Bio 1B Midterm 2

Ecology

The study of the relationship between organisms & their environment. It looks at how living things interact with each other and with non-living elements like water, soil, and climate.

Population

a group of individuals of the same species living in a particular area

Community

all the different populations of species living and interacting in a particular area

Ecosystem

a community of living organisms plus the non-living elements of the environment interacting together

Biosphere

The global sum of all ecosystems; all regions of Earth where life exists

What is Life History and some key aspects of it?

The pattern of growth, reproduction, and survival that an organism follows during its lifetime

Life history refers to the collection of traits that define a species’ life cycle and the timing of major life events.

Key life history traits:

• Average lifespan

• Age at first reproduction

• Number & timing of reproductive episodes

• Size & number of offspring in each episode

• Duration & investment of parental care

• Survivorship

Example: A caterpillar becomes a butterfly, showing a complex life cycle with distinct stages

Principle of Allocation

The idea that organisms have limited resources, so investing in one function (e.g., reproduction) reduces the resources available for others (e.g., growth or survival)

• Principle of Allocation: Organisms have limited resources that must be divided among growth, survival, and reproduction; individual organisms have a limited amount of resources to invest in different activities & functions

Examples of resource allocation:

• Animals: foraging, breeding, allocating biomass to offspring, caring for offspring

• Plants: allocating biomass and nutrients to different parts (roots, stems, leaves, flowers, seeds)

What is a Trade-off & what are the types of reproduction trade-offs?

A situation where investing in one activity limits the ability to invest in another

• Resources invested in one function cannot be used for another (trade-off)

• Size-number trade-off: Species can produce many small offspring or a few large offspring

• Costs of reproduction: Investing heavily in reproduction one year may reduce the ability to reproduce in future years

What is Survivorship, and what are the different survivorship curve variations?

The proportion of individuals in a population that survive to a certain age

Survivorship measures the fraction of individuals surviving to a certain age. There are 3 main types:

• Type I curve: most individuals reach old age (e.g., humans)

• Type II curve: some individuals reach old age (e.g., squirrels)

• Type III curve: very few individuals reach old age (e.g., plants)

Fast-slow continuum

A spectrum describing how species balance growth, reproduction, and survival, from “fast” (short-lived, early reproduction) to “slow” (long-lived, delayed reproduction)

Fast species:

• Short lifespan

• Early reproduction

• Many offspring

• Less parental care

• Often small

Slow species:

• Long lifespan

• Late reproduction

• Fewer offspring

• More parental care

• Often large

This continuum is a helpful guideline, but exceptions exist, & life histories can be more complex.

Explain the nested relationships among populations, species, communities, & ecosystems

Populations, species, communities, and ecosystems are nested levels of biological organization. Populations are groups of individuals of the same species living in an area. Multiple populations of different species form a community, and when a community interacts with its environment, it creates an ecosystem.

Interpret life history tables and survival curves, & compare & contrast key features of life histories

Life history tables and survival curves show how many individuals survive and reproduce at different ages. Type I curves have most individuals living to old age, Type II curves have constant survival, and Type III curves have high early mortality. Life histories vary: “fast” species live short lives, reproduce early, and have many offspring, while “slow” species live long, reproduce later, and have fewer offspring with more care.

Interpret examples of life history trade-offs resulting from variation in allocation of acquisition of resources, and explain how these lead to diversity in life history strategies in variable environments

Trade-offs occur because organisms have limited resources. Investing more in one function, like reproduction, means less for others, like survival or growth. These trade-offs create diverse life history strategies that allow species to succeed in different environments.

What is immigration and emigration?

• Immigration - Individuals moving into a population

• Emigration - Individuals moving out of a population

B-D Model

• B-D model - Population model considering ONLY births & deaths

(Birth–Death Model)

The B–D model is a simpler version used when migration is ignored. It assumes the population is closed, meaning no individuals move in or out.

It only considers:

• B – Births

• D – Deaths

Population change is therefore:

Population change = Births − Deaths

• ΔN = change in population size

• Nₜ = population size at the current time

• Nₜ₊₁ = population size at the next time step

Example:
If N = 100, with

• 12 births

• 7 deaths

Then:

ΔN = 12 − 7 = 5

New population = 105.

Exponential Model. What are the 2 equations and what do they mean? What do the different r options mean?

Population grows continuously at a constant per-capita rate (r) with no density limits.

Assumptions

• Every individual has the same chance of reproducing or dying.

• Birth and death rates are constant over time.

• These rates do not depend on population size (no density dependence).

Intrinsic Growth Rate (r)

r = birth rate − death rate

Units: time⁻¹

• r > 0 → population increases

• r = 0 → population stays constant

• r < 0 → population decreases

Each individual contributes r new individuals per unit time on average, regardless of population size.

Example: E. coli reproducing by binary fission.

Limitation

Exponential growth cannot continue forever because in real populations:

• Resources become limited

• Competition increases

• Disease spreads more easily

As population density increases, growth eventually slows.

Exponential Growth Equations:

1. Instantaneous Growth Equation (Rate of Change) is dN/dt = rN

This equation describes how fast the population size is changing at a specific moment in time.

Variables:

• N = population size

• r = intrinsic growth rate (birth rate − death rate)

• dN/dt = rate of population change over time
  The larger the population (N), the faster it grows, because more individuals are reproducing.

2. Population Size Over Time is Nₜ = N₀ eʳᵗ
This equation predicts the population size after a certain amount of time.

Variables:

• Nₜ = population size at time t

• N₀ = initial population size

• r = intrinsic growth rate

• t = time

• e = Euler’s number (~2.718)

Relationship Between the Two

• dN/dt = rN → describes the rate of growth at a moment in time

• Nₜ = N₀ eʳᵗ → describes the population size after time t

Both describe exponential population growth when r is constant and there are no density limits.

For exponential growth, the slope of the line (on a logged y-axis scale) is the value of r.

So, these are NOT included in the exponential model: Immigration, Emigration, Negative density dependence, & Positive density dependence

Birth and death occur continuously, rather than in discrete units of time

Logistic Model

Logistic growth = population grows fast when small, slows as resources limit growth, and stops at carrying capacity K, producing an S-shaped curve.

Key Parameters

• r = intrinsic growth rate

  • Determines how fast the population grows when it’s very small

  • Constant

• K = carrying capacity

  • Maximum population size the environment can sustain

  • Population equilibrium occurs at N = K

How It Works

• Small population (N ≪ K): growth is fast, similar to exponential growth

• Population grows: growth rate slows because of density dependence

• Population reaches K: growth stops (dN/dt = 0), population at equilibrium

Shape: S-shaped growth curve

Equation

dN/dt = rN (1 − N/K)

Where:

• N = population size at time t

• dN/dt = rate of population change

• r = intrinsic growth rate

• K = carrying capacity

(1 − N/K) represents density dependence: growth slows as N approaches K.

Per capita population growth rate

The per capita population growth rate tells you how much each individual contributes to population growth on average.

So it standardizes growth, allowing you to compare populations of different sizes.

Example:

• Population A grows by 100 individuals

• Population B grows by 100 individuals

That sounds the same — but if:

• Population A has 10,000 individuals

• Population B has 200 individuals

then Population B is growing much faster per individual.

The formula is:

Per capita growth rate = (1/N)(dN/dt)

• N = population size

• dN/dt = total population growth rate (how fast the population is changing)

• (1/N) = dividing by the population size to get growth per individual

So the equation means:

population growth per individual = total growth ÷ population size

You use per capita growth rate when:

1. Describing exponential growth

   In exponential growth:

   (1/N)(dN/dt) = r

   meaning the per capita growth rate equals r, and it stays constant.

2. Comparing populations of different sizes

3. Understanding density effects

   In logistic growth, the per capita growth rate decreases as population size increases.

Quick Example

Population size: N = 100

Population increases by 20 individuals per year

So:

dN/dt = 20

Per capita growth rate:

(1/N)(dN/dt) = 20 / 100 = 0.2

Meaning each individual contributes 0.2 individuals per year on average.

✅ The key exam takeaway:

• In exponential growth, per capita growth rate = r (constant).

• In logistic growth, per capita growth rate decreases as N increases.

What is density dependence and the different types of it?

Density dependence - Population growth changes depending on population size. Density dependence occurs when the per capita population growth rate changes as population size (N) changes.

Step 1: Imagine a population

Say you have a forest with rabbits.

• Few rabbits → lots of food, easy to find mates

• Many rabbits → food is scarce, disease spreads, harder to survive

The way the population growth changes as the rabbits become more crowded is what we call density dependence.

Step 2: Three possible situations

1. Negative density dependence (most common)

• When the population is bigger, growth slows down.

• Negative density dependence = species grow faster when rare, helping them recover and coexist with others.

• slope is negative

• Crowding makes life harder: less food, more disease, more competition.

• Example: 10 rabbits → grow fast, 100 rabbits → grow slower.

2. Positive density dependence

• When the population is bigger, growth actually speeds up.

• Small populations struggle to survive or reproduce.

• Example: 2 wolves → hard to hunt and mate, 20 wolves → easier hunting and mating.

3. No density dependence

• Crowding doesn’t matter.

• Each individual contributes the same to growth no matter how many there are.

• Example: bacteria in unlimited nutrients in a lab — growth is constant.

Step 3: Super simple visual

Type	Line shape	Growth trend
Negative	Slopes downward	Growth slows as population grows
Positive	Slopes upward	Growth faster as population grows
None	Flat line	Growth stays the same

✅ Key Idea:
Density dependence = does crowding affect how fast a population grows?

• Negative: Yes, slows growth

• Positive: Yes, speeds growth

• None: No, doesn’t affect growth

Equilibrium population size

Equilibrium population size - Population size where births = deaths. A population is at equilibrium when its size stops changing.

Key Idea

• Population growth = 0

• Occurs when per-capita birth rate = per-capita death rate

Equation

(1/N)(dN/dt) = 0
Where:

• N = population size

• dN/dt = total change in population

At equilibrium: growth per individual = 0

How It Happens

• Birth rates usually decrease as population size increases (negative density dependence)

• Death rates usually increase as population size increases (positive density dependence)

The intersection of these two rates determines the equilibrium population size.

A population is at equilibrium when births equal deaths, so per-capita growth is zero, usually due to density-dependent births and deaths.

Carrying capacity (K)

• Carrying capacity (K) - Max population size the environment can support

A population is considered at equilibrium when N = K

K is NOT a variable

K is the largest population size that a population can maintain over time

Intrinsic growth rate (r)

• Intrinsic growth rate (r) - The population’s maximum possible growth rate when there are no limiting factors

Key Notes

• r > 0 → population growing

• r = 0 → population stable

• r < 0 → population shrinking

• In logistic growth, r is still calculated the same way, but density dependence reduces actual growth as N approaches K.

✅ Shortcut to remember:

• r = b − d if you know births and deaths per individual

• r = ln(Nt / N0) / t if you know population sizes over time

Population fluctuation

Population fluctuation - Population size rises & falls over time due to environmental variation

Real populations often do not follow perfect models

They can rise & fall over time due to changing conditions

BIDE Model & what are the 4 processes that affect population size?

The BIDE model explains how the size of a population changes over time. The name comes from the four processes that affect population size:

• B – Births: new individuals are born into the population

• I – Immigration: individuals move into the population from elsewhere

• D – Deaths: individuals die

• E – Emigration: individuals leave the population

Population change is calculated as:

Population change = Births + Immigration − Deaths − Emigration

So the BIDE model tracks all ways individuals can enter or leave a population.

• ΔN = change in population size

• Nₜ = population size at the current time

• Nₜ₊₁ = population size at the next time step

Example:
If a population starts with N = 100, with

• 10 births

• 5 immigrants

• 8 deaths

• 2 emigrants

Then:

ΔN = 10 + 5 − 8 − 2 = 5

New population = 105.

Density-Independent Effects

• N is limited by something unrelated to the size of the population

  • Population changes happen regardless of population size

• Examples:

  • Natural disasters

  • Extreme weather

  • Volcanic eruptions

    • The Chaitén volcano eruption damaged forest tree populations.

Competition & the Types of competition

• Competition - A & B both try to acquire the same limited resource

• Competition = when 2 or more individuals share a resource, & consumption by 1 reduces its availability for others, causing reduced growth, survival or fecundity

• Intraspecific competition = competition between individuals of the same species

  • The mechanism behind density-dependent population growth

  • Ex: Southern elephant seal males competing with each other for scarce mates

• Interspecific competition: competition between individuals of different species

  • Ex: Lions competing with hyenas for scarce prey

• Exploitation competition (a type of Indirect interaction): 2 predators share the same prey → better predator harms the other

  • Ex: fox & coyote competing for rabbits.

What is predation? How does predation impact prey? What are some prey defence mechanisms?

Predation - A kills B

• Predators often reduce prey abundance

• Prey strategies:

  • Defend physically:

    • Ex: turtle shell, plant thorns, porcupine spikes

  • Defend chemically:

    • Ex: posin dart frogs, skunks, coffee caffeine, tobacco nicotine 

  • Escape:

    • Ex: Some moths avoid bat predators by evolving ears to detect bats’ ultrasonic echolocation and drop to the ground, or by developing organs or wing scales that jam or absorb bat sonar

  • Avoid by mimicry:

    • Dishonest mimicry - A palatable (edible) species evolves to look like an unpalatable or harmful species to avoid being eaten

    • Honest mimicry - An unpalatable or harmful species has warning signals (like bright colors) that truthfully indicate it is dangerous or bad to eat

  • Fight back

Herbivory. Pros & cons?

Herbivory - A eats B (a plant), may or may not kill B

• 1 species eats part (or all) of another species, which is a plant

• Plant may or may not die, so herbivory is sometimes but not always predation

• Herbivores eating plants typically harms the plants

  • BUT sometimes beneficial…

    • Animals eat seeds but also disperse them

    • Removes dead tissue: grazing can reduce disease

    • Stimulates growth: damage can trigger regrowth/reproduction

Parasitism

Parasitism - A lives on/in B, may or may not kill B

REMEMBER: “all parasites are pathogens, but not all pathogens are parasites”

Pathogen = disease-causing organism or agent (bacteria, virus, fungus, parasite).

Fungal interactions can shift from mutualism to parasitism depending on resources

• Ex: Soybean gives carbon to fungi; fungi provide nutrients only if needed. With fertilizer, fungi may take carbon without benefit → parasitism

Mutualism

Mutualism - A and B help each other

• Can pollinate plants, disperse seeds, defend partners, gather nutrients, help digest food, photosynthesize, or provide habitat

• Ex: Acacia (a tree) that gets protection from herbivores because ants attack intruders, &  ants get a home & food

Commensalism

Commensalism - B helps A, no impact on B

• Some apparently commensal relationships may actually be mutualistic/competitive/etc.

• Ex: a remora & its host, a zebra shark; remora benefits by not having to swim & hark does not seem to be affected

Facilitation

Facilitation - General term for either mutualism or commensalism

• Typically not specified if the 2^nd species is impacted, but often the impact is positive

• Ex: In harsh environments, some plants create shade & keep soil moist, allowing other plants to grow nearby

  • At low stress (low elevation) → plants may compete for resources

  • At high stress (high elevation) → plants may help each other survive (facilitation/mutualism)

Defense

Prey strategies:

• Defend physically:

  • Ex: turtle shell, plant thorns, porcupine spikes

• Defend chemically:

  • Ex: posin dart frogs, skunks, coffee caffeine, tobacco nicotine 

• Escape:

  • Ex: Some moths avoid bat predators by evolving ears to detect bats’ ultrasonic echolocation and drop to the ground, or by developing organs or wing scales that jam or absorb bat sonar

• Avoid by mimicry:

  • Dishonest mimicry - A palatable (edible) species evolves to look like an unpalatable or harmful species to avoid being eaten

  • Honest mimicry - An unpalatable or harmful species has warning signals (like bright colors) that truthfully indicate it is dangerous or bad to eat

• Fight back

Dishonest/honest mimicry

• Dishonest mimicry - A palatable (edible) species evolves to look like an unpalatable or harmful species to avoid being eaten

• Honest mimicry - An unpalatable or harmful species has warning signals (like bright colors) that truthfully indicate it is dangerous or bad to eat

Exploitation competition

A type of indirect competition where organisms compete by using up the same limited resources, without directly interacting or fighting.

How it works

• Each individual reduces the availability of a resource (e.g., food, water, nutrients).

• Other individuals get less of that resource, even if they never directly encounter each other.

Example:

• Two plants growing near each other absorb the same soil nutrients.

  • One plant doesn’t physically attack the other, but by taking nutrients, it reduces the growth of its neighbor.

Why it’s an indirect interaction

• The effect on one species occurs through the shared resource, not through direct contact.

• In contrast, direct competition (interference competition) involves fighting, blocking, or aggressive behavior.

Indirect mutualism

A type of indirect positive interaction where species benefit each other through a shared predator or herbivore.

How it works

• Some species are less tasty to the predator.

• Predator eats a mix of tasty and less tasty species.

• This reduces the number of tastier species eaten, helping them survive.

• The less tasty species also survive because they aren’t completely eaten.

Example:

• Three wildflower species eaten by deer

  • Less tasty flowers reduce how much deer eat the tastier ones

  • All species benefit indirectly

Indirect mutualism = species help each other survive by sharing a predator, where less tasty species reduce predation on tastier species.

Interaction network

• Species interact in networks, not just in pairs

• Interaction network: diagram showing arrows between species with direct pairwise interactions

• Complex networks: multiple species interact directly & indirectly

• Ex: Milkweeds & monarchs:

  • Milkweeds produce toxic cardiac glycosides → most herbivores avoid it

  • Monarchs sequester toxins, specialize on milkweeds, pass toxins into adult stage

  • Milkweeds are mostly pollinated by bumblebees, partially by monarchs

  • Bumblebees are prey for birds and mammals

What are species interactions?

• Definition 1: an individual of species A influences the behavior or life events of an individual of species B

• Definition 2: an individual of species A influences the growth, survival, or reproduction of an individual of species B

• Definition 3: a population of species A influences the growth rate (dN/dt) of a population of species B

What are all of the Types of Pairwise interactions?

Competition, predation, Herbivory, mutualism, Commensalism, Facilitation, & Parasitism

What kinds of interactions can be + for one species and – for the other species?

Predation, herbivory, & parasitism

Community & the outcomes

• Community: multiple species living in the same place at the same time and potentially interacting.

• Does not include the abiotic environment (that would be an ecosystem).

• Often restricted to a single type of organism, e.g. ‘the plant community’ or ‘the microbial community’ but could include many different types of organisms

• The spatial extent of a community can be clear (a pond, for fish)) or unclear (how big is a tree community in a forest?)

• Outcomes: coexistence or extinction

Coexistence

When several species live together in the same area over time, even if their populations fluctuate.

Key Points

• Populations can vary in abundance (some common, some rare).

• Coexistence ≠ perfect stability; species numbers can rise and fall.

• Example: Hutchinson’s “paradox of the plankton”

  • Many plankton species coexist in lakes and oceans

  • Surprising because competition should favor only the best species

Coexistence = multiple species live together over time, even if populations fluctuate; not necessarily stable.

Scarcity

When a resource is limited in an environment, meaning there is less available than what organisms need.

Key Points

• Scarce resources limit population growth and survival.

• Can be abiotic (water, sunlight, nutrients) or biotic (food, mates, territory).

• Leads to competition between organisms.

Example:

• Limited water in a desert means only some plants and animals can survive.

• Few nesting sites can cause birds to compete for space.

Scarcity = a resource is limited, causing competition and affecting survival or growth.

Fundamental / realized niche

Ecological Niches:

• Species coexist when they use different ecological niches

• Fundamental Niche:

  • The full range of conditions/resources where a species could maintain a stable population in the absence of other species; niche limits are based on physiological tolerance limits and resource needs

• Realized Niche:

  • The real/actual niche

  • The actual conditions/resources a species uses when other species are present

  • Usually smaller than the fundamental niche because of competition or predation

High niche overlap → high competition

Low niche overlap → low competition

When can a realized niche be larger than a fundamental niche?

A realized niche is usually smaller than the fundamental niche, but it can be larger if interactions with other species (like mutualism) or release from predators allow the species to use more resources than it normally could.

for example for organisms that depend on others to exist (e.g. symbionts), or that are able to use more habitats than they would otherwise (e.g. plants in the desert that rely on other larger plants for shade/cooling)

Niche partitioning / overlap

• Species reduce competition by using different parts of the environment or resources

• More niche partitioning → less competition → greater coexistence (usually)

• Ex: Warblers (Robert MacArthur)

  • 5 bird species forage in the same trees

  • Fundamental niche: all could forage anywhere in the tree

  • Realized niche: each species feeds in different tree sections

  • This reduces competition and allows coexistence

Niche overlap can indicate the strength of resource competition.

Niches can include environmental or resource variables.

Under the competitive exclusion principle, two species occupying the same niche cannot coexist over time in the same community.

Predator/prey system. What are the 3 possible outcomes?

• Species do not share a resource – one is the resource for the other!

• 3 Possible Outcomes:

  • Predator eats all prey → prey goes extinct → predator goes extinct (0 species).

  • Predator cannot find enough prey → predator goes extinct → prey increase (1 species)

  • Predator and prey coexist (2 species)

• Ex: Lynx & Snowshoe Hare

Populations show cycles: Prey increases → Predator increases later → Prey decreases → Predator decreases

Lotka–Volterra Predator–Prey Model & Cycles

• Explains predator-prey cycles

• Pattern:

  • Low prey → predators decline

  • Low predators → prey increase

  • High prey → predators increase

  • High predators → prey decline

This repeating pattern allows coexistence

Spatial refuge. What are simple & complex environments?

• Spatial refuges: A physical place where a species can escape predators or competitors

  • Allow rare species to recover; enable prey to ‘bounce back’ from rarity & increase their population size

• Carl Huffaker’s Mite Experiment:

  • Predatory mites & prey mites lived on oranges

    • Simple environment:

      • Few oranges

      • No hiding places

      • Predator kills prey → predator later goes extinct

    • Complex environment:

      • Many oranges

      • Prey can disperse & escape

      • Both predator & prey coexist

Disturbance

Disturbance: A change in biotic or abiotic conditions that alters a community. Occur constantly.

• Ex: wildfire, weather changes, species introductions, extinctions, doctors prescribing antibiotics

Succession follows disturbance

What is succession & primary/secondary succession

Succession: Predictable changes in species composition after disturbance

• Primary Succession: following a disturbance, the community becomes empty, or approximately empty; killing every one

  • Species must immigrate from elsewhere to repopulate area

  • Ex: volcanic eruptions, glacial retreat 

  • Agriculture mimics human-controlled primary succession.

    • Farmers: Disturb fields regularly, Plant desired crops, Prevent other species with herbicides/pesticides, Repeat yearly to prevent competition

• Secondary Succession: following disturbance to an existing community, populations decline or only individuals of some life stages survive (e.g. seeds, spores). Much more common.

  • Initially-arriving species (early-successional) are outcompeted by later-arriving (late-successional) species

  • Early species may facilitate late species by improving soil nutrients

    • Like people got to a party early & called their friends to come because it’s a good party; changing the process

Character Displacement

• Character Displacement: 2 species occurring in sympatry ending up with different phenotypes; species evolve different traits when living together to reduce competition.

• Ex: Darwin’s finches evolving different beak sizes.

• Evolutionary response driving a reduction in competition.

Immigration impact on coexistence

Immigration Can Promote Coexistence:

• New species entering a community can help maintain diversity

• Ex: Tropical forests with 1000+ tree species coexist partly due to constant immigration

Species richness / evenness / composition

1. Species Richness

• Definition: Total number of species in a given area.

• Scale examples:

  • Alpha (α): Local site

  • Beta (β): Difference between sites

  • Gamma (γ): Entire region

2. Species Evenness

• Definition: How similar the abundances (# of individuals (either total, or per species)) of different species are.

• High evenness: Species are roughly equally common

• Low evenness: A few species dominate

3. Species Composition

• Definition: Which species are actually present in a community. (what it is composed of)

• Focuses on identity, not number or abundance

Analogy:

• Individuals = candies

• Species = candy colors

• Evenness = how many of each color

• Composition = which colors are in the bowl

Richness = how many species, Evenness = how equally common they are, Composition = which species are present.

Alpha/beta/gamma diversity

Alpha (α) diversity: Number of species in a local site (think of alpha wolf; like a pack in one site)

Gamma (γ) diversity: Total number of species across all sites (think of Gram; like all of the species across the distance between us)

Beta (β) diversity: Difference between alpha & gamma; difference in species between local sites (shows species turnover across sites) (B in Beta like Between)

Spatial scale

Refers to the size of the area being studied when measuring biodiversity or ecological patterns.

Key Points

• Spatial Grain: The smallest unit of measurement (like a 1×1 meter plot).

• Spatial Extent: The total area covered in the study (like an entire forest, state, or region).

• Patterns of diversity can change depending on grain and extent.

Analogy:

• Grain = one pixel on your screen

• Extent = your entire laptop screen

Spatial scale = the size of the area studied, with grain as the smallest unit and extent as the total area.

Species area relationship

Bigger areas usually have more species, but the increase slows down as area gets bigger.

Key Points

• Sublinear = slower-than-proportional increase

  • Example: Doubling the area does NOT double the species; it only adds some more species.

• Why bigger areas have more species:

  • More habitats and resources

  • Lower chance of extinction

• Conservation tip:

  • One large protected area usually preserves more species than several small areas of the same total size.

Analogy:

• Candy store: a bigger store has more candy types, but doubling the store size doesn’t double the types—it just adds a few more.

Species–area relationship = bigger areas have more species, but species increase slows down as area gets larger.

Island biogeography theory

Explains how the number of species on an “island” is determined by immigration and extinction rates.

Key Points

1. Distance from mainland:

   • Closer islands get more new species (higher immigration).

2. Island size:

   • Larger islands have lower extinction rates (more space and habitats).

3. Equilibrium richness:

   • Number of species stabilizes when immigration = extinction.

Note:

• “Islands” don’t have to be actual islands — can be any isolated habitat (mountaintops, forest patches, lakes, urban green spaces).

Island biogeography = species richness on isolated areas depends on size, distance, and the balance of immigration & extinction.

Equilibrium richness

The stable number of species in an area when the rate of new species arriving (immigration) equals the rate of species going extinct.

Key Points

• Determined by:

  • Immigration rate: how many new species arrive

  • Extinction rate: how many species disappear

• Larger areas → lower extinction → higher equilibrium richness

• Closer islands → higher immigration → higher equilibrium richness

• Applies to islands and any isolated habitat (mountains, lakes, forest patches)

Equilibrium richness = the number of species where immigration balances extinction.

Luxury effect

Less poverty → higher alpha and gamma diversity (“luxury effect”)

• Driven by redlining - Denial of mortgages/home-buying to non-white people in certain neighborhoods; subsequent inequality in public investment in urban neighborhoods

Latitudinal diversity gradient (LDG)

• Species richness (# of species) is generally higher near the equator & lower near the North & South Poles

• Observed across many groups of organisms

• Explanations:

  • Tropics have more land area → more species can exist

  • Less stressful environments (warmer, wetter) → more species survive

  • More solar energy → more energy to support more species

  • Higher temperatures → faster mutation → more speciation

  • More time to evolve → no ice sheets in tropics

• Historical note: LDG was absent in the deep past; diversity used to peak where land area was largest

  • Biodiversity patterns were very different in the Earth’s past

  • Antarctica had warm rainforests & no ice sheets

  • Longer evolutionary time → higher alpha diversity

  • Larger area → higher gamma diversity

Species distribution

Species Distribution (Species Range):

• Species Range = the area where a species occurs.

• Species distributions are shaped by multiple factors, all happening simultaneously, not step-by-step

• Key Limits:

  • Dispersal – Can the species reach the location?

  • Abiotic environment – Are non-living conditions (temperature, precipitation, soil, water) suitable for survival, growth, and reproduction?

  • Biotic environment – Are living conditions (predators, competitors, food availability) suitable for survival, growth, and reproduction?

• BTW: Humans can alter any limit & Behavior can influence dispersal & habitat use

Dispersal & Dispersal Limitations

Dispersal - Movement of individuals or gametes away from (and potentially back to) their original location

• Mechanisms:

  • Wind (dandelion seeds)

  • Water (coral gametes)

  • Biotic vectors: ingested/excreted (birds eating seeds), stuck in fur

• Limits: Behavioral avoidance (e.g., birds avoiding predators & whales avoiding ships)

• Human activity can help or hinder dispersal (e.g., introducing crops or animals; colonizers bringing maize across the planet)

• Ex: Cattle spread to the Americas from Europe/Africa in the 1800s, showing dispersal—not environment—limited their original range

Abiotic & Biotic & their limits

• Abiotic = non-living environment (sun, water, soil)

  • Examples:

    • Temperature, sunlight, wind

    • Water, soil, nutrients

    • Rocks, pH, salinity

  • Abiotic Limits:

    • Set the extremes of a species niche (e.g., the lowest/highest temperatures a species can tolerate)

    • Define the fundamental niche (all conditions a species can survive).

    • Ex: California mussels die at high temps; temperature sets their range

• Biotic = living components (plants, animals, etc.; predators, competitors, herbivores)

  • Biotic Limits:

    • Define the realized niche (where a species actually exists due to interactions)

    • Examples:

      • Herbivory: cattle reduce some plants’ distribution

      • Competition: 2 similar hedgehog species do not overlap in range

• Mixed examples: Soil and natural waters include both living & non-living elements

Environmental gradient & the 2 types

A gradient is a gradual change in an environmental condition across space.

• Species distributions change along gradients of environmental factors:

  • Temperature

  • Elevation

  • Storm or hurricane risk

  • Predation risk

• Types of Gradients:

  • Continuous: e.g., temperature from bottom to top of a mountain

  • Patchy: e.g., lake edges or fragmented habitats

Biome & what largely defines biomes

Biome: a region experiencing similar environmental conditions, & therefore containing a similar ‘core’ set of species.

• Species distributions overlap, creating biomes

• Largely defined by climate; the Mojave and Sonoran deserts are both desert biomes

How does Elevation impact biomes?

Elevation:

• Temperature decreases at higher elevations.

• Low elevation has higher temperatures

• Precipitation increases at high elevations on the windward side of mountains because rising air cools, causing water vapor to condense & fall as rain. After crossing the mountain, the air descends on the leeward side, becomes drier, & creates a rain shadow with little precipitation.

How does Latitude impact biomes?

Latitude (distance from the equator) affects climate, which helps determine which biomes occur in different regions of Earth.

Key Points

• Low latitudes (near the equator):

  • Receive more solar radiation

  • Warmer temperatures and often more rainfall

  • Example biomes: tropical rainforests

• Mid-latitudes:

  • Moderate temperatures and seasonal climates

  • Example biomes: temperate forests, grasslands, deserts

• High latitudes (near the poles):

  • Less solar radiation

  • Colder temperatures and shorter growing seasons

  • Example biomes: tundra, polar regions

Latitude affects solar energy and climate, which determines what types of biomes occur in different parts of the Earth.

How do oceans impact biomes?… Maritime / continental climate

• Oceans (Maritime vs. Continental):

  • Oceans buffer climate, so climate extremes are stronger in the interior of continents

  • Maritime climate: Oceans buffer climate → milder winters, cooler summers; less extreme temp. fluctuations

Continental climate: Interior of continents → more extreme temp. fluctuations

Hadley cell

A large-scale atmospheric circulation pattern near the equator that influences global temperature and precipitation patterns.

Key Points

1. Warm air rises at the equator, carrying moisture.

2. As the air rises and cools, moisture falls as heavy rainfall (tropical climates).

3. The air then moves north and south at high altitude.

4. It sinks in the mid-latitudes, becoming dry and creating many desert regions.

Hadley cells move warm, moist air upward at the equator (causing rain) and bring dry air downward in the mid-latitudes (often causing deserts).

Hadley cells form because the equator receives the most sunlight, heating the air and causing warm, moist air to rise. As the air rises, it cools and releases moisture as rain in the tropics. The air then moves north and south at high altitude, eventually cooling and sinking around the mid-latitudes (~30°). As the air sinks it warms and becomes dry, creating desert climates. This circulation pattern creates two Hadley cells (one in each hemisphere) and causes wet climates near the equator and dry climates in the subtropics.

Photosynthesis

Photosynthesis is the process where plants, algae, and some bacteria use sunlight to convert carbon dioxide and water into sugars, storing the energy in chemical bonds that organisms can later use.

• Photosynthesis: sunlight is captured & stored in chemical bonds (carbon compounds) inside organisms

• Photosynthesizing organisms include:

  • Land plants

  • Kelp

  • Phytoplankton

  • Autotrophic bacteria

Respiration

Respiration is the process where organisms break down sugars and other molecules to release energy for metabolism (all chemical reactions in an organism that provide energy and build/break down molecules), returning carbon to the environment and releasing some energy as heat.

Respiration: organisms break down carbon compounds to release energy for metabolism

• This process:

  • returns carbon to the environment

  • releases heat energy

Gross / net primary productivity (GPP & NPP)

• Gross Primary Production (GPP) - Growth; Total energy captured by photosynthesis

• Respiration (R) - Energy available; Energy plants use for their own metabolism

• Net Primary Production (NPP) - Ecological efficiency; Energy left over after respiration → This becomes plant biomass → Energy available to herbavors

  • Highest on land because NPP depends on temperature & water availability

    • Warm + wet → high NPP

    • Cold + dry + low nutrients → very low NPP

    • Ex: Tropical rainforests are a small area but have very high productivity

  • Lowest in the ocean

    • BUT Important Note: The ocean overall contributes a lot to global NPP because it covers so much area

    • BUT Ecosystems like: algal beds & coral reefs have very high NPP but are rare

  • Formula: NPP = GPP − R

    • Ecological efficiency = growth / energy available

    • Measurements:

      • biomass per area per time (example: kg/m²/year)

  • What happens to NPP?... Energy flow

    • Energy moves through ecosystems via trophic levels

Ecological efficiency

Ecological efficiency: fraction of energy later available to other organisms as growth (efficiency = growth / energy available); fraction of energy from one trophic level that is passed on to the next; calculated as energy used for growth ÷ energy available

• Rule of thumb: About 10% of energy moves to the next level

  • Example:

    • Plants capture 1000 units

    • Herbivores receive ~100

    • Carnivores receive ~10

Assimilation fraction

Assimilation fraction is the part of the food an organism eats that actually gets used by the organism for growth or metabolism.

• Not all the food you eat goes into your body—some is wasted (like poop or uneaten parts).

• The assimilation fraction is the energy that actually “counts” for the organism.

Example:

• A caterpillar eats 10 calories of leaves.

• It poops 4 calories, so only 6 calories are used for growth and energy.

• Assimilation fraction = 6/10 = 0.6 or 60%

It’s different from ecological efficiency, which is about how much energy moves to the next trophic level.

Trophic pyramid & order

• Because energy is lost at each step:

  • ecosystems usually form trophic pyramids

  • most energy at the producer level

  • less energy at higher trophic levels

• producers → primary consumers → secondary consumers

• Note:

  • Sometimes inverted pyramids occur, but scientists don’t understand them yet

Trophic cascade

Trophic cascade: when a change in one trophic level affects other levels

Example: More predators → fewer herbivores → more plants

Top-down / bottom-up control

Bottom-up control:

• Ecosystem is controlled by resources available to producers

• More nutrients → more plants → more herbivores → more predators

Top-down control:

• Ecosystem is controlled by predators

• Predators limit herbivores → herbivores affect plants

To know which one is happening ecologists must run experiments

Sociometabolism

Sociometabolism: metabolism of humans accounting for bodily energy use & also indirect consumption through appropriation of ecosystems (agriculture & animals) as well as other energy sources (burning biomass with fire, fossil fuels, etc.); total energy used by human society

• Includes:

  • human metabolism

  • Agriculture

  • Livestock

  • burning biomass

  • fossil fuels

• Slavery as a contributor to sociometabolism:

  • Human energy use can be unethically distributed, stolen from some people & taken by others

  • Black people’s labor as slaves enables some white people to accumulate fortunes that enable industrialization of Europe & U.S.

  • Industrialization & transition to fossil fuel use in turn influences shifts in those countries away from slavery

Stock / flux / equilibrium

• Stock/Pool: Amount of an element in 1 compartment of the system (e.g., carbon in soil).

  • Units: mass or mass/area.

• Flux: Rate at which the element moves between compartments (like moving groceries into/out of the fridge).

  • Units: mass/time or mass/area/time.

• Equilibrium: Stock is stable if flux in = flux out.

  • NO net flux

• Example analogy: Buying food = input, fridge = stock, cooking = output.

Net flux

Net flux = Sum of fluxes in minus fluxes out

= 0 at equilibrium

Residence time & formula

Residence Time: How long an element stays in a compartment at equilibrium.

• Formula: Residence time = Stock / Flux

• Residence time is defined only when the stock is at equilibrium; meaning the flux in = flux out

Example:

• Lake Water

• Stock: 10,000 m³ of water in a small lake

• Flux: 1,000 m³ flows in/out per year

• Residence time: 10,000 ÷ 1,000 = 10 years

• On average, a water molecule stays in the lake for 10 years

Sink / source

Sinks & Sources:

• Both occur when ‘fluxes in’ are not equal to ‘fluxes out’:

• There is a net flux

• Stocks change how much they store, so residence times are no longer defined

• Source: Stock decreasing due to net flux out (e.g., burning fossil fuels).

• Sink: Stock increasing due to net flux in (e.g., growing forests absorbing CO₂).

Net flux = Sum of fluxes in minus fluxes out

Haber-Bosch process

• What it is: Industrial method for making nitrogen fertilizer. Mimics Nitrogen Fixation

• How it works: Converts nitrogen gas (N₂) from the air into ammonia (NH₃) using high pressure, high temperature, and a catalyst.

• Why it matters:

  • Adds nitrogen to soils, increasing crop yields (key part of the Green Revolution).

  • Human-made nitrogen now contributes ~51% of global nitrogen fluxes.

• Energy use: Requires lots of fossil fuels (~1% of global energy production).

Simple analogy: Takes nitrogen from the air (which plants can’t use) and “fixes” it into a form plants can absorb.

Nitrogen fixation

Nitrogen fixation is how nitrogen gets into the soil/plant system from the atmosphere.

• Microbes (like bacteria) do the actual conversion, turning N₂ gas into ammonium or nitrates.

• Plants then get nitrogen from these compounds.

• Humans mimic this with the Haber-Bosch process to make fertilizer.

Chemical fertilizer

• Human-made nutrients added to soil to boost plant growth

• Often contains nitrogen (from Haber-Bosch process), phosphorus, and potassium

• Increases crop yields but can cause environmental problems like runoff to oceans

Acid rain

• Rain, snow, or other precipitation with unusually low pH (more acidic than normal)

• Adds nitrogen to ecosystems, increasing nitrogen inputs to soils and water

• Caused by air pollution: sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) from fossil fuel burning

• Can damage plants, aquatic ecosystems, soils, and human-made structures

Crop rotation

• Agricultural practice of planting different crops in the same field across seasons or years

• Helps restore soil nutrients, especially nitrogen (via nitrogen-fixing crops like beans, peanuts, clover)

• Reduces dependence on chemical fertilizers

• Can reduce pests, diseases, and soil depletion

Rock weathering

• The natural breakdown of rocks into smaller particles and minerals

• Releases nutrients like phosphorus into the soil

• Provides long-term nutrient supply for plants

• Slower on older rocks; faster on younger rocks

Dust transport

• Movement of dust particles (and the nutrients they carry) through the air over long distances

• Can deposit nutrients like phosphorus onto soils far from their original source

• Example: Phosphorus-rich dust from the Gobi Desert helps sustain forests on older Hawaiian islands

Greenhouse gas

• Sunlight reaches Earth → absorbed by the surface → re-emitted as infrared radiation (heat)

• Greenhouse gases (GHGs) absorb infrared radiation & re-emit infrared radiation, trapping more of it in the atmosphere instead of allowing it to radiate to space → act like a “planetary blanket.”

  • Carbon dioxide (CO₂)

  • Methane (CH₄)

  • Nitrous oxide (N₂O)

  • Ozone (O₃)

  • Water vapor (H₂O)

• Global temps rise as CO₂ levels rise (positively correlated; increasing overtime)

Explain the human carbon cycle fluxes (Fossil fuel burning / land use change)

Human Impacts on the Carbon Cycle:

1. Fossil Fuel Burning

   • Direct emissions of carbon to the atmosphere (e.g., cars, factories, power plants).

2. Land Use Change

   • Deforestation: removes carbon sinks → increases atmospheric CO₂.

   • Reforestation: creates carbon sinks → absorbs CO₂.

• Fossil fuels have a larger impact than land use

Climate models

• Climate models link data to physical processes & enable predicting future climates

• Simulation of all physical processes affecting radiation, mass, and heat, in the ocean, land surface, and atmosphere, calibrated with observational data

• Uncertainty about human choices causes the biggest uncertainty for models of future climates

• Representative Concentration Pathway (RCP) - scenarios of future emissions (like Carbon) based on population growth, economy development, and carbon efficiency of the economy

• RCP scenarios reflect different trajectories of dif human activities

Positive / negative feedbacks in the climate system

• Feedbacks: changes in 1 part of the system affect other parts.

• Positive feedback loop: X changes, causing Y to change, causing X to change further – this is a ‘destabilizing’ feedback

  • Ice feedback: warming = polar ice melting = lower albedo = more sunlight absorbed = more warming

  • Vegetation feedback: warming = more tree mortality = more CO2 in atmosphere from decomposition = more warming

  • Cloud feedback #1: warming = more high altitude clouds = more infrared radiation absorbed = more warming

• Negative feedback loop: X changes, causing Y to change, causing X to change back towards its original value – this is a ‘stabilizing’ feedback

  • Radiation feedback: warming = more infrared radiation (heat) emitted = more cooling

  • Cloud feedback #2: warming = more tropical atlitude clouds = more sunlight reflected to space = more cooling

• Plant feedbacks on water cycle:

  • Deforestation disrupts water recycling → less rainfall inland → drier conditions downstream.

Ice / vegetation / cloud / radiation feedback

• Positive feedback loop: X changes, causing Y to change, causing X to change further – this is a ‘destabilizing’ feedback

  • Ice feedback: warming = polar ice melting = lower albedo = more sunlight absorbed = more warming

  • Vegetation feedback: warming = more tree mortality = more CO2 in atmosphere from decomposition = more warming

  • Cloud feedback #1: warming = more high altitude clouds = more infrared radiation absorbed = more warming

• Negative feedback loop: X changes, causing Y to change, causing X to change back towards its original value – this is a ‘stabilizing’ feedback

  • Radiation feedback: warming = more infrared radiation (heat) emitted = more cooling

  • Cloud feedback #2: warming = more tropical atlitude clouds = more sunlight reflected to space = more cooling

Representative concentration pathway

Representative Concentration Pathway (RCP) - scenarios of future emissions (like Carbon) based on population growth, economy development, and carbon efficiency of the economy

a type of climate model

Phenology

The timing of seasonal biological events in organisms

• Examples: flowering, migration, breeding

• Climate change can shift phenology (events happening earlier or later)

Disease

• Disease: a harmful condition affecting an individual, sometimes reducing its growth, survival, or reproduction (or causing pain/suffering)

• Major Types of Disease Causes:

Biological pest control

• Using natural predators or parasites to control pest species instead of chemicals for crops

• Example: Polistes wasps introduced to eat cotton bollworm caterpillars, reducing crop damage

• Goal: reduce pests while avoiding heavy insecticide use

A hyperparasite is a parasite that infects another parasite. In biological pest control, if a specific parasite is destroying crops, scientists might introduce a hyperparasite to kill that primary parasite, thereby protecting the environment without using chemical pesticides

Infectious disease

• A disease caused by pathogens (bacteria, viruses, fungi, or parasites) that can spread between organisms

• Transmission can occur through vectors, contact, or environmental sources

Humans have many parasites that cause infectious disease

Parasite / host

• Parasite: organism that lives on or inside another organism and takes resources from it

• Host: the organism the parasite lives on or in

• Parasites harm the host but usually do not kill it because they depend on it for survival

• Parasite - An organism that feeds on cell contents/tissues/fluids of a host while in or on the host organism; harm but usually do not kill their host. Generally much smaller than the host, can also (sometimes) live outside of the host organism

• Ex: worms & microbes living in cells

• Most species host many parasites

Humans have many parasites that cause infectious disease