Refuge utilization and preferences between competing intertidal crab species

Many invertebrates avoid predation risk by seeking and defending refuges that can be in limited supply, producing strong intra- and inter-specific interference competition. Previous experimental studies in central Chile demonstrated that interference competition for refuges is the primary factor driving habitat segregation between the predatory crabs Acanthocyclus gayi and A. hassleri, with the latter species monopolizing galleries inside mussel beds in the mid intertidal zone and limiting A. gayi to rock crevices. Yet, habitat partitioning between rival species can result from differences in habitat preferences and not solely from interference interactions. Moreover, since A. gayi is also known to shelter in turf-forming algae (predominantly Gelidium), which dominates extensive areas in the low intertidal zone, among-sites variation in the turf morphology and abundance could modify habitat preferences and the pattern of inter-specific interactions. We experimentally evaluated refuge habitat preferences of individual, similarly-sized adult A. gayi and A. hassleri in the laboratory, comparing choice patterns across multiple trials with paired combinations of the main refuge types commonly used by crabs in the field: a) mussel galleries, b) rock crevices, c) short algal turf, and d) tall algal turf. Our results showed that both species display a strong ranking of preferences for some refuge habitats over others. In general, mussel galleries were the preferred refuge type for both crab species, but their preference rankings changed depending on turf morphology. When turf was short, A. hassleri and A. gayi made identical refuge choices, strongly preferring mussel galleries over crevices and these over the short turf. In contrast, when the turf was tall A. gayi selected equally the tall turf or mussel galleries, and these were strongly preferred over crevices. A. hassleri, on the other hand, largely ignored tall turf and kept the highest preference for mussel galleries. A field experiment in which crabs were offered to foraging birds demonstrated that A. hassleri is more susceptible than A. gayi to predation by kelp gulls when outside refuges. Differences in patterns of coloration between crabs may underlie between-species differences in predation susceptibility and their habitat choices. These results suggest that the among-site differences in turf morphology (height and shape of fronds), which is largely driven by varying intensity of upwelling, could affect crab preferences for refuge habitats and the relative importance of inter-specific interference competition.     

Habitat preference of three common fishes for seagrass, <Emphasis Type="Italic">Caulerpa taxifolia</Emphasis>, and unvegetated substrate in Moreton Bay,Australia

A decrease in seagrass cover and a commensurate increase in Caulerpa taxifolia distribution in Moreton Bay have prompted concern for the impact that habitat change may have on faunal communities. Therefore, it is important to understand the patterns of habitat use. We examined habitat selection of three common seagrass species: double-ended pipefish (Syngnathoides biaculeatus), eastern trumpeter (Pelates quadrilineatus), and fan-bellied leatherjacket (Monacanthus chinensis) using a mesocosm experiment. Fish were given three possible habitat pairings (1) seagrass and C. taxifolia, (2) seagrass and unvegetated, and (3) C. taxifolia and unvegetated. Observation trials were conducted during the day and night over two days. In all trials, fish preferred vegetated habitat (seagrass or C. taxifolia) over unvegetated habitat (sand). In seagrass and C. taxifolia trials, all species preferred seagrass significantly over C. taxifolia. Habitat use patterns did not differ between day and night trials. Caulerpa taxifolia provides a valuable structured habitat in the absence of seagrass; however, it is unclear if C. taxifolia meadows provide other resource benefits to fishes beyond that of shelter.     

Recruitment of the infaunal bivalve Soletellina alba (Lamarck, 1818) (Bivalvia: Psammobiidae) in response to different sediment types and water depths within the intermittently open Hopkins River estuary

Studies examining recruitment processes for soft-sediment macroinvertebrate fauna in intermittent estuaries are rare and most studies of active habitat selection have been tested in the laboratory rather than the field. The present field study examined whether recruitment of the infaunal bivalve Soletellina alba was influenced by water depth and sediment particle size in the intermittent Hopkins River estuary, southern Australia. The number of recruits in sediment trays differed between water depths, but active habitat selection was not evident across treatments of varying sediment particle size. The use of sediments with varying particle sizes also provided an opportunity to identify potential discontinuities in body-size distributions of recruits associated with varying habitat architecture. The length (mm) of recruits was converted to the same scale used to express sediment particle size (i.e. phi units: ? = − log₂ of sediment particle size). The size of recruits differed across water depths, but did not differ across treatments with fine (? = 3) versus coarse (? = 1) sediment, and no relationships were apparent between bivalve size and sediments consisting of varying particle size. These patterns of recruitment do not correspond with the distribution of adult S. alba within the Hopkins River estuary. Previous sampling has shown that abundances of juvenile and adult S. alba are variable across time, site and water depth, but are often greater at the deeper water depth (1.05 m below the Australian Height Datum). However, recruitment during the present study was greatest at the shallower water depth (0.05 m below AHD), and the apparent absence of active habitat selection suggests that the distribution of adults is unlikely to be attributable to differences in recruitment associated with sediments of varying particle size.     

The role of parasitoids in evolution of habitat and larval food plant preference by three Pieris butterflies

This article attempts to explain that parasitoids provide the evolutionary pressure responsible for relationships between habitat use and larval food plant use in herbivorous insects. Three species of butterflies of the genus Pieris, P. rapae, P. melete, and P. napi use different sets of cruciferous plants. They prefer different habitats composed of similar sets of cruciferous plants. In our study, P. rapae used temporary habitats with ephemeral plants, P. melete used permanent habitat with persistent plants, although they also used temporary habitats, and P. napi used only permanent habitat. The choice experiment in the field cages indicated that each of the three butterfly species avoided oviposition on plants usually unused in its own habitat, but accepted the unused plants which grew outside its own habitat. Their habitat use and plant use were not explained by intrinsic plant quality examined in terms of larval performance. Pieris larvae collected from persistent plants or more long lasting habitats were more heavily parasitized by two specialist parasitoids, the braconid wasp Cotesia glomerata and the tachinid fly Epicampocera succincta. The results suggest that Pieris habitat and larval food plant use patterns can be explained by two principles. The evolution of habitat preference may have been driven by various factors including escape from parasitism. Once habitat preference has evolved, selection favors the evolution of larval food plant preferences by discriminating against unsuitable plants, including those which are associated with high parasitism pressures. Received: December 3, 1998 / Accepted: January 20, 1999     

Habitat selection by recruits establishes local patterns of adult distribution in two species of damselfishes: Stegastes dorsopunicans and S. planifrons

Local patterns of adult distribution in organisms that disperse young as pelagic larvae can be determined at the time of recruitment through habitat selection or, shortly thereafter, through post-recruitment processes such as differential juvenile survivorship and interspecific competition. This study addresses the importance of habitat selection by recruits in establishing the local pattern of adult distribution in two sympatric Caribbean damselfish species, Stegastes dorsopunicans and S. planifrons. Both species inhabit shallow reefs but show little overlap in their distribution; S. dorsopunicans predominates in the reef crest and S. planifrons occurs primarily on the reef slope. Furthermore, S. dorsopunicans is associated with rocky substrate, while S. planifrons occupies live coral. The substrate cover follows a similar pattern with coral being much less common on the reef crest than on the reef slope. Monitoring recruitment every other day in reciprocal removal experiments and artificial reefs indicates that the observed pattern of local adult distribution is a product of habitat selection for both species. The presence or absence of conspecifics did not influence recruitment patterns for either species. Stegastes dorsopunicans recruited primarily to shallow, rocky areas, appearing to cue on both substratum type and depth. Stegastes planifrons recruited exclusively to coral substratum independent of depth. These results indicate that local adult patterns of distribution can be explained by habitat selection at recruitment, and that substrate type and depth may be important cues. Received: 27 May 1997 / Accepted: 4 January 1998     

Competition between two congeneric stream gobies for habitat in southwestern Shikoku, Japan

Competition between two congeneric gobies, Rhinogobius sp. LD (large-dark type) and CB (cross-band type), for habitat was studied in a tributary of the Shimanto River, southwestern Shikoku, Japan. Habitat use by CB, measured by water depth, current velocity, and substrate, was compared between before and after the removal of LD. After the removal of LD, CB shifted their habitat use to coarser substrates than previously, suggesting a release from effects of LD. This result supports a hypothesis from our previous study that habitat partitioning between CB and LD is a consequence of interference effects of LD on CB.     

Modelling nearshore intertidal habitat use of young-of-the-year Pacific sand lance (Ammodytes hexapterus) in Barkley Sound, British Columbia, Canada

The Pacific sand lance (Ammodytes hexapterus) is an ecologically important prey species for many vertebrate marine predators in the Pacific Northwest. In this study, we examined the use of intertidal habitat by young of the year (YOY) sand lance in Barkley Sound, British Columbia. We selected 14 of the original 29 independent environmental variables for modelling based on univariate analysis for variable selection. These were then used to model sand lance presence-absence using a classification tree approach. Based on our models we found that sand lance avoided mud and intertidal eelgrass. For sites that had very little mud and no intertidal eelgrass, sand lance preferred sediment size mean ≥1,918 μm or sites with sediment size mean ≤1,918 μm but with relatively well sorted sediment (sorting values ≤2.56 SDs, used as a heterogeneity index of the substrate grain size). Adjacent subtidal characteristics were not found to be important at this scale of study. This suggests intertidal substrate characteristics and presence-absence of intertidal eelgrass are the main influences on occurrence of YOY in the intertidal. Our results support the hypothesis that sand lance are associated with particular sediment types, however intertidal sediment types used by sand lance in our study differ from known preferences of sand lance for subtidal sediment types. This difference may be due to unique habitat constraints for intertidal versus subtidal regions. Although it is unknown if the results of this study are widely applicable, the results begin to identify intertidal habitat features that are important for Pacific sand lance.     

Habitat patch size and mating system as determinants of social group size in coral-dwelling fishes

It is thought that the size and dispersion of habitat patches can determine the size and composition of animal social groups, however, this has rarely been tested. The relationship between group size, the mating system, and habitat patch size in six species of coral-dwelling gobies was examined. For all species, there was a positive correlation between coral colony size and social group size, however the strength of this relationship varied among species. Paragobiodon xanthosomus exhibited the strongest relationship and a manipulative field experiment confirmed that coral colony size limited group size in this species. For other species including Paragobiodon melanosomus and Eviota bifasciata, either a highly conservative mating system (P. melanosomus), or increased mobility (E. bifasciata) appeared to disrupt the relationship between habitat patch size and group size. There was no consistent relationship between the mating system exhibited and group size among the species investigated. These results demonstrate that habitat patch size, mobility, and mating systems can interact in complex ways to structure group size even among closely related species.     

Habitat structure and predators: choices and consequences for rodent habitat specialists and generalists

Summary Both habitat structure and risk of predation are thought to influence rodent community composition in different habitats, but experiments on the degree to which these factors determine the use of habitat by rodents are lacking. I sought to discover (1) if cover density altered habitat choice and (2) if cover density affected the vulnerability to predators of two rodents, a habitat specialist and a habitat generalist. In laboratory experiments, the habitat specialist, the red-backed vole (Clethrionomys gapperi), preferred greater densities of both vertical (wall) and horizontal (ceiling) cover. The habitat generalist, the deer mouse (Peromyscus maniculatus), also preferred greater densities of vertical and horizontal cover, but its preferences were weaker and more inconsistent than those of C. gapperi In tests of vulnerability to domestic ferrets, C. gapperi were more vulnerable in arenas with less vertical cover, while P. maniculatus did not differ in vulnerability between the two vertical cover densities used. Vulnerability to predators in differing densities of horizontal cover was not tested because of the reduced differences in preference for this cover type between the rodent species. Risk of predation is one explanation for C. gapper's inherent preference for denser cover.     

Selection of resting sites by the European hare as related to habitat characteristics during agricultural changes

The decline of the European hare Lepus europaeus populations has been shown to be correlated with agricultural intensification, which has caused loss of habitat heterogeneity. We aimed to investigate the diurnal spring habitat selection using Jacobs’ second selection index in an intensively cultivated farmland to reveal how the habitat choice by resting hares was affected by changes in the habitat availability. Clearance counts of hares driven out of known areas were made during an 11-year field investigation within two sectors of a private hunting ground located in the Paris basin (France). The loss of habitat heterogeneity included the disappearance of pastures dedicated to a last remaining sheep farm in the area, the removal of non-cropped areas and field boundaries and the increase of mean field size. Breeding stocks of hare declined during the study. Harrowed fields were always avoided. Pastures, alfalfa fields, thickets and fallow land were selected at the start of our study, whereas the remaining part was avoided at the end. The preference for ploughed fields decreased with the growth of vegetation in the winter-wheat fields. To benefit hares, land management should provide year-round vegetative cover and food from non-cropped areas in intensive arable farms.     

Role of interspecific competition in the coexistence ofApodemus argenteus andA. speciosus (Rodentia: Muridae)

Reciprocal-removal experiments with two replicates were conducted to test for the role of interspecific competition in the coexistence ofApodemus argenteus andA. speciosus. Population density, rate of appearance of new (unmarked) individuals, reproduction, survival rate and habitat use were monitored during pre- and removal periods. In both removal experiments, the removal ofA. argenteus had little effect onA. speciosus, while that ofA. speciosus affected several population characteristics ofA. argenteus. Namely, the removal ofA. speciosus shifted the distribution ofA. argenteus to the habitat with a denser shrub cover in one experiment. Also, the removal increased the population densities and appearance rates of new individuals ofA. argenteus in another experiment. Interspecific interactions between the two species appeared to be a one-way action fromA. speciosus toA. argenteus. In removal periods in both experiments, the rates of appearance of new individuals in each species were the highest on the grid where that species was removed. These results suggest that, though interspecific competition occurred between the two species, intraspecific competition had greater effects than interspecific competition on the abundance and habitat use ofA. argenteus andA. speciosus. This implies that the fundamental niches ofA. argenteus andA. speciosus differ potentially, which may play an important role in the coexistence of the two species.     

Are the preference and selection patterns of hermit crabs for gastropod shells species- or site-specific?

Experimental analyses of hermit crabs and their preferences for shells are essential to understand the intrinsic relationship of the crabs' dependence on shells, and may be useful to explain their shell use pattern in nature. The aim of this study was to evaluate the effect of crab species and site on the pattern of shell use, selection, and preference in the south-western Atlantic hermit crabs Pagurus brevidactylus and Pagurus criniticornis, comparing sympatric and allopatric populations. Differently from the traditional approach to evaluate shell preference by simply determining the shell selection pattern (i.e., the number of shells of each type selected), preference was defined (according to [Liszka, D., Underwood, A.J., 1990. An experimental design to determine preferences for gastropod shells by a hermit-crab. J. Exp. Mar. Biol. Ecol., 137(1), 47–62]) by the comparison of the number of crabs changing for a particular shell type when three options were given (Cerithium atratum, Morula nodulosa, and Tegula viridula) with the number of crabs changing for this same type when only this type was offered. The effect of crab species was tested at Cabelo Gordo Beach, where P. brevidactylus was found occupying shells of C. atratum, M. nodulosa, and T. viridula in similar frequencies, whereas P. criniticornis occupied predominantly shells of C. atratum. In laboratory experiments the selection patterns of the two hermit-crab species for these three gastropods were different, with P. criniticornis selecting mainly shells of C. atratum, and P. brevidactylus selecting more shells of M. nodulosa. The shell preference was also dependent on crab species, with P. criniticornis showing a clear preference for shells of C. atratum, whereas P. brevidactylus did not show a preference for any of the tested shells. The effect of site was tested for the two species comparing data from Cabelo Gordo to Preta (P. brevidactylus) and Araçá beaches (P. criniticornis). The pattern of shell use, selection, and preference was demonstrated to be dependent on site only for P. brevidactylus. The results also showed that the shell use pattern of P. criniticornis can be explained by its preference at both sites, whereas for P. brevidactylus it occurred only at Cabelo Gordo, where the absence of preference was correlated with the similar use of the three gastropod species studied. Finally, the results showed that the shell selection pattern cannot be considered as a measure of shell preference, since it overestimates crab selectivity.     

Species’ surrogacy for conservation planning: caveats from comparing the response of three arboreal rodents to habitat loss and fragmentation

The use of surrogate species in conservation planning has been applied with disappointing results on relatively large sets of species. It could still prove useful for optimizing conservation efforts when considering a small set of species with similar ecological requirements, however few field tests of this nature have been carried out. The aim of this research is to compare the response of three arboreal rodent species—the fat dormouse (Glis glis), the hazel dormouse (Muscardinus avellanarius) and the red squirrel (Sciurus vulgaris)—to habitat loss and fragmentation, with the aim of identifying priorities for conservation and evaluating possible optimization of conservation efforts under different scenarios: habitat restoration and selection of focal patches. We studied the distribution of the three species in a sample of patches in a highly fragmented landscape in central Italy, using a patch-landscape scale approach. The distribution was studied by using hair tubes, nestboxes and nocturnal surveys. The three species showed analogous responses to increasing isolation and decreasing size of habitat patches; what differed however, was the magnitude of responses. Our results show possible application of surrogacy within this restricted group of species, however several caveats arise depending on the conservation strategy and available funding. If habitat restoration is the objective, then the fat dormouse should be the target species for guiding size and isolation of patches. On the other hand, the magnitude of the differences and patch requirements for this species, question the feasibility of these conservation actions. If selection of focal patches for conservation is the objective then selecting the fat dormouse as a focal/umbrella species would overlook areas suitable for the other two species. Feasible optimisation of conservation efforts may be possible only between the red squirrel and the hazel dormouse.     

Trophic interactions and habitat segregation between competing Daphnia species

Summary Two commonly coexisting species of Daphnia segregate by habitat in many stratified lakes. Daphnia pulicaria is mostly found in the hypolimnion whereas D. galeata mendotae undergoes diel vertical migration between the hypolimnion and the epilimnion. I examined how habitat segregation between these two potentially competing species might be affected by trophic interactions with their resources and predators by performing a field experiment in deep enclosures in which I manipulated fish predation, nutrient levels, and the density of epilimnetic Daphnia. The results of the experiment indicate that habitat use by D. pulicaria can be jointly regulated by competition for food from epilimnetic Daphnia and predation by fishes. Patterns of habitat segregation between the two Daphnia species were determined by predation by fish but not by nutrient levels: The removal of epilimnetic fish predators resulted in higher zooplankton and lower epilimnetic phytoplankton densities and allowed D. pulicaria to expand its habitat distribution into the epilimnion. In contrast, increased resource productivity resulted in higher densities of both Daphnia species but did not affect phytoplankton levels or habitat use by Daphnia. The two species exhibit a trade-off in their ability to exploit resources and their susceptibility to predation by fish. D. g. mendotae (the less susceptible species) may thus restrict D. pulicaria (the better resource exploiter) from the epilimnion when fish are common due to lower minimum resource requirements than those needed by D. pulicaria to offset the higher mortality rate imposed by selective epilimnetic fish predators. D. g. mendotae does not appear to have this effect in the absence of fish.     

Macrozoobenthic assemblages in littoral sediments in the enclosed Rhine-Meuse Delta

Littoral macrozoobenthos in the enclosed Rhine-Meuse Delta was investigated by taking 95 sediment samples from 17 sites between 1984 and 1990. In addition, a set of environmental parameters was determined. The aim was to identify the main assemblages and the environmental conditions under which they occur. By the use of TWINSPAN, three main littoral zoobenthic assemblages were distinguished, which were related to geographical zones and differences in sediment grain size distribution. The ‘litoral river sand’ assemblage was found in the most upstream part; it mainly consisted of ‘interstitial’ invertebrates, including the indicator speciesVejdovskiella comata, Propappus sp. andKloosia pusilla. The ‘littoral sedimentation area silt’ assemblage was dominated byGammarus tigrinus, Einfeldia dissidens andPisidium sp. It was found in several river sections and contained the indicator speciesEinfeldia dissidens, Potamopyrgus antipodarum andValvata piscinalis. The ‘littoral sandy basin’ assemblage was concentrated in the littoral fine sands of the Haringvliet and contained the indicator speciesPisidium henslowanum, P. moitessierianum, Cladotanytarsus sp. andLipiniella arenicola. These three assemblages are the reflection of an interaction between habitat, food and disturbance. Palaeoecological analysis of insect remains revealed that 14 out of the 24 insect taxa, that were formerly common in the river sand habitat, are now extinct from the Rhine. The river silt habitat seems less impoverished: two out of the 19 insect taxa found in palaeoecological analysis are now extinct from the Rhine and seven are rate. Exotic species (Corbicula fluminea, C. fluminalis andCorophium curvispinum) have recently colonized the Rhine-Meuse Delta, but their impact on the macrozoobenthos seems limited.Corbicula spp. have become abundant in the ‘littoral river sand’ assemblage only.     

Size-related shifts in the habitat associations of young-of-the-year winter flounder (Pseudopleuronectes americanus): field observations and laboratory experiments with sediments and prey

Field surveys and laboratory studies were used to determine the role of substrata in habitat selection by young-of-the year winter flounder. A synoptic field survey of winter flounder and sediments in the Navesink River-Sandy Hook Bay estuarine system in New Jersey demonstrated that winter flounder distribution was related to sediment grain size. Analysis using a generalized additive model indicated that the probability of capturing 10-49 mm SL winter flounder was high on sediments with a mean grain diameter of /=40 mm SL) preferred coarse-grained sediments. Burying ability increased with size and all flounders avoided sediments that prevented burial. Subsequent laboratory experiments revealed that the presence of live prey (Mya arenaria) can over-ride sediment choice by winter flounder (50-68 mm SL) indicating the complexity of interrelated factors in habitat choice.     

Habitat gradient and reproductive habits of the sevenStellaria species in Japan

The author studied the habitat preference, life form, life cycle and reproductive characteristics of the seven species of the genusStellaria along a valley in the northeastern part of Kyoto City. Along the valley 150 quadrats were set from the village up to the hills. The quadrats were sorted into 15 quadrat groups by their affinity in the environment. The quadrat groups were ordinated by the quotients of community and the Bray-Curtis method in order to reveal the complex gradient in the environment. The complex gradient could be best understood by the degree of human disturbance. The seven species showed a continuous distribution along the complex gradient. There existed three patterns in their habitat preference and reproductive characteristics: (1)S. media andS. neglecta grew as annuals in the habitat of severe human disturbance. (2)S. tomentella, S. sessiliflora, andS. diversiflora grew as perennials in the habitat of slight human disturbance. (3)S. aquatica andS. alsine grew as facultative annuals either in the habitat of severe or slight human disturbance.     

Contrasting patterns of body size for Daphnia species that segregate by habitat

Summary We investigate how body size of two coexisting Daphnia species varies among 7 lakes that represent a gradient of predation risk. The two species segregate vertically in stratified lakes; D. galeata mendotae is typically smaller and more eplimnetic than D. pulicaria. The extent of vertical habitat partitioning, however, varies seasonally within and among lakes in apparent response to predation intensity by epilimnetic planktivorous fishes. Daphnia pulicaria uses the epilimnion at low levels of fish predation but is restricted to the hypolimnion under high fish predation, whereas D. galaeta mendotae always utilizes the epilimnion. The species display contrasting patterns of genetic variation in neonate size and size at maturity. D. pulicaria is larger in lakes with higher fish and Chaoborus densities whereas D. galeata mendotae is smaller. This contrast in body size in lakes with high predation is associated with greater habitat segregation in those lakes. In lakes with low predation risk, the two species are similar in body size at birth and maturity.Authorship order alphabetical     

The effects of plant dispersion and prey density on parasitism rates in a naturally patchy habitat

Despite extensive research on parasitoid-prey interactions and especially the effects of heterogeneity in parasitism on stability, sources of heterogeneity other than prey density have been little investigated. This research examines parasitism rates by three parasitoid species in relationship to prey density and habitat spatial pattern. The herbivore Itame andersoni (Geometridae) inhabits a subdivided habitat created by patches of its host plant, Dryas drummondii, in the Wrangell Mountains of Alaska. Dryas colonizes glacial moraines and spreads clonally to form distinct patches. Habitat subdivision occurs both on the patch scale and on the larger spatial scale of sites due to patchy successional patterns. Itame is attacked by three parasitoids: an ichneumonid wasp (Campoletis sp.), a braconid wasp (Aleiodes n. sp.), and the tachinid fly (Phyrxe pecosensis). I performed a large survey study at five distinct sites and censused Itame density and parasitism rates in 206 plant patches for 1–3 years. Parasitism rates varied with both plant patch size and isolation and also between sites, and the highest rates of overall parasitism were in the smallest patches. However, the effects of both small- and large-scale heterogeneity on parasitism differed for the three parasitoid species. There was weak evidence that Itame density was positively correlated with parasitism for the braconid and tachinid at the patch scale, but density effects differed for different patch sizes, patch isolations, and sites. At the site scale, there was no evidence of positive, but some indication of negative density-dependent parasitism. These patterns do not appear to be driven by negative interactions between the three parasitoid species, but reflect, rather, individual differences in habitat use and response to prey density. Finally, there was no evidence that parasitism strongly impacted the population dynamics of Itame. These results demonstrate the importance of considering habitat pattern when examining spatial heterogeneity of parasitism and the impacts of parasitoids. Received: 3 June 1999 / Accepted: 4 October 1999