Population Biology, I & II

I. Defining Ecology.

The term ecology came into use in the second half of the 19th century.

Henry Thoreau (right) used the word in 1858 in his letters, but didn't define it.

German biologist Ernst Haeckel (left) defined it in 1869 as the total relations of the animal to both its organic and inorganic environment.

More recently, Charles Krebs has defined ecology as the scientific study of the interactions that determine the distribution and abundance of organisms.

In other words:

We can approach the study of ecology from three points of view:

  1. The descriptive point of view is mainly natural history oriented, and proceeds by describing organisms and their relationships.
  2. The functional point of view seeks to identify and analyze general problems. It emphasizes proximate causes - for example the responses of populations and communities to immediate functions of the environment. In other words, how does the system operate?
  3. The evolutionary point of view seeks ultimate causes - the historical reasons why natural selection has favored the particular adaptations and communities we now see. It considers organisms as historical products of the environment.

Each of these approaches has shortcomings.

So when doing ecology, it is best to incorporate all three approaches. This is what we'll do as we work through the basics of ecology, population ecology, community ecology, and finish up by discussing how this knowledge is applied to the field of conservation biology.

The Ecological Hierarchy (most definitions from Dictionary.com)- each of the above three fields have applicability at each of these levels of the hierarchy:

Population - members of the same species that live together in the same area at the same time.

Community - all the populations of all the species that live and interact together in an area.

Ecosystem - An ecological community considered along with its environment, with the entire unit functioning as one.

Biome - A major regional or global biotic community, such as a grassland or desert, characterized chiefly by the dominant forms of plant life and the prevailing climate.

II. Population Ecology.

Based on our definition of ecology, the central question in ecology is determining the causes of the abundance and distribution of organisms.


Organisms live in a matrix of space and time (Fig 1).

Question: What is the most widespread (latitudinally) terrestrial mammal species in the world, excluding humans and human commensals? Answer (Fig2).

Question: What is the most endangered mammalian taxa in North America. Answer.

So distribution and abundance are not the same. Widespread is not the same as common.

Population ecology deals with the number of individuals of a particular species that are found in an area, and how and why those numbers change over time. We differentiate population ecology from community ecology because populations have several properties that communities do not. Most importantly:

Members of a population share a common gene pool, so natural selection can work directly on a population, but only indirectly on a community.

Population density - the number of individuals found per unit area.

The population density of a species at a given location is usually determined by:

  1. Resources - feed, water, nest sites, quality habitat, etc (bottom-up effects), and
  2. Limiting Agents - disease, parasites, predators (top-down effects).

The carrying capacity (K) represents the maximum size of a population that can be supported in a given environment (so different environments will have different K).

If we monitor the growth of a population recently released from somne constraint (or introduced into an habitat) we get a sigmoidal growth curve. However, sometimes top-down effects will keep a population below K.

Dispersion - Within a habitat, individuals are spaced apart from each other in one of three broadly generalizable patterns (see figure, below right):

  1. Aggregated clumped dispersion - individuals are concentrated in specific portions of the habitat. This is the most common scenario, resulting from patchy distribution of resources in habitat.
  2. Uniform dispersion - all individuals are more evenly spaced than one might expect by chance.
  3. Random dispersion - individuals in a population are spaced in an unpredictable and random fashion that is unrelated to the presence of others.

Note that each of these populations may have equal densities, but as a result of how they are distributed in space and time they may have very different dynamics (e.g. growth rates, reproductive options and strategies, survivorship, etc.).

III. Why does population size change?

Populations respond to short and long-term environmental changes, the availability of resources, and to the actual size of the population itself. These are density independent and density dependent changes. Populations also undergo random (chaotic) fluctuations.

Density-independent factors are environmental characteristics that are not influenced by changes in the size of the population.

e.g. weather-induced population declines.

Density-dependent factors are characteristic generated from within a population that influence its growth rate. As population density increases, density-dependent factors result in slowed growth rates. Usually this is due to increased mortality and decreased birth rates. Similarly, as population density decreases, density-dependent factors may cause decreased death rates and increased birth rates.

e.g. infectious disease transmission in a population.

These parameters change concurrently with changes in population density, such that as population density becomes more extreme (high or low) a greater proportion of the population is influenced.

Stochasticity. Population size may fluctuate due to chance alone, as there is some degree of randomness inherent in all systems.

Random population fluctuation is especially important when dealing with small populations, and endangered or threatened taxa.

IV. Population growth.

Predicting population growth rates is relatively straightforward given measurement of a few basic parameters:

Population growth refers to a change in the size of a population over time.

r = ΔN/Δt = (b - d) + (i - e)


r = growth rate
N = population size
t = time
b = natality
d = mortality
i = immigration
e = emmigration

So if r is positive, population size is increasing.
If r is negative, population size is decreasing.

If we are interested in the intrinsic rate of population growth at any instant in time:

δN/δt = rN = ((b - d) + (i - e))N

where N is the number if individuals already present.

A graph of this equation is termed exponential growth.

So although the rate of increase remains the same, the growth of the population (the increase in the number of individuals) changes over time. The larger the population gets, the faster it grows!

Do populations grow exponentially?

Different species have different biotic potentials due to their life-history characteristics.

So an elephant and a bacterial population can both grow exponentially, but in absolute terms it will take an elephant population longer to reach a certain size.

Most populations do not undergo exponential growth.

Why? They run out of resources (e.g. for reproduction or to sustain the current population). This occurs when the populations overshoots K (see above graph). The shape of this curve is sigmoidal and can be described mathematically:

δN/δt = rN ((K - N) / K)

So when N > K, r becomes negative and the population declines until N </= K.

For insights on modeling population growth, see http://www.geom.umn.edu/education/calc-init/population/.

Life history traits determine biotic potential. Why do different species have different combinations of life-history traits?

species # clutches / yr # eggs / clutch

Great horned owl 1 2-3
Sand-hill crane 1 2
Mourning dove 2-4 2-4
California quail 1 4-13

These reproductive strategies represent trade-offs to maximize the life-time reproductive success of the individual (fitness - number of offspring produced that survive to reproduce).

2 main reproductive strategies:

r-selected species - high growth rates, large broods, short lifespans, etc.

K-selected species - lower growth rates, smaller broods, longer life spans, etc.

Question - Is this an artificial dichotomy?

Updated April 12, 2005
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