Townsend C.R., Begon M., Harper J.L. Essentials of Ecology

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Chapter 5 Birth, death and movement

149

(c)

(d)

(e)

Figure 5.1 (cont.)

(c) Stoloniferous organisms in which colonies spread laterally and remain joined by ‘stolons’ or rhizomes: a single plant of strawberry (Fragaria)

spreading by means of stolons (© Science VU) and a colony of the hydroid Tubularia crocea (© John D. Cunningham). (d) Tightly packed colonies

of modules: a tussock of the spotted saxifrage (Saxifraga bronchialis) (© Gerald and Buff Corsi) and a segment of the hard coral Turbinaria

reniformis (© Dave B. Fleetham). (e) Modules accumulated on a long, persistent, largely dead support: an oak tree (Quercus robur) in which the

support is mainly the dead woody tissues derived from previous modules (© Silwood Park) and a gorgonian coral in which the support is mainly

heavily calciﬁed tissues from earlier modules (© Daniel W. Gotshall).

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may reproduce) under thick snow cover. And most other species are so small, or

cryptic, or hidden, or fast moving that they are even more difﬁcult to count.

Ecologists, therefore, are almost always forced to estimate rather than count.

They may estimate the numbers of aphids on a crop, for example, by counting

the number on a representative sample of leaves, then estimating the number of

leaves per square meter of ground, and from this estimating the number of aphids

per square meter. Sometimes more complex methods are used (Box 5.1), and at

other times we may rely on indirect ‘indices’ of abundance. These can provide

Part III Individuals, Populations, Communities and Ecosystems

150

estimates from representative

samples

(a) (b) (c)

5.1 QUANTITATIVE ASPECTS

5.1 Quantitative aspects

An estimate of the size of a population can sometimes

be made by capturing a sample of individuals, mark-

ing them in some way (paint spots, leg rings) and then

releasing them. Later, another sample is captured,

and the proportion that is marked gives some esti-

mate of the size of the whole population (Figure 5.2).

For example, we might capture and mark 100 individ-

uals from a population of sparrows and release them

back into the population. If we later sample a further

100 individuals from the population and ﬁnd half are

marked, we could argue in the following way: half the

sample are marked; the sample is representative of

the whole population; therefore half the population are

marked; 100 individuals were given a mark; therefore

the whole population is composed of about 200 indi-

viduals. But this technique of mark and recapture is

far less straightforward than it appears at ﬁrst sight.

There are many pitfalls in the sampling process and in

interpretation of the data. Suppose, for example, that

many of the individuals we marked died between our

ﬁrst and second visits. Modiﬁcations of the method

would be needed to take account of this. For many

organisms, however, it is the only technique that we

have to estimate the size of a population.

Mark–recapture methods for estimating population size

Figure 5.2

The mark and recapture technique for estimating the size of a population of mobile organisms (in simpliﬁed form). (a) On a ﬁrst

visit to a population of unknown total size N, a representative sample is caught (r individuals) and given a harmless mark. (b) These

are released back into the population, where they remix with the unknown number of unmarked individuals. (c) On a second visit, a

further representative sample is caught. Because it is representative, the proportion of marks in the sample (m out of a total sample

of n) should, on average, be the same as that in the whole population (r out of a total of N). Hence N can be estimated.

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information on the relative size of a population, but usually give little indication

of absolute size. As an example, Figure 5.3 shows how the abundance of Canadian

leopard frogs was affected by the number of occupied ponds and the amount of

summer (terrestrial) habitat in their vicinity. Here, frog abundance was estimated

from the ‘calling rank’: whether there were no frogs, ‘few’, ‘many’ or ‘very many’

frogs calling on each of four occasions. Despite their shortcomings, even indices

of abundance can provide valuable information.

Moreover, as we have already noted, for modular organisms it is often not

even clear what it is we should be counting.

5.2 Life cycles

5.2.1 Life cycles and reproduction

If we wish to understand the forces determining the abundance of a population

of organisms, we need to know the important phases of those organisms’ lives:

that is, the phases when these forces act most signiﬁcantly. For this, we need to

understand the sequences of events that occur in those organisms’ life cycles.

There is a point in the life of any individual when, if it survives that long, it

will start to reproduce and leave progeny. A highly simpliﬁed, generalized life

history (Figure 5.4) comprises birth, followed by a pre-reproductive period, a

period of reproduction, a post-reproductive period and then death as a result of

senescence (though of course other forms of mortality may intervene at any time).

The life histories of all unitary organisms can be seen as variations around this

simple pattern, though a post-reproductive period (as seen in humans) is probably

rather unusual.

Some organisms ﬁt several or many generations within a single year, some

have just one generation each year (annuals) and others (perennials) have a life

cycle extended over several or many years. For all organisms, though, a period

of growth occurs before there is any reproduction, and growth usually slows

down (and in some cases stops altogether) when reproduction starts. Growth and

reproduction both require resources and there is clearly some conﬂict between

them. Thus, as the perennial plant Sparaxis grandiﬂora enters its reproductive

Chapter 5 Birth, death and movement

151

Number of adjacent

ponds with calling

Calling rank at core pond

Area of summer habitat (ha)

100

150

200

250

Figure 5.3

The abundance (calling rank) of leopard frogs (Rana pipiens) in

ponds increases signiﬁcantly with both the number of adjacent

ponds that are occupied and the area of summer habitat within

1 km of the pond. Calling rank is the sum of an index measured on

four occasions, namely: 0, no individuals calling; 1, individuals can

be counted, calls not overlapping; 2, calls of <15 individuals can be

distinguished with some overlapping; 3, calls of ≥15 individuals.

AFTER POPE ET AL., 2000

the conﬂict between growth

and reproduction

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Part III Individuals, Populations, Communities and Ecosystems

152

Birth

Reproductive output

Juvenile phase dominated

by growth

Reproductive

phase

Post-reproductive

phase

Time

Onset of

reproduction

End of

reproduction

Death due to

senescence

Figure 5.4

An outline life history for a unitary

organism. Time passes along the

horizontal axis, which is divided into

different phases. Reproductive output

is plotted on the vertical axis.

Jan

100

Month

Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Parent

corm (PC)

Roots (R)

Leaves (L)

Flowers (F)

Fruit

Nitrogen allocation (%)

Inflorescence

stalk (IS)

Daughter

corm (DC)

Figure 5.5

Percentage allocation of the crucial

resource nitrogen to different

structures throughout the annual

cycle of the perennial plant Sparaxis

grandiﬂora in South Africa, where it

sets fruit in the southern hemisphere

spring (September–December). The

plant grows each year from a corm,

which it replaces over the growing

season, but note the development

of reproductive parts at the expense

of roots and leaves toward the end of

the growing season. The plant parts

themselves are illustrated to the right

for a plant in early spring.

AFTER RUITERS & MCKENZIE, 1994

iteroparous and semelparous

species

stage in the Southwestern Cape, South Africa, ﬂowers, ﬂower stalks and fruit

(aspects of reproduction) can be seen to have been produced at the expense of roots

and leaves (Figure 5.5). There are also many plants (e.g. foxgloves) that spend

their ﬁrst year in vegetative growth, and then ﬂower and die in the second or a

later year (called ‘biennial’ plants). But if the ﬂowers of these species are removed

before their seeds begin to set, the plants usually survive to the following year,

when they ﬂower again and set seed even more vigorously. It seems to be the cost

of provisioning the offspring (seeds) rather than the ﬂowering itself that is lethal.

Similarly, pregnant women are advised to increase their caloric intake by as much

as half their normal consumption: when nutrition is inadequate, pregnancy can

harm the health of the mother.

Among both annuals and perennials, there are some – iteroparous species – that

breed repeatedly, devoting some of their resources during a breeding episode not

to breeding itself, but to survival to further breeding episodes (if they manage to

live that long). We ourselves are examples. There are others, semelparous species,

like the biennial plants already described, in which there is a single reproductive

episode, with no resources set aside for future survival, so that reproduction is

inevitably followed quickly by death.

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5.2.2 Annual life cycles

In strongly seasonal, temperate latitudes, most annuals germinate or hatch as

temperatures start to rise in the spring, grow rapidly, reproduce and then die

before the end of summer. The European common ﬁeld grasshopper Chorthippus

brunneus is an example of an annual species that is iteroparous. It emerges from

its egg in late spring and passes through four juvenile stages of nymph before

becoming adult in midsummer and dying by mid-November. During their adult

life, the females reproduce repeatedly, each time laying egg pods containing about

11 eggs, and recovering and actively maintaining their bodies between the bursts

of reproduction.

Many annual plants, by contrast, are semelparous: they have a sudden burst of

ﬂowering and seed set, and then they die. This is commonly the case among the

weeds of arable crops. Others, such as groundsel, are iteroparous: they continue

to grow and produce new ﬂowers and seeds through the season until they are

killed by the ﬁrst lethal frost of winter. They die with their buds on.

Most annuals spend part of the year dormant as seeds, spores, cysts or eggs.

In many cases these dormant stages may remain viable for many years; there are

reliable records of seeds of the annual weeds Chenopodium album and Spergula

arvensis remaining viable in soil for 1600 years. Similarly, the dried eggs of brine

shrimps remain viable for many years in storage. This means that if we measure the

length of life from the time of formation of the zygote, many so-called ‘annual’

animals and plants live very much longer than a single year. Large populations of

dormant seeds form a seed bank buried in the soil: as many as 86,000 viable seeds

per square meter have been found in cultivated soils. The species composition of

the seed bank may be very different from that of the mature vegetation above

it (Figure 5.6). Species of annuals that seem to have become locally extinct may

suddenly reappear after the soil is disturbed and these seeds germinate.

Dormant seeds, spores or cysts are also necessary to the many ephemeral plants

and animals of sand dunes and deserts that complete most of their life cycle in

less than 8 weeks. They then depend on the dormant stage to persist through the

remainder of the year and survive the hazards of low temperatures in winter and

the droughts of summer. In desert environments, in fact, the rare rains are not

Chapter 5 Birth, death and movement

153

AFTER JUTILA, 2003

seed banks

Seed bank

Mature vegetation

Germination

GR6

Seedlings

GR4

GR3

GR1

GR7

GR2

GR5

Seed rain Seed rain

Establishment

Figure 5.6

Species recovered from the seed bank, from seedlings and from the mature

vegetation in a coastal grassland site on the western coast of Finland. Species may

germinate from the buried seed bank into seedlings, and seedlings may establish

themselves in the mature vegetation. Mature plants may contribute seeds (in the

‘seed rain’) that germinate into seedlings immediately or enter the buried seed bank.

Seven species groups (GR1–GR7) are deﬁned on the basis of whether they were

found in only one, two or all three life stages. The marked difference in composition,

especially between the seed bank and the mature vegetation, is readily apparent.

Thirty-two species in the mature vegetation (19 + 13) were not represented in the

seed bank; 33 species in the seed bank were not found in the mature vegetation,

and 29 of these were not found as seedlings either.

ephemeral ‘annuals’ of deserts

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necessarily seasonal, and it is only in occasional years that sufﬁcient rain falls

and stimulates the germination of characteristic and colorful ﬂoras of very small

ephemeral plants.

5.2.3 Longer life cycles

There is a marked seasonal rhythm in the lives of many long-lived plants and

animals, especially in their reproductive activity: a period of reproduction once

per year (Figure 5.7a). Mating (or the ﬂowering of plants) is commonly triggered

by the length of the photoperiod – the light phase in the daily light–dark cycle,

which varies continuously through the year – and usually makes sure that young

are born, eggs hatch or seeds are ripened when seasonal resources are likely to

be abundant.

In populations of perennial species, the generations overlap and individuals

of a range of ages breed side by side. The population is maintained in part by

survival of adults and in part by new births. A study of the great tit Parus major,

for example, showed that of 50 eggs that were laid by a breeding population of

10 birds in one season, only 30 hatchlings survived to become fully ﬂedged, and

Part III Individuals, Populations, Communities and Ecosystems

154

A desert in bloom. In desert areas where rainfall is rare and

seasonally unpredictable, a dense and spectacular ﬂora of

very short-lived annuals commonly develops after rain storms.

They often complete their life cycle from germination to seed

set in little more than a month.

repeated, seasonal breeders

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only three of these survived to adulthood the following year. These three 1-year-

old birds were joined in that second year, though, by a further ﬁve birds aged

between 2 and 5 years – the survivors of the previous year’s 10 (Figure 5.8).

In wet equatorial regions, on the other hand, where there is very little seasonal

variation in temperature and rainfall and scarcely any variation in photoperiod,

we ﬁnd species of plant that are in ﬂower and fruit throughout the year – and con-

tinuously breeding species of animal that subsist on this resource (Figure 5.7b).

There are several species of ﬁg (Ficus), for instance, that bear fruit continuously

and form a reliable year-round food supply for birds and primates. In more

seasonal climates, humans are unusual in also breeding continuously throughout

the year, though numbers of other species, cockroaches for example, do so in the

stable environments that humans have created.

Other plants and animals (Figure 5.7c) may spend almost all their lives in

a long non-reproductive (juvenile) phase and then have one lethal burst of

reproductive activity. We saw such semelparity earlier in biennial plants, but it

is also characteristic of some species that live much longer than 2 years. The

Paciﬁc salmon is a familiar example. Salmon are spawned in rivers. They spend

the ﬁrst phase of their juvenile life in fresh water and then migrate to the sea,

often traveling thousands of miles. At maturity they return to the stream in which

they were hatched. Some mature and return to reproduce after only 2 years at

sea; others mature more slowly and return after 3, 4 or 5 years. At the time of

Chapter 5 Birth, death and movement

155

Reproductive phase

Reproductive output

(a)

(b)

(c)

Death

Year 1 Year 2 Year 3 Year 4 Year 5

Year 1 Year 2 Year 3

Year 1 Year 2 Year 3 Year n

Juvenile

phase

Reproductive phase

Juvenile

phase

Juvenile phase

Time

Figure 5.7

Simpliﬁed life histories for

organisms living more than 1 year.

(a) An iteroparous species breeding

seasonally once per year. Death

tends not to occur predictably

after any given time, though a

decline toward senescence is often

observed. (b) An iteroparous species

breeding continuously throughout

the year. The pattern of death and

decline is similar to that in (a).

passing several or many years in

a pre-reproductive juvenile phase,

followed by a burst of reproduction,

followed in turn by inevitable death.

continuous breeders

semelparous species like salmon

and bamboo

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reproduction, the population of salmon is composed of overlapping generations

of individuals. But all are semelparous: they lay their eggs and then die; their

bout of reproduction is terminal.

There are even more dramatic examples of species that have a long life but

reproduce just once. Many species of bamboo form dense clones of shoots that

Part III Individuals, Populations, Communities and Ecosystems

156

Survival

Adults

eggs per

female

Year t

84%

survive

50%

survive

71%

survive

Nestlings

(1 month old)

Fledglings

(3 months old)

29.8

10%

survive

Year t +1

Birth

Eggs

Figure 5.8

A diagrammatic life history for a population of great tits near Oxford, UK.

Individuals typically live for several years; hence, the population in any

one year is a combination of survivors from previous years and newborn

individuals. Population sizes (in rectangles) are per hectare; the

proportions surviving from one stage to the next are in triangles; the rate

of egg production per female is shown in the diamond.

AFTER PERRINS, 1965

1.0

0.8

0.4

0.6

0.2

Probability of reproduction

Age

0.5

1.5

2.5

3.5

Leaf area (cm

)

Figure 5.9

The effect of plant age (years) and plant size (as measured by leaf area)

on the probability of Rhododendron lapponicum shoots entering their

reproductive phase. The relationships have been ‘smoothed’ by a

statistical technique called ‘logistic regression’. Note that the probability of

reproduction increases with plant size at all ages. Also, older shoots are

overall more likely to enter their reproductive phase because they tend to

be bigger. However, at any given size, the probability of reproduction tends

to decrease with age, making age itself a much poorer predictor of shoot

fate than size.

AFTER KARLSSON & JACOBSON, 2001

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remain vegetative for many years: in some species, 100 years. The whole popula-

tion of shoots then ﬂowers simultaneously in a mass suicidal orgy. Even when

shoots have become physically separated from each other, the parts still ﬂower

synchronously.

Organisms of long-lived species that are the same age, however, are not neces-

sarily the same size – especially in modular organisms. Some individuals may be

very old but have been suppressed in their growth and development by predators

or by competition. Age, then, is often a particularly poor predictor of fecundity.

An analysis that classiﬁes the members of a population according to their size

rather than their age (Figure 5.9) is often more useful in suggesting whether they

will survive or reproduce.

5.3 Monitoring birth and death: life tables

and fecundity schedules

The previous sections have outlined the different patterns of births and deaths in

different species. But patterns are just a start. What are the consequences of these

patterns in speciﬁc cases in terms of their effects on how a population might grow

to pest proportions, say, or shrink to the brink of extinction? To determine these

consequences, we need to monitor the patterns in a quantitative way.

There are different ways of doing so. To monitor and quantify survival, we

may follow the fate of individuals from the same cohort within a population: that

is, all individuals born within a particular period. A cohort life table then records

the survivorship of the members of the cohort over time (Box 5.2). A different

approach is necessary when we cannot follow cohorts but we know the ages of all

the individuals in a population. We can then, at one time, describe the numbers

of survivors of different ages in what is called a static life table (Box 5.2).

Chapter 5 Birth, death and movement

157

size matters

5.2 QUANTITATIVE ASPECTS

5.2 Quantitative aspects

In Figure 5.10, a population is portrayed as a series

of diagonal lines, each line representing the life ‘track’

of an individual. As time passes, each individual ages

(moves from bottom left to top right along its track)

and eventually dies (the dot at the end of the track).

Here, individuals are classiﬁed by their age. In other

cases it may be more appropriate to split the life of

each individual into different developmental stages.

Time is divided into successive periods: t

, t

, etc.

In the present case, three individuals were born (started

their life track) prior to the time period t

, four during t

and three during t

. To construct a cohort life table, we

direct our attention to a particular cohort (in this case,

those born during t

) and monitor what happens sub-

sequently to the cohort. The life table is constructed

by noting the number surviving to the start of each

time period. Here, two of the four individuals survived

to the beginning of t

; only one of these was alive at

the beginning of t

; and none survived to the start

of t

. The ﬁrst data column of the cohort life table

The basis for cohort and static life tables

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The fecundity of individuals also changes with their age, and to understand

properly what is going on in a population we need to know how much individuals

of different ages contribute to births in the population as a whole: these can be

described in age-speciﬁc fecundity schedules.

5.3.1 Cohort life tables

The most straightforward life table to construct is a cohort life table for annuals,

because with non-overlapping generations it is indeed often possible to follow

a single cohort from the ﬁrst birth to the death of the last survivor. One such life

table, for the annual plant Phlox drummondii, is shown in Table 5.1. An initial

cohort of 996 seeds was followed from seed germination to the death of the last

adult, with the life cycle broken down into successive periods of 14–63 days.

Part III Individuals, Populations, Communities and Ecosystems

158

thus comprises the series of declining numbers in

the cohort: 4, 2, 1, 0.

A different approach is necessary when we cannot

follow cohorts but we know the ages of all the indi-

viduals in a population (perhaps from some clue such

as the condition of the teeth in a species of deer). We

can then, as the ﬁgure shows, direct our attention to the

whole population during a single period (in this case,

) and note the numbers of survivors of different ages

in the population. These may be thought of as entries

in a life table if we assume that rates of birth and death

are, and have previously been, constant – a very big

assumption. What results is called a static life table.

Here, of the seven individuals alive during t

, three were

actually born during t

and are hence in the youngest

age group, two were born in the previous time interval,

two in the interval before that, and none in the interval

before that. The ﬁrst data column of the static life table

thus comprises the series 3, 2, 2, 0. This amounts to

saying that over these time intervals, a typical cohort will

have started with three and declined over successive

time intervals to two, then two again, then zero.

Figure 5.10

See text for details.

Period

Cohort

Age

Birth

Time

an annual life table for a plant

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