AbstractAfter 13 or 17 years underground, periodical cicada nymphs come aboveground to molt into adulthood. Observers have long noted the strikingly directional orientation of nymphs toward trees on which they molt and have speculated on possible mechanisms to account for these movements. Through a combination of observational, experimental, and modeling studies, we demonstrate that cicada nymphs use skototaxis, or movement toward darkness, to reach their targets. Emergent nymphs walked directly toward isolated trees, regardless of initial compass direction or time of day, traveling only 15% farther than the minimum distance necessary. Nymphs whose vision had been temporarily obscured wandered randomly and rarely reached the trees. Indoor trials confirmed that cicada nymphs preferentially orient toward dark objects. This study adds to a growing list of organisms, including tropical plant seedlings, that move or grow toward darkness.
AbstractBiological diversity is widely recognized as a key driver of ecosystem function, yet the mechanisms underlying biodiversity-ecosystem function (BEF) relationships in natural systems, particularly for regenerative processes, remain poorly understood. We examined the BEF relationship in deep-sea xylophagous bivalve communities using 63 experimental wood falls deployed ∼2,000 m deep in the Gulf of Mexico. This system allowed us to combine natural community assembly with controlled wood-fall deployments to examine how trophic partitioning, species dominance, and community-level properties influence total wood consumption, our metric of ecosystem function. We identified 26,324 individuals from 12 xylophagous bivalve species, which occupied complementary trophic niches, although their contributions to wood consumption varied substantially. Wood consumption increased with total abundance and, to a lesser extent, species richness, but richness effects largely reflected underlying variation in abundance. Analyses using the Price equation and species randomization approaches revealed that ecosystem function was primarily driven by a core group of abundant species, with occasional nonrandom contributions from rare species. These results indicate that in naturally assembled deep-sea communities, total abundance and key species' functional capacity outweigh species richness or phylogenetic diversity as predictors of ecosystem function, emphasizing the importance of individual species roles in shaping BEF relationships.
How will species adapt to changing environments? To what extent does adaptation to previous conditions maintain the variation needed to adapt to future conditions? To answer these kinds of questions, we need to identify locally adaptive alleles and quantify their effects. Theory shows that the architecture of adaptation can depend upon the nature of mutation and on how ecology shapes the processes of migration and selection. Depending on this interplay, adaptation can be driven by few alleles of large effect or many alleles of small effect, but little is known about the relative prevalence of such architectures in nature. Unfortunately, our statistical methods are also biased: it is much easier to identify loci of large effect that contribute repeatedly across populations or species, while alleles of small effect are all but invisible to genomic analysis. There is, therefore, a gap between the total amount of locally adaptive variation and that which is explained by genomic studies. To quantify this missing local adaptation, future studies require a deep integration of genomic and phenotypic analyses.
AbstractIn insectivorous birds the reproductive period often coincides with the spring development of vegetation, but whether there is a direct relationship between the two, and through which mechanisms this link could come about, is not clear. Trees release herbivore-induced plant volatiles (HIPVs) when they are attacked by insects, and we test here whether birds use those alarm odors to modulate their behaviors and their reproduction. Using a two-step experimental approach, we first show under controlled conditions that blue tits are attracted to HIPVs mimicking a caterpillar attack on downy oak buds in early spring without having to learn them. We then equipped nest boxes in the field with the same artificial HIPVs. We did not find that birds that built their nest in HIPV nest boxes advanced lay dates or produced larger clutches. However, although this result was not statistically significant, slightly more nest boxes equipped with HIPVs were occupied compared with control ones, resulting in significantly more eggs, nestlings, and fledglings introduced into the population from these treated nest boxes. Our results thus show that blue tits innately detect and track subtle plant olfactory signals, while the link with reproductive decisions appears more tenuous and requires further investigation. Such a link with reproduction is nevertheless in line with recent studies showing that HIPVs can stimulate the reproductive physiology of songbirds and highlights new multitrophic interactions across plants, insects, and birds, an encouraging area of study for understanding adaptations to a constantly changing environment.
AbstractSpecies are restricted to finite ranges, and when they are in demographic equilibrium with the environment, this implies a failure to adapt at the range periphery. There are several competing evolutionary hypotheses for the maintenance and persistence of range edges. One hypothesis is gene swamping of edge populations, in which migrants from the center carrying alleles adapted to the range center but maladaptive along the edge hamper local adaptation. A competing explanation is dispersal limitation, in which reduced migration to the periphery keeps edge populations small, causing drift to dominate over selection. It has proven difficult to assess these potential generative scenarios because of the inherent complexity of species ranges. In this study, we leverage range-wide genomic sequencing data to investigate the evolution of range edges in an empirical system: Haustorius canadensis, the Canadian shovel bug. We find increased genetic drift and inbreeding on the range edges and detect signatures of asymmetric migration toward the range center. The inferred directionality of migration is consistent with the direction of the dominant current regimes in the North Atlantic. We conclude that anisotropic dispersal toward the range center leaves progressively fewer migrants to replenish peripheral populations, consistent with the migration limitation hypothesis, and generates the appearance of an abundant center.
