Investigating how past populations coped in the face of rapid environmental changes - using resurrected fortifers as a model species
Conor A. Waldock
It is increasingly evident that environmental changes affect the persistence of wild populations. Predicting population responses remain difficult as the mechanisms allowing populations to adapt can interact in a complicated way. A promising new approach to investigate population responses to environmental change is the resurrection of dormant propagules - a field called Resurrection Ecology. Dormant propagules are resting life stages (i.e. seeds, eggs, spores) that can remain viable for extended periods of time and are produced during periods of stress, for example during rapid environmental changes. The subsequent study of resurrected populations and the performance of such populations, under experimentally controlled conditions, overcome the time constraints of studying the evolutionary adaptations of populations.
Lake Orta, a subalpine lake in northern Italy, has a well-documented history of environmental change in the form of industrial pollution, starting in 1926, releasing copper and ammonia into the lake waters (causing acidification of the lake environment). Copper pollution ceased in 1958 and ammonia in 1980s and full lake recovery occurred in the 1990s after a liming program to counter the acidified lake environment. Recovery of the lake resulted in an increase in species diversity, from a period of dominance by rotifers (zooplankton) which managed to rapidly adapt to the environmental changes caused by pollution. Rotifers during pollution periods produced an abundance of dormant propagules, providing a model organism to track eco-evolutionary dynamics in retrospect. During my time in Zurich we resurrected the rotifer Brachionus calyciflorus from eggs previously extracted from lake sediments (Fig. 1), decades after the egg production occurred. Investigating these resurrected rotifers in laboratory conditions provides a unique opportunity to study the ecological and evolutionary processes involved in the populations' responses to past anthropogenic change in this lake, which I help to investigate during my time spent in Zurich.
Figure 1. Sediment from Lake Orta has been previously extracted and dated by project collaborators in Italy (a). The periods of pollution have been well characterised (b), and using sediment dating we remove resting eggs from the sediment layers (c) which correspond to the distinct pollution periods.
Figure 2. An example of B. calyciflorus used in the experiments, this individual has been isolated and 'trapped' within a specially developed compression chamber in order for it to be photgraphed for body size measurements. The dark oval attached is an egg containing a female juvenile, almost ready to hatch. The size of the individual is roughly 0.2mm, making isolation, photographing and measuring a challenging task.
I carried out experiments on resurrected rotifers (Fig. 2) to understand which life-history processes (individual survival, growth, and reproduction), underlying a populations' response, changed due to the past environmental pressures in the lake. We set up an experiment which recreated three simplified past environments (peak copper pollution, peak pH pollution, and the post-pollution environment), and placed resurrected individuals adapted to different pollution periods into these environments. Therefore, individuals from each time period are placed into 2 novel environments, and one environment we hypothesized they should be adapted to and therefore excel within. To understand how rotifers might have adapted to such conditions I measured the phenotype (body size) and life-history traits of individual rotifers.
Using the information collected during these experiments can help build population models to understand the mechanisms by which past populations comprised of such individuals responded to the distinctive pollution periods in Lake Orta, and the dynamics of such populations. Furthermore, these experiments will be linked to population level analysis which answers the questions such as "what are the population dynamic consequences of such past environmental changes?", and the information collected here help determine which individual level traits drive these higher level responses.
I am grateful for the support of the Scurfield Bursary which enabled me to develop my skills as a researcher, maintain collaborations abroad, and take part in some exciting science.