This week’s BEACON Researchers at Work post is by MSU graduate student Colin Kremer.
Imagine for a moment that you are a plant, animal, or microbe. Chances are good that the environment you live in (desert, forest, grassland, lake, even the ocean) regularly changes in ways that are important to your survival. Factors such as the amount of food you have access to, the temperatures and weather you experience, or how many predators are trying to eat you, all typically fluctuate through time. This is particularly common in temperate zones with distinct seasons (such as Michigan, where I live and work – although winter seems endless right now!).
Many organisms have specific traits, or adaptations, that enable them to do well (or maximize their fitness) under particular conditions. If their environment changes, these adaptations may not be beneficial anymore, decreasing their fitness. By creating times when species with different adaptations each do well, environmental variation can allow competing species to coexist, promoting biological diversity.
In my research, I use mathematical models and data to study the strategies species use to deal with variable environments, and how these strategies evolve and affect the ability of species to coexist. These questions fascinate me because they provide insight into how biological diversity is generated and maintained, and why species are active and doing different things at different times (For example, why do flowers bloom at different times during the year?). Understanding the consequences of environmental variation is also important for applied reasons: humans are changing ecosystems dramatically across the world. In some cases these effects reduce variability, as we suppress wildfires or control river flooding using dams. They can also increase variability: climate change is affecting temperature variability in addition to driving increases in average temperatures.
What strategies can you think of for dealing with a changing environment? The process of evolution has solved this problem many times, and a few strategies turn out to be particularly common:
Specialize, and avoid the bad times. When the going gets rough, many species either go dormant, or get out of town, migrating to a better environment. To avoid hard times, many animals hibernate or form resting stages, conserving their energy; plants produce seeds that can survive drought and freezing, sometimes for hundreds of years; deciduous trees lose their leaves over winter, and so on. Other species migrate, tracking favorable conditions, such as geese heading south for the winter. With this approach, species specialize on doing well under particular conditions, and avoid stressful conditions they are not adapted to.
Be a generalist. As an alternative to specializing and doing really well under specific conditions, some species focus on just doing okay over a wide range of conditions. There are two major ways of being a generalist. First, species may maintain many different traits or adaptations for surviving a variety of conditions. Like the boy scouts, this strategies involves ‘being prepared’, except instead of a giant backpack full of supplies and gear, organisms have larger genomes, or more diverse collections of proteins, or a variety of physiological adaptations.
A second way to be generalist is to change or adapt in response to changing conditions. For example, trees can produce different kinds of leaves that are better at capturing light or conserving water, depending on whether they are shaded or experiencing drought. These responses can happen at the level of an individual, without requiring a genetic change, a strategy called plasticity. Alternatively, natural selection can act on variation in important traits, when individuals with beneficial traits pass on those traits to their offspring. This allows species improve their fitness through evolution. Both of these responses take time (although plasticity is faster than evolution) and can be costly. If changes occur quickly or unpredictably, plastic or evolutionary responses may be too slow, leaving organisms in a constant state of mal-adaptation.
To wrap up this post, I’d like to share two examples of how I’ve studied these ideas and strategies in my own research.
Fluctuating resources and phytoplankton competition. Phytoplankton are small, unicellular organisms that live in water and depend on nutrients from their surroundings and light from the sun to grow and photosynthesize. In temperate lakes, with strong seasonal cycles, annual pulses of nutrients are common, as strong mixing in the spring distributes nutrients from the bottom of the lake throughout the water column. Light levels also fluctuate, both seasonally and daily. Variation in the availability of critical resources (nutrients, and light) can allow different phytoplankton species to coexist, even if they compete for the same resources. This can occur if some species are good at growing quickly when resources are plentiful, but bad at competing for resources when they are scare. These species do well right after a pulse of nutrients occurs, only to get out-competed later in the season by slow-growing, highly competitive species. The tradeoff between these specialized strategies, combined with varying nutrient supply, creates times of the year when each kind does well, allowing them to coexist.
Using mathematical models, I found out that coexistence is only possible given intermediate amounts of variation – when environments are too constant, or too variable, only one specialist’s strategy remains viable. Also, coexisting specialists can lose out to a generalist species, if it is able to change quickly from being a fast grower to a good competitor in response to nutrient.
Ocean temperatures and our changing climate. Phytoplankton also inhabit the oceans, where they play a critical role as the foundation of most food webs, and drive global cycles of carbon, nitrogen, and other elements. In collaboration with another BEACON scientist, Mridul Thomas, I’ve studied how ocean temperature determines which phytoplankton species inhabit different parts of the ocean. We’re particularly interested in exploring how phytoplankton communities will respond to climate change, which is increasing ocean temperatures and changing their variability. A critical piece of this puzzle is figuring out how quickly the thermal preferences of phytoplankton can evolve. You can read more about this work in Mridul’s previous BEACON blog post, or check out a summary of the paper we published on our research.
For more information about my work, you can contact me at kremerco at msu dot edu or visit my website.