Landscape experiments unlock relationships among habitat loss, fragmentation, and patch-size effects.

Habitat loss is often considered the greatest near-term threat to biodiversity, while the impact of habitat fragmentation remains intensely debated. A key issue of this debate centers on the problem of scale-landscape or patch-at which to assess the consequences of fragmentation. Yet patterns are often confounded across scales, and experimental designs that could solve this scaling problem remain scarce. We conducted two field experiments in 30 experimental landscapes in which we manipulated habitat loss, fragmentation, and patch size for a community of four insect herbivores that specialize on the cactus Opuntia. In the first experiment, we destroyed 2088 Opuntia patches in either aggregated or random patterns and compared the relative effects of landscape-scale loss and fragmentation to those of local patch size on species occurrence. This experiment focused on manipulating the relative separation of remaining patches, where we hypothesized that aggregated loss would disrupt dispersal more than random loss, leading to lower occurrence. In the second experiment, we destroyed 759 Opuntia patches to generate landscapes that varied in patch number and size for a given amount of habitat loss and assessed species occurrence. This experiment focused on manipulating the subdivision of remaining habitat, where we hypothesized that an increase in the number of patches for a given amount of loss would lead to negative effects on occurrence. For both, we expected that occurrence would increase with patch size. We find strong evidence for landscape-scale effects of habitat fragmentation, with aggregated loss and a larger number of patches for a given amount of habitat loss leading to a lower frequency of patches occupied in landscapes. In both experiments, occurrence increased with patch size, yet interactions of patch size and landscape-scale loss and fragmentation drove species occurrence in patches. Importantly, the direction of effects were consistent across scales and effects of patch size were sufficient to predict the effects of habitat loss and fragmentation across entire landscapes. Our experimental results suggest that changes at both the patch and landscape scales can impact populations, but that a long-standing pattern-the patch-size effect-captures much of the key variation shaping patterns of species occurrence.

Litter, Plant Competition, and Ecosystem Dynamics: A Theoretical Perspective.

AbstractCommunity structure depends jointly on species' responses to, and effects on, environmental factors. Many such factors, including detritus, are studied in ecosystem ecology. Detritus in terrestrial ecosystems is dominated by plant litter (nonliving organic material), which, in addition to its role in material cycling, can act as a niche factor modulating interactions among plants. Litter thus links traditional community and ecosystem processes, which are often studied separately. We explore this connection using population dynamics models of two plant species and a litter pool. We first find conditions determining the outcome of interactions between these species, highlighting the role that litter plays and the role of broader ecosystem parameters, such as decomposition rate. Species trade-offs in tolerance to direct competition and litter-based interference competition allow for coexistence, provided the litter-tolerant species produces more litter at the population level; otherwise, priority effects may result. When species coexist, litter-mediated interactions between plants disrupt the traditional relationship between biomass accumulation and decomposition. Increasing decomposition rate may have no effect on standing litter density and, in some cases, may even increase litter load. These results illustrate how ecosystem variables can influence community outcomes that then feed back to influence the ecosystem.

Digest: The complex interplay of phenotypic variation and diversifying selection.

How does diversity arise, and how is it maintained? In a model of microbial competition, Amicone and Gordo find that frequent mutations and the effects of genetic drift generate sufficient phenotypic variation to alter selection such that resource specialists are favored, potentially paving the way for speciation. Their results highlight that processes affecting the distribution of genotypic and phenotypic variation may affect not only population responses to selection but also the direction of selection itself.

Character displacement in the presence of multiple trait differences: Evolution of the storage effect in germination and growth.

Ecological character displacement is a prominent hypothesis for the maintenance of ecological differences between species that are critical to stable coexistence. Models of character displacement often ascribe interspecific competitive interactions to a single character, but multiple characters contribute to competition, and their effects on selection can be nonadditive. Focusing on one character, we ask if other characters that affect competition alter evolutionary outcomes for the focal character. We address this question using the variable environment seed bank model for two species with two traits. The focal trait is the temporal pattern of germination, which is evolutionary labile. The other trait is the temporal pattern of plant growth, which is assumed fixed. We ask whether evolutionary divergence of germination patterns between species depends on species differences in plant growth. Patterns of growth can affect selection on germination patterns in two ways. First, cues present at germination can provide information about future growth. Second, germination and growth jointly determine the biomass of plants, which determines demand for resources. Germination and growth contribute to the selection gradient in distinct components, one density-independent and the other density-dependent. Importantly, the relative strengths of the components are key. When the density-dependent component is stronger, displacement in germination patterns between species is larger. Stronger cues at germination strengthen the density-independent component by increasing the benefits of germinating in years of favorable growth. But cues also affect the density-dependent component by boosting a species' biomass, and hence its competitive effect, in good years. Consequently, cues weaken character displacement when growth patterns are similar for two competitors, but favor displacement when growth patterns are species-specific. Understanding how these selection components change between contexts can help understand the origin and maintenance of species differences in germination patterns in temporally fluctuating environments.

The interplay of movement and spatiotemporal variation in transmission degrades pandemic control.

Successful public health regimes for COVID-19 push below unity long-term regional Rt -the average number of secondary cases caused by an infectious individual. We use a susceptible-infectious-recovered (SIR) model for two coupled populations to make the conceptual point that asynchronous, variable local control, together with movement between populations, elevates long-term regional Rt , and cumulative cases, and may even prevent disease eradication that is otherwise possible. For effective pandemic mitigation strategies, it is critical that models encompass both spatiotemporal heterogeneity in transmission and movement.

Disease in Invasive Plant Populations.

Non-native invasive plants can establish in natural areas, where they can be ecologically damaging and costly to manage. Like cultivated plants, invasive plants can experience a relatively disease-free period upon introduction and accumulate pathogens over time. Diseases of invasive plant populations are infrequently studied compared to diseases of agriculture, forestry, and even native plant populations. We evaluated similarities and differences in the processes that are likely to affect pathogen accumulation and disease in invasive plants compared to cultivated plants, which are the dominant focus of the field of plant pathology. Invasive plants experience more genetic, biotic, and abiotic variation across space and over time than cultivated plants, which is expected to stabilize the ecological and evolutionary dynamics of interactions with pathogens and possibly weaken the efficacy of infectious disease in their control. Although disease is expected to be context dependent, the widespread distribution of invasive plants makes them important pathogen reservoirs. Research on invasive plant diseases can both protect crops and help manage invasive plant populations.

Germination variation facilitates the evolution of seed dormancy when coupled with seedling competition.

Fluctuating environmental conditions have consequences for the evolution of life histories because they cause fitness variance. This variance can favor risk-spreading strategies, often known as bet-hedging strategies, in which growth or reproduction is spread over time or space, with some costs, but greater certainty of success. An important example is seed dormancy in annual plants, in which some fraction of seed remains dormant at any given germination opportunity with the potential of germinating later when environmental conditions may differ. Previous theory shows that environmental variation is critical for the evolution of dormancy. However, these studies have focused on temporal variation in reproduction, ignoring the strong observed effects of environmental variation on the germination fraction, a major contributor to fitness variance. We ask what effects germination fluctuations have on selection for dormancy by adding germination fluctuations to existing density-independent (d.i.) and density-dependent (d.d.) models of annual plant dynamics, extending previous analyses by including temporally fluctuating germination. Specifically, we ask how germination variance affects selection on the temporal average germination fraction, here used to define dormancy. When present alone, or when independently varying with other fitness components, germination fluctuations do not affect selection for dormancy in the d.i. model, despite generating fitness variance because this variance contribution is not reduced by higher dormancy. Germination fluctuations have strong effects in the d.d. model, favoring dormancy when present either alone or coupled with variation affecting plant growth. This is because germination variation causes seedling density to vary, which causes variable reproduction through variable intraspecific competition. Dormancy is advantaged under variable reproduction because it creates a more convex relationship between population growth and reproduction leading to benefits from nonlinear averaging. Predictive germination, a positive statistical association between germination and growth, weakens selection for dormancy under strong competition and strengthens it when competition is weak. Our results suggest that variable germination is a potential explanation for high levels of dormancy observed in nature, with implications for life-history theory for fluctuating environments.