Long-term Population Responses Research Articles

Why we cannot always expect life history strategies to directly inform on sensitivity to environmental change.

Variation in life history traits in animals and plants can often be structured along major axes of life history strategies. The position of a species along these axes can inform on their sensitivity to environmental change. For example, species with slow life histories are found to be less sensitive in their long-term population responses to environmental change than species with fast life histories. This provides a tantalizing link between sets of traits and population responses to change, contained in a highly generalizable theoretical framework. Life history strategies are assumed to reflect the outcome of life history tradeoffs that, by their very nature, act at the individual level. Examples include the tradeoff between current and future reproductive success, and allocating energy into growth versus reproduction. But the importance of such tradeoffs in structuring population-level responses to environmental change remains understudied. We aim to increase our understanding of the link between individual-level life history tradeoffs and the structuring of life history strategies across species, as well as the underlying links to population responses to environmental change. We find that the classical association between lifehistory strategies and population responses to environmental change breaks down when accounting for individual-level tradeoffs and energy allocation. Therefore, projecting population responses to environmental change should not be inferred based only on a limited set of species traits. We summarize our perspective and a way forward in a conceptual framework.

Variation of cisco egg size among Laurentian Great Lakes populations

Many fish species display inter-population and inter-individual egg size variation. Intra-specific differences in egg size seemingly reflect both energetic experiences of individual spawning fish and long-term population responses to differing ecosystems. Optimal egg size theory implies that selection influences a population’s mean egg size in response to its early-life environment, given the well-established trade-off between egg size and fecundity. Currently, there is strong interest in rehabilitation of Laurentian Great Lakes cisco, Coregonus artedi, which is characterized by inter-population variation of morphological and behavioral traits. However, the extent of cisco egg size variation is under-described. In fall 2018 and 2019, we collected egg samples by stripping ripe females at seven total locations in four Great Lakes. We measured unfertilized egg diameters using imaging software and compared mean egg diameters among locations with and without including maternal total length as a covariate. Lake Michigan females produced the largest eggs overall but were excluded from analyses using the total length covariate because of their significantly larger body sizes. Maternal length had a positive effect on egg size, and when accounting for this effect, females in Lake Huron produced the largest eggs followed by Lake Ontario and Lake Superior. We also found that egg size varied among locations within Lake Superior. These findings aligned with observations of morphological and behavioral differences among populations and suggest that cisco phenotypic variation at a fine spatial scale extends to reproductive biology. Consideration of cisco reproductive traits, such as egg size, may inform restoration strategies, including supplemental stocking.

The Disconnect Between Short- and Long-Term Population Projections for Plant Reintroductions

The reintroduction of rare species in natural preserves is a commonly used restoration strategy to prevent species extinction. An essential first step in planning successful reintroductions is identifying which life stages (e.g., seeds or large adults) should be used to establish these new populations. Following this initial establishment phase, it is necessary to determine the level of survival, growth, and recruitment needed to maintain population persistence over time and identify management actions that will achieve these goals. In this 5-year study, we projected the short- and long-term population growth rates of a critically endangered long-lived shrub, Delissea waianaeensis. Using this model system, we show that reintroductions established with mature individuals have the lowest probability of quasi-population extinction (10 individuals) and the highest increase in population abundance. However, our results also demonstrate that short-term increases in population abundances are overly optimistic of long-term outcomes. Using long-term stochastic model simulations, we identified the level of natural seedling regeneration needed to maintain a positive population growth rate over time. These findings are relevant for planning future reintroduction efforts for long-lived species and illustrate the need to forecast short- and long-term population responses when evaluating restoration success.

Differential Long-Term Population Responses of Two Closely Related Human-Associated Sparrow Species with Respect to Urbanization

Urban planning and management need long-term population level studies for evaluating how urbanization influences biodiversity. Firstly, we reviewed the current population trends of the House Sparrow (Passer domesticus) and the Eurasian Tree Sparrow (Passer montanus) in Europe, and evaluated the usefulness of citizens’ science projects to monitor these species in Finland. Secondly, we conducted a long-term (1991–2020) winter field study in 31 urban settlements along a 950 km north–south extent in Finland to study how latitude, weather and urbanization influence on sparrow’s growth rates. The House Sparrow is declining in 15 countries, and increasing in 5, whereas the Eurasian Tree Sparrow is declining in 12 and increasing in 9 European countries. The trend of the House Sparrow was significantly negative in continental Europe. However, the trend of the Eurasian Tree Sparrow was not significant. Both species have declined simultaneously in six countries, whereas in four countries, their trends are opposite. Citizen-based, long-term (2006–2020) winter season project data indicated that House Sparrow has decreased, whereas Eurasian Tree Sparrow has increased in Finland. However, the short-term (2013–2020) breeding season citizen-based project data did not indicate significant changes in the occupation rate of sparrows. Our long-term (1991–2020) field study indicated that wintering populations of the House Sparrow have decreased, whereas the Eurasian Tree Sparrows have both expanded their wintering range and increased their population size. Based on our winter count data, latitude and weather did not significantly influence the growth rates of sparrows. When the human population increased within the study plot, House Sparrow populations decreased, and vice versa. There was also a trend that a decreasing number of feeding sites has decreased the House Sparrow numbers. Urban-related factors did not influence the growth rate of the Eurasian Tree Sparrow. Our results indicate that the colonization of a new, even closely related species does not influence negatively on earlier urbanized species. It is probable that the niches of these sparrow species are different enough for allowing them to co-occur. The House Sparrow mainly nests on buildings, whereas the Eurasian Tree Sparrow can easily accept, e.g., nest boxes. Urban planning should take care of both the food availability and nest sites availability for both sparrow species.

Modeling the Population Effects of Hypoxia on Atlantic Croaker (Micropogonias undulatus) in the Northwestern Gulf of Mexico: Part 2\u2014Realistic Hypoxia and Eutrophication

Quantifying the population-level effects of hypoxia on coastal fish species has been challenging. In the companion paper (part 1), we described an individual-based population model (IBM) for Atlantic croaker in the northwestern Gulf of Mexico (NWGOM) designed to quantify the long-term population responses to low dissolved oxygen (DO) concentrations during the summer. Here in part 2, we replace the idealized hypoxia conditions with realistic DO concentrations generated from a 3-dimensional water quality model. Three years were used and randomly arranged into a time series based on the historical occurrence of mild, intermediate, and severe hypoxia year types. We also used another water quality model to generate multipliers of the chlorophyll concentrations to reflect that croaker food can be correlated to the severity of hypoxia. Simulations used 100 years under normoxia and hypoxia conditions to examine croaker population responses to the following: (1) hypoxia with food uncoupled and coupled to the severity of hypoxia, (2) hypoxia reducing benthos due to direct mortality, (3) how much hypoxia would need to be reduced to offset decreased croaker food expected under 25 and 50% reduction in nutrient loadings, and (4) key assumptions about avoidance movement. Direct mortality on benthos had no effect on long-term simulated croaker abundance, and the effect of hypoxia (about a 25% reduction in abundance) was consistent whether chlorophyll (food) varied with hypoxia or not. Reductions in hypoxia needed with a 25% reduction in nutrient loadings to result in minimal loss of croaker is feasible, and the croaker population will likely do as well as possible (approach abundance under normoxia) under the 50% reduction in nutrient loadings. We conclude with a discussion of why we consider our simulation-based estimates of hypoxia causing a 25% reduction the long-term population abundance of croaker in the NWGOM to be realistic and robust.

Adaptation of reproductive phenology to climate change with ecological feedback via dominance hierarchies

Phenological shifts belong to the most commonly observed biological responses to recent climate change. It is, however, often unclear how these shifts are linked to demography and competitive interactions. We develop an eco-evolutionary model to study adaptation of timing of reproduction in organisms with social dominance hierarchies. We focus on residential birds with winter flocks, where success in competition for territories among offspring depends on ranking given by prior residence. We study the effects of environmental change on breeding population densities, ensuing selection pressures and long-term evolutionary equilibria. We consider changes in food peak date, in winter survival, in total reproductive output and in the width of the food distribution. We show that the evolutionarily stable hatching date will advance with increasing winter survival and reproductive output since these parameters increase habitat saturation and post-fledging competition. Increasing the length of the breeding season also selects for earlier hatching date due to the reduced costs for producing offspring with high ranking. Our analysis shows that there is little correlation between short-term and long-term population responses across different scenarios of environmental change. However, short-term population growth consistently predicts selection for earlier reproduction. Hence, the model identifies changed breeding population density as a key factor to understanding phenological adaptation in systems with prior residence advantages. While selection for change in reproductive phenology is often explained by changed seasonal variation in environmental variables, such as food abundance, we show that environmental change without apparent effects on seasonality can critically affect phenological adaptation. Such factors can mask or even override influences of changed seasonality on phenology. The model thus offers a conceptually new set of explanations for understanding phenological and demographic trends in a changing climate.

Demographic responses of the invasive annual grass Microstegium vimineum to prescribed fires and herbicide

Management of invasive species in forests often includes combinations of prescribed fire and herbicides. However for most efficient management, evaluations of these tools should include whole-population responses of targeted plants. In this study, we evaluated how the timing and frequency of prescribed fire and herbicide application affected population growth of the invasive annual grass Microstegium vimineum (stiltgrass) using periodic matrix population models. We conducted an experiment in M. vimineum-invaded deciduous forests in Indiana, USA to compare effects of spring and fall prescribed fires combined with pre- or post-emergent herbicide on M. vimineum populations and to build matrix population models predicting long-term population responses to these management treatments across multiple life-history stages. We found that spring fires were effective at reducing population growth rates during the year of treatment but there was no effect of burning on M. vimineum populations the following year. Similarly, fall prescribed fires were effective at reducing seed production, as well as numbers of seedlings and adults following fires, but had no long-term effect on population growth rates. Post-emergent herbicide alone was the only treatment that reduced M. vimineum population growth beyond 1year. Seedbank survival had the highest life-stage elasticity across all treatments, indicating that novel management methods specifically designed to exhaust seedbanks for three or more years may be needed to prevent M. vimineum population resurgence after cessation of treatments.

Habitat demands and population characteristics of the rare plant species Gladiolus imbricatus L. in the Frenštát region (NE Moravia, the Czech Republic)

The sword lily (Gladiolus imbricatus L.) is an endangered species belonging to plant communities of wet meadows. Such meadows decreased in area in central Europe during the second half of the 20th century, and unsuitable biotope management has led to recent decreases in sword lily abundance. The conservation of geophytes (i.e. perennial plants with underground storage organ) like the sword lily requires knowledge of their essential environmental factors and population dynamics. Five populations of the sword lily were assessed at sites near Frenstat pod Radhostěm (the Czech Republic). From 2008-2010 all individuals (generative, vegetative, juvenile) were counted, biometric data was measured (leaves, flowers, fruits number, and stalk height), a transient matrix was created, and vegetation (releves) evaluated. The number of all individuals in generative, vegetative and juvenile stage at these sites underwent year-to-year variability. Nevertheless, the influence of management practices was documented, and was also slightly reflected in both biometric parameters and population abundance. The height, number of leaves and number of blossoms did not show clear temporal or site variability. High reproductive success in this gladiolus was found (61.8-77.8 %), yet seed recruitment is crucial for population stability. In contrast to orchids, each flowering individual had a high probability of flowering to the next season (40 %) and did not switch to the dormant or vegetative stage. About 50 % of individuals tended to stay in dormancy for two years. Unfortunately, the short observation period and inability to estimate the number of dormant individuals made it impossible to determine the length of dormancy, an accurate population structure, and long-term population responses to climate factors. Still, our results confirm the ecological linkage of gladiolus with a broad gradient of moisture, and their connection with mesic Arrhenatherum meadows and intermittently wet Molinia meadows. Long-term regular extensive management is essential for the persistence of populations at all study sites. Self- sowing tree species should be pruned away and herb vegetation cut down each season to provide safe sites for gladiolus seed recruitment. K e y words : sword lily, Gladiolus imbricatus L., ecobiology, population dynamics, monitoring, management

Resource partitioning and interactions enable coexistence in a grass-shrub steppe

We revisited a growth-form removal experiment after 12 years with the aim to compare the long-term population responses of three shrub species. Specifically, we were interested to know if the resource partitioning and plant interactions act as complementary mechanisms of the shrub-grass coexistence. In 1983 different plots from grass-shrub Patagonian steppes were submitted to three treatments: control, grass-, and shrub-removal during three consecutive years. In 1997 we studied nine plots from the original experiment and recorded all shrubs to compare shrub density, population size structure, vitality, spatial patterns and neighbourhood interferences of three native and co-dominant shrub species: Mulinum spinosum, Senecio filaginoides, and Adesmia volckmanni. After 12 years of recovery, shrubs as a growth-form group, fully re-established in plots where they were removed and attained 75% higher density in grass-removal plots than in control plots. However, long-term population responses of Mulinum, Senecio, and Adesmia to removals were distinctive. On the other hand, negative and positive interferences among shrub species and between shrubs and grasses generated a complex network. Morphological and functional differences in shrub and grass species and their interactions at population level and long-term could be a key to achieve a better comprehension of shrub-grass coexistence from semi-arid ecosystems.

Long-term changes in leisure time walking, moderate and vigorous exercise: Were they influenced by the national physical activity guidelines?

Data from the Australian Bureau of Statistics three cross-sectional National Health Surveys, conducted in 1989-1990, 1995-1996 and 2000, were used to examine long-term population responses to the 1996 physical activity (PA) recommendations. Past 2-week recall of the number of times and total minutes spent walking for recreation or sport, and doing moderate and vigorous exercise, were averaged for 1 week to create a measure of 'sufficiently active': >or=5 weekly sessions of >or=150 min/week of at least moderate-intensity activity. With the exception of adults aged>60 years, from 1989 to 2000, there were increases in the prevalence of walking (+8%) and in moderate-intensity exercise (+4%), resulting in a significant decline (-6%) in the proportion of those reporting no leisure-time exercise. Participation in vigorous exercise remained unchanged. There was a 'population shift' in median minutes and sessions of walking from 1995 to 2000, but the prevalence of regular walking (>or=5 times and >or=150 min/week) remained unchanged. A small but significant increase (+2%) in the proportion of adults who met the PA recommendations was noted mainly for men under 60 years and for women aged 45-59 years. The proportion meeting the PA recommendations declined significantly in the most socioeconomically disadvantaged area. These data provide evidence contrary to the observed declines in PA documented by the Active Australia surveillance in the late 1990s. The population may have responded to the 1996 concept of moderate-intensity exercise mainly by walking, but sustainable promotional efforts will be required to build on these gains in the forthcoming decades.

How General are Aphid Responses to Elevated Atmospheric Co2?

We studied the impact of elevated CO2 on 2 aphid pest species, Myzus persicae and Brevicoryne brassicae (Homoptera: Aphididae), on a series of host plants in 3 independent studies each differing in experimental complexity. Measurements on individual aphids showed that host plant and aphid species significantly influenced the response to elevated CO2. These differences occurred not only in the level of responsiveness but also directionally. B. brassicae reared on Brassica oleracea produced significantly less offspring at elevated CO2, whereas the opposite was found for M. persicae on the same host. No response was found for M. persicae on Senecio vulgaris. When populations of B. brassicae and M. persicae were followed for a longer period, no differences were observed in population sizes. Comparisons between different experimental systems show that long-term population responses to elevated CO2 can not be reliably predicted from detailed measurements on individual aphids. The consequences of these findings for climate change research are discussed.

Multispecies spatial assessment models for the British Columbia groundfish trawl fishery

A multispecies spatial model is proposed for evaluation of trawl management strategies and tested with data from British Columbia groundfish. Minimum stock sizes are estimated for 16 species using spatial catch per unit effort data; these data indicate that only small proportions of most stocks are exposed to fishing at any time and that exchange rates of fish between the grounds and other habitats are rapid. A gravity model for spatial effort allocation predicts substantial side effects of marine protected areas or discarding of particular species on nontarget species/grounds due to redistribution of fishing effort. Multispecies, multiground delay-difference models are used in conjunction with the gravity model for spatial effort distribution to predict long-term population responses to various regulatory strategies. This simulation suggests that species composition and spatial structure of the fishery are changing slowly due to reduction in abundance of long-lived species, but rapid collapses of target stocks are unlikely due to spatial redistribution of effort in response to local declines. We question whether quota management (individual vessel quotas) can be safely implemented due to extreme uncertainty about current stock sizes and suggest that a better strategy may be to simply denominate fishing rights as individual effort quotas while providing protection for sensitive stocks through carefully placed marine protected areas.

Modelling population exposure-response functions for use in environmental risk assessment

Effective environmental management requires accurate prediction of the probable individual, population, and ecosystem responses associated with environmental hazards. While much is known about the short-term physiological impacts of toxicants at the individual level, little is known about the long-term responses of populations. This occurs, in part, because of the costs and difficulties associated with completing long-term studies. In the absence of such field data it is argued that modelling both bridges the existing information gap and provides a credible means of predicting long-term population responses. An individuals-based Atlantic salmon (Salmo salar) population dynamics model, adjusted to include laboratory-derived acute toxicity data, is used to measure recovery time in a population subjected to concentrations and durations of copper exposure characteristic of an accidental release of mine tailings. Selected recovery criteria are proposed and discussed in terms of their suitability for use in environmental risk assessment. The resulting model data are used to estimate population exposure-response functions and, for purposes of environmental risk assessment, to describe the cumulative probability distribution ofin-situ environmental damage. The output of the model suggests a recovery time of 15 to 20 years and significant increases in the variability of post-perturbation population levels.