Ecology is the study of how organisms interact with each other and with the physical environment in which they live. It is a large field of study and incorporates research at many spatial and temporal scales. Examples of ecological research include impacts of climatic change on species range distributions, patterns of infectious disease outbreaks, the effect of nutrient availability on ecosystem function, etc. In his 1911 book My First Summer in the Sierra, John Muir famously wrote ‘when we try to pick out anything by itself, we find it hitched to everything else in the Universe.’ This quote sums up the unifying tenet of ecology – that all biotic and abiotic factors are linked to interact with one another, and that a change in one factor will likely cause a change in all of the factors to which is linked.
Levels of Ecological Study
Ecology incorporates research at many spatial scales, from a single individual to the entire biosphere. The major levels of ecological study are illustrated in Fig 1.2.1. In some cases, research studies and systems span multiple levels; however, each scale has particular questions and approaches that are commonly used. To illustrate ecological research at each scale, we will consider the research projects of researchers in the lab of Dr. Brian Todd in the Wildlife, Fish, and Conservation Biology Department at UC Davis.
Researchers studying ecology at the organismal level are interested in the adaptations that enable individuals to live in specific habitats. These adaptations can be morphological, physiological, and behavioral. For example, researchers in the Todd Lab are interested in the salt tolerance of different turtle species such as the Western Pond Turtle (Emys marmorata) shown in Figure 1.2.1, which is native to California. Salt tolerance determines the waterways in which the Western Pond Turtles can survive and therefore contributes to the species range and distribution.
A population is a group of interbreeding organisms that are members of the same species living in the same area at the same time. The population may have natural or artificial boundaries: natural boundaries might be rivers, mountains, or deserts, while examples of artificial boundaries include mowed grass, manmade structures, or roads. The study of population ecology focuses on the number of individuals in an area and how and why population size changes through time.The Todd Lab studies several populations of turtles, including the population residing in the UC Davis Arboretum and Public Garden waterway. The Western Pond Turtle is federally listed as vulnerable and is a California Species of Special Concern (our state-level conservation listing), so campus researchers and the Arboretum waterway managers are concerned about the size of the population of Western Pond Turtles in the waterway, particularly if the population is declining. The Todd Lab regularly traps turtles in the Arboretum and documents the size and sex of each turtle found. Small turtles, like the one shown in Fig 1.2.2a, indicate that the Arboretum population is successfully producing new offspring. In addition, Todd’s team marks each Western Pond Turtle with unique shell notches, which allow the researchers o identify how often they catch the same turtle and to track individuals throughout their lives. Figure 1.2.2b shows markings on a turtle captured in April 2019.
A biological community consists of populations of different species living in the same area at the same time. Community ecologists are interested in the interactions within and among these species and the processes driving these interactions. Questions about interactions within a single species often focus on competition among members of the same species for a limited resource. Ecologists also study interactions between various species. These interactions include predation, parasitism, herbivory, competition, and pollination. These interactions can have regulating effects on population sizes and can impact ecological and evolutionary processes affecting diversity. The Western Pond Turtle is not the only turtle species that lives in the Arboretum waterway; several other species have been introduced to the waterway, probably through the release of pet turtles no longer wanted by their owners. The most common turtle species is the Red-Eared Slider (Trachemys scripta elegans), though several other species such as soft-shelled turtles, snapping turtles, and map turtles have also been identified in the waterway (Fig 1.2.3). There are, of course, many other non-turtle species that live in the waterway as well, including European carp, mallards and wood ducks, and possibly even river otters, not to mention a variety of aquatic plants and algae, and a plethora of insects. The potential interactions between these species and the Western Pond Turtles might be important drivers of their population sizes. In particular, previous research by former UC Davis professor Dr. Brad Shaffer looked at competition between the Western Pond Turtle and the introduced Red-Eared Slider for basking sites within the Arboretum waterway (Lambert et al 2013).
Ecosystem ecology is an extension of organismal, population, and community ecology. The ecosystem is composed of all the biotic components (living organisms) in an area along with the abiotic components (non-living factors) of that area. Some of the abiotic components include air, water, and soil. Ecosystem biologists ask questions about how nutrients and energy move among organisms and how environmental factors such as temperature, precipitation, salinity, etc affect species in the habitat and interactions between species. The Arboretum waterway managers are particularly interested in how the built environment around the waterway (paths, trails, concrete retainers, etc) might affect the Western Pond Turtle. Some parts of the waterway have soil banks covered in native vegetation, while others have bare ground, and still others have concrete retaining walls (Fig 1.2.4). Previous research by Dr. Shaffer and his team determined that Western Pond Turtles are more commonly found basking on concrete structures in shallower water in areas less disturbed by human activities (Lambert et al 2013).
Landscape ecology deals with questions of connectivity between ecosystems, including movement of organisms and compounds between ecosystems and the effects of boundaries or edges where different ecosystems come into contact (Fig 1.2.5). A landscape ecologist might study habitat fragmentation (such as deforestation) or the migration of organisms between ecosystems, etc. The Arboretum waterway ecosystem is part of a broader landscape of aquatic and terrestrial ecosystems, including upstream and downstream Putah Creek, grassy lawns, shrublands, forests, and the urban ecosystems of the UC Davis campus and surrounding city of Davis. This landscape could influence migration of the Western Pond Turtle (and other Arboretum inhabitants) to other nearby waterways and could directly affect the Arboretum waterway ecosystem by influencing factors such as water quality (ie. clarity), temperature, sunlight availability, nutrient availability, disturbance (especially by humans), etc.
At its broadest, ecology considers the entire biosphere, all habitats on earth in which living organisms exist. Global ecological questions include the impacts of atmospheric and oceanic currents, the role of greenhouse gases concentrations in the global energy budget, the effects of climate change on ecosystems and organisms, etc
In this section, we have focused on categorizing ecology by its spatial scale, though many other types of categorization are possible. For example, disease ecology focuses on the evolution, virulence, and transmission of infectious pathogens or parasites, and human ecology focuses on human interactions with the environment including societal, cultural, technological, and institutional relationships. These ‘other’ ecologies can operate at any of the scales described above. For instance, disease ecology might focus on the organismal level (physiological impacts of disease on individuals), the population level (transmission of disease between individuals of the same species), or community level (transmission of disease across species), etc.