Home » Gardening Information » Ecology for Gardeners
The Biological Structure of Plant Communities
The biological structure of plant communities gives them the character we observe in nature. A deciduous forest dominated by tall, slender oaks looks very different from one composed of wide-spreading beech trees and rich evergreen hemlocks. A prairie dominated by grasses is altogether different from a forest of stately trees. Within different plant communities a vast diversity of species is found. In a tallgrass prairie dominated by three grass species, you may find more than nine different grasses and up to three hundred different forbs. A mixed mesophytic forest may have over thirty different tree species, even though only two or three are codominant. Ecological health depends on diversity. Plant communities are naturally diverse, but our gardens are comparatively barren. A typical garden has one grass, bluegrass turf, a dozen perennial plants, three different shrubs, and one or two different trees. If we are to make our gardens as attractive to wildlife as they are to us, we must understand biological structure and put that knowledge to work. We must learn to appreciate both biological and visual complexity.
Species Dominance
Dominance refers to the influence of species that contribute the most cover and/or biomass (total mass or weight) to a plant community. In a forest, the dominant species is the tree that contributes the most cover to the canopy. In a prairie it is a grass or a forb. In the tundra, it is a shrub, herb, or lichen. If more than one species make up the dominant cover, then individuals in the suite of species are called codominants. Dominant species are often called visual essence species, for they are the ones that give a plant community its unique look.
Dominance is determined by density in a plant community - that is, the total number of stems or individual plants within a given area. Ecologists use precise sampling techniques to determine dominance. One common technique is the use of quadrats, or plots of land with fixed dimensions. Within each quadrat, the trees, saplings, shrubs, and herbs are counted and measured. In forests, relative density, relative cover, and relative frequency are weighted to determine dominance. In herbaceous communities, dominance is determined by relative cover and frequency.
Biodiversity
Biodiversity, a much used term these days, refers to a healthy mixture of ecosystems, communities within the ecosystems, species within the communities, populations within the species, and individuals within the population. An individual is a single plant of a given species -- a barrel cactus, for example. A population is all the barrel cacti in the area. The community is the barrel cacti, along with associated vegetation such as prickly pears, palo verde trees, ocotillo, and mesquite. The ecosystem is the Sonoran Desert. The Sonoran Desert is one of the four associations in the Western Deserts province.
Ecosystem diversity is the broadest aspect of biodiversity. Globally, as well as within North America, the variety of ecosystems contributes to the health and stability of the biosphere. Forests, grasslands, deserts, and other vegetation formations comprise the global ecosystem. On a smaller scale, biodiversity is important to the health and resilience of ecosystems as well. For example, food chains are built on the foundation of a healthy, diverse ecosystem of many different native plant species. This is referred to as species diversity. Native plants provide food and cover for insects. Insects in turn are a vital food source for many birds and mammals. The greater the diversity of plants, the greater the diversity of other species the ecosystem can support.
Species diversity is a measure of both richness and evenness. Richness refers to the total number of different species in an area or community, while evenness is a measure of the number of individuals of each species. In general, diversity increases with richness and evenness. In other words, the more different species and the more evenly distributed the individuals of each species, the more diverse the community.
On the smallest scale, the genetic diversity of individual species is also critical. Species with the largest number of populations with large numbers of individuals generally have the broadest genetic base. The more diverse its gene pool, the more resilient the species. Species with few populations and few individuals are more likely to lack the genetic diversity necessary to enable them to adapt to environmental change, disease, or other types of stress. In other words, genetic diversity enables a species to adapt and evolve.