Microhabitat use by larvae and females of a rare barrens butterfly,frosted elfin (<Emphasis Type="Italic">Callophrys irus</Emphasis>)

The frosted elfin (Callophrys irus) is a localized and declining butterfly found in xeric open habitats maintained by disturbance. We described the effects of woody plant canopy cover, topography and host plant size and density on the quality of microhabitat of wild indigo (Baptisia tinctoria) host plants containing late instar frosted elfin larvae at four study sites in southeastern Massachusetts, United States. We also assessed whether females preferentially depositing eggs on host plants within specific microhabitats, therefore conferring greater survivorship to the larvae through the late-instar stage. We found that moderate amounts of canopy cover and large plant size characterized larvae-occupied host plants. In the absence of tree canopy cover, late instar larvae density remained low even when host plant density was high. However, females oviposited on wild indigo plants without regard to any of the vegetative or environmental variables we measured. These results indicate that canopy cover was an important characteristic of microhabitats containing late instar larvae, and late instar larvae occupancy was determined by suitable microhabitat conditions, and not female oviposition selection. Managing for canopy cover and microhabitat heterogeneity within relatively open habitats is recommended for the maintenance of frosted elfin populations.     

Topographic microclimates drive microhabitat associations at the range margin of a butterfly

The habitat associations of individuals underpin the dynamics of species distributions. Broad‐scale gradients in climate can alter habitat associations across species’ geographic ranges, but topographic heterogeneity creates local microclimates which could generate variation in habitat use at finer spatial scales. We examined the selection of microhabitats for egg‐laying by populations of a thermally‐constrained butterfly, the skipper Hesperia comma, across 16 sites with different regional temperatures and topographic microclimates. Using models of thermal microclimate, we examined how the association between eggs and warm bare ground microhabitats varied with ambient temperature, and predicted bare ground associations in 287 existing H. comma populations, to investigate the relative impacts of regional temperatures and topographic microclimates on microhabitat use. Eggs were most strongly associated with bare ground in relatively cool sites, indicating climate‐driven changes in microhabitat use. The majority of temperature variation between study sites was attributable to topographic microclimates rather than regional temperature differences, such that changes in microhabitat associations occurred principally between north‐ and south‐facing slopes within the same region. Predicted microhabitat associations across the UK distribution of H. comma showed that, due to the large temperature differences generated by topography, most of the between‐population variation in microhabitat use occurs locally within 5 km grid squares, with a smaller proportion occurring at a regional level between 5 km squares. Our findings show how microclimatic variation generated by topography alters the habitat associations of populations at fine spatial scales, suggesting that microclimate‐driven changes in habitat suitability could shape species’ distribution dynamics and their responses to environmental change.     

Is dietary or microhabitat specialization associated with environmental heterogeneity in horned lizards (Phrynosoma)?

Niche breadth is predicted to correlate with environmental heterogeneity, such that generalists will evolve in heterogeneous environments and specialists will evolve in environments that vary less over space and time. We tested the hypothesis that lizards in a heterogeneous environment were generalists compared to lizards in a homogeneous environment. We compared niche breadths of greater short‐horned lizards by quantifying resource selection in terms of two different niche axes, diet (prey items and trophic level), and microhabitat (ground cover and shade cover) between two populations occurring at different elevations. We assessed the heterogeneity of dietary and microhabitat resources within each population's environment by quantifying the availability of prey items, ground cover, and shade cover in each environment. Overall, our results demonstrate that despite differences in resource heterogeneity between elevations, resource selection did not consistently differ between populations. Moreover, environmental heterogeneity was not associated with generalization of resource use. The low‐elevation site had a broader range of available prey items, yet lizards at the high‐elevation site demonstrated more generalization in diet. In contrast, the high‐elevation site had a broader range of available microhabitats, but the lizard populations at both sites were similarly generalized for shade cover selection and were similarly specialized for ground cover selection. Our results demonstrate that environmental heterogeneity of a particular resource does not necessarily predict the degree to which organisms specialize on that resource.     

Nesting in a thermally challenging environment: nest‐site selection in a rock‐dwelling gecko,Oedura lesueurii (Reptilia: Gekkonidae)

In egg‐laying species, maternal oviposition choice can influence egg survival and offspring phenotypes. According to the maternal‐preference offspring‐performance hypothesis, females should choose oviposition sites that are optimal for offspring fitness. However, in thermally challenging environments, maternal oviposition behaviour may be constrained by the limited availability of suitable oviposition sites. We investigated nest‐site selection in a nocturnal lizard [velvet gecko Oedura lesueurii (Duméril and Bibron)] that inhabits a thermally challenging environment in south‐eastern Australia. The viability of these gecko populations is critical for the persistence of an endangered snake species (Hoplocephalus bungaroides Wagler) that feeds heavily on velvet geckos. Female geckos chose nest sites nonrandomly, with 87% of nests (N = 30) being laid in deep crevices. By contrast, only 13% of clutches were laid under rocks, which were the most readily available potential nest sites. Nest success in crevices was high (100%), but no eggs hatched from nests under rocks. Temperatures in nest crevices remained relatively low and constant throughout the incubation period (mean = 22.7 °C, range 21.0–24.5 °C), whereas thermal regimes under rocks showed large diurnal fluctuations. Geckos selected crevices that were deeper, had less canopy cover, and were warmer than most available crevices; in 85% of cases, such crevices were used simultaneously by more than one female. The thermally distinctive attributes of nest sites, and their frequent communal use, suggest that nest sites are a scarce resource for female velvet geckos, and that the shading of rock outcrops through vegetation encroachment may influence nest success in this species. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 250–259.     

Oviposition site selection in Cactoblastis cactorum (Lepidoptera): constraints and compromises

Summary Oviposition by Cactoblastis cactorum on Opuntia ficus-indica and O. aurantiaca was assessed from the positioning of egg sticks on plants in the field. The number of egg sticks laid on O. ficus-indica plants was affected by: (1) plant size; (2) moth emergence near the plant; (3) cladode condition; and (4) plant conspicuousness. These factors contributed towards the clumping of egg sticks on plants. There was no apparent oviposition preference for one of the two host plant species despite the fact that egg predation was higher and fecundity lower on O. aurantiaca. The selection of a site for oviposition on the host plants was influenced by: (1) cladode condition; (2) height above ground; and (3) shelter from wind during oviposition. Succulent cladodes were the favoured sites for oviposition. The evidence suggests that in C. cactorum, oviposition site selection is largely the net result of a compromise between oviposition behaviour selected for increasing the probability of juvenile survival and oviposition behaviour selected for increasing the probability of laying the full complement of eggs. In addition, environmental and physiological factors such as wind and wing-loading, are thought to place constraints on the range of sites available for oviposition.     

The influence of wild boar <Emphasis Type="Italic">(Sus scrofa)</Emphasis> on microhabitat quality for the endangered butterfly <Emphasis Type="Italic">Pyrgus malvae</Emphasis> in the Netherlands

The decline of open habitats in Europe, such as semi-natural grasslands and heathlands, has caused a general decline in biodiversity, which has been well documented for butterflies. Current conservation practices often involve grazing by domestic livestock to maintain suitable butterfly habitats. The extent to which wild ungulates may play a similar role remains largely unknown. Through their rooting activity, wild boar could be effective to reduce grass encroachment and restore pioneer microhabitats that are vital to many grassland insects in temperate climates. Here, we assessed the microhabitat requirements of Pyrgus malvae, an endangered butterfly of heathland and grassland habitats in the Netherlands, with special attention for the influence of wild boar rooting. To date, oviposition site selection of this species has concentrated on calcareous grasslands, whereas we also include heathlands. Overall, larval occupancy was higher in warm, open and sparsely vegetated microhabitats, which supports earlier findings. In heathland, microhabitat occupancy was positively affected by bryophyte and litter cover. In heath-grassland mosaic, microhabitat occupancy was also influenced by bryophyte and litter cover, but in addition low grass cover increased occupancy by favouring host plants. In grassland, only low grass cover and host plant cover determined microhabitat quality. Across all habitats, occupied microhabitats were characterized by lower vegetation as well as higher average daytime temperatures than unoccupied microhabitats. We discovered that wild boar play an important role in reducing grass cover by shallow rooting in grass patches, thereby increasing host plant availability. Hence, wild boar may have an added value in maintaining and restoring P. malvae microhabitats in grassland habitats in addition to grazing by domestic livestock.     

Microtopography determines the habitat quality of a threatened peatland butterfly at its southern range margin

Knowledge of the ecology of pre-adult stages of an organism at its species’ range margins is a prerequisite for conserving species, especially for understanding its responses to future climate changes. Largely sedentary premature life stages require specific living conditions within a relatively small area. Such conditions are created by vegetation structure heterogeneity and a microclimate gradient, generated by varying microtopography. We investigated the microhabitat selection patterns of egg-laying females and overwintering caterpillars of peatland butterfly Coenonympha tullia relative to vegetation composition, water quantity and microclimatic conditions across microtopographic zones of transition mire at the species’ southern range margin. We showed that (1) small-scale variability in mire microtopography determines oviposition site selection and larval presence; (2) microhabitats of pre-adult stages were largely confined to the intermediate microtopographic zone of the transition mire (flats); (3) egg-laying females and overwintered larvae preferred microsites with high coverage of main hostplants (Trichophorum alpinum, Carex lasiocarpa, C. limosa, C. panicea), and those with more humid and cooler summertime conditions than on hummocks; (4) females and larvae avoided shallow hollows permanently filled with water. The vegetation structure of flats enables the females to select spots with sufficient humidity for egg development, and allows the larvae to change their location according to suitable thermal and moisture conditions. We also discuss poor prospects for species in view of impacts of future climate changes on mire ecosystems.     

Characteristics and distribution of opposition sites of the grasshopper Zonocerus variegatus (L.)

Abstract. 1. The dry-season population of Zonocerus in southern Nigeria oviposits in dense aggregations under single or clumped woody plants or on the edge of tall continuous stands of vegetation. No egg pods were found oviposited singly.
2. The vegetation under which oviposition occurs is the most common available at the time of the year. There seems to be no selection of particular species of plants so long as they provide good low shade.
3. Oviposition can take place in a wide range of gravelly soils.
4. Topography is not important as long as the oviposition site is on high ground away from damp areas subject to flooding.
5. There was a mean of 618 pods per oviposition site from forty-one sites which were dug up, and there was a significant correlation between the area of a site and the number of pods.
6. There was a marked tendency for oviposition sites to be found close to cassava. At two localities of approximately 20 ha each 90% of the sites were found within 30 m of cassava.
7. It is suggested that permanent shade is the most important factor in the selection of a site and this is probably related to the survival of the eggs during their 6–7 months development.     

An ‘ecological trap’ for yellow warbler nest microhabitat selection

Contrary to assumptions of habitat selection theory, field studies frequently detect ‘ecological traps’, where animals prefer habitats conferring lower fitness than available alternatives. Evidence for traps includes cases where birds prefer breeding habitats associated with relatively high nest predation rates despite the importance of nest survival to avian fitness. Because birds select breeding habitat at multiple spatial scales, the processes underlying traps for birds are likely scale‐dependent. We studied a potential ecological trap for a population of yellow warblers Dendroica petechia while paying specific attention to spatial scale. We quantified nest microhabitat preference by comparing nest‐ versus random‐site microhabitat structure and related preferred microhabitat features with nest survival. Over a nine‐year study period and three study sites, we found a consistently negative relationship between preferred microhabitat patches and nest survival rates. Data from experimental nests described a similar relationship, corroborating the apparent positive relationship between preferred microhabitat and nest predation. As do other songbirds, yellow warblers select breeding habitat in at least two steps at two spatial scales; (1) they select territories at a coarser spatial scale and (2) nest microhabitats at a finer scale from within individual territories. By comparing nest versus random sites within territories, we showed that maladaptive nest microhabitat preferences arose during within‐territory nest site selection (step 2). Furthermore, nest predation rates varied at a fine enough scale to provide individual yellow warblers with lower‐predation alternatives to preferred microhabitats. Given these results, tradeoffs between nest survival and other fitness components are unlikely since fitness components other than nest survival are probably more relevant to territory‐scale habitat selection. Instead, exchanges of individuals among populations facing different predation regimes, the recent proliferation of the parasitic brown‐headed cowbird Molothrus ater, and/or anthropogenic changes to riparian vegetation structure are more likely explanations.     

Laboratory and field evaluation of an oviposition trap for Culex quinquefasciatus (Diptera: Culicidae)

An ovitrap (BR-OVT) based on physical and chemical stimuli for attracting gravid Culex quinquefasciatus (Diptera: Culicidae) females was developed and evaluated under laboratory and field conditions. Attractants were assayed using alternative chamber bioassays prior to being used in the BR-OVT oviposition trap. A significant preference of gravid females for sites containing conspecific egg rafts was observed, as a response to the natural oviposition pheromone, as well as for sites treated with the synthetic pheromone erythro-6-acetoxy-5-hexadecanolide. Five- to 20-day old grass infusion was strongly attractive to gravid females for laying eggs. On the other hand, entomopathogenic Bacillus sphaericus (Bs) did not influence the choice of an oviposition site when used in combination with grass infusion and can therefore be used as a larvicide in ovitraps. Results from field trials showed that the BR-OVT with grass infusion and with or without Bs works as a preferred oviposition site for Cx. quinquefasciatus. The BR-OVT was more effective for egg collection when placed indoors and comparison with the number of egg rafts laid in cesspits over 40 days indicates that this very simple ovitrap may be a useful tool for monitoring populations of the most important of the vectors of bancroftian filariasis.     

How avian nest site selection responds to predation risk: testing an 'adaptive peak hypothesis'

1. Nest predation limits avian fitness, so birds should favour nest sites that minimize predation risk. Nevertheless, preferred nest microhabitat features are often uncorrelated with apparent variation in predation rates. 2. This lack of congruence between theory-based expectation and empirical data may arise when birds already occupy 'adaptive peaks'. If birds nest exclusively in low-predation microhabitats, microhabitat and nest predation may no longer be correlated even though predation ultimately shaped microhabitat selection. 3. This 'adaptive peak hypothesis' was tested for a population of Yellow Warblers (Dendroica petechia) focusing on two nest microhabitat features: concealment and height. Experimental nests measured relative predation risk both within and outside the microhabitat range typically occupied by natural nests to examine whether nest site choices made by birds restricted our ability to detect microhabitat effects on predation. 4. Within the natural range (30-80% concealment, >75 cm height), microhabitat-predation relationships were weak and inconsistent, and similar for experimental and natural nests. Over an extended range, however, experimental predation rates were elevated in exposed sites (<30% concealed), indicating a concealment-related 'adaptive plateau'. 5. Clay egg bite data revealed a concealment effect on avian predators, and the abundance of one avian predator group correlated with nest concealment among years, suggesting these predators may cue birds to modulate nest concealment choices. 6. This study demonstrates how avian responses to predation pressure can obscure the adaptive significance of nest site selection, so predation influences may be more important than apparent from published data.     

Predation hazard and seed removal by small mammals: microhabitat versus patch scale effects

Predator avoidance may involve response strategies of prey species that are time and space specific. Many studies have shown that foraging individuals avoid predators by altering microhabitat usage; alternatively, sites may be selected according to larger-scale features of the habitat mosaic. We measured seed removal by two small mammal species (Peromyscus leucopus, and Microtus pennsylvanicus) at 474 stations over an experimentally created landscape of 12 patches, and under conditions of relatively high (full moon) and low (new moon) predatory hazard. Our objective was to determine whether predator avoidance involved the selection of small-, medium-, or large-scale features of the landscape (i.e., at the scale of microhabitats, habitats, or habitat patches). We found rates of seed removal to vary more with features of whole patches than according to variation in structural microhabitats within patches. Specific responses included: under-utilization of patch edge habitats during full moon periods, and microhabitat effects that were only significant when considered in conjunction with larger-scale features of the landscape. Individuals residing on large patches altered use of microhabitats/habitats to a greater extent than those on smaller patches. Studies just focusing on patterns of microhabitat use will miss responses at the larger scales, and may underestimate the importance of predation to animal foraging behavior.     

Oviposition in three species of limnephiloid caddisflies (Trichoptera): hierarchical influences on site selection

1. Oviposition site selection was studied in three trichopteran species; an undescribed species of Hydatophylax (Limnephilidae), Onocosmoecus unicolor (Limnephilidae) and Neophylax rickeri (Uenoidae), in two Coastal Range streams in California, U.S.A. 2. Hydatophylax sp. egg masses were generally found at or above the water surface on substrata near the bank in pools, where undercut banks, overhanging vegetation and rocks shaded attachment sites and provided wind protection. Onocosmoecus unicolor females deposited their egg masses exclusively above the water on moist emergent wood. The egg masses of N. rickeri were almost exclusively found in high velocity areas of riffles, where females oviposited under water and attached their egg masses to the submersed undersides of unembedded, protruding stones with large emergent surfaces. 3. For Hydatophylax sp. and N. rickeri, a hierarchical selection scheme is proposed in which females use cues at three different spatial levels (stream, stream subunit, substratum) to choose oviposition sites. Females of O. unicolor seemed to choose at only two different levels of spatial resolution (stream, substratum). 4. The formation of large aggregations of egg masses in N. rickeri and O. unicolor suggests that females actively choose oviposition sites where conspecific egg masses are already attached. The clustering of egg masses may minimise the risk of predation through the dilution effect, because egg masses of N. rickeri and O. unicolor are both attacked by dipteran predators. 5. Females of the three Trichoptera species studied are able to make a clear choice of oviposition site in the heterogeneous stream environment. In comparison with other stream microhabitats, these are characterised by stable and relatively predictable environmental conditions during the time of egg development, which is seen as the main selective pressure leading to the observed egg‐laying behaviour.     

Dynamics of oviposition in Danaus plexippus (Insecta: Lepidoptera) on milkweed, Asclepias spp

Where a butterfly chooses to lay an egg will influence the subsequent survival of her offspring. In this paper we report on the effect of a number of variables which influence the choice of oviposition site in Danaus plexippus L. Experiments were conducted under both flight cage and field conditions. The field observations consisted of recording the within and between plant egg dispersions across different patch sizes. Laboratory experiments looked at egg laying preferences as affected by plant species, age and condition. Butterflies were selective in their choice of oviposition site. Eggs were laid singly on the underside of medium sized leaves towards the top of a plant. More eggs were laid per plant on single isolated plants than on plants within a patch. The number of eggs per plant increases with plant height but decreases with plant age. Females preferred young plants or plants with fresh regrowth of leaves. These characteristics could override species preferences which were, in decreasing order, Asclepias curassavica. A. fruticosa and A. physocarpa. Species preferences varied between butterflies and with female age. Neither the presence of eggs nor larvae on a plant deterred oviposition. These results are compared with previous observations of egg laying in this species.     

Reduced variability in range‐edge butterfly populations over three decades of climate warming

Populations at the high latitude edge of species’ geographical ranges are thought to show larger interannual population fluctuations, with subsequent higher local extinction risk, than those within the ‘core’ climatic range. As climate envelopes shift northward under climate warming, however, we would expect populations to show dampened variability. We test this hypothesis using annual abundance indices from 19 butterfly species across 79 British monitoring sites between 1976 and 2009, a period of climatic warming. We found that populations in the latter (warmer) half of the recording period show reduced interannual population variability. Species with more southerly European distributions showed the greatest dampening in population variability over time. Our results suggest that increases in population variability occur towards climatic range boundaries. British sites, previously existing at the margins of suitable climate space, now appear to fall closer to the core climatic range for many butterfly species.     

Population Structure, Behaviour and Selection of Oviposition Sites of an Endangered Butterfly, Parnassius Mnemosyne, in Litovelské Pomoravíl. Czech Republic

A population of Parnassius mnemosyne L., an endangered butterfly species, was studied for two seasons by mark–release–recapture (MRR) techniques in the Litovelské Pomoraví Protected Landscape Area, Czech Republic. There were four distinct colonies in the area; the MRR data indicate that the largest colony contained over 1000 adult males during peak flight in 1996 suggesting that the area harbours one of the largest populations of the species in the Czech Republic. The detected rate of intercolony movements showed that the individual colonies were not genetically isolated, but the interchange of individuals was limited. Capture sex ratio and estimated sex ratios were both consistently male-biased. We suggest that this might be caused by different behaviour of the sexes which renders the males more catchable: the females spent most of their time either egg laying or resting, which makes them inconspicuous compared to the patrolling males. Preferred oviposition sites were open, sunny patches such as forest clearings which may be due to different hostplant quality compared to hostplants under closed canopy. The heliophily of the species and the dependency of females on open oviposition sites render the butterfly vulnerable due to a decline in coppice management and replacement of fine mosaic of sunny and shady patches by even-aged forest stands.     

Seasonal Changes in Microhabitat Use and Niche Overlap between Akodon azarae and Calomys laucha (Rodentia,Muridae) in Agroecosystems of Central Argentina

We examined seasonal changes in microhabitat use by the murid rodents Akodon azarae and Calomys laucha in agroecosystems of the former pampa region of Argentina. In summer, trap data showed that both species had low densities and were almost equally distributed between the mature cropfields and their surrounding weedy borders. Analysis of vegetation at the trap sites revealed no selection for microhabitats at the borders, whereas in the cropfields both species shared preferences for covered microhabitats. In contrast, winter data revealed a sharp habitat segregation, being Calomys numerically dominant in post-harvest cropfields and Akodon more abundant in borders. Moreover, there were clear differences in microhabitat selection at the borders, A. azarae occupying the more covered microhabitats and C. laucha the less covered. Breadth and overlap of niches were calculated using principal component analysis, in order to recognize interspecific competition and its influence on community niche space. Available evidence indicates that the structure of this rodent community is strongly influenced by seasonal changes in habitat stucture and rodent abundance. The socially dominant A. azarae retains the best part of the available niche space, a fact becoming more evident under the harsh winter conditions.     

Microhabitat use by Blennius fluviatilis in the Río Matarraña, Spain

SUMMARY.

• 1 Microhabitat use by Blennius fluviatilis in the Río Matarraña, Spain, varied significantly among dates during a 19-month study. Most temporal variation, however, was attributable to changes in microhabitat availability.
• 2 B. fluviatilis demonstrated non-random use of available microhabitats during each sampling period, and generally occupied deeper microhabitats with higher average current velocities dominated by rubblegravel substrates.
• 3 B. fluviatilis also demonstrated non-random use of available nest sites; males generally nested under the largest available pieces of rubble, in areas with less gravel or sand. Water column depth influenced nest site use in two of four study areas.
• 4 We found few microhabitat differences between spawning and nonspawning periods, or between males and females. In addition, microhabitat use was similar among B. fluviatilis size-groups.
• 5 Although B. fluviatilis often were over-represented in deeper, erosional areas, blennies necessarily inhabited a wide range of microhabitats because average current velocities and water depths fluctuated greatly during the study. Tolerance of a broad range of environmental conditions may contribute to the species' persistence in habitats that exhibit extreme seasonal changes in discharge such as the Matarraña.