Amy Thomson
Classic Paper
11-18-05
In 1971, Joseph Connell’s classic paper, “On the Role of Natural Enemies in
Preventing Competitive Exclusion in some Marine Animals and in Rain Forest Trees,”
claimed that species diversity in marine and rain forest systems can be attributed in part
to natural predators and pathogens, as well as density-dependent factors, which allow
several species to coexist. Species-specific predators and pathogens maintain diversity by
preventing seedlings from occurring close to the parent plant, allowing other species to
maintain populations in the gaps between these plants. This disproportionately high
mortality rate of juveniles growing close to their parents causes trees/individuals to be
evenly spaced in the environment. This spacing allows other species to maintain
populations.
This classic paper seems to have been strongly influenced by research for his
other paper on natural enemies “A Predator-Prey System in the Marine Intertidal
Region,” which was also published in 1970 (Connell, 1970 b) as well as a similar, earlier
study “The influence of interspecific competition and other factors on the distribution of
the barnacle Chthamalus stellatus”. These papers explored how barnacle populations
interacted in the presence of very efficient predatory species. Connell concluded that
predators kept the various populations of barnacle species low enough to prevent
competition for space. However, when predators were absent or ignored the very young
barnacles, Connell said that certain species would crowd others out of areas where they
were usually dominant.
Later, the most influential studies inspired by this classic paper explored what was
called the Janzen-Connell model. This model is based on Connell’s classic work as well
as a similar study done by Janzen (Janzen, 1970). Janzen’s article claimed that in addition
to specialized predators that concentrate on prey that occur close to each other – some
predators are also responsive to density and become concentrated near adults where seed
content is highest.(Janzen, 1970)
The Janzen-Connell model or Janzen-Connell effect says that seedlings growing
close to their parents are more likely to catch a disease or fall prey to a predator that
specializes on the plant. This system promotes seedlings that occur at certain distances
away from the parent plant and prevents clumps of species from occurring together –
encouraging species diversity (Clark 1984).
In 1984, Deborah A. Clark and David B. Clark submitted an evaluation of the
Janzen-Connell model in which they used the spacing dynamics of a tropical rain forest
tree to determine if some kind of biological agent would cause greater mortality of
progeny near adults, through distance- or density-dependent predation or infection. They
also studied whether this prevents regeneration in the immediate vicinity of adults and if
these mortality patterns maintain high tree species diversity.
They studied these by tracing the survival of seedlings of the rain forest canopy
tree Dipteryx panamenisis through their first two years at certain radii extending from the
parent tree. In this paper, Clark and Clark found that juveniles and seedlings of Dipteryx,
would not survive within eight meters of an adult bole, and that generally juvenile
survival rates were either distance- or density-dependent – supporting the Janzen-Connell
hypothesis. However, Clark and Clark also found that the Janzen-Connell model is very
complicated to test, and has led to a diversity of approaches in evaluating it (Shupp,
1992).
Howe and Smallwood’s 1982 study led to the development of the “Escape
Hypothesis,” which said that since seedling mortality is high near parent plants because
of density and distant-dependent factors as well as pathogens, recruitment of seedlings
will be limited near adults. This gives other species an opportunity to colonize the area
immediately underneath adults, which enhances species diversity in the area. In future
studies, the “Escape Hypothesis” is considered hand-in-hand with the Janzen-Connell
model.
The Janzen-Connell model helped inspire other models of community regulation
dealing with competition and predation. In 1987, Bruce Menge and John Sutherland used
Clark and Clark’s analysis as well as Connell’s 1971 article to determine whether
patterns of community structure respond predictably to variation in ecological processes
like disturbance, competition and predation, and if the importance of these ecological
processes is affected by variation in environmental conditions like environmental stress
and recruitment density. Their model predicted the influence of consumers/predators on
prey populations – with high recruitment numbers – in various levels of environmental
stress (Menge 1987).
In very stressful environments, Menge and Sutherland predicted that consumers
would not impact prey because they would not be present – similar to Connell’s
conclusion about barnacle competition in stressful areas of the marine intertidal zone in
the absence of predators (Connell, 1971). In more moderately stressed environments,
consumers/predators are still ineffective, but prey species are less inhibited by their
environment and can grow to high densities (Menge 1987). Because of this, the prey
species in this circumstance is limited by intraspecific competition for space. At very low
levels of stress, the model predicts that consumers prevent competition for space (Menge
1987).
In 1992, Eugene Schupp published another critique of the Janzen-Connell model.
In it, he tested the model on the population-level. He found that for the tree Faramea
occidentalis, a subcanopy rain forest tree, survival of seeds increased with the increased
size of the parent population – contrary to the Janzen-Connell model. At the level of the
individual, however, Janzen-Connell spacing took place.
In 1997, Renato Cintra took the Janzen-Connell model and applied it to varying
spatial scales. He found that in two species of rain forest trees, Astrocaryum and
Dipteryx, seedling dispersal and recruitment partially supported both the Janzen-Connell
model and the “Escape Hypothesis.” The trees that he studied were evenly spaced
through the forest with concentric rings of seedlings growing around them. However, his
findings also supported Schupp’s study, which held that seedling survival and recruitment
processes at the level of the individual tree are not necessarily representative of processes
at the population level. According to Cintra’s paper, the proportion of trees of a given
species showing the Janzen-Connell distribution and the frequency that it occurred in the
forest in the long run was important in determining local diversity.
Recent research in this field seems to focus on the role of disease in plant ecology.
In 2001, Packer and Clay published a study – using Connell’s classic work (Connell
1971) as well as Renato Cintra’s more recent study (Cintra 1997) that showed that the
sterilization of soil around parent trees greatly increased seedling survival, but
sterilization of soil further away from parent trees did not affect seedling survival. This
study reinforced the Janzen-Connell test and gave strong evidence that disease and
parasites spread from parent to offspring are a key driver in species distribution and
therefore diversity, and that this process operates in both tropical and temperate
ecosystems (Packer 2001).
Dr. Charles Mitchell from the University of North Carolina at Chapel Hill said
that introduced pathogens account for 56 percent of all plant pathogen emergences. In
addition, it is generally known that mortality rates increase when the pathogen is
transmitted from a relative, and that monocultures have a higher incidence of disease
(Peet lecture). While recent studies have shown that declining species diversity on a
global scale is happening (Knops, 1999), few have shown what impact this will have. In
1999, J.M.H. Knops along with Tilman and several others showed that waning species
diversity made a grassland ecosystem more vulnerable to fungal diseases.
Now that it is becoming clear what role disease plays in plant systems in
maintaining diversity, I think that the major questions left for this area of study are: What
effect exotic pathogens will have on native species, what effect will the disappearance of
some species due to pathogens have on ecosystems and species diversity, will the holes
that these dying species will leave in our ecosystems – like the Frasier firs on Mt.
Mitchell – will make them more susceptible to invasion from further exotic species, and
if invasive pathogens reduce species diversity, does it really matter?
In 1997, Dave Tilman conducted an experiment showing that communities with a
diverse array of species were more resistant to invasion from new species (Tilman 1997).
He concluded that local species dynamics and recruitment determine diversity, species
composition and species abundances.
“This supports a metapopulation-like perspective over a purely
interspecific-interaction perspective or a purely regional perspective, suggesting
that recruitment limitation may be more important, even on a local scale, than
often recognized. (Tilman 1997).”
If this is true, it seems like the loss of species like the Fraser Fir to the Balsam
Woody Adelgid, would leave North Carolina’s Mt. Mitchell more open to invasion from
other species and other species’ pathogens. In future studies, I would like to see someone
set up a study tracing the community structure at various elevations as the Fraser Fir
population declines over the next several decades. It might be the case that the loss of this
species will leave more niches unoccupied (Tilman 1997) and will cause the rest of the
population to be more vulnerable to attack from an exotic species or disease (Knops
1999).
With increasing numbers of exotic species entering ecosystems all over the world,
and with increasingly fragmented habitats making populations more susceptible to
invasion (Tilman 1997), the study of disease and invasion – how they are transmitted and
how they effect ecosystems – will be a vital area of study for ecologists seeking to
maintain habitats and ecosystems.
Works Cited
Cintra, Renato 1997. A test of the Janzen-Connell model with two common tree species
in Amazonian Forest. Journal of Tropical Ecology 13 (5): 641-658.
Clark, Deborah A.; David B. Clark 1984. Spacing dynamics of a tropical rain-forest tree
– evaluation of the Janzen-Connell model. The American Naturalist 124 (6): 769-788.
Connell, J.H. 1971. On the role of natural enemies in preventing competitive exclusion in
some marine animals and in rain forest trees. Dynamics of Populations: Proceedings
of the Advanced Study Institute on Dynamics of Numbers in Populations (eds. P.J. den
Boer and G.R. Gradwell) Centre for Agricultural Publishing and Documentation,
Netherlands: 298 – 310.
Connell, J.H. 1970. A predator-prey system in the marine intertidal region. I. Balanus
glandula and several predatory species of Thias. Ecological Monographs Vol. 1.
40:49-78.
Howe HF; Smallwood J 1982. Ecology of Seed Dispersal. Annual Review of Ecology
Vol. 13: 201-228.
Janzen, Daniel H 1970. Herbivores and the number of tree species in tropical forests. The
American Naturalist. 140 (940): 501-528.
Knops, J.M.H; Tilman, Dave; Haddad,d N.M.; Naeem S., Reich, P.B; Siemann E. 1999.
Effects of plan species richness on invasion dynamics, disease outbreaks, insect
abundances and diversity. Ecology Letters 2 (5): 286.
Menge, Bruce A.; John P. Sutherland 1987. Community regulation: Variation in
disturbance, competition, and predation in relation to environmental stress and
recruitment. The American Naturalist 130 (5): 730-757.
Schupp, Eugene W. 1992. The Janzen-Connell model for tropical tree diversity:
population implications and the importance of spatial scale. The American Naturalist 140
(3): 526-530.
Tilman, Dave. 1997. Community invisibility, recruitment limitation, and grassland
biodiviersity. Ecology 78 (1): 81-92