Townsend C.R., Begon M., Harper J.L. Essentials of Ecology
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Chapter 4 Conditions, resources and the world’s communities
129
4.1 TOPICAL ECONCERNS
4.1 Topical ECOncerns
As a result of human activities, the atmosphere
contains increasing concentrations of certain gases,
particularly carbon dioxide, but also nitrous oxide,
methane, ozone and chlorofluorocarbons (CFCs).
These changes are predicted to lead to increased
temperatures and altered patterns of climate over
the face of the Earth (see Section 13.3.1). Given the
controlling influence of climate on the distribution
of biomes, ecologists expect the biome map of the
world to change significantly as carbon dioxide con-
centrations double over the next 60–70 years.
It is no easy matter to predict the precise details
of future climate or its consequences for biome dis-
tribution. Scientists have come up with a number of
feasible scenarios, which differ according to the basic
assumptions included in their models. The details of
these need not concern us here. It is enough to note
that the simulations shown in Figures 4.12 and 4.13 are
based on a climate change model that assumes an
effective doubling of carbon dioxide concentrations and
takes into account the coupling of atmosphere and
ocean in determining changes in patterns of temper-
ature and rainfall. The model is known as MAPSS. This
is translated into patterns in the distribution of biomes
by simulating the potential mature vegetation that could
live under the ‘average’ seasonal climate prevailing
(see Neilson et al., 1998, for further details).
The distribution of biomes shown in Figure 4.12
is as simulated by the model for current climate con-
ditions (Neilson et al., 1998). In other words, it is the
model’s picture of the way biomes are distributed
now (and reflects reality well; note that the biome
Predicted changes in the distribution of biomes as a result of global
climate change
s
Tundra
Taiga/tundra
Northern conifer forest
Temperate evergeen forest
Temperate mixed forest
Tropical broadleaf forest
Savanna/woodland
Shrub/woodland
Grassland
Arid lands
Figure 4.12
The distribution of major biome types under the current climate, as simulated by the MAPSS biogeography model.
AFTER NEILSON ET AL., 1998
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4.5 Aquatic environments
The dominating characteristics of aquatic environments result from the physical
properties of water. A water molecule is composed of an oxygen atom, which is
slightly negatively charged, bonded with two hydrogen atoms, which are slightly
positively charged. This dipolar structure enables water molecules to attract and
dissolve more substances than any other liquid on Earth. Consequently, water
can hold mineral ions in solution, providing the nutrient resources required for
the growth of algae and higher plants.
On the other hand, the solubility of oxygen, an essential resource for both
plants and animals, decreases rapidly with increasing temperature, and oxygen
diffuses only slowly in water. This problem can place major limits on life in water.
Oxygen is rapidly used up when dead organic matter decomposes. In places where
tree leaves accumulate or untreated sewage is discharged into a river or lake,
decomposition can create anaerobic conditions, which are lethal for fish and other
Part II Conditions and Resources
130
categories are not exactly the same as those we dis-
cuss elsewhere in the chapter). The map in Figure 4.13,
by contrast, is the predicted distribution of biomes in
60–70 years’ time (Neilson et al., 1998). This model
predicts a reduction in area of the northern biomes
of tundra and taiga/tundra (the open woodland that
occurs between the treeless taiga and the dense
northern coniferous forest). It also predicts a decrease
in arid lands and an increase in temperate forest.
These conclusions are in broad agreement with a
variety of models that incorporate different starting
assumptions.
s
Tundra
Taiga/tundra
Northern conifer forest
Temperate evergeen forest
Temperate mixed forest
Tropical broadleaf forest
Savanna/woodland
Shrub/woodland
Grassland
Arid lands
Figure 4.13
The potential distribution of major biomes resulting from climate changes associated with an effective doubling of carbon dioxide
concentration, as simulated by the MAPSS biogeography model.
AFTER NEILSON ET AL., 1998
the special properties of water as
a medium in which to live
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animals that have a high biological oxygen demand. Many aquatic animals main-
tain access to oxygen by forcing a continual flow of water over their respiratory
surfaces (e.g. the gills of fish) or have very large surface areas relative to their
body volume.
Water is viscous, and moving water transports whole living organisms, such as
small plants and animals. It offers resistance to the movement of motile animals
such as fish, otters and aquatic birds; not surprisingly, many motile aquatic animals
are streamlined. Many plants that live in moving water depend on rooting in the
substratum to hold them against water currents, and many smaller animals are
attached to the plants or hide in crevices or under rocks where they are protected
from the drag of moving water.
Water is unusual in remaining liquid over a wide range of temperatures. It
requires a lot of energy to heat it (i.e. it has high thermal capacity), but retains
heat efficiently. One consequence is that the temperature of large bodies of water
(oceans and large lakes) varies little over the seasons. A further peculiar physical
property of water is that it is less dense when frozen than when liquid. Like most
liquids, water becomes denser and sinks as it cools. However, at temperatures
below 4°C, water becomes less dense and when ice forms (at 0°C), it floats. Ice
on a water surface insulates the water beneath; lakes and streams can remain
liquid, free-flowing and inhabitable under a layer of ice.
4.5.1 Stream ecology
Streams and rivers contain a minute portion of the world’s water (0.006%),
but an enormous proportion of the fresh water that can be used by people.
Consequently, they have been tapped, dammed, straightened, rerouted, dredged
and polluted since the beginning of civilization. Understanding the impacts and
sustainability of some of these practices begins with understanding the basics of
stream ecology.
Streams and rivers are characterized by their linear form, unidirectional flow,
fluctuating discharge and unstable beds. The narrow nature of river channels means
that they are very intimately connected to the surrounding terrestrial environment.
Thus, a proper understanding of river ecology requires us to consider the river
and its drainage basin as a unit (see Section 1.3.3).
Oxygen concentration is often high in turbulent, upstream locations and low
farther downstream, where higher temperatures cause reduced solubility. This
is reflected in river fish communities, with active upstream species such as brown
trout (Salmo trutta) having a high oxygen demand, whereas more sluggish species
such as pike (Esox lucius) can tolerate the lower concentrations in their habitats
downstream.
A variety of other chemical and physical conditions vary from stream to
stream, or down the length of a given river. Figure 4.14 illustrates how the species
composition of stream invertebrate communities varies with conditions. There
were 30–40 species at each site (mainly the larvae of stoneflies, caddisflies and
chironomid midges) with much overlap in the list of species present. The data were
subjected to an analysis called community classification, which is conceptually
similar to taxonomic classification. In taxonomy, similar individuals are grouped
together in species, similar species in genera, and so on. In community classi-
fication, communities with similar species compositions are grouped together in
Chapter 4 Conditions, resources and the world’s communities
131
the importance of oxygen
concentration,...
. . . pH and temperature...
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sets. These sets are then, in turn, grouped into more inclusive sets, and so on.
In this case, the conditions that were most influential in determining the
pattern of grouping – and thus were most influential in determining community
composition – were pH, stream temperature and the volume of water flowing
per unit time (discharge).
Because stream discharge responds to events such as thunderstorms and
snowmelt, streams are highly disturbed systems. Stream ecologists have recently
been looking at ways in which different regimes of disturbance of the streambed
are reflected in the composition of the community. For example, the disturbance
regimes of 54 stream sites in New Zealand were assessed by painting particles
(pebbles, cobbles, boulders) representative of the streambeds and determining
the percentage that moved during several periods; this varied from 10% to 85%.
The insect inhabitants of the streams were categorized according to properties
that might help them deal with highly disturbed conditions, including small size
Part II Conditions and Resources
132
D
C
E
E
B
B
B
B
E
E
E
E
C
C
C
C
C
C
C
C
C
C
C
B
B
D
D
C
A
A
D
D
D
D
River Medway
Withyham
Hartfield
Crowborough
Nutley
Maresfield
34 sites
21 sites
(a)
(b)
19 sites
13 sites
High pH
Medium pH
High water flow Low water flow
Low pH
A 2 sites
D 7 sites E 6 sites
B 6 sites
C 13 sites
High summer
temperature
Low summer
temperature
Very low pH
Forest
Row
River
Ouse
km
02
Figure 4.14
The species composition of stream invertebrate communities varies
with conditions such as pH, summer temperature and waterflow.
(a) Classification of 34 stream communities. At each division,
the communities are divided into classes with similar species
compositions, and these divisions can be linked to particular
differences in conditions, as shown. The classes are identified by
the letters A–E. (b) The actual geographic distribution of community
classes A–E in southern England. The classes associated with
acid water conditions (D, E) occur typically in the headwaters of
the streams.
. . . and disturbance of the
streambed
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(small species generally have short life cycles and their populations can rapidly
rebuild), a streamlined or flattened body (less prone to being dislodged) and good
powers of flight of the adult insects that emerge from the stream to mate (more
likely to recolonize after a disturbance). The representation of these traits was
higher in the more disturbed streams, testifying to the ecological importance of
disturbance regime (Figure 4.15).
The terrestrial vegetation surrounding a stream (the riparian vegetation)
has two influences on the resources available to its inhabitants. First, by shading
the streambed it may reduce primary production of attached algae and other
plants. Second, by shedding leaves it can contribute directly to the food supply
of animals and microorganisms. Rivers that begin their course in forested
regions are often dominated by the external supply of organic matter, and many
of the invertebrates have mouthparts that can handle large particles (shredders)
(Vannote et al., 1980). Farther downstream, where the stream is wider and where
shading is less intense, invertebrates that graze or scrape algae from stones
(grazer–scrapers) may be more abundant. As a result of the shredding of large
particles into small organic particles (and also physical processes that break up
leaves), food for collector–gatherers and collector–filterers may also increase
downstream (Figure 4.16).
When riparian vegetation is changed, for example when forest is converted to
agriculture, there can be far-reaching effects. Less particulate organic matter enters
the stream, but there is less shading and more nutrient runoff from farmland.
Results are an increase in productivity of stream plants and a corresponding
change in the stream food web. There may also be effects on discharge (increased
when trees are removed), water temperature (higher if shading removed) and
streambed characteristics (increased input of fine mineral particles). The more
specific consequences of one particular interaction between human activity and
stream ecology are described in Box 4.2.
The intimate relationship between land and water is also obvious on the flood-
plains of rivers such as the Amazon, where seasonal floods inundate huge areas
of surrounding forest and provide massive inputs of nutrients and organic matter
to the river. Many of the world’s floodplains have been deliberately drained or
cut off from their associated river channels, with profound consequences for
river ecology.
Chapter 4 Conditions, resources and the world’s communities
133
(a) Small size (b) Streamlined or flattened (c) High adult mobility
100
50
0
100
50
0
100
50
0
500 100
Average intensity of disturbance (%)
500 100
Average intensity of disturbance (%)
500 100
Average intensity of disturbance (%)
Number of individuals (%)
Figure 4.15
Disturbances play an important role in stream ecology, particularly of stream insects. Disturbed streams contained proportionately more larval
insects that (a) were small, (b) had streamlined bodies, and (c) became adults that were strong fliers: characteristics that would enable these
insects to withstand a disturbance and recolonize afterward. The best-fit lines (see Box 1.2) are very highly significant in every case (P < 0.001).
AFTER TOWNSEND ET AL., 1997
interactions between the stream
and surrounding land
may be disrupted by human
activities
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Part II Conditions and Resources
134
S
hredders Collector–
g
atherer
s
Grazer–scra
p
er
s
Figure 4.16
Examples of the various categories of invertebrate consumers in stream environments.
4.2 TOPICAL ECONCERNS
4.2 Topical ECOncerns
Because streams are so intimately connected with
their terrestrial catchment areas, human activities in
their vicinity can have negative consequences for
stream ecology. For example, landscaping or the con-
struction of roads and buildings in the vicinity of water-
ways increases soil erosion and leads to silt runoff into
streams. The Cherokee darter (Etheostoma scotti)
lives in clear streams with beds made up of pebbles
and cobbles. Streambeds covered in silt deny this
species the ability to forage and spawn; it is now
restricted to just a few streams.
COURTESY OF GEORGIA MUSEUM OF NATURAL HISTORY, E. SCOTTI_RICHLAND CREEK BJF0211
A tiny stream fish with big consequences for property development
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4.5.2 Lake ecology
Just as river ecology is defined by the unidirectional flow of water, lake ecology
is defined by the relatively stationary nature of water within its basin. A critical
component of lake ecology is the way in which water can stratify vertically in
response to temperature (as mentioned in Section 4.2.3). As water sits in a lake
basin, the upper layer is exposed to the sun and heats up. Because warm water is
less dense than cold (and therefore tends to rise) the top layer stratifies – that
is to say, it forms a layer that is quite separate from the colder water beneath.
This layer, the epilimnion, is warm and well illuminated and has high oxygen
content because surface waters exchange oxygen with the atmosphere. It is usu-
ally extremely productive, with high densities of plant and animal life.
In deeper lakes two further layers may form. Below the epilimnion is a trans-
itional layer, the thermocline, in which temperature, oxygen concentration and light
all decrease. The deepest layer, the hypolimnion, is cold and often poor in oxygen.
Chapter 4 Conditions, resources and the world’s communities
135
The following article by Clint Williams appeared in
the Atlanta Journal on July 2, 2001.
Cherokee darter: tiny fish forces changes
to project
While barely 2 inches long, the Cherokee darter
has the power to move roads and redesign a
golf course.
The tiny fish, protected under the federal
Endangered Species Act, swims in the small,
gravel-bottomed streams that wind through a
planned 730-acre gated community straddling
the Cobb-Paulding county line. And it’s forcing
the developer to reshape his plans in order to
protect the fish.
‘We have fine-tuned our layout in order to
minimize our impact on the Cherokee darter’, said
Joe Horton, developer of the Governor’s Club, a
high-dollar golf course development. ‘We’re now
on our sixth-generation site plan’, Horton said.
The Cherokee darter, pale straw yellow with
dark olive markings, was listed as threatened by
the US Fish and Wildlife Service in 1994, not long
after it was identified as a species distinct from
the Coosa darter. The Cherokee darter is found
only in roughly 20 small tributary systems of the
Etowah River, according to a Fish and Wildlife
report. But just a few streams have healthy
populations.
‘It’s in a number of streams but that number
is declining rapidly’, said Seth Winger, a
conservation ecologist at the Institute of Ecology
at the University of Georgia.
The creeks running through the Governor’s
Club property are tributaries of Pumpkinvine
Creek, which flows into the Etowah River below
Allatoona Dam. There are 8000 feet of creeks
on the tract, Horton said. A biological survey
conducted before purchasing the property found
four Cherokee darters . . . ‘We’re proud we have
them’, Horton said.
Having them will be a bit costly, however.
(Reproduced by permission of the PARS Inter-
national Corp.)
1 Is it reasonable that a small population of a
species that occurs in about 20 other streams
should disrupt economic development?
2 More specifically, how widespread would the
species have to be (in how many streams,
in how many states or countries) before
developers should be allowed to ignore it?
3 Do you think is should be the responsibility of
ecologists, such as the one quoted, simply to
inform the public of the facts (as in this article)?
Or is it reasonable for them to become involved
in advocacy for a conservation cause?
lakes may become
thermally stratified, with major
consequences for their ecology
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It is here that sunken dead organic matter is decomposed and its mineral nutrients
released. In temperate regions of the Earth, stratification of lake water breaks down
in the fall when the upper layer cools. Currents then mix the water layers and the
minerals released in the hypolimnion become available at the lake surface.
Lake ecologists are increasingly turning their attention to the larger spatial
scale of whole lake districts. Lakes high in a landscape (such as those in northern
Wisconsin) receive a greater proportion of their water from direct precipitation,
whereas lakes at lower altitudes receive more water as an input from ground
water (Figure 4.17). This is reflected in the higher concentrations of important
ions in lakes low in the landscape. The contrasting ion concentrations can be
expected, for example, to affect the ecology and distribution of freshwater
sponges, whose skeletons require silica, and crayfish and snails, which have a
particular need for calcium.
Nutrient-rich lakes may support a rich flora of microscopic, floating phyto-
plankton (microscopic plants), together with a diversity of invertebrates and
fish species, but a rooted flora of flowering plants is confined to shallow waters
near the shore, the littoral zone. This zone is usually rich in oxygen, light, food
resources and hiding places. However, some fish and invertebrates specialize in
the deeper colder waters of lakes. Lake trout (Salvelinus namaycush) and walleye
(Sander vitreus) are two popular sport fish whose habitat is restricted to the colder
regions of lakes.
Many lakes in arid regions, lacking a stream outflow, lose water only by
evaporation and become rich in sodium and other ions. These saline lakes should
not be considered as oddities; globally, they are just as abundant as freshwater
lakes. They are usually very fertile and have dense populations of blue-green
algae, and some, such as Lake Nkuru in Kenya, support huge aggregations of
plankton-filtering flamingoes (Phoenicopterus roseus).
4.5.3 The oceans
The oceans cover the major part of the Earth’s surface and receive most of the
Earth’s income of solar radiation. However, much of this radiation is reflected
at the water surface or absorbed by water itself and by particles in suspension.
Even in clear water the intensity of radiation falls off exponentially with depth
and photosynthesis is mainly restricted to the upper 100 m – the euphotic zone.
In most waters the euphotic zone is much shallower, especially where water is
more turbid close to coasts and estuaries.
The green plants that photosynthesize in the open oceans are planktonic,
mainly single-celled algae that are capable of using solar radiation very efficiently.
But, in the real world, many areas of ocean that receive the greatest intensity of
solar radiation have the lowest biological activity – because plant productivity is
limited by shortage of mineral nutrients. The great tropical parts of the Atlantic
and Pacific Oceans have a biological productivity of less than 35 g carbon (g C)
m
−2
yr
−1
. This compares with more than 800 g C m
−2
yr
−1
in terrestrial commun-
ities at the same latitudes.
The areas of greatest marine productivity (exceeding 90 g C m
−2
yr
−1
) occur
where there is a reliable supply of minerals (especially nitrogen and phosphorus, and
perhaps iron). This occurs via leaching from the land through rivers and estuaries
or where deep currents in the oceans well up to the surface and bring dissolved
Part II Conditions and Resources
136
saline lakes are common in
some parts of the world
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nutrients into the euphotic zone (see Section 11.2.2). In areas where upwellings
occur, the ocean ‘desert’ becomes transformed to a productive environment, as,
for example, off the coast of Peru. Dense populations of planktonic algae support
small crustacea, which in turn are eaten by schools of anchovies (Engraulis ringens).
The fish support sea lions, and flocks of cormorants, pelicans and gannets.
Chapter 4 Conditions, resources and the world’s communities
137
1000
10,000
100
10
1
Crystal Sparkling Allequash
(b)
02
km
4
(a)
496
494
496
494
500
502
496
500
502
504
498
500
504
Allequash Lake
Crystal Lake
Big
Muskellunge Lake
Trout Lake
Trout Lake
Sparkling
Lake
492
494
490
494
496
496
502
498
Big
Muskellunge
Trout
Land position (high low)
Ca + Mg (µeq l
–1
)
SiO
2
(µg l
–1
)
Nutrient concentrations
Figure 4.17
Lakes at different positions in the
landscape differ in the source of
their water and the concentrations
of chemicals important to their
inhabitants. (a) Map of Wisconsin
Lake District: study lakes are darkly
shaded and contours are shown
(meters above sea level).
(b) Relationships between landscape
position and concentrations of
calcium and magnesium (Ca + Mg)
and silica (SiO
2
) in the five lakes.
Lakes higher in the catchment area
(Crystal and Big Muskellunge) have
lower nutrient concentrations.
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We saw earlier in this chapter that the distribution of terrestrial communities
depends largely on the intensity of solar radiation and its effects on temperature
and water availability. In complete contrast, variations between oceanic commun-
ities are ruled mainly by the availability of mineral nutrients.
Below the euphotic zone is increasing darkness, and the ocean floor is in total
darkness, intensely cold and under great pressure. This abyssal environment sup-
ports the very slow biological activity of a community of extraordinary biological
diversity (including worms, crustaceans, mollusks and fish found nowhere else),
which depends on the rain of dying and dead organisms falling from the euphotic
zone above. Many of the invertebrate animals are tiny, have very low metabolic
rates and possess a lifespan that may last for decades. Yet further diversity occurs
in hydrothermal vents that occur at a number of isolated places 2000–4000 m
deep (see Box 2.2). In these remarkable environments, there are high sulfide
concentrations and very high temperatures, up to 350°C, where superheated
fluid emerges from ‘chimneys’, and there is a sharp gradient down to 2°C, the
temperature normally encountered in abyssal depths close by. The vent areas are
inhabited by productive thermophilic (heat-loving) bacteria and a unique fauna
of polychaete worms, crabs and very large mollusks.
4.5.4 Coasts
Marine environments change dramatically near to coasts. Not only are they
enriched by nutrients from the land; they are also affected by waves and tides that
bring new physical forces to bear. In particular, there are now surfaces to which
organisms can attach; indeed, if they do not do so they are liable to be washed
out to sea or stranded on the shore. At a broad scale, coastal communities are
strongly influenced by waves and tides and the topography of the coast. Within
a single stretch of coast, we can recognize a zonation in the flora and fauna
marked by high and low tide levels (Figure 4.18). Such zonation patterns are
more obvious in sheltered situations where wave action is light, but become more
fuzzy in very exposed situations.
The extent of the littoral zone depends on the height of tides and the slope
of the shore. Away from the shore, the tidal rise and fall are rarely greater than
1 m, but closer to shore, the shape of the land mass can funnel the ebb and
flow of the water to produce extraordinary spring tidal ranges of, for example,
nearly 20 m in the Bay of Fundy (between Nova Scotia and New Brunswick,
Canada). In contrast, the shores of the Mediterranean Sea experience scarcely
any tidal range.
On steep shores and rocky cliffs the littoral zone is very narrow and zonation
is compressed. Both plants and animals are profoundly affected by the phys-
ical force of wave action. Anemones, barnacles and mussels attach themselves
securely and permanently to the substrate and filter planktonic plants and animals
from the water when the tides cover them. Other animals, such as limpets, move
to graze, and crabs move with the tides and use rock crevices as refuges. The
flora in a rocky infralittoral zone (Figure 4.18) is usually dominated by the large
brown seaweeds (kelps), which fix themselves to the rock with specialized
‘holdfasts’.
Environments are quite different on shallow sloping shores on which the
tides deposit and stir up sand and mud. Here the dominant animals are mollusks
Part II Conditions and Resources
138
unique communities occur in the
abyssal depths of the oceans
waves and tides are key
influences in coastal ecology
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