Juvenile survival to first breeding is a key life‐history stage for all taxa. Survival through this period can be particularly challenging when it coincides with harsh environmental conditions such as a winter climate or food scarcity, leading to highly variable cohort survival. However, the small size and dispersive nature of juveniles generally make studying their survival more difficult. In territorial species, a key life‐history event is the acquisition of a territory. A territory is expected to enhance survival, but how it does so is not often identified. We tested how the timing of territory acquisition influenced the winter survival of juvenile North American red squirrels Using 27 years of data on the survival of individually marked juvenile red squirrels, we tested how the timing of territory acquisition influenced survival, whether the population density of red squirrel predators and mean temperature overwinter were related to individual survival probability, and if territory ownership mediated these effects. Juvenile red squirrel survival was lower in the years of high predator abundance and in colder winters. Autumn territory owners were less susceptible to lynx Our results show how the timing of a life‐history event like territory acquisition can directly affect survival and also mediate the effects of biotic and abiotic factors later in life. This engenders a better understanding of the fitness consequences of the timing of key life‐history events.
Long‐term studies of wild animals provide the opportunity to investigate how phenotypic plasticity is used to cope with environmental fluctuations and how the relationships between phenotypes and fitness can be dependent upon the ecological context. Many previous studies have only investigated life‐history plasticity in response to changes in temperature, yet wild animals often experience multiple environmental fluctuations simultaneously. This requires field experiments to decouple which ecological factor induces plasticity in fitness‐relevant traits to better understand their population‐level responses to those environmental fluctuations. For the past 32 years, we have conducted a long‐term integrative study of individually marked North American red squirrels Our long‐term observational study and field experiments show that squirrels can anticipate increases in food availability and density, thereby decoupling the usual pattern where animals respond to, rather than anticipate, an ecological change. As in many other study systems, ecological factors that can induce plasticity (such as food and density) covary. However, our field experiments that manipulate food availability and social cues of density (frequency of territorial vocalizations) indicate that increases in social (acoustic) cues of density in the absence of additional food can induce similar life‐history plasticity, as does experimental food supplementation. Changes in the levels of metabolic hormones (glucocorticoids) in response to variation in food and density are one mechanism that seems to induce this adaptive life‐history plasticity. Although we have not yet investigated the energetic response of squirrels to elevated density or its association with life‐history plasticity, energetics research in red squirrels has overturned several standard pillars of knowledge in physiological ecology. We show how a tractable model species combined with integrative studies can reveal how animals cope with resource fluctuations through life‐history plasticity.
- Award ID(s):
- 1749627
- PAR ID:
- 10455906
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Animal Ecology
- Volume:
- 89
- Issue:
- 11
- ISSN:
- 0021-8790
- Format(s):
- Medium: X Size: p. 2397-2414
- Size(s):
- p. 2397-2414
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract Tamiasciurus hudsonicus , hereafter red squirrels, and how the timing of this event mediated the sources of mortality. We hypothesized that securing a territory prior to when food resources become available would reduce juvenile susceptibility to predation and climatic factors overwinter.Lynx canadensis and possibly mustelidMustela andMartes spp., predation. Autumn territory owners had lower survival in colder winters, but surprisingly non‐owners had higher survival in cold winters. -
more » « less
Abstract To cope with uncertainty and variability in their environment, plants evolve distinct life‐history strategies by allocating different fractions of energy to growth, survival and fecundity. These differences in life‐history strategies could potentially influence ecosystem‐level dynamics, such as the sensitivity of primary production to resource fluctuations. However, linkages between evolutionary and ecosystem dynamics are not well understood.
We used an annual plant population model to ask, when might differences in plant life‐history strategies produce differences in the sensitivity of primary production to resource fluctuations?
Consistent with existing theory, we found that a highly variable and unpredictable environment led to the evolution of a conservative strategy characterized by relatively low and invariant germination fractions, while a variable but predictable environment favoured a riskier strategy featuring more variable germination fractions. Unexpectedly, we found that the influence of life‐history strategy on the sensitivity of production to resource fluctuations depended on competitive interactions, specifically the rate at which production saturates with the number of competing individuals. Rapid saturation overwhelms the influence of life‐history strategy, but when production saturates more slowly, the risky strategy translated to high sensitivity, whereas the conservative strategy translated to low sensitivity.
Empirical estimates from Sonoran Desert annual plant populations indicate that production saturates relatively rapidly with the number of individuals for most species, suggesting that life‐history differences are unlikely to alter sensitivity of production to resource fluctuations, at least in this community.
Synthesis . Our modelling results imply that research to understand the sensitivity of primary production to resource fluctuations should focus more on the intraspecific competitive interactions shaping the density–yield relationship than on the life‐history strategies that determine temporal risk‐spreading. -
more » « less
Abstract Much research has shown that environmental stress can induce adaptive and maladaptive phenotypic changes in organisms that persist for multiple generations. Such transgenerational phenotypic plasticity shrouds our understanding of the long‐term consequences of ongoing anthropogenic pressures.
Here, we evaluated within‐ and transgenerational phenotypic responses to food stress in the freshwater crustacean,
Daphnia . We reared 45 clones ofDaphnia pulicaria each on high‐qualityScenedesmus and low‐quality (but also non‐toxic) cyanobacteria (generation 1). Offspring produced by generation 1 adults were then reared onScenedesmus (generation 2), and life‐history traits were measured across both generations.The results show that
Daphnia in generation 1 exhibited reduced fitness (i.e., delayed maturation, lower reproductive output, increased clutch interval) when reared in the presence of cyanobacteria as opposed to high‐quality food. However, maternal stress had no clear influence on the fitness of offspring. That is,Daphnia in the second experimental generation had similar mean trait values, irrespective of whether their mothers were reared on cyanobacteria or high‐quality food.Signals of transgenerational life‐history effects were obscured, in part, by extensive clonal variation among
Daphnia in the direction of transgenerational responses to cyanobacteria (i.e., adaptive and maladaptive plasticity). Further analyses demonstrated that such individual variance in plasticity might be open to selection and potentially offer a means of contemporary adaptation to cyanobacteria. Taken together, our results denote a link between the overall strength of transgenerational responses to the environment and the potential for rapid evolution in populations.A free
Plain Language Summary can be found within the Supporting Information of this article. -
more » « less
Abstract Mathematical models highlighted the importance of pathogen‐mediated invasion, with the replacement of red squirrels by squirrelpox virus (SQPV) carrying grey squirrels in the UK, a well‐known example.
In this study, we combine new epidemiological models, with a range of infection characteristics, with recent longitudinal field and experimental studies on the SQPV dynamics in red and grey squirrel populations to better infer the mechanistic basis of the disease interaction.
A key finding is that a model with either partial immunity or waning immunity and reinfection, where individuals become seropositive on the second exposure to infection, that up to now has been shown in experimental data only, can capture the key aspects of the field study observations.
By fitting to SQPV epidemic observations in isolated red squirrel populations, we can infer that SQPV transmission between red squirrels is significantly (4×) higher than the transmission between grey squirrels and as a result our model shows that disease‐mediated replacement of red squirrels by greys is considerably more rapid than replacement in the absence of SQPV.
Our findings recover the key results of the previous model studies, which highlights the value of simple strategic models that are appropriate when there are limited data, but also emphasise the likely complexity of immune interactions in wildlife disease and how models can help infer disease processes from field data.
-
more » « less
Abstract Life‐history strategies are known to shift with latitude in many species. While life‐history variation related to body size, reproductive investment, and behavior has been studied for years, another crucial life‐history component is the immune system, which can influence an animal's survival.
We measured selected life‐history traits in side‐blotched lizards in southern Utah and Oregon in the field for two consecutive years and conducted a common‐garden experiment in the laboratory to determine how organisms from different latitudes optimize either immunity or reproduction. We observed lizards from southern populations, which are known to be shorter‐lived, had lower immune function during reproduction when compared to northern lizards in 2012, but the relationship reversed in the following year.
Our laboratory study revealed that southern lizards healed cutaneous wounds faster and had higher microbiocidal ability when compared to their northern counterparts, but lost mass doing so. The northern lizards ate more than the southern ones and maintained their body mass. It is possible that northern lizards are better adapted to taking advantage of available food resources. Alternatively, southern lizards may have exhibited sickness behavior in response to an immune challenge or reacted more strongly to the stress of captivity.
We found differences in life‐history strategies used by animals from different latitudes, and that these changes can shift within a population depending on the weather conditions of the year. Furthermore, when taken from the field and placed into a common‐garden environment, some of these differences in strategy appear to be intrinsic to the animals (i.e., whether they came from southern or northern populations).
