Population Dynamics

Population Dynamics Chapter 15

Learning Objectives • Identify techniques for estimating various populations (quadrats, transects, mark-recapture) • Understand the carrying capacity of ecosystems; factors affecting distribution and abundance of organisms including birth and death rates, migration. • Extend your knowledge of population dynamics.

Populations

Populations • Population – members of one species living and reproducing in the same region at the same time. • Community– a number of different populations living together in the one area. • Speciesrichness – the number of DIFFERENT populations or species in the one community. • Factors that affect the number of species in an area include: - physical size of area - latitude (or distance from Equator) • Carrying capacity - how many organisms a particular environment can support.

Populations continued … Area affects species richness • Number of different populations in terrestrial communities in the same region is related to the physical size of the available area. • In general, if an island has an area 10 times that of another that is in the same region, the larger island can be expected to have about twice the number of different species. Latitude affects species richness • As we move from the poles to the equator, in general, species richness of terrestrial communities increases. • This means that more species exist in a tropical rainforest ecosystem than a temperate forest of the same area.

Populations continued … • Any population can be characterised in terms of several attributes, including: • abundance • density • distribution • age structure of population • rate of growth • The study of a particular population will also involve an examination of other specific features, such as: • habitat requirements • breeding season • reproductive strategy

Abundance and Density • Density– number of individuals of a given species per unit area. • Abundance – the total number of organisms of a given species. • Abundance can be expressed qualitatively, for example: scarce or rare low density infrequent frequent abundant (common) very abundant high density • Abundance can also be expressed in quantitative terms. E.g. numberof ants per square metre • When assessing the abundance of a population, the size of the area over which the population is counted may be large (e.g. grassland) or small (e.g. one tree).

Abundance of populations continued … • Sometimes it’s possible to carry out a total count or true census of a population by counting every member of a population that lives in a given area. • Total counts can be done on large or conspicuous or slow animals and large plant species. • The cost of a total census may be high in time and people power, especially if a large area is involved. • When an entire population cannot be counted, sampling techniques are used (see future slides). • One or more samples are taken at random from a population and the samples are assumed to be representative of the total population.

Abundance of populations continued … • Sampling allows biologists to make estimates of the abundance of a population and the size of the population. • The abundance of a population cannot be based on just one count because of the chance of sampling errors. • To avoid sampling errors, counts of populations are typically repeated several times. • Changes in the abundance of a population can occur over time because of factors such as migration and breeding patterns. E.g. Migration of the orange-bellied parrot (endangered)

Abundance of populations continued … Biologists are interested in the abundance of populations for various reasons: • Conservation requires measuring the abundance of populations of endangered species over time to decide if the populations are stable, increasing or decreasing. Decreasing abundance leads to extinction. • Control or elimination of exotic pest species requires the monitoring of changes in their abundance and range. • Measuring the diversity of a community means counting the numbers of various populations in that community. • To understand why populations ‘explode’ or sharply increase in numbers, measurement of population abundance is required regularly to detect patterns and identify possible causes.

Distribution of populations • Distribution - the spread of members of a population over space (area). • Populations may have identical densities but their distributions can differ. Draw diagrams representing uniform, random and clumped distribution.

Distribution of populations continued … • Clumped and uniform distributions are both non-random patterns. • The most common pattern observed in populations is a clumped distribution. • Changes in the distribution of populations can occur over time. For example, animals that normally have a random distribution can show a different distribution during the breeding season. • Distribution of plant populations will depend on favourable conditions. E.g. Clumps of moss in damp areas • Animal populations will also show clumped distribution in favourable conditions (e.g. Under shade) or herds.

Distribution of populations continued … • A uniform distribution may indicate a high level of intra-specific competition so that members of a population avoid each other by being equidistant from each other. E.g. Plants that release chemicals or animals defending territory. • A random distribution is expected when: • the environmental conditions within the sample area are the same throughout the entire area, and • the presence of one member of a population has no effect on the location of another member of the population. • Both of these conditions are rare.

Age structure of populations • In a population, individual members vary in their ages and life spans. • The age structure of a population identifies the proportion of its members that are: • at pre-reproductive age (too young to reproduce) • at reproductive age • at post- reproductive age (no longer able to reproduce) • The age structure of a population is important since it indicates whether or not a population is likely to increase over time. • If majority of individuals are at reproductive age or younger, then the population will increase. • Or, if most individuals are post-reproductive age, then regardless of size, the population will decrease.

Population Dynamics

Variables affecting population size • Population dynamics is the study of changes in population size over time. • Variables that influence population size include: • Birth rate, that can be expressed numerically • Death rate, also expressed numerically • Migration rate, which is the net gain or loss over a given time by movement of individuals either into the population from other areas (immigration) or out of the population to other areas (emigration). • This may be represented by the equation: Growth rate (time) births + immigration deaths + emigration = -

Variables affecting population size continued … • Growth rate is positive when population size increases • Growth rate is negative when population size decreases • When gains by births and immigration are matched by losses by death and emigration, a population is said to have zero population growth. • Open population - migrations into or out of a population • Closed population - migrations don’t occur, e.g. Lizard population isolated on an island • Closed populations are less common among bird species. Why?

Sampling Populations

Sampling Populations • It is rarely possible to carry out a total count of a population in terms of cost and time or practicability. • It’s more common to take samples and from these to estimate the population size. • Sampling techniques include: • Use of quadrats • Use of transects • Mark-recapture technique • Each of these techniques has its particular application.

Use of quadrats(within a square) • Quadrats are square areas of known size and are often subdivided into smaller areas. • In using a quadrat to sample a plant population, certain decisions must be made: • What size should the quadrat be? Is the species large (tree) or small (moss) • How many areas should be sampled? If not enough, there may be sampling error • How should sampling sites be chosen? Locations should be determined randomly to prevent bias. • Quadrats will only be successful if members of the population to be sampled will remain in the one place.

Use of transects(along a line or within a strip) • A transect is a line or strip laid across the area to be studied. • Linetransects are useful in identifying changes in vegetation with changes in environment. Eg. Plant community on a sand dune • The line is marked at fixed intervals and repeat counts are carried out to minimise sampling errors. • Striptransects (also known as belt transects) are used to estimate animal populations and can be carried out by observers on foot, car, helicopters, planes or underwater. • The type and size of the transect will depend on the animal population that is being surveyed.

‘Mark-recapture’ technique • Various techniques are used to sample the abundance of animal populations because they are difficult to see or are very mobile. • The mark-recapture technique involves: • Collecting a sample of the animal population being studied using traps. • The trapped animals are marked in some way (eg. band, paint spots) and are then released. • Later, another sample of the population is trapped and from the data it’s possible to estimate the size of the population. • Problems can arise with this technique: • the released animals do not mix at random with the rest of the population after release • animals migrate into or from the area over the period of study.

Population Dynamics