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TRENDS in Ecology & Evolution Vol.17 No.10 October 2002
Ecological and evolutionary traps
Martin A. Schlaepfer, Michael C. Runge and Paul W. Sherman
Organisms often rely on environmental cues to make behavioral and lifehistory decisions. However, in environments that have been altered suddenly
by humans, formerly reliable cues might no longer be associated with adaptive
outcomes. In such cases, organisms can become ‘trapped’ by their evolutionary
responses to the cues and experience reduced survival or reproduction.
Ecological traps occur when organisms make poor habitat choices based on
cues that correlated formerly with habitat quality. Ecological traps are part of a
broader phenomenon, evolutionary traps, involving a dissociation between
cues that organisms use to make any behavioral or life-history decision and
outcomes normally associated with that decision. A trap can lead to extinction
if a population falls below a critical size threshold before adaptation to the
novel environment occurs. Conservation and management protocols must be
designed in light of, rather than in spite of, the behavioral mechanisms and
evolutionary history of populations and species to avoid ‘trapping’ them.
Published online: 19 August 2002
Martin A. Schlaepfer*
Field of Ecology and
Evolutionary Biology and
Dept of Natural
Resources, Fernow Hall,
Cornell University, Ithaca,
NY 14853-3001, USA.
*e-mail:
[email protected]
Michael C. Runge
US Geological Survey,
Patuxent Wildlife
Research Center,
11510 American Holly
Drive, Laurel,
MD 20708-4017, USA.
Paul W. Sherman
Dept of Neurobiology and
Behavior, Mudd Hall,
Cornell University, Ithaca,
NY 14853-2702, USA.
Organisms often use indirect cues in their physical
environment to guide their choice of habitat. These
cues can reflect current habitat quality, but more often
they enable individuals to anticipate the future state
of the habitat. For example, by relying on vegetation
structure, an individual can choose a breeding site or
territory long before the appearance of factors that will
determine ultimately the quality of that habitat patch,
such as the availability of food or cover. Generally,
these decisions or ‘preferences’ are adaptive because
they rely on cues that, over evolutionary time, reliably
correlated with survival and reproductive success [1].
However, biologists have long recognized that if an
environment changes suddenly, the normal cues
might no longer correlate with the expected outcome
and, as a result, the evolved responses of individuals
might no longer be adaptive [2,3].
The term ECOLOGICAL TRAP (see Glossary) was coined
to describe the situation in which a bird’s choice of
nesting habitat led to nest failure because of a recent
anthropogenic change in the environment that broke
the normal cue-habitat quality correlation [4]. (The
term was in fact first applied to a ‘natural’ ecological
trap [5], but this usage was supplanted quickly in the
literature so as to refer to anthropogenically induced
ecological traps [4].) If there has always been a tight
correlation between a cue and the future state of the
environment, organisms might not have the phenotypic
plasticity to assess and respond to an evolutionarily
novel situation [6–8]. Thus, a trap arises when the
organism is constrained by its evolutionary past to
make a mistake, although suitable conditions (or
adaptive choices) remain available elsewhere (Fig. 1).
Here, we review recent literature pertaining to
ecological traps, describe the general underlying
mechanism, and show that this mechanism applies to
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a broad range of behaviors that rely on environmental
and social cues. Indeed, ecological traps are part of a
broader phenomenon that we call EVOLUTIONARY TRAPS.
Ecological and evolutionary traps have far-reaching
implications for wildlife population dynamics and
conservation because they can potentially result in
widespread maladaptive behaviors, leading to
population declines or extinctions.
The ecological trap
In their seminal work, Gates and Gysel [4] reported
that mortality of eggs and nestlings in 21 species of
passerine birds was higher near forest edges than in
the interior. They attributed this to the greater
activity of predators and interspecific parasites
(cowbirds Molothrus ater) near forest edges. Nest
densities also were higher near forest edges, and the
authors suggested that this reflected the birds’ evolved
preferences for heterogeneous vegetation. In
undisturbed forests, vegetational heterogeneity might
normally provide good foraging opportunities and
protection against predators. The sudden increase in
forest edges as a result of human activities represents
an ecological trap because the evolved preferences or
DARWINIAN ALGORITHMS [9] of the birds lead them to seek
the heterogeneous habitat now encountered primarily
along edges. However, that choice is no longer
adaptive because of the unusually high density and
diversity of predators and parasites found along edges.
In some cases, the quality of a habitat need not be
altered per se for an organism to make an inappropriate
habitat selection. Ecological traps can also arise when a
novel element in the environment mimics a traditional
cue for habitat choice, thereby misleading the organism.
For example, mayflies (Ephemeroptera) use horizontally
polarized reflected light to identify ponds, presumably
because horizontal polarization normally indicates
suitable habitat for oviposition. Unfortunately, asphalt
also polarizes light horizontally and mayflies sometimes
lay their eggs mistakenly on a dry road although
suitable ponds are available nearby [10]. Likewise, sea
turtle hatchlings rely normally on light cues from the
open horizon to orient and migrate toward the ocean
after emerging from the nest at night. Light pollution
from beachfront structures can cue hatchlings to
migrate inland instead, where survival is unlikely [11].
Interest in the ecological trap concept has been
renewed because of a growing concern about the
adaptiveness of the behaviors of organisms in
increasingly disturbed environments. There are now
multiple examples of reduced nest survival of
ground-nesting birds near forest edges relative to
interior locations [12–14]. Other examples of
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TRENDS in Ecology & Evolution Vol.17 No.10 October 2002
examples of similar situations have been identified in
many different taxa (Table 1 and examples in [6,8,17]).
Some studies in Table 1 do not provide all the
information necessary to evaluate whether they are
bona fide ecological traps, but we included them
because either the author stated they were traps or
evidence presented suggested strongly that they were.
(a)
Habitat 1: high quality
contains: {cues A,B,C}
Strong preference for Habitat 1, which is
the high-quality habitat.
The preference is thus adaptive
Habitat 2: medium quality
contains: {cues C,D,E} but not {cues A,B}
Habitat preference based
on certain cue or cues
(e.g. A, B and C) in the
environment
(b)
Predicting the effects of ecological traps on populations
Habitat 2 is suitable, but of inferior quality,
and less preferred
Habitat 1′: low quality
contains: {cues A,B,C}
Habitat 1 has been altered (1′) and now is
unsuitable, but the cues still suggest
high-quality habitat.
The strong preference for this habitat is
now maladaptive
Habitat 2: medium quality
contains: {cues C,D,E,} but not {cues A,B}
Habitat preference based
on certain cue or cues
(e.g. A, B and C) in the
environment
Habitat 2 is suitable, but generally avoided,
because the cues used to make the habitat
choice suggest that Habitat 2 is inferior to
Habitat 1 (although the reverse is now true)
TRENDS in Ecology & Evolution
Fig. 1. Schematic of an organism’s response to environmental cues in a normal environment (a) and in an
ecological trap (b). Each habitat contains an infinite number of potential cues, a subset of which is used by
an organism for a given decision (in this case, cues A, B and C are used for habitat selection). The stronger
the cue signal, the stronger the preference for a habitat patch (indicated by the thickness of arrows).
An ecological trap occurs when a habitat becomes degraded by human activities, but the original cues
persist, misleading the organism into behaving as though the habitat patch is still of high quality.
ecological traps come from studies of grassland birds
nesting in agricultural areas: individuals settle in
suitable-looking croplands or hayfields, but their
nests are destroyed by tillage and mowing in
mid-season [15,16]. As researchers have become
increasingly aware of the ecological trap concept,
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Researchers now are beginning to ask about the
impacts of ecological traps and are attempting to
determine when they might lead to population declines.
Deterministic models have shown that, as one might
expect, when there are major differences in quality
between habitats and population sizes are small,
behavioral preferences for the habitats that yield no net
reproduction (habitat ‘sinks’[18]) can lead to population
extinction. More surprisingly, this result appears to
hold true even when patches of poor habitat represent
a relatively small proportion of the entire landscape
[6,8,19,20]. Thus, alteration of only a fraction of the
habitat in such a way that the decision-making rules
of an organism no longer yield adaptive outcomes can
result in the demise of the whole population if the
preferences of individuals are strong enough.
The consequences of an ecological trap are
particularly damaging at low population densities,
and thus represent an Allee effect because most of the
individuals can act on their (now, maladaptive) habitat
choices when there is little competition for space [6].
If most individuals have access to their preferred
(now, lower quality) habitat, the population will decline
rapidly. At high population densities, some individuals,
often subordinates, will settle in the less-preferred
(but higher quality) habitat and, as a result, their
higher fitness might sustain the population [6]. These
examples underscore the importance of measuring
basic vital rates in different habitats to identify
population sources and sinks [18,21,22] and illustrate
Table 1. Examples of ecological traps (habitat choice)
Organism
Cue and elicited behavior Alteration of native environment
Grassland birds
(many species)
Nest in habitats with low Appearance of pastures with
Mechanical harvesting of hay in Increased nestling
vegetation structure
similar structural cues as
late spring before chicks fledge mortality
grasslands
Nest sites chosen based Cutting of forests and tall grass
Increased density of predators Increased nestling
on structure
creates more edges with
and cowbird parasites near
mortality
(heterogeneity?) of
heterogeneous structure
edges
vegetation
Nesting apparently based Landscape modified directly by
Increased densities, predation, Lower nest survival
on structural cues
humans into cities, or
disease and disturbance
secondary forest, or indirectly by
exotic species
Ovipositioning on host
Forest clearcutting creates novel Frost kills host plant in clear-cut Starvation of adults
plants
habitat with host plants
areas
and lower survival
Nests preferentially in
Human alterations to the landscape: Cooler nest temperatures
Biased sex ratios and
open areas with short
barriers, denser overstory
increased nestling
vegetation and no cacti
vegetation, among others
mortality
Choose winter habitat
Power plant effluent creates
Interruption of plant operation Cold stress and
with water >20°C
attractive sites north of
(e.g. for maintenance) strands possible mortality
traditional wintering areas
manatees in inhospitable
waters
Woodland birds and
waterfowl (many
species)
Birds (many species)
Checkerspot butterfly
Euphydryas editha
Snapping turtle
Chelydra serpentina
West Indian Manatee
Trichechus manatus
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Unexpected outcome
Consequence
Refs
[15,16]
[4,12–14]
[22,45–47]
[48]
[49]
[50,51]
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TRENDS in Ecology & Evolution Vol.17 No.10 October 2002
a
Table 2. Circumstances under which evolutionary traps are known to arise
Behavior
Organism
Cue and elicited behavior
Alteration of native
environment
Feeding
Leatherback turtles
Ingestion of floating
Transparent plastics
Dermochelys coriacea
transparent prey (jellyfish)
discarded in ocean
and other sea turtle spp.
Great tit Parus major
Hatching of young timed to Global warming
coincide with prey emergence
based on day length
Unexpected outcome Consequence
Refs
Ingestion of plastics
[52,53]
Impaction of
digestive tract and
possible death
Timing of
Bird and insect prey
Reduced prey
breeding
respond differentially availability for
to warming, causing a feeding hatchlings
temporal mismatch
Emergence/ Marmots Marmota
Emergence from hibernation Dissociation between air Premature migrations Increased energetic
Migration
flaviventris and robins or altitudinal migration based temperature and date of or emergence from
costs and reduced
Turdus migratorius
on air temperature
snow melt at high
hibernation
foraging
altitude locations
opportunities
Nest
Wood duck Aix sponsa Females without nests follow Nest boxes are placed in Nests are highly
Lower egg survival
parasitism
other females to rare
conspicuous locations,
parasitized by
and decline in
breeding sites and deposit
where nesting females
conspecifics
population
eggs
cannot hide nest sites
productivity
Mate
Beetles Julodimorpha
Mate recognition based on
Beer bottles resemble
Males attempt to mate No reproductive
selection
bakewelli
morphological appearance
beetle carapace
with beer bottles
output; death
Cuban treefrog
Mating attempt based on cues High density of dead
Males attempt to mate No reproductive
Osteopilus
associated normally with a
females as a result of
with dead females on output; greater
septentrionalis and
receptive female (e.g. no
vehicular traffic
the road
exposure to traffic
Southern toad Bufo
release call, immobile)
terrestris
[25,54]
[55]
[38,39]
[56]
[57] and
refs
therein
a
For examples of habitat choice, see Table 1 and [6,8,17].
how an ecological trap can precipitate the demise of a
small population, even if the initial drop in numbers
was a result of stochastic or deterministic factors.
Evolutionary traps
The term ‘ecological trap’ was coined in the context
of negative outcomes of inappropriate habitat
selection [4]. However, the mechanism that underlies
an ecological trap is applicable more broadly because
organisms also rely on environmental cues to make a
variety of behavioral and life-history ‘decisions’,
such as when to migrate, when to reproduce, whom to
mate with, how many young to bear, what to eat, and so
on (Table 2). The broader phenomenon describing any
decision that is now maladaptive because of a sudden
anthropogenic disruption can be termed an evolutionary
trap. The mechanism underlying an evolutionary trap
is identical to that of an ecological trap (Box 1), but we
propose this new term to respect the original intent
and subsequent usage of the term ‘ecological trap’.
A few examples will illustrate the generality of the
mechanism underlying evolutionary traps:
(1) The activities and population sizes of humans have
influenced habitats worldwide [23]. Global warming
is of special concern because so many organisms rely
on day length, and its predictable correlation with
temperature, to initiate behaviors such as breeding,
flowering or migration. As temperatures rise and
increasingly become disassociated from the expected
temperature based on day length, organisms can
become desynchronized with their environment. For
example, the pied flycatcher Ficedula hypoleuca is a
long-distance migrant that relies on an endogenous
cycle (entrained by day length) to time its return to
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breeding grounds. The phenology of the temperate
breeding areas has shifted rapidly to earlier in the
spring because of global warming, and, as a result,
most birds arrive too late to take full advantage of
the food resources necessary to feed their young [24].
Similar effects have been recorded in Dutch great
tits Parus major, where breeding date has remained
unchanged over a 23-year period, although the
vegetational phenology has advanced in response to
global warming, causing a mismatch between
offspring needs and food availability [25].
(2) When behavioral ecologists manipulate cues
experimentally to discover the Darwinian algorithms
controlling a proximal mechanism, they are, in
essence, creating evolutionary traps. For example,
Møller altered mate attractiveness by manipulating
the tail length and symmetry of barn swallows
Hirundo rustica [26]. Females normally prefer males
with long, symmetrical tails because this secondary
sexual characteristic reflects male quality [27].
Males with tails made asymmetrical experimentally
paired later and experienced reduced seasonal
reproduction relative to controls [26]. To focus on
the effects of tail length and symmetry, Møller chose
his test subjects randomly, thus eliminating any
relationship that exists naturally between tail
length, symmetry, attractiveness (i.e. preference),
mate quality, and reproductive success. As a result,
female choice for males with apparently
symmetrical tails was probably sub-optimal.
(3) Humans crave fatty foods. These cravings are
probably remnants of selection in past environments
where such foods were limited in supply and
nutritious in the small quantities available [28,29].
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TRENDS in Ecology & Evolution Vol.17 No.10 October 2002
Box 1. The normal cue–behavior–environment link, and two types of
environmental change that can result in evolutionary traps
Normally, a cue in the original environment of an organism elicits a corresponding
behavioral response that is associated predictably with an adaptive outcome
(Fig. Ia). Traps (ecological or evolutionary) can arise in either of two ways. In the
first, the environment of the organism has been altered (New environment 1; Fig. Ib)
such that, although the original cue still occurs and elicits its normal behavioral
response, there no longer is a match between the behavior and the environment.
This is the scenario in which an organism settles in what appears to be high-quality
habitat, unaware of the change in the environment (e.g. a higher density of nest
predators along forest edges [a]) that can lead to decreased fitness.
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as diabetes, coronary heart disease, sleep-breathing
disorders, and certain forms of cancer [31].
(4) Humans make use of evolutionary traps to
eradicate or suppress some pest insect species.
For example, a synthetic copy of the sex pheromone
of an insect, which lures the males, is mixed with
an insecticide that kills them. These ‘attracticides’,
as they are called [32], provide a new twist to the
expression femme fatale.
Applications for conservation and wildlife management
Traps versus blatant disturbances
(a)
Original environment
Original cue
Original response
Expected outcome
No trap
(b)
New environment 1
Original cue
Original response
Unexpected outcome
Evolutionary trap
(c)
New environment 2
New cue
(mimics original
cue)
Original response
Fig. I
Unexpected outcome
Evolutionary trap
All the traps highlighted in Tables 1 and 2 have
negative effects and can lead to population declines.
In ecological and evolutionary traps, the agent of
decline is the mismatch between the Darwinian
algorithms of an organism and the actual state of the
(changed) environment. Population declines, however,
also can result from BLATANT DISTURBANCES, where the
agent of decline affects the organism directly [33].
The Cape vulture Gyps coprotheres is an example
of a species that has declined because of both a trap
and a blatant disturbance. These birds prefer
naturally to forage from high perch sites. Some
individuals fall into an evolutionary trap by perching
on high-voltage electricity towers that have been built
recently in their range, rather than available trees.
As a result, hundreds of young adults are electrocuted
annually [34]. But the decline of the Cape Vulture
also has been attributed to blatant disturbances such
as deaths by shooting and poisoning [34]. Thus,
in practice, traps can act in conjunction with other
causes for declines [17]. Given the generality and the
ubiquitousness of the underlying mechanism and its
potential negative consequences, traps will have
important implications for conservation biologists
and wildlife managers.
TRENDS in Ecology & Evolution
Traps and wildlife management
In the second case, the altered environment contains a novel element that
mimics the original cue closely enough to elicit the original behavior, but in an
inappropriate context (Fig. Ic), such as the example mentioned in the text of mayflies
ovipositing on an asphalt road [b]. In any trap, the possibility for adaptive behavior
still exists (e.g. the original environment remains an option in Fig. Ib, and the original
cue is still present in Fig. Ic), but the evolved mechanism of the organism for
evaluating environmental cues causes it to make a maladaptive choice. If the
environmental perturbation is so severe and widespread that no adaptive choice is
available, then the situation is better labeled a ‘blatant disturbance’.
References
a Gates, J.E. and Gysel, L.W. (1978) Avian nest dispersion and fledging success in
field-forest ecotones. Ecology 59, 871–883
b Kriska, G. et al. (1998) Why do mayflies lay their eggs en masse on dry asphalt roads?
Water-imitating polarized light reflected from asphalt attracts Ephemeroptera.
J. Exp. Biol. 201, 2273–2286
Nowadays, these food groups are supplied in massive
amounts in industrialized societies, but we still
prefer them to more healthy choices. In addition,
we are more sedentary than our ancestors were [30].
As a result, our remnant Darwinian cravings can
lead to health problems associated with obesity, such
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Unbeknownst to the manager, management
practices can sometimes lead organisms into traps.
Consideration of the social and physical environments
in which a species evolved [35], and how present
conditions differ from those in their ‘environments of
evolutionary adaptedness’ [36], could provide insights
into the mechanism causing individuals to behave
maladaptively and how to remedy its effect [37].
For example, Semel and Sherman [38,39] reported
that erecting nest boxes for wood ducks Aix sponsa in
clusters over open marshes (i.e. the traditional
management practice) had detrimental effects on
reproduction because it did not consider the
Darwinian algorithm of the birds for nest-site
selection. Wood ducks nest normally in cavities of
dead, standing trees and their clutch size is 10–12 eggs.
Because there is a limited number of suitable,
safe nesting cavities, young females often follow
established nesters to active nests. A follower will
sometimes lay in the cavity and then either contest
ownership of it or simply leave the eggs behind.
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TRENDS in Ecology & Evolution Vol.17 No.10 October 2002
Box 2. Evolutionary responses to evolutionary traps
A population can escape from an evolutionary trap via either phenotypic plasticity or
natural selection. In the first case, plastic behavioral traits, such as experienced-based
learning or philopatric preferences, can serve as effective mechanisms to escape
ecological and evolutionary traps, particularly for long-lived species [a]. In the second
case, selection on existing underlying genetic variation can follow several pathways.
Figure I illustrates hypothetical fitness-response curves to environmental cues A–C.
In the original environment (solid line), behavior A is elicited within a certain range of
cue values, and it is adaptive under these circumstances. When a normal behavior is
elicited, but in an altered environment, two possible evolutionary responses exist.
If a cue still carries relevant information in the novel environment, the range of cue
values under which the behavior is elicited can evolve (Fig. I, dashed line A′′). For
example, with rising global temperatures, turtle species with temperature-dependent
sex determination can be selected to ‘adjust’ their critical temperature upward or
alter their nesting behavior to maintain a balanced sex ratio [b]. Alternatively, a new
set of cues (e.g. B or C) might be necessary to identify the most adaptive situation
in a new environment. For example, as circadian rhythms and average annual
temperature become increasingly dissociated [c], migratory birds might rely more
on different phenological cues to optimize their departure date.
When a novel element in the environment is functioning accidentally as a cue
and eliciting a behavior at an inappropriate context, a narrower acceptance
threshold [d] (Fig. I, dotted line A′), might enable discrimination between the
original and the novel cue. Additional cues (B, C, among others) can be added to
the recognition template to ensure that the original behavior is elicited only in its
adaptive context. For example, a leatherback turtle Dermochelys coriacea might
use a refined set of criteria, including the size, color or smell of an object to
discriminate between a jellyfish and a floating transparent plastic bag (Table 2).
C
C
Fitness
Cu
es
B
B
Original
A′
A′′
A
Cue values
TRENDS in Ecology & Evolution
Fig. I
However, there might be some constraints to how quickly a trait can evolve if
the selective environment is dissociated temporally from the environment where
the cue elicits the behavior [e]. Furthermore, the speed at which each species
evolves in response to an alteration of their environment is likely to be different [a].
If the phenology of predators and prey evolve in response to change (e.g. global
warming) at different rates, predator and prey dynamics can become mismatched
[c,f], which can disrupt biological interactions in the broader ecosystem [g].
References
a Kokko, H. and Sutherland, W.J. (2001) Ecological traps in changing environments:
ecological and evolutionary consequences of a behaviourally mediated Allee effect.
Evol. Ecol. Res. 3, 537–551
b Janzen, F.J. (1994) Climate change and temperature-dependent sex determination in
reptiles. Proc. Natl. Acad. Sci. U. S. A. 91, 7487–7490
c Both, C. and Visser, M.E. (2001) Adjustment to climate change is constrained by arrival
date in a long-distance migrant bird. Nature 411, 296–298
d Sherman, P.W. et al. (1997) Recognition systems. In Behavioural Ecology: An Evolutionary
Approach (Krebs, J.R. and Davies, N.B., eds), pp. 69–96, Blackwell Science
e Visser, M.E. et al. (1998) Warmer springs lead to mistimed reproduction in great tits
Parus major. Proc. R. Soc. Lond. Ser. B 265, 1867–1870
f Buse, A. et al. (1999) Effects of elevated temperature on multi-species interactions: the
case of pedunculate oak, winter moth, and tits. Funct. Ecol. 13 (Suppl. 1), 74–82
g Davis, A.J. et al. (1998) Making mistakes when predicting shifts in species range in
response to global warming. Nature 391, 783–786
By placing boxes in groups over open water sites,
managers attempted to make cavities easier to find.
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Unfortunately, the conspicuousness of nest locations
made it too easy for females to follow others to their
active nests, resulting in super-normal intraspecific
parasitism in the form of egg dumping. Clutches of
30–50 were common. Such huge clutches could not
be incubated properly; many nests were abandoned
and eggs were crushed. As a result, individual
reproductive success suffered and population
productivity declined. When boxes were erected
individually on tree trunks in dense woodlands, the
natural difficulty of following conspecifics through
occluded habitats was re-established. Parasitism was
reduced to its normal, nondeleterious levels (3–4 eggs
per nest) and productivity increased. The traditional
management practice represents a trap because
female wood ducks could choose to nest in a hidden
cavity in the woods or in a visible box over an open
marsh. Females are attracted to active nest sites and
these are easiest to find in the open, although their
choice might be detrimental to their fitness.
Distinguishing evolutionary traps from natural
‘disturbances’
In the short term, organisms are ‘trapped’ in their
evolved proximate mechanisms and Darwinian
algorithms [9] to respond to cues that now occur in
a novel context. However, these traps are not
necessarily evolutionary dead ends. Populations
can avoid extinction if the negative effects of the trap
on reproduction and survival are not too severe,
if there is some genetic variation or behavioral
plasticity in the response to the novel environment
within the population [6], and if the population is
large enough and can persist long enough for
adaptation to occur (Box 2).
From the perspective of an evolutionary purist,
an ecological or evolutionary trap does not differ
fundamentally from any natural ‘disturbance’ and its
associated natural selection. Traps, however, are
induced by humans and generally occur in a shorter
time span than natural environmental changes.
If the magnitude of the change exceeds the range of
values normally encountered, some populations
might not have the ability to survive the novel
circumstances. Thus, ecological and evolutionary
traps will be of concern to conservation biologists and
wildlife managers who wish to minimize short-term
losses of natural populations to human activities.
Solutions for the future
The original ecological trap concept is important
because it reveals how rapid anthropogenic
environmental change can cause organisms to
evaluate incorrectly the quality of their altered
habitat. An analysis of the mechanism underlying
the ecological trap suggests that novel environments
can miscue other behaviors. Realizing this will
undoubtedly lead to the recognition of many more
potential examples. However, ecological and
evolutionary traps can be difficult to identify because
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TRENDS in Ecology & Evolution Vol.17 No.10 October 2002
Glossary
Blatant disturbance: an anthropogenic alteration in the environment that results in decreased
fitness of an organism independent of its behavior.
Darwinian algorithm: behavioral decision-making rule based on environmental cues that is
adaptive in the original evolutionary environment of the organism.
Ecological trap: in an environment that has been altered suddenly by human activities, an organism
makes a maladaptive habitat choice based on formerly reliable environmental cues, despite the
availability of higher quality habitat. An ecological trap is a specific type of evolutionary trap.
Evolutionary trap: in an environment that has been altered suddenly by human activities, an
organism makes a maladaptive behavioral or life-history choice based on formerly reliable
environmental cues, despite the availability of higher quality options.
Acknowledgements
M.A.S. was supported by
a US Environmental
Protection AgencyScience To Achieve
Results fellowship. P.W.S.
was supported by the
National Science
Foundation and the
Agricultural Experiment
Station at Cornell
University. We thank
G.S. Boomer, P.M. Buston,
S.M. Flaxman, T.A. Gavin,
M.E. Hauber, C.S.
Jennelle, M.M.M. Kéry,
D.I. MacKenzie,
J.D. Nichols, J.R. Sauer,
T.S. Sillett, K.R. Zamudio
and several anonymous
reviewers for helpful
comments.
the agent of decline might be a slightly modified set of
environmental variables in which a formerly adaptive
behavior is elicited, or an inconspicuous, novel factor
that elicits a formerly adaptive behavior, but in an
inappropriate context (Box 1). Some caution is also
required before invoking a trap because a behavioral
strategy that reduces survival or reproduction in
the short term is not necessarily maladaptive if it
enhances lifetime reproductive success. The entire
life cycle of an organism should be taken into
account because a novel environment could have
compensating effects on the survival and
reproductive output of different life stages [14].
Behavioral ecology and evolutionary psychology
are being integrated increasingly into the fields of
conservation biology and wildlife management
[40–44]. Ecological and evolutionary traps are prime
examples of useful concepts that can result from the
merging of these fields. For example, if a population
decline is due to a trap it probably is remedied more
easily than if it is due to a blatant disturbance.
Manipulative experiments (e.g. choice experiments)
are essential to identify the cues for a given behavior.
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Speciation in nature: the threespine
stickleback model systems
Jeffrey S. McKinnon and Howard D. Rundle
The threespine stickleback Gasterosteus aculeatus species complex is an
important natural model for speciation research because it includes several
replicated sets of coexisting, divergent forms that are also experimentally
tractable. Recent research has begun to emphasize lesser known divergences
within the complex in addition to the well-studied limnetic–benthic pairs, as
well as exploring a broader range of speciation mechanisms. With the goals of
making general inferences about speciation in nature and bringing this body
of research to a wider audience, we have surveyed studies from the entire
species complex. We find that stickleback speciation is often rapid, that the
geographical context of speciation is variable and often complex, and that
many, diverse traits have often diverged early in the speciation process. We find
no unambiguous evidence of founder-effect speciation, but much evidence that
divergent natural and sexual selection have been central to the evolution of
reproductive isolation in this species complex.
Published online: 31 July 2002
In recent years, significant progress has been made
in our understanding of how speciation occurs in
nature [1]. An important component of this endeavor
has been the study of the threespine stickleback
http://tree.trends.com
Gasterosteus aculeatus complex, beginning with work
by McPhail, Hagen and their colleagues [2,3].
Research in this species complex has focused on a
diverse collection of distinct model ‘systems’, each
involving a pair of phenotypically divergent forms
that coexist in nature and exhibit various degrees of
reproductive isolation (Fig. 1, Table 1). The
limnetic–benthic lake pairs are the best studied but
not the only example. Work has also been done on
several other stickleback systems and, with an
accelerating pace of research over the past decade,
the literature is now extensive. Here, we update and
expand upon McPhail’s 1994 review [4], integrating
the results from a survey of the entire complex and
presenting the general patterns and conclusions that
emerge concerning speciation in nature.
The threespine stickleback species complex
The natural history of the threespine stickleback is
characterized by repeated episodes of colonization by
the marine stickleback (including freshwater-breeding
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