By Prof. Schalk vdM. Louw, Centre for Plant Health Management, University of the Free State, Bloemfontein, South Africa.
Agricultural landscapes are by implication complex adaptive systems, tailored by anthropogenic interference. The relationship between structure and function, e.g. trophic structures, diversity - productivity connections and nutrient fluctuation patterns of such landscapes is fundamental in their organization, whether self-driven or regulated. In cropping system landscapes, we are obligated to understand the processes that influence the abundance, richness and diversity of biota that impact on plant health and ultimately crop yield.
Crop health management therefore requires that, amongst others, an understanding is developed on how the architecture of landscapes influences pest population dynamics and their interaction with the surrounding environment, natural enemies and agents of control. Agricultural landscapes, on whatever scale, implies habitat fragmentation to a lesser or larger degree and we need to understand the implication of such events with regard to ecosystem processes.
Understanding the processes which drive agricultural landscapes, especially in semi-arid regions, is a prerequisite towards analysing patterns, which in turn lead to sound policy and management decisions. This article therefore makes the point that, in general, crop system analysis in the context of diverse agricultural landscapes across South Africa is lacking and needs to be addressed. The main research question derived from this, in turn, is in which manner spatial (i.e. shape, size and structure) and temporal (i.e. seasonality) features of agricultural landscapes and the crop value chains emanating from them, link to the population dynamics and trophic structure of insects and other biotic agents.
Understanding the population dynamics and trophic structure of insects in crop agroecosystems is vital in the sustainable management of such systems. However, this would be an incomplete picture of the actual scenario, should the diversity, identity and architecture of the particular landscape within which these systems fall not be taken into consideration. Bring into the equation the fact that agricultural landscapes are always under the pressure of anthropogenic influences at different scales and the issues suddenly become intricate. What follows below is a breakdown of the array of direct and indirect factors that influence the identity of agricultural landscapes within which crops are cultivated and which have a bearing on their value to the farmer/producer and ultimately the end-user.
A holistic approach
Fundamental would be that the management of landscapes influenced by humans be geared towards the enhancement of biodiversity and ecosystem function. This must be regarded in the context that agricultural landscapes in an area unavoidably result in fragmented habitats, which, in turn, are the drivers of complex localized processes and community structures. Biodiversity, as such, is an organizing principle in agroecosystem management, since it determines the degree of relative stability in a disturbed environment. Landscapes planning should therefore protect and enhance biodiversity and support ecosystem processes of succession, energy flow, and hydrological and nutrient cycling. In this context gaps should be bridged between agricultural policy, land-use and biodiversity indices. A further objective should be a marriage between the approaches of social scientists and ecologists when providing relevant advice.
Space and time are basic entities in ecosystem analysis. Thus, analysing landscapes and their biotic inhabitants in terms of spatial and temporal dynamics is crucial in providing an informed perspective. In this context landscapes patterns arise which originate from exogenous (e.g. climate) and endogenous (e.g. competition) processes and feedbacks.
A holistic approach to investigations of this nature is always a strong recommendation, since it provides insight into system self-organization. Any deviation, which is out of sync with the optimal functioning of the system, is an interference which could jeopardize sustainability. In harsher, semi-arid regions this could have far reaching effects with regard to establishing new/underutilised crops which are required to address different utilisation commodity groups.
Landscape fragmentation and insect population dynamics
There are implications of insect populations in habitat fragments. In such a scenario the sequence of events are: crop cultivation > natural habitat destruction > sharply contrasted habitat mosaics > threat to biodiversity > collapse of trophic structures. The implication of such a scenario is that populations of beneficial insects collapse, whilst detrimental / pest insects are boosted. In other words, fragmentation sensitive species could be influenced in terms of trophic position, population dynamics variability and vagility. There are also further ramifications that relate to pure habitat loss, fragment size and shape, fragment isolation, fragment quality, edge effect and landscape structure.
A further crucial landscape ecology issue exists in terms of the temporal frameworks link for above ground – below ground multifunctional linkages between organisms. This is not only an important barometer for soil health and ultimately crop health, but also for microbial-insect-plant interactions. Interaction strength levels, on the whole, matter very much in landscapes and determine their complexity. Overall the specific biome and regional setting of a particular landscape is the ultimate determining factor of soil and crop health integrity.
In fragmented landscapes habitat edge effects (i.e. greater species richness and abundance in the transitional zone between habitats than in the habitats themselves) also reveal interpretive complications and may be the foremost explanation of many of the negative effects arising from fragmented habitats. One such negative effect is that habitat edges can serve as ecological traps, whereby an anthropogenic change in an organism’s environment leads individuals to use misleading cues of habitat quality. Specifically the size and internal habitat of a fragmented patch and its relationship with ‘edge effect’ needs further investigation. Fragmentation affects populations along a number of routes that relate toedge effect, i.e. area identity, degree of isolation and area age.
Soil quality and the role of bioindicators
Another important factor in agricultural landscapes is soil quality. Insect herbivory, as part of the trophic structure of a particular landscape, increases litter quality and decomposition indices and as such influences ecosystem nutrient cycling and accelerates the production of soil organic matter. It also acts as a persistent control mechanism for ecosystem processes.
If bioindicators can be identified in a system in this context and used to indicate disturbances in an environment, the relevance emanating from this could be significant. In terms of sound management strategies, analysing insect structures in heterogeneous environments (landscapes) to determine optimal diversification in systems, can act as an important guidance towards establishing environmentally acceptable pest management strategies.
Ecosystem services are the benefits that humans (society) obtain from ecosystems. Agricultural landscapes and their soils also provide multiple ecosystem services, the successful management of which depends on the recognition of the relationships between the processes and the structures that maintain a healthy system. Although science continuously yields new environmentally friendly agricultural practices and techniques, the sustainable development of a system will ultimately depend on a farmer’s ability to understand and utilize these advances.
For more on soil health in South Africa and the Soil Ecosystem Research Group (SERG) go to www.sergsa.org.