Miller Ch 7 - Community Ecology:
Species Interactions, Succession, and Sustainability
Intro - Flying Foxes: Keystone species in tropical forests
Story highlights the importance of role that one species can play in a larger
ecosystem - many plants depend upon flying foxes (type of bat- Fig 7-1) to
pollinate and to spread new seeds. Foxes are considered to be pests, widely
hunted. As they decline, so will the plants that they polinate.
7-1 Community Structure: Appearance and species diversity
Community (or ecosystem) structure (or spatial distribution) is defined in terms of
4 characteristics:
1. physical appearance - sizes, stratification, distributions of species
2. species diversity - number of diff species
3. species abundance - numbers of individuals of each species
4. niche structure - number of niches, how they are similar, how different
Various ecosystems are shown in Fig 7-2 - background on how these systems
are controlled by climate is found in Chap 6, which Bdale is not requiring us to
cover….if you want the right background, though, read Chap 6 (my Ch 6 notes
are also available on website) and notice how temperature and precipitation
control the global distribution of these ecosystems
How communities differ
Both terrestrial and aquatic ecosystems differ in physical appearance and in the
types of plants and animals populating these systems (many figs in ch 6)
Differences are both vertical (stratification, Fig 6-23 and Fig 6-32) and lateral
(by latitude, Fig 6-14)
Systems also have differing sizes of vegetation patches.
"Edge effects" occur at boundaries of ecosystems, e.g.,
Forest
Edge
Open Field
Edge may have diff characteristics than either field or forest, with diff species of
plants & animals
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However, as patches shrink and edges increase, more stress on species can
occur due to fire and predation
Also, more patches and edges decrease mobility of species to move to new
areas - that is, they can get isolated or trapped in smaller and smaller
ecosystems
To conserve habitats and species, 2 things are recommended:
1. preserve large areas of habitat
2. use migration corridors to link small habitat patches or ecosystems to each
other
Biodiversity
the world's most biodiverse habitats are:
- tropical forests
- coral reefs
- deep sea
- large tropical lakes
other areas with many diff species include:
- tropical dry habitats
- temperate shrublands
Paradox: some of the most diverse habitats tend to have low species abundance
(only a few members of each species)
Also, Miller indicates 3 major factors affecting species diversity:
-Latitude in terrestrial communities (Fig 7-3, also see Ch 6)
-Depth in aquatic systems (see Ch 6)
-pollution in aquatic systems Fig 7-4
Species diversity on islands:
In an isolated ecosystem like an island, size and isolation are two factors that
affect species diversity
MacArthur and Wilson developed the species equilibrium model (fig 7-5) that
predicts species diversity:
Biodiversity = Immigration rate of new species - extinction rate of current species
They also found that increasing size of island and decreasing distance
from mainland are 2 factors that increase biodiversity (Fig 7-6)
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7-2 - General Types of Species
we group species in an ecosystem into 4 main categories:
 native
 non-native
 indicator
 keystone
-native species are those that are normally found in the system
-non-native are those that migrate into the system from elsewhere, and can
crowd out native species (example - wetlands plant in NJ Meadowlands thrives
on polluted water, has crowded out other species….)
-indicator species serve as "early warning indicators" of potential environmental
problems. They are first to be affected by changes in environmental conditions.
Example - birds….there is a decline in insect-eating songbirds, thought to
be due to habitat loss and/or fragmentation both in winter habitats in Latin
America and in summer habitats in North America
Another example - fish such as trout. If temperatures are in correct range,
trout should exist unless water has low levels of dissolved oxygen, which usually
indicates contaminants are present.
Another example - amhibians such as frogs, toads, salamanders.
-keystone species are those that are particularly important to health of an
ecosystem, because they perform certain important functions:
- pollinate plants
- disperse seeds
- modify habitat
- predate upon other species, keeping them in check
- help plants get minerals and water
- recycle animal wastes
examples:
elephants modify habitat by clearing trees
bats regenerate forests by dropping seeds in their guano
sea otters eat sea urchins and keep the urchins from demolishing kelp
forests in the eastern Pacific Ocean
Top predator keystone species are very important, because they eat (regulate)
populations of other species, which in turn regulate other species, etc. etc.
Wolves, lions, alligators, sea otters, sharks are predator keystone species
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7-3 - species interactions: competition and predation
species interact in 5 different ways:
1) interspecific competition
2) predation
3) parasitism
4) mutualism
5) commensalism
look at 1 & 2 first
1) interspecific ("between species") competition: 2 or more species compete for
limited resources (food, space, shelter, etc)
with no competition, a species can occupy its "fundamental niche" without
interference
with competition, overlap of niches occurs (fig 7-8)
competition ultimately forces one of the species to move out, find diff food
sources, or reduce its numbers.
How to avoid competition - species use "resource partitioning" (diff birds in fig 79) as a way to all live together without too much competition
How?? Original "fundamental niches" that overlapped (fig 7-8, top)
become slightly separated (fig 7-8, bottom) into "realized niches"
Examples of resource partitioning:
 lions (large prey) and leopards (small prey)
 hawks hunt small animals during day, owls hunt same prey at night
 some birds feed on ground, others in trees
 warblers (fig 7-9) partition various parts of a tree
2) predator-prey species interaction
predator feeds directly on the prey, which is outside the predator (i.e., not an
internal organism)
individually, predator definitely not good for the prey ….
But on the larger population level, predators "thin out" populations of prey by
removing aged and/or sick animals, so that's a good thing in terms of maintaining
healthy, resilient populations of animals.
It's also important to realize that predators "play an important ecological role" in
controlling populations that might otherwise grow out of control…
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Local NJ example in my town - too many deer, running rampant, driving risk at
night. Why? Natural predators like wolves, coyotes, are getting pushed away by
development. Town has actually hired hunters to come in and shoot them in
order to control the population.
There are a number of strategies prey employ in order to survive (see Fig 7-10)..
 camouflage (tree frog)
 irritating chemicals (bad smell, taste - skunks)
 behave or look scary (blowfish, snake caterpillar)
Species interactions: Parasitism, mutualism, commensalism
1. parasitism - parasite (guest) benefits, host is harmed - "special type of
predation", but here the predator is small, remains attached or near the host,
tries to NOT kill the host..
2. mutualism - both the guest and the host benefit - examples include pollination
of plants by insects, lichens, and bacteria. With lichens, mutual relationship
between algae and fungi…algae physically live on the fungi, while the algae
create food for both. Birds and rhinos are another example…birds live on
rhinos and other large animals, while they eat the parasites that might hurt the
rhinos.
3. Commensalism - one species benefits, but the other is neither helped nor
harmed. Certain plants may live on tree trunks without hurting the tree. (Fig
7-12)
7-5 - Ecological Succession - communities in transition
this explains the rise and fall of populations through time 2 types:
 primary succession - original life from lifeless ground
 Secondary succession - establishment of new plants and animals in place of
the original communities
Primary succession is "starting from ground zero" fig 7-13. Rocke weathering
has to produce soil, which then can sustain life. Takes hundreds to thousands of
years.
Primary succession Sequence early successional lichens, moss, etc.
mid successional grasses, shrubs
late successional trees
secondary succession sequence - area has an earlier history, is disturbed, new
species come in and populate - excellent example - abandoned farmland that is
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being "reclaimed" by nature - fields filling in, barns falling down, things moving
back to the natural state.
7-6 - Ecological stability and sustainability
Miller introduces 3 factors that affect stability in ecosystems:
 Inertia (persistence) - system resists disturbance
 Constancy - numbers of species stay sustainable, given resources
 Resilience - ability for a system to "bounce back" after a disturbance
How does species diversity fit with this? In 1960s, it was thought that high
diversity yields high stability. Why? Because multiple paths through a big food
web allows plants & animals multiple ways to respond to stress on the
ecosystem.
However, things are more complicated than that…so we really don't know if
simple ecosystems are less stable than complicated ones, nor do we really know
what species need to be eliminated for an ecosystem to collapse.
Newer research suggests:
1. more species diversity tends to correlate with higher primary productivity and
perhaps more resilience
2. species populations fluctuate more in diverse ecosystems, suggesting
perhaps less resilience
Miller's bottom line:
SOME biodiversity appears to provide insurance against total ecosystem
collapse (makes sense), but how much is still very unclear.
Example: graph suggests that 10-40 "essential" producer species is optimal
Net
productivity
??
??
10
20
30
40
No. of producer species
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Big issue in ecosystem survival is trying to understand relationships between
species diversity and stability, and then relating these to:
Inertia and Resilience
Examples:
1. tropical rainforest has high species diversity and high inertia (that is, it's
resistant to change…)
But it has LOW resilience, so when a catastrophe like forest clearing or huge fire
occurs, this ecosystem may NOT be able to recover easily….
Why?
Because in this ecosystem, lots of nutrients are not found in the soil (unlike other
systems), but rather in the rapidly-growing plants…so if you lose the plants, you
lose the life-giving nutrients, which might otherwise be found in soil and could
promote new growth.
2.grasslands have low species diversity and low inertia (they burn easily and are
not resistant to change…)
But these are very resilient, because a lot of nutrients are stored below ground in
root systems. So after fire sweeps through, they can bounce back quickly (high
resilience)
Lastly, Miller talks about the "precautionary principle"
Aka - "Better safe than sorry"
VERY Important to Ecologists….
Principle essentially states that if there is some evidence that human activities
may be causing harm to ecosytems, then we need to take precautions to not
unduly harm ecosystems, even though we are not sure of cause and effect
relationships.
This causes a lot of friction between warring factions of the populace Industry may argue that we're overly cautious through rigid enforcement of
environmental regs, endangered species act, etc
Environmentalists may argue that we are not doing enough to protect things,
because we don't really know long-term effects of human activities on global
systems (e.g., global warming).
This is at the heart of MUCH environmental debate…..
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