Applying a new demographic framework to understand and project consequences of climate change in size- and age-structured populations

This project investigated effects of climate change on size-structured populations, using unique long-term data from pike (Windermere, U. K) and brown trout (Mjøsa/Gudbrandsdalslågen, Norway). For many species, body size is an important factor determining vital rates of survival, reproduction, and growth of individuals, as well as interactions between them (e.g. competition). Despite this general importance, population size structure has rarely been considered in studies investigating population responses to climate change. In this project we have developed and applied integral projection models (IPMs), to study different general questions as well as questions specific to each study population. IPM is a demographic modeling framework that links individual phenotypic properties (for instance, body size) to vital rates (survival, fecundity and growth rate), which allows us to study how the phenotypic distribution changes over time, and/or in response to external factors such as temperature. In the first study from this project we developed a size-structured IPM to consider effects of climate warming on the pike population. As many other northern lakes, Windermere has seen a strong warming trend in the last decades. The results of our study showed that warming may have several positive effects on the life history and on population growth, however negative effects are expected for intermediate sized individuals. The population structure is expected to shift towards fewer large fish and more intermediate-sized fish. These changes were caused by the differential effects of temperature on survival and growth of pike depending on size. Warming increases the growth rate of small fish, while the final size is smaller than before. The mortality of large fish also increases with temperature, but the overall effect on large fish is positive because of increased fecundity. These changes in the pike population can potentially have large effects on other species in the ecosystem, such as important prey species (pike are top predators and select prey depending on size). Other studies in the project have extended this first model (which depended on temperature and body size) in different ways, including i) density dependent survival, ii) individual heterogeneity in growth and survival, iii) size-dependent interactions among individuals (cannibalism and competition), iv) effects of maternal size on offspring survival, and v) other environmental variables. These studies all revealed important mechanisms to the dynamics of size-structured populations and their responses to climate changes. For both study populations we found large persistent differences in growth between individuals, and this type of heterogeneity is likely common in fish. These differences can arise from several mechanisms, including genetic differences and environmental impacts during early development (in particular during the first year). For the pike population we found strong effects of temperature during the first year on subsequent growth, which again can have large impacts on the population growth rate. This result could be important to management also of other fish populations experiencing climate warming. In contrast, another proposed important factor, namely maternal size, was not important to population growth in our analyses. Large females tend to produce larger eggs than small females, and offspring from larger eggs likely have a higher survival. However, at the population level this mechanism did not have a large effect. Future research should investigate whether there are other mechanisms by which large females can have disproportionate impacts on population growth (relative to their size). Another study within the project was a review of biological responses to increased variability in climate variables (such as temperature), including extreme events. This study highlights that increased variability can often have positive effects as well as negative, however we need more knowledge on the underlying mechanisms to these responses. Interactions among individuals, such as competition and cannibalism, often depend on size. In one of the last studies (manuscript), such size-based effects was included in the IPM, and we evaluated how population dynamics and stability depended on the variance in the size distribution of offspring (age 1 individuals). A general conclusion from this analysis is that variation in offspring size promotes population stability. For pike from Windermere, the variance in size at age 1 has decreased over the study period, which could indicate that the population growth is becoming more unstable. Whether this reduced variance is due to climate change remains to be investigated.

Fresh waters are particularly vulnerable to climate change, which affects temperature as well as other physical and chemical water properties. Temperate Northern lakes have shown an increasing temperature trend in the last decades, and climate models proj ect a continued temperature increase of surface waters as well as changes in stratification patterns. In this project we take an innovative approach to study ecosystem responses to climate change, by extending the demographic framework of integral project ion modeling (IPM) to include key drivers of climate change, and applying it to unique long-term individual-based data from two freshwater fish species, brown trout (Norway) and pike (UK). As top predators, the responses of these species to climate change may have large consequences for the ecosystems. IPM is a powerful approach that can incorporate population structure from a mixture of discrete and continuous state variables, such as age and body size. Size structure is rarely considered in relation to climate change, but it is most likely important because many organisms have highly size-dependent vital rates. Here, we will study consequences of climate change in combination with harvesting, eutrophication, and stocking, as well as threshold effects of extreme events. Based on developed climate change scenarios for the two local regions we will use our models to project future responses to climate change. We will also consider opportunities for adaptive management based on the new approach. This projec t brings together recently developed theory and existing data in a synergistic fashion that will help answer some of the unresolved questions of climate change research. Analyzing these questions in the context of size-structured populations is at the fro ntier of ecological research, and acknowledges the need for management to be size-specific. Our results will provide valuable knowledge for management of size-structured organisms, also beyond aquatic ecosystems.