Difficulty: Introductory | Prerequisites: None (first exam unit)
Ecology is the study of how organisms interact with each other and their environment. This unit sets up the foundational vocabulary and tools you will use for the rest of the course, including how abiotic factors shape global patterns and how life tables let us predict population futures. If you are coming in cold, start here: everything in the population growth and competition units builds on these ideas.
Ecology studies how organisms interact with their surroundings, and those surroundings break into biotic (living) and abiotic (non-living) components. Abiotic factors like temperature, sunlight, and rainfall create predictable global patterns (deserts at 30° latitude, rainforests at the equator). Populations can be described by a handful of numbers, and life tables let you project whether a population will grow or shrink by tracking survival and reproduction at each age.
Biotic
Relating to living organisms and their interactions (predation, competition, disease). In simple terms, biotic means anything alive that affects other organisms.
Abiotic
Non-living physical and chemical factors that influence ecosystems: temperature, sunlight, rainfall, wind, soil composition. Think of it as the stage on which living things perform.
Population
A group of individuals of the same species living in the same area at the same time. This is the basic unit of ecological study when we start counting and predicting.
Per capita
Per individual. Birth rate (B) is total births in the population; per capita birth rate (b) is births per individual. The distinction matters because per capita rates let you compare populations of different sizes.
Survivorship (lx)
The proportion of individuals surviving from birth (age 0) to age x. Written as a decimal between 0 and 1. Think of it as the probability that a newborn makes it to a given age.
Age-specific fecundity (mx)
The average number of female offspring produced by a female of age x. This tells you how reproductive output changes across a lifespan.
Life table
A table listing survivorship (lx) and fecundity (mx) for each age class. It is the core tool for predicting whether a population will grow, shrink, or stay stable. Think of it as a demographic spreadsheet for a species.
Sunlight intensity, temperature, rainfall, atmospheric circulation, and mountain ranges shape where biomes occur.
Hadley cells are large-scale atmospheric circulation patterns. Warm air rises at the equator, drops moisture as rain (creating tropical rainforests), moves poleward, cools, and descends at roughly 30° latitude. The descending dry air produces the world's major deserts.
Mountain ranges create rain shadows: moist air rises on the windward side, drops precipitation, and the leeward side stays dry.
These patterns are predictable. If you know latitude and geography, you can often predict the biome.
N = population size (total number of individuals).
B = total number of births; b = per capita birth rate (B / N).
D = total number of deaths; d = per capita death rate (D / N).
r = per capita rate of increase = b – d. A positive r means the population is growing; negative means it is shrinking.
The distinction between B (total births) and b (per capita births) matters because per capita rates are what you plug into growth models and compare across populations.
A population's age structure is the distribution of individuals across age classes.
Two populations can have the same N and r right now but very different futures, because their age structures differ. A population heavy with young, pre-reproductive individuals will likely grow; one dominated by old, post-reproductive individuals will likely shrink.
This is why N and r alone are not enough to predict the future. You need to know how survival and reproduction vary with age.
A life table tracks two things for each age class: survivorship (lx) and age-specific fecundity (mx).
Life tables focus on females because females are the reproductive bottleneck in most populations.
Each row gives you a pair: the chance of being alive at that age, and how many offspring you produce if you are.
By combining lx and mx across all ages, you can project the population forward.
Organisms face trade-offs: energy spent on reproduction now means less energy for survival later (and vice versa).
These trade-offs show up in life table data. A species that reproduces heavily at a young age often has low survivorship at older ages. A species that invests in longevity tends to have lower fecundity per age class but higher lx values at later ages.
Net reproductive rate (R0)
R0 = Σ (lx × mx) across all age classes
If R0 > 1, the population is growing (each female more than replaces herself).
If R0 = 1, the population is stable.
If R0 < 1, the population is shrinking.
This is a weighted average: each age class's reproductive contribution is discounted by the probability of actually surviving to that age. A species could have enormous fecundity at age 10, but if almost nobody survives to age 10, that contribution barely moves R0.
Per capita rate of increase
r = b – d
Where b = B / N (per capita births) and d = D / N (per capita deaths).
Worked example from the review guide:
N = 20,000 hummingbirds, B = 2,000 births, D = 1,400 deaths.
b = 2,000 / 20,000 = 0.1
d = 1,400 / 20,000 = 0.07
r = 0.1 – 0.07 = 0.03 (population growing)
Hadley cell circulation is why the Sahara, Arabian, and Australian deserts all sit near 30° latitude. The same mechanism explains why tropical rainforests cluster at the equator. If you can sketch a Hadley cell, you can explain a large chunk of global biome distribution.
Life tables are used in conservation biology to decide whether an endangered species is recovering or declining, and in fisheries management to set sustainable harvest levels.
Students often think "per capita" and "total" rates are interchangeable. They are not. B is total births; b is births per individual. You need per capita rates for growth equations.
Students sometimes assume that a high mx (fecundity) at one age class guarantees population growth. It does not. If survivorship to that age is extremely low, the contribution to R0 is tiny.
Life tables are sometimes confused with simple population counts. A life table is not just "how many are alive." It tracks survival probability and reproductive output by age, which is what makes prediction possible.
Students often wonder why life tables track only females. The reason is that in most species, females are the limiting factor for reproduction. One male can fertilise many females, so the number of females determines reproductive output.
⚠️ Be able to calculate b, d, and r from given values of B, D, and N. This is a near-certain exam question.
⚠️ Know how to read a life table and compute R0. The review guide includes a life table calculation exercise, and the exam will likely have one too.
⚠️ Be prepared to explain why knowing N and r alone is not enough to predict population growth. The answer involves age structure.
⚠️ Be able to describe how abiotic factors (especially Hadley cells) explain global patterns like desert locations. This is a classic short-essay topic.
⚠️ Understand why life tables use only females. Expect a fill-in-the-blank or true/false on this.
True or false: Abiotic factors include predation and disease. (False. Those are biotic.)
Fill in the blank: The proportion of individuals surviving from birth to age x is called ____. (Survivorship, lx)
True or false: If R0 = 0.8, the population is growing. (False. R0 < 1 means the population is shrinking.)
Fill in the blank: r = ____ – ____. (b – d, or per capita birth rate minus per capita death rate)
True or false: Life tables track both males and females equally. (False. Life tables typically track only females.)
Q: What is the difference between population ecology and community ecology?
A: Population ecology studies the dynamics of a single species (birth rates, death rates, growth). Community ecology studies interactions between multiple species in the same area (competition, predation, coexistence).
Q: Describe the Hadley cell and explain how it influences abiotic conditions.
A: Warm, moist air rises at the equator, cools as it gains altitude, and drops heavy rainfall (creating tropical rainforests). The now-dry air moves poleward and descends at roughly 30° latitude, creating arid conditions and the world's major deserts. This is a Hadley cell.
Q: A population has N = 20,000, B = 2,000 births, D = 1,400 deaths per year. Calculate b, d, and r.
A: b = B/N = 2,000/20,000 = 0.1. d = D/N = 1,400/20,000 = 0.07. r = b – d = 0.03.
Q: Given the life table below, will the population grow or shrink?
Age 0: lx = n/a, mx = 0.0 | Age 1: lx = 0.2, mx = 1.0 | Age 2: lx = 0.1, mx = 1.5 | Age 3: lx = 0.05, mx = 2.0 | Age 4: lx = 0.01, mx = 3.0
A: Calculate R0 = Σ(lx × mx) = (0.2)(1.0) + (0.1)(1.5) + (0.05)(2.0) + (0.01)(3.0) = 0.2 + 0.15 + 0.1 + 0.03 = 0.48. Since R0 < 1, the population will shrink.
Q: Why do we only consider females in life tables?
A: Females are typically the reproductive bottleneck. One male can fertilise many females, so the number of females and their reproductive output determines population growth rate.
This material feeds directly into the Population Growth unit, which takes r and N and builds exponential and logistic growth models from them. Understanding b, d, and r here is essential before tackling density dependence.
Life tables and age structure also connect to the Competition unit: competition is one of the biotic factors that can alter survivorship (lx) and fecundity (mx), changing a population's trajectory.
ecology, abiotic factors, biotic factors, Hadley cell, atmospheric circulation, rain shadow, population ecology, community ecology, survivorship curve, lx, mx, life table, age-specific fecundity, net reproductive rate, R0, R-nought, per capita birth rate, per capita death rate, age structure, demography, BBIO 180, intro biology, University of Washington