The Various Impacts of Henry S. Horn and Robert H. Mac Arthur’s paper,
“Competition among Fugitive Species in a Harlequin Environment” on the Field
of Ecology
Biol 112
Walker Swain
9/17/05
Literature analysis paper
The field of Ecology, like all other evolving sciences, is shaped by the
publication of key influential breakthroughs in experiments, research, and analysis.
These seminal publications change the way scientists in the field think about a
particular issue and inspire further experimentation, research and analysis to challenge,
support, extend, or collapse conclusions asserted therein. Robert MacArthur was
involved in the publication of many such influential papers, particularly in the area of
population ecology. In 1972, not long before his death, MacArthur with accomplished
mathematical modeler Henry S. Horn published the classic paper “Competition among
fugitive species in a harlequin environment.” The paper examines the qualitative
behavior of differential equations in describing the potential for stable coexistence or
competitive exclusion based on rates of migration and local extinction. This paper will
provide a brief synopsis of Horn and MacArthur’s “Competition among fugitive species
in a harlequin environment,” and attempt to situate the paper historically and trace the
impact of its original concepts through the history of ecology.
Prior to the publication of Horn and MacArthur’s “Competition among fugitive
species in a harlequin environment,” students of fugitive species, like Hutchinson in
1965, argued that two species, even when depending upon the same resource, “can
coexist stably under certain conditions, when local extinction and dispersal are
important” (Horn & MacArthur, 1974). Noting similarities between Lotka-Volterra
equations for interaction of two species and a stochastic model for this interaction in a
patchwork of similar habitats, the authors developed models to translate rates of
migration among patches into the parameters of Lotka-Volterra Equations (Cohen,
1970). The purpose of Cohen’s development was to illuminate host parasite
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interactions, particularly human infections, but this also applied to species distribution
in discrete, patchy, similar environments (Cohen, 1970).
Horn & MacArthur’s paper examines the qualitative behavior of these types of
differential equations, describing proportions of insular patches occupied by each of
two species in two similar types of habitats within a patchy, “harlequin” environment.
They allow Species 1 to reach its equilibrium point in the absence of species 2. They
then examine when the rates of increase for species 2 are still positive even when the
proportion of patches occupied by 2 is near zero to determine whether a given species
can invade a habitat already saturated by another species. Horn and Mac Arthur find
that the invasive capacity and potential persistence of species 2 is dependant on the
conditions of certain ratios of migration and extinction. If confined to one habitat, a
species can invade only when rates of colonization of unoccupied patches within the
habitat are greater than the sum of the rate of local extinction independent of local
competitors and percentage of co-inhabited patches where species 1 out-competes
species 2. However when migration rates are augmented by a substantial amount of
immigration of species 1 from a second habitat, the species could persist in this mixed
environment (Horn & Mac Arthur, 1974?).
The focus of the interpretations is on the “most biologically interesting results,”
where species 2 invariably out-competes species one in habitat 2 and species 1 always
bests species 2 in habitat 1. In this situation sufficiently high migration among patches
of habitat 1 allows species 2 to persist even when confined to a habitat where it is
competitively inferiority. Conversely there are levels of inter-patch migration that allow
for the competitive exclusion of species 2 from the whole in an open mixture of habitats
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1 and 2, even though each species has a habitat where it is competitively superior. The
results of these analyses are applicable to species living in successional and ephemeral
habitats, and suggest qualitative limits to the amount of subdivision of resources among
species (Horn & Mac Arthur, 1974).
In the years following its publication, several retrospectives gave opinions as to
the importance of particular original concepts within the paper. Fretwell gives import
to Horn & Mac Arthur’s showing that “if there were a trade-off between competitive
ability and colonizing ability, a so-called harlequin environment could be stably
subdivided by two similar species, one sedentary and one mobile,” noting that “In time,
the sedentary species replaces the mobile, in a given habitat" (Fretwell, 1975). A later
article by Horn addresses the implications of his and Mac Arthur’s findings on the “The
Ecology of Secondary Succession,” and their implications for further research. Horn
explains that he and Mac Arthur found that competition for unoccupied patches favors
increasingly higher levels of migration between the patches. This increased rate of
migration in turn reduces the number of unoccupied patches bringing more species in
direct competition with one another. Horn argues that these greater migration rates
“tend to smooth out the patchiness of early successional communities and to produce
competitive limits” to further increases in diversity. Horn also suggests that an
empirical study on the limits to diversity in fugitive species would be beneficial in
evidencing the projections of their work (Horn, 1974).
Recent literature has repeatedly built upon and challenged hypotheses and
concepts that were laid out by Horn and Mac Arthur. One such paper, by J.S. Clark and
Jason McLachlan (2003) focuses on an alternative hypothesis for forces behind
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stability, in opposition to that articulated by Horn and Mac Arthur. The article attributes
to Horn and Mac Arthur the hypothesis that stabilizing mechanisms, such as tradeoffs
in species ability to disperse to sites where competition is low versus ability to compete
for scarce resources, lead to stability in different times and places as different
competitors thrive. Alternatively, Clark and McLachlan focus on the implications of
what they call the “neutral model,” which suggests that “stabilizing mechanisms may
be superfluous.” This hypothesis emphasizes ‘equalizing’ mechanisms that occur due to
the slowness of competitive exclusion of similar species. The lack of ecologically
relevant differences in these similar species allows abundances to “experience random
‘neutral drift’, with slow extinction”(Clark & McLachlan, 2003).
One current paper that builds on the ideas laid out by Horn and MacArthur is
Loreau and Mouquet’s 1999 Immigration and the Maintenance of Local Species
Diversity. Loreau and Marquet develop a model that incorporates aspects of previous
models, “built after classical metapopulation models that have been used to link
regional and local processes and to study interactions between species that compete for
space” (Loreau & Mouquet, 1999). This competition portion of the model builds
directly on the model Horn and MacArthur laid out in 1972. Their model, however,
operates under the assumption that competition for space is indirect, as in classical
exploitation competition, in contrast to most previous metapopulation models, like that
of Horn and Mac Arthur, which have, implicitly or explicitly, considered direct
interference interactions (Loreau & Mouquet, 1999).
Cited by over one hundred papers, Horn and Mac Arthur’s “Competition among
fugitive species in a harlequin environment” has been applied to questions about plant
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competition, landscape change, biodiversity maintenance, and parasite management.
These papers along with those cited in this paper provide evidence of the broad impact
of this seemingly specialized paper. In 1972, Horn and Mac Arthur contributed greatly
to the field of ecology without stepping foot into the field to observe or collect data.
But, like field research and experimentation, Horn and Mac Arthur’s work provides the
basis, explanation, or inspiration for a large portion of ecological work still being
conducted today.
-Late
-Citation format needs some work. You don’t need to list titles of scientific papers in
the text. The paper could also use some basic editing.
-The first paragraph starts out well, clearly outlining the goals of the paper (as per the
assignment). The summary of the classic is also fairly good for some tough material.
There are at least 2 good examples of recent work as well. However, the gap between
the 1970s and late 90s is not bridged in the paper.
13/26 C+
Nice summary.
17/26 B
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Literature Cited
Clark, J.S., McLachlan, J.S. 2003. Stability of forest biodiversity. Nature. 423:
635-638.
Cohen, E. J. A Markov. 1970. contingency-table model for Replicated LotkaVoltera Systems Near Equilbrium. The American Naturalist. 104(940): 547-560.
Fretwell, S.D. Nov., 1975. The Impact of Robert Macarthur on Ecology.
Annual Review of Ecology and Systematics. 6: 1-13
Henry S. Horn; Robert H. Mac Arthur. Jul, 1972. Competition among fugitive
species in a harlequin environment. Ecology 53: 749-752.
Horn, H.S. Nov.1974. The Ecology of Secondary Succession. Annual Review of
Ecology and Systematics. 5: 25-37
Loreau, M., Mouquet, N. 1999. Immigration and the Maintenance of Local
Species Diversity. The American Naturalist 154: 427–440
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