Paper 38: Food Web Complexity and Species Diversity
Robert T. Paine
image source: http://www.washington.edu/news/2013/07/30/fifty-years-of-ecological-insights-earn-uw-biologist-international-award/
Background:
Paine (pictured above in Makkaw Bay) is a retired professor emeritus at the University of Washington. Paine
coined the term keystone species. Paine thought apex species increase species
diversity.
Hypothesis:
Local species diversity is directly related to the efficiency with which
predators prevent the monopolization of major environmental requisites by one
species.
Application:
Local diversity patterns of rocky intertidal marine organisms.
Conclusion:
Predatory species are in relatively greater proportion in diverse situations.
The Structure of Selected Food Webs
Subwebs are groups of
organisms capped by a terminal carnivore. Subwebs have top predators that appear
to be distinct, known later as an apex predator.
North temperate subweb
The Pacific Coast in
Washington community intertidal organisms found on rock substrate included:
mussels, barnacles, and one starfish. The carnivores in the system were Pisaster ochraceus (starfish) and Thais emarginata (muricid gastropod). The
food web is tied to the barnacle community and both predators feed on them. The
ratio of carnivore species to total species is 0.18.
Subtropical subweb
The Northern Gulf of
California’s community is more complex than the previous subweb. The structure includes
a starfish (apex carnivore), carnivorous gastropods, herbivorous gastropods,
bivalves and barnacles. Paine says this ubweb has increased trophic complexity.
The ratio of carnivore species to total species is 0.24.
Tropical subweb
The Mate de Limon off the
Pacific shore of Costa Rica did not have a secondary carnivore. The community
consisted of two muricid gastropods (Acanthina brevidentada and Thais biserialis)
and mytilid and barnacles. This is a simplistic system compared to the north
temperate and subtropical subwebs.
Predation and Diversity Gradients
The removal of the apex predator (Pisaster - starfish) decreased diversity in the community, and the
system became simplistic. Paine concludes that predation is a positive feedback
loop because predation prevents resource monopolies and thus allows for
diversification in a community. There was not a relationship between latitude
and diversity in this study, but increased stability could increase the
capacity for higher-level carnivores.
Questions:
1. This study assumes the probability
of bivalves being eaten is proportional to abundance. Do you agree with this
assumption? If not, why?
3. Paine states high calorie
foods have greater nutritional value. Do you agree that more calories means greater
nutrition?
4. Paine seems to ignore stochastic
events and anthropogenic influence on the intertidal marine ecosystems. Do you
think stochastic events or anthropogenic factors could have influenced his data,
especially his results in the tropical subweb?
This paper addresses an incredibly interesting question…why is diversity higher at the equator and generally decreases towards the poles. The hypothesis posed is that predators prevent monopolization of space by any one species and thus environments that support more predators will support more diversity. By looking in Washington State, Baja California, and Costa Rica, Paine set up a nice gradient. Unfortunately he finds that diversity is actually lowest in the tropics for his marine intertidal zone relative to the other two with Baja being the highest. He notes that Baja also has the most predators. The site in Washington state is also high in predators relative to the tropics. Paine emphasizes that his observations only include local diversity so we can’t generalize to the tropics at large, but we can conclude that predators are at least correlated with higher diversity. He also notes in his work how important starfish are in promoting diversity, and he emphasizes this with an exclusion experiment. Overall a nice read with an important and still controversial topic though I would say he did not do much to answer the question of why diversity is higher in the tropics, rather he answers a sub question about the driver(s) of local diversity.
ReplyDeleteAnother great paper! I love the topic of diversity…This paper explores how trophic structure contributes to diversity. Particularly, that predation controls the abundance of prey species preventing them from “monopolizing” resources and thus allowing other species to exploit common resources and as a result diversity increases. This is a very interesting idea to me because I had never thought of diversity from this perspective. Predator-prey relationships are mostly investigated in the context of population growth rather than diversity per se so the concept of predation increasing diversity was really enlightening. Since this paper is in the Experiment Manipulations section I am guessing Paine was the first to actually test this concept in the field by removing the top predator in a natural setting and measuring the response, consequently supporting his hypothesis. I have witnessed anecdotally how grazing can increase plant species diversity (up to a certain threshold of course) by preventing dominant species from creating a monoculture. He attempted to address the latitudinal diversity gradient but then things got fuzzy from there for me…
ReplyDeleteI really like this point about the idea of a predator preventing a prey species from monopolizing a resource. It kind of makes sense intuitively I think. One species is different from another and that probably makes them better suited to a specific habitat. To allow another species to exist, there has to be some control on that first species or else that first species will outcompete the other one just because they are intrinsically different. Their differences also allow them to be predated differently and therefore they can both exist and diversity can flourish. Once you can do that with one species in a certain area, you can think down to that smaller scale again and say that any species that is best suited for a certain environment will end up having to share that environment if something other than competition is in play. Since you have a multitude of these micro-habitats, predation allows multiple species to live in all of these and promotes diversity.
DeleteI really like this point about the idea of a predator preventing a prey species from monopolizing a resource. It kind of makes sense intuitively I think. One species is different from another and that probably makes them better suited to a specific habitat. To allow another species to exist, there has to be some control on that first species or else that first species will outcompete the other one just because they are intrinsically different. Their differences also allow them to be predated differently and therefore they can both exist and diversity can flourish. Once you can do that with one species in a certain area, you can think down to that smaller scale again and say that any species that is best suited for a certain environment will end up having to share that environment if something other than competition is in play. Since you have a multitude of these micro-habitats, predation allows multiple species to live in all of these and promotes diversity.
DeleteAn interesting read. While I've heard of this experiment and the conclusions that came from it, I've never had the chance to read it. I think Paine makes a great stride in his conclusions about diversity, disproving his original hypothesis that seems to be very intuitive. There is a part in the paper where he mentions that the "greatest diversity is associated with the most variable temperature region", which is a great observation. To me, this seems to make sense, as more variability in temperature gives more organisms the opportunity to have their ideal conditions for life. If we think about each region Paine tested in this way, it makes sense that the California location had the highest diversity. This can also carry out to the diversity of predators, as a larger range for prey and secondary predators gives fuel for a higher number and variety of predators. All in all, a very interesting paper and one that seems to be one of the first of its kind.
ReplyDeleteI found Paine's demonstration that removal of the top predator leads to more diversity convincing, and appreciate that Martina supplied another example of this with grazing.
ReplyDeleteI wonder how Paine chose his study sites? Was it partially latitude (49, 31, and 10 degrees north) and partially convenience of access to rocky intertidal zones? What would the results have looked like with more sites in each region? It is funny how the tropical site was the least diverse, although a premise throughout the paper is that tropical areas are the most diverse. It would be interesting to look at Paine's field notes, to address Sami's question about whether there were stochastic and anthropogenic factors affecting diversity at the tropical site. Also, the tropical subweb doesn't really fit Paine's definition of a subweb as "groups of organisms capped by a terminal carnivore." Could the top carnivore have already been removed from this system?
As he admits, Paine couldn't draw any general conclusions about latitude, although as Eric notes, this seems to be the main question of the paper. He writes that "greater stability of production should be correlated with greater diversity," but instead, as Ali points out, he finds that the least stable temperature (the Baja site) is associated with greater diversity. I didn't follow his reasoning at the bottom of page 857, where he seems to assert that greater and more stable primary production will lead to less interspecific competition.
Regarding Sami's first question: I think as long as space is the main limiting factor, it's reasonable to assume that the probability of bivalves being eaten is proportional to abundance. But as soon as some other factor becomes limiting, like topography or defense mechanisms or availability of other food, it might not be a fair assumption.
Definitely an interesting read, with some thoughtful musings on the causes of diversity. Paine's study was actually pretty short on data and didn't have a very strong experimental design, yet was a still a powerful illustration of the keystone predator concept and did offer some good insight. The food subweb complexity gradient and Pisaster removal experiment both show clearly that more diversity can results from the presence of a/more predator(s), but with a sample size of three and one respectively, it's hard to view these as more than anecdotal observations. As Julie and Sami suggest, the anthropogenic, abiotic, or biotic peculiarities of these systems could explain what's going on. Nonetheless, I did find the experiment convincing, as the most parsimonious explanation for the observed diversity differences is presence of predation.
ReplyDeletePaine goes on with some interesting ideas and explanations for the causes of diversity, invoking energy conversion efficiency and stability of production, along with predation and environmental heterogeneity as potential mechanisms. Like Julie, I had a hard time reconciling the production stability and temperature variability comments. I think it'd be worth further hashing out these ideas in class.
I thought the food subweb diagrams were a neat, information-dense graphical display of trophic structure for each of the communities examined. I'm not sure why bivalves being eaten in proportion to abundance is a necessary assumption here, but it does make some amount of sense for a generalist predator. At very low/high densities I wonder if predatation would be lower/higher than expected though due to predators being less/more familiar with a prey. I think the use of energy (calories) as a common currency useful in that it tends to normalize across species' size-abundance tradeoffs, though having actual population numbers could be useful too. Assuming higher calorie prey items are always better or more nutritious is overly simplistic I think, as we overfed Americans can surely attest too; the relative balance of nutrients and vitamins in food is also important.
This paper is last to be blessed with an Ecomotion Studio video: http://ecomotionstudios.com/paine.php
Another example I remember hearing about in high school/college. I remember when I had first heard of this study it seemed counter intuitive that the removal of a predator would result in a reduction of diversity. However the case is convincing. Like in last week’s discussion prey need an opportunity to hide from predators. In a situation like the controlled setting, the immobile bivalves have the opportunity to ‘hide’ among other species of bivalves so to say. In that respect the numbers being eaten might be proportionate to their abundance. However, when we think of the barnacle paper there does seem to be some preferential eating habits at least in some similar predator/prey relationships that may play a factor in this study. He did choose to lump all bivalves together to “dodge taxonomic problems”. That might help to simplify matters, but we know that different species occupy different niches and may have unique ways of dealing with the predators (think back to the two tenebrionids in a sack of flour with a parasite. Diversity did reduce as a result of predator removal do to interspecies competition for space. I wonder if the dominant species would be predictable, would the predators preference for prey (if there is any) prior to removal influence the dominant species after its removal? Would dominant prey after removal be greatly influenced on initial population numbers or does one species strongly out-compete others? I again think back to the tenebs in flour and the barnacle studies. This maybe a tangent but the study does open up more questions.
ReplyDeleteThis was a nice, easy to follow read with implications that seem very intuitive today. I also remember this paper from my early undergrad. I think there was even a primitive computer simulation modeled after the Makkaw Bay experiment, with different pixel colors and sizes representing the species concerned. It was not a very engaging computer game—you just set a number of predators between 0 and as many as would fit and let it run—they should have just had us read the paper.
ReplyDeleteI would suspect that the latitudinal difference in complexity of food webs also has a lot to do with habitat heterogeneity. By default, a length of coastline in the Pacific Northwest will have more dramatic tidal influences and consequently greater heterogeneity than an identical stretch of coastline on the Pacific side of Central America.
Matthew, brings up some interesting questions. It seems that some of the math we covered last week may help predict a victorious prey species after predator removal. I also noticed the taxonomic lumping of bivalves and thought it a bit odd to ignore the diversity there, but then I wouldn’t want to key out a bunch of clams either.
I found the figures really helpful in understanding complexity. The differences in complexity between figures 1, 2 and 3 are astounding. Only seeing 0.18 or 0.24, I thought that there would not be a large difference until I looked at the figures- another reason why images are so beneficial! The part of the paper I found most interesting is how large of an impact ambient temperature can have on these ecosystems. This paper, rewritten with added sites and more supportive data, could have serious conservation implications especially when referring to global climate change. As oceans warm, what can we expect to happen to apex predators and overall diversity?
ReplyDeleteExcellent read. It make me think about some examples in nature. Sea cucumber are keystone species in marine environment. They are feeding upon sediments, decay matter, etc. Due to overfishing they have become extinct in some places. T his caused the invasion of sea urchin that became very abundant in most of this localities
ReplyDeleteI like this paper in that, as Martina mentioned, it looks at predation's impact on diversity rather than just on population growth. All in all, Paine's idea of a diversity decrease in the absence of predators makes sense. Right away, when you take away the predator, the diversity has gone down by one species. Along with this, the predator, as a population regulator, decreases competition among the prey. Perhaps the prey are divided in competing for resources while at the same time attempting to evade predators.
ReplyDeleteAs Matthew pointed out, this idea of the predator impacting diversity is evident in Connell's barnacle paper. In the absence of the predator, the barnacles are separated in their different zones as the larger barnacle outcompetes the smaller for space. Once the predator snail is introduced, the smaller barnacle has a greater opportunity to occupy the same spatial area as the larger.