Evan Olson
Josh Schneider
Chapter 12 – Redundancy in Ecosystems
I.
Introduction
A.
Redundant means surplus to requirements. Redundant words in a
sentence can be deleted with no loss of clarity of meaning. If a species
is removed from an ecosystem but the ecosystem continues to survive
then that species is considered redundant.
B.
Two common questions that are asked are: 1. How much species
redundancy is built into an ecological process? 2. To what extent are
patterns of biological diversity important in determining the behavior
of ecological systems?
1. There are theoretical and empirical studies that might help us
answer so fundamental a question are few; nor does there appear to be a
clear consensus among ecologists about what the answer might be. But
with unprecedented numbers of the earth’s flora and fauna threatened with
extinction, getting the answer right is not merely of theoretical interest.
C.
Rivet Hypothesis
1.
All species contribute to the integrity of the biosphere in a
small but significant way.
2.
You have to look at how many species or rivets you have to
take away before the structure of the ecosystem collapses.
D.
Redundant Species Hypothesis
3.
The biomass of primary producers, consumers, decomposers
are maintained.
4.
These are maintained in general all ecological process will
function perfectly well with very few species.
Q. Which of these two hypotheses would describe the ability of an ecosystem
to survive better?
E. There are certain factors that go into determining whether or not a species
is redundant.
1.
Major biogeochemical cycles that constitute the lifesupport systems of the planet.
2.
Key ecosystem processes of primary production, primary
consumption and secondary production.
3.
Food-web assembly
4.
Ecosystem stability and resilience
5.
Succession and the whole gamut of processes that structure
communities.
II.
Evidence From the Fossil Record
A.
Evidence from the fossil record suggests that most of the ecological
systems that have existed over the past 600 million years have been
conspicuously less species-rich than extant assemblages both in the sea
and on land and that these relatively impoverished ancient
assemblages suffered occasional mass extinctions.
B.
Fossil records state two conclusions
1.
The current rates of species extinction are several orders of
magnitude faster than anything seen in the fossil record, even
during the most severe mass extinctions.
2.
We often cannot tell from the fossil record, although deposits
of coal and limestone clearly point to substantial differences
in some ecosystem processes in time past; but again, it is
unclear how these differences relate to reductions in species
richness of the species composition of particular ecosystems.
Q. Do you think we can predict the future of ecosystem survival and species
richness of an ecosystem based on the fossil record?
III.
IV.
Patterns of Energy Flow, Biomass and the Structure of Food Webs
A.
Productivity in Biomass
1.
Information exists to suggest that major energy flow and an
apportionment of biomass maybe broadly insensitive to the
number of species involved.
2.
There are trends in species richness within these
relationships, with the low productivity systems tending to be
from relatively species-poor tundra and some deserts, and the
high productivity systems dominated by species-rich tropical
forests.
3.
Species richness depends on the productivity of the biomass
that contains the species.
B.
Food Web
1.
A food web is a map that describes which kinds of organisms
in a community eat which other kinds.
2.
Common Features of a food web
a.
Top species have no predators
b.
Basal species support the web and have no prey.
3.
It’s not to say however that certain rules govern all food webs
only that the rules are more complicated than current theory
and data suggest.
4.
Very recent work suggests that scale invariance may be an
artifact of poor data, particularly the failure of ecologists to
study and document the structure of very large food webs
with sufficient rigour and care. Evidence already exists, for
example, that linkage density is not constant; rather it
probably increases with increasing numbers of species.
Q. What happens to a food web if the primary producers of that web
become extinct? Does this web continue to thrive?
Theoretical Models of Ecosystem Stability and Resilience
A.
V.
A different number of species and different strengths of the links of
species to an environment determine an ecosystems stability and
resilience.
B.
Lower diverse systems will be greater negatively affected by loss of a
species.
C.
Biomass stability is not simply generated by having more species but
also the interaction of the species.
D.
More complex systems are affected less by the deletion of a species
E.
For a given level of connectance, species deletion stability decreases
as the number of species in the web increases.
F.
An example would be a large mammalian grazer changed the total
green plant biomass less in high diversity than in low diversity
grassland plots.
G.
Possible modeling approach would be to explore the affects of species
deletion on the rates of nutrient cycling and upon the stability and
resilience of the nutrient cycle itself.
Q. What are certain things that keep an ecosystem stable?
Observations and Experiments on Real Systems.
A.
Having high species richness doesn’t seem to affect the ecosystem; it
is the interaction of the species that affects it.
B.
It is possible that systems with many species fluctuating out of
synchrony will show a greater stability of ecosystem function than
systems with fewer, similarly fluctuating species.
C.
Keystone Species
1.
Certain species that are contained within an ecosystem are
more important to that ecosystem then others. These species
are called the keystone species. These species are in a way
the “drivers” of the ecosystem and the rest of the species that
are contained within are considered the “passengers” of that
system.
2.
There is no doubt that removal of a keystone species affects
the ecosystem.
3.
It is not clear by what mechanism keystone species might be
expected to exert there effects. In some cases it is trophic-by
eating other potentially dominant members of the
community. In other cases however it is clearly not.
4.
We have no idea what proportion of species in an ecosystem
is “key”. There can be assumptions made as to what species
could have a greater effect on that ecosystem. However it is
too hard to experiment with this because you would have to
take every species within the system and remove them one at
a time in order to find out what species had the greatest
effects.
5.
The message in the keystone species concept for an emerging
understanding of species redundancy in ecosystems is that all
species are decidedly not equal, and that and understanding
of redundancy will not be possible until we better understand
how to define, identify and measure the impact of key
species.
Q. What would be a general characteristic of a keystone species?
VI.
Manipulation Experiments
A.
General considerations
1.
Removal of a species often leads to changes in the
distribution, abundance and performance of at least one other
species.
2.
We often can’t tell how big of an affect takes place due to the
idea that our measurements are not accurate enough or easily
containable.
B.
Examples
1.
Perturbation experiments
a.
Removal of plants in a field biomass
b.
Some of these manipulative experiments also deal
with the removal of individual species or functional
groups have different effects on the community in
terms of species-richness and biomass.
c.
The magnitude of the effect is not always consistently
correlated with the abundance or even the dominance
of the species.
2.
Changing a keystone species and measuring its results on a
nutrient cycle.
3.
Any organism that has a major impact on nutrient cycling, as
well as the rate or direction of successional change, is a
keystone species, and if nothing has else can fill its role, the
process has no redundancy built into it.
4.
Removal or addition of natural enemies.
5.
Use of insecticides
Q. Regarding experiments of manipulation, does removal of one species
necessarily cause that ecosystem to fail?
VII. Conclusion
A.
Rivet Hypothesis
1.
Data tends to go against rather than support.
2.
It is difficult to tell from this restatement of the rivet
hypothesis whether Ehrlich and Wilson subscribe to it in the
extreme form which assumes that the removal of each and
every species matters.
B.
Considerable redundancies are built into ecosystem processes.
C.
Attention must be shifted to figuring out the minimal level of the
species that an ecosystem needs to survive.
D.
Also a set of problems centres on the difference between average
ecosystem processes under normal conditions and the maintenance of
ecosystem function under rare or unusual extreme events.
E.
Namely all species are not equal, that the loss of some species is more
important than that of others, and that species loss may be tolerated up
to some critical threshold.
Any Questions or comments?