A selection of the seminar questions from Seminal Readings in Ecology (BIOL 953 2012)
Competitive exclusion principle and species coexistence
1. When he first defines the competitive exclusion principle, Hardin includes in his criteria that the two coexisting
species must occupy the same niche – in “Elton’s sense”. The Hutchinson niche was elaborated three years prior
to Hardin’s publication and was in fact developed as a means to explain patterns in species diversity that cannot
be explained by the competitive exclusion principle; it seems far from coincidental that Hardin chose to omit this
reformulation of the niche concept from his paper! Do you think the Hutchinson concept of niche resolves the
troubles with the competitive exclusion principle?
2. If we look hard enough at two species that appear to inhabit the same niche in space and time, then surely we
are going to be able to find ecological differences, and in the end conclude that the species are in fact using
different niches. This is similar to Hardin’s general rule: “Ecological differentiation is the necessary condition for
coexistence”. Even though the competitive exclusion principle seems to be mathematically true, one could
argue that the principle is fundamental to theories of inter-specific competition and niche partitioning.
However, environments are generally heterogeneous and unpredictable in space and time. This leads me to the
question: Could there be mechanisms other than ecological niche differentiation that would allow two or more
species to coexist?
Sustainability
3. Hardin presents the tragedy as resulting from the “remorseless working of things”. The free market economist
would argue that supply and demand of the commons will equilibrate for a population at a given time and is
therefore the best regulator in a “complex, crowded, changeable world”. Barring appealing to conscience and
education which are not homogeneous elements in a population, is money the only social arrangement for
protecting the ecological commons that companies and people universally care about?
4. Hardin argues that there is no “technical solution” to the Tragedy of the Commons. – Is this ‘black and white’
thinking practical? In this regard, human values must change in Hardin’s opinion – I agree. But in practical
terms, how does this limit developments in ecosystem management? What balance should be struck between
changing values and progression science and technology to solve ecosystem management issues that need
urgent care?
5. Hardin’s description of the “tragedy of the commons” is based on the implicit assumption that humans behave
as independent, rational, free – enterprisers. What are the consequences and constraints of this assumption for
using the ‘tragedy’ as a framework for ecological management?
Metacommunity theory
6. Leibold et al. suggest that even in typically low dispersal environments, metacommunity theory can still play an
important role in understanding community assembly because the history of colonisation events may be
important. With respect to invasive species in terrestrial or aquatic environments, how can managers who want
to preserve community structure apply the metacommunity concept to understand the susceptibility of their
local systems to invasion?
7. Is it possible for Leibold’s different paradigms of metacommunity theory to be true for the same
metacommunity if the size of that community is changed? For example, if the metacommunity has 3 local
communities the mass effect paradigm may be the best explanation for patterns of community assembly, but if
6 local communities are studied the neutral paradigm may be best.
8. Under the mass-effect paradigm, immigration can supplement birth rates of a species “beyond what might be
expected in a closed community”. If dispersal is unidirectional (it does not necessarily need to be) and incoming,
then would an equilibrium (the seemingly implied prior state) be disrupted? Would the result of this
immigration be emigration of new-arrived or of established individuals, or would there simply be an increase in
mortality commensurate with decreased food availability per individual or increased predation risk?
9. The patch dynamic and neutral paradigms for metacommunities assume that “local sites do not differ in any
respect except for the species composition that exists at any given moment”. This seems to be a major flaw in
these models. After all, do not different species influence the local sites? For example, soil characteristics are
modified by the species present (e.g. cedar tree litter acidifies the surrounding soil, while deciduous species may
enhance soil fertility).
Origins of cooperation within and among species
10. How does evolution of the guilt trait (i.e. experiencing bad conscience with regards to one’s moral values)
correspond to the evolution of cooperation in humans? Would it be considered a positive or a negative
influencing cooperation (in which one wants to benefit from the outcome)?
11. Is the evolution of cooperation in humans much different from the examples given regarding interspecies
cooperation (e.g. plants and mycorrhizae, wasps and fig trees, ants and acacia trees)? Does our particularly well
developed ability to communicate with each other interfere with our capacity for developing cooperation (intra
and inter-species)?
The problem of Scale
12. “Many ecologists…. focus on their small questions amenable to experimental tests and remain oblivious to the
larger scale processes which may largely account for the patterns they study”. Levin is correct in my view to
remind us that different processes are important at different scales. But it seems the quoted perspective missed
out on a fundamental point. By turning the sentence around, ecologists are asking small scale questions
because experimental evidence can be used to address them. It follows that if theory and methodology were
developed, then ecologists would ask larger scale questions. In other words, “if you build it, they will come”?
Has it been built?
13. An issue with the “transfer of information between different scales” is that the mechanisms working at a study
site may not be the same mechanisms operation within the region as a whole. The difference may be due to
‘emergent properties’ as discussed by Levin. Is it possible to create a model that takes emergent properties into
account so that it is possible to transfer information from small-scale experiment to large-scale models? For
example, if a site displays properties A and B, can we know based on study of another system that property C
will also be present at that site?
Disturbance as a factor influencing community structure
14. In the literature it seems that as the idea of disturbance has been tested over the past 30 years, many especially
recent studies have either rejected or at least questioned the role of intermediate levels of disturbance in
maintaining diversity. If I am correct in this interpretation, why is it still such a broadly taught theory in
undergraduate ecology courses? Do you think it is more or less important to the structuring of communities
than other mechanisms such as niche partitioning, competitive exclusion etcetera?
15. Connell’s “gradual change” hypothesis to explain temporal patterns of community change in response to
disturbance may be useful in considering the effects of climate change on species diversity. Based on the rate of
climate change, could the concept be useful in predicting climate change effects on species with differing
generation times?
16. Connell explicitly states that his hypotheses apply to plants and other sessile organisms that occupy most of the
available space and largely determine the structure of the ecosystem. How might the intermediate disturbance
hypothesis apply to animals with larger ranges and shorter life cycles? Does the nature of ‘equilibrium’ change
for animal populations, and if so, how?
Top predator effects on species diversity in communities
17. How does having a ‘top predator’ within an ecosystem affect its susceptibility to species invasion? Does having a
top predator create a complex and stable food web that makes it difficult for a new species to become
established or does the prevention of monopolisation of resources by top predators generate unoccupied niches
that an invading species could take advantage of?
18. Paine’s pattern of increased stability of annual production leading to an increased capacity of ecosystems to
support higher-level carnivores seems to suggest that more stable habitats allow for more predators, which in
turn promotes diversity by preventing monopolisation. Are these more stable habitats closer to equilibrium,
and if so shouldn’t competitive exclusion be operating? In other words, is Paine arguing against the competitive
exclusion principle by concluding that the closer a system is to equilibrium, the more diversity it can sustain?
19. According to Paine’s 1966 paper, predation increases biodiversity by preventing competitive exclusion between
prey species. However, predation in some instances can reduce biodiversity. Can this be reconciled with Paine’s
hypothesis, or is some new or amended hypothesis necessary? Furthermore, the concept of disturbance and
Connell’s intermediate disturbance hypothesis suggest that it would be possible to find high species diversity in
regions without predators so long as other disturbances occur. If so, should Paine’s hypothesis be discarded in
favour of a more generalised theory, such as the intermediate disturbance hypothesis (with predation
considered as a (‘disturbance’)?
20. In the marine rocky tidal habitat where Paine’s studies were conducted, the limiting factor for organisms is
space. In many other systems however, there are several limiting factors. Does the inclusion of multiple limiting
factors constrain the application of Paine’s hypothesis to other ecosystems?
21. How does diversity among predator species affect the species diversity of lower trophic levels? Is the predator
diversity a force driving niche specialisation at lower trophic levels, or does predator diversity itself arise from
the extent of species diversity at lower trophic levels?
Interpreting the evolutionary origins of organism traits
22. “The design is so elaborate, harmonious, and purposeful that we are tempted to view it as the starting point of
any analysis, as the cause in some sense of the surrounding architecture. But this would invert the proper path
of analysis. The system begins with an architectural constraint: the necessary four spandrels and their tapering
triangular form.”
While there are undoubtedly architectural constraints in creating a design, couldn’t the initial ‘vision’ for the
building have selected for which architectural design (and therefore which constraints) to use? Perhaps then,
this relationship is a cyclical one?
23. Gould and Lewontin describe bivalves that have shell patterns that are merely a by-product of another
evolutionary selection process, and therefore are not a consequence of direct natural selection on that
particular trait. As ecologists, we might consider morphology to be important in competition (e.g. maybe a
certain shell pattern helps one bivalve species to outcompete others in some way) – but this might lead us to the
‘adaptionist programme’ when we try to explain why this trait came about. What steps can we as ecologists
take to avoid this potential error, and what steps should we take to better understand a specie’s traits
(especially life history features that are so important to understanding ecology)?
24. Phenotypic plasticity is rarely seen in a capacity that allows individuals to alter phenotype to perfectly match the
environment, although one would think that this trait would be strongly selected for in heterogeneous
environments in particular. Instead, intermediate levels of phenotypic plasticity are often observed, where
phenotype is altered to match a particular environment but is insufficient to allow competitive exclusion of
individuals of the same species that are native to that environment. There is little evidence for a ‘selection cost’
of plasticity, and so I think Gould and Lewontin may argue that the extent of plasticity may be restricted by
‘bauplan’ legacy constraints. What kinds of constraints are these likely to be?