Many species interactions are visible – a bee visits a flower, a lion chases a gazelle, a mistletoe perches atop a tree. However, a whole host of other interactions remain invisible. These interactions may include microbes or other organisms that are too small to see, or the interactions may occur belowground, hidden from sight. Plants engage in extensive belowground interactions with other plants, soil arthropods, microbes, and fungi. Plants also serve as a link between aboveground and belowground subsystems, an area that is currently receiving much attention in ecological research.
Over hundreds of millions of years, pollinators and many plant species have coevolved intricate mutualisms through which both partners benefit – plants see their genetic material dispersed to new mates, and pollinators receive rewards in the form of nectar. Different species have evolved floral morphologies that restrict floral reward access to particular pollinator species; for example, flowers with long corollas have rewards that are accessible only to long-tongued pollinators. The function of different floral morphologies is, in part, to ensure that pollinators that gain access to a plant’s nectar are also contributing to the plant’s reproductive success. However, the phrase “cheaters never prosper” doesn’t really apply to natural systems, and many pollinators “rob” nectar by drilling holes in a plant’s corolla. As is often the case, some of the first descriptions of nectar robbing can be traced back to Charles Darwin, who, in 1872, was the first to suggest the nectar robing was a socially-learned behavior.
I don’t spend much time reading Nature Medicine, and I imagine most ecologists are similar to me in that regard. However, I ran across an article from that journal that’s gotten a lot of play in the popular media, and it got me thinking about some concepts from ecology and evolutionary biology, and their relevance to disease treatment.
First, let’s take a step back. You have trillions of microbes living and reproducing inside of you, right now. This may not surprise you, but if it does, don’t worry – your life would be much less pleasant if they were all wiped out. In particular, your digestive system is teeming with microbes that help you digest and process food. These microbes, of course, have genes that determine their traits and how well they function as digestive assistants. They also have very short lifespans, so the microbial communities inside of you can evolve in response to your diet and other activities. With that out of the way, let’s move on to the study.
Throughout an organisms’ life, the expression of genes, regulated by the biotic and abiotic environment, gives rise to traits that determine how fast it can run or how tall it can grow. Many traits also affect species interactions; for example, are you fast enough to outrun predators? Do you look tasty to herbivores? Most traits (e.g., running speed) cease to be important once an organism dies, but some traits linger and have “afterlife” effects on the environment. A prominent example of afterlife effects can be found in decomposing plant material, which is a crucial part of nutrient cycling. Microbes and fungi are critical to many stages of nutrient cycling, such as the mineralization of organic matter and the nitrification of NH4+, which plants cannot use, to NO3–, which is usable by plants. However, microbes and fungi can be “picky eaters” in a sense, as they prefer substrates with labile simple sugars instead of defensive molecules such as lignin. Simple sugars have carbon and nitrogen supplies that are easily accessible, while larger, more complex molecules require degradation by energetically-costly enzymes. Therefore, genetic and environmental influences on the chemical composition of plant material can persist after a plant sheds its leaves and affect how quickly its nutrients are cycled.