Shaded Patches Research Articles

Effects of microclimate on disease prevalence across an urbanization gradient.

Increased temperatures associated with urbanization (the "urban heat island" effect) have been shown to impact a wide range of traits across diverse taxa. At the same time, climatic conditions vary at fine spatial scales within habitats due to factors including shade from shrubs, trees, and built structures. Patches of shade may function as microclimate refugia that allow species to occur in habitats where high temperatures and/or exposure to ultraviolet radiation would otherwise be prohibitive. However, the importance of shaded microhabitats for interactions between species across urbanized landscapes remains poorly understood. Weedy plants and their foliar pathogens are a tractable system for studying how multiple scales of climatic variation influence infection prevalence. Powdery mildew pathogens are particularly well suited to this work, as these fungi can be visibly diagnosed on leaf surfaces. We studied the effects of shaded microclimates on rates of powdery mildew infection on Plantago host species in (1) "pandemic pivot" surveys in which undergraduate students recorded shade and infection status of thousands of plants along road verges in urban and suburban residential neighborhoods, (2) monthly surveys of plant populations in 22 parks along an urbanization gradient, and (3) a manipulative field experiment directly testing the effects of shade on the growth and transmission of powdery mildew. Together, our field survey results show strong positive effects of shade on mildew infection in wild Plantago populations across urban, suburban, and rural habitats. Our experiment suggests that this relationship is causal, where microclimate conditions associated with shade promote pathogen growth. Overall, infection prevalence increased with urbanization despite a negative association between urbanization and tree cover at the landscape scale. These findings highlight the importance of taking microclimate heterogeneity into account when establishing links between macroclimate or land use context and prevalence of disease.

Negative effects of the spatial clumping of thermal resources on lizard thermoregulation in a fragmented habitat

In ecosystems threatened by the expansion of croplands, habitat fragmentation and climate change, two of the main extinction drivers, may have thermoregulation-mediated interacting effects on demographic trends of terrestrial ectotherms. We studied the thermal biology of a metapopulation of the widespread Mediterranean lacertid Psammodromus algirus in ten fragments of evergreen or deciduous oak forests interspersed among cereal fields. We obtained thermoregulation statistics (selected temperature range, body and operative temperatures, thermal quality of the habitat, and precision, accuracy, and effectiveness of thermoregulation) that could be compared among fragments and with conspecific populations living in unfragmented habitat. We also measured the selection (use vs. availability) and spatial distribution of sunlit and shaded patches used for behavioral thermoregulation in fragments, and we estimated operative temperatures and thermal habitat quality in the agricultural matrix surrounding the fragments. Variation of the thermal environment was much larger within fragments than among them, and thermoregulation was accurate, precise, and efficient throughout the fragmented landscape; its effectiveness was similar to that of previously studied unfragmented populations. The average distance between sunlit and shaded patches was shorter in deciduous than in evergreen fragments, producing a more clumped distribution of the mosaic of thermal resources. Consequently, in evergreen habitat the cost of thermoregulation was higher, because lizards were more selective in their choice of sunlit sites (i.e. they used sunlit patches closer to shade and refuge than expected at random, and the extent of such selection was larger than at deciduous habitat). Temperatures available in croplands were too high to allow lizard dispersal, at least in the post-breeding season. This result confirms the role of croplands as a thermal barrier that promotes inbreeding and associated fitness losses in isolated fragments, and it forecasts a dark future for populations of forest lizards in agricultural landscapes under the combined effects of habitat fragmentation and global warming.

Living in a Thermally Diverse Environment: Field Body Temperatures and Thermoregulation in Hermann’s Tortoise, Testudo hermanni, in Montenegro

Reptiles live in a range of different habitats from tropical forests to temperate zones where the climate may change on seasonal or a daily basis. The thermal environment is a major determinant of how efficiently they can achieve optimum or preferred body temperatures and, in terms of physiologically optimum body temperatures, these may not be possible in a natural environment. In this paper, null models have been employed to evaluate thermoregulatory efficiency in Hermann’s tortoise, Testudo hermanni, in high summer in central Montenegro when temperatures change on a daily basis. The study area is defined as a low-cost thermal environment and thus we assumed that tortoises should be able to achieve an efficient level of thermoregulation. However, the results varied and depended on where the tortoises operated and the weather conditions. High levels of efficiency were found during sunny weather in areas with abundant patches of shade and sunlit areas. These reflected the temperatures of models placed in these areas and in females during cooler cloudy weather when thermoregulatory effort increased. Model temperatures placed in partially shaded sunlit areas were in better agreement with tortoise body temperatures than models in other areas. Tortoise body temperatures were in closer agreement with set point temperatures than any of the null models placed in either open sunny, shaded or partially shaded areas, indicating that tortoise movement was non-random and due to active thermoregulation.

Lizard thermoregulation revisited after two decades of global warming

Abstract Although the effects of global warming on thermoregulation are usually explored using predictions of climate envelop modelling, such effects should best be analysed empirically, studying the same population with the same methods after a long enough period of temperature rise. We used a 30‐year long database about body temperatures (Tbs) of field‐active Psammodromus algirus lizards inhabiting a well‐conserved temperate open forest, and we focused on the summers of 1997 and 2017 to compare Tbs, environmental operative temperatures (Tes), their proximity to the selected thermal range (Tsel), and the selection of sunlit and shaded patches all along the day. From these data, we estimated the precision (standard deviation of Tbs), accuracy (average distance between Tbs and Tsel) and effectiveness (extent to which Tbs are closer to Tsel than Tes) of thermoregulation. Of the highest 5% of all Tbs in the database, 95% were recorded in 2017, when the adjustment to Tsel was much better for Tbs selected in a laboratory thermogradient than for field Tbs (percentages of Tbs above Tsel of 2% and 52% respectively). In 2017, especially after 12:00 h, the selection of shaded patches (87% of lizards in full shade vs. <1% in full sun) was more intense than in 1997, contributed more to overall thermoregulation, and produced a larger difference between Tes and Tbs. In spite of this, Tbs were lower—and closer to Tsel—in 1997 (when most shaded patches offered favourable thermal opportunities, with Tes within or below Tsel) than in 2017 (when only 33% of full shade Tes, and 8% of all Tes, were within or below Tsel). As a consequence, estimates of the accuracy and effectiveness of thermoregulation decreased over the 20‐year period examined. We conclude that given the low availability of Tes within or below Tsel, lizards cannot longer prevent the rise of their Tbs above Tsel, at least in hot summer days. Thus, the effects of global warming are already hindering the ability of lizards to buffer environmental change by behavioural means, even in temperate forests with a fine‐grained mosaic of sun and shade patches. Read the free Plain Language Summary for this article on the Journal blog.

Stream Temperature Response to 50% Strip-Thinning in a Temperate Forested Headwater Catchment

Stream temperature is a critical parameter for understanding hydrological and biological processes in stream ecosystems. Although a large body of research has addressed the effects of forest harvesting on stream temperature, less is known about the responses of stream temperature to the practice of strip-thinning, which produces more coherent patches of shade and sunlight areas. In this study, we examined stream temperature response to 50% strip-thinning in a 17 ha headwater catchment. The thinning lines extended through the riparian zone. Paired-catchment analysis was applied to estimate changes in daily maximum, mean, and minimum stream temperatures for the first year following treatment. Significant effects on daily maximum stream temperature were found for April to August, ranging from 0.6 °C to 3.9 °C, similar to the magnitude of effect found in previous studies involving 50% random thinning. We conducted further analysis to identify the thermal response variability in relation to hydrometeorological drivers. Multiple regression analysis revealed that treatment effects for maximum daily stream temperature were positively related to solar radiation and negatively related to discharge. Frequent precipitation during the summer monsoon season produced moderate increases in discharge (from 1 to 5 mm day−1), mitigating stream temperature increases associated with solar radiation. Catchment hydrologic response to rain events can play an important role in controlling stream thermal response to forest management practices.

Aquatic food web response to patchy shading along forested headwater streams

In forested streams, changes in age and structure of riparian vegetation have been shown to directly influence the amount of light reaching the stream benthos. The potential for light to directly impact primary productivity in forested streams is generally understood, but most field experiments exploring reach-scale in-stream light dynamics have evaluated large changes in riparian vegetation. Fewer studies have quantified influences of smaller changes in irradiance, particularly how patchy in-stream light developed with complex riparian forests affects stream biota. We applied patches of shade, covering ∼50% of three manipulation reaches, which were each paired with an unmanipulated reference reach. We quantified changes in stream light availability, benthic periphyton, and aquatic macroinvertebrate, fish, and salamander biomass using a before–after control–impact study design. Patchy shading decreased localized and reach-scale light and reduced periphyton, macroinvertebrate, fish, and salamander biomass in manipulation sites relative to the reference reaches. Results suggest that moderate changes in stream light, such as those that occur through stand development and small-scale disturbance processes, can impact stream biota through bottom-up processes.

Drivers of species richness and community integrity of small forest patches in an agricultural landscape

AbstractQuestionsLong‐term fragmentation and land use in Europe have created a landscape pattern where small forest patches are embedded among agricultural landscapes. These small forest patches can be one of the few habitats left to maintain the species richness and ecosystem functions within intensively managed agroecosystems. We ask, which factors determine vascular plant species richness, community composition and forest community integrity in small forest patches in an agricultural landscape?LocationNE Estonia.MethodsWe combine island biogeographic theory (patch area and isolation) with the properties of the surrounding landscape and local environmental conditions within a patch to study the drivers of species richness and community integrity.ResultsPatch area together with local environmental factors (understorey light conditions and soil reaction) determined both species richness and community integrity. Total species richness and forest generalists were related to patch area alone, whereas forest specialists were additionally dependent on patch light conditions. Species richness of grassland specialists in the forest patches increased with the amount of natural habitat in the surrounding landscape, while the presence of synanthropic species was positively related to soil reaction. Forest community integrity was higher in larger, more shaded patches with low soil reaction, which together defined suitable conditions for forest communities and hindered the intrusion of species from other habitats.ConclusionsUnder a suitable set of conditions, encompassing both favourable landscape and local environmental conditions, even small forest patches can provide habitat for both forest and grassland communities in agricultural landscapes. Comprehensive approaches, considering species composition, environment and landscape conditions simultaneously, are needed for making reliable predictions of biodiversity patterns.

Lizards perceived abiotic and biotic stressors independently when competing for shade in terrestrial mesocosms

Hormones such as glucocorticoids and androgens enable animals to respond adaptively to environmental stressors. For this reason, circulating glucocorticoids became a popular biomarker for estimating the quality of an environment, and circulating androgens are frequently used to indicate social dominance. Here, we show that access to thermal resources influence the hormones and behavior of male lizards (Sceloporus jarrovi). We exposed isolated and paired males to different thermal landscapes, ranging from one large patch of shade to sixteen smaller patches. Both the presence of a competitor and the patchiness of the thermal environment influenced hormone concentrations and movement patterns. When shade was concentrated in space, paired lizards competed more aggressively and circulated more corticosterone. Even without competitors, lizards circulated more corticosterone in landscapes with fewer patches of shade. Conversely, shifts in circulating testosterone depended only on the relative body size of a lizard; when paired, large males and small males circulated more and less testosterone, respectively. Furthermore, isolated males moved the farthest and covered the most area when shade was concentrated in a single patch, but paired males did the opposite. Because the total area of shade in each landscape was the same, these hormonal and behavioral responses of lizards reflect the ability to access shade. Thus, circulating glucocorticoids should reflect the thermal quality of an environment when researchers have controlled for other factors. Moreover, a theory of stress during thermoregulation would help ecologists anticipate physiological and behavioral responses to changing climates.

Elevated temperatures drive fine-scale patterns of habitat use in a savanna bird community

Many animals use thermally buffered microhabitats, e.g. patches of shade within trees, to avoid temperature extremes. These ‘thermal refugia’ may mediate the impact of climate change on animals. Predicting how species and communities will respond to rising temperatures therefore requires an understanding of the availability of these refugia and how animals use them. We investigated patterns of tree use by birds in the southern Kalahari across different times of day and days of varying maximum air temperature. On ‘hot’ days (>35 °C) birds showed increased preference for trees that provided the greatest density of shade (Boscia albitrunca), and this effect was particularly pronounced at the hottest times of day. Comparisons of focal bird species with differing foraging niches revealed interspecific differences in tree use. Two arboreally foraging species showed a similar preference for B. albitrunca on both ‘cool’ and ‘hot’ days. In contrast, two ground-foraging species increased their use of B. albitrunca trees on hot days, with one species (Scaly-feathered Finch Sporopipes squamifrons) changing its behaviour from avoiding to preferring this tree. We discuss the role of B. albitrunca trees as thermal refugia and the implications of temperature-driven changes in tree use in the context of rising temperatures due to climate change.

Sexual reproduction in the phyllodioicous bryophyte Homalothecium lutescens (Hedw.) H.Rob. in relation to habitat age, growth conditions and genetic variation

Sporophyte production and female fertility were investigated in seventeen calcareous grassland demes of the moss Homalothecium lutescens (Hedw.) H.Rob. on the Baltic island of Öland, with the aim of understanding the relationships between sexual reproduction, habitat age, genetic variation and factors related to growth conditions. The overall proportion of fertile female shoots (with perichaetia) was 35%. Fertility status at the level of individual shoots was positively associated with shoot length and density, while within deme fertility was positively associated with bush cover. There was no association between female fertility and habitat age, genetic diversity (HS) or allelic richness. Out of 1344 investigated shoots, only two were normal-sized fertile males. Dwarf males were also extremely rare, and found almost exclusively on shoots with sporophytes. Few sporophytes were observed (in the two demes with highest fertility and bush cover). No relationship between genetic variation and the frequency of sporophytes and males was found. The lack of a relationship between sexual reproduction and genetic variation suggests that sexual reproduction may not occur in the same grassland fragments as the recruitment of new clones (from spores or vegetative fragments). The majority of the dry, open grassland habitats, where H. lutescens is typically found in the study area, appear to be suboptimal for both dwarf males and fertilization. Sexual reproduction is more likely to occur in shaded (although grazed) grassland patches, where moisture levels are likely to be higher and the moss colonies are generally more vigorous.

Predator Detection is Limited in Microhabitats with High Light Intensity: An Experiment with Brown‐Headed Cowbirds

AbstractVariations in ambient light conditions across different microhabitats can modify the detectability of predators and prey. Prey have been shown to be more visible in sunlit than in shaded patches, leading to higher predation risk and more investment in vigilance (predation risk hypothesis). Additionally, prey have been hypothesized to take longer to detect predators in sunlit compared to shaded patches because of the excess of sunlight causing glare effects (disability glare hypothesis). We tested the predictions of these two non‐mutually exclusive hypotheses in a seminatural experiment with brown‐headed cowbirds by measuring vigilance behavior and detection of a ground predator in patches under the shade of vegetation and in the open. Light intensity and achromatic contrast were higher in the sunlit patches, which could enhance glare effects, but chromatic contrast was higher in the shaded patches. Brown‐headed cowbirds took longer to show alert reactions to and flee from a ground predator in sunlit compared to shaded patches. However, the two parameters associated with perceived predation risk (vigilance prior to the predator exposure and time to resume foraging after the attack) did not differ between sunlit and shaded patches. Our findings support to a greater extent the disability glare hypothesis than the predation risk hypothesis. Overall, ambient light conditions can affect two critical components of behavioral predator–prey interactions in terrestrial habitats: detection of and escape from predators. The effects of disability glare are expected to be more pronounced in bird species with wider visual fields or without sun‐shading structures; however, species may compensate through various behaviors (e.g. avoidance of sunlit patches and changes in head orientation).

Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river

1. Algal growth in lotic systems is controlled either from the bottom-up (e.g. nutrients and light, which determine growth rates) or from the top-down (e.g. grazing pressure, which reduces accumulated biomass). Nutrient-enriched streams that support large and diverse grazing macroinvertebrate populations and those with shaded riparian corridors rarely suffer from excessive algal growth. 2. In this study, the density of benthic algivorous macroinvertebrates was experimentally manipulated in shaded and open nutrient-enriched stream habitats of the Owennagearagh River, south-west Ireland. The ability of macroinvertebrate grazers and riparian shade to control benthic algal growth [particularly the nuisance alga, Cladophora glomerata (L. Kütz)] was investigated. Three sites with markedly different concentrations of plant nutrients (one site upstream and two sites downstream of the sewage outfall) were selected. The density of grazing invertebrates colonising ceramic tiles was reduced using high-voltage localised electric pulses. Replicates of treatment (grazer-excluded) and control (grazed) tiles were deployed in open and shaded (<25 and >80% canopy cover, respectively) patches of stream bed, in each site. 3. After 2-week Cladophora cover, periphytic chlorophyll a and biofilm ash-free dry mass (AFDM) were quantified for all experimental tiles. Values for all three parameters were highest on grazer-excluded tiles from open patches. Grazed tiles from open patches accrued little Cladophora and had significantly lower levels of chlorophyll a and AFDM. Nutrient inputs were found to have an impact on the density of grazing invertebrates, with higher densities of Baetis nymphs at the most nutrient-enriched site. 4. Our results demonstrate that in eutrophic, high-light streams, filamentous algae can quickly accumulate to nuisance levels in the absence of invertebrate grazers. In future, greater attention should be paid to the role of grazing invertebrates in controlling nuisance algae in streams, in addition to algal–nutrient relationships.

Atmospheric water vapor as driver of litter decomposition in Mediterranean shrubland and grassland during rainless seasons

AbstractLitter production in many drought‐affected ecosystems coincides with the beginning of an extended season of no or limited rainfall. Because of lack of moisture litter decomposition during such periods has been largely ignored so far, despite potential importance for the overall decay process in such ecosystems. To determine drivers and extent of litter decay in rainless periods, a litterbag study was conducted in Mediterranean shrublands, dwarf shrublands and grasslands. Heterogeneous local and common straw litter was left to decompose in open and shaded patches of various field sites in two study regions. Fresh local litter lost 4–18% of its initial mass over about 4 months without rainfall, which amounted to 15–50% of total annual decomposition. Lab incubations and changes in chemical composition suggested that litter was degraded by microbial activity, enabled by absorption of water vapor from the atmosphere. High mean relative humidity of 85% was measured during 8–9 h of most nights, but the possibility of fog deposition or dew formation at the soil surface was excluded. Over 95% of the variation in mass loss and changes in litter nitrogen were explained by characteristics of water‐vapor uptake by litter. Photodegradation induced by the intense solar radiation was an additional mechanism of litter decomposition as indicated by lignin dynamics. Lignin loss from litter increased with exposure to ultraviolet radiation and with initial lignin concentration, together explaining 90%–97% of the variation in lignin mass change. Our results indicate that water vapor, solar radiation and litter quality controlled decomposition and changes in litter chemistry during rainless seasons. Many regions worldwide experience transient periods without rainfall, and more land area is expected to undergo reductions in rainfall as a consequence of climate change. Therefore, absorption of water vapor might play a role in decomposition and nutrient cycling in an increasing number of ecosystems.

Gap‐dependence in mangrove life‐history strategies: a consideration of the entire life cycle and patch dynamics

1 Tropical rain forest trees have been classified according to the importance of canopy disturbances for their life histories. To assess gap-dependence vs. shade tolerance, we propose using demographic analysis of the degree to which gaps are critical for population growth. 2 Population growth is the composite of the successes and failures of individuals experiencing different life-cycle pathways (i.e. gap, building, and mature forest patches). To assess the contribution of each pathway, it is necessary to study patch dynamics. 3 We use stage and patch structured matrix models and loop analysis to assess the life-history strategies of two Neotropical mangrove trees, Avicennia germinans and Rhizophora mangle. We use data from field studies in Lake Maracaibo, Venezuela, that were specifically designed to parameterize these models. 4 In R. mangle, seedling to juvenile progression was limited to high light patches (> 20% canopy openness), even though they comprised < 10% of the area, and population growth depended entirely on gaps. A. germinans seedlings progressed to juveniles from all light patches, and progression rates increased with light level. However, low and medium light patches (< 20% openness) were most important to population growth because they were most frequent. We suggest that R. mangle is gap-dependent while A. germinans is shade-promoted (i.e. shaded patches are demographically more important to A. germinans population growth even though seedling progression rates increase with canopy openness). 5 Previous studies considered R. mangle shade-tolerant because its propagules can establish and initially survive in the shade, but our classification was based on its entire life cycle. For A. germinans, the important measure was not the growth response to light, but the frequency and demographic importance of different light patches. 6 Synthesis: These examples show that observations of a plant's germination biology or photosynthetic/growth response to light do not necessarily translate into demographic behaviour because physiology can be decoupled from demography by patch dynamics. Thus, it is critical to consider the entire life cycle in the context of forest patch dynamics when assessing the life-history strategies of tropical trees.

Change in radiation penetration of mechanically-injured maize

The aim of the study was to provide new information about those environmental processes, mainly in radiation, which lead to production decrease in mechanically- and hail-injured maize. The size of the leaf area and its vertical distribution were used to describe the stand. In place of direct leaf angle measurements the extinction coefficient was used to characterize the modification that resulted from leaf laceration. In the course of the investigations it was found that while healthy plant stands allowed 30.6% of the total incident radiation to reach the ground, this figure was 41% for hail-damaged stands. There was little difference in the quantity of radiation measured on sunny and shaded patches in the two stands, but the ratio of sunny to shaded patches was altered. The radiation intensity reaching the soil surface in the lacerated stand increased despite the very slight difference in leaf area index (LAI). Extra radiation input in the soil of hail-damaged stand was a consequence of modification in the leaf angle.

Why overgrowth of intertidal encrusting algae does not always cause competitive exclusion

Encrusting algae are well-known to be able, for long periods, to withstand shading and overgrowth by other organisms. How this is achieved remains a mystery. It had been proposed that connections with unshaded (non-overgrown) parts of the thallus may allow transfer of nutrients to the shaded part. From this model, I proposed and tested the hypothesis that shaded patches of the intertidal red alga, Hildenbrandia rubra, would survive overgrowth longer, or better, when connected to unshaded thallus than when experimentally separated from surrounding alga. Experimental treatments were shading (black or transparent 80 mm perspex discs or no cover) and scraping (scraped around the disc to remove contacts, a control for effects of scraping, no treatment). The 9 orthogonal combinations of cover and scraping were applied to 3 independent, random replicates (i.e. 27 plots) in each of four randomly chosen sites. In all 4 sites, over 13 months, shaded H. rubra survived in greater abundance (as % cover) where in contact with surrounding thallus. In one site, there was no effect of shading unless the thallus was isolated. In two sites, shading reduced cover, but was more deleterious where the thallus was isolated. In the fourth site, there were artefacts due to a perspex cover, but still less cover of alga where it was isolated. This encrusting alga can withstand a long period of complete shading, provided there is connection to unshaded thallus. Interpreting or predicting overgrowth interactions in terms of competitive outcomes is therefore dependent on consideration of whether the overgrown species is actually being affected. It also depends on the duration of overgrowth and, as shown here, the extent to which connectivity with unshaded thallus is effective at preventing or reducing any consequences. Observations and experiments that do not ascertain these are difficult to interpret.

Seasonal variation in the contribution of different behavioural mechanisms to lizard thermoregulation

Summary We studied seasonal changes in the thermoregulatory behaviour of the lacertid lizard Psammodroums algirus in a Mediterranean evergreen forest. Body temperatures (Tb), environmental operative temperatures (Te) and upper and lower limits of the selected thermal range (Tsel) were lower in May than in July. On average, mean deviations of Tb from Tsel (0·7 °C in both seasons) were much smaller than those of Te (8·3 °C in both seasons). Thus both the accuracy (average difference between Tb and Tsel) and effectiveness (the extent to which Tb are closer than Te to Tsel) of thermoregulation were high, and similar in both seasons. However the thermoregulatory contribution of two distinct behavioural mechanisms varied markedly between seasons. Daily activity contributed significantly to the regulation of Tb in May (when a population of Te thermometers matching lizard activity patterns would be, on average, 1·0 °C closer to Tsel than were randomly available Te), but not in July (when such a population would be only 0·2 °C closer to Tsel than were randomly available Te). The selection of sun–shade patches, the contribution of which was larger than that of daily activity in both seasons, was more important in July (when it produced a distribution of Te that would be, on average, 3·1 °C closer to Tsel than were randomly distributed Te) than in May (when a population of thermometers matching the lizards’ pattern of exposure to sunlight would be 1·3 °C closer to Tsel than were randomly available Te). These changes are discussed in the light of seasonal differences in the daily patterns of among‐patch variation in Te. In spring, lizard activity was low in the early morning because even the selection of sunlit patches was of limited utility to attain Tb within Tsel; in summer, lizards could remain active at midday, despite low overall thermal suitability, by selecting shaded patches. Thus the contribution of patch selection to thermoregulation was important in the early basking period of both seasons, and at summer midday hours. Our data suggest that shuttling between sun and shade, rather than selecting sun or shade, may be an additional mechanism of behavioural thermoregulation, the importance of which would be greatest at times of day when lizards use patches at random (e.g. spring midday hours), and that their mean Tb is closer to the grand mean of full sun and full shade Te than to the mean equilibrium Te within any type of patch.

BEHAVIORAL THERMOREGULATION BY TREECREEPERS: TRADE-OFF BETWEEN SAVING ENERGY AND REDUCING CRYPSIS

We studied the effect of solar radiation on the winter biology of Short-toed Treecreepers Certhia brachydactyla inhabiting a montane forest in Spain. We hypothesized that, in temperate latitudes of cold winter climate, with low cloudiness and under windless conditions, birds should select sunlit sites (i.e., forest sectors or trunk patches with high levels of exposure to sunlight) to reduce the metabolic cost of thermoregulation. At a within-habitat scale, a hypothesis of “only metabolic benefits” predicts that birds should select sunlit patches at shade temperatures (Tshade) below the birds' lower critical temperature (Tlc). They should shift to a random use of sunlit and shaded patches at temperatures above Tlc. Alternatively, there could be added costs (e.g., travel costs, predation risk) in using sunlit patches. If higher visibility leads to diminished crypsis at sunlit patches (“trade-off with predation risk” hypothesis), birds should select only shaded patches at Tshade values above Tlc (to enhance crypsis). They should increasingly select sunlit patches as Tshade decreases below Tlc. Treecreepers were selective in their use of sun–shade patches, across different spatial scales. At the among-plots scale, treecreeper abundance was positively related to the availability of sunlit trunks, after we controlled for the effects of tree density, prey availability, and altitude. At the within-plot scale (i.e., selection of foraging patches on trunks and thick branches), Ivlev's electivity for sunlit patches decreased linearly as Tshade increased. Birds preferred to forage on sun-exposed surfaces (electivity >0) when Tshade was lower than ∼4°C, but they tended to forage on shaded surfaces (electivity < 0) when Tshade was higher than ∼9°C. The selection of sunlit trunk patches at low temperatures was not a by-product of higher food availability, because numbers of prey were much less predictable than temperature as a function of trunk exposure (sun vs. shade), and the negative relationship between use of sunlit patches and temperature remained significant when we controlled for the effects of prey availability. Thus, the selection of sunlit patches at low temperatures can be interpreted as a behavioral thermoregulation strategy allowing birds to save energy. However, the Ivlev's electivity for sunlit patches became negative at temperatures well below Tlc, and no sunlit patches were used when Tshade ≈ Tlc, which led us to reject the “only metabolic benefits” hypothesis. Photometric measurements of treecreeper taxidermic mounts realistically positioned on trunk surfaces, and detection times by simulated (human) predators, suggest that treecreepers were more detectable under direct solar radiation than in deep shade. Crypsis diminished in sunlit patches because of a higher image contrast and an increased difference in perceivable coloration between bird and background, relative to shaded patches. Average detection times were significantly lower for sunlit mounts. Moreover, focal birds scanned more frequently in sunlit than in shaded patches. Thus, the observed temperature-dependent variation in the selection of sunlit substrata is consistent with the “trade-off with predation risk” hypothesis predicting that prey should avoid patches where they are more detectable to potential predators. We interpret the distribution and behavior of treecreepers as indicative of a trade-off between the energy savings (through higher operative temperature and reduced metabolic costs) and the increased risk of predation (through higher visibility and diminished crypsis) afforded by sunlit foraging patches.

Growth of rice in modified microclimates of agroforestry

In an agroforestry system, radiation received at the ground level is highly variable and consists of alternate patches of light and shade. The level of incident and diffuse radiation affects radiation-use efficiency in crops. Differences in air movement and radiation may also result in spatial variation in temperature and humidity. The importance of soil and floodwater temperatures in the rice crop has been emphasized by Collinson et al (1995). Modifying a crop’s microclimate in agroforestry systems is therefore likely to affect crop photosynthesis and evapotranspiration. This study was carried out to measure some components of the microclimate in rice under trees and observe the growth parameters of rice in such modified microclimates.

Clash of the Clans: An Extract from a Bit of Earth

Street near the docks was the meeting place of the gainfully employed and the desperately unemployed. Hundreds of miners, who had fled from the troubles in Bandong Valley, Larut and elsewhere, had converged on the area. The backlanes and sidelanes were choked with idle men. They squatted beside the open drains, shoved their way along the five-foot paths, overflowed onto the roads and blocked the progress of the pushcarts, rickshaws, buggies, gharries and bullock carts inching their way through the heart of the trading district. All were hoping to earn a few cents, which would buy the day's bowl of rice. But jobs were scarce. Traders and merchants were jittery about their investments in the turbulent Malay States. The town was full of rumours of impending war. To the few policemen patrolling the area, mainly Sikhs and Amboynese employed in the Colonial Police Department, the babble of Chinese tongues was incomprehensible. They walked through the throng, unseeing and unhearing. They did not see the knots of coolies swelling. Neither did they hear talk of murder and slaughter nor rumours of impending war between the Hakka and Cantonese miners. Tai-kor Wong was killed in bed! His head chopped off and flung out of the window ! 'The gods have eyes, lor Let the White Cranes taste our sword of vengeance! Kill those daughter-fuckers, mother-fuckers and sons of pigs! As the morning wore on, the talk grew more virulent and strident. Tales of past battles were recalled and relived. Venom and rage spread among the coolies, smiting the knots of men huddled in the shadows. The sun rose higher in the cloudless sky, and soon not many patches of shade were left. More coolies squeezed into the covered walkways of the shops. Tempers ran short. A word here, a word there, a few words overheard and repeated, an imagined insult, a stare, a push and before anyone knew it, something had ignited the tinderbox. A scuffle broke out just as Baba Wee's gharry swung into view. A young coolie was pushed into its path. The two horses neighed and reared up on their hind legs. The Indian driver was nearly knocked off his seat. Whoa!