Predicting seasonal diet in the yellow-bellied marmot: success and failure for the linear programming model

Seasonal diet selection in the yellow-bellied marmot (Marmota flaviventris) was studied at two sites in Montana during 1991 and 1992. A linear programming model of optimal diet selection successfully predicted the composition of observed diets (monocot versus dicot) in eight out of ten cases early in the active season (April–June). During this period, adult, yearling and juvenile marmots selected diets consistent with the predicted goal of energy maximisation. However, late in the active season (July–August), the model predicted the diet composition in only one out of six cases. In all six late-season determinations, the model underestimated the amount of monocot in the diet. Possible reasons why the model failed to reliably predict diet composition late in the active season are discussed. Received: 26 April 1996 / Accepted: 12 June 1997     

Cost of morphological specialization: feeding performance of the two morphs in the trophically polymorphic cichlid fish,Cichlasoma citrinellum

Summary The feeding performance on soft and hard prey of two morphs of the trophically polymorphic Neotropical cichlid fish, Cichlasoma citrinellum, was investigated in the laboratory. The molariform morphs, specialized to feed on hard prey, are able to crack snail shells that are twice as hard as those cracked by the papilliform morphs. During ecological bottlenecks in food resources this ability should allow molariform morphs to exploit alternate, less preferred prey sources that are not available to papilliform morphs. Analysis of stomach contents revealed that molariform morphs feed significantly more often on hard snails than do papilliform morphs (Meyer 1989a). The performance advantage of the trophically specialized morphs when feeding on hard prey is countered by their less efficient performance on soft diets. The morphologically generalized papilliform morph feeds more efficiently on soft prey. The abundance of preferred soft prey, seasonal fluctuations in prey availability and the frequency of ecological bottlenecks may determine the relative abundance of these two morphs in natural populations in Nicaraguan lakes.     

Experimental evolution of the grain of metabolic specialization in yeast

Adaptation to any given environment may be accompanied by a cost in terms of reduced growth in the ancestral or some alternative environment. Ecologists explain the cost of adaptation through the concept of a trade‐off, by which gaining a new trait involves losing another trait. Two mechanisms have been invoked to explain the evolution of trade‐offs in ecological systems, mutational degradation, and functional interference. Mutational degradation occurs when a gene coding a specific trait is not under selection in the resident environment; therefore, it may be degraded through the accumulation of mutations that are neutral in the resident environment but deleterious in an alternative environment. Functional interference evolves if the gene or a set of genes have antagonistic effects in two or more ecologically different traits. Both mechanisms pertain to a situation where the selection and the alternative environments are ecologically different. To test this hypothesis, we conducted an experiment in which 12 experimental populations of wild yeast were each grown in a minimal medium supplemented with a single substrate. We chose 12 different carbon substrates that were metabolized through similar and different pathways in order to represent a wide range of ecological conditions. We found no evidence for trade‐offs between substrates on the same pathway. The indirect response of substrates on other pathways, however, was consistently negative, with little correlation between the direct and indirect responses. We conclude that the grain of specialization in this case is the metabolic pathway and that specialization appears to evolve through mutational degradation.     

The evolution of diet breadth: Monophagy and polyphagy in swallowtail butterflies

Much of the world's biodiversity has resulted from specialization of insect populations onto different plant species, partially through evolution of preference in ovipositing females. Here I report an experimental analysis of how an oviposition preference hierarchy has evolved during the evolutionary diversification of an insect group to produce taxa ranging from monophagous to polyphagous. Tests on the Papilio machaon group of swallowtail butterflies show that the preference hierarchy for plant species is evolutionarily dynamic within this species complex, yet constrained among most populations within species, creating a geographic mosaic of populations differing to various degrees in patterns of host preference. The results indicate that different diet breadths can evolve within a group of closely-related species through a combination of conservatism in preference hierarchy among some populations, occasional but rare rearrangements in preference among others, correlations in preference for some plant species, and availability of similarly ranked hosts.     

Confirmation of pleisiomorphic daily torpor in mammals: the round-eared elephant shrew Macroscelides proboscideus (Macroscelidea)

The characteristics of daily torpor were measured in the round-eared elephant shrew Macroscelides proboscideus (Macroscelidea) in response to ambient temperature and food deprivation. Elephant shrews are an ancient mammal order within a superordinal African clade including hyraxes, elephants, dugongs and the aardvark. M. proboscideus only employed torpor when deprived of food; torpor did not occur under an ad libitum diet at ambient temperatures of 10, 15 and 25?°C. Torpor bout duration ranged from <1?h to ca. 18?h. The times of entry into torpor were restricted to the scotophase, despite normothermic body temperature patterns indicating a rest phase coincident with the photophase. Full arousal was always achieved within the first 3?h of the photophase. When food deprived, the onset of the rest phase, and hence torpor, advanced with respect to the experimental photoperiod. The lowest torpor body temperature measured was 9.41?°C. Daily torpor in M. proboscideus confirms a pleisiomorphic origin of daily heterothermy. Torpor facilitates risk-averse foraging behaviour in these small omnivores by overcoming long-term energy shortfalls generated by the inherent variability of food availability in their semi-arid, El Niño-afflicted habitats.     

Trophic specialization influences the rate of environmental niche evolution in damselfishes (Pomacentridae)

The rate of environmental niche evolution describes the capability of species to explore the available environmental space and is known to vary among species owing to lineage-specific factors. Trophic specialization is a main force driving species evolution and is responsible for classical examples of adaptive radiations in fishes. We investigate the effect of trophic specialization on the rate of environmental niche evolution in the damselfish, Pomacentridae, which is an important family of tropical reef fishes. First, phylogenetic niche conservatism is not detected in the family using a standard test of phylogenetic signal, and we demonstrate that the environmental niches of damselfishes that differ in trophic specialization are not equivalent while they still overlap at their mean values. Second, we estimate the relative rates of niche evolution on the phylogenetic tree and show the heterogeneity among rates of environmental niche evolution of the three trophic groups. We suggest that behavioural characteristics related to trophic specialization can constrain the evolution of the environmental niche and lead to conserved niches in specialist lineages. Our results show the extent of influence of several traits on the evolution of the environmental niche and shed new light on the evolution of damselfishes, which is a key lineage in current efforts to conserve biodiversity in coral reefs.     

Evolution of climatic niche specialization: a phylogenetic analysis in amphibians

The evolution of climatic niche specialization has important implications for many topics in ecology, evolution and conservation. The climatic niche reflects the set of temperature and precipitation conditions where a species can occur. Thus, specialization to a limited set of climatic conditions can be important for understanding patterns of biogeography, species richness, community structure, allopatric speciation, spread of invasive species and responses to climate change. Nevertheless, the factors that determine climatic niche width (level of specialization) remain poorly explored. Here, we test whether species that occur in more extreme climates are more highly specialized for those conditions, and whether there are trade-offs between niche widths on different climatic niche axes (e.g. do species that tolerate a broad range of temperatures tolerate only a limited range of precipitation regimes?). We test these hypotheses in amphibians, using phylogenetic comparative methods and global-scale datasets, including 2712 species with both climatic and phylogenetic data. Our results do not support either hypothesis. Rather than finding narrower niches in more extreme environments, niches tend to be narrower on one end of a climatic gradient but wider on the other. We also find that temperature and precipitation niche breadths are positively related, rather than showing trade-offs. Finally, our results suggest that most amphibian species occur in relatively warm and dry environments and have relatively narrow climatic niche widths on both of these axes. Thus, they may be especially imperilled by anthropogenic climate change.     

Determinants of predation on phytophagous insects: the importance of diet breadth

To evaluate the role of predation in the evolution of diet specialization and to determine the effectiveness of various larval defenses, we offered lepidopteran larvae to colonies of the tropical ant Paraponera clavata. We recorded behavioral and physical characteristics of prey items and used log-linear models to analyze their importance as deterrents to predation by P. clavata. The most important determinant of probability of prey rejection by P. clavata was a prey's diet breadth; specialists were rejected by the ants significantly more than generalists. Other less important, but significant, predictors of prey rejection included ontogeny, morphology and chemistry. Late instar caterpillars were rejected more frequently than early instars, hairy caterpillars were rejected more frequently than caterpillars with other morphologies, and one caterpillar species with an unpalatable extract was rejected more frequently than two species with palatable extracts.     

Establishing macroecological trait datasets: digitalization,extrapolation, and validation of diet preferences in terrestrial mammals worldwide

Ecological trait data are essential for understanding the broad‐scale distribution of biodiversity and its response to global change. For animals, diet represents a fundamental aspect of species’ evolutionary adaptations, ecological and functional roles, and trophic interactions. However, the importance of diet for macroevolutionary and macroecological dynamics remains little explored, partly because of the lack of comprehensive trait datasets. We compiled and evaluated a comprehensive global dataset of diet preferences of mammals (“MammalDIET”). Diet information was digitized from two global and cladewide data sources and errors of data entry by multiple data recorders were assessed. We then developed a hierarchical extrapolation procedure to fill‐in diet information for species with missing information. Missing data were extrapolated with information from other taxonomic levels (genus, other species within the same genus, or family) and this extrapolation was subsequently validated both internally (with a jack‐knife approach applied to the compiled species‐level diet data) and externally (using independent species‐level diet information from a comprehensive continentwide data source). Finally, we grouped mammal species into trophic levels and dietary guilds, and their species richness as well as their proportion of total richness were mapped at a global scale for those diet categories with good validation results. The success rate of correctly digitizing data was 94%, indicating that the consistency in data entry among multiple recorders was high. Data sources provided species‐level diet information for a total of 2033 species (38% of all 5364 terrestrial mammal species, based on the IUCN taxonomy). For the remaining 3331 species, diet information was mostly extrapolated from genus‐level diet information (48% of all terrestrial mammal species), and only rarely from other species within the same genus (6%) or from family level (8%). Internal and external validation showed that: (1) extrapolations were most reliable for primary food items; (2) several diet categories (“Animal”, “Mammal”, “Invertebrate”, “Plant”, “Seed”, “Fruit”, and “Leaf”) had high proportions of correctly predicted diet ranks; and (3) the potential of correctly extrapolating specific diet categories varied both within and among clades. Global maps of species richness and proportion showed congruence among trophic levels, but also substantial discrepancies between dietary guilds. MammalDIET provides a comprehensive, unique and freely available dataset on diet preferences for all terrestrial mammals worldwide. It enables broad‐scale analyses for specific trophic levels and dietary guilds, and a first assessment of trait conservatism in mammalian diet preferences at a global scale. The digitalization, extrapolation and validation procedures could be transferable to other trait data and taxa.     

Perspective: the evolution of warning coloration is not paradoxical

Animals that are brightly colored have intrigued scientists since the time of Darwin, because it seems surprising that prey should have evolved to be clearly visible to predators. Often this self-advertisement is explained by the prey being unprofitable in some way, with the conspicuous warning coloration helping to protect the prey because it signals to potential predators that the prey is unprofitable. However, such signals only work in this way once predators have learned to associate the conspicuous color with the unprofitability of the prey. The evolution of warning coloration is still widely considered to be a paradox, because it has traditionally been assumed that the very first brightly colored individuals would be at an immediate selective disadvantage because of their greater conspicuousness to predators that are naive to the meaning of the signal. As a result, it has been difficult to understand how a novel conspicuous color morph could ever avoid extinction for long enough for predators to become educated about the signal. Thus, the traditional view that the evolution of warning coloration is difficult to explain rests entirely on assumptions about the foraging behavior of predators. However, we review recent evidence from a range of studies of predator foraging decisions, which refute these established assumptions. These studies show that: (1) Many predators are so conservative in their food preferences that even very conspicuous novel prey morphs are not necessarily at a selective disadvantage. (2) The survival and spread of novel color morphs can be simulated in field and aviary experiments using real predators (birds) foraging on successive generations of artificial prey populations. This work demonstrates that the foraging preferences of predators can regularly (though not always) result in the increase to fixation of a novel morph appearing in a population of familiar-colored prey. Such fixation events occur even if both novel and familiar prey are fully palatable and despite the novel food being much more conspicuous than the familiar prey. These studies therefore provide strong empirical evidence that conspicuous coloration can evolve readily, and repeatedly, as a result of the conservative foraging decisions of predators.     

Out of the dark: 350 million years of conservatism and evolution in diel activity patterns in vertebrates

Many animals are active only during a particular time (e.g., day vs. night), a partitioning that may have important consequences for species coexistence. An open question is the extent to which this diel activity niche is evolutionarily conserved or labile. Here, we analyze diel activity data across a phylogeny of 1914 tetrapod species. We find strong phylogenetic signal, showing that closely related species tend to share similar activity patterns. Ancestral reconstructions show that nocturnality was the most likely ancestral diel activity pattern for tetrapods and many major clades within it (e.g., amphibians, mammals). Remarkably, nocturnal activity appears to have been maintained continuously in some lineages for ～350 million years. Thus, we show that traits involved in local‐scale resource partitioning can be conserved over strikingly deep evolutionary time scales. We also demonstrate a potentially important (but often overlooked) metric of niche conservatism. Finally, we show that diurnal lineages appear to have faster speciation and diversification rates than nocturnal lineages, which may explain why there are presently more diurnal tetrapod species even though diurnality appears to have evolved more recently. Overall, our results may have implications for studies of community ecology, species richness, and the evolution of diet and communication systems.     

Magadi tilapia ecological specialization: filling the early gap in the speciation continuum

Cichlid fish are well known for their high speciation rates, which are usually accompanied by spectacular and rapid diversification in eco‐morphological and secondary sexual traits. This is best illustrated by the famous repeated explosive radiations in the African Great Lakes Tanganyika, Malawi and Victoria, each lake harbouring several hundreds of mostly endemic species. Correspondingly, cichlids diversified very rapidly in many other lakes across their range. Although the larger radiations, unparalleled in vertebrates, are certainly the most intriguing, they are also the most intricate and difficult to address because of their complex nature. This is where smaller, simpler systems may prove to be the most useful. In this issue of Molecular Ecology, Kavembe et al. ( 2016 ) report very recent genetic diversification accompanied by ecological specialization in cichlids of the small and ecologically extreme Lake Magadi, in Kenya. Combining geometric morphometrics, stable isotope analysis, population genomics using RADSeq data and coalescent‐based modelling techniques, the authors characterize the eco‐morphological differences between genetically distinct populations of Magadi tilapia (Alcolapia grahami), which are consistent with the different environmental conditions they experience, and infer their history of divergence. The simplicity of the focal system and the use of a multidisciplinary approach make this work particularly important for our understanding of the early stages of speciation, in both cichlids and other organisms.     

Regional specialization of the sperm membrane in the tick Amblyomma hebraeum koch (Acari: Ixodidae)

Summary The spermatozoon of Amblyomma hebraeum is about 200 m long and comprises: (1) a thick, club-shaped anterior part, about 20 m long bearing at its apex a tactile hemisphere, and (2) an elongated tail-like part, about 180 m long. The surface of the tactile hemisphere is covered by numerous bulbous expansions, attached to it by short stalks. The base of the hemisphere is surrounded by a fringe of thin motile processes; the remaining surface of the spermatozoon is covered with long cellular processes which run more or less parallel to one another.The membrane-associated particles found on the membrane beneath the cellular processes are regularly arranged as groups of parallel strands. The external surface of the so-called peripheral granules, as revealed by freeze-etching, is smooth with a very small number of particles. Internally the particles exhibit a regular hexagonal pattern which has not been observed, so far, on any other membrane of these sperm cells.The regional specialization of the spermatozoon surface membrane in relation to sperm motility is discussed. The results obtained indicate that processes of three types: (1) bulbous expansions, (2) motile processes, and (3) cellular processes are regional specializations, all engaged in aspects of sperm motility.The technical assistance of Mr. R. Haemmerle of Balzers Research Laboratories, Mrs. F. Seif and Miss C. Pugin, is gratefully acknowledged     

Evolution in heterogeneous environments: Effects of migration on habitat specialization

Summary Richard Levins introduced fitness sets as a tool for investigating evolution within heterogeneous environments. Evolutionary game theory permits a synthesis and generalization of this approach by considering the evolutionary response of organisms to any scale of habitat heterogeneity. As scales of heterogeneity increase from fine to coarse, the evolutionary stable strategy (ESS) switches from a single generalist species to several species that become increasingly specialized on distinct habitats. Depending upon the organisms' ecology, the switch from one to two species may occur at high migration rates (relatively fine-grained environment), or may only occur at very low migration rates (coarse-grained environment). At the ESS, the evolutionary context of a species is the entire landscape, while its ecological context may be a single habitat.Evolution towards the ESS can be represented with adaptive landscapes. In the absence of frequency-dependence, shifting from a single strategy ESS to a two strategy ESS poses the problem of evolving across valleys in the adaptive surface to occupy new peaks (hence, Sewell Wright's shifting balance theory). Frequency-dependent processes facilitate evolution across valleys. If a system with a two strategy ESS is constrained to possess a single strategy, the population may actually evolve a strategy that minimizes fitness. Because the population now rests at the bottom of a valley, evolution by natural selection can drive populations to occupy both peaks.     

Essential role of seaweed cultivation in integrated multi-trophic aquaculture farms for global expansion of mariculture: an analysis

Ecologically friendly aquaculture crops, such as seaweeds, herbivores, omnivores, and detritivores can be cultured using relatively less of our limited natural resources and produce relatively less pollution. They also top FAO’s estimates of aquaculture crops for the 21st century. These crops already comprise nearly 90% of global aquaculture tonnage, >90% of all aquaculture production in China and >60% of production even in North America. Consumers prefer them, most likely due to their low prices. Production costs of organisms low on the food chain are low due to the ability of these organisms to efficiently utilize low-cost, mostly plant-based diets and to recycle their own waste. Thus, ecologically friendly aquaculture is not a dream but a dominant global reality. The less ecologically-friendly aquaculture of salmon, sea bream, fed shrimp, among others, has attracted public opposition to aquaculture, but these crops totaled approximately only 10% of global production in 2004. The profitability of industrialized monocultures of these crops is threatened further by rising costs of energy and feed, environmental regulation compliance, disease, and public opposition. Current monoculture practices and perceptions intrinsic to the aquaculture industry can be turned around into a vision of sustained profitable expansion of carnivores production with trophically lower organisms in ecologically-balanced aquaculture farms. This category of aquaculture, which is the modern intensive form of polyculture practiced in Asia, feeds the waste of carnivore culture to lower trophic level organisms, primarily algae and mollusks. Species are selected based on their ecological functions in addition to their economic potential. Ecologically-balanced farms turn the costly treatment of carnivore waste outside the farm to a revenue-generating process of biofiltration, conversion, and resource recovery into plant and mollusk crops inside the farm. In doing so, they solve several of the major problems faced by modern aquaculture. The aquaculture industry can protect its own interests – and reap major benefits – by understanding the importance of ecological balance, the potential of seaweeds as components in feeds, and the importance of the culture and R&D of low trophic level organisms. The industry should also accept the relevance of environmental, social, and image aspects of aquaculture to its success. Governments have the tools to reward multi-trophic farms with seaweeds by means of tax credits and nutrient credits and to penalize unbalanced monoculture approaches by means of ‘polluter pays’ fines, thereby providing the multi-trophic farms with a significant economic advantage. Such measures have been discussed, but their implementation has been slow.     

Individual dietary specialization in a generalist predator: A stable isotope analysis of urban and rural red foxes

Some carnivores are known to survive well in urban habitats, yet the underlying behavioral tactics are poorly understood. One likely explanation for the success in urban habitats might be that carnivores are generalist consumers. However, urban populations of carnivores could as well consist of specialist feeders. Here, we compared the isotopic specialization of red foxes in urban and rural environments, using both a population and an individual level perspective. We measured stable isotope ratios in increments of red fox whiskers and potential food sources. Our results reveal that red foxes have a broad isotopic dietary niche and a large variation in resource use. Despite this large variation, we found significant differences between the variance of the urban and rural population for δ¹³C as well as δ¹⁵N values, suggesting a habitat‐specific foraging behavior. Although urban regions are more heterogeneous regarding land cover (based on the Shannon index) than rural regions, the dietary range of urban foxes was smaller compared with that of rural conspecifics. Moreover, the higher δ¹³C values and lower δ¹⁵N values of urban foxes suggest a relatively high input of anthropogenic food sources. The diet of most individuals remained largely constant over a longer period. The low intraindividual variability of urban and rural red foxes suggests a relatively constant proportion of food items consumed by individuals. Urban and rural foxes utilized a small proportion of the potentially available isotopic dietary niche as indicated by the low within‐individual variation compared to the between‐individual variation. We conclude that generalist fox populations consist of individual food specialists in urban and rural populations at least over those periods covered by our study.