Concluding
Remarks
In this relatively short but dense work, Hutchinson provides
a comprehensive, contemporary definition of the niche—effectively uniting Grinnell’s
definition of the niche as the distributional unit in which a species is
confined, with Elton’s definition of the niche, which focused on interactions
with the biotic community. Moreover, Hutchinson does so mathematically by defining
the limiting variables that permit a particular species to survive. By
considering all biotic and abiotic limiting variables (x1, x2,… xn), the n-dimensional hypervolume (N) or
fundamental niche is defined. Hutchinson also identifies some limitations to
this model: 1) it is assumed that all points falling within N are equally good
and all points falling outside of N are equally bad, 2) it assumes that
environmental variables can be linearly ordered, 3) the model represents only a
snapshot in time, and 4) only a few species can be considered an once.
Hutchinson examines interspecific competition as the
potential overlap of two fundamental niches (N1 and N2).
In some cases there will be no overlap (i.e., the fundamental niches of the two
species will be separate). While in other cases, some points in N1
will correspond to or overlap with points in N2. In this case, subsets
of points that are unique to one or the other as well as common to both (the
intersection subset) can be generated to examine ideas introduced by Volterra
and Gause, specifically that of niche specificity.
Hutchinson disregards the possibility of N1 being
equal to N2, as it is extremely unlikely that any two distinct
species will have exactly the same requirements, he also didn’t seem to think
that would be very interesting. Instead, he proposes two much more likely
cases. In the first case, all of the points within N2 exist within
the space of N1. In this scenario, competition will favor species 1
and eventually only species 1 survives. Alternatively, competition favors
species 2 where it occurs and both species survive. In the second case proposed
by Hutchinson, portions of N1 and N2 intersect while
other portions do not. In this case, some portion of the realized niche will
act as a refuge for species 1 and another portion will act as a refuge for
species 2 and both will survive. Further, Hutchinson explicitly states that
this approach does not validate the “Volterra-Gouse Principle” but provides a
starting point for experimental and observational researchers that seek test
this principle.
Finally, Hutchinson attempts to address the connection
between the realized niche and the rarity or commonness of species in a
community. In doing so, Hutchinson relied heavily on theoretical work done by MacArthur
and applied concepts of niche specificity to that work. Unfortunately
Hutchinson provided relatively little explanation in this section. Perhaps this
was a result of the limitations of MacArthur’s model, which was still probably
the best model available at the time.
The
Population Consequences of Life History Phenomena
At the time of publication, life-history characteristics had
received little attention from population ecologists. Cole summarized the
diversity of life-history phenomena that influence reproductive potential
(fecundity, longevity, age at first reproduction…) and the challenges
associated with quantifying their effect on population growth. Moreover, Cole
recognizes that each of the countless reproductive strategies must be effective
in their respective environments.
On introducing early attempts to explain population growth
mathematically, it became clear that some initial attempts were too simplistic
or too labor-intensive to be truly useful. Although calculations of population
growth may be influenced by noise and calculation problems in short timescales,
deeper timescales seem to be free of such issues. Ultimately, Cole concludes
that approximation based on a geometric model of growth yield results
equivalent to the more labor-intensive methods and are useful in examining
life-history evolution as it relates to population growth.
Cole’s calculations indicate that increasing the number of
reproductive events (i.e., the evolution of iteroparity) and especially the age
at which the first reproductive event occurs, have a strong bearing on
population size. However, a paradox identified in this paper suggests it would
be more efficient and, for that matter, more likely for a semelparous species
to increase it’s litter size than it would be to undergo the numerous
evolutionary changes to achieve an iteroparous lifecycle.
On several occasions, Cole reiterates the need to study the
evolution of such life-history traits as they relate to populations structure
and growth, and biology in general. This mathematical approach provided a
framework for many of the population studies that have followed.
I liked this paper; particularly Cole's comments on semelparity vs iteroparity. It's a very interesting trade off when you consider the amount of effort that must go into either. Although I don't necessarily agree that it would be more efficient for a semelparous species to switch to iteroparity. Especially if you consider organisms that do not live in high density populations, like, say, the Kakapo... putting all your eggs in one basket (so to speak) seems like it could be disastrous for such a species.
ReplyDeleteI liked this paper; particularly Cole's comments on semelparity vs iteroparity. It's a very interesting trade off when you consider the amount of effort that must go into either. Although I don't necessarily agree that it would be more efficient for a semelparous species to switch to iteroparity. Especially if you consider organisms that do not live in high density populations, like, say, the Kakapo... putting all your eggs in one basket (so to speak) seems like it could be disastrous for such a species.
ReplyDeleteOverall, I appreciated Hutchinson's paper commenting on the Symposium. Firstly, he uses anthropology and archaeology to compare human demographic studies to animal demographics. Also, from the beginning of his discussion on niche concepts, it is clear he is combining Grannellian and Eltonian concepts into a single model. In all, the paper is easy to follow.
ReplyDeleteI was, however, confused about his views on competition. Is it still held that competition is difficult to identify in nature?
I agree with you that I was initially confused about his views on competition. However, Hutchinson states competition is not necessarily the limiting factor that determines the range of a species, rather competition is intermittent and apparent during a dry years when range is limited (using the Plethodon example).
DeleteI also took the message that competition is difficult to identify in nature.
DeleteAnd likewise, I enjoyed the application of ideas in population biology to humans, especially the bit at the end about the baby boom, maternal culture, and the economics of family size. I've always thought that it'd be interesting to see more use of concepts in ecology that we use to study other organisms used to study humans. Is that part of what you archaeologists do? It seems like many social scientists ignore or downplay biophysical factors in trying to understand human societies.
I thought by far one of the coolest ideas presented in any of the papers that we have read so far was at the end of Hutchinson's paper when he suggested, technically he was referencing what someone else suggested, that women have multiple children because of a neurotic compulsion to make a better child than the last one.
DeleteFor Hutchinson: This was an interesting read for me. I really liked all the comparisons to human demography, to me this brings the hard science down to a level that normal people can absorb…perhaps lacking in most modern papers, and lots of these old ones too. The full definition of the fundamental niche as a n-dimensional hyperspace was really classic and sorely needed to hammer home that point for me. I also enjoyed his suggestions for future work to expand on his hypothesis, I’m interested in seeing what of this has been done.
ReplyDeleteFor Cole: This paper went through what I recall being all the topics of basic population biology. The rationale for improving population modeling was well done. The work-up from the simple non-overlapping generations to more complex models with life history traits made the math progressions easier to follow. I really enjoyed the discussion on the differences between iteroparity and semalparity. I liked the added discussion on what factors might select for an organism to have a particular reproductive habit.
ReplyDeleteAs Hutchinson discusses examples and implications of interspecific competitive exclusion, at first I wasn't sure whether he was taking into account the type of random dispersal described by Gleason in 1926. For the sake of his argument, he lists examples such as the geographically overlapping species Gammarus deubeni and Gammarus pulex. When viewed in terms of his definition of the fundamental niche, the relationship between these species appears to illustrate the competitive exclusion principle. However, Hutchinson seems concerned about observations of "empty niches." He does list chance dispersal as one of the possible exceptions to the Volterra-Gause principle, but he also seems to consider that the existence of empty niches might constitute evidence against this principle being a primary force that structures populations.
ReplyDeleteHutchinson: I really enjoyed reading the general ideas discussed in this paper. The fact that a whole closing speech of a conference was dedicated, mainly, to defining what a niche is and how it can be applied is pretty incredible. I agree with Eric when he talked about Hutchinson's definition of the fundamental niche and how that really reiterated the idea behind it. I also agree with the general consensus of how it's nice he brings in human reproduction into the mix. Particularly when he says, "the criterion is never purely economic" for humans to reproduce, meaning that we can decide when we feel "ready", even if we don't know if our economic status will allow that. Animals don't have this issue to contend with, but bringing humans into this talk about reproduction allows us to think about how advanced (or primal) we are.
ReplyDeleteCole: This paper helped me to relate my perception of population biology with what it was in Cole's time and has eventually turned into today. He spends a lot of time talking about how the "modern" conception of population growth regards the potential rate of increase as more or less fixed, when this is really the main issue at hand. The "potential" rate of increase depends on all of these life-history characteristics, including age of first reproduction, which is most often (then) not even recorded. I am curious to know how the field of population biology has changed today, and if it has indeed placed more emphasis on paying attention to the life-history features of a species or population.
I thought it was very interesting and philosophical to think about the "human demographer" that Hutchinson talks about in the beginning of this paper. The animal demographer is looking at populations from the outside, but the human demographer is part of this ever-changing population. It seems as humans we may have a blindness to our own species- making it hard to see the "whole picture." Similar to the views in the above statements, Archaeology and anthropology play huge roles in leaving the "micordemography'' allowing us to see past our small blip in time and view a larger picture.
ReplyDeleteCole’s approach of plotting population variables against individual life-history variables seems productive. I wonder if a Lindeman-style trophic dynamic analysis might cast some light on the biological trade-offs between the semelparous and iteroparous reproductive strategies.
ReplyDeleteI was struck by one of Cole’s conclusions, that “the birth rate, the death rate and the age composition of the population, as well as its ability to grow, are consequences of the life-history features of the individual organisms…hence it is to be expected that natural selection will be influential in shaping life-history patterns to correspond to efficient populations.” Is Cole implying that natural selection acts primarily at the level of the species, rather than at the level of the individual organism?
Hutchinson:
ReplyDeleteBeing partial to Plethodons, I was intrigued by the comparison of two sympatric Plethodons, P. dunni and P. vehiculum. My project deals with two geographically distinct Plethodons, yet the themes hold true and competition isn’t the only limiting factor. Plethodon physiology depends on moisture and temperature (for respiration and to prevent desiccation) are the limiting factors in suitable habitat, unless dry years reduce suitable habitat. In sympatric Plethodons, like P. dunni and P. vehiculum, interspecific competition is likely to occur in dry years when suitable habitat is reduced. On the other hand, geographically distinct Plethodons could show intraspecific competition also occurring during dry years since resources and range would be limited.
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ReplyDeleteSo in Hutchinson I really liked the description of the niche in terms of n-dimensional space and the use of set theory, and also found the demography discussion pretty interesting. I also really appreciated his point about the temporal aspects of coexistence, competition, and the niche - that fluctuating environmental conditions can change competitive interactions and create windows of opportunity for species that otherwise wouldn't survive. I did have a few questions, particularly about his references to MacArthur's broken stick model:
ReplyDelete1) Where do the Figs 2-4 come from? I really had a hard time following what he was saying about MacArthur's work beyond the basics of broken stick.
2) Given Hutchinson's description of the niche as an n-dimensional hypervolume, his thinking in Fig 1, and his knowledge of MacArthur's work, why didn't he extend the broken stick model to n-dimensions? Why did it take until 1980 for Sugihara to take this obvious step, which apparently provides a more biological grounded explanation for the canonical distribution? Maybe I'm misunderstandingthe broken stick...
3) Is Hutchinson's reference to homogeneous and heterogeneous diversity the same concept as Preston's contagion?
Paper 9: Hutchinson – Concluding Remarks
ReplyDeleteI really like the concept of n-dimensional niche in the way the Hutchinson describes it. I think the idea that various habitats can and do fulfill the niche requirements of species to varying degrees resulting in different abundance patterns across habitats does a good job, in my opinion, of capturing what really happens in nature. For example, a particular plant species can exist across are large range of conditions which superficially may seem like quite contrasting environments or habitats but yet that species is able to survive in all and it is often evident in their abundance, distribution, size, and structure that some places are more favorable than others. The n-dimensional hypervolume theory does a good job of explaining this phenomenon by allowing us to think of a species’ niche as a multidimensional space that is realized when conditions are right (whatever those conditions may be, both biotic and abiotic).
Paper 10: Cole – The population consequences of life history phenomena
I appreciated the approach in this paper to look at various life history traits and scenarios using specific examples and trying to describe them mathematically as a starting point for beginning to think about how these different life history traits influence population growth. I also thought the paper was well written and clear and easy to follow which made it pleasant and interesting to read. I especially like the practical implications of identifying and understanding the age distribution of a population and how that information may be used for the conservation of desirable species and conversely the control of undesirables.
Questions:
• In figure 5 I don’t quite understand why the curves flatten out showing that each successive litter contributes less to potential population growth so that “families which are very large by ordinary standards actually contribute little more to potential population growth than do families of quite moderate size.”
• Why is reproduction earlier in life more important from the population standpoint? It is mentioned on pg. 133 that from equation 11 “it is strikingly brought to one’s attention that the final terms representing reproduction later in life are relatively unimportant in influencing the value of r.” Why and how is this and does this actually hold in certain instances?
Cole’s preface was striking to me because my initial reaction to the Theoretical Advances chapter was exactly as he had described. I often felt skeptical and daunted by the mathematical equations, but I thought Cole’s approach was comprehensible and appealing to biologists and mathematicians alike. Unlike Lindeman, I thought this paper balanced life history and mathematical theory into a concept that was duly interesting and challenging.
ReplyDeleteI agree with Sami's observation about the mathematical equations was the most comprehensible out of all that we have read so far. I didn't feel nearly as "bogged down" by the math.
ReplyDeleteAlthough Cole looks at the benefit/disadvantages of reproductive rate to the species itself, a couple of questions I had about this research were: How does the reproductive rate of a species impact its role within the environment as a whole? How does the way species produce impact the whole of the environment.
Concluding Remarks by Hutchinson: This paper provides a great insight about the concept of niche . It actually help me to understand the concepts of fundamental and realize niche. The relationships between biotopes and niches is a great way to understand how biotic and abiotic variables constraint the niche for x specie understudy. Once analyzed the figures of the paper I got a better understanding of the concepts explained in the paper
ReplyDelete