Nicholson and Bailey

            The Balance of Animal Populations was published in The Proceedings of the Zoological Society in 1935. Bailey was an Australian physicist while Nicholson was an Australian entomologist. Much of this paper draws on earlier mathematical work done by Voltera and others working on early population modeling. Bailey and Nicholson begin by summarizing what one can assume was all the population modeling work published and available to them. They note that many earlier models are greatly simplified, or lack biological applicability. This pair of researchers works to bring mathematical modeling to ecology by progressively complicating an initially simple biological system: that of the parasite-host relationship.

            Section one defines the fundamentals of the investigation:

1. to exist and reproduce animals must obtain food, mates, and shelter

2. the searching of animal populations is always random

3. the probability of making random contacts can be expressed mathematically and is independent of individual search efficiency

4. competition between searching animals produces balance and interspecific oscillation

5. search efficiency depends on many factors

6. any given animal has an area of discovery for an object it seeks. The value of that area is influenced by general environmental conditions

7. with greater population density of species x, species x spends more time searching over previously searched areas.

            The investigators go on to note that their work will be limited to the search for food and shelter as they believe animals have little trouble finding mates unless in a very small population. The authors also note that certain individuals are likely to be better searchers, which leads to a damping of interspecific oscillation. The entomophagous parasite is used as a model as it hunts for food (host) required by offspring.  Hence the offspring are born in ideal conditions. The number of eggs laid is dependent on host and parasite, but in general one can assume that a host-parasite encounter will result in parasite offspring.

            Section two looks at the interaction of one host and one parasite species. Based on the assumptions above and an assumption of unlimited eggs the authors posit that there is a condition where the number of parasites and hosts are at a steady-state where the densities of the interacting animals do not change greatly. Complications of this scenario follow. This state implies that a particularly efficient parasite species can only be supported at a low density. It also follows that high birth rate does not necessarily cause a species to be abundant, but rather tends to cause the species to be scarce in the adult state. The next situation described involves when a parasite requires more than one host individual to develop. The outcome is a host density that varies inversely as the number of parasites increases. From this it is also derived that the efficiency of a parasite in controlling its host is increased when a greater number of parasites develop from one host. The third scenario describes when a host attacked can be attacked more than once. In summary this should lead to a situation where the steady density of the host must be higher than the simple situation. If the parasite species is unable to attack all previously un-attacked hosts, this situation implies egg limited parasites (which the author implies is not common) or defensive capabilities of the host population, which allows its density to grow greater than the simple prediction. The next scenario includes when hosts are limited by other factors besides parasites. In any case this will decrease parasite density and the host effect is conditional on the extrinsic factors timing pre or post parasite attack.  If the lifecycle of the parasite is longer than the hosts’ vulnerability period then the density of the hosts should be lower than expected. Lastly, if the vulnerability of the host and effective period of  the parasite do not overlap completely will cause both host and parasite density to rise.

            The third section of the paper deals with situations when several different species of parasite attack a common host. This situation is said to have no steady state as a steady state only applies when a parasite is operating under a set of conditions it creates. Accounting for the affect of so many variables operating independently at the same time is too complex to predict a steady state. This situation does increase the area of search-ability for a parasite if it can attack multiple hosts.

            Section four involves interspecific oscillations, which apply to a more natural system that might be perturbed by outside factors to allow either the host or parasite to stray from its steady state density. In general it is found that since hosts are the “food” for parasites, the parasite density should trail host oscillation by roughly a quarter “period.” Any perturbation from steady density is attributed to a interspecific causes not external factors and thus the oscillation should always return to the steady state. In a situation where a parasite has a hyper-parasite, the hyper-parasite is predicted to attack when host population density is at a minimum to influence the oscillation to pull it back up by reducing parasite load. The effects of increasing oscillation are predicted to be a fragmentation of host population that would be lower than predicted density. The oscillation may also be due to external factors beyond parasite control.

            Section five involves continuous interaction of host and parasite which is predicted to be greatly age distribution limited producing oscillations around this characteristic in both populations increasing in magnitude over time.

            In general the step-up approach applied in this paper makes it easier to understand what are otherwise fairly complicated models by end. I would hope that other commentators might provide some insight on the applicability of this to the field of parasitology and other infection systems. It seems that for its time this is a very complicated model applied to an ecological system.

Grinnell (1917) – The Niche-Relationship of the California Thrasher

Grinnell’s essay is focused primarily on observations of the natural history of the California thrasher, but in so doing he offers some astute insight into the determinants of species’ habitats and ranges. He starts by describing the range of the California thrasher, which is strictly limited to a particular life-zone (the Upper Sonoran division of the Austral zone), especially on the lower elevation edge of such habitat. Grinnell alludes to competitive exclusion when pointing out that the California thrasher avoids habitat that might otherwise be suitable in places where a similar species, Leconte’s thrasher, is prevalent. He states that the California thrasher’s range shows that it is restricted primarily by temperature, and secondarily by humidity (“faunal” restriction), with three sub-species found in different humidity zones. Grinnell goes on to describe the California thrasher’s habitat requirements and behavior in some detail – it’s shy and omnivorous, eating insects, berries, and seeds at or near ground level in chaparral with adequate cover, though open at ground-level. Several observations connecting the species’ form and function are made (e.g. curved beak for searching for insects beneath litter). Grinnell concludes by stating that the habitat requirements, temperament, and physical structure of the California thrasher demonstrate the niche it occupies among fauna in the region, and that no two species in a community have precisely the same niche relationship.
I have to say that while I found this piece to be a good read and a nice illustration of the basic idea of a niche, I am somewhat surprised by it’s selection as a foundational paper, given that these ideas had been previously developed (by Elton), and were more rigorously elaborated on soon thereafter (by Gause; see Kinglsand pgs. 6-9). I would’ve expected something explaining the niche concept in more depth and in a more generalized way.
Elton defined the niche in terms of an animal’s economic role, or place in the food chain (i.e. subdivision of carnivore, herbivore, insectivore etc. according to Kingsland). What is the difference between Elton’s and Grinnell’s niche concept (i.e. habitat), and what did Gause add? What about our current notion of the niche? How does the idea of the niche connect to related concepts like community assembly, functional diversity, and trophic ecology?

Reading for Thursday, September 3rd: Paper 7

Raymond L. Lindeman 1942

The Trophic-Dynamic Aspect of Ecology

In this work Lindeman studies the food-cycle relationships of aquatic systems and tries to describe the dynamics of trophic levels as the ecosystem passes through the phases of community succession. He starts off by discussing the term and concept of ecosystem and its importance or relevance in ecology in understanding the processes of nature.  Lindeman makes the point that ecology is dynamic and it is arbitrary and unnatural to create distinct lines that distinguish the living biotic community from the non-living environment because doing so forces a biological emphasis on basic functional organization like the organic-inorganic cycle of nutritive substance that is so completely integrated that biotic and abiotic components are not clearly separate. This idea emphasizes the term and concept of ecosystem as described by Arthur Tansley in 1935 where the whole system of biotic and environmental factors be considered together rather than separately.

Trophic Dynamics

Lindeman defines the basic process of trophic dynamics as the transfer of energy from one part of the ecosystem to another.  It begins with the assimilation of sun energy into the structures of living organisms via photosynthesis (primary producers). Primary consumers then feed directly on plant material while secondary consumers feed on primary consumers, etc. Saprophytes feeding on dead material, particularly specialized decomposers, transform organic substances into an inorganic state making nutrients available for autotrophic plants and thus completing the cycle. Lindeman concerns himself with the quantitative measurement of productivity and efficiency for each trophic level. Productivity is defined as the rate of production which is typically measured in terms of annual yield, a value which must then be corrected for respiration, predation, and post-mortem decomposition.  Next Lindeman addresses the issue of biological efficiency – trophic level efficiency with respect to lower levels. As a result he identifies two fundamental trophic principles:

1.       Percentage loss of energy due to respiration is progressively greater for higher levels in the food cycle because they expend more energy to acquire their food.

2.       Consumers at progressively higher levels in the food cycle are progressively more efficient in the use of their food supply because the increased activity of predators considerably increases the chances of encountering suitable prey.

In conclusion, the increased respiration and efficiency with each successive level of predator appears to be important in restricting the number of trophic levels in a food cycle as pointed out by Elton 1927 that food cycles rarely have more than 5 trophic levels.

Eltonian Pyramid

The Eltonian pyramid describes the general relationships of higher trophic levels to one another and to community structure where the number and size of animals in an ecosystem follow a pattern. The animals at the base of the food chain are relatively abundant while those toward the peak are progressively fewer in number. This is explained by the fact that abundances of organisms in lower trophic levels must be adequate to support predation by the next level up and still maintain existence.  

 Trophic-Dynamics in Succession

Lindeman tries to describe how trophic level interactions, specifically productivity, change over the course of hydrarch succession (Figure 3.) He mentions that typically there is a high rate of productivity at first which then slows over time until a climax state is reached. He also recognizes that there may be stages of succession that remain unchanged for long periods of time representing a pseudo climax or equilibrium state. Lastly, progressive efficiencies of consumer levels seem to increase throughout the aquatic phases of succession but this is an areas that deserves more study.

Questions

1.       Is productivity for a given trophic level still quantified in the same way today? Do we still try to quantify efficiency?

2.       Do both trophic principles as presented by Lindeman still hold today?  

3.       What kind of trophic dynamics have been observed in terrestrial ecosystem succession with respect to productivity and efficiency? 

Clements & Gleason - Two contrasting views of vegetation associations

Clements 1936 – Nature and Structure of the Climax

Introduction 

        Clements published this foundational paper in the Journal of Ecology in 1936. From Clements position at the Carnegie institute he was one of the leading botanical-ecological researchers of his time. He had publications examining plant community ecology, plant development, and several on succession and climax. His previous work had taken him across North America to examine different plant communities of all the major biomes. The Nature and Structure of Climax corroborates all of Clements observations into one foundational paper addressing the major factors and vocabulary involved in defining and understanding the climax state. As defined by Clements, “the climax constitutes the major unit of vegetation and as such forms the basis for the natural classification of plant communities.” He goes on to point out the intimate relation of climax and climate as the overriding thesis of the bodywork.

Defining Climax 

        The climax did not come to be rather it evolved out of a preceding climax state just as species arise. Much as a species is defined by a common trait a climax is defined by common species or even close relatives. Hence comparing dominant species can test a climax. Clements expands on his climax definition by adding that animals must also be considered in the climax and thus when referring to both plants and animals climax and biome are synonymous. However when referring to plants of an area alone he proposes emphasizing climax due to the more intimate relations of plants and climate versus more transient animals that have some ability to modify or escape the effects of climate. Climax is a circular state for plants one where they both respond to and indicate climate. Clements proposes that plants, by nature of their sessile state, directly and fully integrate physical factors through their growth forms. This utility of the plant climax is illustrated by comparing the different and similar dominant plant forms of the prairie as latitude and longitude change emphasizing the associated temperature and precipitation changes. Stability seems to be the hallmark of climax, yet Clements disputes this noting that in the absence of human intervention change is constantly at work within a climax and is neither destructive nor beneficial to it.

Layers of Climax

        The climax comes to be through a series of states intervening to reach a state most well fit to a particular climate. Any of these states may seem to be a climax on the human time scale.  The proclimax is defined as any state resembling the climax that is gradually replaced by the climax in the absence of disturbance. The proclimax is defined by four types: The subclimax precedes the climax, a notable example being Aspen in the Rocky Mountains preceding the coniferous climax state. The disclimax is the community resulting from the disturbance or replacement of the true climax brought about by human intervention and/or mass migration. The encroachment of Salsola on western range and cropland as a result of overgrazing is a notable example of disclimax. Preclimax may be best defined as a position similar to sub climax but as a result of unusual edaphic factors such as valleys and canyons providing extra water or on the margins of two adjacent climaxes. Such edaphic factors buffer the area from the climate that pushed the surroundings to full climax. The postclimax is similar to the preclimax but differs in that these areas are where “new” climate change is buffered due to unique edaphic factors that have kept the old climax from transitioning to the new.

Community of climax

        The major functions of climax serve to select and group organisms, while the minor functions affect abundance and visibility. The most abundant species is known as the dominant. A perdominant may be a type that is found throughout many similar climax types such as the shrub of chaparral and desert. The eudominant form refers to a dominant unique to its particular association. Subdominants are life forms subject to control by the dominant. The term influent applies to animal members of the biome that exert an influence back onto the community, similar per, eu, and sub “fluents” apply as above. Within a climax there are lower divisions meant to more specifically address unique local or regional characteristics that modify the climax form. In descending order of hierarchy these are at a community level: association, consociation, faciation, location, and at a society level, sociation, lamination, and clan. These organizational units run from regional (association) to layers of the forest floor (lamination).

Conclusion

        Clements presents his current knowledge of the organization of the biological word in this treaty. Though his view is almost wholly plant centric, it makes many generalizations that apply to a more well-rounded view of the biological world including microbes and animals. The plant-centric nature of his piece is not without justification, but neglects the key role of microbes in facilitating plant-abiotic interactions.  Nevertheless Clements defines and illustrates many traits of succession and biomes that hold true today and served as a catalyst for research further refining his proposals.

Gleason 1926 – The Individualistic Concept of the Plant Association

        Gleason begins by pointing out the high degree of disagreement among plant ecologists in how to classify vegetation associations, and suggests there may be something fundamentally wrong with a classification system that tries to pigeon-hole every community into a strictly defined type. While acknowledging that plant associations are real and a useful concept, he argues that the rules invented to define them may be overgeneralizations and that the amount of variation within plant associations indicates that plant communities might be better understood as loose associations changing continuously in space and time, rather than discrete, precisely-defined units. He emphasizes that in addition to plant-plant relations, the physical environment (climate, soils, physiography) is quite important in determining the composition of communities. To support these argument, Gleason cites a number of examples, including transition zones with continuous change in vegetation (i.e. ecotones), inter-annual variation in species’ relative abundance (i.e. non-equilibrium), associations that are fragmentary in space and time, the effect of broad-scale environmental gradients on plant associations, similar associations occupying different environments, and distinct associations in the same environment.

 Gleason goes on to explain his alternative theory of how the sorts of vegetation assemblages we observe could come to be through his individualistic concept of community assembly. According to this view, the abiotic environment and a species’ physiological tolerances are paramount in determining the presence and abundance of plants, with biotic effects from other vegetation mattering secondarily and only in so much as they modify the physical environment favorably or unfavorably. Since species tend to have slightly different physiological limits, their relative abundance will vary idiosyncratically in space and time. Equally important, according to Gleason, are the processes of dispersal and migration, including the proximity of seed-producing parent plants to potential habitat, species’ typical dispersal mode and distances, chance long-distance dispersal events, and incumbent effects giving an advantage to individuals who arrive in a habitat first. Gleason emphasizes in particular, with several examples, how “various accidents of dispersal” can largely determine community composition in many cases. He is careful to acknowledge that associations and succession are real phenomenon, that there is environmental sorting, but argues that it is not as predictable or deterministic as others have assumed. Gleason finishes by dismissing the “usual” (i.e. Clementsian) concept of vegetation units resembling species or organisms, stating instead the “each species of plant is a law unto itself,” with its distribution determined by migration and environmental selection, and that resemblances between communities are due to similarities in these underlying factors, not some intrinsic affiliation of groups of species for one another. 

Gleason articulates quite well a number of concepts that have been further developed and supported in more recent ecological research, including the competing role of niche and neutral processes in community assembly (i.e. environmental sorting and chance dispersal, in his terms). Are there other concepts used by Gleason that are still active areas of research in ecology today? What are some shortcomings in Gleason’s ecological worldview?

Gleason uses the example of dune succession to illustrate the role of chance dispersal events (pgs. 19-20). How does this story compare to that told by Cowles? What parts of The Individualistic Concept would Cowles likely agree or disagree with?

Gleason makes some strong arguments against the Clementsian view of plant associations. Are there areas where you think Gleason was not entirely convincing? Where is the more traditional notion of plant associations still useful, or at least more useful than the individualistic concept?

Overall questions to consider for Clements and Gleason

        What sorts of evidence do Clements and Gleason each use to make the case for their concepts of vegetation associations? What are the strengths and weaknesses of their respective positions? Is one more convincing? Why?

        To what extent, and in what contexts, do the Clementsian and Gleasonian views of vegetation association still hold today? Has one or the other won out? Where were they each right and/or wrong?

Note: Please don't feel bound by the summaries, discussion points, and questions written here. We'd be interested in hearing your own questions, thoughts, and observations that we may have missed.

Forbes & Cowles - Ecology emerging in the late 19th Century

Forbes 1887 – The Lake as a Microcosm

Forbes begins with an introduction in which he makes some general observations about lakes and the animal life found within them. The notion of the sensibility of the complex of life in such environments is emphasized, by which Forbes seems to mean the interaction and interdependence of all the organisms in the local ecosystem, suggesting it is impossible to really understand any individual species without some concept of its place in the community as a whole. He states his purpose of examining the lacustrine life of northern Illinois within the context of this idea, based on his and colleagues’ observations and sampling of watershed lakes in the uplands of northern Illinois. Differences between watershed and fluviatile lakes are described. The latter are characterized by seasonal overflows and fluctuating conditions, with effects cascading through trophic levels and resulting in oscillating population cycles. Forbes considers this one of the best examples of the how the system of organic life can adapt.

Forbes then gives some contextual description for the seven more isolated watershed lakes that are the primary focus of his paper, including a brief mention of the methods of observation used. He goes on to describe the animal life found in these lakes, including the fishes, molluscs, arthropods, and crustaceans (esp. Entomostraca), remarking on their relative abundance and diversity, the habitats within the lakes different life forms occupy, and how these compare to other lakes in the region and in Europe. Forbes then gives several examples of the organic relations among the lacustrine fauna, including the common black bass and all its food prey, competitors, and predators, as well as the bladderwort – a carnivorous plant which apparently competes with fish by feeding upon many of the same crustaceans and insects. These relations are then placed in the evolutionary context of all the species’ struggle for existence against one another and the severe conditions this creates. But in spite of this, Forbes emphasizes that natural selection in fact promotes a community of interest even among species with seemingly antagonistic relationships due the interdependence and food relationships in the community. He concludes by remarking on the apparent contradiction that exist in natural systems, where organisms that are seemingly hostile or indifferent to one another produce cruel conditions, but conditions that nonetheless appear to promote life and a balanced harmonious equilibrium.

Kingsland’s introduction describes Forbes as being influenced by both Darwinian evolutionary biology, and by the Victorian natural history idea that natural systems tends to be stable and self-regulating (articulated by Spencer). The Lake as a Microcosm provides a good synthesis of these ideas, as well an early articulation of the concepts of a biological community and of food webs. I was surprised at the sophistication of the thinking Forbes exhibited in an era when ecology was not yet a formal field of study. Though his methods were different from what would be considered sufficient today - largely qualitative – I found it challenging to identify misconceptions in Forbes’ thinking. Examples could include the notion that aquatic ecosystems are completely isolated and oblivious to their terrestrial counterparts, as well as an overemphasis on equilibrium – population/ecosystem/community dynamics have become major themes in ecology. Here are some additional thoughts and questions to consider:

·         Are there other misconceptions you noticed in The Lake as a Microcosm?

·         Both traditional natural history and evolutionary biology seem to play key roles in the origins of ecology. How does ecology, and Forbes in particular, depart from these earlier fields? In what ways does Forbes’ thinking seem more like natural history, rather than ecology? Is there a place for natural history in ecology today?

·         What are Forbes’ methods and data? Is he justified in making the conclusions he does based on this?

·         Kingsland describes Forbes’ motivations as being both practical (build scientific basis for agriculture) and basic (use theoretical framework of evolutionary biology; pg. 3). What do you think was the stronger motivation in The Lake as a Microcosm? How about in ecology as a whole, both in its early days and today?

Cowles 1899 - The Ecological Relations of the Vegetation on the Sand Dunes of Lake Michigan

Cowles starts by expressing the scientific value of studying plant communities in sand dunes and why sand dunes are an ideal place to learn about ecology. “The sand dunes furnish a favorable region for the pursuit of ecological investigations because of the comparative absence of the perplexing problems arising from previous vegetation.” Essentially the formation of dunes wipes out all pre-existing traces of vegetation thereby creating a clean slate and forcing plants to establish from existing physical conditions.

This papers describes the environment of the sand dunes of Lake Michigan and the ecology of the flora that exist in this habitat. Cowles aims to understand and explain the dynamics of the sand dune phenomena and “uncover the laws which govern the panoramic changes.”  He refers to the sand dune environment as the “dune-complex” where it is a moving landscape – a restless maze always changing. This restless maze advances as a whole in one direction (the direction of prevailing winds) but with individual portions advancing in all directions (small dunes may advance in any direction provided they are protected from winds blowing in other directions).

The main ecological factors influencing floral establishment and persistence:

·         Temperature: In the summers the dunes are hot and sunny where warming begins in early spring and cooling begins in early fall followed by periods of very cold temperatures and severe winds favoring arctic floras.

·        Wind: Wind is the most potent of all factors in determining the character of dune vegetation. The windward sides of the vegetation endure intense sand blasts and are often damaged. Wind is the chief destroyer of plant societies by blowing away soil from underneath exposing roots or by burying vegetation that exists in depressions or low-lying areas with the advancement of dunes impelled by the winds.

·        Soil: The soil is mostly comprised of Quartz sand and is extremely porous lacking nutrition and organic matter.

·        Water:  Overall sand dunes are a very dry environment especially for short root systems and periods of drought favor xerophytic floras.

·        Topography: Sand dune slopes are driest with southern exposures producing more xerophytic flora. Depressions between sand dunes are much wetter and offer the opportunity for humus to accumulate resulting in the presence of different floral species. 

·        Plants and organic matter: Humus can build producing great soil but can also be covered or buried too deeply to be utilized until it is uncovered again someday.

The scanty flora is not due to the lack of water in the soil but to the instability of the soil and to the xerophytic air.

Cowles spends some time discussing plant societies and ecological succession which he terms genetic succession – “….arrange plant societies in order of development - this order more faithfully expresses genetic relationships than any other.” In the dune-complex the beach is the most extreme environment with conditions for plant life becoming less and less severe as they pass through several stages before culminating in the normal climax type of the lake region which is the more genial deciduous mesophytic forest.

Also present in the dynamics of the dune-complex is what Cowles terms Rejuvenation. Rejuvenation is when an established dune has its vegetation destroyed and passes into a state of activity (succession). Rejuvenation most commonly takes place as a result of a wind sweep and the majority of rejuvenated dunes are developed from established coniferous dunes that exist in the windward slopes near the lake. Plants do not succeed in stopping the movement of a dune, only a decrease in wind energy resulting from increased distance from the lake or to barriers can do so. 

In conclusion the sand dune environment is not only variable but reaches more than one extreme – water, temperature, and wind resulting in a dynamic succession of plant communities over relatively short periods of time with various plant species that are each adapted to a particular extreme. 

Questions:

·        What ecological principles are being put to the test, or is Cowles hoping to test, by studying the vegetation of sand dunes?

·        What are the main lessons learned or the main contributions that Cowles made to the discipline of ecology with this study?

·        Do you agree that genetic succession is most faithfully expressed by the order of development of plant societies? Is the term genetic succession still used and if so in what context?  Are genetic and ecological succession synonymous?

Overall questions to consider:

                  What are some similarities and differences between Forbes 1887 and Cowles 1899? How does Cowles appear to build on Forbes ideas, and what new concepts does he bring in? Collectively, what do these two pieces say about the emerging field of ecology in the late 19^th Century?

Note: We will discuss these two pieces on Tuesday. Clements and Gleason will be discussed Thursday, with summaries and questions coming in a separate post.

"Warm-up" papers on advice for graduate students

Some Modest Advice for Graduate Students by S.C. Stearns

Reply to Stearns: Some Acynical Advice for Graduate Students by R.B. Huey

Welcome

Fall 2015 has begun. And, indeed, this is the site for the UNM Grad Core Ecology class.