Lecture 11: Communities through time

The “Balance of Nature“

• Before Darwin’s work, nature was typically viewed to be “in balance”, where each species “plays its part” to maintain a stable ecosystem, and where ecosystems always return to a “natural state” after a small disturbance – a view that persists in popular culture until today (Circle of life in lion king)

• e.g., Herodotus, ~484-425 BC: predators never excessively consume prey populations to maintain a “wonderful balance”

Nature is dynamic.

• Change is the rule, rather than the exception

• Examples

  • Seasonality

  • altered animal home ranges during the COVID-19 pandemic

  • (some of the) wildfires in Brazil (2019), Australia (2020), California (2022), and Canada (2023)

  • ecological succession

  • six mass extinctions

Seasonality

• Changes of temp, and weather have consequences on the plants and animals they support (soil fungi, micro level, insects, parasites, and larger level animals)

• Animals use park differently in winter than in summer. Ex. A prey in winter might use environment in different way and hide so that they do not leave tracks in snow so predator can find them

altered animal home ranges during the COVID-19 pandemic

• When everyone was inside during lockdown, cities looked abandoned, to animals decided to explore these habitats that now looked different without humans

• Ex. Dolphins temporarily returned to canals in Venice

(some of the) wildfires in Brazil (2019), Australia (2020), California (2022), and Canada (2023)

• Wildfires becoming more frequent because of climate change

• This alters habitats

ecological succession

• In shores of Lake Ontario, over last 5 yrs, there are sand dunes where it used to be flat

• Sand dunes are there to provide more habitats for species that need it, as well as prevent erosion

Yet, predictable patterns emerge across spatial and temporal scales all the time. How is that possible

Successions

Tommy Thomson Park

• Started out as a landfill and was managed for natural succession

• Now globally important ecosystem (great place for bird watching)

• More trees to the left = more trees

• Originally built as a landfill in the 1950s, using construction rubble, dredged sand, and excavation waste.

• Created to support harbour expansion and serve as a breakwater, not as a park.

• Over time, the unused landfill naturally transformed into an accidental wilderness, with plants and wildlife colonizing the area (1970s–1980s).

• Became an important migratory bird habitat and ecological hotspot.

• Now managed by the TRCA, restored and protected as one of Toronto’s largest natural urban parks.

• A unique example of a human-made landfill turning into a thriving ecosystem.

Succession

• Communities change in fairly predictable ways.

• Ecological succession is the series of changes in the species composition of a community through time at a particular location that occur in a fairly predictable way as a result of biotic and abiotic influences, as the location goes from bare rock or lifeless water to being filled with interacting species.

• Ex successional change on Hawaiian islands

  • After volcanic eruption we can imagine that island will not stay lifeless forever as long as you let enough time pass

  • Initial colonizers (seeds fall in between cracks and fall into soil) and over time first plants can persist in these are. These new plants can create an environment so that bigger plants can take hold. As time passes, more soil, resources and space accumulated to sustain larger plants → forest

Primary succession

• refers to succession that begins in/on substrates that contain no organisms and no organic material.

• tends to be slow, as the first colonists must arrive from elsewhere, and it is only through the actions of these species that the environment becomes suitable for the establishment of species in later seres

The frequency and intensity of disturbance determines to

• which stage a community is set back, and thus, whether primary or secondary succession occurs.

• Ex. Forest, big storm goes through, takes down 90% of the trees, insects and animals die. But you are not going back to zero, when rebuilding you are still starting from an established community → secondary succession

• Little and medium size disturbances are frequent→ kill some life but not everyone → secondary succession

• Much less frequently → volcano erupts or hurricane (once in lifetime event) → kills everyone → primary succession

Secondary succession

• occurs following a disturbance where some, but not all, organisms have been destroyed.

• Ex. Forest fires, hurricane destruction, mining

Early successional species tend to be

r-selected

Late successional species tend to be

K-selected.

Early successional species tend to be r-selected

• The first species that establish are not going to be the big trees

• Small, high reproduction rate, low survival rate, short generational time, rapid development, early maturity and high dispersal ability

• Per capita growth rate from logistic function → these species have large per capita growth rate, in matter of days, insects can reproduce in large amounts

late successional species tend to be K-selected

• large, low reproductive rate, high survival rate, long generational time, slow development, late maturity, low dispersal ability

• Carrying capacity → these species not successful because they grow fast, they are successful because they are persistent

  • Parents take care of children ensuring they live to maturity

Example: Primary Succession on Mount St. Helens

• Active Volcano erupted And exploded

• Half of the mountain collapsed

• Everything within radium of 30-40 km was destroyed

• Lifeless environment → all above ground plants died, lots of animals and some humans

• Starting from barren ground → succession occurs as little plants establish

  • The soil is basically ash. The plants that first established were nitrogen fixers (they don’t need nitrogen from the soil, they can harness it from the air)

  • Then we go from shrubs and now there is a forest

Example: Secondary Succession in North Carolina’s Piedmont Forests

Abandoned farm still had some resources left (not completely barren) turned into a forest

Example: Animal Succession in North Carolina’s Piedmont Forests

• Shifts in the bird community of North Carolina’s Piedmont Forests over time are associated with successional plant community changes and differing habitat preferences among bird species

• As environment changes, the animals that are better competitors for the new environment will alive

General Patterns of succession

• Primary succession takes longer than secondary succession, because you are starting from zero

• Chance events play a significant role in determining successional pathways

  • Mount St Helens succession was quicker than expected because not everyone died. There were gophers underground making pockets where seeds and nutrients would remain → succession was quicker

• Succession relies on complex sets of biotic interactions

• Plant cover, biomass, species richness, and species diversity all tend to increase over time

Connell & Slayter’s 3 models

Facilitation, Inhibition, and Tolerance Models of Succession

Facilitation model:

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

• The sequence of species’ facilitations leads to a climax community.

Tolerance model:

Early species modify the environment in ways that neither benefit nor inhibit later species.

Inhibition model:

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

• Succession requires disturbance for succession to continue

• Disturbance creates more space and resources for other species to inhabit area

To test which mechanisms are determining the observed successional pathways, a field experiment was conducted:

• add spruce seeds to each successional stage, and monitor their rates of germination, growth, and survival over time.

• Spruce seedlings experience both positive (facilitative) and negative (inhibitory) effects, and these change across successional stages.

• Pioneer stage → Facilitation model

  • Early species improve conditions (e.g., higher survival).

• Dryas/Alder stage → Inhibition model

  • Mid-successional plants compete (lower germination/survival, more predation, competition for light & roots).

• Spruce stage → Tolerance model

  • Spruce can establish despite low nutrients and competition; success depends on ability to tolerate conditions.

• Key idea: The direction and strength of interactions shift with succession—from helping spruce seedlings → hindering them → neutral/tolerant dynamics.

Early succession

Facilitation likely most important, especially when physical conditions are stressful

Mid succession

Mixture of positive and negative interactions

Late succession

Bigger, long-lived species; competition most important

Discuss how succession has progressed in Tommy Thompson Park since its inception.

• Began in the 1950s as a landfill made of rubble, sand, and construction waste.

• Primary succession occurred on bare, disturbed substrate.

• Pioneer species (grasses, shrubs) colonized first, stabilizing soil.

• Followed by early shrub communities and nitrogen-fixing species improving soil quality.

• Alder, cottonwood, and willow established, creating early forests.

• Today: a mosaic of meadows, shrublands, young forests, wetlands, and rich bird habitats—all formed through natural colonization.

• Represents classic natural succession on human-made land.

Right vs left of Tommy Thompson

• Left side is older because there are more trees there

• Left side is about 50 yrs ahead of the right hand side→ right is still catching up while left has moved on to forest community

• Right is light green because it has less sunlight than the left

  • Light green areas = early-successional plants (grasses, herbs, small shrubs) growing on thin landfill soil.

  • These plants have lighter leaves that reflect more sunlight → appear pale green.

  • Dark green areas = denser, later-successional vegetation (shrubs, trees) with more chlorophyll, which absorbs more light.

  • So the color difference is due to vegetation type + light reflection vs. absorption.

• More species richness/ or trophic levels on left

Outline likely future successional pathways for Tommy Thompson Park.

Both sides will continue with succession → move towards climax → larger trees, different types of species

Community Assembly

• Can we guess what species will be present at the end of succession

• While large-scale patterns of community succession can be fairly predictable, different communities will have different species compositions. → the species present will not be the same, it depends on habitat

• Community assembly may vary, for example, because of priority effects/ Or chance events (who wins the competition depends on who arrives first, or who has more initial abundance)

Priority effects

• the impact that early-arriving species have on the establishment, growth, and survival of later-arriving species in a community.

• These effects can be inhibitory>>, where the first species negatively affects the second (e.g., by competing for resources), or facilitative>>, where the first species creates conditions that benefit the second (e.g., by improving soil or microclimate).

• The arrival order of species can strongly influence the final community structure and composition

Assembly “rules”

• are guiding principles outlining how the timing of species arrival or the initial suite of colonizing species can determine the species composition of the community.

• At least some such rules must exist (e.g., obligate parasites cannot establish without their host; specialist predators cannot establish without their prey; obligate mutualist cannot establish without their mutualistic partner),…

• But general rules are difficult to establish empirically due to the many possibilities that a community can assemble: there are typically many more possible combinations of species assembly than there are communities, making it difficult to demonstrate that observed patterns of species co-occurrence are nonrandom.

Assembly rules can be studied in experimental microcosms.

• James Drakes’ experiments on how the order of colonization affects community composition showed that:

  • widely differing community compositions can be achieved by solely altering the sequence of colonization

  • variability among replicates was low, suggesting repeatable mechanisms at play

  • for the algal species shown on the right, species dominance (blue curve) was due to interspecific competition (suggesting no assembly rules in this case), but dominance among the remaining two species was influenced by the order of introduction (suggesting assembly rules in this case)

    • Blue species is always dominating no matter when they are introduced → blue is dominating over other 2 and priority effects do not apply to them

    • Light and dark green → order of arrival does matter. Whoever arrives first dominates → some assembly rules do apply

Restoration ecology

• provides many large-scale “experiments” on community succession and applies successional principles for management.

• It aims to manage highly degraded or newly established sites by providing conditions that make sites physiologically tolerable for a diverse array of species to accelerate succession towards a desired endpoint community.

• Example: Sacramento River National Wildlife Refuge, California (succession on abandoned farmlands)

• Example: Tommy Thompson Park, Toronto (succession on newly established landfills

Disturbance

• Definitions may vary based on the context, ecosystem, and research question, but are generally understood to refer to some type of event that disrupts ecological processes and/or ecosystem, community, or population structure, and which directly or indirectly creates opportunities for new individuals to be established.

• Example, Human-Caused Disturbance: Oil spill

• Example, Natural Disturbance: Hurricane

• Example, Abiotic Disturbance: Hurricane

• Example, Biotic Disturbance: Grasshopper outbreak

Disturbance is an integral part of all ecological systems.

• Depending on type, frequency, and intensity, disturbance may reset successional processes, or enable them

• Example: lava flow removing (nearly) all living matter, leading to primary succession

• Example: forest fire might reduce the abundance of dominant species and facilitate the colonization success of other species, typically leading to secondary succession

Example: The Effects of Human Disturbance on Mammalian Communities on the Osa Peninsula, Costa Rica

• Osa peninsula → biodiversity hotspot Harbours 3% of all species found on earth

• Question: how human disturbance effect animal communities and their succession

• Use camera traps to count every species you see. Maybe in disturbed area you will see different species or different number of species compared to an undisturbed area

• Human disturbance benefits some species and is detrimental for others.

• Coatis are cool with humans (blue they’re ok with that). Same goes for armadillo, opossum all do ok when humans disturb area because they are in blue

• don’t do ok when humans disturb area: in red→ peccary, tapir, Jaguar (large herbivores) or agouti (oversized rodents)

• Look at how steep middle part of bottom photo

  • Area with no disturbance, most common species is agouti. In disturbed area, agouti drops in commonness

  • In disturbed area Tapir basically nonexistent

  • Common opposum not common in undisturbed areas, but more common in disturbed areas

• Therefore, human disturbance alters the rank abundances of species, with some species becoming more common and others declining.

• These effects lead to decreasing community diversity with increasing disturbance in our study system

Many types of disturbance exist, making it difficult to generalize how disturbance affects ecosystems, communities, and populations.

• One important generalization that has been proposed in the 1970s is the intermediate disturbance hypothesis.

• It suggests that species diversity will be greatest at intermediate levels of disturbance

Intermediate Disturbance Hypothesis

• suggests that species diversity will be greatest at intermediate levels of disturbance

• Lots of disturbance = lots of mortality for species = r selected species will persist

• Low abundance = a stable environment leads to competitive exclusion by dominant species for many other species and limits diversity (mostly K-selected species will persist)

Why do r-selected species persist in environments with lots of disturbance?

• High disturbance = high mortality for many species

• r-selected species thrive because they:

  • Reproduce quickly

  • Colonize open space fast

  • Don’t rely on long-term stability

• Result: r-selected species dominate in frequently disturbed areas.

Why do K-selected species dominate stable, low-disturbance environments?

• Low disturbance = stable environment

• Leads to competitive exclusion: dominant species outcompete others

• Diversity decreases because few species can compete long-term

• Result: K-selected species (efficient competitors) persist and dominate.

The intermediate disturbance hypothesis has been fairly successful

• in explaining species diversity in some well- understood systems, but when viewed across hundreds of studies, diversity peaks at intermediate disturbance values in only a minority (~20%) of studies.

• Example: vegetation growth on boulders in the intertidal zone

  • Size of rock can determine how badly it will be impacted by disturbance

  • Pebble = in hurricane anything that lives on that pebble will die = lots of disturbance

  • Largest rock = hurricane comes and the rock will still be there

  • Intermediate size rock will have the most species = intermediate disturbance hypothesis holds true

Alternative Stable States

• Although succession and other ecological dynamics often follow predictable pathways, disturbances can affect which path is taken

• Ex Throw phosphorus in lake, turn it from oligotrophic to eutrophic → get systems overgrown with algae → oxygen depletion → fish death

  • Whether or not we throw the phosphorus in, determines what we get at the end of the day

Equilibria

• Recall: A system is at equilibrium if its rate of change is zero. A system may have more than one equilibrium.

• Equilibria may be stable or unstable

• Example: Equilibria of population growth models

  • 2 stable states (species extinct or at carrying capacity) and there is a separator between the two (the allee threshold)

• Example: Equilibria in the Lotka-Volterra model of competition

  • Starting lots of species 2, few species 1, species 1 goes extinct

  • Starting with lots of species 1, few species 2, species 2 goes extinct

• Example: Equilibria in the Lotka-Volterra model of predation

Bistability

• A system may have one or more stable equilibria.

• We refer to a system as bistable if there are two different stable equilibria to which the system can be attracted.

• Hysteresis occurs when a larger perturbation is needed to shift the system from one stable equilibrium to another than the other way round.

Hysteresis

• that an ecosystem can have two stable states, but the effort needed to move between them is unequal.

• To leave the first stable state, you need a big disturbance.

• But once the system has shifted to the second state, simply removing the disturbance does NOT return it.

• Instead, you need an even larger push in the opposite direction to get back to the original state.

• Destruction is easier than return from destruction

Hysteresis Examples

• Trophic cascades due to overfishing

• Eutrophication due to overfertilization

• Coral bleaching due to climate change and ocean acidification

• Coexistence and alternative stable states in the bioeconomics of fisheries and aquaculture

Trophic cascades due to overfishing

1 stable states (lots of otters and kelp forest) to another (few otters and diminished kelp forest) → takes long time to bring otters back

Eutrophication due to overfertilization

Fertilizer causes eutrophic systems and going back to healthy ecosystem is very complicated

Coral bleaching due to climate change and ocean acidification

• Coral bleaching occurs when it gets too warm or acidic the corals will eject their symbiotic algae. So the corals are still there (there is possibility to get those photosynthesizing algae and be healthy again)

• But if coral is too acidic or warm for a long time, corals will die

Coexistence and alternative stable states in the bioeconomics of fisheries and aquaculture

• In salmon aquaculture, fish are raised in open net pens, where salmon are kept at very high densities. This crowded environment allows diseases like sea lice to spread quickly. Because the pens are open to the ocean, these diseases also spread to wild fish populations outside the pens.

• As wild fish become infected and decline, humans rely even more on farmed salmon to maintain food supply. This creates a feedback loop:

  • More pens → more disease → fewer wild fish → even more dependence on pens.

• This system can create alternative stable states:

1. State 1: Healthy wild fish populations coexist with limited aquaculture (fish farms).

2. State 2: Depleted wild fish populations with heavy dependence on aquaculture (fish farms).

• Once the system shifts to the second state, it’s hard to reverse because disease pressure and ecological damage remain high. Thus, aquaculture can push fisheries into a new stable state where wild fish can no longer recover.

Polar bears are likely to go extinct in several regions of the Arctic over the coming century. Discuss how Arctic marine communities and food webs might change as a consequence.

• On land polar bears are no match for caribou

• in water polar bears are no match for seals

• When polar bears are gone, that’s a niche that becomes unoccupied (niche of top predator)

• Killer whale will occupy that niche and make their way and colonize the arctic

• Arctic will not be barren but experience a different stable state

• Cascades and top down control can also happen:

  • Polar bears provide top-down control by limiting seal populations. If they disappear, seals would increase sharply, intensifying their predation on Arctic cod and other fish. This triggers a trophic cascade: depleted fish populations weaken food availability for seabirds, belugas, and narwhals, while orcas move in and become new apex predators.

Can science predict whether killer whales or polar bears will be better top predator

• Science’s role is to tell society what could or would happen

• Science does not tell you what is better or worse

• What we want is a societal decision → do we try to slow down global warming or do we let the killer whales come in and polar bears disappear

• Even in covid, science did not tell us to take vaccines. It said that if you take a vaccine then we will avoid large number of people getting sick → then we as a society decided that that was good