Agricultural Intensification and its Effects on Species Diversity PDF

AGRICULTURAL INTENSIFICATION AND ITS EFFECTS ON SPECIES DIVERSITY In agricultural intensification, the composition of the plant community, as determined by the farmer, is described as ‘Planned Diversity’ of cropping systems (I.e. the farmer controls, the spec...

AGRICULTURAL INTENSIFICATION AND ITS EFFECTS ON SPECIES DIVERSITY In agricultural intensification, the composition of the plant community, as determined by the farmer, is described as ‘Planned Diversity’ of cropping systems (I.e. the farmer controls, the species which are allowed to grow and the methods and inputs used to maintain the agroecosystem). Crop diversity is critical not only to production, but it is an important determinant of total biodiversity. It influences the composition and abundance of the associated biota such as those of the pest complex and the soil invertebrates and microorganisms which in turn affect plant and soil processes. THE PEST COMPLEX In both agricultural and natural ecosystems, herbivorous insects and microbial pathogens can have significant impacts on plant productivity. The reduction in plant species diversity which occurs in agricultural intensification leads to changes in the community composition of the pest complex, whereby herbivorous insects, their natural enemies (predators and parasites) and microbial community attack crops. This low planned diversity of monocultural agricultural systems typically results in greater crop losses from an insect pest complex that is less diverse but more abundant Lower insect pest densities occur in more diverse crop systems such as polyculture, resulting partly from changes in the following- o host-finding and insect movement o higher predation rates o higher parasitism rates and o higher ratio of natural enemies to herbivores Most viruses that are transmitted by insects tend to be found at lower incidences in polycultures because of plant species diversity on their insect vectors. In contrast genetic diversity within species can also significantly reduce pathogen impacts on crop productivity. Typically, one has a mixture of resistant and susceptible crops of one species in an area, with more resistant crops compared to susceptible crops in these mixtures therefore reducing the pathogen spread. Both multiline cultivars and varietal mixtures have been used effectively to control the spread and evolution of fungal pathogens in small grains. The potential for using crop diversity to manage insect and microbial pests has not been extensively exploited. Currently pest management is accomplished via the use of pesticides As a result, pesticide resistance has become a ubiquitous (present) problem. Although integrated pest management (IPM) advocates the use of host plant resistance, biological control and cultural controls along with pesticides and has been promoted for decades with some success it has been only been adapted in relatively few crops and has yet to significantly reduce the amount of pesticides worldwide.  Reasons for this is that pesticides are cheaper, and policies encourage the use of pesticides while IPM requires knowledge intensive management. (I.e. It is easier to spray a pesticide on crops vs learning about which crops to plants to grow together to reduce the impact of pests etc.) SOIL BIOTA In natural ecosystems, soil nutrient cycling, soil structure and other properties are substantially regulated by the activity of a highly diverse soil community of microbes and invertebrate animals. The level of diversity in terms of abundance, composition and activity levels of soil communities have been shown to be markedly different in agricultural systems compared to natural ecosystems. Usually it is about less than half of that of the natural ecosystem from which is was derived. However, this trend is not absolute in tropical pastures where the abundance and biomass of fauna is usually enhanced. Soil fauna in these communities are usually dominated by a single species or a small number of species that have adapted to the changed environment The changes in soil community in agriculture can be attributed to the following reasons o Conversion from natural forest resulting in the removal or burning of large plant biomass followed by tillage is a major disturbance of the of the soil environment which affects the soil biota (e.g earthworms which can be killed or exposed to extreme temperatures and decreased moisture at soil surface) o Climatic extremes of temperature and moisture can also influence soil biota activity (e.g. poorly drained soil can promote the spread of some pathogens which can cause disease) o Decrease in the quantity and chemical composition of organic matter to the soil can significantly result in changing the competitive balance between different organisms Some functions, particularly those in the nitrogen cycle are carried out by very specific organisms (nitrogen fixing-bacteria, nitrifying bacteria and denitrifying bacteria). Other processes such as decomposition and mineralization are carried out by the interactions within a diverse community of organisms. Given the important roles organisms (e.g. termites, earthworms, bacteria, mycorrhizal fungi and nematodes) play in ecological services in the soil, a reduction in their diversity may profoundly alter the biological regulation of decomposition and nutrient availability in soil. Now proper IPM practices can be designed to maximise the presence and function of soil biota. These include the change from extensive to reduced tillage which reduces the severity of physical effects at the soil surface as well as increases organic inputs because crop residues are retained on soil surfaces. CHANGES IN NATURAL RESOURCES In natural ecosystems most of the nutrient supply results from the turnover of soil organic matter and is mediated or controlled by soil organisms. In the case of nitrogen, inputs occur via  atmospheric deposition and biological fixation via nitrogen fixing bacteria in the roots of legumes. Nitrogen gases are released to the atmosphere via emissions and erosion etc, but in general the input and outputs are negligible compared to nutrients moving within the internal cycle. In agricultural systems, the cycling of nutrients is altered by the removal of nutrients via harvesting, resulting in reduced organic matter and then the heavy application of inorganic nitrogen that is readily available for uptake by plants or carried away in runoff. Remember soil organisms process soil organic matter and provide soils with altered nutrients usually not at the levels crops demand. So hence the application of fertilizers such as Nitrogen fertilizers but plant can only uptake so much These losses occur when Nitrogen fertilizers are applied in excess of plant needs, cannot be retained, or mediated by soil microbial community because soil biological community has been disrupted. Another challenge of excess Nitrogen fertilization is that it causes an increase in sap-feeding insects like aphids and leafhoppers and planthoppers. This was evident in an outbreak of brown planthopper in Southeast Asia in a rice paddy. As a result, farmers were instructed to apply less Nitrogen fertilizers to their crops to avoid future outbreaks. Similarly crop pathogens such as bacteria fungi and viruses cause more damage when Nitrogen inputs are high, and their pathogen dynamics and disease severity can vary depending on the particular form of Nitrogen fertilizer applied. Some have a greater affinity for ammonia than nitrate and vice versa. INTERACTIONS BETWEEN AGROECOSYSTEMS AND SURROUNDING REGIONS Although agroecosystems are typically managed in isolation from other ecosystems within a region, the physical, chemical, and biogeochemical processes and changes that take place within them have numerous consequences for adjacent and distant ecosystems. Similarly, neighbouring ecosystems can influence agroecosystems. The structure and diversity of agroecosystems can influence the movement of wildlife between natural ecosystems and affect their use of such systems. Perennial vegetationally diverse ecosystems with complex structures can provide important habitats for many birds that are typically found in undisturbed ecosystems. (e.g. many traditional cacao and coffee systems are habitats for migratory and resident forest birds and these bird become important consumes of insect pests in the system)