Paper 33: Predation, Body Size, and Composition of Plankton

Paper 33: Predation, Body Size, and Composition of Plankton: The effect of a marine plantivore on lake plankton illustrates theory of size, competition, and predation (1965)

By John Langdon Brooks and Stanley I. Dodson

This paper begins by the authors remarking that while the cladoceran Daphnia is present in most of the lakes in southern New England, it is absent from most lakes near the eastern Connecticut coast. They discuss the dominant cladocerans and copepods found in those lakes.

They then discuss the herring-like Alosa pseudoharengus, or alewife fish. This is a marine fish that swims up the streams and rivers that feed into Long Island Sound and ends up in lakes that are within about 40 km of the coastline. They mainly feed on planktonic copepods and Cladocera. The dominant planktons in the lakes with alewife present are smaller sized species, and in lakes without alewife present, the larger Diaptomus spp. and Daphnia spp. are dominant. They speculate that this could be due to predation from the alewife populations.

Changes in Crystal Lake Plankton

They test this hypothesis at a lake in northern Connecticut that had previously been dominated by the larger plankton species prior to alewife introduction in 1942. They sampled Crystal Lake on 30 June 1964, and counted and identified all of the crustacean zooplankton captured both in 1942 and 1964 (Table 1). They also sampled the populations of four lakes with Alosa present, and four lakes without Alosa present. 

They also wanted to know what part body size of the dominants played, and so determined the size range of each species (Figure 4). In 1942, the dominants reached a length up to 1.8 mm. In 1964, no zooplankters over 1 mm could be found. This was smaller in the lakes with Alosa, where the largest species found was less than 0.6 mm in length with an average length of 0.285 mm. In lakes without Alosa, the average length was 0.785 mm. They conclude that Alosa predates heavily on larger species larger than 1 mm in length.

Effects of Predation by Alosa

They now begin to investigate what the significance of this critical size is. They decide that there must be other factors for species between 0.6 - 1 mm in length. Alosa avoid the shores, so predation may fall more heavily on species that avoid the shore also. They discuss the manifestation of this with examples of different species. They bring up the example of one of the Finger Lakes, and state that this one upholds the high numbers of plankton of intermediate size.

Size and Food Selectivity

Here they establish four trophic levels within the lakes to establish the importance of size of food organisms:

Level 4: Piscivores (mainly fish)

Level 3: Planktivores (also mainly fish)

Level 2: Herbivorous zooplankters

Level 1: Microphytes 

They say that animals choose their food on the basis of size, abundance, edibility, and the ease at which it is caught. There is a large difference in food selection between herbivorous zooplankton and higher level predators. Higher level predators tend to consistently choose the largest food morsel available because it requires the least amount of energy to obtain. Visual discrimination plays a large role. In herbivorous zooplankton, on the other hand, visual discrimination has little to no role; their food capturing is determined by the mechanism of removal of the particles from the water in the rate of flow near the mouth. 

Size-efficiency Hypothesis

Planktivores and piscivores are labeled as “food selectors” while herbivorous zooplankton are labeled as “food collectors,” due to their determined food range. 

Size-efficiency hypothesis:

1. Planktonic herbivores compete for fine particulates in the open water.Small particles are most important, composed of algae, detritus and other organic aggregates that provide constant food within the system.
2. Larger zooplankters are more efficient and take larger particles.This is due to greater effectiveness of food collection, reduced metabolic demands per unit mass and greater reproductive success. ( Ex. Daphnia catawba is 4x the size of Bosmina longirostris, so Daphnia will have a filtering area  16x greater than Bosmina.)
3. Therefore when there is low predation, herbivores will be out-competed by larger forms. 
4. When predation is intense, it will take out larger forms, allowing for small forms to be dominant.
5. When predation is moderate, it will keep the larger forms low, allowing for smaller forms to persist.

Prediction tested by Hrbacek et al. and “the result is precisely what the size-efficiency hypothesis predicts.”

Size of coexisting Congeners 

In aquatic and terrestrial systems the common pattern is that larger species take the larger food, while the smaller species eat the smaller food. 

An exception to this rule is congeneric zooplankton living in coexistence are roughly the same size in certain European Lakes. Is this because they are clones??

Summary 

Predation of alewife upon zooplankton, the larger more dominant crustaceans are eliminated and replaced by the smaller species Bosmina longirostris. In regard to planktonic herbivores, the larger species are better at food accumulation due to their more effective strategies. In this case larger species will outcompete the smaller ones when predation is low. However when predation is high, the lager species is removed and the smaller species will become dominant. These demands of competition and predation determine body size of dominant herbivorous zooplankton. 

Paper 31: Pattern and Process in the Plant Community (1947) by Alex S. Watt

Paper 31: Pattern and Process in the Plant Community (1947)

Alex S. Watt

Alexander Watt was a Scottish botanist and plant ecologist. When this talk was given, he was a professor of botany at University of Cambridge. This paper was originally an address given to the British Ecological Society on January 11, 1947.

The Plant Community as a Working Mechanism

Watt begins his talk classifying the plant community as a "working mechanism". He continues by describing his aim of applying dynamic principles to the plant community to formulate laws by which it maintains and regenerates itself. He says it is relatively unknown how individuals and species are put together, what determines their relative proportions and their spatial and temporal relations to each other. Ultimately he wants a qualitative statement of the nature of the relations between the compounds of the community.

He presents a record of seven communities, the first of which their features make for orderliness, and the latter which specifically depart from that order. He assumes with these case studies essential uniformity in the fundamental factors of habitat (separate from context of whole environment) and essential stability of the community.

The Evidence from Seven Communities

The regeneration complex

This is a mosaic of patches that form an intergrading series that can be assigned to a few types of phases. He presents Tregaron Bog and goes through the sequence with which species take over the open water in the pool. The final species in this series then forms a hummock, which is then also inhabited by a series of species. This can be proven by the layers of plant material in the peat. 

He states that each of these phases of the “regeneration complex” were once considered a community in itself, but Watt believes that each cycle is brought about by the last one and the spatial relation between patches and their change in level. 

Dwarf Callunetum

This is an analysis of the dwarf Callunetum on an exposed slope of the Cairngorms (a national park in Scotland). The two main species he talks about are Calluna and Arcostaphylos. These two have the same habitat, but Arcostaphylos is almost always found leeward of Calluna. Arcostaphylos first grows over the eroded soil, and then Calluna (the dominant plant) grows over the older parts of Arcostaphylos. This, he says, is proof of a dynamic relationship between the two species and the process of succession. It is a three-phase system with the phases arranged in linear series. 

Eroded Rhacomitrietum

This is a similar phenomena to the Calluna in that wind determines the direction of growth. This community consists of a network of patches of bare soil and of vegetation. These are characterized by a series of bryophytes ending with Rhacomitrium. This plant is then eroded by the wind and and the exposed accumulated humus is dispersed and the mineral soil bare again. 

Bracken

This study is about the Pteridietum (ferns) that occurs in a hinterland where the fronds are patchily distributed and the axes of the plants form a loose network (Figure 4). There are patches with fronds and without fronds. The patches with fronds vary features of frond and rhizome, and exist in four different stages - pioneer, building, mature, and degenerate. These exist in what Watt calls the marginal zone and the hinterland, and he says the processes are the same in these two - when the vegetation cycle changes, there is also a change in the factors of habitat. 

Grassland A (Breckland)

This is a patchy grassland that has hollows and hummocks with the habitat resembling the regenerative complex. Here, the four stages Watt classifies are hollow, building, mature, and degenerate. The life of the community centers around Festuca, the dominant plant, and in the spaces around this, lichens grow. Every part of this community can be assigned to one of the four stages, and the cover percentage of fescue to bare soil are inversely related. Holophytic bryophytes are excluded from the building and mature phases, and reach their peak in the hollow where competition from fescue is the least.

Grass-heath on acid sands (Breckland)

This community consists of rings of vegetation, and Watt says it epitomizes the spatial and temporal changes within a community. The two main species being studied are Agrostis and Cladonia, the first of which gives way to the second. Drought has a large effect on this system - it bypasses the first stage completely and goes straight to the second one.

Beechwood

In this study, Watt wants to investigate if there is a correspondence between structure and current meteorological factors within long-lived dominants. There is a cyclic relation between the tree species and the causes having long-continued effects in structure. Watt also remarks that if a whole gap phase regenerates at the same time, there an age class of abnormal area is initiated.

Supplementary Evidence

In this section, Watt mainly gives evidence of where else these dynamic principles could apply; not just to temperate regions, but to extreme regions like the high arctic. The tropical forest is not expected to fit into a simple dynamic interpretation. He states that Aubreville has somewhat disproven this, that although the tropical forest is diverse and has much larger number of species, there is a change in the dominant species over time. This leads him to conclude that the community itself largely determines the “distribution, density, and gregariousness of its component parts.”

Comparison and Synthesis

Watt introduces the idea of an upgrade of a community and a downgrade. The upgrade results in a building up of plant material for an ultimate positive balance, and a downgrade is an ultimate negative balance. He also looks at a quantitative graphical expression of the change in an upgrade series and says that a logistic curve of growth expresses the general course of change (a similar negative curve for downgrade fits also). Figure 11 gives a nice approximation of this. Most species are limited in space, but also in time based on when the gap phase is receptive. This may lead us to assume that the plant community in a constant environment will show a definite proportion between constituent phases. 

Some Implications

He takes some time to point out that Clements and Shelford do not incorporate the effects of animals and micro-organisms into their study. He thinks the degree of intimacy varies, but their key positions in the system should be considered a part of it. Fungi also play a role in the maintenance of the ecosystem. Watt then moves on to “shatter” the unified system that Tansley calls the ecosystem, and points out that the living plants and animals (biome) must be separated from the non-living habitat. He also points out Gleason’s “randomicity” of the species in an association, and says that although two species may be randomly distributed, they may show a high degree of association. Gleason minimizes the significance of the relations between the components of a community. 

Watt ends with saying that drawing a line between the plant community and the ecosystem is difficult, but we must have even an idealistic objective of melding all of the fragments into one original unity. So many problems in nature are problems of the ecosystem and not just of soil, animals, or plants, and because of this, we must know how all are connected. 

Population Ecology of Some Warmers of Northeastern Coniferous Forests by: Robert H. MacArthur

Intro

Robert MacArthur noticed that several species of warblers living in close proximity, with similar roles in the bird community, are an exception to the general rule that species with the same niche cannot coexist because they will outcompete one another (competitive exclusion). The five species Cape May, myrtle, black-throated green, blackburnian, and bay breasted are found together in breeding season in similar types of mature boreal forests. These five species are of the same genus, they all eat insects, are of similar size and shape, and have similar ecological preferences. MacArthur partook in this study to determine the factors controlling species abundances, and what allows them to persist together in a homogenous habitat. 

Pop control

Populations are regulated by two types of events:

1. Density independent: storms, severe weather, and disease
2. Density dependent: food shortage, lack of habitat for nesting, and interspecific relationships

MacArthur claims that interspecific competition does not allow for coexistence. He states that for coexistence to occur, each species, when abundance is high, should limit itself rather than limiting other species. (see fig. 1) For this to happen species need to be limited by a slightly different factor. For MacArthur to test this he decides that the most feasible way is to compare the bird populations in two different regions differing in the limiting factor.

Four parts to this study:

1. Density dependent events play a large role in population control of  the species.
2. General ecology of the species ( food, behavior, territory, etc..) 
3. Habits of the different species in different seasons are compared to find the general characteristics of a species.
4. Comparison of species abundances relative to niches. 

Density Dependence

Warblers are regulated primarily by density dependent events, they increase when rare and decrease when common, relative to limiting factors. These increases and decreases are not random. Ex: Food supply is a limiting factor for bay breasted and Cape May warblers.

General Ecology

Two similar species will be limited by different factors so that each species inhibits its own growth instead of others. Many of the factors are food presence, proper feeding zones, shelter and nesting sites. 

Sites: 

1. Bass Harber head, Maine; summer of 1956; 9.4 acre plot of mature white spruce.
2. Marlboro, Vermont - July 7 1956; red spruce woodlot of comperable structure. 
3. more plots 1957; May 30-June 5 many more plots were studied.

Feeding Habits

Food competition is huge in birds, but more importantly to understanding competition are differences in feeding behavior and feeding positions. This determines differences in types of food between similar species and what food is obtainable. To describe feeding zones, the number of seconds each observed bird spent in each of 16 zones was recorded. Zones varied in height and branch positions. Height zones were measured in ten foot intervals from top of tree to the bottom. Then each branch was divided into 3 zones: bare or lichen cover base (B), middle zone of old needles (M), and a terminal zone of new needles or buds (T). see fig 2-6. I feel that this study could be more precise today with infrared cameras to locate birds more easily in brush. 

Another way to study differences in feeding behavior, is to measure movement along branches. The tree directions were vertical, radial, and tangential. Measurements were measured on how far birds moved across branches in all three directions. see fig 7 and table 3. 

Another comparison used during flight feeding was hawking (moving prey sought in air) or hovering (stationary prey sought in foliage). see table 4

MacArthur gives a great comparison of all five species, indicating feeding habits, positions, movement along branches and flight feeding. MacArthur found that each warbler species divided its time differently among various parts of the tree. This information is very lengthy and will be left out of the summary. see pgs 692-695

Food

Species may eat different food for three reasons:

1. They feed in different places or different times of day.
2. They feed in different ways to find different types of food.
3. They may accept different kinds of food depending on what they are exposed to. 

By feeding in different places in a different way, they are being exposed to different foods. 

Morphological differences are not substantial, and do not seem to give different species advantages over there others. Insects appear to be in birds stomachs proportionately to availability. Insects located in specific feeding zones are eaten more by the species of warbler located there. 

Nest location

Nest position in height reflects preferred feeding zones. see figure 8.

Territoriality 

Territory regulates populations:

• Small niche differences; species inhibit own population growth.
• Density dependent regulation: regulating mechanism must have information of its own population density
• Predator should keep prey at stable level for greatest rate of production. More food should equal more predators. 

Territory size seems to be fixed in a region. Intraspecific territoriality is greater than interspecific,  reducing competition and acting as a stabilizing factor. 

Natality and Mortality

The more densely populated an area is, including all warbler species, the better chances of young surviving. see table 6

Time of activities

Differences in feeding times( time of day) , breeding seasons( time of completion of clutches), nesting dates and behavior can allow different species to coexist. see fig 9. 

Population control

All factors that control local populations have space distribution ( food, nesting sites, predators) thus populations are regulated by suitable space. Certain activities require more space than others, and those are most likely limiting. The five species of warbler do not seem to have special nesting requirements. Like nesting space and nesting material, territory probably require less space for warblers in normal years than food gathering. pgs. 702-704

Conclusions

Species can coexist only if each inhibits its own population more than another. Moreover coexisting species divide up resources of a community in a way that each is limited by a different factor. fig 10

Of the five, Cape May and bay breasted warblers are dependent on periods of super abundance of food. The remaining species  maintain populations proportional to the volume of foliage where they normally feed. Differences in behavior, feeding strategies, time of activities all reduce competition. Along with differences in habitat, niche space, and territoriality allow for the coexistence of these 5 warbler species.

Climatic Changes in Southern Connecticut Recorded by Pollen Deposition at Rogers Lake by Margaret B. Davis

Introduction

Fossilized pollen grains can be used to understand the terrestrial past. It can be very useful when near complete records are preserved in bogs or lakes. Percentages of pollen can change between stratigraphic levels, allowing us to look into the past at the types of vegetation that were present and compare it with today. 

Margret Davis, in seeking to expand information contained in pollen percentage diagrams, estimates rates of deposition of pollen in the sediment. Changes in deposition rates infer changes in species abundances of plants better than pollen percentages. She compares these rates as they change from level to level. Her results allow for interpretations of the past and have expanded previous interpretations based on pollen percentages; giving us good insights to the historic climate of Rogers Lake in Southern Connecticut. 

Rogers Lake

The lake is 6 km east of the Connecticut River. It is a damned lake, surrounded by houses, and there are probably a lot more now. What used to be farm land is now deciduous forest, all second growth, made up by oaks, red maple, hickory, birch, ash, pine, beech, elm and sugar maple. ( in order of greatest to least in abundance) 

Methods

Two cores of sediment were collected from Rogers Lake, one in the North basin and one in the South. Samples were taken from cores using the spatula technique where sediment was packed into a 1-ml porcelain spatula. Supposedly this technique is pretty accurate and sample error is of the same magnitude as error in pollen determination . The use of digital cameras would probably aid in narrowing down that error. 

The spatula samples were treated with KOH and acetolysis. Assays were made by using the aliquot slide method. They used Eucalyptus pollen as the control-which apparently is not present in the lake. Counts of the aliquot slides provided estimation of pollen concentration and calculations for percentages. 1/3 of pollen grains could not be determined because they were torn or distorted. 

Results/Discussion

Pollen percentage: percentage of any given type divided by total grains in sample. These are plotted against depth or age in pollen percent diagram.

Sediment matrix accumulation rate: the net thickness of sediment accumulated per unit of time or amount of time per unit of thickness of sediment, after compaction and diagenesis .(fig 2) slope equals rate of accumulation. The deepest sample was 14,300 years old, showing a rapid rate of sediment accumulation. 

Problems: radiocarbon C14 occurs in atmosphere so testing samples older than 2,500 years are shown to be younger than the true ages. This can cause changes in the slope and alter results. Davis reports that the changes are small and are not significant. 
The second problem stems from interpretation. Some older accumulation rates were used for comparison that were incomplete. The size of margins are not predictable and can fluctuate based on the site, however Davis assures us that the margin or error is small and should be considered normal. 

Pollen concentration: the number of grains per unit volume of wet sediment. AKA absolute pollen frequency (APF) fig 3. Pollen concentration is plotted against radiocarbon age, not corrected for C14 in atmosphere. 

Pollen disposition/accumulation rate or pollen influx: net number of grains accumulated per unit of area of sediment surface per unit of time. To get this Davis multiplies pollen concentrations by the sediment accumulation rate. (fig 3) The total influx of pollen increased between 12,000-9,000 years ago from 1,000 to 50,000 grains/cm² per year, then dropping to 20,000 and eventually increasing again.  Because there is little for Davis to compare her results to, she cannot effectively say why these changes are occurring. 

Pollen stratigraphy and interpretation

Davis separates different zones based on stratigraphy and carbon dating. I will try to be as brief as possible for this section. 

The first and oldest zone is the Herb pollen zone (14,300-12,150 years ago) where the oldest and siltiest sediments had the highest percentage of herbaceous plants and grasses with few pollen from trees. 

The next zone is A-1: transition from herbs to spruce(11,700-12,150 years ago) maximum of birch pollen and rising percentages of spruce

Zone A-2-3: spruce oak zone (10,200- 11,700 years ago) high percentage of spruce pollen and by a late-glacial percent max for oak, hornbeam and ash.

Zone A-4: spruce fir zone (9,100-10,200 years ago) decrease in oak,  and max for spruce, fir and larch frequencies. 

Zone B: pine zone (7,900-8,100 years B.P.) high in white pine frequencies, over 50% of pollen is pine, a large change in the trees may indicate change in climate, but much of this area has not been sampled thoroughly. 

Zones C 1,2,&3 oak zones (7,900- present) 

Conclusions

Several things to be noted. 

1. The sudden increase in  pollen deposition rates for trees between 14,000 and 9,000 years ago. This fits with the idea that as tundra receded, it was replaced by woodland, and then forests as trees increased in frequency. 
2. There was a large increase in Oak pollen 11,500 years ago. This could be due to an overall increase in oak abundance or it could be due to a change in the wind or weather. 
3. 8,500-9,500 years ago there was a large influx in pine pollen, as boreal pollen decreased. This signifies a large change in plant species as the forest is becoming more deciduous. This large change could be due to climate change. 
4. C-1a shows an increase of ragweed 7-8,000 years ago, possibly due to a prairie period that was hot and dry (xerothermic or zerothermic?). This could have been overlooked because of the large focus on tree pollen and not herbaceous plants. 
5. The changes in pollen frequency that were thought to represent the same zerothermic period in S. New England were found in younger sediment (2,000-4,500 years BP). Only over the last 2,000 years have the pollen in S. Connecticut been similar to modern assemblages. This is evidence that may suggest that these forests are of very recent origin. 

Qs:

What were some previous studies that could have influenced this study?

Has this paper influenced any recent studies?

What does m Davis mean when she states that these forests might be of recent origin? 

How glad are you that you do not study pollen?

Paper 28: The Influence of Rainfall, Evaporation, and Atmospheric Temperature on Fluctuations in the Size of a Natural Population of Thrips imaginis (Thysanoptera)

Paper 28: The Influence of Rainfall, Evaporation, and Atmospheric Temperature on Fluctuations in the Size of a Natural Population of Thrips imaginis (Thysanoptera)

by J. Davidson and H.G. Andrewartha (1948)

Thrips imaginis

Introduction

Thrips imaginis are a small insect with adults about 1 mm long that are indigenous to southern Australia. They feed on the flowers of weeds most of the time, but also feed on apple blossoms and rose blossoms. The authors noticed that significant fluctuations in their populations had been taking place for the past 40 years prior to this study. These fluctuations were separated by thousands of miles, and so they speculated that the long term fluctuations in weather lead to a growth or decline in the populations based on temperature and soil moisture. They then decided to focus their study on measuring the degree of association between the number of thrips and the components of the weather they considered to be related to the size of the population. They would then analyze this data with partial regressions.

Materials

Over a 14-year period from 1932 - 1946, daily counts of the number of adult thrips in roses in Adelaide were taken. 1944 was excluded, and from 1932 - 1938, a sample of 20 roses was taken daily over the entire year while from 1939 on, only 10 roses were included in the daily sample and the observation time was only from September to December. Samples were not taken on Sundays or holidays. Table 1 displays the number of samplings taken each month for 14 years. 

Method

The total variability of the daily records of thrips may be ascribed to four components:

1. The growth of the population with time within each year.
2. The variation in the growth of the population from one year to another.
3. The variation due to variable “activity” of the insects in seeking out roses, and
4. The residual variability which could be random with respect to the three components selected for this study.

The study mainly focused on the second and third components, one at a time. The authors go on to state that from their experience, thrip populations don’t change by a constant quantity, but by a constant proportion. They then describe why they are choosing to display their data on a logarithmic scale, and then use a partial regression to analyze their data.

Daily Fluctuations in the Number of Thrips in the Roses

It’s stated that in any one year during the time when a population is increasing in size, the variability in the daily counts may be ascribed to:

1. natural increase with time in the number of thrips in the garden
2. variable “activity” of the thrips in seeking out the flowers
3. residual or random variation

They then calculated a curve of predetermined degree for the 60 days which preceded the day when the population attained its maximum and turned them into 3-day intervals - giving 20 groups to calculate the mean number from each. Table 2 displays the extent and direction of the daily variation. 

They then choose to look at the days preceding the sample taking and the day of as their independent variates. They choose to examine the effects of daily maximum air temperature, daily total rainfall, and changes in the barometric pressure. They continue by excluding the effects of barometric pressure and choose to just look at air temperature and daily total rainfall. 

The x variables (X1 - X6) all represent some aspect of the climate for either the previous day or the day before that. They perform regressions for the effects of y (thrips numbers) on all of the X variates for the 8 year data. Ultimately they come to the conclusion that the daily number of thrips per rose increased by 25% for each 10 degree fahrenheit increase in temperature, and decrease about 66% for each 1 inch in total rainfall.

Annual Fluctuations in the Number of Thrips in the Roses

They follow a similar process here, selecting independent and dependent variates and investigating their relationships. The perform many partial regressions to find out what is the main cause of the annual fluctuations in these populations. They mention the germination time of the thrips’ main food source, which is in autumn. Ultimately they find in this section that the maximum density is reached by the population in the early spring. This is largely determined by the weather the preceding autumn, although abnormal early spring weather may modify this.

Annual Fluctuations in the Numbers of Thrips After the Elimination of Variance due to “Activity”

In this section the authors are aiming explain for any variation in their data due to the activity of the insects seeking out the flowers. They calculate a correction factor for each dataset, and then continue to complete partial regressions with their new dependent variates. 

Discussion

The authors present their alternative theory for the density of the thrips population in this section. They found through all of their statistical analyses that the most influential factors on the population of thrips is the variable X2, which is total daily rainfall. They then state many of the factors that also may influence the populations, including the pollen count, at what time the “break” of the season happened, etc. Figure 5 is a hypothetical plot of the control of the population density (density independent) components of the environment. 

They conclude that there are two major facts that they found through this study: 

1. There are four “density-independent” components of the physical environment that account for 78% of the variance within thrips populations. 
2. Rainfall (the variable X2) was the most important component.

They spend a good time at the end discussing the results put forward by Nicholson and Bailey, and how they are completely wrong. Davidson and Andrewartha say competition plays no part in determining the maximum density of a population. The “balance” that keeps a population from getting too large is the race against time to reproduce in what they call the “favorable period”, or a season. They say weather is the most important influence on the density, and that it never decreases to zero because they can’t all die out by the time it’s spring and the population starts to increase again.

What do you think of their conclusion about population density? Would you tend to agree more with Nicholson and Bailey (density dependent), or Davidson and Andrewartha (density independent)?

What do you think of their data collection methods? Although the study was over a long period of time, do you think their methods are sound?

Energy Flow in the Salt Marsh Ecosystem of Georgia by John M. Teal

John Teal is an ecologist from Woods Hole Oceanographic institution in Massachusetts. He is one of the first to analyze energy flow in the salt marsh system. This study is largely dependent on the work of others. I had a hard time finding anything else on him. He is currently still working for the institution and still publishing to this day. 129 publications with over 6,000 citations, according to Research Gate. 

Introduction

Teal calls our attention to all of the previous studies on energy flow and states that, although they are very comprehensive, they have been limited to few natural ecosystems, and lack the “broad” details. Teal decided to study the Georgia salt marshes and apply these broad details to his study. The Georgia salt marshes are the second largest in the US. The marsh is a really interesting site because of its harsh ever-changing environment. Salinity in marsh can vary from values as high as 70% in dry isolated areas to as low as 5% in isolated areas with heavy rains. In accordance with that, there is a large fluctuation in water temperature but Teal does not really talk about temperature all too much. Although these factors can be sporadic, the animals that adapt to this area benefit from a lack of predators and intense competition. The amount of animals and plants that actually spend most of their time in the marsh are few, with most living on the edges, in the tall grasses, or burrowing under the thick mud to seek shelter from the salty conditions. Those that live in and around the marsh are summarized in Table 1.

Food Webs

Methods

Spartina alterniflora is the main grass in the marsh and therefore one of the main producers focused on this study. The herbivores are broken into 2 categories: those that feed directly on living spartina, and those that feed on detritus. 

Spartina grasses and grasshoppers had complete sampling, enough to determine production. The rest of the organisms were estimated from turnover time ( production = one maximum population per turnover period) or assuming the ratio between respiration and production for the group in question is 0.25/0.30 ( it seems this ratio was found in one of his previous studies: Teal 1958).

Primary production 

Spartina  was measured by harvesting  and weighing these plants in monthly intervals; respiration was measured by short term harvesting so 305 kcal/m² yr (insect consumption) needed to be added to arrive at the “true net production.” This respiration value only makes up 19% of gross production- and may fluctuate based on the seasons. The rest comes from the decomposition of the grasses by bacteria and algae, making the grass more edible to the rest of the community.

Decomposition
To find out how spartina is broken down, it was air dried and 10g of it were placed in 500ml flask with 200ml of ocean water, 1ml of marsh mud at 20 C with no light. Sampling occurred at 0,2,7,14,28,56, and 112 days. Oxygen consumption was measured  and the material was analyzed for protein, fat, fiber, ash, nitrogen-free compounds, and caloric content. The results are summarized in figure 3.
Bacteria, along with algae are active in degradation process.  Bacteria alone degrade 2090 kcal/m^2 yr. Average respiration which is 60 mm^3/gr hr multiplied by biomass of spartina gives us 1000 kcal/m^2 yr.  Their activity accounts for 3890kcal/m^2 yr breaking down about 59% of spartina. 

Community Energy Flow

Gross production is 6.1% of incident light energy and most of this is metabolized by the plants. Net production=1.4 % of incident light.

The plants are eaten by the herbivores with not much lag time and they assimilate 4.6% of potential food. 

There is more lag in detritus feeders due to the fact that it must first be broken down by algae and bacteria. This seems harder to quantify and I do not see a figure in the text. see fig 4

Stability 

Stability minimizes disturbances that can lead to extinctions and according to Dunbar 1960, stability in ecosystems has selective value and over time they will develop more stable configurations. According to MacArthur stability is established in one of two ways: by having many species with restricted diets or few species with broad diets. The latter is an example of the salt marsh. The reasons for this could be due to having only one dominant plant species, lacking variety and niche space. The other reason could be that biomass is restricted by tidal currents and most of it is washed away. So for this community the only way to reach stability is for the consumers to develop broad, unrestricted food habits. 

Findings

55% of net production is utilized by marsh community with producers consuming the most energy, bacteria degrade only 1/7 as much energy as producers, and consumers degrade 1/7 as much as the bacteria. 

Another finding is that Spatina respires over 70% of its energy, which is a huge loss in energy. Likewise the bacteria consumed and respired 45% of net primary production. Last, 45% of the net production was unaccounted for. This 45% is thought to be lost through tidal currents and washed up in estuaries where more species persist possibly due to more stability? (this was not studied in paper) 

Qs

In the paper Teal talks about respiration of Spartina, why is this over 70%? It seems like a huge waste of energy, but I do not know much about plants so I could be wrong. In the texts it briefly states that this may be due to the large amounts of salt, but it does not touch on any mechanisms or other possibilities. 

Does the Georgia salt marsh seem like a stable system to you?

Did Teal have enough data to support all of his claims? A lot of it seems to be extrapolated. 

How well does this paper stand to todays standards? 

Do you think there were species that he could have overlooked, especially microorganisms in the soil?

Was respiration the best way to test for overall production? Does this method have any faulty assumptions or leave anything important out? 

I don’t really understand figure 3, why does protein increase as fat drops? 

Has this study been updated to include carnivorous birds and insects? 

Intermediate Disturbance Hypothesis Debate

Following our discussion in class today, I found some interesting back-and-forth in the literature on the Intermediate Disturbance Hypothesis that some of you may find neat (lit debates are always fun!):

The intermediate disturbance hypothesis should be abandoned (Fox 2013 TREE 28:86-92)

Defining and defending Connell’s intermediate disturbance hypothesis: a response to Fox (Sheil & Burslem 2013 TREE 28:571-2)

The intermediate disturbance hypothesis is broadly defined, substantive issues are key: a reply to Sheil and Burslem (Fox 2013 TREE 28:572-3)

Disturbance, productivity, and species diversity: empiricism vs. logic in ecological theory (Huston 2014 Ecology 95:2382-96)

My general takeaway after skimming these is that the more simplistic interpretations of the IDH do not have a lot of support, but that interpreted with more nuance and in the right context, the IDH could be a useful part of a broader theory.

Incidentally, Jeremy Fox, who calls the IDH a "zombie idea," has a great blog, Dynamic Ecology, which is where I first hear about the IDH. Check out this post on the most cited ecology papers of the 70s, 80s, and 90s. Apparently Ric Charnov is a reader.