Download Independent species in independent niches behave neutrally: a

Oikos 120: 964–965, 2011
doi: 10.1111/j.1600-0706.2011.19880.x
© 2011 The Author. Oikos © 2011 Nordic Society Oikos
Subject Editor: Stefano Allesina. Accepted 6 April 2011
Independent species in independent niches behave neutrally:
a response
Ryan A. Chisholm and S. W. Pacala
R. A. Chisholm ([email protected]), Smithsonian Tropical Res. Inst., MRC0580/12, Unit 9100, Box 0948, DPO AA 34002/9998, USA.
– S. W. Pacala, Dept of Ecology and Evolutionary Biology, Princeton Univ., Princeton, NJ 08544, USA.
In a recent paper (Chisholm and Pacala 2010), we
analyzed a hybrid niche-neutral model with strong niche
structure: the landscape is divided into several discrete
niches; each species is assigned a single niche preference;
species in their preferred niche competitively exclude those
from other niches; and species with the same preferred
niche behave neutrally with respect to one another (i.e. neutral zero-sum dynamics occur within a niche). The model
is spatially implicit and consists of a local community that
is semi-isolated from a much larger metacommunity. We
proved analytically that, in the limit of high diversity, the
species abundance distribution resulting from this model
looks neutral. This result suggests that neutral drift is the
mechanism responsible for generating statistical patterns of
species abundances in ecosystems even when a degree of
niche structure is present.
Haegeman and Etienne (2011) analyzed a similar
model to ours but without the zero-sum constraint. They
observed that in their model species in different niches are
independent of one other and therefore behave neutrally.
Haegeman and Etienne make two main claims based on
their analysis. The first is that the results of our model
(Chisholm and Pacala 2010) are also attributable to the
independence of species in different niches. The second is
that simple niche models such as these cannot be used to
draw conclusions about the prevalence of niche or neutral processes in ecological systems. We address these two
claims in turn.
With regards to the first claim, it is certainly true that
in the model of Haegeman and Etienne (2011), species in
different niches are completely independent and therefore
behave neutrally. However, the difference between their
model and ours, as noted already, is that we impose a zerosum constraint whereas they do not. As a consequence,
we observe niche-structured patterns at low diversity and
neutral patterns at high diversity, whereas they always
observe neutral patterns. The reason for our observation
of niche-structured patterns is that the species in different
niches in our model are fundamentally not independent,
but instead competitively exclude one another from their
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respective niches and thereby constrain one another’s
abundances. The model of Haegeman and Etienne (2011)
does shed light, mathematically, on why we obtain our
asymptotic result in the limit of high diversity. But the
absence of the zero-sum constraint makes their model less
biologically realistic (in a tropical forest tree community,
there is a strong zero-sum constraint because individuals are effectively competing for space), prevents nonneutral patterns from emerging at any level of diversity,
and therefore prevents one from exploring any transition
from niche-structured to neutral pattern (Chisholm and
Pacala 2011).
The second claim that Haegeman and Etienne (2011)
make is that simple niche models such as these cannot be
used to draw conclusions about the prevalence of niche or
neutral processes in ecological systems. The point of our
paper was not to draw conclusions about the prevalence
of niche structure. We know from empirical evidence that
niche structure is prevalent in different ecosystems across
the planet. We stated explicitly in our paper that observations of neutral species abundance distributions ‘cannot be
used to infer an absence of niche structure’, at least in highdiversity systems (Chisholm and Pacala 2010). The point
of our paper was to investigate the robustness of the neutral
species abundance distribution to niche structure. For the
case of strong non-overlapping niche structure, we provided a rigorous proof of previous conjectures that neutral
and niche models generate similar patterns of diversity.
Of course, our model represents a simplified view of
reality and Haegeman and Etienne’s (2011) model is even
more simplified. An important extension of our model
can be understood by imagining a continuum of niche
strength, with different models sitting at different points
on the continuum. Niche strength can be measured as the
extent to which individuals in their preferred niche exclude
individuals from other niches. The neutral model sits at
one extreme end of this continuum, where niche strength
is zero (no competitive exclusion). Our model was specifically designed to be at the opposite end of this continuum,
where niche strength is infinite (complete competitive
exclusion from non-preferred niches). We have proven
that species abundance distributions at both ends of this
continuum look neutral (Chisholm and Pacala 2010), and
simulations suggest that species abundance distributions
at intermediate points of the continuum also look neutral
(Purves and Pacala 2005). Thus, we conjecture that neutral drift is the predominant mechanism responsible for
observed patterns of species abundance distributions in
high diversity systems at all points along the continuum.
However, in the absence of an analytical proof it does, of
course, remain conceivable that some other mechanism
intervenes at intermediate levels of niche strength. We
agree with Haegeman and Etienne that the analysis of
more realistic models such as these, while challenging, is a
priority for future research.
References
Chisholm, R. A. and Pacala, S. W. 2010. Niche and neutral models predict asymptotically equivalent species abundance distributions in high-diversity ecological communities. – Proc. Natl
Acad. Sci. USA 107: 15821–15825.
Chisholm, R. A. and Pacala, S. W. 2011. Theory predicts a rapid
transition from niche-structured to neutral biodiversity patterns
across a speciation-rate gradient. – Theor. Ecol. 4: 195–200.
Haegeman, B. and Etienne, R. S. 2011. Independent species in
independent niches behave neutrally. – Oikos 120: 961–963.
Purves, D. W. and Pacala, S. W. 2005. Ecological drift in nichestructured communities: neutral pattern does not imply neutral process. – In: Burslem, D. et al. (eds), Biotic interactions
in the tropics: their role in the maintenance of species diversity.
Cambridge Univ. Press, pp. 107–138.
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