Partitioning the sources of demographic variation reveals density-dependent nest predation in an island bird population

Ecological factors often shape demography through multiple mechanisms, making it difficult to identify the sources of demographic variation. In particular, conspecific density can influence both the strength of competition and the predation rate, but density-dependent competition has received more attention, particularly among terrestrial vertebrates and in island populations. A better understanding of how both competition and predation contribute to density-dependent variation in fecundity can be gained by partitioning the effects of density on offspring number from its effects on reproductive failure, while also evaluating how biotic and abiotic factors jointly shape demography. We examined the effects of population density and precipitation on fecundity, nest survival, and adult survival in an insular population of orange-crowned warblers (Oreothlypis celata) that breeds at high densities and exhibits a suite of traits suggesting strong intraspecific competition. Breeding density had a negative influence on fecundity, but it acted by increasing the probability of reproductive failure through nest predation, rather than through competition, which was predicted to reduce the number of offspring produced by successful individuals. Our results demonstrate that density-dependent nest predation can underlie the relationship between population density and fecundity even in a high-density, insular population where intraspecific competition should be strong.

Plasticity of parental care under the risk of predation: how much should parents reduce care?

Predation can be an important agent of natural selection shaping parental care behaviours, and can also favour behavioural plasticity. Parent birds often decrease the rate that they visit the nest to provision offspring when perceived risk is high. Yet, the plasticity of such responses may differ among species as a function of either their relative risk of predation, or the mean rate of provisioning. Here, we report parental provisioning responses to experimental increases in the perceived risk of predation. We tested responses of 10 species of bird in north temperate Arizona and subtropical Argentina that differed in their ambient risk of predation. All species decreased provisioning rates in response to the nest predator but not to a control. However, provisioning rates decreased more in species that had greater ambient risk of predation on natural nests. These results support theoretical predictions that the extent of plasticity of a trait that is sensitive to nest predation risk should vary among species in accordance with predation risk.

Fear, food and sexual ornamentation: plasticity of colour development in Trinidadian guppies

The evolution of male ornamentation often reflects compromises between sexual and natural selection, but it may also be influenced by phenotypic plasticity. We investigated the developmental plasticity of male colour ornamentation in Trinidadian guppies in response to two environmental variables that covary in nature: predation risk and food availability. We found that exposure to chemical predator cues delayed the development of pigment-based colour elements, which are conspicuous to visual-oriented predators. Predator cues also reduced the size of colour elements at the time of maturity and caused adult males to be less colourful. To the best of our knowledge, these findings provide the first example of a plastic reduction in the development of a sexually selected male ornament in response to predator cues. The influence of predator cues on ornamentation probably affects individual fitness by reducing conspicuousness to predators, but could reduce attractiveness to females. Reduced food availability during development caused males to delay the development of colour elements and mature later, probably reflecting a physiological constraint, but their coloration at maturity and later in adulthood was largely unaffected, suggesting that variation in food quantity without variation in quality does not contribute to condition dependence of the trait.

Genetically and environmentally mediated divergence in lateral line morphology in the Trinidadian guppy (Poecilia reticulata)

Fish and other aquatic vertebrates use their mechanosensory lateral line to detect objects and motion in their immediate environment. Differences in lateral line morphology have been extensively characterized among species, however intraspecific variation remains largely unexplored. In addition, little is known about how environmental factors modify development of lateral line morphology. Predation is one environmental factor that can act both as a selective pressure causing genetic differences between populations, and as a cue during development to induce plastic changes. Here, we test if variation in the risk of predation within and among populations of Trinidadian guppies (Poecilia reticulata) influences lateral line morphology. We compared neuromast arrangement in wild-caught guppies from distinct high- and low-predation population pairs to examine patterns associated with differences in predation pressure. To distinguish genetic and environmental influences, we compared neuromast arrangement in guppies from different source populations reared with and without exposure to predator chemical cues. We found that the distribution of neuromasts across the body varies between populations based on both genetic and environmental factors. To the best of our knowledge, this study is the first to demonstrate variation in lateral line morphology based on environmental exposure to an ecologically relevant stimulus.

Selection in nature: experimental manipulations of natural populations

Numerous studies have documented evolution by natural selection in natural populations, but few are genuine selection experiments that are designed and then executed in nature. We will focus on these few cases to illustrate what can be learned from field selection experiments alone or field and laboratory selection experiments together that cannot be learned from laboratory experiments alone. Both types of study allow us to evaluate cause and effect relationships because a planned experiment can be accompanied by a more direct evaluation of the factors that cause evolution. A unique benefit of field experiments is that they give us the opportunity to measure the rate and magnitude of selection in nature. We have found that this rate is far greater than one might imagine based on observations of the fossil record. A combination of field and laboratory selection experiments has revealed the importance of population size and structure in shaping the genetics of adaptation. For example, laboratory selection experiments on insecticide resistance tend to attain resistance though polygenic inheritance. The evolution of insecticide resistance in nature often eventually yields to single genes of large effect that are rare but, once they arise, represent a higher fitness solution to resistance and spread among populations. Finally, field studies enable us to test evolutionary theory in a context in which all of the tradeoffs associated with a trait are realized; in the laboratory, organisms may be shielded from the fitness tradeoffs associated with the evolution of a trait. For example, we have compared the patterns of senescence in guppies from high and low mortality rate environments in the laboratory and in the field. In the laboratory, guppies from high predation environments had delayed senescence relative to those from low predation environments. In the field the apparent relationship is the opposite. One hypothesis for this difference is that a tradeoff associated with the evolution of the high predation life history is a decrease in the investment in the immune system. Such a sacrifice would be evident in nature where there is exposure to disease and parasites but less so in the laboratory, which is relatively disease and parasite free.

Seasonal and population variation in male testosterone levels in breeding orange-crowned warblers (Vermivora celata)

Comparative hormone studies can reveal how physiology underlies life history variation. Here, we examined seasonal variation in plasma testosterone concentration between populations of male orange-crowned warblers (Vermivora celata) breeding in Fairbanks, Alaska (V. c. celata) and on Santa Catalina Island, California (V. c. sordida). These populations face different ecological constraints and exhibit different life histories. Alaska birds have a short breeding season, low annual adult survival, and high reproductive rates. In contrast, Catalina Island birds exhibit high adult survival and low reproductive rates despite having a long breeding season. We examined seasonal variation in male testosterone concentrations as a potential mechanism underlying differences in male reproductive strategies between populations. From 2006 to 2008, we sampled males during the pre-incubation, incubation, and nestling stages. Alaska males exhibited a seasonal testosterone pattern typical of northern passerines: testosterone levels were high during pre-incubation and declined during incubation to low levels during nestling provisioning. Testosterone concentrations in Catalina Island males, however, did not vary consistently with breeding stage, remained elevated throughout the breeding season, and were higher than in Alaska males during the nestling stage. We hypothesize that in Alaska, where short seasons and high adult mortality limit breeding opportunities, the seasonal testosterone pattern facilitates high mating effort prior to incubation, but high parental investment during the nestling stage. On Catalina Island, elevated testosterone levels may reflect the extended mating opportunities and high population density facing males in this population. Our results suggest that population variation in seasonal testosterone patterns in orange-crowned warblers may be a function of differences in life history strategy and the social environment.

Experimental studies of evolution in guppies: a model for understanding the evolutionary consequences of predator removal in natural communities

Guppies (Poecilia reticulata) in Trinidadian streams are found with a diversity of predators in the lower reaches of streams, but few predators in the headwaters. These differences have caused the adaptive evolution of guppy behaviour, morphology, male colouration and life history. Waterfalls often serve as barriers to the upstream distribution of predators and/or guppies. Such discontinuities make it possible to treat streams like giant test tubes by introducing guppies or predators to small segments of streams from which they were previously excluded. Such experiments enable us to document how fast evolution can occur and the fine spatial scales over which adaptation is possible. They also demonstrate that the role predators play in structuring this ecosystem resembles many others studied from a more purely ecological perspective; in these streams, as elsewhere, predators depress the numbers of individuals in prey species which in turn reduces the effects of the prey species on other trophic levels and hence the structure of the ecosystem. A focus on predators is important in conservation biology because predators are often the organisms that are most susceptible to local extinction. Their selective loss occurs because large predators have been deliberately exterminated and/or are more susceptible to environmental disturbances. Furthermore, we will argue that predator re-introductions might be destabilizing if, in the absence of predators, their prey have evolved in a fashion that makes them highly susceptible to predation, even after time intervals as short as 50-100 years. A better understanding of the evolutionary impacts of top predators will be critical goal for the policy and practice of large carnivore restoration in the future.

Effect of extrinsic mortality on the evolution of senescence in guppies

Classical theories for the evolution of senescence predict that organisms that experience low mortality rates attributable to external factors, such as disease or predation, will evolve a later onset of senescence. Here we use patterns of senescence in guppies derived from natural populations that differ in mortality risk to evaluate the generality of these predictions. We have previously found that populations experiencing higher mortality rates evolve earlier maturity and invest more in reproduction, as predicted by evolutionary theory. We report here that these same populations do not have an earlier onset of senescence with respect to either mortality or reproduction but do with respect to swimming performance, which assesses neuromuscular function. This mosaic pattern of senescence challenges the generality of the association between decreased extrinsic mortality and delayed senescence and invites consideration of more derived theories for the evolution of senescence.

Constraints on Adaptive Evolution: The Functional Trade‐Off between Reproduction and Fast‐Start Swimming Performance in the Trinidadian Guppy (Poecilia reticulata)

The empirical study of natural selection reveals that adaptations often involve trade-offs between competing functions. Because natural selection acts on whole organisms rather than isolated traits, adaptive evolution may be constrained by the interaction between traits that are functionally integrated. Yet, few attempts have been made to characterize how and when such constraints are manifested or whether they limit the adaptive divergence of populations. Here we examine the consequences of adaptive life-history evolution on locomotor performance in the live-bearing guppy. In response to increased predation from piscivorous fish, Trinidadian guppies evolve an increased allocation of resources toward reproduction. These populations are also under strong selection for rapid fast-start swimming performance to evade predators. Because embryo development increases a female's wet mass as she approaches parturition, an increased investment in reproductive allocation should impede fast-start performance. We find evidence for adaptive but constrained evolution of fast-start swimming performance in laboratory trials conducted on second-generation lab-reared fish. Female guppies from high-predation localities attain a faster acceleration and velocity and travel a greater distance during fast-start swimming trials. However, velocity and distance traveled decline more rapidly over the course of pregnancy in these same females, thus reducing the magnitude of divergence in swimming performance between high- and low-predation populations. This functional trade-off between reproduction and swimming performance reveals how different aspects of the phenotype are integrated and highlights the complexity of adaptation at the whole-organism level.

The population ecology of contemporary adaptations: what empirical studies reveal about the conditions that promote adaptive evolution

Under what conditions might organisms be capable of rapid adaptive evolution? We reviewed published studies documenting contemporary adaptations in natural populations and looked for general patterns in the population ecological causes. We found that studies of contemporary adaptation fall into two general settings: (1) colonization of new environments that established newly adapted populations, and (2) local adaptations within the context of a heterogeneous environments and metapopulation structure. Local ecological processes associated with colonizations and introductions included exposure to: (1) a novel host or food resource; (2) a new biophysical environment; (3) a new predator community; and (4) a new coexisting competitor. The new environments that were colonized often had depauperate communities, sometimes because of anthropogenic disturbance. Local adaptation in heterogeneous environments was also often associated with recent anthropogenic changes, such as insecticide and herbicide resistance, or industrial melanism. A common feature of many examples is the combination of directional selection with at least a short-term opportunity for population growth. We suggest that such opportunities for population growth may be a key factor that promotes rapid evolution, since directional selection might otherwise be expected to cause population decline and create the potential for local extinction, which is an ever-present alternative to local adaptation. We also address the large discrepancy between the rate of evolution observed in contemporary studies and the apparent rate of evolution seen in the fossil record.

Fecundity-survival trade-offs and parental risk-taking in birds

Life history theory predicts that parents should value their own survival over that of their offspring in species with a higher probability of adult survival and fewer offspring. We report that Southern Hemisphere birds have higher adult survival and smaller clutch sizes than Northern Hemisphere birds. We subsequently manipulated predation risk to adults versus offspring in 10 species that were paired between North and South America on the basis of phylogeny and ecology. As predicted, southern parents responded more strongly to reduce mortality risk to themselves even at a cost to their offspring, whereas northern parents responded more strongly to reduce risk to their offspring even at greater risk to themselves.

Parental investment strategies in two species of nuthatch vary with stage-specific predation risk and reproductive effort

Life-history theory predicts that differences in reproductive effort and residual reproductive value among species should result in differences in the level of risk that parents are willing to tolerate to themselves versus their offspring. Specifically, highly fecund and shorter-lived species are expected to place greater value in current offspring than themselves, whereas less fecund and longer-lived species are expected to place greater value in their own survival and future breeding opportunities. Here, we test the prediction that parental investment decisions are correlated with life histories by comparing risk-taking behaviour in two species of nuthatch that differ in reproductive effort: the white-breasted nuthatch, Sitta carolinensis (more fecund, lower survival) and the red-breasted nuthatch, S. canadensis (less fecund, higher survival). We experimentally manipulated stage-specific predation risk by presenting models of an adult predator (hawk) and an egg predator (wren) and measured the willingness of males to feed incubating females on the nest. We found that both species of nuthatch responded to predators by increasing the length of time between visits and aborting more visits to the nest. However, as predicted by their life histories, S. carolinensis displayed a significantly stronger response to the egg predator, whereas S. canadensis responded more strongly to the adult predator. Thus, species can differ in their willingness to tolerate risk to themselves and their young, and such differences appear to be related to differences in investment in current reproduction and the probability of future survival. Copyright 2000 The Association for the Study of Animal Behaviour.