So if Ehrlich and Raven were focused on the competitive
nature of plant versus herbivore with insect larva as an example, John L.
Harper is considering plant to plant interactions alone. He does admit to
completely avoiding the topic, but the paper is quite full without the
consideration of herbivores on plant communities.
This paper is synthesizing the application of population and
community structure to the study of plant ecology. He is bringing in plenty of
previous equations and graphs to look at plant community structure. Until now,
plant communities have been described qualitatively without empirical data.
Harper is concerned with measuring the science of plant ecology and collects
data from many different studies to put this paper together.
As he mentions at the beginning, many plant ecologists had
not taken the same approach to population ecology as the animal ecologists. Plasticity
and vegetative reproduction complicate the study of discrete plant populations
when compared to animal populations. Even at a similar age, two plants can have
different sizes and weights depending on the quality of the environment it grew
within. However, Harper still describes plant population growth with the
logistic model:
Harper mentions that r, intrinsic rate of increase, now has
two forms of increase for higher plants, seed output and vegetative
reproduction. Seed output has a broader geographic range and vegetative reproduction
is an immediate and proximate increase in population size. K represents the
literal density of the plant coverage in relation to how much sun the plant can
obtain. There is only so much surface area that is usable for photosynthesizing
and producing dry weight. In fact, using the dry weight of plant parts, Harper
shows the relative energy distribution between plant organs within a couple of species
over the growing season. The mass reflects the amount of energy devoted to
growing that particular organ.
Additionally, the more the plant grows the more it crowds
out others, which can be derived from the equation, 1/w=a+bx. Mean plant weight is
represented with w and density is x.
This is the reciprocal yield law; the mean plant weight, w, and density, x, share an inverse relationship. The smaller the mean plant weight
allows for a higher the density and vice versa. There is only so much room for
plants to grow and only so many nutrients and moisture in the soil. Plants are
controlled by density-dependent limitation with each other. This point is
further made with effects on density due to frequency distribution in Figure 8.
The higher the density of seeds sown the higher the frequency of lower weight
plants.
Darwin emphasized intraspecific
competition, or interspecific competition between closely related species, as a
means of evolution. The competitive edge that one particular individual has
over other individuals of its own species for limited resources provides it
improved fitness. The mechanical abilities for a plant to gain leaf coverage
for the sun, tap into soil for nutrients and water allows it to outgrow other
individuals. Secondary metabolite production gives the plants a chemical
advantage via herbivory reduction and direct attacks on other plants. The
winners of the competition provide the opportunity to reproduce.
Harper presents a modification to Hutchinson’s
logistic growth for two species that live together. When the density of
different species differ then there is a shift from inter- and intraspecific
competition. The lower density species faces interspecific competition with the
higher density species more than intraspecific competition within its own
species, however the higher density species faces intraspecific competition
with little interspecific competition with the less abundant species. This
feels intuitive. However, it is not to say that intraspecific
competition does not exist for the less abundant species, but that the weight
of intraspecific competition probably does not compare with interspecific
competition.
Yet despite the density-dependent factors and competition of
plant community structure, higher diversity within a plant system is more
productive than a single species system. The one animal example Harper uses
with Drosophila helps illustrates
this point; the dumpy and wild type flies do not interfere with each other’s
population size. Ecological combining ability increases niche specialization and
diversity. The increase of diversity may also mean the increase of stability for
the mixture of populations. As Harper mentions, better exploitation of the
resources of the environment helps strengthen the community.
A Darwinian Approach to Plant Ecology: J.L Harper used for the very first time the conceptual framework of Darwinian Theory applied to plant ecology. I really enjoyed the approach made by a plant ecologist connecting concepts from animal population biology to plants. The paper mentioned different examples of population dynamics, turnover rate, self regulation, seed and plant size survival rates in order to explain the different populations dynamics that occur in nature order to explain the concept of struggle. At the early stage of this complex dynamic among plants, some seedlings will survive an other ones died, once they grow and are mature, there will be oscillations periods of competition, where at some point they will reach a stabilizing situation, therefore this will explain how plant are coexisting together in nature.
ReplyDeleteI guess my main question is: why was this the first time a Darwinian approach was applied to plant ecology?
DeleteI know harper talks about, density, plasticity and reproduction, but why was this idea novel so late in the game? Was it because plants are not animals and so therefore they cannot follow the same rules? I am curious as to why Darwin's concepts were not applied and tested on every living thing.
I think it has to do with how people specialize. It took a while for ecology to just set some groundwork to actually work with before they could start playing around with ideas and arguments. Now that there is a really good foundation for ecology and evolution, and several other fields in science, studies are becoming more interdisciplinary.
DeleteHarpers address does a nice job tying together many of the drivers of plant populations, many of which also transfer to animal populations. His talk put great emphasis on turnover and the tradeoffs that plants have between producing many individuals with short lifetimes versus producing fewer individuals with longer lifetimes. In addition to these life history tradeoffs, he puts great emphasis on the importance of competition both between individuals and species in driving differentiation of behavior and morphology. His case of a minority species facing greater pressure from the majority species while the majority species faces greater pressure from itself illustrates how competition and tradeoffs are not static but rather change with changes in environment and competitors. He also talks about how in greatly disturbed areas many competitive constraints are often removed hence allowing rapid and missive proliferation of individuals in an environment following disturbance. In whole I think this speech did a great job of summarizing most of the drivers of population ecology.
ReplyDeleteThis paper was awesome! There is a lot going on, many interesting ideas and topics that I think really inspire a deep thinking into plant ecology and evolution. So taking it from the top…Harper first discusses the challenges with studying plant demography as a result of the ability of plants to reproduce vegetatively and their plasticity and thus “a statement about the number of plants implies very little about the real nature of the population.” I agree with this and thought it interesting the approach Harper takes to measure the growth of individuals to determine population turnover and really see the continuous flux that goes on in a stable plant community. Harper goes on to state “the intrinsic rate of increase of higher plants is a function of seed output and of vegetative reproduction” and while this is true I don’t agree that reproductive strategy is necessarily a limiting factor on reproductive success and therefore I don’t agree that reproductive output would be density dependent. I think the evolution of a given reproductive strategy/output is driven by the abiotic factors influencing a species and inter or intraspecific density stress would affect germination rate and survivorship but not output itself. So the question is about coevolution and the influence of density on driving reproductive strategy. Which is the primary driver – density and biotic interactions or abiotic factors? And what would be the ratio of influence of each?
ReplyDeleteBut moving on, it was very interesting to me to learn that self-stabilizing properties exist in plant communities where oscillations in abundance between two species can exist as a result of both inter and intraspecific competition and this relationship is similar to the predator-prey relationships that Volterra describes in Paper13. These “mutual strategies” help maintain a balance that allows for an increase in species diversity by opening up niche space rather than driving a species to extinction. This then feeds into the idea of an integrated community that has co-evolved essentially and Harper points out that these relationships and observations strengthen the concept of community over Gleasonian view. In my opinion they are not exclusive of each other – a community is dynamic and species come and go especially following disturbance or some kind of perturbation which follows Gleasonian view because communities don’t necessarily respond to change as a unified community (or large organism) in the sense that the community tries to stay intact by keeping equal composition. Communities are integrated and evolve under stable conditions but in changing conditions species will fit where they can to survive and the community as a whole can be rearranged and a new community or slightly different community arises.
I certainly appreciated the fact that Harper recognizes his lack of discussion on plant-animal interactions but perhaps in some respects this may fall under the umbrella of interspecific competition especially since there are some parallels between competition and predation.
Great thoughts Martina. I agree that Harper doesn't pay a lot of heed to the role of abiotic factors in community and population dynamics (nor have many of the papers we've read so far, Cowles and Lindemann excepted). In this case though it makes some sense to focus on plant-plant reproductive strategies and interactions (both inter- and intra-specific), as these had apparently been neglected, but are critical to understanding evolution. The interplay between evolutionary considerations (individual fecundity) and ecological considerations (competition for resources, population dynamics) was very cool.
DeleteI also liked the parts about self-stabilizing mechanisms. Fig 10 and the equations below seem important, but it's be helpful to go over in class, as I'm not certain I fully grasp everything.
Also with you on Gleason - I can see what Harper is saying, but certainly individual responses exist andare important too. Was Harper really a latter-day Clementsian?
This was a great paper. While Harper does spend his time discussing plant ecology and the evolution of plant communities, I echo what has been said above that his ideas seems to be somewhat applicable to animal communities also. The ideas about density-dependent intra and interspecific competition really show the depth that we are starting to see in these papers. While it does seem intuitive, some of the larger concepts within Ecology are very intuitive and that's what makes it exciting - it gives us the opportunity to makes sense of our world. Going back to his ideas about density-dependent competition, I wonder how these rules would apply to a place like the Amazonian rainforest. There is so much pure biomass there, and I am curious about the proportions of large species versus small species and who they are competing against.
ReplyDeleteI also really enjoyed the way Harper ended his talk. He admits that he has mainly shown what he finds exciting in the plant ecology field, and that for any one scientist to introduce a new concept is a lofty undertaking. He then goes on to say that the ideas Darwin first introduced supply enough questions for plant ecologists to study for the next 100 years, which seems eloquent and humble but also very realistic.
Haper’s “neodarwinian synthesis” approach to plant ecology resonated with me. The term plasticity is used to describe a phenotypic change. Plasticity is a mechanism for coping with variable biotic and/or abiotic factors. Plasticity is a short term response and is independent of heritable genetic change. Harper’s examples include change in size or reproductive capacity. I found Harper’s economic take on the logistic growth equation to be really insightful! An organism’s “investment” in plasticity directly influences the intrinsic rate of of natural increase (“r”). Large “r” values coincide with high risk investments, while low r values are considered low risk investments. What other examples of plasticity influence “r”? Also, how do you identify and/or quantitatively measure plasticity in an organism? I would default to genetics, so I’m curious if anyone has another take on it!
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteI really found this paper to be cool. I like how Harper keeps going back to observations that Darwin made, and makes a convincing point about how little things have developed in evolutionary plant ecology. That said, he seems deride important work by plant community ecologists. A few thoughts:
ReplyDelete- The ratio diagrams and parts about self-stabilization were neat, but I kept wonder what the mechanism was that would cause such coexistance. What species characteristics would allow this, as opposed to competition exclusion?
- I really appreciated his bringing in examples from agricultural studies (probably helps that he was in an ag-botany department). Most of the papers in this book are pure ecology, and I'm not sure there's sufficient recognition of the contribution and motivation provided by by applied fields like agronomy to ecological understanding. Certainly in forest ecology there's a large inheritance owed to forestry research that goes back before ecology really existed.
- The bit about yield, self-thinning, and changing biomass distribution among individuals with increasing density (Fig 8) was neat, and made me think of inequality in human populations. I believe that in general poorer countries (higher 'density' or people/resources) tend to have higher inequality, with wealth concentrated among fewer individuals, whereas higher-income countries tend to have a more significant middle class and therefore less inequality. Perhaps there are biological forces at play in human wealth distribution?
This is the first paper we've read that has mentioned the Mendelian concepts of dominance and recessiveness. Harper is synthesizing so much. I agree with Martina that the Gleasonian and Harperian views are not necessarily mutually exclusive. I keep puzzling about whether the intraspecific/interspecific tradeoffs are fundamentally different when considering plant or animal communities. Plants seem so much less versatile in their abilities to adapt to varying food sources; when compared with animals in general, they are relatively homogeneous in their needs.
ReplyDeleteWhat I found most interesting in Harper's paper was the discussion of inter- and intragroup struggles. Particularly crucial to this was the idea that struggle between species does not necessarily terminate with the eradication of the weaker species. Rather, Pimentel and Seaton & Antonovics provide three different evolutionary solutions to this problem: (1) extinction of one group; (2) mutual oscillating inter-group selection leading to increased stability of the mixture; and (3) mutual divergence in behavior leading to the avoidance of inter-group struggle.
ReplyDeleteWhatever the solution, these struggles are important in understanding the mutual strategies employed by different groups to allow increasing diversity to allow the community to operate as an integrated whole.
One of the most alluring aspects of this paper, however, is Haper's excitement of Darwinian ecology as the most important origin of plant ecology.