Introduction
Two types of fluctuations
reduced densities and amplitude of fluctuations, compared to when predators
were absent.
A.)
Exclusion of
predators caused a pattern of fluctuations of decreasing amplitude
Example:
Reciprocal density-dependent interaction of the phytophagous mite and host
plant.
B.)
Contrasting type
of pattern of fluctuation
Example: Predation on the phytophagous mite.
Huffaker questioned if Gause
theory sufficiently described predator prey relationships. He supported his
idea with Nicholson’s criticism of Gause’s experimental design as being too
small to approximate qualitative or quantitative results. Huffaker was also
influenced by DeBach and Smith’s experiment on the searching capacity of
predatory parasites using Nicholson’s formulas. Huffaker took a quantitative
approach to a laboratory experiment of continual (not self-exterminating)
predator-prey relationships.
Experimental Design and Procedure
In this experiment, the
six-spotted mite, Eotetranychus
sexmaculatus, was the prey species and Typhlodromus
occidentalis was the predator species. Oranges were kept in the dark, at 83
degrees F, and in greater than 55 percent humidity. Food quality and feeding
area were altered to various degrees by wrapping the orange in paper and/or
paraffin. Six-spotted mites were cultivated on lint covered oranges. A
continuous system was developed by removing and replacing oranges. At 11 day
intervals, ¼ of the oldest or unsuitable oranges were removed and replaced. The
experiments were initially in duplicates, but as experiments failed, new improved
experiments were created and substituted in.
A “universe” was created
using oranges and similarly sized rubber balls in a 40 inches long by 16 inches
wide trays. The tray had a 1 inch side wall covered in petroleum jelly to
prevent mite movement in or out of tray, and 40 Syracuse watch glasses on each
orange or rubber ball. Increasing the
area with rubber balls complicated the search for food by prey and predator.
Predators and prey unable to leave or enter the universe, but both predator and
prey were allowed to move freely in the universe.
To make counting easier,
diameter lines were drawn on the surface of the exposed surface and divided
into 16+ numbered sampling sections. A portion of the mites were counted then
multiplied to estimate the total populations, and the total populations were
counted in small samples. Statistical analysis showed estimated samples have a
loss in confidence. Subsamples of an orange were better estimated by two or
more non-contiguous areas evenly distributed with a proportion of ½ or ¼ the
total exposed area on each orange. Small changes in population might be
undetectable due to sampling procedure, but the sampling is adequate and
accurate for major trends or patterns of population change.
Results
The present experiment
showed oscillation between prey and predator under laboratory conditions. If we
take into account the absence of predator, the prey population will persist
through time, but once the author added another predatory mite both of them, predator
and prey will become extinct. Complex habitats were created, which reduce
predator’s dispersal and therefore predation upon preys. These microhabitats
which were created experimentally increased heterogeneity and produced
stabilizing effects in the oscillations previously reported.
Discussion
The section introduction
really saves us from Huffaker’s writing in this paper. The take home message
that I think, with the help Real’s intro, Huffaker is trying to portray is that
the patchiness of an environment directly affects the survivability of prey due
to increased search time by the predator, and more possible refugia both in
space and time for prey. This paper seems to scream that it is the experimental
aspect of McArthur and Pianka’s paper about optimal use of patchy environments,
however that paper was published 8 years later so that is likely not an
accurate statement. The concept of predator-prey oscillations I think is
something familiar to us all, with a classic example, again from our high
school and undergrad textbooks, being the oscillations of snowshoe hare and
lynx. It would be interesting to look at how the patchiness of the environment
might provide local refuge from predation for the hare across their range, and
other systems outside of a laboratory setting.
While this paper was a bit lengthy and the experiments had a bit of redundancy, it was a good paper for examining the effects of a patchy environment on predator-prey relationships. It showed that the prey was safe for a time until the predators were able to locate the new patch. Although the conclusions of the oscillations of predators decimating the prey population and thus themselves might now be considered common sense, it is interesting to see it play out in this laboratory experiment. I wonder, though, if this experiment could be replicated for other species? What about in the presence of other players within the universe?
ReplyDeleteAlso, I do wish Huffaker would have been more explicit about the changes he made to the experiments when they failed.
This paper was a great read. This really hits home the predator prey relationship. The paper has a great methods write-up that really details how the experiment was setup. I felt like this experiment could easily be repeated from the methods, though as Melyssa says the method changes are glossed over. The controls also seemed well done and were well matched with the treatments to compare back to. The simplicity of the graphs was also a welcome respite from the many complicated graphs presented in other papers we have read. I thought the most important message of this paper was that the greater complexity of the system the more likely that the predator-prey relation can persist. He then gives the example of what would happen in a system where two mountain lions and two mule deer were in a small area. It’s easy to see how this has obvious management implications where preserving large swaths of diverse habitat will help maintain predator-prey relationships among other benefits.
ReplyDeleteI think the experiments were well done. I like how it was seen that complex habit was needed in order for prey and predator to persist together. Learning predator-prey oscillations is one of my first memories of middle school science ecology along with peppered moths and food webs. One problem I had always thought of with predator prey experiments in closed systems was that it would be difficult for populations to persist. We were taught as predators increase prey decreases causing predators to die back and prey to increase in numbers…… but in closed system experiments it just seemed obvious that an increase in predators will cause a decrease in prey as expected. However, the large numbers of predators will likely be able to seek out and eat the remaining prey just by strictly being in such great number, causing both to ultimately achieve the most stable population numbers possible, zero. That is why diversity is important in both a biotic and abiotic sense, some prey need to persist even when predators are I great numbers by being able to hide, either physically among another prey item if you will.
ReplyDeleteI was intrigued by the innovative methods Huffaker used to increase the complexity of his system--from wooden posts and electric fans to vaseline barriers. It seemed important to make the environment patchy, but not too patchy, to dampen "the annihiliative force of predation." Huffaker seemed very pleased by his ability to create an environment complex enough to sustain three oscillations, and confident that given more complexity, more oscillations could be sustained.
ReplyDeleteFor me, this series of experiments expanded not only Gause's work, but also Holling's. Holling measured the numerical response of the predator to increased prey density, but Huffaker was able to show that patchy prey density protected both predator and prey from extinction.
I was especially interested in the implications for agricultural monocropping: that the large decrease in spacial heterogeneity represented by monocropping could lead to wild, annihilative swings in predator/prey populations, leaving niches open to invasive species, which would also be subject to severe swings in population.
This paper makes me think of some implications of what happened in the experiments. We saw that the predators eventually drove the prey to extinction, but as the environment got bigger, we saw more oscillation and the prey persisting for a longer time. Thinking about this from a historical standpoint, this implies to me that for a prey to persist, it needs to have a large area that it is already living in and it must be large enough before a predator comes in to the picture or else it might suffer the same fate as the phytophagous mites and go extinct. This could mean that predators would have to necessarily come about after a prey in some way, be it evolving to eat that prey, or arriving at an area with that prey after the prey has already established itself. The dynamics of this system are very interesting and the added levels of complexity in the model were very cool.
ReplyDeleteThis paper makes me think of some implications of what happened in the experiments. We saw that the predators eventually drove the prey to extinction, but as the environment got bigger, we saw more oscillation and the prey persisting for a longer time. Thinking about this from a historical standpoint, this implies to me that for a prey to persist, it needs to have a large area that it is already living in and it must be large enough before a predator comes in to the picture or else it might suffer the same fate as the phytophagous mites and go extinct. This could mean that predators would have to necessarily come about after a prey in some way, be it evolving to eat that prey, or arriving at an area with that prey after the prey has already established itself. The dynamics of this system are very interesting and the added levels of complexity in the model were very cool.
ReplyDeleteI think this is really cool that Huffaker was able to show that with increased barriers, space and "dispersal corridors" the populations of the mites were able to sustain for longer. I agree with Eric in saying that this paper can tie into many conservation and land management issues. Keeping large open areas of space with many trees, streams and other forms of shelter/dispersal is necessary for many organisms to persist. When you think about this idea, it seems like common sense, but Huffaker was among the first to show this in a lab experiment. Also along with Huffakers questions, How does disease play into this system? Does this act as a barrier for the prey, or is it more of a destabilizer for both predator and prey? I think adding disease or a parasite to a similar system would be really interesting to this study.
ReplyDeleteThe parallels with McArthur and Pianka pointed out by Schuyler were evident while reading this and it seems they are good compliments of one another. I also have to agree with some of the other comments regarding the implications for conservation. It seems this basic idea of maintaining habitat heterogeneity, at least outside of preserves, came quite a bit later than this paper and early papers like it and we are now trying to recreate buffers and corridors. Julie mentioned the parallels with agricultural monocultures and invasive species. in my experience looking for invasive herps in Florida, many of them first become established in ag fields or areas with similarly reduced heterogeneity and seem to invade other areas from there. This is probably at least partly due to proximity to places where people release unwanted pets but it seems very likely that extirpation of a native competitor allowed the invader to gain a foothold.
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ReplyDeleteWhile long-winded, I really appreciated Huffaker's paper for two reasons:
ReplyDelete1) By introducing heterogeneity into his experimental system, Huffaker incorporated additional realism found in nature into the microcosm experiments used to study predetor-prey relations. Migration dynamics thus becomes inherent in the microcosm, rather than being seen as an arbitrary variable to be controlled or manipulated as in early experiments (e.g. Gause).
2) The fact that Huffaker was examining fundamental ecological questions about population dynamics with an eye towards applications in agriculture and biological control was really neat. I started seeing this study in a whole new light when he begins discussing agricultural implications on page 377, though I wish he would've elaborated further. From a farmer's point of view, it would seem on the one hand that extinction of an herbivorous pest by a predator would be good, but since that can also lead to the extinction of the pest's predator, the crop could then be subject to recolonization by the pest. This could lead to large and potentially unpredictable oscillations in pest levels; a farmer would likely prefer a small, predictable level of crop loss to the pest, with negligible oscillations. Huffaker shows that environmental heterogeneity can facilitate persistence in Fig 18, but it'd be nice to shrink those oscillations. It's unfortunate that modern industrialized agriculture seems to prefer to continue monocropping, while insecticiding and genetically engineering its way out of the pest problem, rather than using biological principles elucidated by Huffaker and others to manage pests.
Also, a question: In the intro they mention that others have found aggregation effects to not be essential for stable predator-prey populations, and give some examples that also seem to imply some sort of spatial heterogeneity effects. I didn't fully grasp what they were getting at, and was also wondering, under what conditions might these different mechanisms generating predator-prey population stability operate and become more/less important?
Be sure to checkout the Ecomotion Studios video summarizing Huffacker: http://ecomotionstudios.com/huffaker.php
I'm just continually disappointed that we keep reading papers with awesome names like "Experimental Studies on Predation" and then we end up talking about mites and rodents and moths. Can't we talk about hyenas just once? I want to know what happens when you put a hyena on an orange.
ReplyDeleteI'm not entirely up to date on the literature about mite generalism, but how might patchiness affect generalist predators as opposed to specialist predators? Could a specialist ever live in a very patchy environment?
I really enjoyed reading this paper, particularly the experimental design! Does anyone else think it's so cool how they added complexity and area with the rubber balls? I'm not sure why this seems like such an ingenious way to simply add more levels to the experiment, but it really appeals to me. This experiment is a relatively physically small one (mites are small), but the effect and scope of the experiment are very large. I'm curious to see whether if this experiment was repeated on a larger physical scale, such as with some predator/prey relationship in the savannah (assuming you could isolate both species), the results would be the same. I imagine if you could have the exact same conditions as this experiment, you would get similar results, and I suppose that's why this study was so influential. It used the small world of mites to illustrate predator-prey oscillations in a way that hasn't really been presented yet in our book.
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