Boreal forests are primarily composed of coniferous species that resist damage from hard winter freezes.
Desert, and the Mojave Desert.
Temperate Grasslands
Large expanses of grassland exist in both the Northern
and Southern Hemispheres at temperate latitudes. These
areas have great seasonal variability in temperature, with
long periods of freezing temperatures during a cold, dry
winter, and warm, moist summers. Fire and grazing by
large herbivores are important natural disturbance processes in these ecosystems, limiting the establishment of
woody tree and shrub species and favoring an ecosystem
dominated by grasses and small herbs.
Temperate Shrublands and Woodlands
Shrublands and woodlands develop in temperate regions with a winter rainy season. In these regions, the
timing of precipitation (winter) and the growing season
(summer) are asynchronous, limiting growth of largerstatured plants. The vegetation in these ecosystems tends
to be small statured, with thick, stiff evergreen leaves.
They are generally well adapted to long dry periods and
will slowly grow and photosynthesize in a moisture-limited environment. Some coastal temperate shrublands
include the fynbos of South Africa and the chaparral of
North America. Inland shrublands and woodlands areas
are associated with the seasonally cold climates and often
fall in the rain shadow of mountain ranges. The Great
Basin of North America sits between the Cascade/Sierra
Nevada crest to the west and the Rocky Mountains to
the east and is characterized by shrubs and infrequent,
22
small trees.
Temperate Deciduous Forests
Temperate deciduous forests occur where cold periods
are prolonged enough to make ongoing photosynthesis
inefficient, but where the growing season is long enough
and the soils nutrient-rich enough to make regrowth possible in the spring. These forests occur in eastern North
America and on both the eastern and western edges of
Eurasia. There are multiple vertical layers to these forests, with a subcanopy of trees as well as shrubs and forbs
below the upper canopy.
Temperate Evergreen Forests
There is quite a bit of variability in the environmental conditions that support evergreen forest growth in
the temperate regions, from warm coastal areas to cool
inland ecosystems. At the high precipitation end of the
spectrum, these forests are sometimes referred to as temperate rainforests, and at the low precipitation end, drier
forest types support frequent fire return intervals of ten
to twenty years, which promote the persistence of the
species present. Soils in temperate evergreen forests tend
to be nutrient-poor, in part because of the acidity in the
leaves that becomes incorporated into the soil profile.
The diversity of these forests is usually lower than that
of either deciduous or tropical forest types. Dominant
tree species in North America are needle-leaved conifers
such as pines, firs, hemlocks, and junipers.
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Figure 1.25
Billions
1.8
Projected
India
1.6
1.4
China
1.2
1
0.8
0.6
0.4
United States
0.2
0
1950 60
70
80
90 2000 10
20
30
40
50
60
70
80
90 2100
Source: United Nations
Population growth of China, India, and the United States,
from 1950 and projected to 2100, based on United Nations data.
If current growth rates continue, India will be the
planet’s most populous country by the year 2028.
stant in time or space due to resource availability, limiting abiotic conditions, the movement of organisms, and
other factors. Seasonal and inter-annual variability in
habitat conditions impacts germination and growth rates
of plants, reproductive vigor of many organisms, survival
rates of young, and movement patterns of both organisms
and embryos across large areas. At times, a population’s
habitat may be a series of disconnected patches, and individual movement or dispersal of seeds is required to
link these habitat patches. These movements vary widely,
from seed dispersal distances of just a couple of centimeters in small plants to tens of thousands of kilometers for
migratory whales.
Dispersal limitations can prevent a species from reaching areas of suitable habitat that are a great distance away.
For example, the Hawaiian Islands have only a single
species of terrestrial mammal, the Hawaiian hoary bat
(Lasiurus cinereus). No other mammals have been able
to cross the Pacific Ocean to disperse to the Hawaiian
Islands on their own, although dogs, cats, pigs, cows,
and many other exotic mammals that were brought to
Hawaii by humans are thriving on the Islands. Dispersal
limitations can also occur at much smaller spatial scales,
though, when physical disturbance or habitat modifications alter habitat structure.
Within populations, the distribution of individuals
depends on the availability of resources, dispersal, and
biotic interactions. Organisms may be fairly evenly dispersed, randomly dispersed, or grouped together to form
a clumped distribution, depending on resource availability and environmental conditions. In natural populations, organisms tend to be clumped within a population
more often than other types of distribution, largely because habitats are also typically patchy.
26
Metapopulations
Patchy habitats sometimes divide a species into small
populations, with occasional migration and interaction
among populations. These groups of populations are collectively called metapopulations, or “populations of populations,” and often occur in areas where patches of suitable habitat of various size and quality are interspersed in
a matrix of unsuitable habitat. Two sets of processes and
interactions must be considered when examining metapopulations: 1) the local or within-patch scale; and 2)
the metapopulation or between-patch scale. Population
size and density at the local scale is determined by birth
and death rates, while those interactions at the metapopulation scale must be expanded to consider movements
between patches as well. Persistence of a metapopulation is a constant balance between extinctions of local
populations and the colonization of new suitable habitat
patches.
Temporal Change
Population Growth and Decline
If birth rates and age-specific survival rates are constant over time, a population will grow at a fixed rate,
and the proportions of individuals in each age class will
be constant. When this age structure does not change
from one year to the next, the population is said to have
a stable age distribution. However, changing environmental conditions or biotic interactions often alter these
rates, impacting the population’s age structure. Age
structure, the proportions of the population in each age
class, is an important feature of populations, as it determines whether a population is growing, is stable, or
will decline. If a large proportion of the organisms are
young, the population is likely to grow, but if a disproportionate number are past reproductive age, the species
is likely to be declining. Because survivorship at each age
class is impacted by environmental conditions as well as
interspecific and intraspecific interactions, biologists and
natural resource managers often manipulate these factors to manage a population over time.
When a species reproduces at fixed intervals, such as
annual plants, the population changes in size by a constant proportion, resulting in geometric growth of the species. This means that the number of individuals has the
potential to constantly become larger with each reproductive cycle, unless the death rate for the population
balances the increasing population. Populations grow
exponentially when there is continuous reproduction (as
in humans) outside of discrete time periods. When generations overlap, increases in numbers can occur much
more rapidly. Logistic growth occurs when the growth
rate decreases as the population reaches carrying capacity, the maximum number of individuals in a population
that the environment can support.
India’s human population is a very visible example of
exponential population growth. India is currently the
second most populous country in the world (after China),
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