On the ecological mechanisms of tropical forest diversity and the construction of logging
roads
By
John Vandermeer
And
Rick Riolo
Generations of ecologists have pondered the mystery of hyperdiversity in tropical
rain forests. The classical theories of ecology holds that when a group of species are
effectively identical in their environmental requirements, that is, shared similar niches,
the process of interspecific competition should render any system monospecific – the best
competitor should eventually overwhelm all others. While this theory sits well over vast
stretches of northern forests, where one or two species of trees usually dominate a forest,
tropical rain forests contain hundreds of tree species, and it has always been difficult to
imagine how each of those species could be occupying a distinct ecological niche, which
is the basic requirement for biodiversity preservation under the classical model.
Modern ecologists have come to propose a variety of mechanisms above and
beyond those of the classical theory, to account for this hyperdiversity. Most often these
alternative approaches involve the recognition that the trees of the forest have a place in
physical space, that is, the temporal dynamics that dominated classical theory would have
to be enhanced with a spatial dynamics. The new spatiotemporal dynamic approaches
can account, at least theoretically, for the hyperdiversity in a straight-forward manner.
Parallel with the ecological study of tropical rain forest biodiversity there has
been a more practical concern with the utilization of these forests, especially with the
activities of a forestry industry that sees the forests as little more than a source of timber.
Emerging from this concern has been a call for sustainable harvest of rain forest timber
leading to a variety of management schemes, e.g., limiting harvesting based on tree size
and species or using a rotational harvesting mechanism that presumably allows for forest
regeneration in the interharvest period. For the most part it is the accumulation of
biomass that has been the variable of concern in these schemes.
The more complicated question of what happens to the biodiversity per se, and the
evident secondary consequences of harvest activities have not always been given the
attention they deserve. In particular, the necessary construction of logging roads, while
perhaps not abusively destructive to the forest initially, very frequently carry with them
secondary consequences, namely entrance points for human colonists. The result is not
the seemingly benign disturbance of narrow ribbons through the forest, but rather a
ribbon that tends to become widened by the activities, mainly agricultural, of the
colonists.
These two issues, the modern spatiotemporal theoretical approach to biodiversity
maintenance and the secondary consequences of road building, have not been considered
as intersecting issues. Here we examine two specific approaches to the spatiotemporal
dynamics of rain forest biodiversity and ask whether the consequences of road building
depend on which theory is true.
The ecological mechanisms:
The two ecological mechanisms we examine in this research are both stimulated
by the key insights of Hubbel and Foster (1984), that in general trees are very likely to
occupy similar ecological niches. While such niches as occupied by pioneers and swamp
species are evidently specialists on some particular ecological background, the vast
majority of tree species in any particular tropical forest indeed seem to occupy similar
niches, at least similar enough so as not to account for classical Gaussian competitive
exclusion. This insight, while not fully appreciated at the time, has since led to renewed
interest in non-niche dynamics, of which the two mechanisms we examine in this work
are examples.
Borrowing from the theory of metapopulations, Nee et al., (1992) and May and
Nowack (1994) proposed a spatiotemporal mechanism in which the average dispersal
distance of each tree species was negatively correlated with the competitive ability of that
species. In other words, there is a trade-off between ability to disperse and ability to
compete. Given such a negative correlation, it is theoretically possible to contain, in
perpetuity, an arbitrarily large number of species in a single community.
An alternative approach is suggested by a combination of the suggestion of
Hubbell et al., (????) that dispersal limitation is a key element determining the probability
that a space will be occupied by a particular tree species, and the Janzen/Connell effect
(Janzen, ????; Connell, ????) that correlates the probability of attack by a natural enemy
with the local population density of a particular species. This mechanism does not rely
on competition any further than the evident requirement that no two individual trees can
occupy the same physical space. The expansion/repression dynamic results in a
population that is clustered in space and, depending on parameter values, may be very
rare (Vandermeer et al., 2008). Effectively the rendering of a large number of very rare
species permits those species to coexist.
Both of these mechanisms (the competition/colonization tradeoff and the dispersal
limitation/Janzen-Connell effect) are known to allow a large number of species to coexist
in perpetuity, despite the fact that they all occupy the same “niche.” The question that
emerges from sustainability arguments is whether it makes a difference which of the two
mechanisms is operative to the consequences of indirect road-building. Since both
mechanisms explicitly involve dispersal phenomena it is to be expected that the extent to
which the consequences of road building interrupt the dispersal, the consequences for
biodiversity could be substantial. But it is not immediately evident whether the
consequences would differ depending on which of the two mechanisms is operative.
The model:
We are in the process of formulating a model in which a series of lattice points are
occupied by individual trees. Each species has a dispersal kernel and either a position in
the competitive hierarchy, or a natural enemy that acts in a density-dependent fashion
(i.e., to the local density of the tree). Populating each of the alternatives with 200 – 300
species of trees, the model will run for some time until a stable point is reached and then
a road will be built on the lattice. Construing a road as a narrow percolating cluster on
the lattice, we will then investigate the consequences of the road increasing its width,
simulating the secondary effects of colonists using the logging roads to penetrate the
forest and engage in agricultural activities. The consequences of the pattern of road
building and widening will be examined in terms of how the overall biodiversity is
affected, and how that effect is dependent on which of the two ecological mechanisms is
operative. We also will explore the effects of different road patterns on biodiversity, both
systematically varying road patterns over a wide range and looking at patterns generated
by our agent-based models of loggers, government and NGO monitoring and enforcment.