Ww. Murdoch, POPULATION REGULATION IN THEORY AND PRACTICE - THE ROBERT-H-MACARTHUR-AWARD-LECTURE PRESENTED AUGUST 1991 IN SAN-ANTONIO, TEXAS, USA, Ecology, 75(2), 1994, pp. 271-287
Population regulation is a fundamental process related to most phenome
na in ecology, including evolutionary ecology. I review the conceptual
basis for defining regulation as bounded fluctuations in abundance, i
n contrast to the unbounded fluctuations of random-walk populations. R
egulation arises as a result of potentially stabilizing density-depend
ent processes, even when brought about by ''non-equilibrium'' mechanis
ms. Although the phenomenon is unambiguous in theory, detecting regula
tion by finding evidence for density dependence in a series of populat
ion estimates faces unsolved statistical problems. So, while there is
growing evidence for widespread regulation, severe detection problems
remain. I illustrate these with data from bird populations. Whether re
gulation is typically achieved by local stabilizing mechanisms or via
metapopulation dynamics remains to be determined. I summarize recent s
tudies on a particularly well-regulated system-red scale (Aonidiella a
urantii) and its controlling parasitoid, Aphytis melinus. We tested an
d failed to find evidence for eight hypotheses that might account for
the system's stability, including spatial heterogeneity in attack rate
s, a refuge, and metapopulation dynamics. We also failed to find evide
nce for density-dependent parasitism. but such density dependence migh
t be still be present. Recent laboratory and modeling studies have unc
overed a number of other potentially stabilizing mechanisms centering
on the response of individual Aphytis to their size-structured host. T
his plethora of size- and stage-dependent interactions leads naturally
to a consideration of the factors controlling Aphytis' size-dependent
behavioral decisions, and consequently to the elaboration of size-str
uctured models. The latter provide a vehicle for bringing together inv
estigations of selection of life histories, and population dynamics. T
his is illustrated by a model of Aphytis and red scale dynamics that c
an explain a dramatic case of competitive displacement. The red scale/
Aphytis system exemplifies a particularly challenging problem in popul
ation regulation, namely to account for the co-occurrence of stability
and severe suppression of the prey population. A potentially generic
solution is to assume stabilizing density dependence in the parasitoid
or predator population; however, this has the consequence of increasi
ng the host or prey population equilibrium. My colleagues and I have s
hown that observed prey densities in a plankton system are too low for
such a mechanism to be operating. Further work is needed to test this
and other hypotheses.