Microhabitat Quality Research Articles

Micro-scale differences in thermal habitat quality and a possible case of evolutionary flexibility in the thermal physiology of lacertid lizards.

We studied the thermal ecology of the lacertid lizards Lacerta oxycephala and Podarcis melisellensis on the Adriatic island of Vis (Croatia) during summer. These species obviously differ in microhabitat use: L. oxycephala climbs on rocks and stone walls, whereas P. melisellensis is mainly ground-dwelling in vegetation. Since theoretical considerations predict a difference in thermal quality between the species' microhabitats, this system seems to present a good opportunity to test the influence of thermal microhabitat quality on body temperature, thermoregulatory behaviour and evolution of thermal characteristics. Data on thermoregulatory behaviour, body temperatures (T b) and habitat quality were collected in the field and selected temperatures (T sel) were estimated in a laboratory thermogradient. Accuracy and effectiveness of thermoregulation were quantified. Thermoregulatory behaviour consisted of timing of activity, selection of places in full sun and near sun-shade transitions, and basking. As predicted, L. oxycephala occupied the microhabitat with the lower thermal quality and had on average a lower T b. However, L. oxycephala also selected lower temperatures in the experimental thermogradient. Thus, if T sel can be regarded as the thermoregulatory target, both species proved to be accurate and effective thermoregulators. These results corroborate the "labile view" on the evolution of thermal physiology: both L. oxcephala and P. melisellensis appear to be adapted to their respective thermal microhabitat. This is a surprising conclusion, since earlier studies have found the thermal characteristics of Lacertidae to be evolutionarily rigid.

Increased prevalence of bot flies (Cuterebra fontinella) on white-footed mice (Peromyscus leucopus) near forest edges

Several studies have suggested that forest edge is preferred habitat for white-footed mice (Peromyscus leucopus). Because previous research has shown that parasitism by bot flies (Cuterebra fontinella) is higher in open areas than in woodlands, lower rates of infestation at woodland edges could partly explain the greater preference for woodland edges by the mice. We recorded the prevalence and load of bot fly larvae in mice trapped along edge-to-interior gradients in four forested areas in east-central Illinois. At all sites the prevalence of bot flies (the proportion of infected adult mice) was greatest near the edge (less than 50 m from the woodland margin), which should reduce, not improve, the quality of edge microhabitats. Mating bot flies tend to congregate in open areas, and the higher prevalence of parasitism by bot flies near the forest edge may reflect the greater penetration of light there than in the interior.

Denning Ecology of Black Bears in the White River National Wildlife Refuge, Arkansas

Lack of adequate den sites may adversely affect cub survival and reproductive success in black bears (Ursus americanus). Management of remnant bear populations in the lower Mississippi floodplain necessitates understanding denning ecology. We monitored 23 radiotagged black bears for 33 den years from 1993 to 1995 in White River National Wildlife Refuge, Arkansas to investigate denning chronology and den requirements. Bears entered dens between 1 January-20 February and emerged between 12 March-28 April. Mean (±SE) denning duration was 92.9 ± 6.6 days and varied between sex-age classes (P = 0.002). Pregnant females denned longer (117.8 ± 5.2 days, P = 0.029) than males and non-pregnant females (84.2 ± 7.7 days). More than 44% of the bears that denned (n = 27) used >1 den during a denning season, and one female used 4 dens. Most (90.2%, n = 51) dens were elevated tree cavities with a hollow opening on the top or middle of the main trunk. Three species of trees were used with 77.8% (n = 46) being overcup oak (Quercus lyrata). Basal area of trees ≥10 cm diameter at breast height (dbh; P < 0.001) and number of stems ≥84 cm dbh (P < 0.001; the min. dbh of den trees documented in this study) differed among vegetation plots with and without den trees. For seasonally flooded habitats, our results suggest the need for maintenance or augmentation of trees ≥84 cm dbh to increase the number of potential dens and to enhance the quality of den microhabitat in the lower Mississippi Alluvial Valley.

Stoneflies as ecological engineers – hungry predators reduce fine sediments in stream beds

1. We conducted experiments in a Colorado Rocky Mountain stream to measure the effect of foraging by predatory stoneflies (Megarcys signata) on fine sediment deposition and removal.2. Cages containing one of four treatments were placed in the stream substratum and removed 3 days after fine sediment load to the stream was experimentally increased. Cages containing a stonefly but no prey accumulated less sediment than cages initially with no benthic invertebrates. Although cages with Megarcys plus prey also tended to have less sediment than controls, sediments were not reduced in cages with prey only.3. Distance from sediment source, depth and current velocity at cages, final prey density and numbers of prey per predator gut at the end of the experiment had no effects on sediment accumulation in cages.4. To determine the mechanisms underlying this effect, behavioural tests were conducted in a stream‐side artificial stream system. To simulate the different hunger levels of stoneflies in cages, Megarcys were fed or starved for 3 days before behavioural trials that were repeated during high (night) and low (day) stonefly feeding periods. During night trials, foraging movements by starved Megarcys were more prolonged and active than those of fed stoneflies, regardless of the presence of prey. However, neither hunger level nor prey presence affected stonefly foraging behaviour during day trials.5. Results of field experiments suggest that the presence of stoneflies enhances sediment removal from interstitial spaces. Behavioural observations indicate that nocturnal searching for prey by hungry Megarcys dislodges fine sediments from interstitial spaces.6. Future studies should explore beyond the direct impacts of predators on stream invertebrate communities, and investigate the potential impact of predators on benthic microhabitat quality.

Landscape Ecology of the Wart-Biter Decticus verrucivorus in a Patchy Landscape

1. Populations of wart-biter Decticus verrucivorus living on habitat islands in an intensively exploited agricultural landscape in south-eastern Norway were studied. Auditory recordings of singing males indicated that 27 (39%) of 70 habitat islands contained populations in the first year of the study, while 2 years (one wart-biter generation) later the number was 16 (23%). Dispersing male wart-biters were also observed, and the dispersal distances measured. 2. Predictors of island occupancy were identified by applying logistic regression models to the pattern of presence/absence, probing the predictive power of both landscape variables (island size and isolation) as well as various microhabitat quality variables (vegetation, aspect and slope). The isolation of habitat islands relative to surrounding wart-biter populations was measured by two types of connectivity indices: one based on the empirical probability distribution of dispersal distances, the other on the negative-exponential dispersal function. 3. Habitat area and the connectivity indices were by far the best predictors of occupancy. The probability of occupation increased with increasing area and decreasing isolation. Of the unoccupied habitat islands, the ones with observations of single males were less isolated than the ones without. From the pattern of occupancy, the average dispersal distance was estimated to be 40m, while the observed average dispersal distance of males was 37 m. 4. In addition to the effects of area and connectivity, the probability of occupation was positively influenced by increasing slope to the south, increasing amounts of the plant Achillea millefolium, and decreasing fraction of vegetation lower than 10 cm. The probability of local population extinction increased with declining habitat area and increasing fraction of low vegetation. 5. The situation of the wart-biter in this highly fragmented study area seems to largely fit a metapopulation model: extinctions and absence of wart-biters were adequately predicted by habitat island size and isolation, and observed dispersal distances closely matched estimated dispersal distances from the model.

Mapping leaf surface landscapes.

Leaf surfaces provide the ecologically relevant landscapes to those organisms that encounter or colonize the leaf surface. Leaf surface topography directly affects microhabitat availability for colonizing microbes, microhabitat quality and acceptability for insects, and the efficacy of agricultural spray applications. Prior detailed mechanistic studies that examined particular fungi-plant and pollinator-plant interactions have demonstrated the importance of plant surface topography or roughness in determining the outcome of the interactions. Until now, however, it has not been possible to measure accurately the topography--i.e., the three-dimensional structure--of such leaf surfaces or to record precise changes in patterns of leaf surface elevation over time. Using contact mode atomic force microscopy, we measured three-dimensional coordinates of upper leaf surfaces of Vaccinium macrocarpon (cranberry), a perennial plant, on leaves of two age classes. We then produced topographic maps of these leaf surfaces, which revealed striking differences between age classes of leaves: old leaves have much rougher surfaces than those of young leaves. Atomic force microscope measurements were analyzed by lag (1) autocorrelation estimates of leaf surfaces by age class. We suggest that the changes in topography result from removal of epicuticular lipids and that the changes in leaf surface topography influence phylloplane ecology. Visualizing and mapping leaf surfaces permit detailed investigations into leaf surface-mediated phenomena, improving our understanding of phylloplane interactions.

Variation in leaf carbon isotope discrimination in Encelia farinosa: implications for growth, competition, and drought survival.

Population-level variation in the leaf carbon isotope discrimination (Δ) values was examined in Encelia farinosa, a common Sonoran Desert shrub. There was approximately a 2‰ range in Δ values among different plants. These differences in Δ values among neighboring plants were maintained through time, both under conditions when neighbors were present and after neighbors had been removed. Individuals with high Δ values were found to have an accelerated growth rate when these plants were released from competition for water. Individuals with low Δ values were better able to persist through long-term drought. These data suggest possible tradeoffs between conditions favoring high- and low-Δ-value plants within a natural population. Given the temporal variability in precipitation between years and spatial variability in microhabitat quality in the Sonoran Desert, variation in Δ values among E. farinosa plants will be maintained within a population.

Microhabitat use by the long-nosed potoroo, Potorous tridactylus, and other small mammals in remnant forest vegetation, south-western Victoria

Microhabitat use by the long-nosed potoroo, Potorous tridactylus, and six other species of small mammal was studied in remnant forest vegetation in south-western Victoria, Australia. Throughout its geographic range, P. tridactylus is consistently associated with dense vegetation in the ground and shrub strata. However, at a local scale, captures of P. tridactylus were not clearly associated with a particular floristic group, and were not strongly correlated with any structural feature of the vegetation. Rather, individuals utilised a range of sites of differing floristic composition and vegetation density. Dense cover provided diurnal shelter and protection from predators, whereas food resources were most abundant in adjacent more open areas. The use of vegetation mosaics or ecotones that allow the inclusion of contrasting microhabitats within an individual home range appears to be characteristic of potoroids in temperate environments. Such mosaics may result from topographic or edaphic variation, or from sera1 successional stages in vegetation following disturbance. Of the other small mammals, the bush rat, Rattus fuscipes, and the brown antechinus, Antechinus stuartii, favoured floristic groups that provided dense low cover. Captures of the swamp rat, Rattus lutreolus, were clumped, and centred on several sites along the forest edge on impeded drainage where potential foods were common. The long-nosed bandicoot, Perameles nasuta, and the southern brown bandicoot, Isoodon obesulus, were uncommon and clear microhabitat preferences were not displayed. The house mouse, Mus musculus, was of transient occurrence, mostly during autumn, and no obvious habitat preference was apparent. The quality and availability of microhabitats in remnant vegetation, together with landscape structure, are important in ensuring the persistence and conservation of small mammals in fragmented landscapes.

Asymmetric resource competition in a filter‐feeding stream insect (Hydropsyche siltalai; Trichoptera)

SUMMARY. 1. Field experiments in a lake outlet stream were carried out on filter‐feeding larvae of the caddisfly Hydropsyche siltalai Döhler, to assess whether microhabitat quality, establishment and growth rate is affected by position (upstream—downstream) within a dense aggregation of larvae.2. Artificial substrates holding high‐density aggregations of hydropsychid larvae were used. Marked III‐instar H. siltalai larvae were implanted either in the upstream end or in the downstream end of an aggregation and recovered after 17 days.3. Larvae recovered from the downstream end of an aggregation were smaller than those that had been implanted in the upstream end.4. Fewer larvae had settled in the downstream end of aggregations compared to the exposed upstream part.5. Naturally colonizing H. sitalai larvae were, in contrast to implanted larvae, more numerous behind aggregations. Small (first and second instar) Hydropsyche angustipennis (Curtis) and/or H. pellucidula (Curtis) larvae were more numerous in front of aggregations.6. Current velocities, measured with a microcurrent velocity meter, and density of drifting prey, were significantly lower behind aggregations of hydropsychid larvae. Food availability decreased by about 50% behind aggregations.

Nesting Ecology and Microhabitat of the Eastern Phoebe in the Central Appalachians

-In 1984, a study of the eastern phoebe (Sayornis phoebe) was conducted on the Appalachian Plateau in western Maryland, principally in the Savage River State Forest. Fledging success and mortality patterns were used as indicators of differences in quality of nest-site microhabitats. A total of 229 fledglings resulted from 84 nesting attempts, an average of 2.73 fledglings per attempt. Fledging success was 61.4%, with early nests generally more successful than late nests. Fledging success was similar among nesting structures, with adherent nests having higher success than statant nests, except on rock outcrops. However, late-adherent nests on rock outcrops did have higher fledging success than late-statant nests. The success of new-active and old-active nests was the same. Losses attributed to predators amounted to 70.1% of all losses. Losses attributed to small predators (55.6%) were significantly higher than that from large predators (14.6%). Other losses were attributed to nests falling (13.9%), human disturbance (5.6%), hatching failure (4.2%), adult death/desertion (3.5%) and inclement weather (2.8%). Losses ascribed to small predators increased from early to late nesting attempts on all nesting structures, but less so for adherent nests on rock outcrops. The principal nest predator was probably the eastern chipmunk (Tamias striatus). Stepwise discriminant function analysis was used to distinguish between microhabitats of high-success (>60%) and low-success (?60%) phoebe nests. Evergreen canopy cover (%), height to nest rim and distance to nearest understory tree were greater at high-success nests; while angle of recess in which the nest was built was smaller. Microhabitat variables with no differences among nesting structures, and therefore probably important in overall nestsite selection, included height of rock outcrop or other structure at the nest location, height to nest rim, width of shelf used for support of nest, length of overhanging structure above nest, height to underhanging structure below nest, height of understory trees and percent evergreen canopy cover. Variables associated with access to the nest tended to affect success more than others.

Tests of Density‐Dependent Habitat Selection in a Patchy Environment

A simple regression analysis can be used to assess the response of animal density to differences in habitat quality. The same test can evaluate general predictions of habitat selection theory as well as search for differences in the shapes of habitat suitability density functions, something previous tests have been unable to do. Combined with demographic or other estimates of fitness, regression tests can provide new insights into the evolution of habitat selection. Regression and fitness tests were used to explore the pattern of density—dependent habitat use in two temperate—zone rodents. The intensity of population regulation appeared to be inversely related to a habitat's carrying capacity. Variation in density—dependent habitat choice suggests new and unexpected dispersal strategies that vary with habitat heterogeneity. The predictions of the theory are complicated when habitat quality varies independently of population density. Sweepstake fitness rewards may be reaped by animals that would previously have been assumed to have made a suboptimal habitat choice. At the level of microhabitat, habitat selection models appeared to be much less capable of predicting variation in population density. Such a relation could be due to complex and inverse relationships between microhabitat quality and carrying capacity, or it could simply reflect scaling patterns in habitat selection.

Comparative Demography of Littorina rudis, L. Nigrolineata and L. Neritoides on Three Contrasted Shores in North Wales

(1) A possible relationship between reproductive mode and demography was sought between species of Littorina that brood young, lay benthic egg masses and broadcast planktonic larvae. Population size-frequency and density and cohort survivorship were monitored for Littorina rudis on a sheltered saltmarsh, L. rudis and L. nigrolineata on a boulder shore and for L. rudis and L. neritoides on an exposed cliff face. (2) Adult size classes formed a single broad peak and, except in L. neritoides, young recruits formed a second peak each summer. (3) In the saltmarsh, L. rudis population density fell from a maximum after summer recruitment to a minimum in spring; this was less evident in L. rudis and L. nigrolineata on the boulder shore and undetectable in the cliff populations of L. rudis and L. neritoides. (4) Spring densities fluctuated between years by maximum factors of 7, 6, 9, 9 and 2 in saltmarsh L. rudis, boulder shore L. rudis, cliff L. rudis, L. neritoides and L. nigrolineata respectively. (5) There was no evidence of density dependence in any of the population trajectories. (6) Artificial crevices increased the local density of L. rudis and L. neritoides on the cliff by attracting snails seeking shelter, but did not affect the total population size. (7) Mortality rate diminished progressively through the juvenile phase to a constant rate in adults. (8) Generation times were 24, 18, 18, 25 and 41 months for the saltmarsh L. rudis, boulder shore L. rudis, L. nigrolineata, cliff L. rudis and L. neritoides respectively. (9) Age-specific reproductive value increased beyond the age of first sexual maturity and declined only when growth had almost ceased. (10) All populations except that of L. nigrolineata had values of R0 < 1. Isoclines of juvenile versus adult survivorship for R0 = 1 were used to assess the reliability of the observed survivorship values. (11) Apart from a possible advantage of oviparity versus ovoviviparity during phases of population expansion, no relationship between demography and reproductive method could be found. Oviparity may be advantageous where egg survivorship is high and ovoviviparity in the converse situation. Broadcasting of planktonic larvae by L. neritoides may be correlated with the temporal and spatial unpredictability of microhabitat quality between shores.