 0 0 100 200 300 Herbivore Density (snails/m 2) (Lubchenco 1978 Am Nat) log # seaweed species 4 Nov-75 May-76 Oct-76 May-77 # seaweed species Species richness 5 1. 00 0. 75 0. 50 0. 25 0. 0 0. 2 0. 4 0. 6 0. 8 1. 0 log ammonium loading (micromol/L/h) (Bracken & Nielsen 2004 Ecology)

~ 59 seaweed species 20 – 26 common species 2 species 30 cm 17 species

Community Dynamics § Factors affecting changes in community structure – – – Intermediate Disturbance Hypothesis Community Instability (Alternate stable states) Keystone Species Exotic (introduced species) Ecological Succession § Primary § Secondary Copyright © 2009 Pearson Education, Inc.

Intermediate Disturbance Hypothesis § § Ecological Community stability is an uneasy balance Areas with moderate levels of disturbance have greater species diversity Copyright © 2009 Pearson Education, Inc.

Community Instability Disturbances can cause a community to change in ways that persist even if the change is reversed. In some cases these are considered “alternate stable states” Copyright © 2009 Pearson Education, Inc.

Keystone Species § A species that has a disproportionately large effect on community structure § Removal of a keystone species can cause drastic changes in a community – can increase or decrease diversity Copyright © 2009 Pearson Education, Inc.

§ Keystone species exert an important regulating effect on other species in a community. Populations are limited by competition for resources Fig. 53. 14 © 2002 Pearson Copyright © 2009 Pearson Education, Inc. , publishing as Benjamin Cummings

Interactions of Multiple Species Trophic cascade: A carnivore eats an herbivore (a direct negative effect on the herbivore). The decrease in herbivore abundance has a positive effect on a primary producer.

Interactions of Multiple Species In kelp forests, sea otters feed on sea urchins, which feed on the kelp. Sea otters have a positive indirect effect on kelp. Kelp, in turn, can positively affect abundance of other seaweeds, which serve as habitat and food for marine invertebrates and fishes.

Species Introductions § Exotic species – A species moves out of its home range and takes up residence in a new place – No natural enemies or controls – Can outcompete native species § Geographic dispersal § slow or rapid movement Copyright © 2009 Pearson Education, Inc.

Caulerpa taxifolia • Native to Indian Ocean, used as ornamental in aquaria • Specific strain was found to thrive in cold aquarium environments • Selective breeding under exposure to chemicals and ultra-violet light made the strain even hardier • Escaped from Oceanographic Museum in Monaco (France) • Smothering parts of Mediterranean Sea • Continues to spread Copyright © 2009 Pearson Education, Inc.

Invading Seaweed

Spread of Caulerpa in the Mediterranean Sea

The introduction of Caulerpa to the Mediterranean dramatically changed native species interactions, and thus community structure and function. Seagrass meadows support a multitude of species and were overgrown by Caulerpa.

A Mediterranean Seagrass Meadow

The seagrass Posidonia and Caulerpa have different growth cycles: Posidonia loses blades in the summer when Caulerpa is most productive. This allows Caulerpa to overtop Posidonia and dominate.

Caulerpa acts as an ecosystem engineer, accumulating sediments around its roots, which changes the invertebrate community. There is also a decrease in numbers and sizes of fish after Caulerpa invades, suggesting the habitat is no longer suitable.

Connections in Nature: Stopping Invasions Requires Commitment In 2000, Caulerpa was discovered near San Diego, California. A team of scientists and managers was immediately assembled to design an eradication plan. It eventually took six years and $7 million to eradicate the alga.

Prickly Pear Cactus in Australia 1916 – introduced to Australia 1925 – 30 million acres covered (500 ton/acre) 1926 – Cactus moth (Cactoblastis) introduced 1934 – only 1/10 of the cactus remained Copyright © 2009 Pearson Education, Inc.

Kudzu in Georgia § Imported for erosion control § No natural herbivores, pathogens, or competitors § Grows over landscapes and cannot be dug up or burned out § May turn out to have some commercial use Copyright © 2009 Pearson Education, Inc.

Aquatic Invasives: Unintended Stow-aways § Ballast water has brought many invaders to Bay Area § Great Lakes affected along with ALL other shipping ports world-wide Copyright © 2009 Pearson Education, Inc.

Community Structure Changes through Time § Community structure varies in time and space – Across the landscape (zonation) – In one position as time passes § Succession is the gradual and (seemingly) directional change in community structure through time from field to forest Copyright © 2009 Pearson Education, Inc.

Community Structure Changes through Time § The sequence of communities from grass to shrub to forest has been called a sere, and each of the changes is a seral stage § Each seral stage has its characteristic structure and species composition – It may last only one to two years or for several decades Copyright © 2009 Pearson Education, Inc.

Types of Succession § Primary succession: new environments – Begins with “foundation species” – Ends with “climax community” § Secondary succession: communities destroyed or displaced – May not have “foundation species” – Also ends with “climax community” Copyright © 2009 Pearson Education, Inc.

Primary Succession Occurs on Newly Exposed Substrates § Primary succession begins on sites that have never supported a community – Rock outcrops and cliffs – Sand dunes – Newly exposed glacial till Copyright © 2009 Pearson Education, Inc.

Primary Succession § Primary succession begins on sites that have never supported a community – Rock outcrops and cliffs – Sand dunes – Sand is deposited by wind and water and sand particles may be piled in long, windward slopes to form dunes – Dune position can shift, and this often covers existing vegetation Copyright © 2009 Pearson Education, Inc.

Primary Succession § Colonization of sand dunes § The establishment and growth of plant cover acts to stabilize the dunes – Grasses are the most successful pioneering plants § Once the dunes are stabilized, mat-forming shrubs invade the area § The vegetation eventually shifts to trees, first pines and then oak – Differences in tree species based on moisture availability and region Copyright © 2009 Pearson Education, Inc.

Primary Succession Occurs on Newly Exposed Substrates § Newly deposited alluvial soil on a floodplain represents another example of primary succession § Glacier Bay National Park, Alaska – Over the past 200 years the glacier that once covered the entire region has been retreating – The newly exposed landscape is initially colonized by a variety of species Copyright © 2009 Pearson Education, Inc.

Marine communities are subject to disturbance by tropical storms. Any major changes in the living community has significant Fig. 53. 17 affects on local environmental conditions. Copyright © 2009 Pearson Education, Inc.

Volcanic Island Succession Foundation Species Climax Community a relatively stable complex community at the final stage of succession that is in equilibrium with the current physical environment. Copyright © 2009 Pearson Education, Inc.

§ Secondary succession – occurs where an existing community has been cleared, but the soil is left intact. § Relatively rapid succession © 2002 Pearson Copyright © 2009 Pearson Education, Inc. , publishing as Benjamin Cummings

No fire – no seedlings, growth of white fir Low intensity – allows Giant Sequoia to re-seed Medium intensity – kills competing fir trees Giant Sequoia Copyright © 2009 Pearson Education, Inc. High intensity – crown fire destroys forest

Community Structure Changes through Time § The process of succession is common to all terrestrial and aquatic environments § W. Sousa examined the process of succession in a rocky intertidal algal community – Results show a pattern of colonization and extinction, with other species displacing populations that initially colonized as time progressed Copyright © 2009 Pearson Education, Inc.

Community Structure Changes through Time § Patterns of succession are not random § Early successional species or pioneer species are usually characterized by high growth rates, smaller size, high degree of dispersal, and high rates of per capita population growth § Late successional species have lower rates of dispersal and colonization, slower per capita growth rate, and they are larger and longer-lived Copyright © 2009 Pearson Education, Inc.

Community Structure Changes through Time § Succession in terrestrial plant communities at the Hubbard Brook Experimental Forest (New Hampshire) after clear-cutting – Beech and sugar maple replaced by raspberry thickets and seedlings of sun-adapted, fast-growing species Copyright © 2009 Pearson Education, Inc.

Ecological Issues: American Forests § When European colonists first arrived on the eastern shores of North America (1600 s), the landscape was dominated by forest – Native Americans had used fire to clear areas for planting crops, but this had minimal impact on the landscape § The clearing of forest by settlers was driven by the need for agricultural lands and forest products – By the 1800 s, most of the forest in eastern North America had been felled Copyright © 2009 Pearson Education, Inc.

Ecological Issues: American Forests § The trend was reversed in the Dust Bowl period in the 1930 s § With the advent of mechanization of agriculture, small family farms gave way to large commercial farms § By the 1930 s, the amount of agricultural land in the east had peaked, and it has been declining ever since Copyright © 2009 Pearson Education, Inc.

Secondary Succession Occurs After Disturbances § Old-field succession in the Piedmont region of North Carolina – Annual crabgrass is the first species to grow after a crop field has been abandoned – In the second years, horseweed quickly claims the field, white aster and ragweed also invade – By the third summer, broomsedge invades and soon dominates – Pine seedlings establish and within 5 to 10 years, the pines are tall enough to shade the broomsedge – Eventually, hardwood species take over the field Copyright © 2009 Pearson Education, Inc.

Secondary Succession Occurs After Disturbances § Studies of physical disturbance in marine environments have demonstrated secondary succession in various communities – – – Seaweed Salt marsh Mangrove Seagrass Coral reef Copyright © 2009 Pearson Education, Inc.

Secondary Succession Occurs After Disturbances § D. Duggins (University of Washington) examined the process of succession in the subtidal kelp forests of Torch Bay, Alaska – One year after the removal of kelp forest, a mixed canopy and understory of kelp species formed – During the second and third years, the community returned to the original composition Copyright © 2009 Pearson Education, Inc.

Secondary Succession Occurs After Disturbances § J. Zieman (University of Virginia) described the secondary succession of seagrass communities in Florida Bay § Wave action and heavy grazing creates openings in the grass cover, exposing underlying sediments — subsequent erosion results in blowouts – Rhizophytic macroalgae contributes to sediment – Halodule wrightii colonizes stabilizes the sediment surface – Thalassia testudinum eventually colonizes the area and again resembles the surrouding seagrass community Copyright © 2009 Pearson Education, Inc.

The Study of Succession Has a Rich History § E. Warming (Denmark) and H. Cowles (United States) developed the concept of ecological succession § Several hypotheses have addressed the processes that drive succession – – – Monoclimax hypothesis (F. Clements) Initial floristic composition (F. Egler) Facilitation model (J. Connell and R. Slatyer) Inhibition model (J. Connell and R. Slatyer) Tolerance model (J. Connell and R. Slatyer) Copyright © 2009 Pearson Education, Inc.

The Study of Succession § Monoclimax hypothesis: the community is a highly integrated superorganism – The process of succession represents the gradual and progressive development of the community to the climax stage Copyright © 2009 Pearson Education, Inc.

The Study of Succession § Initial floristic composition: the process of succession is individualistic – Depends on the particular species that colonize the site and the order in which they arrive – No species is competitively superior to the other Copyright © 2009 Pearson Education, Inc.

The Study of Succession § Facilitation model: Early successional species modify the environment so that it becomes more suitable for successive species to invade and grow Copyright © 2009 Pearson Education, Inc.

The Study of Succession § Inhibition model: The first species to arrive hold the site against all invaders — they make the site less suitable for both early and late successional species – The initial species hold their position as long as they live and reproduce Copyright © 2009 Pearson Education, Inc.

The Study of Succession § Tolerance model: Later successional species are neither inhibited nor aided by species of earlier stages – Communities are composed of species most efficient in exploiting available resources Copyright © 2009 Pearson Education, Inc.

Succession Is Associated with Autogenic Changes in Environmental Conditions § Autogenic environmental change is a direct result of the presence and activities of organisms within the community – Vertical profile of light in a forest is a direct result of the vegetation structure § Allogenic environmental change is governed by physical processes – Decline with average temperature with elevation Copyright © 2009 Pearson Education, Inc.

Succession Is Associated with Autogenic Changes in Environmental Conditions § Though a general model of plant succession has emerged, no single cause of succession fits all the examples – In both primary and secondary succession, plant colonization alters the environment (e. g. , light, soil) Copyright © 2009 Pearson Education, Inc.

Succession Is Associated with Autogenic Changes § Alteration of light environment – In the early stages of plant succession, shadeintolerant species can dominate – Shade-intolerant species grow above and shade the slower-growing, shade-tolerant species – Eventually, shade-intolerant species cannot grow and survive in the shaded conditions § Succession results from changes in the relative tolerances and competitive abilities of species under autogenically changing environmental conditions Copyright © 2009 Pearson Education, Inc.

Succession Is Associated with Autogenic Changes § Autogenic changes in soil development — stabilization and addition of organic matter – Seagrass communities – Succession on glacial sediments — changes in soil nitrogen during primary succession Copyright © 2009 Pearson Education, Inc.

Species Diversity Changes during Succession § Chronosequences (chronoseres) are groups of sites used to compare patterns of diversity through succession – Some studies have shown that plant diversity increases with site age (old-field communities) § R. Whittaker (Cornell University) found a different pattern – Species diversity increases with herbaceous stages, decreases into shrub stages, increases again in young forest, and finally decreases as the forest ages Copyright © 2009 Pearson Education, Inc.

Species Diversity Changes during Succession § Species colonization and replacement drive succession – Colonization by new species increases local species richness – Species replacement, as a result of competition or lack of tolerance, acts to decrease species richness § The peak in diversity during the middle stages of succession corresponds to a transition period Copyright © 2009 Pearson Education, Inc.

Species Diversity Changes during Succession § Disturbance can have a similar effect to that of reduced growth rates by extending the period over which the species coexist § Disturbance acts to “reset the clock” in succession Copyright © 2009 Pearson Education, Inc.

Species Diversity Changes during Succession § Patterns of diversity depend on the frequency of disturbance – High diversity will remain low because later successional species never have a chance to colonize – Low diversity will decline as later successional species displace earlier species – Intermediate colonization can occur but competitive displacement is held to a minimum § Intermediate disturbance hypothesis (M. Huston and J. Connell) Copyright © 2009 Pearson Education, Inc.

Succession Involves Heterotrophic Species § Succession also occurs in heterotrophic communities § The decomposition of fallen trees, animal carcasses, and droppings by fungi and invertebrates recycle nutrients and alter the substrate Copyright © 2009 Pearson Education, Inc.

Succession Involves Heterotrophic Species § As the structure of the plant community changes during succession, so do the habitats available to heterotrophs § As the vertical structure becomes more complex, new species appear Copyright © 2009 Pearson Education, Inc.

Systematic Changes in Community Structure Are a Result of Allogenic Environmental Change at a Variety of Timescales § Allogenic environmental change is governed by physical (abiotic) processes – Annual fluctuations in temperature and precipitation have little influence on successional patterns – Fluctuations or shifts in environmental conditions that occur at periods longer than the organism’s life span are likely to result in successional shifts Copyright © 2009 Pearson Education, Inc.

Systematic Changes in Community Structure Are a Result of Allogenic Environmental Change at a Variety of Timescales § Seasonal succession of phytoplankton (Lawrence Lake, Michigan) is correlated with species’ optimal temperature, nutrient, and light requirements Copyright © 2009 Pearson Education, Inc.

Systematic Changes in Community Structure Are a Result of Allogenic Environmental Change at a Variety of Timescales § Seaward expansion of marshlands (River Fal, Cornwall, England) is a result of silt deposition lowering water depths over the past century § Sediment deposition in freshwater environments can lead to infilling and a transition from an aquatic environment marsh grassland, swamp woodland, peat bog meadow § Shifting distribution of tree species in North America following the last glacial maximum 18, 000 years ago Copyright © 2009 Pearson Education, Inc.

Community Structure Changes Over Geologic Time § Earth’s surface has changed profoundly over the past 4. 6 billion years § Paleoecology is the study of the distribution and abundance of ancient organisms and their relationship to the environment – Reconstruction of plant distribution after the last glacial maximum (Laurentian ice sheet) of the Pleistocene – Climatic oscillations between cold and temperate – Historic occurrence and distribution of plant species is indicated by the pollen record of sediment cores Copyright © 2009 Pearson Education, Inc.

The Concept of Community Revisited § Clements viewed the community as a quasiorganism made up of interdependent species § Gleason saw the community as an arbitrary concept and stated that each species responds independently to the underlying features of the environment – Individualistic or continuum concept § The reality of what a community is probably lies somewhere in the middle Copyright © 2009 Pearson Education, Inc.

The Concept of Community Revisited § The organismal community is a spatial concept § The continuum view is a population concept § Each view can be applied to a given community and though different, are consistent – Patterns of co-occurrence along a gradient of altitude – Patterns of forest communities in Great Smoky Mountains National Park Copyright © 2009 Pearson Education, Inc.

The Concept of Community Revisited § The structure of communities is the product of a complex interaction of pattern and process § Species respond to an array of environmental factors that vary over space and time Copyright © 2009 Pearson Education, Inc.