Ecologists Study the Interactions of Organisms and Their Environment

Scientists estimate that there are between five to fifty million species of organisms on Earth, of which less than two million have been officially named (May 1988). Many organisms are small: including microbes that inhabit almost every crevice of the Earth; tiny worms that help build soils; and insects that spend their entire lives in tree tops. Alongside these small denizens coexist larger, flashier species that have drawn human attention throughout the ages: multicellular plants and fungi, birds, reptiles, amphibians, and fellow mammals. These species, as well as many smaller ones, are consumers that depend for sustenance on energetic biochemical compounds generated from light energy by photosynthesizing producer species, or from inorganic chemical reactions by chemosynthetic species.

The diversity of producer species, on which all life depends, is immense, and ranges from cyanobacteria to towering trees in tropical and temperate rainforests. Plant life clothes much of Earth’s land surface, providing structure to ecosystems (e.g., interacting systems of organisms and their physical environment), habitat for consumers, and regulating the exchange of energy and chemicals with the atmosphere. Nutrients from terrestrial systems wash into lakes and oceans, where additional primary production by phytoplankton and algae helps support large communities of zooplankton, fish, sea mammals, and birds. Over time, nutrients are returned from the oceans to the land through the movements of organisms, atmospheric gaseous exchange, or slower geological processes, such as the uplift of ocean sediments (Schlesinger 1997).

Ecological scientists who study this complex web of life take diverse approaches. The aim of some studies is to illuminate general principles that explain how ecosystems work. For example, such research might investigate whether greater biodiversity tends to make ecosystems more or less susceptible to invasion by exotic species. In other cases, research focuses on specific issues that offer insights useful for ecosystem management. For instance, such studies might examine whether new agricultural cropping strategies will expand habitat for wildlife (Figure 1).

To answer questions like these, ecologists observe nature, conduct experiments, and construct mathematical models. Studies are conducted at different scales because ecosystems come in many sizes. Ecological studies may examine individual organisms, single species populations, multiple species communities, eecosystems, or the Earth as a whole. Ecological studies may also examine different time frames, from short-term interactions, on the order of seconds to minutes, to perspectives that span large portions of Earth’s 4.5 billion year history.

What these different ecological research approaches share is the recognition that materials and energy flow through multiple systems on Earth, and that interactions among organisms and their environments are two-way: the environment influences organisms, and organisms alter their environment.

Organisms inhabit nearly every environment on Earth, from hot vents deep in the ocean floor to the icy reaches of the Arctic. Each environment offers both resources and constraints that shape the appearance of the species that inhabit it, and the strategies these species use to survive and reproduce. Some of the broadest patterns of environmental difference arise from the way our planet orbits the Sun and the resulting global distribution of sunlight (Chapin et al. 2002). In the tropics, where solar radiation is plentiful year-round, temperatures are warm, and plants may photosynthesize continuously as long as water and nutrients are available. In polar regions, where solar radiation is seasonally limited, mean temperatures are much lower, and organisms must cope with extended periods when photosynthesis ceases.

Across ecosystems, environmental resources and constraints shape the structure and physiology of organisms. One of Earth’s oldest environmental legacies is the array of chemical elements it contains (Schlesinger 1997). At its birth, Earth inherited carbon atoms produced by stars that burned out long before our sun was formed. These carbon atoms, with their unique capacity to build chains and four-way links with other elements, provide the backbone of all the organic molecules that make up life today (Figure 2). Nitrogen and phosphorus are also essential elements in living organisms, where they play central roles in the makeup of proteins, nucleic acids, and energetic compounds. These elements are not always readily available to organisms, so nutrient limitations can powerfully constrain biological strategies. For example, inert nitrogen gas makes up 78% of Earth’s atmosphere, but nitrogen forms readily useable by organisms are typically much scarcer in terrestrial ecosystems (Chapin et al. 2002). Over evolutionary time, symbioses that developed between nitrogen-fixing bacteria and plants helped increase the availability of nitrogen in many ecosystems. Nonetheless, given strong competition for nitrogen and other elements, ecologists find that nutrient limitations constrain life in many environments (Chapin et al. 1986).

Organisms are shaped further by the physical properties of the media in which they live, including the media’s densities and temperatures. For example, marine mammals like Stellar sea lions (Eumetopias jubatus) have developed streamlined bodies that move efficiently through water, which is more than 700 times denser than air, but that slow them down on land (Figure 3a; Riedman, 1991). As a result, sea lions sleep on shore, but hunt for food primarily in the water, where their speed is optimized.

Ecologists also study how temperature influences the ecology and evolution of species. Organisms generally slow down or freeze when conditions are cold, but overheat and lose function as temperatures rise. Many species have therefore evolved traits that help protect themselves against extreme temperatures and influence their ecology. For example, while sea lions rely on thick layers of fat for insulation, sea otters (Enhydra lutris) swimming in the same cold waters depend on unusually thick fur to retain heat. As a result, sea otters spend more time grooming (Figure 3b), and their thick fur attracted hunters who drove them nearly to extinction (Riedman 1990). On land, research shows that plants and cold-blooded animals develop dark coloration and position themselves to maximize solar energy gain in cool weather. In hotter regions, studies reveal that animals may avoid intense sun, while plants protect themselves by transpiring large amounts of water, maximizing air flow through their foliage, or going dormant until cooler temperatures returns. Some temperature adaptations can be surprising. For example, scientists recently found that grasses growing near geothermal vents gain heat tolerance from a virus within a fungus inside their roots (Marquez 2007).

Water availability further shapes ecological dynamics on Earth. Early life arose in aquatic ecosystems, and all living cells still require water to function. Water availability is influenced by temperature, because in very cold climates water is frozen and not available, and in very warm ones water evaporates quickly. Ecological studies of water relations have found that organisms employ an amazing array of strategies to capture and retain water resources. For example, in the searing hot Namib desert of South Africa, the Stenocara beetle survives by capturing water from rare wisps of fog that condense in special structures on its back (Parker et al. 2008).

At the community level, community ecologists study how resource availability influences ecosystem characteristics, including the number and types of species present. For example, the amount of carbon and energy fixed in photosynthesis by plants and other producers (e.g., productivity) constrains the amount of consumers an ecosystem may support. Because of this limit and because energy is lost at each transmission step through a food web, low productivity ecosystems generally support less consumer biomass than higher productivity systems. Ecologists have identified this relationship as one possible reason that biodiversity is greater in highly productive tropical rainforests than in less productive systems like deserts (Gaston 2000). Within communities, environmental variability can drive complex variation in ecological dynamics. For example, researchers recently discovered that small increases in temperature can markedly increase the aggressiveness of some coral reef fish (Biro et al. 2010). These behavioral changes may increase fish exposure to predation and other risks.

Because the environment is both dynamic and diverse, ecologists recognize that there is no single set of ecological attributes or strategies that make an organism "the best." All living populations and species are continuously changing in response to pressures from other organisms, and to variability in Earth's geology and climate. Over time, this dance of evolving interactions has produced an amazing array of organisms that depend upon, and compete with, each other across the surface of the planet. To reconstruct Earth's ecological history, ecological scientists and other researchers seek data of many types, including tree rings that describe ancient patterns of drought, ice cores that contain bubbles of Earth's earlier atmosphere, and DNA preserved in millennia-old animal bones. These data show how organisms have responded to environmental change, including the meteorite-driven extinction that helped usher in the age of mammals 65 million years ago.

The environment is dynamic because physical processes drive change in Earth's attributes over time. However, research demonstrates that life itself drives equally important environmental changes. Because other organisms are part of each individual’s environment, changes in species distributions can profoundly alter ecological interactions within communities. In some cases, the loss of a native species, or introduction of a non-native one, can threaten the survival of other organisms. For this reason, the conservation of endangered organisms and control of invasive species are of broad concern.

Ecologists have found that interactions among organisms come in several different forms. In antagonistic relationships, organisms compete for resources, spread disease to their neighbors, or consume each other. In more mutualistic associations, one organism shelters another, two organisms exchange resources, or tighter dependencies evolve, such as coevolved relationships between specialized pollinators and flowers. In some cases, species even cultivate others. For example, ecologists recently found that coral reef damselfish tend underwater algal gardens, where they remove less desirable algae species and chase away predators (Hata et al. 2010). In other cases, species with large structures become habitat for smaller organisms. For example, the human digestive tract harbors so many bacteria that they outnumber the cells in the human body by tenfold (Dethlefsenet et al. 2008). Investigating how digestive tract microbes influence their hosts is now a promising area of microbial ecology and medicine. At a bigger scale, the evolutionary rise of flowering plants (angiosperms) and the development of extensive rainforest canopies produced novel environments in which animals tested new ecological strategies. Scientists suggest that evolution of the open branch structure of rainforest trees helped drive the evolution of forelimb structure in apes, permitting tree-to-tree swinging, and bequeathing manual dexterity to humans (Figure 4; Burger 2006).

Research demonstrates that organisms have additional power to change the environment by altering stocks and flows of water, energy, and elements at both small and large scales (Beerling 2007; Morton 2008). For example, paleoecology documents how the evolution of photosynthetic organisms released oxygen that precipitated iron oxides and then accumulated in the atmosphere, changing its composition and generating Earth’s ozone layer (Cowan 1990). The ozone layer then reduced UV radiation on terrestrial surfaces, and helped to protect organisms emerging onto land from potentially lethal does of UV. Today plant life controls a large fraction of energy and water fluxes between land and the atmosphere. Scientists estimate that in the extreme case of removing all plant life from land, rainfall on Earth would drop by 50% (Kleidon et al. 2007). Animals also play critical roles in influencing the physical properties of ecosystems. For example, recent work shows how underground termites in Kenya increase grassland productivity and biodiversity over large areas by raising soil fertility in evenly spaced circles (Figure 5; Pringle et al. 2010). In the twenty-first century, key ecological questions center on human manipulation of the Earth’s environment. Future research will grapple with conflicts between human needs for food, fuel, and fiber, and preservation of natural biodiversity and ecological function (World Health Organization 2005).