Understanding Biodiversity: Case Study

In New Zealand about half of our land area is in arable, pastoral or horticultural use. Although the total area of New Zealand land in pasture has decreased since 1972, this period has also seen a significant intensification of agricultural land use. This shift has occurred as farmers respond to economic signals by converting suitable non-irrigated pasture, exotic forestry and existing farms into more intensive farm operations, generally dairy (4). Intensification of agriculture threatens both the environment and the sustainability of food production. The long-term detrimental environmental effects of agriculture generally go unmeasured, but they include:

• Loss of biodiversity and the vital functions it provides, through the increase in land area in monocultures
• A decrease in water quality as a consequence of fertiliser and pesticide use
• The loss of soil fertility
• Damage to landscapes
• Ultimately, negative impacts on human health

The impact on biodiversity in this context refers not just to the biodiversity of natural ecosystems but also the biodiversity of the agricultural land. Agricultural biodiversity consists of the “microbes, plants, and animals that provide ecosystem services such as nitrogen fixation, decomposition, facilitation of nutrient uptake by plants, pollination and pest control” (5). Intensification usually results in a loss of these ecosystem services.

Some ecosystem services may be provided by adjacent natural ecosystems, such as ladybirds (a predator of some agricultural pests) colonising farmland, but most are provided by the agricultural ecosystem itself. If the services are impacted on by farming practices, for example through misuse of pesticides, then the farmer must provide them artificially and at some cost to the farm, the environment and ultimately human health. These are called “external costs” by resource economists. External costs have traditionally not been recognised by either the farming community or society. Likewise, until recently, the environmental and societal impact of agricultural practices has had little impact on farmer choice of production methods (2).

Intensification of land use leads to simplified landscapes and a decline in the ability of the ecosystem to provide ecosystem services. However, services such as biological control of pests and diseases can provide economic benefit to both the farmer and to society as a whole. If we can improve the conditions for natural enemies, this has the potential to be economically and environmentally beneficial for both the farmer and society (through reduction in use of chemicals to control pests and diseases, with a corresponding reduction in costs and pesticide residues in the environment) (6). Such improvements can also provide eco-tourism opportunities and maintain or increase export markets.

Leafrollers are one of the major pests of grapevines in New Zealand and Australia. Leafroller refers to the larval or caterpillar stage (Figure 4) of a number of moth species. The larvae feed on grapes, flowers and stalks causing a reduction in yield. More importantly the damage caused by the larvae encourages the spread of a fungal disease, Botrytis cinerea or grey mould (Figure 5). This reduces grape yield but also affects the flavour and ageing of the wine.

Traditional control of leafrollers is through frequent spraying of insecticides. Sprays are applied when over 5% of the bunches of grapes have one or more caterpillars.

There are two problems with this approach. Notice how in Figure 4, the caterpillar rolls the leaf edges together and joins them with a web. This structure makes it very difficult for pesticide sprays to come in contact with the caterpillar. The second problem is that spraying results in pesticide residues in the grapes. Society is increasingly demanding fewer pesticide residues in food, which requires a different approach to controlling the caterpillars.

Research conducted by the Bio-Protection Research Centre is focusing on encouraging natural predators of leafroller to be present in the vineyard in numbers large enough to provide a natural control agent. This is achieved by increasing biodiversity through the introduction of specific plants that enhance the effectiveness of these natural predators. The research has centred around planting introduced annual species such as tansy leaf (Phacelia tanacetifolia), and buckwheat (Fagopyrum esculentum) between the rows of vines (Figure 8) and planting selected endemic New Zealand trees and shrubs within and around the edges of the vineyards. Introducing plants like these provides beneficial insects, such as parasitic wasps and hoverflies, with a reliable source of nectar and pollen, alternative food and shelter.

Agricultural ecosystems tend to be low in biodiversity, with simplified and less resilient food webs. Ecosystems in this state lack the food resources, such as nectar and pollen needed by the adult stage of the parasitoids that are important in controlling pests like leafroller. Parasitoids need access to sufficient and appropriate food to ensure survival and gain maximum reproduction (fecundity) (7). In order to ensure that natural biological control agents are as effective as possible we need to understand how increasing biodiversity improves the efficacy of the agent. This has been the focus of much of the research carried out at the Bio-Protection Research Centre. One study looked at the specific effects of providing alyssum flowers as a food source for the parasitoid D. tasmanica on its longevity and fecundity. The results are shown in Figure 9.

These results showed that providing D. tasmanica with access to alyssum flowers increased the lifespan and fecundity (reproductive capacity) of the parasitoid. An increase in longevity allows the parasitoids more time to attack their hosts, the leafrollers, therefore making them a more efficient biological control agent. The other interesting finding to come out of this research was, that in the absence of floral resources, the sex ratio of D. tasmanica became very male-biased. However, in the presence of alyssum flowers the sex ratio was closer to the ideal of 1:1. These results again show how the lack of appropriate floral resources in an agricultural ecosystem can affect the population dynamics and potentially the efficacy of natural predators (7).

The Double-Edged ‘Sward’: The Potential Disadvantage of Increasing Floral Diversity 

As you have seen, increasing floral diversity in agricultural ecosystems can have a beneficial effect on natural biological control agents. But other research suggests that increasing biodiversity can also benefit the pest the biological control agents are targeting. How can this happen? 

Think back to the discussion about insect life cycles. The majority of pests found in agricultural ecosystems are insects, so their adult stage also requires nectar and pollen. Consequently the nectar and pollen resources provided for the adult parasitoid may also be the type of resource that the pest likes, leading to increasing survival and fecundity of the pest. This can result in unintended enhancement of pest populations when floral resources are introduced to improve the effectiveness of natural enemies. 

One study (8) that looked closely at this compared the responses to different floral resources of two insects: the diamondback moth (Plutella xylostella), whose larvae is a major pest of Brassica crops in New Zealand, and its parasitoid Diadegma semiclausum. The longevity and fecundity of both the diamondback moth and the parasitoid were assessed using the following treatments: the flowering plants phacelia, bentham, buckwheat, and alyssum, plus diluted honey and water. The results are shown in Figures 10 and 11.

The practical application of this research can be seen in the Greening Waipara Project. This was established in 2005 by the Bio-Protection Research Centre, the Hurunui District council and Landcare Research. It aims to enhance the sustainability, biodiversity and marketing of winegrowing by giving vineyards “ecological makeovers”. Like other forms of modern agriculture, vineyards are also monocultures (Figure 12) with low biodiversity. Greening Waipara restores vineyard habitats by planting mainly introduced species such as buckwheat and phacelia, and natives among and near the vines. These species should provide nectar and pollen to support beneficial insects – those that control pests of the grapevines.

There are now 51 Waipara Valley properties including farms and other horticultural operations signed up to the project, their owners having seen how adding biodiversity to agricultural ecosystems makes them more sustainable, profitable and marketable. 

Four vineyards (Mud House Wines, Pegasus Bay Winery, Waipara Springs and Torlesse Wines) and the local primary school have each developed biodiversity trails, complete with information boards and an educational quiz for children. The trails wind through vines and native plants, leading visitors to areas where Greening Waipara is in action.