Bio Exam 3

Population Biologists

People who study how populations change over time, including whether they are increasing or decreasing and what factors (biotic and abiotic) influence those changes.

Main factors that increase a population

• High food availability

• Lack of predators

• Favorable environmental conditions

• Adequate shelter

• Low disease spread

Main factors that decrease a population

• Limited resources (food, space)

• Predation

• Disease spread (especially in dense populations)

• Environmental stress

• Competition

Exponential growth

When the population increases at a rate proportional to its size, resulting in a J-shaped curve where growth accelerates rapidly over time.

Geometric Growth

Type of exponential growth that occurs in discrete steps (e.g., populations doubling each generation), though it appears continuous when graphed

Why is exponential growth unrealistic long-term

• Resources become limited

• Competition increases

• Waste accumulates

• Predation and disease increase

Per capita growth rate (r )

Growth rate per individual in a population (e.x. a rabbit population starts with 200 rabbits. After 1 year it increases to 260 rabbits)

Formula for per capita growth rate

r = b - d

Where:

• b = birth rate (probability of reproduction)

• d = death rate (probability of dying)

r > 0

Population is increasing (births exceed deaths)

r < 0

Population is decreasing (deaths exceed births)

r = 0

Population is stable (births equal deaths)

Exponential growth equation

Δt/ΔN​=rN

Where:

• N = population size

• r = per capita growth rate

A key assumption of exponential growth models

All individuals are identical (same birth and death probabilities), which is unrealistic in real populations

Logistic growth

Population growth that starts exponentially but slows as resources become limited, forming an S-shaped (sigmoid) curve.

Three phases of logistic growth

1. Lag phase - slow initial growth

2. Exponential phase - rapid growth

3. Plateau phase - growth slows and stabilizes

Carrying capacity (K)

Maximum population size that an environment can sustainably support

Why do populations level off at carrying capacity

Limiting factors (food, space, disease, competition) prevent further growth

Life table

Table that tracks survivorship and reproduction of a population by age class, allowing prediction of population trends

Cohort

Group of individuals born at the same time and tracked throughout their lives

Age class

Specific age range (e.g., 0-1 years, 1-2 years) used to categorize individuals in a population

Survivorship (lₓ)

The proportion of the original cohort that survives to a specific age

Formula for survivorship

lx ​= Nx/​No

Where:

• No = original cohort size

• Nx = number surviving to age x ​​

Mortality (dx)

Proportion of the cohort that dies during a specific age interval

Formula for mortality

dx = lx - lx + 1

Fecundity

Average number of offspring produced per individual (usually females only) in a given age class

Why focus on females in fecundity

Because female reproductive capacity typically limits population growth

Survivorship Curve Type 1

• High survival until old age

• Example: humans, elephants

Survivorship Curve Type 2

• Constant mortality rate

• Example: birds, some mammals

Survivorship Curve Type 3

• High early mortality, few survive to adulthood

• Example: insects, fish

Life history traits

Traits that affect an organism’s schedule of reproduction and survival (e.g., growth rate, age of reproduction, lifespan)

Key limitation of exponential/logistic models addressed by life tables

They assume all individuals are identical, ignoring age structure and reproductive differences

Why don’t extremely large animals (like giant movie monsters) exist?

Because f physical and biological constraints, including structural support, energy demands, and surface area-to-volume limitations

Evolutionary trade-offs

Compromises where improving one trait reduces performance in another

Phenotypic plasticity

Ability of an organism to change its traits in response to environmental conditions without genetic change

Three major challenges animals face

• Obtaining and using energy

• Avoiding predators (survival)

• Reproducing successfully

Herbivore vs carnivore trade-off

• Herbivores: efficient at processing plants but cannot digest meat

• Carnivores: efficient at processing meat but cannot digest plants

Omnivore

Organism that eats both plants and animals but is not highly specialized in either

Generalist vs specialist trade-off

Generalists:

• Eat many foods

• High competition

• Flexible

Specialists

• Eat specific food

• Low competition

• High risk if food disappears

Endotherm vs ectotherm trade-off

Endotherms (warm-blooded):

• Generate own heat

• High energy needs

• Active anytime

Ectotherms (cold-blooded):

• Rely on environment

• Lower energy needs

• Activity depends on temperature

Hibernation

State of reduced metabolic activity to conserve energy during unfavorable conditions

Migration

Movement to more favorable environments to avoid harsh conditions

Surface area to volume ratio

Relationship between an organism’s outer surface and internal volume, affecting heat loss, metabolism, and resource exchange

Why do small animals have higher metabolic rates

They have a higher surface are to volume ratio, causing faster heat loss and requiring more energy

Why do large animals have lower metabolic rates per unit mass

Their volume increases faster than surface area, reducing heat loss

Trade-offs of being small

• Pros: less food needed, more hiding places

• Cons: high heat loss, higher predation risk

Trade-offs of being large

• Pros: fewer predators, better heat retention

• Cons: need more food, fewer hiding places

Interspecific exploitation

Interaction between species where one benefits (+) and one is harmed (-), such as predator-prey, herbivory, and parasitism.

+/ - interactions

• + (benefit): increased fitness

• - (cost): decreased fitness

Always measured in terms of fitness (survival + reproduction.

Three major types of consumption interactions

• Herbivory

• Predation

• Parasitism

Why are exploitative interactions extremely numerous

• Each species interacts with many others as food or host

• Example: 500 species → 25,000 interactions

Lotka-Volterra model

Cyclical relationship between predator and prey populations

What happens when prey populations increase

• more food for predators

• Predator population increases after a time lag

Population Inertia

Predator populations continue increasing even after prey begins to decline, because reproduction lags behind food intake

Why do predator populations lag behind prey populations

• It takes time to convert prey into offspring

• Predator reproduction is delayed

When predator population gets too large

• Prey declines

• Predator eventually declines due to lack of food

WHen predator population declines

• Prey population rebounds

• Cycle repeats

Key limitation of Lotka-Volterra model

Assumes predator and prey only affect each other, ignores food availability, disease, and environment

Additional factor that strongly affects prey populations

Food availability (carrying capacity of prey)

Herbivory

Consumption of plant material by animals

Can herbivory ever benefit plants

Yes, through seed dispersal and pollination

Why is plant food more available than animal food

Plants are more abundant and base of food chains

Why do herbivores have longer digestive tracts

Plant material is harder to digest (cellulose, lignin) and requires more processin

Carnivory

Killing and consumption of other animals

Why is animal tissue higher quality food

It contains more protein, fat, and is easier to digest

Carnivore adaptations

• Claws/talons

• Sharp teeth

• Venom

• Speed

• Advanced senses

Search image

A predator’s focus on the most common or profitable prey type, ignoring others

Parasitism

Relationship where parasite benefits and host is harmed but usually not killed

Ectoparasites

Parasites that live on the outside of the host (e.g., ticks, lice)

Endoparasites

Parasites that live inside the host (e.g., tapeworms, hookworms)

Why don’s parasites usually kill their hosts

The host is their resource and habitat

Parasitoid

Parasite-like organism that eventually kills its host

Example of parasitoid behavior

• Wasps lay eggs inside host

• Larvae consume host internally

• Host dies

Parasite vs parasitoid

Host lives vs host dies

Constitutive defenses

Defenses that are always present

• Thorns

• Shells

• Quills

Inducible defenses

Activated only when predators are present

• Running

• Curling up

• Behavioral responses

Mechanical plant defenses

Physical barriers:

• Thorns

• Bark

• Spines

• Tough tissues

Chemical plant defenses

Secondary metabolites that deter herbivores

Secondary metabolites

Chemicals evolved specifically to prevent herbivory (e.g., tannins, alkaloids, terpenes)

Tannins

• Bind to proteins

• Prevent digestion → reduce nutritional value

Crypsis

Camouflage - blending into the environment

Aposematic coloration

Bright warning colors indicating toxicity or danger

Example: Monarch caterpillar (toxic from milkweed)

Müllerian mimicry

Multiple dangerous species look alike

Batesian mimicry

Harmless species mimics a harmful one

Evolutionary arms race

Continuous cycle where:

• Prey evolve defenses

• Predators evolve counter-adaptations (Example is birds removing caterpillar hairs before eating)

How do herbivores overcome plant defenses

• Digestive symbiosis (microbes)

• Enzymes like cellulase

• Eating less toxic parts

• Behavioral strategies

How predators overcome prey defenses

• Social hunting

• Ambush tactics

• Luring prey

• Specialized behaviors

Mutualism

A +/+ interaction where both species benefit, increasing fitness

Benefits in mutualism

Increased fitness (survival and/or reproduction)

Cooperation vs mutualism

• Mutualism: Between species

• Cooperation: within a species

A mutualistic relationship ends when..?

Costs > benefits

Commensalism

A +/0 interaction '

• One benefits

• One unaffected

Amensalism

A -/0 interaction

• One harmed

• One unaffected

Symbiosis

Two species living in close constant contact

Facultative mutualism

Species can survive without each other

• Example: Honeyguide bird & humans

Obligate mutualism

Species cannot survive or reproduce without each other

• Example: Herbivores + cellulose-digesting bacteria

Competition in ecology

A -/- interaction in which both individuals suffer a cost while competing for a limited resource

Ultimate determinant of ecological costs and benefits

Fitness (survival + reproductive success)

How does competition influence evolution

Competition favors better competitors, so traits improving competitive ability become more common over generations.

Intraspecific competition

Competition within the same species