Temperate Herbivores Research Articles

A test of the frost wave hypothesis in a temperate ungulate.

Growing evidence supports the hypothesis that temperate herbivores surf the green wave of emerging plants during spring migration. Despite the importance of autumn migration, few studies have conceptualized resource tracking of temperate herbivores during this critical season. We adapted the frost wave hypothesis (FWH), which posits that animals pace their autumn migration to reduce exposure to snow but increase acquisition of forage. We tested the FWH in a population of mule deer in Wyoming, USA by tracking the autumn migrations of n = 163 mule deer that moved 15-288 km from summer to winter range. Migrating deer experienced similar amounts of snow but 1.4-2.1 times more residual forage than if they had naïve knowledge of when or how fast to migrate. Importantly, deer balanced exposure to snow and forage in a spatial manner. At the fine scale, deer avoided snow near their mountainous summer ranges and became more risk prone to snow near winter range. Aligning with their higher tolerance of snow and lingering behavior to acquire residual forage, deer increased stopover use by 1 ± 1 day (95% CI) day for every 10% of their migration completed. Our findings support the prediction that mule deer pace their autumn migration with the onset of snow and residual forage, but refine the FWH to include movement behavior en route that is spatially dynamic.

Range‐extending tropical herbivores increase diversity, intensity and extent of herbivory functions in temperate marine ecosystems

Abstract Climate change is modifying species distributions around the world, forcing some species poleward, where they can alter trophic interactions. Many tropical herbivorous fishes have successfully expanded their ranges into temperate ecosystems, and while it is clear they drive increases in herbivory rates in specific localities, little is known about how they might affect the diversity of herbivory functions across large spatial scales, considering their interaction with assemblages of native herbivores in temperate habitats. We assessed the spatial overlap and habitat associations of native temperate and range‐expanding tropical herbivorous fishes in six subregions of south‐western Australia to determine how incursions of tropical species may have affected the diversity, redundancy (index of uniqueness) and the ‘spatial extent’ (addition of functions in new areas) and ‘intensity’ (increasing density of functional groups) of specific herbivory functions in recipient ecosystems. Tropical herbivores had high abundances in temperate ecosystems, forming schools from 40 (parrotfish) to 200 (rabbitfish) individuals strongly associated with seagrass meadows and reefs with high cover of turf algae. Overlap with temperate herbivores was highest in the northern subregions, forming unique assemblages, with no apparent species displacements. The addition of tropical species increased functional diversity and uniqueness (the complement of redundancy), introducing novel herbivory functions to many locations. Seagrass browsing increased in spatial extent (27%) and intensity (15×), while seaweed browsing and grazing increased in intensity by up to 2.5× in regions with high abundances of tropical herbivores. Our results suggest that the diversity, intensity and spatial extent of different herbivory functions can change as tropical species with different habitat affinities, behaviours and diets shift their distributions poleward. Changes in functional redundancy are likely to be heterogeneous in space and might not increase initially because the diversity of herbivory functions is relatively low in some temperate marine ecosystems. However, there is the potential for greater redundancy as further tropical species arrive, their abundances increase and the spatial and functional overlap of communities rises. A free Plain Language Summary can be found within the Supporting Information of this article.

Overwintering tropical herbivores accelerate detritus production on temperate reefs.

The tropicalization of temperate marine ecosystems can lead to increased herbivory rates, reducing the standing stock of seaweeds and potentially causing increases in detritus production. However, long-term studies analysing these processes associated with the persistence of tropical herbivores in temperate reefs are lacking. We assessed the seasonal variation in abundances, macrophyte consumption, feeding modes and defecation rates of the range-extending tropical rabbitfish Siganus fuscescens and the temperate silver drummer Kyphosus sydneyanus and herring cale Olisthops cyanomelas on tropicalized reefs of Western Australia. Rabbitfish overwintered in temperate reefs, consumed more kelp and other macrophytes in all feeding modes, and defecated more during both summer and winter than the temperate herbivores. Herbivory and defecation increased with rabbitfish abundance, but this was dependent on temperature, with higher rates attained by big schools during summer and lower rates in winter. Still, rabbitfish surpassed temperate herbivores, leading to a fivefold acceleration in the transformation of macrophyte standing stock to detritus, a function usually attributed to sea urchins in kelp forests. Our results suggest that further warming and tropicalization will not only increase primary consumption and affect the habitat structure of temperate reefs but also increase detritus production, with the potential to modify energy pathways.

Macroalgal resource use differences across age and size classes in the dominant temperate herbivorous fish Aplodactylus lophodon (Aplodactylidae)

Herbivorous fishes comprise a substantial proportion of temperate fish communities, although there is little understanding of their trophic resource use and whether this changes throughout post-settlement ontogeny. With increasing loss of macroalgal forests, understanding how temperate fishes use macroalgae will be vital in predicting future effects on temperate fish biodiversity. The Australian rock cale (Aplodactylus lophodon) is one of the most abundant herbivorous fish inhabiting shallow temperate south-eastern Australian reefs. We examined gastrointestinal contents throughout ontogeny and demonstrated that this species maintains a herbivorous diet through all life stages. Differences in algal taxa consumed were apparent through ontogeny, with the juvenile diet dominated by filamentous red and green algae and the adult diet dominated by brown and calcareous red algae. Relative gut length increased through ontogeny, potentially facilitating dietary transition to less digestible algae, but no concurrent increase in jaw power was observed. The results highlight the diversity of trophic resource use in a temperate marine herbivore, but the near-complete dominance of dietary algae throughout ontogeny indicates the reliance on primary producers across all life stages. Given the importance of fucoid resources in the adult diet, any loss of macroalgal forests within south-eastern Australia may affect foraging success and persistence.

Tropicalization strengthens consumer pressure on habitat-forming seaweeds

Ocean warming is driving species poleward, causing a ‘tropicalization’ of temperate ecosystems around the world. Increasing abundances of tropical herbivores on temperate reefs could accelerate declines in habitat-forming seaweeds with devastating consequences for these important marine ecosystems. Here we document an expansion of rabbitfish (Siganus fuscescens), a tropical herbivore, on temperate reefs in Western Australia following a marine heatwave and demonstrate their impact on local kelp forests (Ecklonia radiata). Before the heatwave there were no rabbitfish and low rates of kelp herbivory but after the heatwave rabbitfish were common at most reefs and consumption of kelp was high. Herbivory increased 30-fold and kelp abundance decreased by 70% at reefs where rabbitfish had established. In contrast, where rabbitfish were absent, kelp abundance and herbivory did not change. Video-analysis confirmed that rabbitfish were the main consumers of kelp, followed by silver drummers (Kyphosus sydneyanus), a temperate herbivore. These results represent a likely indirect effect of the heatwave beyond its acute impacts, and they provide evidence that range-shifting tropical herbivores can contribute to declines in habitat-forming seaweeds within a few years of their establishment.

Potential for biotic resistance from herbivores to tropical and subtropical plant invasions in aquatic ecosystems

Invasions of tropical and subtropical aquatic plants threaten biodiversity and cause ecological and economic impacts worldwide. An urgent question is whether native herbivores are able to inhibit the spread of these alien species thus providing biotic resistance. The potential for biotic resistance to these plants depends on plant traits that affect palatability to herbivores, i.e., plant nutritional quality and the presence of secondary metabolites related to anti-herbivory defenses. Studies across latitudinal gradients suggest that aquatic plants from lower latitudes may be less nutritious and better defended than high latitude plants. Therefore, we hypothesized that native herbivores prefer temperate plants over plants from tropical and subtropical regions which would limit the strength of biotic resistance that native temperate herbivores can provide against alien tropical and subtropical plants. Drawing upon the published literature we (1) investigated whether native temperate herbivores reduce the establishment or performance of tropical and subtropical alien plants in the field, and (2) analyzed herbivore consumption of tropical and subtropical versus temperate plants in laboratory feeding trials. In our literature survey, we found only three field studies which, in contrast to our hypothesis, all demonstrated that the native herbivores (beavers, coots or generalist insect herbivores) significantly reduced the success of invading tropical and subtropical plant species. The analysis of the feeding trials yielded mixed results. Ten out of twelve feeding trials showed that (sub)tropical and temperate plants were consumed in equal amounts by both temperate and tropical generalist ectothermic herbivores. The remaining trials showed higher consumption rate of both temperate and tropical plants by tropical snails. Although a body of evidence suggests that tropical plants are nutritionally poor and better defended (i.e., less palatable) compared to temperate plants, we conclude that in the majority of cases, herbivores would eat tropical plants as much as temperate plants. Thus, in agreement with the available field studies, evidence suggests that there is potential for biotic resistance from native generalist herbivores to tropical invasive plants in non-tropical areas.

Winter severity modulates the benefits of using a habitat temporally uncoupled from browsing

AbstractResources whose abundance is not affected by the density of the consumer population, namely donor‐controlled resources, are ubiquitous. Donor‐controlled resources can act as food subsidies when they sustain consumer populations at higher densities than what would be predicted without donor‐controlled dynamics. Herbivore populations that have access to food subsidies may reach and maintain high densities, with potential major ecological and economic consequences. A better understanding of the roles of food subsidies on temperate herbivores will likely be achieved by simultaneously taking into account other drivers of demographic variations such as winter severity. Here, we tested the hypothesis that the use of a donor‐controlled food resource that may act as a food subsidy, namely balsam fir (Abies balsamea), and winter severity act together to shape the patterns of overwinter mass loss in a large herbivore population (white‐tailed deer, Odocoileus virginianus). We monitored weather conditions, diet, habitat use, and mass loss of female deer during two highly contrasted winters. During an exceptionally milder winter, characterized by shallower snow depth and warmer windchill temperatures, female deer shifted their diet toward resources usually covered by snow during typical winters. Surprisingly, the rate of body mass loss remained similar during the milder and the harsher winter. The rate of body mass loss rather decreased with the use of balsam fir stands during the harsher winter, but increased with that same variable during the milder winter. Our study revealed that deer can alleviate overwinter mass loss by using a donor‐controlled habitat type temporally uncoupled from browsing, but that this benefit is climate dependent. This study represents an additional step to address the largely unexplored concept of how temporal uncoupling between resources and consumer dynamics may contribute to sustain consumer populations at higher densities than predicted without considering donor‐controlled dynamics.

Temperate predators and seasonal water temperatures impact feeding of a range expanding tropical fish

Range expansions of species due to climate change threaten the function and composition of marine ecosystems globally, yet factors constraining or facilitating this redistribution are poorly understood. Native predators may constrain prey shifting poleward through consumption, or by restricting their feeding activity. However, the extent that native predators impact range-expanders will likely be structured by physiological mismatches between these groups, associated with water temperatures. We examined how temperate predators and seasonal water temperature affected foraging of the planktivorous tropical reef fish, Abudefduf vaigiensis, within temperate southeastern Australian waters, an emerging new range. Foraging excursions of A. vaigiensis were compared between predator-rich marine reserves and predator-depauperate fished reefs during summer and winter (~18 and 22 °C water, respectively). A. vaigiensis foraged with shorter excursions in marine reserves than fished reefs and higher excursions during summer than winter. Effects of predation risk and water temperature on A. vaigiensis foraging were isolated in an aquarium experiment. Groups were held at 18 or 22 °C and visually exposed to a temperate predator, a predator control (temperate herbivore) and an empty tank. Foraging excursions and feeding rates were reduced when exposed to predators at 22 °C, but did not differ between predator and the predator control or empty tank at 18 °C. Results suggest temperate predators may restrict range expansions of A. vaigiensis by reducing its’ food intake during summer months. But winter water temperatures may limit feeding, independent of predation risk. Protection of predators from fishing should improve resistance of some marine ecosystems to impacts of range expanding prey.

Forage-mediated density and climate effects on body mass in a temperate herbivore: a mechanistic approach.

The interplay between density and climate in shaping the dynamics of herbivore populations is widely acknowledged, and current research is fueled by the identification of mechanisms underlying their effects on individuals and populations. We assessed whether forage availability mediated the effects of density and winter climate on body mass of white-tailed deer (Odocoileus virginianus) yearlings by experimentally reducing deer density to 7.5 and 15 deer/km2 during eight growing seasons, and by using causal (graphical) hierarchical models and Bayesian hierarchical modeling to assess relationships. The abundance of preferred forage decreased with deer density and varied quadratically (positive parabola) with winter North Atlantic Oscillation (NAO), whereas the fall mass of yearlings increased with forage abundance and spring mass. Fall mass did not differ between experimentally reduced deer densities, yet experimental yearlings were 30% heavier than yearlings harvested at ambient densities. Hence, forage abundance simultaneously mediated the effects of density and climate on fall body mass, which was also influenced by carry-over effects of spring body mass. Our findings increase our ability to anticipate how temperate large herbivores will respond to ongoing changes in intrinsic (e.g., large-herbivore density) and extrinsic (e.g., climate) factors.

Confounded winter and spring phenoclimatology on large herbivore ranges

Annual variation in winter severity and growing season vegetation dynamics appear to influence the demography of temperate herbivores but parsing winter from spring effects requires independent metrics of environmental conditions specific to each season. We tested for independence in annual variation amongst four common metrics used to describe winter severity and early growing season vegetation dynamics across the entire spatial distribution of elk (Cervus elaphus) in Wyoming from 1989 to 2006. Winter conditions and early growing season dynamics were correlated in a specific way. Winters with snow cover that ended early tended to be followed by early, but slow, rises in the normalized difference vegetation index (NDVI), while long winters with extended periods of snow cover were often followed by late and rapid rises in NDVI. Across the 35 elk ranges, 0.4–86.8 % of the variation in the rate of increase in NDVI’s in spring was explained by the date snow cover disappeared from SNOTEL stations. Because phenoclimatological metrics are correlated across seasons and shifting due to climate change, identifying environmental constraints on herbivore fitness, particularly migratory species, is more difficult than previously recognized.

Biogeographic and phylogenetic effects on feeding resistance of generalist herbivores toward plant chemical defenses

Many terrestrial and most marine herbivores have generalist diets, yet the role that evolutionary history plays in their foraging behaviors is poorly documented. On tropical hard-bottom reefs, generalist fishes and sea urchins readily consume seaweeds that produce lipophilic secondary metabolites. In contrast, herbivores on temperate reefs less commonly encounter seaweeds with analogous metabolites. This biogeographic pattern suggests that tropical herbivores should evolve greater feeding resistance to lipophilic defenses relative to temperate herbivores, but tests of this biogeographic pattern are rare. We offered lipophilic extracts from nine subtropical seaweeds at two concentrations to sea urchins (four subtropical and three cold-temperate populations) and quantified urchin feeding resistance. Patterns of feeding resistance toward lipophilic defenses were more similar within genera than across genera of urchins, indicating a substantial role for phylogenetic history in the feeding ecology of these generalist herbivores. The biogeographic origin of urchins also influenced feeding resistance, as subtropical species displayed greater feeding resistance than did temperate species. Similarly, a subtropical population of Arbacia punctulata had greater feeding resistance for Dictyota and Stypopodium extracts relative to temperate A. punctulata. We conclude that evolutionary history plays a more central role in the foraging ecology of generalist herbivores than is currently appreciated.

Fecal chlorophyll describes the link between primary production and consumption in a terrestrial herbivore

Spatiotemporal variation in primary productivity is known to have strong and far-reaching effects on herbivore ecology, but this relationship is often studied indirectly at broad scales, in part due to the difficulty in measuring selection for green biomass by individual animals. In aquatic systems, the concentration of chlorophyll in herbivore feces has been used as a direct measure of the consumption of photosynthetic primary production, but this method has not been applied to terrestrial systems. We measured chlorophyll concentration in feces from elk (Cervus elaphus) experiencing large fluctuations in primary production in the winter to spring transition over three years. We compared temporal trends in fecal chlorophyll with trends in fecal nitrogen, grass chlorophyll, grass digestible nitrogen, and landscape-level primary productivity (as described by the normalized difference vegetation index or NDVI). We also directly examined the relationship between fecal chlorophyll and NDVI. Temporal trends in fecal chlorophyll were strong and well described by piecewise regression (adjusted coefficient of determination, r(2)a = 0.881-0.888), showing uniformly low concentrations throughout winter followed by an abrupt, rapid increase beginning on different Julian days (88, 91, or 110) each year. Changes in fecal chlorophyll closely matched the temporal trend in the chlorophyll and digestible nitrogen concentration of forage grasses collected directly from elk feeding sites. Fecal chlorophyll also tracked broad temporal patterns in fecal nitrogen and NDVI, but discrepancies between the indexes may highlight preferences or constraints on selectivity for green biomass in elk. Spatially and temporally matched NDVI and fecal chlorophyll estimates were uncorrelated until NDVI reached approximately half its seasonal range. Combined, these data describe important patterns in selection for nutritious, green biomass in a temperate herbivore that would be difficult to study without data on fecal chlorophyll. Fecal chlorophyll produced novel and precise descriptions of (and detected large interannual differences in) winter length, severity, and the rate of spring green-up, as they were experienced by a large, grazing herbivore. Measuring fecal chlorophyll provides a noninvasive, inexpensive, and direct approach to describe an important aspect of foraging ecology in terrestrial herbivores and may be particularly powerful for studying climate effects in seasonal environments.

Spring Normalized Difference Vegetation Index (NDVI) predicts annual variation in timing of peak faecal crude protein in mountain ungulates

Summary In recent years, the Normalized Difference Vegetation Index (NDVI) has been used to assess the relationships between habitat quality and animal life‐history traits. Since numerous ecological studies now use NDVI rather than perform direct vegetation assessments, field validations are essential to provide confidence in the biological significance of NDVI estimates. While some studies have compared NDVI with plant biomass, very few examined the relationship between NDVI and changes in vegetation quality. Using data from two long‐term studies of alpine ungulates, we assessed the relationship between two NDVI indices and the date of peak in faecal crude protein (FCP), which represents temporal variability in the availability of high‐quality vegetation. We also evaluated if NDVI data could predict annual variation in the timing of spring green‐up. In both populations, integrated NDVI in June was negatively correlated with the date of the peak in FCP, indicating that high integrated NDVI values corresponded to early springs in alpine habitats. Maximum NDVI increase during spring green‐up was positively correlated with the timing of peak FCP, illustrating that rapid increases in NDVI represented delayed springs. Predicted values of date of peak FCP estimated each year from NDVI data satisfactorily fitted observed values, and prediction intervals included all observed values. These results suggest that NDVI can reliably predict variation over years in the timing of spring. Synthesis and applications. Our long‐term studies demonstrate that a multi‐year time series of Normalized Difference Vegetation Index (NDVI) can reliably measure yearly changes in the timing of the availability of high‐quality vegetation for temperate herbivores. This finding therefore supports the use of NDVI as a proxy for vegetation attributes in population ecology and wildlife management studies.

Investigator Group Expedition 2006: Flexible Foraging Ecologyofa Temperate Herbivore, The Herring Cale, Olisthops Cyanomelas, in South Australia

The odacine fish, herring cale (HC), Olisthops cyanomelas, is a facultative browser, feeding mainly on the secondary laminae of Ecklonia radiata, and the apical receptacles of the fucoids, Cystophora spp. and Sargassum spp. Diet, daily feeding rates and time budgets of HC were examined at two South Australian sites >1000 km apart and in different water movement conditions, with additional dietary data from an exposed site. The diet and feeding behaviour were habitat-dependent. At one partly-sheltered site with abundant Ecklonia, HC was most active immediately after dawn and toward dusk, whereas at the other more sheltered site, dominated by fucoid algae, it was inactive at dawn increasing to a moderate level of activity in mid-morning, and peaking toward sunset. Feeding bite rates were variable and mean rates ranged from 8 – 14.5 min−1 according to site. HC characteristically engaged in intermittent foraging bouts, interspersed with periods of inactivity, which averaged ~ 55% of daylight hours. Interactions with other species were few. Analysis showed no significant differences between small and large HC in bite rates, but significant between-site differences in bite rates, and in the proportion of time spent foraging and swimming. In this study HC did not destroy patches of the kelp, Ecklonia, as it does in eastern Australia, even when the kelp comprises the main diet, because it fed only on the distal parts of the kelp’s secondary laminae and meristem.

Maternal care in white-tailed deer: trade-off between maintenance and reproduction under food restriction

Lactation is the most energetically costly component of maternal investment in mammals. For large temperate herbivores, summer is characterized by relatively abundant forage but also high energetic needs for lactation and recovery from winter mass loss. We experimentally restricted food supply by about 20%, and compared the nursing and foraging behaviours of control and food-restricted adult female white-tailed deer, Odocoileus virginianus, and their fawns during lactation. We considered two fawn ages: 0–30 days (after which time spent suckling dropped markedly) and 30–80 days (the end of the nursing period). From 0 to 30 days of age, food-restricted fawns performed 17 more suckling bouts/day and spent twice as much time suckling than control fawns. Compared with controls, food-restricted fawns gained 26% less mass from birth to 80 days. Body growth was inversely related to time spent suckling and to the frequency of nursing bouts, but positively related to survival. Food-restricted fawns had twice as many suckling solicitations and rejected suckling attempts as control fawns. Solicitations for allosuckling and successful allosuckling bouts were also more than twice as high in the food-restricted group as in the control group. Mothers and fawns from the food-restricted group spent more time foraging than control individuals. We conclude that a reduction in food availability during summer, which may occur under high intraspecific competition, should lead to drastic changes in foraging and nursing behaviours as well as reduced growth rate of juveniles of large northern herbivores.

Host plant defences and voltinism in European butterflies

Abstract1. With respect to seasonal availability for herbivores, plants defended by synthesising qualitative compounds differ from those protected by accumulation of quantitative macromolecules, leaf toughness, and low water and/or nutrient content. While the palatability of the former plants remains relatively constant during the season, the palatability of the latter group decreases with leaf age.2. It was hypothesised that in seasonal temperate environments, quantitative plant defences should restrict the annual numbers of insect generations. To test this hypothesis, European butterflies were used as a model, both non‐corrected regressions and tests controlling for phylogeny were carried out, and potentially confounding factors such as body size or occurrence in short‐season environments were treated as covariables.3. Non‐phylogenetically controlled regressions corroborated that butterflies feeding on quantitatively protected hosts (woody plants + grasses) form fewer generations than species feeding on qualitatively protected forbs. Plant defences fitted voltinism better than butterfly size, and remained significant even after controlling for short seasons. Using independent contrasts, feeding on woody plants plus grasses, and feeding on woody plants only, predicted fewer generations. These patterns, however, applied exclusively for foliage‐feeding species.4. The association between plant defences and voltinism represents a hitherto overlooked pattern in the ecology of temperate herbivores. It may explain why large insects tend to form fewer generations and feed on structurally complex hosts, and why some species remain monovoltine although they are not restricted by short season.

Are Tropical Herbivores More Resistant Than Temperate Herbivores to Seaweed Chemical Defenses? Diterpenoid Metobolites from Dictyota acutiloba as Feeding Deterrents for Tropical Versus Temperate Fishes and Urchins

Because herbivory is more intense in the tropics, tropical seaweeds may be better defended against herbivory than are temperate seaweeds. A “diffuse” coevolutionary corollary to this hypothesis is that tropical herbivores should be more resistant to seaweed defenses than temperate herbivores because tropical herbivores more commonly encounter heavily defended seaweeds. We begin to test the latter prediction using three newly discovered diterpenoid secondary metabolites from the tropical brown alga Dictyota acutiloba. We tested the feeding deterrent properties of these compounds against common herbivorous fishes and sea urchins from warm-temperate North Carolina versus tropical Guam using standardized laboratory feeding assays. The temperate herbivores were deterred by lower concentrations of secondary metabolites than the tropical herbivores. In no case was a tropical herbivore more deterred by a compound than a temperate herbivore, suggesting that temperate herbivores may be more strongly affected by seaweed chemical defenses. Feeding by the temperate pinfish Lagodon rhomboides was significantly reduced by two of the three diterpenes at a concentration that was only 13–18% of the natural concentration found in the alga. Feeding by four species of tropical fishes (two parrotfishes and two surgeonfishes) was unaffected by metabolite concentrations that deterred the temperate fish. At 100% of natural concentrations, only one of the three compounds deterred the two parrotfishes, and none of the three compounds deterred the surgeonfishes. Contrasts between the temperate sea urchin Arbacia punctulata and the tropical sea urchin Diadema savignyi showed a similar pattern; low concentrations of acutilol A acetate strongly deterred the temperate, but not the tropical, urchin. Tropical herbivores appear more resistant than temperate herbivores to seaweed chemical defenses.

Climate, Plant Phenology and Variation in Age of First Reproduction in a Temperate Herbivore

Density-independent weather effects can have important consequences for the demography of terrestrial herbivores because precipitation, temperature and insolation influence plant phenology, forage quality and biomass production, which in turn affects the habitat carrying capacity. Since forage digestibility influences intake and weight gain, life-history traits of young, growing animals are likely to reflect variation in the prevailing weather. This paper specifically investigates spatial and temporal variation in age at maturation in female red deer (Cervus elaphus) in Norway in relation to climate variables known to influence primary production. Our findings are corroborated by analysing differences in age at maturation in 21 cohorts of red deer on the Isle of Rum, Scotland. In Norway the majority of females ovulated as yearlings and calved for the first time as 2-year-olds. The proportion calving for the first time at two years varied from 0.23 to 0.67 between regions and fluctuated from 0.46 to 0.76 between cohorts. On Rum, where age of maturation was delayed at least a year, the proportion calving for the first time as 3-year-olds varied between cohorts from 0.0 to 0.89. In a subset of yearlings culled in Norway at the time of conception, the spatial and temporal differences in ovulation rates were related to the geographical and annual variation in body weight. 5. Both the spatial and temporal variation in the proportion of 2-year-olds calving in Norway, and cohort differences in the proportion calving as 3-year-olds on Rum, were negatively related to variation in May-June degree days 12 months earlier. Although primary production on the preferred herb-rich Agrostis-Festuca grasslands was positively correlated with temperature in May and June on Rum, the proportion of females calving as three years old, was negatively correlated with annual differences in May June primary production. We argue that retarded phenological development, during periods of cooler weather, enhances diet quality because leaf :stem ratios and digestibility of plant parts decline more slowly. Thus, weight gain during the early summer growth spurt should be rapid during cool May-June weather, increasing the probability of conception in the autumn. Since density-independent variation in food availability also influences fitness components which commonly have a more pronounced influence on population demography, for example offspring survival, we argue that our results highlight the potential importance of variation in weather on herbivore abundance.

Gradient Analysis Using Plant Modular Structure: Pattern in Plant Architecture and Insect Herbivore Utilization

Distribution of five species of tropical insect herbivores was recorded in relation to gradients of modular length on their host plants. Patterns in plant architecture and herbivore responses were compared with a well-studied temperate herbivore, Euura lasiolepis Smith (Hymenoptera : Tenthredinidae). The five tropical species were each from a different family, and host plants belonged to different families and vegetation types. Using modular gradients of shoot or leaf length we showed that plant architecture was comparable directly and was similar even when trees, shrubs, and grasses were compared. Also, the membracid, cicadellids, psyllid, and cecidomyiid in the tropics showed patterns of attack on modular gradients consistent with the patterns derived from high ovipositional preference linkage with larval performance, known to exist in the temperate species. Strong positive relationships between module length and percentage of modules attacked in each length class were observed, with variance accounted for ranging from 68 to 96%. One pest species on forage grasses in the vicinity of Brasilia, Deois flavopicta Stal (Homoptera : Cercopidae), showed a weak response to the shoot-length gradient of its host grass (r 2 = 0.41) and an insignificant response per unit length of shoot, suggesting a generalized capacity to use wide variation in module size. A method was developed for direct comparison of species employing a comparison between a hypothetical ideal response to modular gradients and the actual response. The actual slope of the attack versus module length regression, expressed as a percentage of the hypothetical slope, provided estimates of herbivore responses to modular plant structure consistent with hypotheses on plant vigor and phylogenetic constraints.

Interspecific allometry of population density in mammals and other animals: the independence of body mass and population energy-use

Global regressions of ecological population densities on body mass for mammals and for terrestrial animals as a whole show that local population energy-use is approximately independent of adult body mass—over a body mass range spanning more than 11 orders of magnitude. This independence is represented by the slope of the regressions approximating –0.75, the reciprocal of the way that individual metabolic requirements scale with body mass. The pattern still holds for mammalian primary consumers when the data are broken down by geographic area, by broad habitat-type and by individual community. Slopes for mammalian secondary consumers are also not statistically distinguishable from –0.75. For any given body mass temperate herbivores maintain on average population densities of 1.5 to 2.0 times those of tropical ones, though slopes do not differ. Terrestrial animals of all sizes exhibit approximately the same range of population energy-use values. These results agree with those reported for population energy-budgets. It is suggested that rough independence of body mass and the energy-use of local populations is a widespread rule of animal ecology and community structure.