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1

how do populations fluctuate?

naturally, over time

2

Wild population numbers undergo significant
amounts of variation from year to year

variations
that occur in invertebrates as well as vertebrates ....
in plants as well as bacteria, fungi and even viruses

3

wild population numbers are dependent on

changing environmental conditions over time (weather/climatic conditions) - these factors can influence such things and competitive interactions, predation rates, herbivory, parasitism

4

variation in the density of a population depends on:

1. the amoung of environmental fluctuation (both density dependent and density independent factors)
2. the inherent stability in the population

5

all populations experience fluctuations due to factors such as

availability of resources, predation, competition, disease, parasites, and climate

6

Fluctuations include

random and cyclic changes through time

7

Some populations tend to remain

relatively stable over long periods

8

In contrast, some populations exhibit

much wider fluctuations

9

Small organisms (e.g., algae) tend to reproduce much faster than larger organisms (e.g., red deer), so their populations often respond

faster to favorable and unfavorable conditions (high surface areas)

10

Larger organisms have a lower surface-area-to-volume ratio, which allows them to

maintain homeostasis in the face of unfavorable environmental changes

11

some of the most pronounced population fluctuations in nature are the predictable cycles of certain

mammal and bird species at high latitudes

12

When an age group contains a high or low number of individuals, the population likely experienced

high birth or death rates in the past

13

Long-term fluctuations in age structure can be determined for a forest by

examining tree rings

14

Populations in nature rarely follow a smooth approach to their

carrying capacity

15

overshoot

when a population grows beyond its carrying capacity

16

when does overshoot occur

often when the carrying capacity of a habitat decreases from one year to the next

17

die-off

a substantial decline in density that typically goes well below the carrying capacity

18

when do die-offs often occur

when a population overshoots its carrying capacity

19

Populations With High Growth Rates Track
Environmental Fluctuations More Closely
Than

Those With Low Growth Rates

20

Environmental changes continually increase and
decrease the carrying capacity of the environment
for each population. How a population responds to such
changes through density-dependent effects, depends on …

the intrinsic capacity of the population to increase
in size ... that is, its r value (which ultimately is
influenced by the extent to which the species is “r”
or “k” selected)..

21

The faster the potential rate of growth or decline
of the population – i.e., the higher the fecundity and
the shorter the life span of individuals

the greater is
its capacity to track change in its environment (e.g.
r-selected species).

22

Populations with r > 1 tend to

track environmental
changes

23

populations with r < 1 may
be

unresponsive to short-term environmental change

24

A population
with a higher growth rate (r = 0.5) tracks

a population with
a lower growth rate (r = 0.1)

25

Population Cycles May Result From

Intrinsic
Demographic Processes

26

Environmental changes generally tend to be

irregular rather than predictable

27

biological responses to environmental changes are also frequently

random and unpredictable

28

The sizes of many populations do, however,
change with

periodic frequency

29

a number of small mammal cycles have become part of the

lore of population ecology

30

Charles Elton, in 1924, showed that the lynx, and its
main prey, the snowshoe hare, display

large, regular fluctuations in population size - with each cycle lasting about 10 years