Ecological theory and concepts related to natural biological control in forests
The basic concepts underlying the population dynamics of forest insects involve top-down (natural enemies) and bottom-up (host plant) forces acting on the reproductive success and survival of insect populations. In addition, lateral forces (competition) significantly influence the population dynamics at high population densities (Martin et al., 2013).
Release from enemy pressure (top-down) is considered to be a common underlying factor that drives fluctuations in many forest insect populations (Berryman, 1996). The continuity or stability of enemy pressure affects the strength and regularity of this release, and is often linked to the number of enemy species involved; few enemy species often result in low stability in enemy pressure (Macfadyen et al., 2009). This is a concrete example of the diversity–stability theory, which suggests that more diverse ecosystems will be more stable or more resilient to perturbations (McCann, 2000) and it has been suggested that more diverse forest systems suffer less insect damage (Jactel et al., 2005). A relevant hypothesis within this theory is the ‘insurance hypothesis’, proposing that, because different tree species and different natural enemy species are differentially susceptible to disturbance, high diversity will maintain the overall functioning of an ecosystem even when biotic and abiotic conditions are temporally disturbed (Yachi and Loreau, 1999). From the perspective of the insurance hypothesis any kind of management that increases diversity could be useful to increase the resistance of a community or ecosystem.
To describe the potential benefits of a diverse plant community Root (1973) coined the term ‘associational resistance’, which is now a widely accepted ecological concept encapsulating plant diversity effects on herbivores and their natural enemies (Barbosa et al., 2009). Associational resistance basically consists of two hypotheses, one addressing the bottom-up processes (the resource concentration hypothesis) and the other addressing the top-down processes (the natural enemy hypothesis) both of which are likely to affect the success of a herbivorous insect in a heterogeneous habitat.
The resource concentration hypothesis suggests that plants can benefit from neighbours that are not of the same species, for example, by masking of host trees for potential herbivores, because increasing tree diversity will make it more difficult for the pests to locate their resource (Riihimäki et al., 2005; Vehviläinen et al., 2007; Castagneyrol et al., 2013). This ‘masking’ can involve chemical mechanisms such as confusion of olfactory stimuli (Jactel et al., 2011) or physical (Dulaurent et al., 2012) mechanisms with other plants functioning as barriers (Barbosa et al., 2009).
The natural enemy hypothesis suggests that the presence of flowering plants in a diverse forest provides additional resources like nectar or pollen that are likely to attract and support more natural enemies. Further, increased levels of alternative prey in diverse forests might increase the overall levels of natural enemies and therefore the predation pressure on the pest insect (Letourneau et al., 2009). Positive effects of understorey enrichment are found in cases showing increased parasitoid densities and parasitism rates (Cappuccino et al., 1999), higher predation rates in leaf miners (Riihimäki et al., 2005) and leaf beetles (Stephan et al., 2016), and increased availability of alternate hosts leading to increased parasitism rates (Maltais et al., 1989).
These ecological concepts and theories describe general patterns in ecology. Many of them have been successfully used to develop ‘conservation biological control’ methods in agricultural systems (Barbosa, 1998) and should be taken into account when designing forest management methods to improve forest health.
source : https://academic.oup.com/forestry/article/89/3/253/1749690/Forest-management-and-natural-biocontrol-of-insect