Question Key
-
Explain the Intermediate Disturbance Hypothesis (IDH). What pattern is
explained by this hypothesis and what might give rise to the pattern?
How does this relate to topics we have discussed previously?
The IDH is one idea that we
use to understand why communities differ in species diversity. The basic
idea is that the amount of “disturbance” a community experiences
influences which species (and how many) can persist there. Let’s
consider the two extremes of very disturbed versus little
disturbed.
If an area is extremely
disturbed (either by very frequent disturbances or by less frequent but
large disturbances), the hypothesis predicts that very few species can
persist with so much disturbance and it is likely that conditions will
favor the few species that can survive the disturbance or the few
species that can grow fast enough to reproduce before the community is
disrupted again.
Now if we think about the
opposite extreme, a community where disturbances are exceedingly rare,
then we expect that competition will dominate and only the few species
that are the very best competitors will persist over the long
term.
And then let’s look at the
intermediate condition – the thing emphasized in the hypothesis! In a
community that experiences intermediate levels of disturbance (such as
common but not huge disruptions), there will be some areas that are more
stable and favor strong competitors and other areas that are more
recently disrupted and will favor pioneer species (good dispersers, fast
growers, but poor competitors)… this intermediate level of disturbance
thus fosters the persistence of a variety of life history strategies,
overall increasing community diversity.
-
We can turn the IDH on its head and consider the way in which community
diversity influences the response to a disturbance. That is, a diverse
community may be more resilient to disturbance than a species-poor
community.
-
What is meant by a community being “resilient” to disturbance? How could
we measure this?
Generally, when ecologists
say that a community is resilient to disturbance, they mean that the
productivity of the community is maintained, despite disturbances.
Productivity is the amount of biomass that is produced by the community
over the course of a growing season. When that value is relatively
stable (low coefficient of variation), then we would say the community
is more resilient, able to maintain its production of biomass despite
disturbances.
-
Why might a diverse community be more resilient than a species-poor
community?
One of the ideas for why a
diverse community might be more resilient than a species-poor one is
that if some species can’t do well under some conditions, having a
diversity of species means it’s likely that another species will be able
to grow well under the changed conditions. Consider a monoculture (a
community that has only a single species in it, like a corn field). If
that monoculture community is hit with a bad drought, and that one
species can’t survive well in drought, that will mean biomass goes way
down in the drought year. But in a multi-species diverse community,
there are likely some members of the community who can thrive during a
drought (are adapted to these conditions) and so during the drought
year, they will do well and thus maintain biomass production in the
community.
-
As mentioned in the content video on succession, Clements viewed
ecosystems “as analogous to an entire complex organism that, in the
event of a disturbance, would pass through a series of stages to return
to its climax comunity.” Why is this view inaccurate – what’s wrong with
it? What would be a more accurate way to describe our current
understanding of the causes and process of ecological succession?
An ecosystem is NOT
analagous to an entire organism. Ecosystems do not have mechanisms of
maintaining homeostasis, they do not follow a program of development
contained in an inherited genome, they do not reproduce themselves, nor
do they seem to compete with other ecosystems. Thus, comparing
succession in an ecosystem to organismal development from birth to
maturity, is not an accurate representation. This comparison suggests
that ecosystems are supposed to turn out in a particular way, that they
will naturally follow that path every time. While we do describe
ecosystems as having properties that emerge from the interactions of
their constituent species, we have yet found “rules” that strictly
govern how ecosystems are assembled. We do not have evidence that
species intentionally change conditions for the benefit of other species
and against their own interests (and evolution would not favor
this…).
There does seem to be a
pattern of change in terms of succession – primary succession starts out
with species that can survive harsh conditions (pioneer species) and
moves seemingly predictably through stages of low vegetation before
becoming dominated by shrubs and small trees followed by domination by
larger longer-lived trees (strong competitors).
One of the strongest
arguments against the Clements view comes from what we’ve learned of
invasive species. If ecosystems operated as Clements thought, then it
would seem that invasive species should never happen – the ecosystem
itself would prevent that. Instead, we see that there is nothing special
maintaining things as they are – ecosystems can and do tolerate the
addition of new species (introduced taxa are not all invaders). Clearly,
some taxa change ecosystems more than others do and in ways that we
might not appreciate – by altering ecosystem services that we depend
upon or reducing species diversity that we value (for aesthetic or other
reasons).
Thus, ecological succession
results from a dynamic process in which propagules (seeds or fragments
of plants) arrive in a place, some of them are able to persist and grow
there, their cycles of life and death eventually build soil and change
conditions in ways that make them no longer the best competitor,
allowing new species to arrive and take root, with the same process
occuring of changing conditions with cycles of life and death, until new
best competitors are enabled to dominate. Which species are able to
disperse into the ecosystem will be critical in determining the possible
species composition. When disturbances occur, the competitive balance
can be shifted to favor different dominant taxa (ex: an invader arrives)
or to alter the composition in more subtle ways (ex: an introduced
species becomes part of the community). The properties of the ecosystem
result from the activities of its inhabitants and thus alterations to
the community structure can also alter ecosystem properties.
-
Read either Binding up the Wounds or Kickapoo. How does the reading
connect to what we’ve been covering in class? Give an example of how
this reading relates to each of the following.
- competition
- community diversity and structure
- ecological succession
Responses will
vary!
Describe the flow of energy through a food chain. What are the physical
constraints and biological factors that influence the flow?
In a food chain, energy
ultimately derives from an external source like the sun (there are a few
other sources like hydrothermal vents, utilized by chemotrophs). Primary
producers (autotrophs) are able to capture some of the energy in
sunlight and convert it into biomass via photosynthesis; some energy
will also be lost to heat and respiration of primary producers. Primary
consumers (heterotrophs) will then eat primary producers, converting
some of the energy they contain into consumer biomass but also losing
some of it to heat and respiration. Secondary consumers will then eat
the primary consumers, etc… When organisms die, decomposers will break
down their biomass into simpler components, converting some of the
energy into decomposer biomass and losing some to heat and respiration.
Decomposers will thus return simpler components (nutrients) back into
the system, to be taken up again by primary producers.
The amount of energy
captured by producers will limit how much could possibly be available at
higher levels, as will the growth rate of the produers (if they grow
faster, that potentially provides greater amounts of energy to
longer-lived consumers). In addition, the local patterns of temperature
and precipitation will influence how much biomass can be supported, with
greater biomass typically found with warmer temperatures and greater
precipitation (up to a point!). Temperature influences the rate of
chemical reactions and adequate precipitation is also typically
important in facilitating life-supporting chemical conditions! The local
availability of the nutrients needed for organisms to grow (such as
nitrogen, phosphorus, potassium, etc) will also limit how much biomass
can be supported at each level. A system that is low in nitrogen will
not support as many primary producers, for example, despite having
sufficient sunlight.
How do we study energy flow in ecosystems? What do we measure and how
can we determine how much of the energy available at one trophic level
is transferred to another?
To study energy flow in
ecosystems we would generally estimate how much biomass is present at
different trophic levels. Gross or net primary production, estimated as
amount of carbon fixed by plants, is a typical estimate of the energy
capacity of ecosystems.
When we want to examine how
much of the energy from one trophic level is available to the next
trophic level, we examine production efficiency and trophic efficiency.
Production efficiency is a method of estimating how much of the energy
available to a consumer is actually converted into biomass. At the level
of the ecosystem, we can use production efficiency estimates and known
numbers of organisms at each trophic level to estimate the trophic level
efficiency: of all the biomass energy from one trophic level, how much
makes it into biomass at the next level? Typically, trophic efficiency
is less than 10% and will mean that total biomass tends to decrease as
you move up the food chain, such that top predators are the least
abundant category.