UNM Grad Ecology

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?