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Diversity: Background Information

Note: hot linked bold faced terms are in the Glossary

Types of Diversity

Biological diversity, or biodiversity for short, is a concept that has become very popular and much better understood in the 1980s, significantly owing to the work of Harvard biologist Edward O. Wilson. The idea of biodiversity encompasses several kinds of "diversities," ranging over a variety of scales. We will not look at biodiversity at each scale in this module, but introduce some of the different types of diversity briefly here.

Genetic diversity

The variability within the genetic pool of one species. For example, the establishment of so-called gene banks of certain species (e.g., apples, rice, various animals) has been suggested for species that are no longer planted, sown, or otherwise occurring naturally to preserve our ability to cross-breed them with other commercial varieties.
Population diversity The observable variation among members of a population of a given species, including stature and behavior. Humans are an obvious example.
Species diversity The number of different species in a given habitat or biome. This is most often referred to in the context of tropical deforestation where we may unknowingly extinguish species that only existed in small niches in the rain forest and then got destroyed along with the forest.
Trophic diversity The number of trophic levels present in an ecosystem. Trophic diversity is an indicator of the complexity of the food web. (See the section on the food web in Unit 2.)
Habitat diversity The variability among habitats in a landscape or a region. The American Midwest can be very monotonous with regard to habitats: wheat or corn fields encompassing hundreds of hectares exhibit low habitat diversity. On the other hand, a highly fragmented landscape may be rich in habitat diversity, yet each habitat may be too small to sustain a species or assemblage of species over a long time.
The last example indicates the interrelatedness among the different types of diversity, and between diversity and other functional system characteristics like an ecosystem's stability, productivity, ability to reproduce, fragility, and resilience. It is beyond the scope of this module to investigate each of these concepts and functional relationships in detail, but it is important to note that functional characteristics of ecosystems -- like biodiversity or resilience -- are interconnected just like storage compartments of these systems by way of flows between them. While this interconnectedness, say between biodiversity and complexity of an ecosystem, is not simple and straightforward, it ultimately rests on the flows of materials, energy, and information (e.g., genetic information).

Understanding Biological Diversity at Different Geographic and Temporal Scales

The diversity of organisms that we find in an area depends on how large an area we observe and on the processes of colonization and invasion of species that created the particular degree of biological diversity within that area. Both the number of organisms and their variety expand as we broaden our view. If we expand our scope to include places beyond a particular area of focus, we see the context of our local populations in relation to their neighbors. Examination of neighboring areas can sharpen our impression of the uniqueness of our space and of the contrasts that make it different. Many reserves have been established where an ecosystem was exquisitely different from its surroundings and may also be particularly fragile or home to endemic, rare, or endangered species. The Everglades National Park is a good example, even though the sharp differences between inside and outside the Park have increased since its establishment simply because certain human activities like intensive agriculture or housing developments were not allowed inside the Park.

The geographic distribution of a population of organisms cannot be fully understood by looking at one type of organism alone. Populations of species exist in the context of many other species and within the context of the biophysical characteristics of their habitats (Ricklefs 1987). Geographical analysis of distribution patterns can guide our thinking about the diffusion processes that created the particular mix of species in a given area. This type of analysis over long spans of time (as in the study of paleobiogeography) provides information on what organisms diminish or disappear from local populations as new organisms evolve there or invade. Thus, a broadening of geographic scales will increase the biodiversity we find, and a broadening of temporal scales beyond the present into the past will help us understand how the biodiversity of the study area came about as a result of interactions between species.

Biotic Interactions and Biodiversity

Tracking nutrient flows is a useful way to find all of the trophic levels (positions in the food web) present in a landscape (see the graphics of the food web in the Nutrient Cycling Simulation Activity in Unit 2). The vegetation (primary production) at a site is the foundation of all other populations living there. Primary production, simply defined, is the conversion of atmospheric carbon to plant biomass through the process of photosynthesis. This process requires that plants have access to resources other than CO₂ to support production. These include solar radiation, water, nutrients, and appropriate temperature.

Small spaces may not have sufficient resources to support large herbivores (animals that consume vegetation) or carnivores (animals that consume other animals). Also, animals not residing in the area may consume plant and animal matter there and then "export" the consumed nutrients by leaving to another area. Thus, the apparent diversity (range of organisms we observe in a space) may be lower than the effective diversity (range of organisms that use a space).

Invasion and Biodiversity

All of the organisms in a space moved there, or invaded, at some time in the past. We have excellent historic examples of changes that have resulted from human introductions. Some, like the potato, were intentional human imports; others, like the gypsy moth, were accidental. Other examples of "exotic" invaders into North America include the sea lamprey into the Great Lakes, the zebra mussel in the Mississippi Basin and now beyond, kudzu in the southeastern United States, and Russian thistle (tumble weed) in the Great Plains (Culotta 1991).

The Africanized Honey Bee

Few invasions are as well documented as the invasion of the Africanized honey bee (see the animated slide show AFHBEES.FLI). The African honey bee was introduced into Brazil in 1957 (Rowell et al. 1992). It aggressively took over hives and queens of the European bee, another intentionally introduced bee, and spread rapidly. Early speculations were that the resultant spreading and mixing would dilute the African genes and that aggression would be diminished in the altered genetic stock. Efforts to halt the northward advance have been unsuccessful, and the Africanized bee spread into Texas in 1991. The genetic material on the frontier of this invasion is over 90% African, contrary to the genetic dilution hypothesis (Makela 1994).

Entomologists expect that the northward migration of the Africanized bee will be halted by climate because the metabolic processes of the African bee evolved in a warmer climate than those of the European bee species which is the species predominantly found in the Americas (Taylor and Spivak 1984; Southwick et al. 1990). Some speculate that the northward advance of the Africanized honey bee will be halted at midcontinent in the range of about 40^o north latitude; it remains to be seen if that hypothesis is valid. Questions about the species' adaptability to cooler conditions and about climatic changes that may enlarge the suitable habitat for the Africanized bee remain unanswered.

The question of how rapidly genetic adaptation can take place is particularly interesting. Will the mixing of the two bee varieties at the northern edge of the invasion favor the selection of European bee genes that facilitate better metabolic adaptation to cold climates? Also questionable is the ability of the Africanized bee to resist parasites and viral infections that now plague the European bee populations in North America. The aggressive bee is now invading a weakened resident population. Finally, an increase in greenhouse gases thought to lead to a warming of our climate may not only expand the northern range of the Africanized bee, it may also weaken the resident European bees further, thereby facilitating the invasion of the Africanized bees.

Resistance Barriers, Corridors, and Staggered Invasion

Invasions of organisms like the Africanized bee create the diversity of organisms in a place. It is not likely that all occupants of a space invaded at the same time. Analysis of fossil plant materials confirms the staggered invasions of plants through time. In the period from 14,000 to 6,000 years before the present, glaciers melted, exposing vast areas of glacial debris that were free of plant and animal life (see the glacial retreat animated slide show ICEAGEWI.FLC). The suite of plants and animals that persisted beyond the margins of the ice had the best chance for invading the territory uncovered by the retreating ice sheet. They differed considerably in their abilities to disperse into new spaces and in their tolerances of newly available environments. Later arriving plants had to pass through a resistance barrier of already occupied spaces (see the migration animated slide shows FINCHES.FLI, GRASSINV.FLI, and TREEINV.FLI).

At the peak extent of glacial ice, the southern and central Plains were dominated by short grasses (e.g., blue grama, black grama, sideoats grama, buffalo grass) that probably invaded from the Southwestern deserts at an earlier date (see the Appendix for an additional reading that includes drawings and maps of these and the following grasses). Tall grasses (e.g., big and little bluestem, Indiangrass, and switchgrass) dominated the Florida Peninsula and probably were abundant in areas of the Gulf of Mexico, which was exposed when sea level was low during the last glacial maximum (about 18,000 years ago). In the Great Basin, west of the Rocky Mountains, cool-season grasses (western wheatgrass, bluebunch wheatgrass, needle-and-thread, and green needlegrass) were present and dominated some parts of the landscape.

The extensive erosion and deposition along the Mississippi Valley provided a corridor for the Southeastern tallgrasses to invade the Midwestern Plains and move up the Arkansas and Missouri tributaries into the Great Plains. The cool-season grasses found their way into the plains through gaps in the Rocky Mountains. A suite of disturbances, including erosion and deposition along valley bottoms and hills, movement of sand dunes by wind, trampling and wallowing by bison, provided opportunities for the aggressive invaders to capture territory formerly vegetated by the warm-season shortgrasses and spruce and pines that found the Northern Plains increasingly inhospitable as the glaciers melted (see the animated slide show GRASSINV.FLI).

Many of the plants that colonized the Northeast in the wake of the receding glacier front were deciduous trees that ranged from the lower Mississippi Valley to the Atlantic Coastal Plain during the last glacial maximum. Others were spruce, pines, and hemlock that spread from the Ohio River Basin and Mid-Atlantic coastal plain. Birches were probably confined to the exposed continental shelf of the Atlantic and in Alaska at the glacial maximum, spreading from both areas to quickly colonize the exposed glacial landscapes (see the TREEINV.FLI slide show).

These changing patterns of abundance for prevalent plant types during past global climate change events exemplify a process that is common to all species. In addition to the purposeful and inadvertent introductions of species, humans have significantly altered the process by creating avenues for more rapid migration or by creating barriers to movement (e.g., the annual plowing of extensive areas has produced significant barriers to the dispersal of perennial plants and animals that depend on the perennial plants).