Insect Emigration in Patchy Habitats:
A Review of the Effects of Patch Geometry and Context

INTRODUCTION

	Insect emigration is of extreme ecological importance.  As a fundamental process in population and 
community dynamics, emigration has important implications for many lines of research including population 
modeling and plant-herbivore interactions.  Since natural environments are spatial mosaics of different habitats 
(Wiens et al. 1993), the relationship between characteristics of individual habitat patches and emigration 
processes needs to be understood. The phenomenon of most concern here is the movement of insects between 
patches of habitat.  Very often these are defined as very local patches of host plants in studies of plant-
herbivore interaction (e.g. Root 1973), but are usually more inclusive of other environmental features, such as 
"meadows," in metapopulation research (e.g. Hanski and Gilpin 1991).  Since patches are often easiest to define 
for host specific phytophagous insects, all of the research to date has used such insects (mostly Coleoptera and 
Lepidoptera). This paper presents of review of recent research into this area.

THEORETICAL BASIS

	Models of insect movement in patchy habitats provide a theoretical basis for claims that patch 
characteristics influence emigration rates (Stamps et al. 1987, Turchin 1986).  Patch geometry, especially 
size, and patch context are two relatively easily measured characteristics which have been investigated.

	Stamps et al. (1987) created a model of animal movement which, although intended to be general to 
many animal taxa, drew heavily from research in insect systems (e.g. Kareiva 1985, Turchin 1986).  The 
details of their models are not discussed here.  Instead, it will be sufficient to note the qualitative predictions 
they present.  Primarily, the relative amount of edge around a given habitat patch was suggested to be a better 
predictor of emigration than either patch size or shape alone.  This perimeter:area ratio affects the frequency 
with which an animal encounters the patch boundary, events which provide opportunity for emigration.  These 
models predict that increasing the perimeter:area ratio will result in increased levels of emigration.  Also of 
importance in these models is the relative permeability of the patch boundary.  Boundaries range from "hard" 
to "soft," representing edges which inhibit or allow emigration respectively.  The formation of the patch 
boundary, and its degree of permeability, is a function of the adjacent habitats, and is therefore referred to 
here as "patch context."

	Turchin (1986) used a model of insect movement to investigate the effect of patch size on emigration, 
but also compared the model predictions to empirical data of emigration in a coccinelid beetle.  The investigation 
uses a "random walk" model to simulate insect movement.  Similar to the model of Stamps et al. (1987), 
Turchin's model predicts that emigration will increase in small patches in accordance with the increasing 
perimeter:area ratio.  The empirical test of the model resulted in surprisingly precise prediction of emigration 
rate (Turchin 1986).  Observed movement of the beetles differed by less than 10% of the model predictions, 
suggesting that the assumptions of the model including the random walk were valid.  

PATCH GEOMETRY

	Kareiva (1985) provides one of the earliest experiments designed to test the effect of patch size on 
insect emigration. Using the phytophagous beetles Phyllotreta cruciferae and P. striolata, his study set out to 
investigate one aspect of the resource concentration hypothesis (Root 1973), that herbivores will tend to 
remain in large pure stands of their host plant.  Replicated treatments of four different patch sizes and two 
types of surrounding vegetation indicated that emigration was consistently increased in smaller patches.  The 
author concluded that a null model of random movement could not be rejected by the findings since the 
difference in emigration with patch size were most simply explained by changes in the perimeter:area ratio of 
the patch.  Given a random walk model of movement, an insect in a smaller patch has a higher probability of 
encountering and passing through the patch boundary during any particular move, resulting in higher levels of 
emigration.  This suggests that it would be possible for there to be a "critical patch size," below which 
emigration exceeds local births and immigration, and population persistence is not possible (Kareiva 1985).  
Recognizing the potential effect of the changing environmental conditions on insect movement, the author 
pointed to the need to investigate behavior at the patch boundary in order to check the validity of the random 
walk model. Kareiva's (1985) work is often cited in the literature that follows. 

	The findings of Kareiva (1985) and Turchin (1986) led many researchers to consider the consequences 
of patch size.  McCauley (1991) sought to investigate its effect on the genetic structure within a population of 
the red milkweed beetle, Tetraopes tetraophthalmus (Cerambycidae).  This study again found that emigration 
rate increased with smaller patch sizes and that this did indeed have significant genetic consequence.  However, 
McCauley cited the influence of sex ratio and conspecific abundance on emigration in red milkweed beetles 
(Lawrence 1987, 1988) as a mechanism for the observed pattern.  Presumably, the tendency for the beetles to 
stay longer in large patches may be due to the covarying factors related to mate availability.  This author did 
not address the potential fit of a null model of random movement even though it may provide sufficient 
explanation. The problem in distinguishing among various mechanistic hypotheses reminds one of the need to 
control for many of the factors which are bound to be confounded with patch size in the natural world.

	  Continuing a long line of research on the red milkweed beetle, Matter (1996) investigated beetle 
movement in a naturally occurring system and made comparisons to the many preceding studies of this species.  
Unoccupied patches were significantly smaller than occupied patches, and patch size was found to have a 
stronger effect on occupancy than patch isolation.  A comparison of patch size and mean emigration rates in this 
and previous studies of the red milkweed beetle also suggests that dispersal is greater in "landscapes" 
comprised of smaller sized patches.  These findings are significant in that they reveal that the so far general 
pattern is expressed at a larger scale in systems of patches, but also suffers from the limitations of 
nonexperimental field studies since it does not provide insight to the responsible mechanisms.

	A second major line of research into the relationship of patch size and insect emigration stems from 
studies of butterflies in Europe (e.g. Hanski and Gilpin 1991).  Primarily addressing population structure and its 
implications for conservation, the butterfly literature has become a rich source of ecological research.  
Findings in this field compliment those discussed above which were primarily inspired by tests of the resource 
concentration hypothesis (Root 1973).  

	Perhaps the first in this field to write of a possible influence of patch size on emigration was Warren 
(1987) in his study of the heath fritillary, Melicta athalia.  Although the study was not designed to test for 
such an effect, the fact that the smallest site had the shortest residence times was enough for speculation that 
emigration rate there was relatively high.   The author cautioned that this result could likely have been due to 
response to high population density or decreasing site quality, but the suggestion was enough to stimulate 
further research in this area.

	Very little notice was paid to Warren's speculation until recently. The growing sophistication of 
metapopulation modeling (e.g. Hanski and Thomas 1994) seems to be encouraging efforts to refine our 
understanding of the proximal influences on processes such as emigration.  Hill et al. (1996) designed their 
study to explicitly investigate the patch size-emigration relationship.  In attempting to understand the 
metapopulation structure of Hesperia comma in England, the authors recognized the potential for emigration 
rates to vary among patches and included this in their investigation.  A strong and consistent effect of 
increasing emigration rates from smaller patches was evident.  This suggested to the authors that a "critical 
patch size" (Kareiva 1985) may exist for this species as well.  The behavioral mechanisms suggested here 
which could produce this pattern were only speculative.  Like Kareiva (1985), the authors note that patch 
geometry in the form a the perimeter:area ratio combined with random movement may be sufficient, but did 
point out that behavioral observations at the patch boundary were lacking and could suggest other mechanisms.  
Additionally, confounding effects such as differential permeability of patch boundaries could not be separated.

	The population structure of the Glanville fritillary, Melitaea cinxia, in southern Finland is relatively 
well understood (Hanski and Thomas 1994).  A recent experimental study has shed further light on the proximal 
influences on emigration in this system (Mikko et al. 1996).  By releasing laboratory reared marked individuals 
into empty patches, Mikko et al. (1996) again found that the effect of decreasing patch size was to increase 
emigration rates.  The authors uncritically attribute this effect to simple perimeter:area ratios and the 
increased likelihood of a butterfly in a small patch to encounter the boundary and leave the patch.  Precise 
behavioral mechanisms were not considered further.

	Building on the butterfly metapopulation literature, Sutcliffe et al. (1997) investigated butterfly 
movement between patches of a relatively common species of ringlet butterfly, Aphantopus hyperantus in 
England.  Essentially mirroring the results of studies of the English butterflies discussed above, small patch 
size was again associated with increased emigration in a way that was "exactly what... would have been 
expected if A. hyperantus were responding to habitat geometry." The authors point out that patch boundary 
permeability does not confound conclusions pointing to a simple geometric mechanism since frequency of 
encounter with the boundary still increases in small patches.  However, the authors fail to recognize the 
possible effects of differential permeability among patches which has the potential to correlate with patch size 
in some systems.  Observation of behavior at patch boundaries was again lacking.

	All of the above studies combine to suggest that increased emigration with decreasing patch size is a 
general phenomena.  While few of these studies have suggested alternative behavioral explanations (e.g. 
McCauley 1991), the hypothesis that increased frequency of encounters with a patch boundary leads to 
increased emigration has been taken for granted.  Only Kareiva (1985) provided sound experimental data to 
support this claim, but still recognized the possibility of more complex behaviors at the patch boundary as 
being potentially important.  

	Consideration of the only study to provide contradictory trends in the patch size-emigration 
relationship (Bach 1988) illustrates the need to consider interactions at or near the patch boundary.  In her 
experimental work with different species of phytophagous chrysomelid beetles,  she examined the effects of 
host plant patch size as well as that of the surrounding habitats.  Interestingly, she found that in patches which 
were surrounded by monocultures of a nonhost species, emigration was actually lower in small patches and 
increased with increasing patch size.  Such anomalous results do not necessarily contradict the findings 
discussed above, but instead points to the interacting effects of patch context on insect emigration.  In fact, her 
findings are entirely consistent with the theoretical basis advocated by Stamps et al. (1987), an aspect which 
will be more fully developed in the following sections.

PATCH CONTEXT

	As demonstrate above, many studies have considered the possible effects of patch size, and especially 
perimeter:area ratios, on emigration.  Fewer investigations have included consideration of the patch context, 
or boundary permeability.  Ecologists are increasingly recognizing the importance of the landscapes in which 
observed ecological phenomena take place (e.g. Wiens et al. 1993), and studies of insect movement between 
populations is no exception.  

	As suggested by Stamps et al. (1987), the type of boundaries which delineate a patch may have strong 
interacting influences with patch size on emigration.  These authors used the terms "hard" and "soft" to 
describe patch boundaries which are relatively impermeable to permeable respectively.  Their models indicated 
that patch geometry would have greater effect in patches with softer boundaries, and would be totally 
inconsequential in very hard boundaries which do not permit emigration regardless of geometry.   Permeability 
is a function of the composition of the edge of the patch and is therefore actually a characteristic of the 
surrounding habitats.

	Kareiva's (1985) study discussed above included treatments of two different types of surrounding 
vegetation.  Comparisons were made of plots surrounded by "cultivated garden" and "goldenrod fields," or in a 
second experiment between plots in "cultivated garden" and in "grass meadow."  The four size classes were 
each replicated twice in each background treatment.  As noted above, the size of the patch had consistent 
effects on emigration rates.  However, the effect of the surrounding vegetation was not so predictable.  While 
surrounding vegetation alone showed no significant effects on emigration, it did alter the relationship between 
patch size and emigration in the second experiment.  Thus, Kareiva concluded that the effect of surrounding 
vegetation depended on the size of the patches and the contrast in habitats being examined.

	Revisiting Bach's (1988) experimental study, one finds strong empirical support for the effect of patch 
boundary on emigration rates.   The effect of alternative surrounding vegetation treatments of either nonhost 
plants or mown field were strong enough to override any tendency for decreasing patch size to increase 
emigration.  A clue to the mechanism responsible for this anomalous result is found in the fact that patch size 
alone did not reveal a consistent relationship to emigration rate, but appeared to only have an influence in plots 
with a particular surrounding habitat.  Her study indicates that the surrounding nonhost vegetation acted as a 
"reflective barrier," causing beetles which landed on a nonhost plant just outside of the patch boundary to 
reverse their direction of travel and return to the patch.  This mechanism is supported by her observation that 
the beetles spend significantly less time on nonhost plants, and when given the choice will only remain on host 
plants.  Since small patches have a higher perimeter:area ratio, insects in small patches were more susceptible 
to this "trapping effect," due to higher rates of contact with nonhost plants.  Thus, this study clearly 
illustrates that patch size does not act independently of patch permeability and that behavioral mechanisms 
such as host preference may determine how these environmental factors effect emigration.  

	As predicted by Stamps et al. (1987), Bach (1988) found that a patch boundary which had very low 
permeability prevented emigration from these patches.  The "reflective barrier" described by Bach is in fact a 
very "hard edge" in the usage of Stamps et al.  However, a direct connection to this theory may be lacking 
since Bach did not find the usual patch size-emigration relationship even in patches with "softer" boundaries.  
Clearly, the relationship of boundary permeability and patch size requires further investigation.

	Drawing again from the lepidopterological literature, Mikko et al. (1996), in addition to demonstrating 
an inverse relationship between emigration and patch size, also found a significant effect of adjacent habitat on 
emigration of the Glanville fritillary.  In essence, patches which formed a greater proportion of their edge with 
adjacent grassland habitats, as opposes to forested ones, had significantly higher rates of emigration.  Two 
reasons for this association are stated by the authors.  The first is that the adjacent habitats which are open 
are more easily moved into by the butterflies, apparently due to less contrast with the patch habitat.  Although 
not formally noted in this study, this would imply that forested boundaries had the effect of retaining 
butterflies within the patch.  A second possible mechanism for increased emigration from patches with less 
forested edge may be passive dispersal caused by a lack of wind shelter in such patches.  The former 
mechanism requires a behavioral reaction to the patch boundary while the latter does not.  Noting this 
distinction, Mikko et al. also called for further investigations into insect behavior at this interface.

	Thus, both theoretical considerations (Stamps et al. 1987) and empirical data point to the important 
influence of the patch context which determines what type of boundary delineates a given patch.

BEHAVIOR AT PATCH BOUNDARY

	Although emigration rates may often be associated with environmental variables such as patch 
characteristics, one must not forget that it is fundamentally a behavioral process.  This notion almost seems to 
be lost in many descriptive studies.  As seen in most of the studies above, ecological correlates of emigration 
are relatively common while investigation into the mechanistic details is often lacking in spite of widespread 
recommendations for research into this area.  Since emigration is defined as movement across an ecological 
boundary, it is reasonable to ask how this boundary affects movement behavior.

	Kareiva (1985) explicitly stated an assumption which is only implicit in most explanations of the patch 
size-emigration relationship. The use of such random null models are reasonable only in the sense that they 
closely approximate insect movements even though the movements themselves may be the result of complex 
deterministic factors (Turchin 1986).  This assumption rules out the possibility of behavior varying in space, 
specifically at the patch boundary.  This assumption seems counterintuitive, and yet many studies have 
employed such random models to successfully predict insect movement (Turchin 1986).  While this implies that 
insect movement may indeed be unresponsive to patch boundaries, there is empirical evidence to the contrary.

	Lawrence (1982) compared the movement of cerambycid beetles within and at the boundary of their 
host plant patches.  His results clearly show that within a patch the insects' movements provided a good fit to a 
random model.  However, this contrasted strongly with the observed behaviors at the patch edge where the 
beetles showed a strong tendency to move in a direction toward the center of the patch.  He concluded that the 
beetles were perceiving the patch boundary and altering their behavior to remain in it.

	 A similar test of random walk models was performed by Marsh (1995).  Investigating flight behavior 
of coccinelid beetles released at the patch boundaries of different types, he found that flight direction was most 
often into the host patch regardless of surrounding vegetation.  Flight duration was apparently unaffected and 
some beetles even flew over and beyond the host patch.  The author concludes that movement at the patch 
boundary is nonrandom and that its effect would be to decrease emigration rates in this system.  Importantly, 
he points out that in-flight behavior may not be similarly affected, possibly accounting for the success of 
Turchin's (1986) model in depicting movement of the same species.

	The previously discussed work of Bach (1988) also provides insight into insect behavior at a patch 
boundary.  Her study indicated that surrounding nonhost vegetation presented a "reflective barrier" to 
emigration.  Other research cited in this work demonstrated that residence time was significantly greater on 
host than nonhost plant, indicating that host selection was playing a role in the insects' movement patterns, and 
possibly explaining the effect of the patch boundary.  Given the profound importance of host selection behavior 
in phytophagous insects (Bernays and Chapman 1994), such mechanisms may be a common influence on 
emigration.

	Similar reasoning applies to the suggestions of Lawrence (1987, 1988) that potential demographic 
correlates of patch size, especially conspecific density and sex ratio, influence emigration in some systems.  
McCauley (1991) used these findings to suggest that the tendency for this species to stay in large patches was 
due to a greater possibility of finding mates in these patches.  However, experimental works such as that of 
Kareiva (1985) indicate that such influences cannot generally explain emigration patterns in different systems.

SUMMARY

	Understanding of proximal influences on insect emigration is a prerequisite for establishing realistic 
population models as well as generalized ecological theory.  The effects of local habitat patch characteristics, 
both patch geometry and patch context, have received relatively little attention, but in a number of cases have 
been found to be important.  An association of increasing emigration rate from patches of smaller size has been 
repeatedly demonstrated in several insect taxa. However, important exceptions to this trend emphasize the 
influence of patch context as well.  It is apparent both in theory and in the field that these factors are 
interrelated and the size-emigration relationship is strongly mediated, and sometimes overwhelmed, by the 
insects' response to adjacent habitats.

	Insect behavior at the patch boundary is an essential mechanism determining these patterns.  While 
random movement models are sometimes successful in predicting emigration rates, empirical data suggest that 
insects do indeed form behavioral responses to patch boundaries which affect emigration. 

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