IDEA
Attractive sinks, or how individual behavioural
decisions determine source—sink dynamics
Abstract
Miguel Delibes,1 Pablo Ferreras2
and Pilar Gaona1
1
Department of Applied
Biology, Estacio‘ n Biolo‘ gica de
Donaana, CSIC, Apdo. 1056,
41080 Sevilla, Spain.
E-mail: mdelibes@ebd.csic.es
2
Instituto de Investigacio‘ n en
Recursos Cinege‘ ticos, CSICUCLM, Calle Libertad, 7 A,
13004 Ciudad Real, Spain.
Gundersen et al. (2001, Source-sink dynamics: how sinks affect demography of sources.
Ecol. Lett., 4, 14—21.) suggested that sinks can severely affect the demography of
populations in source habitats. We propose this is a common result when animals lack
cues associated with reduced fitness inside sinks and consequently select habitat
inappropriately. These attractive sinks can result either from undetected risks of
mortality (as in the experiment of Gundersen et al. 2001) or from undetected poor
breeding probabilities (due to bioaccumulation of pesticides, for instance). Thus,
individual habitat choice is a key process underlying source—sink dynamics.
Keywords
Attractive sinks, deceptive sources, demography, dispersal, ecological traps, habitat
selection, metapopulations, risk detection, source—sink dynamics.
Gundersen et al. (2001) have shown that sinks can
seriously affect the demography of sources: in the
presence of an experimental sink patch, source populations of root voles (Microtus oeconomus) failed to increase
over the breeding season as did the control populations.
According to the initial models of source—sink dynamics
(e.g. Pulliam 1988) this was a rather unexpected result,
since the sinks should be replenished by the surplus of
sources, which should be barely or no affected. Gundersen et al. (2001) explained their findings as a result of high
spatially density-dependent dispersal rates from source to
sink patches. However, density-dependent export of
individuals from sources to sinks is assumed by the
traditional theory of source—sink dynamics, without
predicting a serious effect on source demography. We
think density-dependent dispersal per se was not the key
process underlying the effects observed by Gundersen
et al. (2001), but that the perception individuals had from
the sink and the consequent behavioural decisions to settle
there, influenced dispersal rates.
Some time ago it was recognized that sinks could affect
the demography of sources and even threaten their
persistence (as in the experiment of Gundersen et al.
2001), when animals or plants suffered passive dispersal,
i.e. when they did not select sinks, but were moved towards
there by external forces (e.g. winds or water currents; Holt
1993). This evidence suggests that habitat selection should
be an important mechanism determining the dynamics of
source—sink systems.
Most source—sink models assume that animals dispersing
actively are able to recognize, and if possible avoid,
substandard (sink) habitats, where mortality exceeds recruitment. Thus, they should first select the source and only
poorer competitors will occupy the sink (Dias 1996). But,
what happens if animals fail to properly select habitat? In
the models by Pulliam & Danielson (1991), the whole
population (source + sink) may become extinct when
habitat sampling is incomplete and site-selection ability is
consequently low. Doak (1995) explored source—sink
models assuming that individuals did not perceive differences in habitat quality and thus moved solely on the basis
of relative densities in each habitat. In this case, some
combinations of mobility rates and proportions and qualities
of good and bad habitats turn the whole source—sink
population into a sink. Quoting these and some other
analyses, Hoopes & Harrison (1998) concluded that relaxing
the assumption that dispersing individuals select habitat
optimally (i.e. accepting they can move inappropriately from
better to worse habitat) made the sink habitat lose its
positive role and caused the population to decline. More
recently we have proposed that habitat selection is a key
factor underlying source—sink dynamics: when individuals
avoid sink habitat (as usually happens) the source subpopulation is not depressed by the sink; however, when animals
choose habitat in a maladaptive way, because they are unable
to distinguish sink from source habitat or even they prefer
the sink, the whole population may become extinct (Delibes
et al. 2001). This means that, everything being the same (size
and demographic parameters of sources and sinks, distance
among them, etc.), the dynamics of source—sink systems will
be completely different according to the perception individuals have of the quality of sink habitat, and their
consequent behavioural decisions (in the limit, the so-called
matrix”” or no-habitat”” would be extreme sink habitat,
easily recognized and therefore never used).
The results of Gundersen et al. (2001) coincide with the
predictions of Doak (1995) and ourselves (Delibes et al.
2001) when sinks are not recognized as such. In the
experimental conditions of Gundersen et al. (2001), source
and sink habitats were exactly the same (the sink was a
sink only because all the immigrants were removed
periodically by humans). In this case, voles must be
unable to evaluate the risk of mortality in the sink, because
they do not perceive there any of the cues used as
indicators of risk in their evolutionary history (e.g. the
odour of a weasel or some frustrated attack of a raptor).
We called these sinks leading to maladaptive habitat
selection attractive sinks (or deceptive sources) (Delibes
et al. 2001). In addition, inappropriate (maladaptive) habitat
choice has recently and independently been proposed as
generating source—sink dynamics by Remes (2000). This
author makes several references to field—forest ecotones as
attractive, but dangerous, ecological traps”” for birds, a
term commonly used in the literature since Gates & Gysel
(1978).
Gundersen et al. (2001) also suggest their results could
be related to the difficulty of dispersing animals in
recognizing mortality sinks, as they would be different
from the usual sinks ruled by scarce resources and low
recruitment rates. We agree that mortality risk may be less
well assessed than poor breeding conditions (see Gaona
et al. 1998), but this is not always the case. Deceptive
sources (i.e. unrecognizable demographic sinks) can also
appear when animals can not evaluate the probability of
breeding failure, as it usually happens with pesticides. For
instance, industrial PCBs affect reproduction in many
mammals and are suspected to be the cause of pseudohermaphroditism in polar bears (Wiig et al. 1998). But, as
the journalist Doug Mellgren (1998) wrote, polar bears
fear nothing they can see in their icy domain, but they
cannot see polychlorinated biphenyls.”” Thus, bears could
be attracted to the highly polluted Norwegian islands of
Svalbard, which should be deceptive sources ruled by low
recruitment rate. Again, this should be the case of some
ecotones acting as ecological traps”” for birds, for instance
when a high density of brood parasites seriously affects the
host population breeding success (e.g. Trine 1998).
In summary, the matter is not the type of sinks (either
ruled by high mortality or low recruitment rates) but the
behavioural decisions of the animals, depending on the
agreement between the cues they use as indicators of habitat
quality and the current fitness reward obtained (Remes
2000).
ACKNOWLEDGEMENTS
E. Revilla, A. Rodr’iguez and B.J. Danielson helped to
improve the manuscript. This research was supported by
project IFD1997-0789 of the Spanish National Plan of
Research and Development.
REFERENCES
Delibes, M., Gaona, P. & Ferreras, P. (2001). Effects of an
attractive sink leading into maladaptive habitat selection. Am.
Naturalist, 158, 277—285.
Dias, P.C. (1996). Sources and sinks in population biology. Trends
Ecol. Evol, 11, 326—330.
Doak, D.F. (1995). Source—sink models and the problem of habitat
degradation: general models and applications to the Yellowstone
grizzly. Conservation Biol., 9, 1370—1379.
Gaona, P., Ferreras, P. & Delibes, M. (1998). Dynamics and
viability of a metapopulation of the endangered Iberian lynx
(Lynx pardinus). Ecol. Monographs, 68, 349—370.
Gates, J.E. & Gysel, L.W. (1978). Avian nest dispersion
and fledging success in field—forest ecotones. Ecology, 59,
871—883.
Gundersen, G., Johannesen, E., Andreassen, H.P. & Ims, R.A.
(2001). Source-sink dynamics: how sinks affect demography of
sources. Ecol. Lett., 4, 14—21.
Holt, R.D. (1993). Ecology at the mesoscale: the influence of
regional processes on local communities. In: Species Diversity in
Ecological Communities (eds Ricklefs, R.E. & Schluter, D.). University of Chicago Press, Chicago, USA, pp. 7—88.
Hoopes, M.F. & Harrison, S. (1998). Metapopulation, source—sink
and disturbance dynamics. In: Conservation Science and Action
(Ed. Sutherland, W.J.). Blackwell Science Ltd, Oxford, UK,
pp. 135—151.
Mellgren, D. (1998). PCBs now genetically damaging polar bears in
Norwegian islands. [WWW document]. URL: http://www.rense.
com/earthchanges/polarb.htm
Pulliam, H.R. (1988). Sources, sinks and population regulation.
Am. Naturalist, 132, 652—661.
Pulliam, H.R. & Danielson, B.J. (1991). Sources, sinks, and habitat
selection: a landscape perspective on population dynamics.
Am. Naturalist, 137, 550—566.
Remes , V. (2000). How can maladaptive habitat choice generate
source—sink population dynamics? Oi/€os, 91, 579—582.
Trine, C.L. (1998). Wood thrush population sinks and implications
for the scale of regional conservation strategies. Conservation Biol.,
12, 576—585.
Wiig, 0., Derocher, A.E., Matthew, M.C. & Skaare, J.U. (1998).
Female pseudohermaphrodite polar bears at Svalbard. J. Wildlife
Dis, 34, 792—796.
BIOSKETCH
Miguel Delibes conducts research on the ecology and conservation of mammals, mainly carnivores. The challenge of
conserving endangered carnivores in reserves surrounded by
mortality sinks generated his interest on source—sink dynamics.
©2001 Blackwell Science Ltd/CNRS