Species Interactions Can Drive Adaptive Radiation
Adaptive radiation is the process by which one species gives rise to multiple species that exploit different features of the environment, such as food resources or habitats (see Section 5.9, Figure 5.22). Different features of the environment exert the selective pressures that push populations in various directions (phenotypic divergence); reproductive isolation, the necessary condition for speciation to occur, is often a by-product of the changes in morphology, behavior, or habitat preferences that are the actual objects of selection. Likewise, variations among local populations in biotic interactions can result in phenotypic divergence and therefore have the potential to function as mechanisms of adaptive radiation. Resource competition is often inferred as a primary factor driving phenotypic divergence. For example, species of the globeflower fly Chiastocheta present a unique case of adaptive radiation as a result of resource competition. At least six sister species of the genus Chiastocheta lay their eggs (oviposition) on the fruits of the globeflower, Trollius europaeus (Figure 12.14); however, the different species of globeflower flies differ in the timing of their egg laying. One species lays its eggs in 1-day-old flowers, whereas all the other species sequentially deposit their eggs throughout the flower life span. In a series of field experiments, Laurence Despres and Mehdi Cherif of Université Joseph Fourier (Grenoble, France) found evidence that supports the hypothesis that the evolutionary divergence of species of Chiastocheta was a result of disruptive selection on the timing of egg laying (reproduction). The researchers established that intense intraspecific competition occurs within each of the species, but differences in the timing of egg laying and larval development functions to minimize competition among species (the concept of resource partitioning will be examined in Chapter 13).
Although numerous studies have illustrated the role of competitive interactions in adaptive radiation, the importance of other interactions, such as mutualism or predation, remain largely unexplored. The research of Patrik Nosil and Bernard Crespi of Simon Fraser University (British Columbia, Canada), however, has shown that adaptive radiation can result from divergent adaptations to avoid predators. Nosil and Crespi’s research focused on two ecotypes (populations of the same species adapted to their local environments) of the stick insect Timema cristinae (see Section 5.8 and Chapter 5, Field Studies: Hopi Hoekstra for discussion of ecotypes). Timema walking sticks are wingless insects inhabiting southwestern North America. Individuals feed and mate on the host plants on which they reside. The two distinct ecotypes of Timema are adapted to feeding on different host plants, Ceanothus and Adenostoma. The two host plants differ strikingly in foliage form, with Ceanothus plants being relatively large and tree-like with broad leaves and Adenostoma plants being small and shrub-like with thin, needle-like leaves (Figure 12.15).
The two Timema ecotypes differ in 11 quantitative traits (see Figure 12.15), comprising aspects of color, color pattern, body size, and body shape. These differences between the two ecotypes appears to be a result of divergent selection. The different traits exhibited by each of the ecotypes appear to provide crypsis (avoidance of observation) from avian predators on the respective host-plant species. Field experiments were conducted to determine how differences in phenotypic traits influenced the survival rates of the two ecotypes on the two plant species. Each of the two Timema ecotypes was placed on each of the two host-plant species. The results of the experiment clearly indicated that the direction and magnitude of divergence in traits represent adaptations that function to reduce rates of predation on Timema on their respective host-plant species. The ecotypes of T. cristinae, like the example of the limnetic and benthic ecotypes of sticklebacks examined in Chapter 5, can be considered to represent an early stage of adaptive radiation because studies indicate that reproductive isolation is not complete (see Section 5.6, Figure 5.15).
Ecological Issues & Application Urbanization Has Negatively Impacted Most Species while Favoring a Few
As we will see in the chapters that follow, species interactions are ubiquitous in nature and play a fundamental role in the structuring of ecological communities. Perhaps no other interaction, however, has as great an impact on the diverse array of plants and animals that inhabit our planet as their interaction with the human species.
As we first presented in Chapter 9 (Ecological Issues & Applications), the primary cause of population declines and recent species extinctions is habitat loss as a result of human activities—namely, changing land-use patterns. There are two major land-use changes that are responsible for habitat loss in terrestrial environments: expanding agriculture and urbanization.
According to the Food and Agricultural Organization (FAO) United Nations’ statistics, at present some 11 percent (1.5 billion hectares) of the globe’s land surface (13.4 billion ha) is used in crop production (arable land and land under permanent crops), and even more land (3.2 to 3.6 billion ha) is used to raise livestock. Together, agricultural lands account for almost 40 percent of Earth’s land surface. The negative impacts of the expansion of agriculture to meet the needs of the growing human population have been central to the discussion of the decline of biological diversity on our planet, a topic we will examine in more detail in Chapter 26. The increasing urbanization of the human population over the past century (Figure 12.16), however, has led to the emergence of a new field of ecology—urban ecology—to study the ecology of organisms in the context of the urban environment.
Ecology has historically focused on “pristine” natural environments; however, by as early as the 1970s, many ecologists began turning their attention toward ecological interactions taking place in urban environments. What has emerged is a picture of species interactions dominated by humans, which negatively impacts most species and benefits only a few.
Estimates of urban land area vary widely from 0.5 to slightly more than 2.0 percent of the world’s land, depending on the criteria used to define urban development. Historically, cities have been compact areas with high population densities that grew slowly in their physical extent. Today, however, urban areas are expanding twice as fast as their populations. According to the United States Census Bureau, about 30 percent of the U.S. population currently lives in cities, whereas another 50 percent lives in the suburbs. More than 5 percent of the total surface area of the United States is covered by urban and other developed areas; this is more than the land covered by the combined totals of national and state parks.
The expansion of urbanization produces some of the greatest local extinction rates and frequently eliminates the large majority of native species. Eyal Shochat of Arizona State University’s Global Institute of Sustainability and colleagues used data from Phoenix, Arizona, and Baltimore, Maryland, to contrast the distribution of species in these two urban areas as compared to the surrounding natural ecosystems. Their findings show a general pattern of decline in the number of species in urban environments as compared to both surrounding agricultural and natural ecosystems (Figure 12.17).
Species vary in their ability to adapt to the often drastic physical changes along the gradient from rural to urban habitat. Moving from the rural landscape of natural ecosystems and cultivated lands into the suburban landscape, one moves through a heterogeneous mixture of residential areas, commercial centers, and the managed vegetation of parks and cemeteries. The main cause for the loss of species in these suburban environments is habitat alteration. Yet in contrast to the decline in the number of species, both suburban areas and urban centers are usually characterized by higher population densities of resident species as compared to adjacent natural lands. For example, in a study of population of northern cardinals (Cardinalis cardinalis) in the metropolitan area of Columbus, Ohio, and surrounding forested landscape of central Ohio, Lionel Leston and Amanda Rodewald of Ohio State University found that birds were four times more abundant in urban than rural forests. Their research showed that food abundance was as much as four times greater in the urban habitat as compared to the forests of the surrounding region because exotic vegetation, refuse, and bird feeders may all provide food sources for birds in these urban environments.
Some mammals, such as raccoons (Procyon lotor), skunks (Mephitis mephitis), and rabbits (Sylvilagus spp.) have also benefited from the spread of the suburban landscape, finding shelter beneath sheds and porches, and an abundance of food—for raccoons, garbage; for skunks, insects and larvae on lawns and in gardens; and for rabbits, an abundance of high quality food plants in gardens and flowerbeds. Larger species, rapidly adapting to human presence, are moving into the suburban landscape and dramatically increasing in number. White-tailed deer (Odocoileus virginiaus), carriers of Lyme disease, find an abundance of forage on grass, shrubs, and gardens. Resident Canada geese (Branta canadensis), attracted to large open areas of grass—including golf courses and parks—create both a nuisance and health problems. In recent years, coyotes (Canis latrans), attracted by garbage and small prey including rodents and pets (cats and small dogs), are becoming more common in suburban areas. Even black bears (Ursus americanus) are attracted to backyard bird feeders and dumpsters in suburban areas adjacent to forested, rural landscapes.
In addition to increased abundance and predictability of food resources, recent research indicates that a reduction in predator populations in urban environments favors resident species. Evidence has been gathered that supports the idea that urban environments are safer for some species than are rural habitats. Both birds and squirrels in urban environments benefit from reduced nest predation and are able to spend a greater proportion of their time foraging compared with individuals in the surrounding natural ecosystems, indicating that the urban habitat is less risky than the surrounding rural habitats.
Species adapted to habitats along the suburban gradient drop out as they come to urban centers where habitat changes sharply. Vegetation is limited to scattered parks, some tree-lined streets, and vacant lots. Species that benefit from the habitat provided by these core urban centers are often referred to as “urban exploiters.” Among plants, urban exploiters tend to be ruderal species (see discussion of plant life history classification in Section 10.13) that can tolerate high levels of disturbance. Examples include wind-dispersed weeds (grasses and annuals) that colonize abandoned lots and properties, and plants that can grow in and around pavement.
Bird species that thrive in urban habitats are often adapted to nesting in environments that are similar to the cityscape. For example, species that use cliff-like rocky areas, such as the rock dove (pigeons, Columba livia) and peregrine falcon (Falco peregrinus), are “pre-adapted” to using the barren concrete edifices of urban buildings, whereas cavity-nesting species, such as the house sparrow (Passer domesticus), house finch (Haemorhous mexicanus), and European starling (Sturnus vulgaris) are able to inhabit human dwellings.
Mammalian urban exploiters consist of species that are able to find shelter in human dwellings and exploit the rich food source provided by refuse, such as the house mouse (Mus musculus), the black rat (Rattus rattus), and brown rat (Norway rat: Rattus norvegicus).
Urban environments typically have more in common with other cities than with adjacent natural ecosystems, so species that flourish in urban habitats are often not native to the region. Rather, these species tend to disperse from city to city, typically with assistance—either intentionally or unintentionally—from humans (see Chapter 8, Ecological Issues & Applications). Species such as rock doves, starlings, house sparrows, Norway rats, and the house mouse are found in all cities in Europe and North America. As a result, many studies have found that the number (and proportion) of non-native species tends to increase as you move from rural habitats toward urban centers. In general, the proportion of species that is non-native goes from less than a few percent in rural areas to more than 50 percent at the urban core.
This combination of negative interactions with the majority of native species—while enhancing a small subset of often non-native species, which we have manipulated to serve our needs, facilitated through dispersal, or created urban environments in which their populations flourish—is resulting in what urban and conservation ecologists refer to as biotic homogenization, which is the gradual replacement of regionally distinct ecological communities with cosmopolitan communities that reflect the increasing global activity of humans.