Experimental Studies on Predation: Dispersion Factors and Predator-Prey Oscillations, C.B. Huffaker

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.

Paper 37: The influence of interspecific competition and other factors on the distribution of the barnacle Chthamalus stellatus

Here is another great study that I am sure many of us remember from our youth. Hold on dearly to these memories for I can attest to the negative effects of being a graduate student. Already the proportion of gray hair atop my head has increased dramatically, and the absolute number of said hairs would also have increased tenfold I am sure if not for the additional increase in my rate of hair loss. So, I repeat, hold onto these memories of your youth for they are all you have now that life is heading down hill so quickly, and the stresses of life are increasing “exponentially” (I totally pay attention to your math lessons Helen.)

Okay, let us begin with my summary, or one can head to the end of the paper for a very nice summary by the author Joseph H. Connell, who is still alive and very old. 92 years old to be exact. Connell is an American ecologist, but, since his study on how interspecific competition regulates the distribution of the barnacle Chthamalus stellatus was conducted on the shores of Scotland, I personally think he should be referred to as Sir Joseph Connell because the United Kingdom is the land of knights and chivalry, and everyone knows that academics are the epitome of chivalry and honor. However, I am not the queen and therefore cannot bestow such an honor so I will call him Connell.

Connell starts off his paper with an observation on the shores of Scotland. He noticed that the adults of Chthamalus and the adults of Balanus balanoides are separated into two distinct zones with the smaller Mediterranean Chthamalus distributed mainly above the tide level, and the arctic Balanus distributed at and below the tide level. Connell also noted that while he observed young Chthamalus below the tide level intermixed with the Balanus, he saw very few adult Chthamalus below the tide level. Something must be at play that is separating these two species of barnacle on the majestic coast of Scotland, and Connell was set out to discover what this was or die trying.

And bravely we soldier on into the rather detailed, and dry, methods section. So, who had to look up what a spring tide and neap tide are? Don’t lie, Christmas is just around the corner, and if we all get coal then London will be polluted again and all the typical moths will be eaten. I know I had to look up what these two kinds of tide are, but we can discuss the specifics in class; in short these two kinds of tide are tied (pun intended) to the relative positions of the sun and moon, and were used in conjunction with the mean tide level as demarcations for the distributions of the two barnacle species in question. I don’t think it is very effective to go through piece by piece what exactly Connell did, so I will try my best to be concise. Basically, Connell had several areas of different depths where he mapped the locations of barnacles for several years. At each area there was a control portion that he did not remove the presumed dominant species, Balanus, and an experimental portion where he did remove Balanus. In order to test for competition at depths where Chthamalus did not occur he performed translocations of stone with barnacles already attached. Connell also tested for the effect of a predatory snail on barnacle distribution at several testing areas by using a mesh cage to prevent predation.

(It’s a Scottish fold, the primary predator and snuggler in Scotland)

Enough with that methods jazz, onto the results, which is the real meat of any paper. Connell Briefly discusses physical factors that play into the distributions of Chthamalus and Balanus. The main point to take away from this section is that Chthamalus appears to be capable of surviving at a higher level than Balanus due to a greater tolerance to heat and desiccation. This explains the upper limit of the distribution of Balanus; they simply can’t survive being dried out and baked as well as Chthamalus. In addition to this Connell also pointed out that the data suggests that Chthamalus is capable of surviving at deeper depths than it is currently found at, meaning that there are other factors contributing to the lower limit of Chthamalus.

Now we move on to the crux of the paper, competition for space between the two species. Figures 2 and 3 sums up this paper quite well actually. Each graph represents a study area with the horizontal axis being time in months, and the vertical axis being total number of Chthamalus in the study area. The dotted line on the graph is the number of Chthamalus in the study area where Balanus was removed, and the solid line is the number of Chthamalus in the study area with Balanus present. A majority of the graphs show the same trend; Chthamalus survives longer when it is not living, however short that life might be, sympatrically with Balanus. Connell provides several tables of ancillary data in regards to how Balanus either directly kills or removes Chthamalus when they are competing for space, and other details about mortality rates. Figure 4 is not from Connell’s study, but shows that survival of Chthamalus is negatively correlated to the growth rate of Balanus.

Connell, again briefly, discusses the effects of predation by the snail Thais. What is significant in this section is that predation by this snail does not explain the distributions of these two barnacle species, but, interestingly, it does lessen competition. Connell proposes that the mechanism by which Thais reduces competition is by preferentially preying upon larger barnacles, which happen to usually be Balanus. This would obviously reduce competition between Balanus and Chthamalus because it lowers the population of Balanus.

I remember reading about this study as an undergrad, and it was just a given that Balanus is a better competitor than Chthamalus. It was never explained exactly why or how Balanus out competed Chthamalus. Lucky for us Connell does just that in his discussion section with his part on “The Causes of zonation.” Connell ends his section on competition for space with a quote from Elton and Miller (1954) defining interspecific competition as “in which one species affects the population of another by a process of interference, i.e., by reducing the reproductive efficiency or increasing the mortality of its competitor.” In this final section Connell explains how Balanus does just this to Chthamalus. And for anyone teaching a 203 or 204 lab, you can use this as an example of a redundant sentence since I said basically the exact same thing before the quotation. According to Connell, Balanus directly affects the fitness of Chthamalus by increasing mortality of adults through physically smothering or removing adults from an area, but also indirectly affects fitness by deforming and reducing the size of adult Chthamalus, which in turn decreases the amount of larvae produced thus reducing reproductive efficiency. Balanus is able to do this because of two traits; One, Balanus produces more larvae resulting in a larger population density relative to Chthamalus; and two, Balanus has a faster growth rate than Chthamalus. These two traits allow to Balanus to out compete Chthamalus when the two species exist sympatrically.

The last section is Connell’s wonderful summary that you could read and skip mine, but hopefully this has been somewhat entertaining and informative, now for some questions.

What exactly makes this a foundation paper and why is it included? (Everyone has to ask this question so cut me some slack)

How does this paper deal with interspecies interactions in comparison with Kettlewell and Park? What is the ultimate conclusion to the interactions in each of these systems?

Is Billy Connolly Scottish, or over the top Scottish?

I thought that the experimental design was robust, but I am sure people can find holes and flaws, what might these be?

It was mentioned both in the introduction to this section and Connell’s introduction to this paper that part of the beauty of this system is that the two competing species are sessile, how might one accurately test for completion in a more complicated and mobile system?

           

            Do I actually know how to use a semi-colon, or was I randomly throwing them in there for brownie points?

             

            This entire section for experiments in the lab and field is animal centric when a majority of what we have read up till now has been looking at plants, why do you think that is?

            How could Mel Gibson go from making Braveheart to The Beaver?

           

Biston betularia color morphs (For paper 34)

Biston betularia color morphs (Images are not mine)
A. typical
B. insularia
C. carbonaria

 

 Ecology reading Part 6

Hi all,

On Tuesday (October 27th ), we will be discussing the following papers: Selection Experiments on Industrial Melanism and Competition between  populations of the flour beetles, Trilobium confusum and Trilobium castaneum

Paper 34. Selection experiments on Industrial Melanism

1. Author: 

Dr. H.B.D Kettlewell (1907- 1979). British Medical Doctor, Lepidopterist and Geneticist.  He is well known for a classical study on natural selection using as a model organism peppered moths (Biston betularia).  Kettlewell conducted a series of experiments in 1950 on the phenomenon of industrial melanism.  His research was conducted in two type of woodlands in England:  Birmingham (polluted) and Dorset (non-polluted; results are not presented in this paper ).  His great contribution is divided in two. Firstly, he coordinated surveys of the dark morphotypes across England, which was a country wide survey that allowed to establish a reference study for further studies of the species.  Secondly, he did provide evidence of bird feeding on peppered moths selectively.

2.Scope of this paper: To demonstrate experimentally (aviary and woodlands experiments) how color variation is under natural selection. Due to pollution generated during industrial revolution, dark morphos of Biston betularia (carbonaria and insularis) were assumed to be fitter that the white types (typical) because of lower predation during daytime.

3. Methodology

3.1 Field experiments: Experimental release in an industrial area

Mark- release capture experiments were conducted in polluted sites close to Birmingham. In this experiments only males were used. They were marked with a dots of paint.  The release occurred at  sundown. Two different types of trapping methods were used. In the fist, he used a trap made of perforated zinc, where only males were allowed to enter, but not escape. The second one used muslin cages.  One virgin female of each genotype (carbonaria, insularis and typical) was kept in each trap. Basically they did this because they wanted to keep as many males nearby the forest (female’s pheromones avoid males migration out the woodlands). Otherwise they would have escaped. Mark recapture was done using mercury vapor lights

3.2 Aviaries experiments

The aviary experiments were conducted in  Research Station , Madingley, Cambridge. Dark and light color trunks of different species were placed inside the aviary. The experiments used predators such as the Great Titis in order to measure predation upon the tree forms of bestularia, which were released inside the aviary

4. Results

4.1  Field experiments

Approximately 700 individual of the tree morphotypes were released.There was a large number of recaptures for the melanic forms (carbonaria and insularis). Assumption was that the white morphos (typical) could have been predated and that’s why there were not many recaptures.

4.2 Aviary experiments

For the very first time, selective elimination on incorrect background was first demonstrated. Therefore, the conclusion was that the birds are selective agents

 5. Conclusions

This study is the classical example of natural selection, which is explained in classical textbooks of evolutionary biology. The author spent a considerable part of his life studying the effect of industrial melanism on peppered moths. The results of the experiments supports the hypothesis that birds act as selective agents.

6. Question for discussing in class

1. Dark- melanic forms are less abundant in post-industrial Britain. Would you still consider the conclusions  presented by Kettlewell  well - founded? If you are supposed to measure the selection coefficient in nowadays. Do you think it will affect the conclusions presented by Kettlewell?

2. How about to take into account  other selective agents  than bird's predation, for example predation  by insectivorous bats of the genus Pipestrellus. How would the results differ from the ones already presented in the paper?

Paper 35: Experimental Studies of Interspecies competition

Competition between populations of the flour beetles Trilobium confusum Duval and Trilobium castaneum  Herbst

     

     1. Author:  Thomas Park (1908-1992) (Ph.D. University of Chicago). Animal ecologist known for his experiments with beetles in analyzing population dynamics. He was one of the author's that wrote "Principles of Animal Ecology"  (1949). He also proposed the use of quantitative methods  and experimental studies in ecology.

      

    2. Background information:  This seminal paper is the first publication of a series of studies about interspecific competition. The other two papers were published in  1954 (Experimental studies of interspecific competition II Temperature humidity and competition in two species of Trilobium) and 1957 (Experimental studies of interspecies competition III:  Relation of initial species proportion  to the competitive outcome in populations of Trilobium). We need to take into account that these papers are merely descriptive and the results presented are rather exploratory than definitive. This research supports fundamental principles of competitive exclusion, limiting similarity and ecological niche.

3.Scope of this paper: To use flour beetles (Trilobium confusum and Trilobium castaneum) in a long- term study of interspecific competition (211 laboratory populations for four years with counts every thirty days), in order to explore and describe how both species are individually adapted to their environment.

4. Methodology: Herein, a few comments on census procedures, fecundity, metamorphosis and longevity data.

•   Animals were kept in dark incubators (29. 5 º C) with the following conditions: relative humidity (60 to 75 %); medium: 95 %  whole wheat  flour sifted  trough No 88 silk bolting cloth.  Medium was mixed with dry brewer’s yeast powder in the amount of five percent by weight.
•  Populations were established in two sizes of glass containers and in three volumes of medium. Census were conducted every thirty days interval for every population. Bolting cloth sieve  was used to  in order to retain the imagoes, pupae and large larvae.
•  The census method allowed an accurate total count of all the larvae, pupae and imagoes (adults) and established of the population in another fresh medium for another 30 days period.
•  T. castaneum exhibited a higher rate of fecundity and grew faster   than T confusum. 

5. Experimental Designs 

    

•       Control (singles species cultures) and experimental populations ( I-E-a, II-E-a, III-E-a: Both species introduced in equal proportions; I-E-b, II-E-b, III-E-c: T confusum has an advantage over T. castaneum: III-E-a, III-E-b, III-E-c;  T castaneum has an advantage over confusum)

•       Three volumes o medium were established: I (8gr), II (40 gr) and III (80 gr). Adults of the same sex ratio were introduced initial population: 4 males and 4 females, 20 males and 20 females and 40 males and 40 females. Then control experiments are single species culture, while the experimental are divide in tree. It also included code, initial ratios of adults,  number of replicates and number of observation days.

6. Results

The author monitored control populations in order to increase the reliability of the results obtained for the experimental populations.

• Trilobium confusum controls: The populations are healthy, the volume of medium did not impose no difference in the patterns of growth  and maintenance.

• Trilobium castaneum controls:  For the 8 gram series the author noticed a population decline because of the presence of a parasitic infection  (Adelina: coccidian parasite). The removal of the parasite permitted T castaneum to increase the population again.

• Trilobium confusum and T castaneum sterile controls: The population are large enough without the infection of the parasite. Higher proportion of adults

6.1 Control Cultures considered over the total period

The author presented some relationships: 

• Between volumes within species: T confusum decreases with increase volume. Association of density with the size of the environment exist. This pattern is not a clear cut for castaneum.

• Between species within volumes: At 8 gr and 40 gr confusum is at significantly higher level than does castaneum . There is not difference in 80 gr

• Between parasitized and nonparasitized within species within volumes: T confusum infected and T confusum sterile cultures maintain populations of equivalent total size.  Non  infected castaneum larger than infected over the entire period of observation

• Between non parasitized Species, within volume:  The removal of Adelina allows castaneum to attain larger densities, actually larger than confusum.

• Percentage composition: T confusum has higher proportions of adults than castaneum populations. There is little divergence in percentage composition  between tree volumes within the species.

• Variability within and between control populations. This is divides in grouped data and data by census intervals. There is not  consistent trend  for either specie that can be related to volume of medium . T confusum is less variable than castaneum. 

6.2 Mixed populations experimentals 

The major ecological finding when T confusum  and T castaneum are brought into competition , one of the two invariable becomes extinct with the other then assuming its  "normal" population behavior. Therefore both species wont be able to coexist even thought they are well adapted to their habitat  and its surrounding physical environment.  T confusum became the successful organism, while T castaneum disappeared. These results were presented discussing tree underlying observations

1. Parasitized populations in T castaneum always become extinct: Initially T castaneum can multiple in association with T confusum, but the increased and spread of Adelina by days 90 and 360 caused the extinction of castaneum. 
2. Extinction curves for T castaneum and T confusum: The most common pattern observed is the extinction of   castaneum, although the extinction of  confusum is limited to  eight series I out of a total of 45.
3. Extinction of T castaneum and T confusum in the absence of Adelina. T castaneum wins more often than does  T confusum (This happened in 12 out the 18 replicates).

7. Conclusion: In conclusion, this is one of the first paper in ecology that used experimental approaches in order to shed light on the underlying mechanism of interspecific competition. This ground breaking contribution support the observation that two species of the same genus can not coexist if they come into competition. Under the Lotka -Volterra scenario, the coexistance equilibrium in unstable and two competitive exclusion  criteria are stable.

8. Question for discussion in class:

1. How do you think this research supports the fundamental principles of  competitive exclusion and ecological niche?

2. How do ecologist model interspecific competition?

      

t       

 

      

An Ordination of the Upland Forest Communities of Southern Wisconsin, J. Roger Bray and J. T. Curtis (1957)

Atmospheric distributions!

Remember G. Evelyn Hutchinson’s definition of the fundamental niche as an n-dimensional hypervolume? In October of 1957, the same year Hutchinson makes his concluding remarks, Bray and Curtis publish a description of three dimensions constructed from quantitative plant community data. In contrast to the confined reductionist approach of C. S. Holling, Bray and Curtis seem animated by the spirit of exploration of the unknown, and a willingness to try to describe whatever they find. They have no hypotheses. Their stated goals are 1) to describe the compositional structure of a community; and 2) to look at patterns of interaction between biotic and abiotic phenomena. Their project is to map the complexity of “a field of interrelated units and events,” to build a broad foundation on which future scientists might discover causal connections.

In the introduction to this section, James H. Brown mentions that some people are less than enthusiastic about the impact of plant ordination studies. However, according to googlescholar.com, Bray and Curtis’ ordination paper has had 6,197 citations, 355 of them in 2015. Multivariate analysis, although begun using only adding machines and hole punches, must have had some impact.

In their brief literature review, Bray and Curtis go all the way back to Gleason’s 1910 prescient vision for a quantitative description of plant communities. They outline three basic schools of thought that have arisen since then, with different foci: 1) relationships of species to each other; 2) relationships of whole stands to each other; 3) demonstration of degree of relationship directly from analysis of quantitative data. They decide to combine relationships of whole stands with direct analysis of data.

Source and Treatment of Data

Bray and Curtis gather data from 59 representative, relatively undisturbed stands of vegetation in the southwestern half of Wisconsin. Using a random sampling technique, for each of the 59 stands, they make 38 measurements: density for 12 species of trees and total basal area for those 12 species (basal area, according to the USDA Forest Service glossary, is tree area in square feet of the cross section at breast height of a single tree), and frequency of 14 species of shrubs and herbaceous plants in regularly placed 1 m. quadrats. Scores for each stand are adjusted to relative values.

For their pairwise comparisons between stands, the plant ecologists discard the traditional correlation coefficient, “r,” as too insensitive. Instead, they use the Gleasonian coefficient of community “w,” the ratio of the species in common to the adjusted index for one stand (80). They arrive at 1711 values of w, and arrange them in a matrix.

Application of the Method

Rejecting previous analytical methods, they then constructed a 3-dimensional ordinate system with x, y, and z-axes, in which to project the data from their matrix. They inverted the values of the 1171 coefficients in the matrix (subtracted them from 80) to reflect their assumption that the greater the similarity of species between stands, the less distance there will be between those stands when plotted in the theoretical space. They constructed the scales for the axes using stands with farthest apart values (lowest w coefficients). A unit on any axis is defined as 1.

Results

The plots on pages 580 and 581 show 3-dimensional axes of tree species; the plots on pages 582 and 583 show herbs and understory plants in 2 dimensions. All plots depict pairwise “distance” relationships between stands. Each dot or circle represents a stand; the size of dot or circle reflects species dominance as measured by basal area. For each species, the stands are plotted on three 2-dimensional graphs (the three possible combinations of the x, y, and z axes) that can be thought of as cubes, with the three graphs forming the front, top, and side of the cube. Please excuse me if I pause just a moment to say this is REALLY COOL! Please see the 3D model (Figure 7) on page 584, and the diagram (Figure 8) on page 586.

Discussion

Mechanical Validity of the Ordination

Bray and Curtis test the accuracy of the 3-dimensional ordinate system by using a 3D version of the Pythagorean theorem. Distances in the ordinate system are compared to the coefficients of community in the matrix. A significant correlation is found (1% for 58 stand pairs and .1% for 11 reference stands). When the axes are tested for their ability to supply meaningful separation of the strands, however, the “z” axis is found lacking. It is retained for several reasons. The axes are also tested for their ability to produce random orientation of stands. They pass this test (they do not produce randomness).

Biologic Validity of the Ordination

By visualizing the 3D graphs, the species show all or part of an “atmospheric distribution,” where there is a center of concentration, and a diminishing radiation outward from that center. This is reminiscent of Gleason’s dune vegetation descriptions. Bray and Curtis point out that they have shown a 3D version of the concept of ecologic amplitude, and that compression into 2D gives contour-shaped patterns, while compression into 1D gives bell-shaped curves. They characterize the distribution and relationship of the patterns as “continuous variation,” and suggest that this is “evidence for individualistic theory of species distribution and for the continuum nature of community structure,” listing many past studies supported by their model.

While exercising caution about attributing causality, Bray and Curtis speculate about possible biologic meanings of their constructed axes. Correlation tests are done for all available environmental variables with the three axes. Patterns emerge:  for x, light, moisture, and temperature, relative to past fires; for y, soil moisture and aeration; for z, recent disturbance and amount of organic matter.

Questions

A. A couple of the assumptions in this paper are: 1) it makes sense to depict similarity of species as nearness in space; 2) the three types of measured quantities have equal weight (understory plant frequency, tree density, tree dominance). Are these good assumptions?

B. Would someone in our class please demonstrate the geometric technique of arc projection Bray and Curtis used for constructing their y and z-axes?

C. What do you think about this type of study in general? No hypothesis, no prediction, no testing of prediction—just an exploration.

D. Bray and Curtis write, “The nature of the causal role of factors shaping a community is an ultimate goal in ecology.” Do you agree? Why or why not?

Thermodynamic equilibria of animals with envirrnment, Porter and Gates (1969)

In which two scientists endeavor to prove that animals do not, in fact, violate the laws of thermodynamics.

It is interesting to think of this paper as the authors sought to include it, demonstrating three concepts that were being explored in ecology at the time. Using mathematical models to better understand factors that might affect organisms in real life. Using physical models to try and recreate at least a simulacrum of the study organism and be able to analyze factors that affect them. Lastly, the idea of a "climate space" which we have been talking about through a lot of these paper. The idea behind this is the climate part that N-dimensional hypervolume that we consider when we are thinking about a niche.

The paper starts out talking about the idea of the climate space right away and gives us the first four dimensions of the hypervolume they will be considering: radiation, wind, air temperature, and humidity. These four factors are a pretty good approximation of climate. I am sure there are a number of other things that we could all come up with that might go in to climate but almost everything that goes into what a climate is comes from these four factors. These abiotic factors affect three factors they consider on the animal: body temperature, rate of moisture loss, and metabolic rate. They are analyzing the animals to be part of a steady state system and fully admit that this is not necessarily the case in the wild. But, the idea behind it is sound. Over a long period, the animal must be in a steady state. To not be in a steady state would mean that the animal was either cooling or warming over the long term. This is fairly contrary to what we know about animals, barring some age-based body temperature differentials.

These ideas about what goes into and out of an animal form this idea in the paper called the "energy budget" and this is just the idea that Energy IN = Energy OUT. But what they are really trying to do is to determine what the parts of these in and out equations. A quick rundown of the variable might be helpful.

Energy In

• Metabolic Rate
• Radiation Absorbed

Energy Out

• Epsilon-sigma-Tr^4 is the energy lost by radiation of the animal
• hc(Tr-Ta) is the energy lost by convection coefficient
• Eex is the energy lost from expiration
• Esw is the energy lost to sweating
• C is the energy lost by conduction to substrates
• W is the work done by the animal

The reset of this section is talking about how they obtained these values and decided on specific ones. It discusses some other factors (like color) that will have to go into calculating these values and discusses the effect of size on things like the convection coefficient.

In the next section, Heat Transfer Within The Animal, Porter and Gates go on to talk about a really important part of how animals interact with their environment, their adaptations to the environment around them. The are specifically concerned with the fat layer and the feather/fur layer that many animals have to help them adapt to different climates. Since they are treating all animals as cylinders, they look at the idea of concentric cylinders with the heat generating stuff in the middle most cylinder, a cylinder of fat around that, and a cylinder of feathers/fur on the outermost layer. These approximations give them a means to talk about how animals adapt to their environments to prevent or enhance heat loss.

This section leads into the next two sections which are more explanation of the equations. Specifically they talk about the energy that can be exchanged on the surface of the animal and the radiation that will be absorbed on the surface of the animal. These are both a function of the shape(cylinder)/size of the animal, and the energy exchange also has to do with the idea of the concentric cylinders that they are using as their animal model.

Finally they get in to their modeling of different animal's climate spaces. Using molds of animals as well as mathematical approaches, they took measurements of the energy absorbed by animals at different temperatures and amount of radiation absorbed. They admit that some things have to be known before their analysis can work and that animals can change some of these values about themselves, but they are still confident, and rightfully so, that their calculations and experimentation can show some valuable results. In their analysis they are able to find temperature ranges and sun radiation absorptions that would let an animal live in the temperatures that are within it physiological range. They do this with a number of animals (iguana, shrew, finch, pig, etc.) as well as some theoretical animals, showing that it works and can be extrapolated for other organisms.

In conclusion, they are defining their climate spaces as a three dimensional space of radiation absorbed, wind speed, and ambient temperature. While we know that there are definitely more factors, this is a really great step towards understanding the fundamental niche of an organism. This paper shows methodological advancements in a way that we haven't really seen before, using both mathematical models as well as physical models to test concepts. These two modeling methods are still used extensively today.

The Components of Predation as Revealed by a Study of Small-Mammal Predation of the European Pine Sawfly, by C. S. Holling (1959)

C. S. Holling proposes to create a comprehensive theory that breaks down predation into two basic components: 1) functional response, or change in number of prey consumed per predator as prey density rises; and 2) numerical response, or change in density of predators as prey density rises. The two main variables he considers are prey density and predator density.

Holling calls the calculation of rate of growth without considering limiting factors “whimsy.” He comments that several researchers (including Nicholson, of the 1934 paper we read on parasite-host dynamics) have focused too myopically on various aspects of predation, and states a need for a more general theory.

Approach: Holling combines field and laboratory experiments. His model system has the advantage that it avoids many complications. In the field, three small mammals prey on one cocooned insect in an even layer of pine needles, under a uniform canopy of pines. The three main predators are the masked shrew (Sorex), the short-tail shrew (Blarina), and the deer mouse (Peromyscus). In the laboratory, variables that are constant in the field could be varied to extend the scope of the results.

In the field, Holling and his associates sample and estimate mammal and cocoon numbers from areas of different prey densities caused by spraying viruses of differing concentrations. In the lab, they vary prey density and the amount and type of alternate food available. The number of prey eaten, in addition to the identity of each predator, can be determined from scrutinizing the marks on the opened cocoons.

Results for basic components: In Figure 1, the functional responses of the three predator species are plotted against prey density. As the density of prey rises, the number of cocoons opened increases in an “S” curve for each predator, which levels out at different densities. The rate of increase is greatest for Blarina, least for Peromyscus, and Sorex is between the two. Analysis of Peromyscus stomach contents in the field, as well as functional response in the lab, support these data.

Figure 3 plots numerical response, or predator density against prey density. Increasing prey density apparently has an effect on prey density for two species. Holling states that for these two species, Sorex and Peromyscus, he has demonstrated that predator density is a “response” to prey density, but as he does not mention any correlation calculations or p-values, we would be justified in remaining skeptical about whether a causal relationship has been established.

The effect of predator density is tested briefly. Different densities do not result in different functional responses, so predator density is not taken into account in the totals for Figure 4.

In Figure 4, Holling combines the functional and numerical “responses” for each species by multiplying them, converts them to percentages, and plots them against prey density. Each shows a peaked curve, which in Blarina only reflects the functional response, since it showed no numerical response.

Results of varying subsidiary components:

Figure 5 is an aesthetically pleasing 3-D graph, showing that one deer mouse does not eat as many cocooned saw flies when they are buried deeper in the sand as when they are buried shallower. Figure 6 shows that one deer mouse decreases its saw fly consumption less when dog biscuits (unpalatable) are available than when sunflower seeds (palatable) are available.

Discussion:

Figure 7 is a theoretical model showing regulation of prey by predators. A horizontal line marks the ranges in percent predation where the prey birth rate = prey death rate. He states that regulation happens “when there is a rise in percent predation over some range of prey densities and an effective birth-rate that can be matched at some density by mortality from predators.” In his rambling discussion, Holling considers various models that have been proposed for oscillations of animal populations. He mentions Nicholson and Bailey’s prediction that oscillations in host (prey) numbers will increase in amplitude, and suggests that small mammal predation (in Holling’s system) can damp oscillations in prey population through the functional and numerical responses.

Holling compares his results to other systems explaining predator-prey interactions, with emphasis on Errington’s concept of compensatory predation. He then postulates four major types of predation in Figure 8, based on combining four different functional response curves with three types of numerical response.

Questions:

1. What do you think of Holling’s use of the term “anthropocentric”? Do you think he is successful in making his focus more objective?

2. What is the main point of this paper? What has Holling actually said, other than that small mammals eat more food when more food is available until they are full, and also sometimes congregate where there is more food?

3. Is statistical analysis really completely absent from this paper?

4. Holling describes various oscillating populations at some length in his discussion, but none of his graphs show oscillations. Why?