By Andre Bonneau, PAg., Range Management Extension Specialist, Prince Albert
Plant biodiversity is a measure of the number of individual plants and different plant species in a geographic area. For example, a clean wheat field may have fewer than 10 plant species present. The plant population will be dominated by one species, wheat, whereas the remaining species, presumably weeds, make up a minimal number of the remaining individuals. Meanwhile, native pasture, in good condition, may have more than 150 plant species. Some species on native pasture will be more common than others, but generally there is a more even distribution of species and number of individuals. In this example, the wheat field is relatively low in biodiversity while the native pasture is considered rich in biodiversity.
Under natural conditions, the number of plant species colonizing an area would increase until all environmental niches, or growing sites, are occupied. As species biodiversity increases, the resources available to those species, such as water, sunlight and mineral nutrients, are used more efficiently and more resistant to loss from the system. As well, insects, disease and some weeds don't overwhelm the site as easily. Interactions between species can also increase biomass production with minimal inputs. One common example is the interaction between legumes, grasses and forbs on pasture. The legumes potentially add nitrogen and phosphorus that isn't available in significant amounts otherwise.
Why is biodiversity important? As biodiversity increases, so does environmental stability. The environmental tolerance of each plant species occupies certain ranges. A diverse landscape will support plants that differ in drought tolerance, heat and cold, fire, insect infestation resistance, rooting depth, dormancy, direct sunlight, shade, darkness and seed dispersal. As growing conditions change, there are plant species present that can tolerate the shift. For example, during dry conditions, performance of shallow-rooted grasses weakens while deep-rooted plant species that are able to access water deeper in the soil profile thrive.
There is some indication that livestock diets can benefit from diverse landscapes. Studies in the United States suggest that livestock will actively select plant species that can meet their dietary requirements. Comparisons between rumen samples and clippings on pasture show stomach content is usually higher quality than the forage available. Cattle and other livestock can select forage from different grass, shrub and broadleaf (forb) species to maintain or increase forage quality.
The benefit of biodiversity can be seen in wildlife populations. Outdoor enthusiasts often notice that wildlife are more commonly found in transition zones, or areas where one plant community type transitions into another. These zones are generally areas with increased biodiversity. In some cases, wildlife competes with livestock for forage. But, the presence of wildlife indicates stability and biodiversity.
There are a few examples of farming practices that take advantage of biodiversity. Crop rotations involving legumes, cereals, oilseeds and perennial forages increase diversity and can reduce insect, weed and disease pressure. Complex forage blends with a combination of legumes and grass species create a relationship involving nitrogen and phosphorus. Cover crops with many annual and biennial species attempt to fill as many available niches in the soil profile as possible. The goal is to make better use of soil resources and potentially increase carbon reserves.
Range management specialists assess tame and native pastures throughout the province to determine the level of plant diversity on the site. As grazing pressure increases, the population of some plant species increase while the population of others decrease. In time, the population shift will reduce biodiversity and may reduce both forage yields and the stability of the remaining plant population. As biodiversity decreases and niches become vacated, weeds are expected to become an issue.
Managing diverse plant stands can be more difficult than managing a monoculture as the environmental needs of each species varies. However, biodiversity can provide many of the benefits that farmers and ranchers are looking for, such as environmental stability, low cost forage production and wildlife habitat. Managing biodiversity is about balancing the needs of the plant population with the needs of livestock.