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
- 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.
- 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.
- 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
Prior to this paper there had certainly been plenty of observational work affirming the principal of competitive exclusion, but this seems like to be one of the first experiments save for Gause to experimentally demonstrate this. The predictability of the winner of competition was also notable in that certain conditions often promoted a particular winner, but this was not a certainty. To me this reflects what you often see in nature with inherent unpredictability but general trends being common. The importance of abiotic factors that Park suggests is notable in that most modern work in this area seems to focus on how abiotic factors (or other biotics) affect interspecific competition.
ReplyDeleteI can see why this paper is thought to be influential and was included in our book. I agree with most of what Eric mentioned above, and found it especially interesting that while one species obviously wins out over time, it is not always the same one. Along the same lines, I was amazed at the effect that parasites had on the populations. This again exemplifies coevolution and how apparent it is in many of the systems we study. Within this paper, I thought it was so cool that the author was proving the concept of competitive exclusion, something that doesn't seem to have been explicit in other papers we've read. I'd be more interested to address the second question posed by Carlos above - how do we model and predict these competitive exclusions?
ReplyDeleteGoing off Ali's question- this was what I was trying to ask when we were discussing the multi-variate analysis; whether we could model the existence of multiple organisms in a community based on their assumed niche. I have absolutely no idea, but I find the potential to do so fascinating.
ReplyDeleteWhile I feel like this paper was a great example of testing interspecific competition, I always tend to feel like the more basic density-dependent experiments have room for improvement, or at the very least room to explore more interactions. For example, how would this interaction change if a predator were introduced to the experiment, or perhaps a different parasite?
I agree this paper looked had a good look at interspecific competition. One species is always out-competed, however not always the same species. The fact that T. castaneum tends to successfully exclude T. confusum more often when the prostist parasite is absent, and the opposite true for the presence of the parasite show distinct adaptive differences between the two species. However these trends were not always true, showing more is in the works then a simple explanation of X out-competes Y implying more factors at work then the experiment could control, or perhaps influenced by the inherent chaos found in these systems.
ReplyDeleteI feel like this example is really showcasing the niche again. Things don't have the exact same niche and thus when there are multiple other factors, one might be more in its niche than the other. T. confusum is better able to survive with the parasite, leading to this idea that its niche is maybe wider in that aspect? I am not sure but it is interesting to read about.
DeleteI always find parasitism really fascinating, especially how extensively they can alter populations. Parks ideas of competition- supported by his lab experiments- fall in line with many of todays ideas of what competition is and the result of it. He did a good job finding controls and other exp. factors that might convolute the results. To go with what Kat questioned, I would think that a predator introduction would make the data more messy, simpler is always easier to produce good data. WIth the addition of a new predator or parasite many more questions come into play, like is one easier to prey on than the other? is the predator only after the designed prey, or could they be more of a generalist. I think this would be a very cool follow up experiment.
ReplyDeleteI was impressed by how carefully Park took the parasite Adelina into account in his experimental system, and that he was able to demonstrate that even though T. casteneum reproduces much more rapidly than T. confusum, the presence of the parasite depressed casteneum enough that T. confusum was able to outcompete it to extinction. I loved Park's spirit of inquiry on page 768, where he lists an entire paragraph of questions about Adelina that are beyond the scope of the current investigation.
ReplyDeletePark's paper is fairly long-winded, but also interesting. He spent a lot of time discussing the dynamics of the control populations, which even though it was somewhat tangential to the main point about competition, was also instructive in just characterizing single-species populations. The big takeaway I got was that competition can be mediated by tradeoffs between inherent reproductive rate (r) and susceptibility to parasitism (or predation). As others have mentioned, I appreciated the emphasis on the stochasticity of which species "wins." I think this is one of the things that makes ecology cool - outcomes are not deterministic but probabilistic, and sometimes random events can tip things one way or another, with or against the odds. The result of this experiment that one population or the other always goes extinct highlights the seeming improbability of coexistance of similar species, and begs the question of what additional mechanisms might be necessary to maintain coexistence. One things I kept trying to figure out was why Park used different volumes of media and initial population sizes. I was thinking smaller populations might have relatively more variability in outcome, but there didn't seem to be much difference.
ReplyDeleteA long winded paper but provided good insight into the previously, nearly exclusively observational accounts of competitive exclusion. Dunbar points out one of the conclusions; that one population or the other always goes extinct. While this is definitely true in a closed experimental system with one niche to exploit, in the wild, wouldn't this sort of competition result in adaptation to exploit other resources, thereby reduce competition and result in on occasion result in sympatric species? I have to admit, I'm not a huge fan of lab experiments. While they often provide good insight they generally seem too simplistic to be applicable in real world situations.
ReplyDeleteTo address Kat's question regarding modeling animal communities using multivariate methods. It is possible and it is fairly labor intensive. First, it will be necessary to collect data for every aspect of spatial and temporal habitat use for each species involved as well as phylogenetic and morphological data. The habitat data can then be run through some multivariate ordination (i.e., a PCA) and the spread of the data can be visualized. Moreover, the data can be assigned, a priori, to phylogenetic clades or morphotypes, which may be more informative. Through interpretation of the blobs in the ordination and hypothesis testing of the vectors you could infer some of the important factors in the segregation of the species in the community and you may even be able to make some predictions regarding other communities composed of closely related taxa or similar morphotypes. I'm not a math person so I may be overreaching here. What say you Noah?
I agree that the paper was long winded. I don't have much to add, but I do echo what others are saying and wonder if how the experiment would pan out in the natural environment when other species (both predator and prey) are in play.
ReplyDelete