Page 88 - Environment: The Science Behind the Stories

800 (because their populations tend to stabilize over time near
carrying capacity, commonly abbreviated K). Because their
populations stay close to carrying capacity, these organisms
600
Population size (number of cells) 400 natural selection favors investing in high-quality offspring
must compete to hold their own in a crowded world. Thus,
that can be good competitors.
In contrast, species that are r-selected have high biotic
200
potential and devote their energy and resources to producing
many offspring in a short time. Their offspring do not require
0 parental care after birth, so r-strategists simply leave their sur-
0 3 6 9 12 15 18 21 vival to chance. The abbreviation r denotes the per capita rate
Time (hours)
at which a population increases in the absence of limiting fac-
(a) Yeast cells, Saccharomyces cerevisiae tors. Population sizes of r-selected species fluctuate greatly,
such that they are often well below carrying capacity. This is
6000 why natural selection in these species favors traits that lead to
rapid population growth. Many fish, plants, frogs, insects, and
others are r-selected.
Population size extremes on a continuum and that most species fall some-
It is important to note, however, that these are two
4000
where between the extremes of r-selected and K-selected spe-
2000
cies. Moreover, many organisms show combinations of traits
that do not correspond to a place on the continuum. A red-
wood tree, for instance, is large and long-lived, yet it produces
0 many small seeds and offers no parental care.
20 60 100
Time (days)
Conserving Biodiversity
(b) Mite, Eotetranychus sexmaculatus

Environmental changes that affect populations have been
2500
taking place as long as life has existed, but today human
2000 development, resource extraction, and population pressure
Population size 1500 change. Science is crucial in helping us understand how
are speeding the rate of change and altering the types of
we modify our environment. However, the threats to biodi-
1000

so environmental scientists recognize that we must also
500 versity have complex social, economic, and political roots,
understand these aspects if we are to develop sustainable
0 solutions.
1910 1920 1930 1940 1950
Fortunately, millions of people around the world are
Time (year) taking action to safeguard biodiversity and to preserve and
(c) St. Paul reindeer, Rangifer tarandus restore Earth’s ecological and evolutionary processes. We will
explore these efforts more fully in our discussion of biodiver-
Figure 3.19 Population growth in nature may depart from sity and conservation biology in Chapter 11. For now, let us
the logistic growth curve in various ways. Yeast cells from an see how Hawaiians have been confronting the challenges to CHAPTER 3 • Ev ol u T i on, Bi odiv ER si T y, A nd Po P ul AT i on E C ology
early lab experiment show logistic growth (a) that closely matches their biodiversity.
the theoretical model. Some organisms, such as the mite shown
here, show cycles (b) in which population fluctuates above and
below the carrying capacity. Populations that rise too fast and Introduced species pose challenges for
deplete resources may crash just as suddenly (c), such as the pop-
ulation of reindeer introduced to the Bering Sea island of St. Paul. native populations and communities
Data from: (a) Pearl, R., 1927. The growth of populations. Quarterly Review of
Biology 2: 532–548; (b) Huffaker, C.B., 1958. Experimental studies on predation: On top of our direct effects on populations, communities, and
Dispersion factors and predator-prey oscillations. Hilgardia 27: 343–383, Figure 7, ecosystems, human beings exert many indirect effects—for
© 1958 by the Regents of the University of California; (c) Adapted from Scheffer, instance, by introducing species into areas where they do
Victor B., 1951. The rise and fall of a reindeer herd. The Scientific Monthly 73: not occur naturally. Some such introduced species thrive in
356–362, Fig. 1. Reprinted with permission from AAAS. their new surroundings, killing or displacing native species
(pp. 106–107, 304–307). Island species are particularly vul-
require a long time to gestate and raise their young, but the nerable to introduced species: They have evolved in isolation
considerable energy and resources they devote to caring for in small areas with a limited community of other species, and
and protecting them helps give these few offspring a high so they lack defenses against mainland species that are well
likelihood of survival. Such species are said to be K-selected adapted to deal with a broad array of enemies. 87

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