I D E AAttractive sinks, or how individual behavioural

decisions determine source±sink dynamics

Miguel Delibes,1 Pablo Ferreras2

and Pilar Gaona1

1Department of Applied

Biology, EstacioÂn BioloÂgica de

Don~ana, CSIC, Apdo. 1056,

41080 Sevilla, Spain.

E-mail: mdelibes@ebd.csic.es2Instituto de InvestigacioÂn en

Recursos CinegeÂticos, CSIC-

UCLM, Calle Libertad, 7 A,

13004 Ciudad Real, Spain.

Abstract

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 ®tness 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.

Ecology Letters (2001) 4: 401±403

Gundersen et al. (2001) have shown that sinks can

seriously affect the demography of sources: in the

presence of an experimental sink patch, source popula-

tions 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. Gunder-

sen et al. (2001) explained their ®ndings 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, in¯uenced 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 recruit-

ment. Thus, they should ®rst 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 differ-

ences 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 subpopu-

lation 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

Ecology Letters, (2001) 4: 401±403

Ó2001 Blackwell Science Ltd/CNRS

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 indi-

viduals 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 RemesÏ (2000). This

author makes several references to ®eld±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 dif®culty 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 pseudo-

hermaphroditism 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 ®tness reward obtained (RemesÏ

2000).

A C K N O W L E D G E M E N T S

E. Revilla, A. RodrõÂguez 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.

R E F E R E N C E S

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402 M. Delibes, P. Ferreras and P. Gaona

Ó2001 Blackwell Science Ltd/CNRS

B I O S K E T C H

Miguel Delibes conducts research on the ecology and conser-

vation of mammals, mainly carnivores. The challenge of

conserving endangered carnivores in reserves surrounded by

mortality sinks generated his interest on source±sink dynamics.

Editor, N.G. Yoccoz

Manuscript received 19 June 2001

Manuscript accepted 16 July 2001

Undetected risks generate attractive sinks 403

Ó2001 Blackwell Science Ltd/CNRS