History rather than hybridization determines population structure and adaptation in Populus balsamifera

Hybridization between species is known to greatly affect their genetic diversity and, therefore, their evolution. Also, within species, there may be genetic clusters between which gene flow is limited, which may impact natural selection. However, few studies have looked simultaneously at the influence of among‐species and within‐species gene flow. Here, we study the influence of hybridization between Populus balsamifera and Populus trichocarpa on population structure and adaptation in P. balsamifera. We did this by sampling a total of 1517 individuals from across the ranges of these two species, and by genotyping them using a combination of 93 nuclear and 17 cpDNA SNPs. We found that hybridization is mostly limited to the contact zone where the species’ distributions overlap. Within P. balsamifera, we found multiple levels of population structure. Interestingly, the border between the Eastern and Central clusters is very sharp, whereas the border between the Central and Western clusters is diffuse. Outlier analysis revealed that three loci associated with the sharp border were also associated with climate. We hypothesize that the observed clusters derive from three refugia during the Pleistocene ice ages. Between the Central and Western clusters, post‐glacial long‐distance gene flow has led to the diffusion of their border. In the Eastern cluster, we hypothesize that endogenous genomic barriers have developed, leading to the sharp border and a spurious climate association. We conclude that the large‐scale genetic structure of P. balsamifera is mostly shaped by historical factors and the influence of interspecific hybridization is limited.     

Renewal ecology: conservation for the Anthropocene

The global scale and rapidity of environmental change is challenging ecologists to reimagine their theoretical principles and management practices. Increasingly, historical ecological conditions are inadequate targets for restoration ecology, geographically circumscribed nature reserves are incapable of protecting all biodiversity, and the precautionary principle applied to management interventions no longer ensures avoidance of ecological harm. In addition, human responses to global environmental changes, such as migration, building of protective infrastructures, and land use change, are having their own negative environmental impacts. We use examples from wildlands, urban, and degraded environments, as well as marine and freshwater ecosystems, to show that human adaptation responses to rapid ecological change can be explicitly designed to benefit biodiversity. This approach, which we call “renewal ecology,” is based on acceptance that environmental change will have transformative effects on coupled human and natural systems and recognizes the need to harmonize biodiversity with human infrastructure, for the benefit of both.     

Using restoration as an experimental framework to test provenancing strategies and climate adaptability

Restoring degraded Australian landscapes through revegetation is a key concern of land holders, NGOs and government agencies. With the advent of climate change, it is increasingly important that revegetation activities take into consideration the species and provenance of plant materials to ensure that environmental plantings will be resilient to future climate conditions. A major strength of the past 30 years restoration programmes is the development of a distributed network of educated and experienced practitioners. We have recently invited stakeholders from among this network to participate in a process to cost‐effectively build Environmental Research Infrastructure – a nationally distributed network of restoration plantings that explore a broad range of research activities including a better understanding of the adaptive responses of different species and provenances. This would also facilitate long‐term monitoring of change and adaptation across Australia, providing a wealth of information to inform future conservation and restoration programmes.     

Carrying capacity in a heterogeneous environment with habitat connectivity

A large body of theory predicts that populations diffusing in heterogeneous environments reach higher total size than if non‐diffusing, and, paradoxically, higher size than in a corresponding homogeneous environment. However, this theory and its assumptions have not been rigorously tested. Here, we extended previous theory to include exploitable resources, proving qualitatively novel results, which we tested experimentally using spatially diffusing laboratory populations of yeast. Consistent with previous theory, we predicted and experimentally observed that spatial diffusion increased total equilibrium population abundance in heterogeneous environments, with the effect size depending on the relationship between r and K. Refuting previous theory, however, we discovered that homogeneously distributed resources support higher total carrying capacity than heterogeneously distributed resources, even with species diffusion. Our results provide rigorous experimental tests of new and old theory, demonstrating how the traditional notion of carrying capacity is ambiguous for populations diffusing in spatially heterogeneous environments.     

The ecological consequences of environmentally induced phenotypic changes

Population dynamics and species persistence are often mediated by species traits. Yet many important traits, like body size, can be set by resource availability and predation risk. Environmentally induced changes in resource levels or predation risk may thus have downstream ecological consequences. Here, we assess whether quantity and type of resources affect the phenotype, the population dynamics, and the susceptibility to predation of a mixotrophic protist through experiments and a model. We show that cell shape, but not size, changes with resource levels and type, and is linked to carrying capacity, thus affecting population dynamics. Also, these changes lead to differential susceptibility to predation, with direct consequences for predator‐prey dynamics. We describe important links between environmental changes, traits, population dynamics and ecological interactions, that underscore the need to further understand how trait‐mediated interactions may respond to environmental shifts in resource levels in an increasingly changing world.     

Non‐genetic polymorphisms in rotifers: environmental and endogenous controls,development, and features for predictable or unpredictable environments

Pronounced non‐genetic polymorphisms, or polyphenisms, occur in some monogonont rotifers reproducing by diploid, female parthenogenesis. In many brachionids, there is great variation in spine length. In trimorphic species of Asplanchna, females can vary in size and shape, from a small saccate morph to giant cruciform and campanulate morphs. In species that also reproduce sexually, diploid eggs can develop into two types of females. Amictic females produce diploid eggs that develop parthenogenetically into females; mictic females produce haploid eggs that develop parthenogenetically into males or, if fertilized, into resting eggs. In a species of Synchaeta, amictic females produce diploid eggs that can be either thin‐shelled and subitaneous or thicker‐shelled and diapausing. In all cases, morph determination occurs during the oogenesis or embryological development of diploid eggs in the maternal body cavity. For the first time, these polymorphisms are reviewed together and compared regarding a number of features associated with transitions from default to induced morphs: (i) type of variation (morphological, physiological, or both; continuous or discrete); (ii) inducing signal (environmental, endogenous, or both); (iii) universality of response to that signal (all or only some individuals); (iv) fitness cost; (v) reversibility; and (vi) ecological significance. Most of the polymorphisms fall into two major categories regarding these features. Transitions suitable for predictable environments involve: universal responses to environmental signals; continuous morphological variation; low reproductive cost; rapid reversibility; and adaptations for defence, hydrodynamics or prey ingestion. Transitions suitable for unpredictable environments are bet‐hedging strategies and usually involve: partial (stochastic) responses to environmental or endogenous signals; discontinuous physiological variation; initiation of diapause, and thus high reproductive cost and slow reversibility. Two cases of morphological variation also involve the simultaneous production of different morphs and likely are adaptations for an uncertain future: continuous spine‐length variation due to maternal age in Brachionus calyciflorus, and production of discrete cruciform and campanulate females in Asplanchna spp.