Chapter 17- Changes in Communities

Communities are…

always changing, some more than others

Agents of change act on communities across all

temporal and spatial scales

Stress

factors that reduce growth, reproduction, or survival of individuals

Disturbance

Abiotic events that injure or kill some individuals and create opportunities for other individuals

What could be subtle agents of change on a reef (causing stress)?

Fig 17.3- Change Happens

• The slow rise to dominance of certain coral species, and the slow decline of others, due to the effects of competition, predation, and disease.

What could be more serious/ catastrophic agents of change (a disturbance)

Fig 17.3- Change Happens

• massive deaths of corals in the last decade due to bleaching (loss of symbiotic algae)

• The great tsunami of 2004, resulting in the replacement of some coral species with other species, or no replacement at all.

Examples of Abiotic Agents of Stress, Disturbance, and Change in Communities

Waves, Currents, Winds, water supply, chemical composition, temperature, volcanic activity,

Examples of Biotic Agents of Stress, Disturbance, and Change in Communities

• Negative interactions

  • Competition

  • Predation

  • Herbivory

  • Disease

  • Parasitism

  • Trampling

  • Digging

  • Boring

What can Biotic interactions result in with another species?

• Replacement of one species with another

How can disease initiate community changes

• By causing death or slow growth of a species

Importance of Engineer or Keystone species

• They can influence community change

Importance of smaller and more frequent disturbances

It can open patches of resources for other individuals

Succession

is change in the species composition of communities over time

What causes succession?

A variety of abiotic (physical and chemical) and biotic agents of change

Two types of succession

1. Primary

2. Secondary

Primary Succession Definition

• Colonization of habitats devoid of life as a result of catastrophic disturbance or because they are newly created habitats (e.g. volcanic rock)

Secondary Succession

• Reestablishment of a community in which some, but not all, organisms have been destroyed

Climax stage

a stable end point that changes little

Components of Primary Succession

• No life to Pioneer Stage

Components of Secondary Succession

Some life to Intermediate Stage

Grimes 4 possibilites

1. Low stress, low disturbance

2. Low stress, high disturbance

3. High stress, low disturbance

4. High stress, high disturbance

Outcome of Low stress, low disturbance

Competitive

Outcome of Low stress, high disturbance

Ruderal (A plant that is associated with human dwellings or agriculture, or one that colonizes waste ground)

Outcome of high stress, low disturbance

Stress-tolerant

Outcome of high stress, high disturbance

Not Viable

Competitive plants

With superior ability to acquire light, minerals, water, and space

Stress- Tolerant Plants

Slow rates of water and nutrient use (e.g., succulents)

Ruderal Plants

With short life span, rapid growth rates, heavy investment in seed production (weeds)

Selections that describe two ends of a reproductive strategy continuum

1. r-selected

2. K-selected

Compoents of r-selected species

• Short-lived

• High Mortality

• Fast growing

• Early maturation

• High reproductive rates

• Viable/ Stressful environments

Components of K-selected species

• Long-lived

• Low mortality

• Slow growing

• Late maturing

• Low reproductive rates

• Stable environments

Environments for r-selected species

• Variable/ stressful environments

Environments for K-selected species

• Stable environments

Basics of Primary Succession

• Can be very slow

• Initial conditions can be very inhospitable

• First colonizers (Pioneer or Early successional species) tend to be stress-tolerant

• Transform the habitat in ways that benefit their growth and that of other species

First colonizers (pioneer or early successional species) tend to be…

Stress- tolerant

Basics of Secondary Succession

• Occurs after primary succession

• Or can occur after fires, storms, logging, disturbance to climax community

• The legacy of preexisting species and their interactions with colonizing species play larger roles than in primary succession

Preexisting species secondary succession vs primary succession

• Preexisting species and their interactions with colonizing species play larger roles in secondary than in primary succession

Explain this Graph of Disturbance Intensity vs Disturbance Frequency and the Type of Ecological Succession that occurs

• Little successional Changes Occurs When:

  • When disturbance is frequent and mild — communities don’t have time or need to change much

• Blue Region

  • High disturbance intensity

  • High disturbance frequency

  • The region with the blue circle represents areas of very frequent and intense disturbance, where succession is often suppressed or reset constantly, leading to little successional change. Stable communities cannot form, and primary succession is unlikely due to the lack of recovery time

Succession can take decades to centuries, so how can we “observe” succession without watching the entire process unfold?

• Henry Cowles assumed that plant assemblages farthest from the lake’s edge were the oldest; the ones nearest the lake were the youngest, representing a time series of successional stages.

Figure 17.6- Henry Cowles’ experiment and Outcomes

• The first stages were dominated by a hardy ecosystem engineer, American beach grass.

• Beach grass traps sand and creates hills that provide refuge for plants less tolerant of constant burial and scouring.

• Cowles could predict how communities would change over time without actually waiting for the pattern to unfold, which would have taken decades to centuries.

• This technique is called “space for time substitution.”

  • Studied dunes of different distances from the lake shore.

  • Assumed that further inland = older dunes, and closer to shore = newer dunes

  • SPACE FOR TIME SUBSTITUTION

Space for Time Substitution

• Assumes time is the main factor causing communities to change

• Assumes unique conditions in a location are inconsequential

Charles Elton Experiment for Succession and Outcomes

• Believed that organisms and the environment interact to shape succession.

• Succession in English pine forests following felling depended on moisture conditions.

  • Wet areas developed into sphagnum bogs, drier areas developed into grass and sedge marshes.

• Elton emphasized that the only way to predict the trajectory of succession was to understand the biological and environmental contex in which it occurred. In other words, time is not the only player in succession

• Elton also recognized the contribution of animals to succession

CC Clements, Gleason, and Charles Elton’s approaches to succession that helped Henry Cowles’ Space for Time Substitution experiment

Aspect	Frederic Clements	Henry Gleason	Charles Elton
View of Community	A superorganism – tightly integrated and structured	A coincidental collection of species with similar needs	A dynamic system shaped by both species and environment
Succession Pattern	Orderly and predictable (like an organism’s life cycle)	Unpredictable and variable, shaped by chance and dispersal	Context-dependent; affected by environmental conditions and species traits
End Point (Climax)	Yes – a stable climax community determined by environment	No fixed climax – multiple possible endpoints	No single climax – trajectory depends on conditions
Role of Species	Species are interdependent; cooperation dominates	Species are independent; individual responses dominate	Species interact with and respond to the environment
Role of Environment	Drives a fixed successional path	Sets the stage, but outcomes are not fixed	Environmental variation affects which path succession takes
Mechanism of Change	Facilitation – early species prepare habitat for later ones	Dispersal & tolerance – species arrive and survive based on their traits	Species-environment interactions – both facilitation and inhibition occur
Legacy	Inspired the facilitation model	Inspired the tolerance and inhibition models	Highlighted the importance of external context in shaping succession (e.g., deforestation example)

Three Models of Succession (Connell and Slatyer)

1. Facilitation Model

2. Tolerance Model

3. Inhibition Model

Facilitation Model

• Inspired by Clements

• Early species modify the environment in ways that benefit later-arriving species but hinder their own continued dominance

Facilitation Model early species

• These early successional species have characteristics that make them good at colonizing open habitats, dealing with physical stress, growing quickly to maturity, and ameliorating the harsh physical conditions often characteristic of early successional stages.

Facilitation Model Later Species

• Eventually, however, a sequence of species facilitations leads to a climax community composed of species that no longer facilitate other species and are displaced only by disturbances

Tolerance model

• Earliest species modify the environment, but in neutral ways that neither benefit nor inhibit later species.

Tolerance Model Early Species

• Grow and reproduce rapidly life strategy

Tolerance Model Later Species

• Later species persist merely because they have life history strategies such as slow growth, few offspring, and long life that allow them to tolerate increasing environmental or biological stresses that would hinder early successional species.

Inhibition Model

• Early species modify conditions in negative ways that hinder later successional species.

Inhibition Model Early Species

• These early colonizers may monopolize resources needed by subsequent species.

• This suppression of the next stage of succession is broken only when stress or disturbance decreases the abundance of the inhibitory species.

Inhibition Model Later Species

• As in the tolerance model, later species persist merely because they have life history strategies that allow them to tolerate environmental or biological stresses that would otherwise hinder early successional species.

William Cooper Glacial Bay “Space for Time” Substitution Opportunity Experiment

• He established permanent plots that are still being used today.

• Glacier Bay, Alaska is one of the best-studied examples of succession.

• Melting glaciers have led to a sequence of communities that reflect succession over many centuries.

Fig 17.10- Results of Cooper’s Successional Communities at Glacier Bay, Alaska

• This graph shows how primary succession is slow, with each stage building the foundation for the next.

• Supported models of facilitation, inhibition, and tolerance, depending on the stage.

• Tree colonization is delayed due to the need for developed soil and stable microclimates.

• Pioneer Stage→ Facilitation

• Dryas Stage→Inhibition & Facilitation

• Alder Stage Facilitation → Inhibition

• Spruce Stage → Tolerance

Fig 17.11- Chapin Experiment: Soil Properties Change with Succession: What model of succession do these models support

These patterns show that:

• Early species modify the environment in ways that benefit later species.

• The community becomes more nutrient-rich, moist, and organic over time.

• These experiments, along with observations of unmanipulated plots, showed that neighboring plants had both facilitative and inhibitory effects on the spruce seedlings but that the directions and strengths of those effects varied with the stage of succession

➡ Facilitation Model

This model proposes that:

• Early colonizers (like mosses, lichens, Dryas, and alder) alter the environment (e.g., fix nitrogen, add organic matter),

• Making conditions more favorable for later successional species (like spruce),

• Paving the way for increased species richness and complexity.

This is exactly what we see here: early stages enrich the soil, allowing trees to eventually colonize.

Chapin Experiment: Pioneer Successional Stage Positive and Negative Effects on Spruce

Positive:

• Higher Survival

Negative:

• Lower Germination

Dryas Successional Stage Positive and Negative Effects on Spruce

Positive:

• Higher Nitrogen Level

• Higher Growth

Negative:

• Lower Germination

• Lower Survival

• Higher Seed Predation and Seed Mortality

Alder Successional Stage Positive and Negative Effects on Spruce

Positive:

• Higher Soil organic matter

• Higher Nitrogen Level

• More mycorrhizae

• Higher Growth

Negative:

• Lower germination

• Lower Survival

• Higher Seed Predation and seed mortality

• Root Competition

• Competition for Light

Spruce Successional Stage Positive and Negative Effects on Spruce

Positive:

• Higher germination

Negative:

• Lower growth

• Lower Survival

• Higher Seed Predation and seed mortality

• Root Competition

• Competition for Light

• Lower Nitrogen Level

Where Do Waves Fit in the Figure?

Answer: Bottom right quadrant

• High frequency (they occur often)

• Low to moderate intensity (they generally don’t remove all biomass or reset the system)

• This corresponds to an area of "little successional change" in the diagram.

Figure 17.13 Wrack Creates Bare Patches in Salt Marshes

• New England Salt Marsh: secondary succession.

• Salt marshes have different species compositions and physical conditions at different tidal elevations.

• Cordgrass, Spartina patens, dominates near the sea border; spike rush, Juncus gerardii, is found at the terrestrial border.

Figure 17.13- What is Wrack?

• (dead plant material) smothers and kills plants, leaving patches where secondary succession occurs. Salinity in the bare patches is high because of evaporation

Bertness and Shumway Experiment

• New England Salt Marsh

• Hypothesized that Distichlis could either facilitate or inhibit later colonization by Spartina or Juncus depending on the salt stress experienced by the interacting plants.

• Bertness and Shumway (1993) manipulated patches after they had been colonized to understand patterns of succession and the role Distichlis played

• Spike grass, Distichlis spicata, colonizes the patches first. It is eventually outcompeted by both Spartina and Juncus in their respective zones.

Figure 17.4: New England Salt Marsh Is Context Dependent: Experimental Observations

Experimental Design:

1. Two tidal zones were studied:

   • Low intertidal (Spartina zone) — closer to the shoreline

   • Middle intertidal (Juncus zone) — closer to land

2. In each zone, researchers created experimental bare patches and then:

   • Removed Distichlis from some patches and left Spartina or Juncus.

   • Removed Spartina or Juncus from other patches, leaving Distichlis.

3. Control patches were left unmanipulated to observe natural colonization.

4. Additionally, in each zone, half the patches were watered with fresh water to reduce salt stress, while the others were left dry.

In the Spartina zone (low intertidal)…

• Spartina always colonized and dominated the plots, whether or not Distichlis was present or watering occurred.

• Distichlis was able to dominate only if Spartina was removed, so it was clearly inhibited by Spartina (inhibition).

In the Juncus zone (middle intertidal)…

• Juncus was able to colonize only if Distichlis was present or watering occurred.

• The presence of Distichlis helped shade the soil surface, thus decreasing salt accumulation (facilitation).

If plots were watered…

• Distichlis was easily outcompeted by Juncus (tolerance).

Sousa Experiment

• found that the first species to colonize and dominate a patch was always the bright green alga Ulva lactuca (FIGURE 17.15A). It was followed by the red alga Gigartina canaliculata.

  • Found that colonization by Gigartina was accelerated if Ulva was removed (inhibition) controlling succession,

    If Ulva is able to inhibit other seaweed species, why doesn’t it always dominate?

    - Sousa found that grazing crabs preferentially fed on Ulva, thus initiating a transition from the early Ulva stage to other mid-successional algal species.

  • In turn, the mid-successional species were more susceptible to the effects of stress and parasitic algae than the late successional Gigartina.

    - Gigartina dominated because it was the least susceptible to stress and consumer pressures.

Farrell Experiment

• Farrel saw that in the Oregon intertidal zone the uccessional order of these:

  • Cthamalus (barnacle) -> Balanus (barnalce -> macroalgae (algae)

  • Tolerance to facilitation

• Removal experiments showed Balanus was able to outcompete Chthamalus over time

  • = tolerance model.

• Balanus did not hinder macroalgal colonization, but facilitated it,

  • = facilitation model.

• Farrell Experiment: But why and how would Balanus facilitate macroalgal colonization?

• Farrell created experimental plots from which Balanus, limpets, or both were removed, then observed macroalgal colonization in those plots.

  • He found that macroalgae colonized all of the plots without limpets but had a much higher density in the plots with barnacles than in those without barnacles

    - These results suggested that Balanus did indeed act to impede limpets from grazing on newly settled macroalgal sporelings

Farrell Experiment- Why doesn’t Chthamalus have the same facilitative effect on macroalgae that Balanus does?

• Farrell suspected that the reason was Balanus’s larger size (it is nearly three times wider than Chthamalus).

  • It seems likely that the smaller and smoother Chthamalus does not retain as much moisture, or block as many limpets, as the larger and more sculpted Balanus—or the mimics, for that matter.

Explain Farrell Experiment Setup Figure 17.16- Algal Succession on the Oregon Coast is Driven by Facilitation

What overall conclusion can be drawn from this study?

• 1. Succession followed a predictable species sequence:

  • First colonizer: Chthamalus dalli (small barnacle)

  • Replaced by: Balanus glandula (larger barnacle)

  • Followed by: Three macroalgal species — Pelvetiopsis limitata, Fucus gardneri, Endocladia muricata

  2. Tolerance model supported:

  • Chthamalus did not inhibit colonization by Balanus

  • Instead, Balanus outcompeted Chthamalus over time

  • → Supports the tolerance model, where later species are not prevented by early ones, but outlast them due to competitive superiority

  3. Facilitation model supported:

  • Balanus facilitated macroalgal colonization

  • Macroalgae colonized better in the presence of Balanus, especially when limpets (grazing snails) were removed

  • → Suggests Balanus protected algae by:

    • Reducing desiccation stress

    • Physically blocking limpets from grazing new algal sporelings

  4. Size matters — Balanus vs. Chthamalus:

  • Balanus is ~3x larger than Chthamalus

  • Larger barnacle mimics (made of plaster) had an even greater facilitative effect on macroalgal growth than real barnacles

  • → Conclusion: Size and structure influence how well a species can buffer environmental stress and prevent consumer access

  🔁 Key Takeaway:

  Farrell’s work revealed that facilitation and tolerance play important roles even in competitive, space-limited systems like the rocky intertidal zone. Succession is not governed by inhibition alone, but by a dynamic interplay of biotic interactions shaped by species traits (like size) and environmental stress.

Succession summary

• Driven by many mechanisms - no one model fits any one community.

• Facilitative interactions are often important drivers of early succession, especially when physical conditions are stressful.

• As succession progresses, larger, slow growing and long-lived species may begin to dominate.

• Competition may play a more dominant role later in succession.

Facilitative interactions are often…

Important drivers of early succession, especially when physical conditions are stressful.

As succession progresses…

larger, slow growing and long-lived species may begin to dominate.

Alternative stable states

Different communities develop in the same area under similar environmental conditions

Competition may play a more…

dominant role later in succession

Stable Community

When the community returns to its original state after perturbation.

Sutherland (1974) study on Marine Fouling Communities Experimental set up

• Investigated succession and community stability in marine fouling communities—organisms like sponges, hydroids, and tunicates that colonize hard surfaces like docks and ship hulls.

• Setup:

  • Ceramic tiles were suspended from a dock

  • Placed out in spring to allow natural colonization by invertebrate larvae.

• Observation:

  • Over 2 years, the tiles developed two distinct communities:

    • Dominated by Styela (a solitary tunicate)

    • Dominated by Schizoporella (a bryozoan)

• Key Observations:

  • Styela tiles stayed stable—other species couldn’t invade once Styela had established.

  • Schizoporella formed its own stable community when tiles were placed in late summer.

  • Styela declined in winter but recolonized strongly each spring.

• Fish Exclusion Experiment:

  • New tiles were set out with half enclosed in cages to exclude fish predators.

  • Caged (no predators): Dominated by Styela

  • Uncaged (predators present): Dominated by Schizoporella

Ecological concepts of Sutherland’s study outcomes

• Styela is competitively dominant if left undisturbed (no predators).

• Schizoporella dominates in the presence of predators that prevent Styela from establishing.

• Once Styela gets large enough, it acts as its own “cage,” protecting itself from predators (natural defense via size).

• Suggests strong species interactions (especially with predators) determine which community forms.

• Alternative Stable States

  • Sutherland’s work supports the theory that multiple stable community types (states) can exist under the same environmental conditions, depending on species interactions and history.

  • Styela-dominated and Schizoporella-dominated communities are seen as two “valleys” in a stability landscape (see Figure 17.18).

  • A “ball” (the community) may settle into one valley or the other depending on key factors like predator presence.

  • If a strong interactor is removed or added, the system may shift states and not return—this is called hysteresis.

What factor influenced which species became dominant in the communities in Sutherland’s study?

The timing of tile placement—tiles placed in late summer favored Schizoporella dominance.

What is the main ecological lesson from Sutherland’s study?

Small differences in initial conditions (like timing) can lead to different long-term stable community structures.

What do the graphs in Figure 17.17 illustrate?

They show how predation affects the dominance of Styela (tunicate) vs. Schizoporella (bryozoan) in marine fouling communities.

In the absence of predation, which species dominated during summer months?

Styela dominated from June to September.

What happened to Schizoporella when Styela was dominant (without predation)?

A: Schizoporella had very low cover and only increased again as Styela declined (starting October).

How did predation affect community composition?

A: Predators suppressed Styela, allowing Schizoporella to dominate consistently throughout the year.

Q: What do these results say about the role of predators in maintaining community structure?

A: Predation prevented Styela from dominating, stabilizing a community dominated by Schizoporella.

Two different stable communities seemed to result:

• One dominated by Styela, a solitary tunicate.

• One dominated by Schizoporella, a bryozoan, on tiles placed out in late summer.

Q: What does this experiment demonstrate about alternative stable states?

A: Predation can shift communities between alternative stable states—one with Styela dominance (without predators), the other with Schizoporella dominance (with predators).

Alternative stable states can be controlled…

by strong interacting species, like predators

Hysteresis

Is an inability to shift back to the original community type, even when original conditions are restored.

Cultivation

• Depensation hypothesis: adult cod may eat competitors for juvenile cod

• Decreases in river herring (a proposed food source) resulting from damns is preventing cod recovery

• Ocean warming could be limiting the recovery of cod