Moving beyond the resource dispersion hypothesis
Eloy Revilla
Estació n Bioló gica de Doñ ana CSIC, Avenida de Marı́a Luisa s/n, E-41013 Seville, Spain
In a recent article in TREE [1], Johnson et al. review
the evidence showing that, when resources are patchily
distributed, the economics of exploiting these patches
enable individuals to share resources over a common
area, satisfying their needs without imposing large
costs on each other. This resource dispersion hypothesis (RDH) is presented as a causal mechanism of
group living, especially when individuals obtain no
apparent benefits from such living. There are two
points on which I would like to comment.
First, the authors only refer to the evidence in
favour of the RDH. However, in no single case are all
the assumptions and predictions satisfied (Table 1 in
[1]). To test the RDH, all the assumptions must be
fulfilled and, then, only a simultaneous positive test of
all its predictions would validate the hypothesis [1].
However, we need evidence against only one of its
assumptions and/or predictions for it to be invalid. This
evidence exists [2]. For example, in badgers (a RDH
model species), there is a high cost of living in groups
[3 – 5], the territoriality and use of space by different
individuals are affected by different resources [6 – 8],
and decreases in food availability increase territory
overlap [9]. These pieces of evidence invalidate the
assumption of there being no cost to group living, that
the determinant of space use is the same for all
individuals, and
that group ranges are exclusive,
respectively. Neither was there a consistent relationship found between territory size and resource dispersion, nor was group size consistently related to
resource richness in one site [10]. In another study, a
decrease in food availability was followed by territory
expansion rather than by the expected group size
reduction [9]. Furthermore, in one group-living population, territory size was related to its richness and not
to patch dispersion, whereas an adjacent population,
suffering the same strong seasonal resource heterogeneity, was solitary living [8].
Second, from its conceptual and mathematical
definition, the RDH provides an explanation for why
there is a surplus of resources inside one territory.
Although this is important, there is a large conceptual
gap between this explanation and demonstrating that
this surplus is the causal means of group formation.
Otherwise, in our heterogeneous world, we would
observe many more species living in groups.
Corresponding author: Eloy Revilla (revilla@ebd.csic.es).
Current evidence shows that resource patchiness
alone is not enough to explain group living. In ecology,
the adaptive process of understanding includes the first
step of developing theory based on empirical information, followed by testing of theoretical predictions.
When there are conflicts between empirical evidence
and predictions, the theory has to be refined to
accommodate the new knowledge. I think that it is
now time to move beyond the RDH as a casual
mechanism of group living.
References
1 Johnson, D.D.P. et al. (2002) Does the resource dispersion hypothesis
explain group living? Trends Ecol. Evol. 17, 563 – 570
2 Revilla, E. (2003) Does the resource dispersion hypothesis explain
anything? Oikos 101, 428 – 432
3 Macdonald, D.W. et al. (2002) Density-dependence regulation of body
mass and condition in badgers (Meles meles) from Wytham woods.
Ecology 83, 2056 – 2061
4 Silva, J.D. et al. (1994) Net cost of group living in a solitary
forager, the Eurasian badger (Meles meles). Behav. Ecol. 5,
151 – 158
5 Woodroffe, R. and Macdonald, D.W. (1995) Female/female competition
in European badgers Meles meles: effects on breeding success. J. Anim.
Ecol. 64, 12 – 20
6 Tuyttens, F.A.M. et al. (2000) Spatial perturbation caused by a
badger (Meles meles) culling operation: implications for the
function of territoriality and the control of bovine tuberculosis
(Mycobacterium bovis). J. Anim. Ecol. 69, 815 – 828
7 Revilla, E. and Palomares, F. (2001) Differences in key habitat
use between dominant and subordinate animals: intraterritorial
dominance payoffs in Eurasian badgers? Can. J. Zool. 79,
165 – 170
8 Revilla, E. and Palomares, F. (2002) Spatial organization, group living
and ecological correlates in low-density populations of Eurasian
badgers, Meles meles. J. Anim. Ecol. 71, 497 – 512
9 Kruuk, H. and Parish, T. (1987) Changes in the size of groups and
ranges of European Badger (Meles meles L.) in an area in Scotland.
J. Anim. Ecol. 56, 351 – 364
10 Johnson, D.D.P. et al. (2001) Long-term resource variation and group
size: a large sample field test of the Resource Dispersion Hypothesis.
BMC Ecology 1, 2 (http://www.biomedcentral.com/1472-6785/1/2)