The Structure and Dynamics of Populations

Concepts

• Populations are dynamic - they change over time, space and with the environment.

• Population change over time is related to rates of birth, death, immigration and emigration.

• Populations interact across space. A group of spatially isolated, conspecific populations that occasionally interact through migration of seeds or pollination is called a metapopulation.

• Individuals within a population are unique; they vary in their age, size, stage of development, and other physical and genetic features. This variation gives a population structure.

• Life history strategies are a way of understanding a population.

Introduction

In Chapter 2, we discussed ways of describing populations in terms of their distribution and abundance. Populations were treated as whole entities. We then discussed the spatial distribution of individuals within a population, and how this would influence estimates of distribution and abundance. For the most part, we treated individuals as identical entities. Populations, however, are made up of individuals that vary in age, size, genetic structure (genotype) and appearance (phenotype). As a result, populations are structured by this variation. Population structure refers to the organization of indi viduals within a population, based on specific characteristics. For example, in a human population we could compare the age structure of men and women.

Demography is the study of population size and structure, and how they change over time. Populations are also dynamic: their size and structure change over time. Population size refers to the total number of individuals or the density of individuals within a specific population. A change in population structure will affect population dynamics; as population size increases or decreases, the structure will be affected. In this chapter, we will first look at how population size changes over time. We then look

© 2003 CAB International. Weed Ecology in Natural and Agricultural Systems (B.D. Booth, S.D. Murphy and C.J. Swanton)

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Fig. 3.1. Population size changes over time.

at how immigration and emigration can influence a population's demography. Third, we examine the different ways that populations can be structured. Finally, we look at life history strategies.

Population Dynamics: Size Changes over Time

In nature, a population's size will rarely remain constant. Within a short time frame, population size may remain stable, steadily increase or decrease, or it may cycle regu larly, or in an unpredictable fashion (Fig. 3.1). The rate of population change is dependent on the ratio of individuals entering the population through births (B) or immigration (I) to individuals leaving through deaths (D) or emigration (E). Thus, the change in a population's size (N) from one time period (t) to the next (t+1) can be represented by the equation:

N(t+D = N + B - D + 7 - E Birth (or natality) is the addition of individuals to the population. For plants, births may refer either to the number of seeds pro-

Fig. 3.2. The (a) exponential and (b) logistic growth curves.

Time (generations)

Fig. 3.2. The (a) exponential and (b) logistic growth curves.

Time (generations)

duced or seeds germinating (Chapter 6), or to individuals produced via vegetative reproduction (Chapter 5). Mortality is the loss of individuals from the population through death. Mortality rates and causes will change over time. In the following sections we look at population growth curves, first using the exponential and logistic models of growth and then by looking at real populations.

Exponential and logistic growth curves

As long as births outnumber deaths (ignoring immigration and emigration), population growth will be positive. Over generations, a population with a constant positive growth rate will exhibit exponential growth (Fig. 3.2a). The greater the difference between birth rate and death rate, the more rapid the increase. The difference between birth rate and death rate is the instantaneous rate of population increase (r). Therefore the exponential population growth can be shown as:

dN/dt = rN or NM = Nt ert where dN/dt is the change in N during time(t).

Many plants have the potential to produce a huge number of offspring. This is especially true for some weeds where a single individual may produce more than 1,000,000 seeds per season (Table 3.1). Given that plants produce so many seeds, why then do their populations not continue to increase exponen tially? Many seeds will not be viable, while others will not germinate because environmental conditions are not appropriate, or because the seed dies due to predation or disease. In spite of this, there can still be many viable seedlings produced per adult plant. During the early stages of population growth, density may increase exponentially (Fig. 3.3), but at some point, the growth will slow and density may even begin to decrease. Why is this so? Exponential growth cannot be maintained because populations are limited by a lack of resources. At some point there will not be enough resources (e.g. nutrients, light or space) to satisfy the needs of every new individual and so population density will level off.

The logistic curve is a model of population growth under limiting resources. Once a seed germinates, there are many biotic and abiotic forces that cause mortality and reduce population growth rate. For example, each seedling requires resources (space, nutrients, water, light) to survive, and individuals that fail to acquire adequate resources will fail to reproduce or may die.

The lack of adequate resources will cause the population growth curve to level off. The growth of all populations will eventually level off. The carrying capacity (K) is the maximum number of individuals the environment can support. To incorporate K into the population growth equation, the exponential equation can be modified by including an additional term that causes the growth rate to level off. It looks like this:

Table 3.1. Plant size and seed production of various weed species (from Holm et a/., 1977).

Species Common name Plant height (cm) Seeds per plant (number)

Table 3.1. Plant size and seed production of various weed species (from Holm et a/., 1977).

Species Common name Plant height (cm) Seeds per plant (number)

Amaranthus spinosa

Spiny amaranthus

to 120

235,000

Anaga//is arvensis

Scarlet pimpernel

10-40

900-250,000

Chenopodium a/bum

Common lambsquarters

to 300

13,000-500,000

Digitaria sanguina/is

Large crabgrass

to 300

2000-150,000

Echinoch/oa crus-ga//i

Barnyardgrass

to 150

2000-40,000

E/eusine indica

Goosegrass

5-60

50,000-135,000

Euphorbia hirta

Garden spurge

15-30

3000

Po/ygonum convo/vu/us

Wild buckwheat

20-250

30,000

So/anum nigrum

Black nightshade

30-90

178,000

Striga /utea

Witchweed

7-30

50,000-500,000

Xanthium spinosum

Spiny cocklebur

30-120

1 100 H

Recruitment from seeds produced by pines which have invaded the eucalypt/ forest

Recruitment from seeds produced in an adjacent, pine plantation

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