Transient dynamics in altered disturbance regimes: recovery may start quickly, then slow

When a pattern of spatial or temporal environmental variation changes, it takes time for populations to reach their new stationary distributions, and during this time, the competitive landscape is also in flux. As a first step toward understanding community responses to altered variational regimes, I investigate the convergence of an annual–perennial plant system to its stationary spatiotemporal distribution following a change in environmental variation. I find that, to good approximation, convergence is the sum of two separate processes: global convergence, which governs changes in the total population, and local convergence, which governs population redistribution. While the slower process (global or local) eventually governs convergence, the faster process may initially dominate if it starts further from its stationary distribution, so that the populations converge quickly at first, then slow down. That is, when disturbances are spatially heterogeneous, a system may be initially more resilient under some initial conditions than others.

Species properties and recovery from disturbance: Forest herbs buried by volcanic tephra

Question: How do forest herb species differ from each other in their spatial and temporal dynamics during recovery from volcanic disturbance, and how are dynamics related to species traits? Location: Northeast of Mount St. Helens, Washington, USA. Methods: Following deposition of volcanic tephra in 1980, we measured herb density and cover in permanent 1‐m² plots during 1980‐2005 in three old‐growth forests with differing tephra depths. For 26 species, we calculated the frequency of plots with residuals (individuals that survived the eruption) versus re‐establishment on the tephra, timing of establishment, turnover, influence of nearby conspecific plants, importance of three components of cover increase, and relationship of flowering frequency to succession. Results: Tephra depth affected species behavior. Deep tephra produced fewer residuals, a greater increase in shoot size, and more shoot turnover; favored species that established late; and allowed establishment of early seral herbs. Nearby presence of conspecifics increased permanence of first establishment and rate of plot occupancy. Most species spread significantly in deep tephra but not in shallow tephra. Among species, frequency of flowering increased with fewer residuals, later establishment, and higher turnover. Species behavior seldom differed among growth forms based on leaf longevity and vegetative spread. Conclusion: Population dynamics at the small‐plot scale differed from those of the entire population. The timing and permanency of establishment and mechanism of expansion differed among species and with tephra depth. There was some consistency among species with similar habitat breadth and degree of flowering, but little consistency associated with the usual growth form classification.     

Digest: Species delimitation in the face of demographic processes*

Existing approaches to species delimitation use the extent of divergence between taxa. However, processes, such as gene flow during divergence or secondary contact, as well as population expansion and migration, complicate this task. Smith and Carstens introduce the R package delimitR, which uses machine learning to integrate gene flow into species delimitation inference.     

Digest: Fossils,evolutionary models,and diatoms*

Adaptation to new environments by organisms has been and is of much interest in scientific research. Does the diatom lineage Stephanodiscus niagarae/yellowstonensis follow models compatible with adaptation to a new environment? Voje (2019) shows that in this lineage, none of the current models tested—decelerated evolution, Ornstein-Uhlenbeck, and random walk—fully explained how multiple traits—valve diameter, the number of costae, and spines per valve—evolved. More accurate models are needed to understand the evolutionary history of these diatoms.     

Sideways walking: preferred is slow,slow is optimal,and optimal is expensive

When humans wish to move sideways, they almost never walk sideways, except for a step or two; they usually turn and walk facing forward. Here, we show that the experimental metabolic cost of walking sideways, per unit distance, is over three times that of forward walking. We explain this high metabolic cost with a simple mathematical model; sideways walking is expensive because it involves repeated starting and stopping. When walking sideways, our subjects preferred a low natural speed, averaging 0.575 m s⁻¹ (0.123 s.d.). Even with no prior practice, this preferred sideways walking speed is close to the metabolically optimal speed, averaging 0.610 m s⁻¹ (0.064 s.d.). Subjects were within 2.4% of their optimal metabolic cost per distance. Thus, we argue that sideways walking is avoided because it is expensive and slow, and it is slow because the optimal speed is low, not because humans cannot move sideways fast.     

Climate and coastal dune vegetation: disturbance, recovery, and succession

The sand dune habitats found on barrier islands and other coastal areas support a dynamic plant community while protecting areas further inland from waves and wind. Foredune, interdune, and backdune habitats common to most coastal dunes have very different vegetation, likely because of the interplay among plant succession, exposure, disturbance, and resource availability. However, surprisingly few long-term data are available describing dune vegetation patterns. A nine-year census of 294 plots on St. George Island, Florida suggests that the major climatic drivers of vegetation patterns vary with habitat. Community structure is correlated with the elevation, soil moisture, and percent soil ash of each 1 m² plot. Major storms reduce species richness in all three habitats. Principle coordinate analysis suggests that changes in the plant communities through time are caused by climatic events: changes in foredune vegetation are correlated with temperature and summer precipitation, interdune vegetation with storm surge, and backdune vegetation with precipitation and storm surge. We suggest that the plant communities in foredune, interdune, and backdune habitats tend to undergo succession toward particular compositions of species, with climatic disturbances pushing the communities away from these more deterministic trajectories.     

Digest: Stress and inbreeding depression in Drosophila melanogaster*

Do stressful conditions exacerbate inbreeding depression? Using Drosophila melanogaster, Schou et al. (2018) examine the mechanisms underlying the interaction between stress and inbreeding depression. The authors found that gene expression in inbred individuals was highly stochastic under benign conditions, but differential gene expression in inbred individuals was reduced compared to controls under stressful conditions.     

Digest: Disentangling plumage and behavior contributions to iridescent signals*

To what extent do plumage properties and behavior interact to produce visual signals? Simpson and McGraw (2019) propose an elegant and novel experimental set‐up to dissociate behavior and color and assess their relative effects in the resulting iridescent signal. They find that modification of either component leads to a modification of the resulting signal as seen by the receiver, suggesting that sexual selection acts simultaneously on both signal components.     

Digest: Developmental life history and adaptive radiation in salamanders*

What determines the rate at which species adapt to new climatic conditions? Weaver et al. found that the evolution of short larval periods promotes climatic niche evolution in salamanders in the genus Desmognathus.     

Biodiversity loss through speciation collapse: Mechanisms,warning signals,and possible rescue*

Speciation is the process that generates biodiversity, but recent empirical findings show that it can also fail, leading to the collapse of two incipient species into one. Here, we elucidate the mechanisms behind speciation collapse using a stochastic individual‐based model with explicit genetics. We investigate the impact of two types of environmental disturbance: deteriorated visual conditions, which reduce foraging ability and impede mate choice, and environmental homogenization, which restructures ecological niches. We find that: (1) Species pairs can collapse into a variety of forms including new species pairs, monomorphic or polymorphic generalists, or single specialists. Notably, polymorphic generalist forms may be a transient stage to a monomorphic population; (2) Environmental restoration enables species pairs to reemerge from single generalist forms, but not from single specialist forms; (3) Speciation collapse is up to four orders of magnitude faster than speciation, while the reemergence of species pairs can be as slow as de novo speciation; (4) Although speciation collapse can be predicted from either demographic, phenotypic, or genetic signals, observations of phenotypic changes allow the most general and robust warning signal of speciation collapse. We conclude that factors altering ecological niches can reduce biodiversity by reshaping the ecosystem's evolutionary attractors.     

Species decline: A perspective on extinction,recovery, and propagation

This keynote address was presented at the Conference on the Conservation of Endangered Species in Zoological Parks and Aquariums on April 18, 1982 at the National Aquarium in Baltimore. It outlines 1) future trends in the world's environment, resources, and population; 2) factors affecting species decline; 3) reasons for preserving life forms; and 4) techniques, with emphasis on captive propagation, used to assist in species recovery.     

Digest: Differentiating Fisherian sexual selection dynamics between haploids and diploids*

Female mate preference for males displaying costly traits is a puzzling phenomenon found in nature. Fisher was the first to propose a formal model describing this paradoxical system of sexual selection. Despite the many models constructed from his theory, much remains unknown about its mechanics, especially in different genetic settings. Through simulations, Veller et al. compared population dynamics between haploids and diploids and found that Fisherian sexual selection acts differently on diploids compared to haploids due to linkage mechanisms.