Topic 3.5 Notes – Population Growth and Resource Availability

What Resource Availability Means for Population Growth

A population changes through births, deaths, immigration, and emigration, but here the key idea is that resources affect whether organisms survive and reproduce in the first place.

Total resource base means the total amount, or rate of supply, of needed resources in an environment.
Resource availability means the part organisms can actually use.

That difference matters a lot. A resource can exist and still be unavailable if it is:

• polluted
• frozen
• defended by other organisms
• fragmented by habitat loss
• outside an organism’s territory
• otherwise unusable

Resources and usable space are finite, even when they are renewable. Renewable just means replenished over time, not unlimited.

Common limiting resources and space needs include:

• Food or nutrients for energy and growth
• Freshwater for metabolism and survival
• Light for photosynthetic organisms
• Oxygen for respiration
• Soil nutrients for plant growth
• Shelter for protection
• Nesting or breeding sites for reproduction
• Territory for animals that defend area
• Living, feeding, or reproductive space for both plants and animals

Environmental conditions also change how useful resources are:

• Temperature
• Precipitation
• Salinity
• Disturbance such as fire or flood

Those limits help explain why populations do not all grow the same way. When resources are abundant, growth can look exponential for a time. As limiting factors become stronger, population growth levels off near carrying capacity.

Exponential vs. logistic population growth

What Makes Resources Limiting

A population is limited by the essential resource in shortest supply relative to demand. If food is plentiful but nesting sites are full, adding more food will not increase growth.

Resource distribution matters too. When resources shrink, access becomes less equal. Dominant individuals or organisms in better territories may get enough, while others do not.

Intraspecific competition

This is competition within the same species. It is often intense because individuals need nearly the same resources.

Interspecific competition

This is competition between different species for the same limited resource.

Space can also be limiting even when food is still available:

• animals may need territory
• plants compete for root space, light, and soil moisture
• some species need specific breeding habitat, like cliff ledges or spawning areas

Resource availability can drop because of:

• depletion by the population itself
• increasing population size
• competition
• drought, winter, fire, flood, or other disturbances
• seasonal change
• habitat loss, degradation, or fragmentation
• pollution that makes resources unusable

How Resource Availability Changes Growth

When resources are abundant compared with population size:

1. survival increases
2. fecundity increases
3. individuals may mature sooner or produce more offspring
4. population growth accelerates

When resources become scarce:

1. organisms spend more energy getting resources
2. fecundity drops
3. mortality rises directly through starvation, dehydration, or exposure
4. mortality also rises indirectly through disease, predation, and stress
5. growth slows, stops, or becomes negative

That causal chain shows up all over APES. One detail students miss is that slower growth does not mean the population is shrinking. It may still be increasing, just less quickly.

Exponential Growth, Logistic Growth, and Carrying Capacity

Exponential growth

With abundant resources and space, populations can grow near their intrinsic rate of increase, $r$. This produces a J-shaped curve.

$$N_t = N_0(1+r)^t$$

The same proportion is added each time interval, so the actual number added gets bigger over time.

Logistic growth

As resources become limiting, growth shifts to logistic growth, which forms an S-shaped curve. Growth starts fast, then slows as density rises.

Carrying capacity, $K$, is the maximum population size the environment can sustain over time. At $K$, births and deaths are roughly balanced, so net growth is about zero.

Environmental resistance

This is the combined effect of limiting factors such as resource scarcity, competition, disease, predation, limited territory, and physical conditions.

Carrying capacity can change

$K$ rises with more usable habitat, food, or water. It falls with drought, habitat destruction, depletion, or new competition. A resource-based estimate may suggest one support level, but another factor may set the real limit lower.

Overshoot, Dieback, and Reading Population Graphs

Overshoot happens when a population temporarily exceeds carrying capacity. This usually comes from time lags. Reproduction responds to earlier good conditions, but resource depletion shows up later.

Dieback is a major decline after overshoot or resource loss. Overshoot can even damage the resource base and lower $K$ itself.

Overshoot and dieback on a population graph

On a graph, overshoot shows up when the population rises above the carrying capacity line, and dieback shows up as the drop that follows. Some populations then oscillate around carrying capacity for a while, and others crash much more sharply.

The classic example is St. Matthew Island reindeer. Reindeer had abundant lichen and no major predators, so the population grew rapidly. Then lichen was depleted, a harsh winter hit, and the population crashed.

Reading graphs

• J-shaped curve = accelerating growth with temporarily abundant resources
• S-shaped curve = growth slowing near carrying capacity
• Plateau = net growth near zero, not zero births and deaths
• Curve above $K$ then falling = overshoot followed by decline
• Falling $K$ line = worsening conditions or declining resource base