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There’s a delicate balance that separates order from chaos. Things that seem permanent and unshakable prove to be transient and fragile. In nature, ecosystems are held in a state of precarious equilibrium called keystone species, or keystones. These organisms hold entire food webs together, very often as top predators but not always. Scientists have learned that the removal of these keystones from a given ecosystem causes vibrant landscapes to transform from fertile to barren. This understanding has provided conservationists with a strategy to reverse the degradation of ecosystems, whether it be natural or man-made.
On October 9th, a documentary will be premiering on PBS Nature called The Serengeti Rules. The film features several scientists whose discoveries have contributed to the understanding of keystone species and have hypothesized rules for maintaining a sustainable habitat.
Mary Power, one of the researchers involved with the project, set aside time to discuss the role of keystones in an ecosystem.
SCIENTIFIC INQUIRER: Just for some background, can you define what an ecosystem is and the features a well-balanced ecosystem possesses?
MARY POWER: An ecosystem was well defined by Arthur Tansley in 1935 as a system of interacting biota and physico-chemical factors and processes. Ecosystems can range in scale from a microbiome in a droplet to the entire globe, depending on what interactions we want to investigate. We try to circumscribe ecosystems for study in some sensible way that best supports our effort to predict their futures—how ecosystems and their living and abiotic components will respond to change.
I’ll take “well-balanced” to mean that the ecosystem supports biota and processes that enhance human well-being (including aesthetically or culturally). Although values vary among human cultures, we often want clear, clean water and charismatic native species in our ecosystems. What this often means is that we need to keep nutrients high in the landscape, and high in the food web. If we remove natural vegetation, erode hillslopes in our watersheds, and allow sediments, agrochemicals, or sewage to flow down and collect in lowland lakes, reservoirs, or coastal oceans, eutrophication triggers harmful algal blooms that threaten public health and close fisheries.
If, on the other hand, better land stewardship reduces nutrient fluxes to levels food webs can assimilate, aquatic grazers (snails, crustaceans, and in freshwaters, aquatic insect larvae) can suppress excessive algae. But if aquatic insect populations are unchecked, we’ll have pestiferous insect emergences. Small predators (e.g. dragonfly larvae, small fish) can suppress grazing insects; large fish may limit these small predator populations, but even larger fish can indirectly protect the little predators, and make osprey, bear, and human fishers happy.
So a simple rule of thumb: “keep nutrients high in the landscape and high in the food chain”, can guide plans for sustaining what I think you’d call a balanced ecosystem. (At least nowadays, most managers and naturalists hope that predators at these high “trophic levels” will be native to the region, not exotics.)
SI: The notion of a keystone species is a central concept in The Serengeti Rules. What is a keystone species and what role do they play?
MP: There are two definitions, one historical and one revised. Bob Paine coined the word to describe a predator that structures entire food webs (and ecosystems) by keeping a prey population in check that could otherwise overwhelm the system. Because the term was so evocative, ecologists started using it in their papers to designate any species that did anything important.
To save the term from getting expanded beyond useful meaning, a group of ecologists including Bob Paine met in Hilo Hawaii (convened by the ecosystem scientist and world conservation leader Hal Mooney). Following Bob and his students, we recognized two types of “strong interactors” in ecosystems: keystones, and dominant or foundational species that have obvious roles in structuring ecosystems (coral in reefs; trees in forests, kelp in kelp forests). The keystones also had strong effects, but they could be rare, or even cryptic, because their effects were disproportionately strong relative to their biomass (Pisaster, sea otters, bass).
This broadened “keystone” to designate non-predators as well as predators—for example, keystone pollinators like tiny fig wasps that keep figs going in tropical rainforests through months of dry season when other trees are not fruiting. The utility of this definition is that we can try to learn which species might matter a lot, for example to maintaining biotic diversity, even if they are not particularly conspicuous as we observe their ecosystems.
SI: Was there surprise when it became evident that a keystone species doesn’t have to be a top predator or a carnivore?
MP: No, I think all the ecologists working in the area recognize that strong ‘keystone’ interactions don’t have to come from the top of the web.
SI: Can something as small as a microbe be a keystone or do organisms at the microscopic level have their own ecosystem isolated from the macro?
MP: Microbes are extreme keystones, if they have strong effects albeit with tiny amounts of collective biomass. (Mycorrhizal fungi that partner with plant roots in Ken Read’s “wood-wide web”, on the other hand, may be “dominants”—they have huge biomass in forest or grassland soils, they’re just hard for us surface-dwellers to see.)
We’re learning more every day about the important and fascinating interactions of microbes with hosts and the rest of the macro-world, starting with host-microbiome interactions and co-evolution. And it’s not always the microbes that are driving the directions of these ecological interactions or the co-evolutionary trajectories.
SI: What happens to an ecosystem experiencing downgrading?
MP: In many ecosystems, downgrading through loss of larger predators leaves plants more exposed to unchecked herbivory. If plants that sheltered and fed diverse nectivores, frugivores and herbivores are overgrazed, their loss can trigger huge food web and ecosystem consequences (e.g. changes in albedo, Earth surface temperatures and exposure to desiccating winds, erosion via blowing dusts or erosive gulleys.)
If riparian forests are stripped off of stream banks by elk and deer, erosion of fine bank sediments can degrade stream habitats for fish and other aquatic life. Loss of riparian vegetation might lead to loss of beaver, which would eliminate important fish habitat, allow channel incision, and lower the water table, altering large expanses of the surrounding terrestrial and aquatic habitat.
If otter extirpation allows losses of kelp forests, fin fish populations that used kelp for shelter collapse, along with fisheries, sea birds, and mammals that depended on them. Even the adjacent coast can change from depositional mud flats with kelp to erosional rocky shores, after kelp wave breaks are lost. In freshwaters, changes in top down controls of planktivorous or surface-grazing fish by predators have huge effects on water clarity, habitat structure and nutrient loading and fluxes in both rivers and lakes.
SI: What happens when an invasive species is introduced into a system?
MP: Invasive species are sometimes cryptic for months or years after their initial introduction. Then some environmental event—for example, a change in weather pattern–allows them to explode and spread. Invasive plants can then change fire cycles in ways that devastate native vegetation (e.g. cheat grass in the American west), or add nitrogen to soils where native vegetation had adapted to low nutrients, leaving the native plants susceptible to overgrowth by weeds (e.g. Myrica in Hawaii). Loss of native vegetation can trigger loss of the butterflies, birds, and other animals that depended on it.
Invasive animals undermine both natural food webs and human infrastructure (e.g. zebra mussels clogging pipes and canals in our fresh waterways; armored catfish and exotic crayfish burrowing through levees in Florida and California). One sad knock-on effect is that after invasion, restoring habitats to help rare native species might instead make those habitats “sinks” for their populations.
An example is when predaceous non-native bullfrogs colonize restored ponds intended for endangered red-legged frogs in California. The ponds look good to the red-legged colonizers, but they are death traps Invasives add one more challenge to our efforts to recover and sustain native species.
SI: Are humans a keystone species? Or are we an invasive species?
MP: We’re certainly THE dominant species on our planet if you now consider how much of the Earth’s surface is human-dominated. You’d have to count area converted for agriculture, pasture, and aquaculture as well as land covered by human-built structures. We’ve also had less visible but no less important impacts on marine, freshwater, and temperate and tropical food webs, via extermination of top predators and manipulations of populations to increase production of our food plants and animals.
Tens of thousands of years ago, humans had huge effects when they first expanded out of Africa to colonize (or invade?) Asia, Europe, and then the Americas, via fire and via hunting of megafauna to the point of extinction. Perhaps back then we were keystones, because a carnivorous social primate with projectile weapons probably had a huge effect on naive megafauna, even when our numbers were relatively low.
SI: The Serengeti Rules closes with efforts to counteract downgrading. How effective have efforts at upgrading been?
MP: Some of the results have been extremely encouraging. Tony Sinclair’s work documenting the restoration of many ecosystems—grasslands and wood groves–following the recovery of wildebeest to their historic densities is definitely a leading example. Also, the recovery of kelp forests where otters have re-established is rapid (giant kelp are annual seaweeds) and dramatic.
I know that river and stream ecosystems are extremely resilient—they have to be, because river biota have experienced scouring floods and drought stress through their evolutionary histories and have a suite of life history tactics to recover. So river restorations via dam removals, deliberate levee breaches, or restoration of more natural flow regimes to rivers have triggered surprisingly rapid recovery in valued populations and in more natural habitats that better support them.
SI: Can an artificial keystone be created or mimicked?
MP: There’s a lot of excitement (and apprehension) that comes with synthetic biology, particularly since ~2014 with the powerful CRISPR-Cas9 gene editing tools. Sergei and Nikita Zimov are expecting geneticist George Church to present his first resurrected baby mammoth (an elephant embryo with a lot of mammoth traits inserted, by editing in genes from frozen mammoth DNA). They have a good place to put this baby (hopefully, more than one because mammoths will probably be highly social)—their Pleistocene Park, where megafaunal grazers already in place seem on track to converting frozen tundra back to steppe grasses, which dominated the region before 12,000 BC. Steppe grasses could sequester a lot more carbon underground, and could support the kind of productive grazing ecosystem that fueled Genghis Khan’s conquest of Asia and eastern Europe around 1200 AD.
SI: What is the biggest question when it comes research into keystone species?
MP: How do organisms interact with environmental regimes to determine the strength and outcome of species interactions? And how does this depend on their traits? What will be the consequences of changes in climate, land cover, or biota (including changes imposed by gene editing) that will feed back either to “bite us on the rear” (as Sean Carroll says in the movie) or to sustain a world with the kind of nature that we like, and need.
IMAGE SOURCE: Creative Commons