Animal Ecology 3435 - Module 3

Lecture 10

Abiotic factors strongly influence animal ecology

Animals are strongly affected by abiotic conditions such as:

• Temperature

• Moisture

• Salinity

• pH

• Nutrient concentration

• Physical structure of the environment

Abiotic conditions influence:

• Where animals can live

• When they are active

• Reproduction and movement

• Energy allocation

• Population distributions

Examples:

• Biomes are shaped by temperature and rainfall

• Reproductive timing depends on benign vs harsh seasons

• Endotherms spend energy coping with temperature

Sometimes animals also modify abiotic conditions themselves.

Endotherms and ectotherms respond differently to temperature

Ectotherms

• Internal temperature changes with environmental temperature

• Broad range of internal temperatures

• Performance strongly depends on body temperature

Endotherms

• Maintain a relatively constant internal temperature

• Narrow range of tolerated internal temperatures

• Use metabolism to stabilize body temperature

Thus, the same ambient temperature can have very different consequences for the two groups.

Ectotherms are not “cold-blooded”

Ectotherms do not simply match the environment passively.

They:

• Generate some heat metabolically

• Seek out optimal body temperatures

• Perform best within a specific internal temperature range

Higher temperatures speed up:

• Gas exchange

• Nerve transmission

• Enzyme activity

• Muscle contraction

Which leads to faster:

• Movement

• Digestion

• Perception

• Response to prey

Thus ectotherms actively regulate body temperature behaviorally (not by internally generating heat)

How ectotherms regulate body temperature

Behavioral regulation

Examples:

• Lizards move between sun and shade

• Fish seek preferred water temperatures, swimming decisions seek best thermal environment, autonomous response to sensors on body

Metabolic heat generation

Ectotherms can generate heat through metabolism.

Example: Glanville fritillary butterfly

• Basks in the sun before takeoff

• Wings absorb heat

• Heat transferred to thorax and flight muscles

Thus, ectotherms can temporarily warm the body parts needed for activity.

Ectotherms can overheat and die

Biological processes only function within a limited temperature range.

Above the optimum:

• Small increases in temperature become very dangerous

• Heat injury occurs

• Eventually death occurs

Therefore ectotherms must actively avoid overheating, often by:

• Seeking shade

• Burrowing

• Reducing activity

5 Strategies ectotherms do when it is too cold

When temperatures are too low, ectotherms cannot function normally.

Strategies include:

1. Live in thermally buffered environments

• Oceans vary less than terrestrial environments

2. Cold hardiness through life stage

Example:

• Holometabolous insects overwinter in cold-tolerant stages

3. Use buffered microhabitats

Examples:

• Burrowing frogs

• Turtles, salamanders, snakes underground

4. Freeze avoidance

• Supercool body fluids

• Eliminate ice-nucleating agents

• Use antifreeze proteins

5. Freeze tolerance

• Permit body water to freeze

• Restrict ice to spaces outside cells

• Some species tolerate 50–80% frozen body water

Temperature affects ectotherms over multiple time scales (short, medium, long)

Short-term

Reversible changes in performance:

• Slower movement

• Slower foraging

• Less frequent reproduction

Medium-term

Changes over days to seasons:

• Slower development

• Reduced reproductive activity

• Changes in pheromone signaling

Long-term

Evolutionary changes:

• Larger body size

• Longer lifespan

• Changes in egg size and number

The timescale matters because some responses are reversible and others are not.

Endotherms maintain body temperature through BLANK therefore….

Endotherms generate heat internally through catabolism.

Therefore:

• Internal body temperature stays relatively constant

• Animals can remain active over a wider range of ambient temperatures

However, maintaining this stable temperature is energetically expensive

Thermal Neutral Zone (TNZ), UCT, and LCT

Thermal Neutral Zone (TNZ)

• Range of ambient temperatures where the animal uses the least energy

• No active thermoregulation required

Lower Critical Temperature (LCT)

• Below this, the animal must generate extra heat

Upper Critical Temperature (UCT)

• Above this, the animal must actively cool itself

Outside the TNZ, metabolic rate increases rapidly.

How endotherms respond to cold (short term, seasonal, long term)

Short-term responses

• Behavioral changes

  • Seek shelter

  • Fluff insulation

  • Goosebumps

• Increase heat production

  • Shivering

Seasonal responses

• Thicker insulation

• Metabolic acclimation

• Lower thermostat in winter

Example:

• Willow ptarmigan reduces the energetic cost of winter

Long-term responses

• Larger body size

• Hibernation

• Heterothermy

• Migration

Costs of cold in endotherms

Lethal costs

• Death if body temperature drops too far

Sub-lethal costs

Common and cumulative:

• More energy needed

• Lost opportunities to forage or reproduce

• Time spent huddling or warming up

Opportunity costs are important because time spent coping with cold cannot be used elsewhere.

How endotherms respond to heat (short term, seasonal and long term

Short-term responses

• Behavioral avoidance

• Evaporative cooling:

  • Sweating in mammals

  • Panting or throat cooling in birds

• Redirect blood flow to surface

Seasonal and long-term responses

• Reduced insulation

• Lighter coloration

• Same general principle as cold, but reversed

Examples:

• Gular fluttering in birds

• Vultures urinate on legs for evaporative cooling

Costs of heat

Sub-lethal responses often progress in stages:

1. Wing-fanning

2. Clustering

3. Salivating

4. Panting

If these fail:

• Extreme heat causes mortality

Example:

• Extreme heat killed nearly one-third of Australia’s spectacled flying foxes

Why heat is often a bigger problem than cold

Two reasons:

1. Endotherm body temperatures are already close to lethal temperatures

2. Heat causes protein denaturation (“cooking” proteins)

Thus, there is often less margin of safety above normal body temperature than below it.

Body size and thermal biology (small and large animals)

Small animals:

• Higher mass-specific metabolic rate

• Higher internal temperature

• Higher conductance

• Lose heat faster

Large animals:

• Lower mass-specific metabolic rate

• Lower conductance

• Better buffered from the environment

Conductance depends strongly on surface area : volume ratio.

Small animals have more surface area relative to volume, so they exchange heat faster with the environment.

Bergmann’s Rule

Animals at higher latitudes tend to be larger.

Reason:

• Larger animals lose heat more slowly

• Helps reduce heat loss in cold environments

Thus larger body size is often an adaptation to colder climates

Why endotherms do not get smaller than ~2 g

Very small endotherms:

• Lose heat extremely quickly

• Need extremely high metabolic rates

• Already have very high body temperatures

Eventually:

• Heat loss becomes too great

• Metabolic rate cannot increase enough

• Body temperature approaches dangerous limits

Thus there appears to be a minimum size (~2 g) for endothermy.

Thermal tolerance, body size, and water balance are linked

Endotherms cool themselves through evaporation.

Therefore:

• To avoid overheating, they must lose water

Small animals are especially vulnerable because they:

• Generate more heat

• Gain and lose heat faster

• Lose water more rapidly

Thus body size, heat tolerance, and water balance are tightly linked.

Wind increases conductance

Wind increases the rate of heat exchange between an animal and the environment.

Thus wind:

• Makes animals lose heat faster when cold

• Can increase cooling when hot

Wind therefore strongly changes the energetic costs of thermoregulation

Thermal niches link physiology to distributions

Animals have thermal niches:

• Specific ranges of temperature where they survive and reproduce best

Thermal niches influence:

• Population growth

• Geographic distributions

• Habitat use

Thermal tolerance depends on ecological context.

Temperate vs tropical thermal niches

Temperate animals

• Experience large seasonal variation

• Therefore tend to have broader thermal niches

Tropical animals

• Experience less variation

• Therefore expected to have narrower thermal niches

This makes tropical animals especially vulnerable to climate change because they already live near their thermal limits.

Lecture 11

Three ways to define a niche - Grinnellian, Eltonian, Hutchinsonian

Grinnellian niche

• Based on habitat and abiotic conditions

• Focuses on where a species can live because of physical conditions

Eltonian niche

• Based on food, enemies, and other species

• Focuses on a species’ role in the community

Hutchinsonian niche

• Modern view

• A niche is the multidimensional set of environmental conditions and resources required for survival and reproduction

• Includes both:

  • Abiotic factors

  • Biotic factors

Fundamental vs. realized niche

Fundamental niche

• Based only on abiotic tolerances

• Describes all places a species could theoretically live

Examples of abiotic limits:

• Temperature

• Rainfall

• Salinity

• Oxygen

Realized niche

• Subset of the fundamental niche

• Includes effects of biotic interactions:

  • Competition

  • Predators

  • Prey availability

Fundamental niche = where a species could live
Realized niche = where it actually lives

Individual tolerances shape species-level niches

Fundamental niches are built from the tolerances of individuals.

Two species can have the same total niche breadth, but differ in how individuals vary:

• Some species have individuals that each tolerate a broad range

• Others have specialized individuals that together cover the full range

Why this matters:

• Species with more individual variation may cope better with environmental change

If conditions shift:

• Species made up of highly specialized individuals may be more vulnerable

Fundamental niches have many axes

A niche is multidimensional:

• Temperature

• Oxygen

• Light

• Humidity

• Rainfall

• Salinity

• Sulfur

• etc.

But it is difficult to visualize more than 2–3 axes at once.

For animals, the most important and most commonly studied axis is: Temperature

Thus thermal niches are usually the focus

Thermal niches are based on individual thermal tolerances

Individuals perform best at an optimal temperature.

Away from the optimum:

• More energy is spent on thermoregulation

• Less energy remains for:

  • Growth

  • Reproduction

  • Survival

At the limits:

• Individuals die

Thus individual responses to temperature scale up to determine:

• Population growth

• Species distributions

Populations near the edge of a species’ range often:

• Perform worse

• Show more variability

Other important axes of the fundamental niche - Oxygen (O₂)

Insects depend on passive diffusion of oxygen through spiracles and tracheae.

Therefore:

• Oxygen availability limits body size

• Larger insects require more oxygen

This explains why giant insects existed 250–300 MYA:

• Atmospheric oxygen was much higher

Temperature and oxygen also interact:

• Warm temperatures increase oxygen demand

• Thus low oxygen becomes more limiting at higher temperatures

Other important axes of the fundamental niche - Light

Species differ in the light conditions they require.

Examples:

• Diurnal species

• Nocturnal species

Human impacts alter light conditions:

• Light pollution at night

• Human activity during the day

Because humans are mostly active during the day:

• Diurnal mammals are declining most strongly

Thus light is an important niche axis

Other important axes of the fundamental niche - Humidity and rainfall

Humidity and rainfall strongly affect:

• Thermoregulation

• Water balance

• Survival

Example: cave beetles

• Live in cool, stable, humid caves

• Climate change is increasing temperature and decreasing humidity

• Cave beetles have narrow humidity tolerances

High humidity and temperature together can become deadly

Rainfall matters for endotherms too

Rainfall influences the distributions of birds and mammals

Example:

• Lance-tailed Manakin occurs only on the drier side of a rainfall gradient, despite no physical barrier.

Too little rain can matter because:

Direct effects

• Dehydration

• Harder to cool body by evaporation

Indirect effects

• Lower food availability

• Fewer insects

• Changes in predators

Too much rain can matter because:

Direct effects

• Less time for foraging

• Flooding or storm mortality

• Wet fur/feathers increase thermoregulation costs

Indirect effects

• More disease/pathogens

• Lower prey activity

• Less plant growth and productivity

Salinity is an important niche axis in aquatic systems

Many aquatic species have a specific range of salinity they can tolerate.

Performance usually:

• Peaks at an optimal salinity

• Declines if salinity is too low or too high

Example:

• Mediterranean mussels

Different physiological mechanisms explain why performance declines outside the optimum

Cool critter: Sulphur mollies

• Live in sulfur-rich spring streams in southeastern Mexico

• Most animals die immediately in hydrogen sulfide (H₂S)

• Sulphur mollies tolerate both:

  • Normal water

  • Sulfur-rich water

They evolved:

• Physiological detoxification

• Adaptations to avoid sulfide toxicity

No physical barriers separate sulfur and normal streams, yet the fish remain in sulfur streams.

This has major ecological consequences:

Environment in sulfur streams:

• Low oxygen

• High predation from birds

• Low food

Adaptations:

• Surface air breathing

• Coordinated “wavemaking” behavior to reduce bird predation

• Eat sulfur bacteria mats

Consequences for life history:

• Smaller body size

• Delayed maturity

• Fewer offspring

• Lower metabolic rate

Why niche breadth matters (broad vs narrow)

Niche breadth = range of conditions a species can tolerate.

Broad niche breadth

• Larger geographic range

• Larger population size

• Lower extinction risk

• Better ability to survive changing conditions

Narrow niche breadth

• More specialized

• Higher extinction risk

• More tightly packed species → potentially more species coexistence

Broad-niched generalists often give rise to narrower-niched specialists through evolution

Why niche breadth may be smaller in the tropics

General expectation:

• Tropical species have narrower niches

Why?

• Tropical climates are more stable over the year

• Temperate climates vary greatly seasonally

Thus:

• Temperate animals must tolerate both hot summers and cold winters

• Tropical animals usually experience a narrower range of temperatures

Therefore tropical species are expected to evolve narrower thermal tolerances.

Example:

• Tropical mountain species may tolerate only a small temperature range

Evidence for tropical vs. temperate niche breadth

Prediction:

• Tropical endotherms should have narrower thermal niches

Evidence so far:

• Mixed

Some tropical species clearly have very narrow tolerances, especially at high elevations.

But not all species fit the prediction.

How ecological niche modeling works - Mechanistcs vs Correlative

Two main approaches:

Mechanistic / process-based

• Use experimentally measured fitness responses to abiotic factors

• Most accurate

• Rare because data are lacking

Correlative

• Determine where a species occurs

• Measure climate in those places

• Use correlations to predict other suitable areas

Correlative approaches make up >95% of studies

Why niche modeling is useful

Main uses:

1. Predict where rare species may occur

2. Predict future changes in distribution

Can be used for:

• Conservation

• Invasive species

• Climate change responses

Niche Modeling Application 1: Finding rare species

Example: Sabaleta

• Rare freshwater fish in the Colombian Andes

• Hard to survey directly

Researchers:

• Collected known occurrence points

• Identified abiotic conditions associated with them

• Used those conditions to predict other suitable areas

Important variables included:

• Rainfall

• Open water

• Minimum temperature

Result:

• Model predicted a much larger suitable range than official conservation maps

Thus niche models can improve conservation planning

Niche Modeling Application 2: Predicting future spread

Example: Emerald Ash Borer

• Native to East Asia

• First found near Detroit/Windsor in 2002

• Kills nearly all ash trees

Researchers used climate conditions from:

• Native range

• Invaded range

To predict where it could spread next.

Combined models best predicted its future expansion.

This shows how niche modeling can help anticipate invasive species spread