Some plants that live in high-salinity environments have roots which exclude salt uptake,
others have the ability to secrete excess salt through special glands on their leaves, and some
isolate salt in internal organs.
canopy, which is why it is often very dark in these types
of forests, even in the middle of the day. The amount of
sunlight that does penetrate the canopy depends on the
type, quantity, and orientation of branches and leaves.
Seasonal changes in many ecosystems influence the
amount of sunlight that is available at the Earth’s surface. Temperate forests tend to have many deciduous
trees, which lose their leaves in the fall, allowing much
more light to penetrate the canopy than in the summer
months. In high-latitude ecosystems, winter months
bring much shorter days than summer months, reducing
the amount of time each day that sunlight is available
for use by biological organisms. In tropical ecosystems
near the equator, day length, and thus light availability,
is fairly constant year round.
Other Abiotic Factors: Wind, Salt, pH,
Nutrients
Many other abiotic factors can limit or facilitate the
successful survival, growth, or ability of an organism to
reproduce in an environment. Wind can cause significant erosion, either by transporting existing particles or
by wearing down surfaces. Plants in areas prone to high
wind conditions have evolved strategies-such as flexible
stems that bend without breaking, succulent tissues that
retain moisture, and narrow leaves (e.g., grasses, needles)
and are often small statured-to avoid the desiccating ef-
16
fects of high wind.
Salinity (a measure of the dissolved salt content in water) alters properties of water and can limit an organism’s
ability to absorb water. We usually think of the ocean
when we talk about salt water, as the Earth’s oceans contain 97 percent of its water. The salinity of the ocean
ranges from 33–37 parts per thousand. Soils adjacent
to oceans, such as those in salt marshes, are similarly
very saline, creating unique ecosystems that are capable
of persisting in these high-salinity environments. Some
plants that live in high-salinity environments have roots
which exclude salt uptake, others have the ability to secrete excess salt through special glands on their leaves,
and some isolate salt in internal organs. Many animals
that live in estuary and tidal marsh habitats will move
with the changing tide to maintain their own salinity
requirements. Some fish can drink salt water and excrete
the salt through their gills. Sea birds often excrete excess
salt through specialized salt glands in their nasal cavities. Marine mammals, while they live in high-salinity
conditions, get most of the fresh water that they need to
survive from the food that they eat.
Nutrients are chemical elements that are required by
all organisms for metabolism and growth. Nutrients
must occur in the environment for plants and animals
to survive, but nutrient concentrations vary considerably
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Figure 1.20
The open ocean, or pelagic zone, is stratified
vertically. The benthic zone refers to the ecological
zone at the bottom of a body of water. In the ocean,
the benthic zone starts in the rocky intertidal area
and continues along the ocean floor out to sea.
structure by extracting calcium carbonate from seawater.
Corals build their hard skeleton structure out of calcium
carbonate. Corals live in close association with algae. The
algae live inside each polyp and provide energy through
photosynthesis, while the corals provide protection from
grazing to the algae. The reefs provide structure and protection for other organisms as well, making coral reefs
much more diverse and productive than the open ocean
that surrounds them.
The open ocean, or pelagic zone, is stratified vertically. Light availability decreases rapidly with depth
in the water column, which leads to rapidly changing
habitats. Throughout the pelagic zone, the dominant organisms are tiny phytoplankton (microscopic plants) and
zooplankton (microscopic animals and juvenile stages of
larger animals), with the highest concentrations of photosynthetic organisms in the layers nearest to the water
surface. These very tiny organisms absorb nutrients directly from the seawater and in turn are a food source for
much larger oceanic organisms. Deeper in the water column, light becomes limited, and biota become few and
far between. Small and microscopic marine crustaceans
feed on the decaying material that falls from layers above
them. Fish, larger crustaceans, octopuses, and squid are
common deep sea predators.
The benthic zone refers to the ecological zone at the
bottom of a body of water. In the ocean, the benthic zone
starts in the rocky intertidal area (the rocky zone occu-
pying the area between high and low tide) and continues
along the ocean floor out to sea. The ocean bottom is
sparsely populated by very unique organisms that often
are not well studied due to the logistical difficulties of
examining the ocean floor. The base of the benthic food
chain is made up of detritus from dead phytoplankton,
marine mammals, birds, fish, and invertebrates. Polychaete worms and crustaceans are diverse and abundant
in these areas. Sea cucumbers and sea stars graze on the
organic matter on the ocean floor or filter food out of the
water. Benthic predators often use bioluminescence (the
biochemical emission of visible light by living organisms)
to lure prey.
Freshwater ecosystems make up a small portion of the
earth’s surface, but create important linkages between
terrestrial and marine environments. Rivers and streams
transport nutrients and material from terrestrial uplands
downhill to the ocean. The smallest and highest elevation
streams are called first-order streams. When two firstorder streams merge, they create a second-order stream.
The Amazon River in South America, the world’s largest
river by discharge, is a twelfth-order stream. Each stream
has stretches of riffles, fast-moving portions flowing over
coarse substrate, and pools, deep, slow-moving stretches
with fine sediment. Fish and other swimming organisms inhabit fast-moving portions of the main channels,
and invertebrates tend to live in or on the river substrate,
feeding on dead organic matter.
There is a continuum of changing environments from
the high-elevation headwater streams to the lowland
large rivers. Headwater streams (orders 1–3) are often
Figure 1.21
Terrestrial biomes are classified in terms of temperature and precipitation gradients, as well as by
the major plant life form that they contain.
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Section II
introduction
Community Ecology
In any ecosystem or habitat, a community is a unique
collection of plants, animals, bacteria, and fungi that
interact with one another and with their environment
in the same place and at the same time. The community’s physical environment is usually loosely defined as
a bounded geographical region. For example, a community might be all of the organisms that live within
a particular lake, sand dune, forest, or desert. These
organisms may compete with one another for limiting
resources such as light, nutrients, food, and water, may
rely on each other for food, or may interact in a way
that is mutually beneficial. Collectively, these interspecific interactions form the field of ecology known
as community ecology.
Species Richness and
Evenness
Community ecologists seek to understand how the
number of species, their spatial arrangements, and the
interactions among them form the structure of the
ecosystem. A simple measure of this structure is the
number of different species occurring in a defined geographical area, referred to as species richness. While
richness gives us an idea of the complexity of a community, there are not equal numbers of each species
represented. The evenness, or relative abundance, is
the percentage that the individuals of each species
contribute to the total number of organisms of all species present, and it gives us an idea of the “rareness” or
Table 2: Species Richness and Evenness
Two communities with the same number of species and total organisms can be divided in numerous ways.
Consider the following example of two simplistic ecological communities. Each community has only four
species present, so the species richness is the same. However, community 1 is much more homogeneous. In
contrast, community 2 has a much greater evenness among the numbers of each species present.
Species Richness and Evenness
Community 1
Community 2
Species 1
97 individuals
25 individuals
Species 2
1 individual
25 individuals
Species 3
1 individual
25 individuals
Species 4
1 individual
25 individuals
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Biodiversity Hotspots in India
There are two biodiversity hotspots in India: the Eastern Himalaya hotspot and the Western Ghat
hotspot. The Eastern Himalaya hotspot contains the northwestern and northeastern states of India as well
as northern Pakistan, Nepal, and Bhutan, and includes Mt. Everest, the world’s tallest mountain. Because
these mountains rise very abruptly, they contain many unique ecosystems in a relatively small area, from
grasslands to subtropical forests to alpine meadows. There are an estimated 10,000 plant species, almost a
thousand bird species, and about three hundred mammal species in this area. Logging for agriculture, livestock grazing, and settlement has fragmented habitats in the area, degraded many ecosystems, and caused
large-scale erosion on steep slopes.
The Western Ghat hotspot spreads across six Indian states, encompassing a mountain region that parallels the country’s western coast. The Western Ghat hotspot contains an estimated five thousand plant species, over five hundred bird and one hundred mammal species, 179 amphibian species, and 288 freshwater
fish species. Clearcutting for tea, coffee, and teak plantations has significantly fragmented habitats and
threatened the biodiversity in this region.
community. Ecosystem diversity refers to the diversity of ecological communities that are found within a
fixed area. For the purposes of this discussion, we will
be referring to biodiversity as the diversity of species
found within an ecological community.
Global Patterns of Biodiversity
with few periods of mass extinctions and other declines.
The types of species have shifted dramatically through
time. Animals and vascular plants are absent in the fossil record for nearly the first 4 billion years of Earth’s
history. About 540 million years ago, many marine invertebrate taxa evolved. Around 400 million years ago,
vascular plants evolved, and many terrestrial vertebrates
start to occur in the fossil record shortly thereafter.
The sub-discipline of community ecology seeks to answer questions about why these distributions of diversity
exist such as the following: Why are there more species
in the tropics than in the temperate regions? Why do we
usually find a few dominant species and many less common species in any one ecosystem? Why do woody species typically replace grasses and herbs over time? As we
start to understand more about an ecosystem, it becomes
possible to start to answer some of these questions.
Globally, scientists have estimated that there may be
somewhere between 5 and 30 million different species,
although these numbers are just guesses, as only about
1.7 million species have been identified and described.
However, these species are not evenly distributed
across the planet. Rather, there is a major gradient of
diversity from the very diverse equatorial tropics to the
species-poor polar regions. The number of species in
a fixed area of the tropics often has an order of magnitude or more individual species than the same fixed
area in a temperate or subarctic ecosystem. In North
America, there is a trend of greater diversity of tree Causes of Biodiversity
species in the east and less diversity in the west, with
What drives these patterns of biodiversity? Cleara hotspot of tree diversity in the southern states of the ly this is a difficult question with multiple interacting
U.S. In contrast, over the same area, there is a trend theories that explain these patterns. Causal factors have
of greater bird diversity in the west and fewer species interacted over many hundreds of thousands of years to
in the eastern part of the continent. Small island eco- produce the unique patterns of species distribution that
systems have lower biodiversity than large islands or we see today. Some hypotheses for the observed patterns
mainland ecosystems, with very distant islands like of diversity are as follows:
the Hawaiian archipelago being relatively species poor.
Additionally, there are places of high biodiversity that 1. The evolutionary speed hypothesis says that
there are more species in some areas (like the trophave been greatly threatened by human activity, called
ics) because speciation (the formation of new and
biodiversity hotspots. There are currently thirty-five
distinct species by evolution) happens faster in
areas that have been designated as biodiversity hotspots
these areas or has been happening longer. These
on the planet, and they are often a focus of biological
mechanisms can occur if increased temperature
conservation efforts.
increases the rate of speciation, allowing species to
Over geologic time, evolutionary changes have altered
diversify more rapidly than in more temperate repatterns of global species diversity. Overall, there has
gions. Places with a long evolutionary history (like
been a trend of increasing numbers of unique species,
many places in the tropics) will be more diverse
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Figure 3.3
a) A trophic pyramid; b) A food web
second is the fixing of carbon to generate carbohydrates. The rate of photosynthesis determines the supply of energy available to organisms.
The rate that sunlight is converted by autotrophic
organisms (organisms which synthesize their own
food; for example, plants on land and algae in water)
via photosynthesis into organic compounds is referred
to as primary productivity. Gross primary productivity (GPP) is the total rate of photosynthesis, or the
total energy obtained by autotrophs. However, autotrophs must use some energy in the process, reducing
the total productivity rate. The rate of energy stored
after accounting for the energy expended is referred to
as net primary productivity (NPP). This productivity of an ecosystem is also sometimes referred to as a
rate of production and is measured in units of energy
per unit area per unit time (such as grams per square
meter per year). The stored energy found at a given
area at a given time is often referred to as biomass,
which is simply the amount of organic material that
can be found at a given area at any given time. We can
measure the total amount of biomass in an agricultural
pasture or in a body of water if we are interested in the
amount of organic matter that is stored in that area.
In terrestrial ecosystems, temperature, precipitation, and nutrients control rates of primary productivity. Generally, NPP increases with increasing mean
annual precipitation, mean annual temperature, and a
longer growing season. Places that are very warm and
moist, such as tropical rainforests, have extremely high
rates of NPP. In contrast, places that are warm but dry,
48
such as deserts, have low rates of productivity. In addition to favorable climatic conditions, adequate nutrient
availability is required for plant growth. The presence
of necessary soil elements impacts the rates of nutrient and water uptake, photosynthesis, and thus, plant
growth. Generally, we see a pattern of increasing NPP
with increasing nutrient availability.
In aquatic ecosystems, the major controls on primary productivity are temperature, light, and nutrient
availability. As we discussed earlier, light availability
decreases with depth in the water column; therefore,
primary production also decreases with depth. Microscopic phytoplankton (free-floating algae, protists,
and cyanobacteria) perform the majority of the ocean’s
primary production, form the basis of the oceans’ food
web, and fix large amounts of carbon. Oceanic plants,
like their terrestrial counterparts, need nutrients for
growth. The two most important nutrients for phytoplankton growth are nitrate and phosphate, though
smaller amounts of other nutrients are also important.
These nutrients are present in dissolved seawater, but
the phytoplankton in the upper layers of the ocean often use up all that are available during photosynthesis. They are replenished during periods of upwelling,
when deep, cold, nutrient-rich waters are driven to the
surface to replace the warmer, nutrient-poor surface
waters.
In some conditions, there is no sunlight available to
support photosynthesis. Certain bacteria have evolved
to live in these conditions and are able to synthesize
energy from the oxidation of inorganic materials. We
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