Biodiversity, Ecosystem Thresholds, Resilience and Forest Degradation

25 Biodiversity, ecosystem thresholds, resilience and forest degradation I. Thompson Following certain ecological orests comprise multiple ecosys- goods and services that are expected principles in management tems associated with variance in (e.g. FAO, 2009), the loss of biodiversity 4 Fedaphic and microclimatic con- is a key criterion for measuring forest ditions across broad landscapes. The degradation. Conserving biodiversity adaptation to climate change. composition and nature of forest eco- is a cornerstone of sustainable forest systems vary over time, depending on management (e.g. Montreal Process, natural disturbances and changes to the 2009) and a key to maintaining forest climate regime. However, they remain ecosystem functioning. more or less the same within the bounds This article explores the ways in which of natural variation (see Figure), referred forests maintain their stable states over to as a stable state. In a stable state, a time and outlines what happens when forest can produce a range of associated disturbances overwhelm the natural goods and services that humans value. mechanisms of recovery. It describes Biodiversity underpins most forest eco- how sustainable management of forests, system goods and services, and many including the conservation of biodi- tropical forests, in particular, maintain versity, is key to supporting a forest’s high levels of biodiversity. Loss of bio- recovery mechanisms, and presents eco- diversity may have considerable negative logical principles that can be applied to consequences for the productive capac- forest management. ity of forests (e.g. Thompson et al., 2009; Bridgeland et al., 2010; Cardinale et al., RESILIENCE AND RESISTANCE 2011) and for the provision of goods ] and services. Therefore, because forest An important characteristic of forests degradation can be defined as the loss is their resilience, which is the capacity of the ability of a forest to produce the to recover following major disturbances Illustration of tipping points, or thresholds, in ecosystems Pressures Tipping point Existing biodiverstity CHANGED STATE Actions to increase resilience SAFE Less diverse Fewer ecosystem OPERATING services Degradation of SPACE human well being Changed Ian Thompson is a research scientist, working biodiverstity for the Canadian Forest Service, in Sault Ste. Marie, Ontario, Canada. Source: Secretariat of the Convention on Biological Diversity, 2010. Unasylva 238, Vol. 62, 2011/2 26 (e.g. Gunderson, 2000). Under most caused by the loss of functional groups in large part, on these key species and natural disturbance regimes, forests (see Mechanisms and Tipping points) the functions that they perform rede- maintain their resilience over time. For- resulting from environmental changes veloping as a forest recovers following est resilience is an emergent ecosystem such as large-scale climate change, poor disturbances, including forest manage- property resulting from biodiversity at forest management or a sufficiently large ment interventions. multiple scales, from genetic to land- or continual alteration of natural dis- At a genetic level, the capacity for scape diversity (Thompson et al., 2009). turbance regimes (Folke et al., 2004). resilience comes from the ability of To sustain the goods and services that a species to persist over a range of humans derive from forests, forest eco- Mechanisms environmental variability, such as by systems must recover after disturbances There is strong evidence that forest resil- tolerating a range of temperatures or a and not become degraded over time. ience is tied to the biodiversity that certain level of drought. At the species Related to the concept of resilience normally occurs in the ecosystem (e.g. level, there are various behavioural and is resistance, which is the capacity of Folke et al., 2004; Thompson et al., functional responses that can assist a a forest to resist minor disturbances 2009). In particular, certain species and species to repopulate a disturbed area over time, such as the death of a few groups of species perform key functions or respond to environmental changes. trees or a chronic level of herbivory in forests and so are essential for the Further, ecosystem assembly processes by insects. Forests are generally stable forest to maintain all of its functional very much reflect the landscape pool of and change little as a result of non- processes (Díaz and Cabido, 2001). For available species (e.g. Tylianakis et al., catastrophic disturbances. Minor example, bird predation can maintain 2008), as well as landscape connectiv- changes are mitigated, such as when low abundances of insects in a forest, ity. At the landscape scale, heterogene- canopy gaps created by the death of reducing the possibility of catastrophic ity among forest patches can provide a individual or small groups of trees are levels of insect herbivory of trees, and measure of redundancy among species quickly filled by new young trees. For- thus increasing tree productivity (e.g. and a source for colonizers that, as a ests may also be resistant to certain Bridgeland et al., 2010). Pollinators, forest begins to redevelop or recover environmental changes, such as weather including some insects, bats and birds, after disturbance, should enable com- patterns over time, owing to redundancy are also excellent examples of highly munities to converge on the original among the functional species (redun- functional species in ecosystems, and forest types. Hence, the consideration of dancy refers to the overlap or duplica- without them, many plants could not resilience necessarily involves thinking tion in ecological functions performed reproduce. Forest resilience depends, from small to large scales. by a group of species; see Mechanisms) (e.g. Díaz and Cabido, 2001). Ecosystems may be highly resilient but have low resistance to a given disturbance. For example, many boreal forests are not especially resistant to fire, but they are highly resilient to it and usually recover fully over a number of years. Generally, most natural forests, especially primary old forests, are both resilient and resistant to various kinds of changes. Loss of resilience may be FAO/A. YANCHUK Hardwood forest composed primarily of trembling aspen in the boreal forest of northern Canada. Forests may not be particularly resistant to certain disturbances to which they are resilient Unasylva 238, Vol. 62, 2011/2 27 Degraded juniper (Juniperus thurifera) forest in the High Atlas, Morocco Loss of resilience and forest degradation An ecosystem state is defined by the dominant floristic (tree) composition and stand structure expected for a given stand. A change in forest state results from a loss of resilience, with a partial or complete shift to a different ecosystem type from what is expected for that area. Such changes in state result in a reduction in the production of goods and FAO/A. PERLIS services. Therefore, “change in ecosystem state” can be used as an indicator of degradation. For example, if a forest is expected to be of mixed species but is has been degraded can be determined Tipping points can be reached rap- instead dominated by only a few species, through remote sensing. Using satel- idly or as a result of chronic change or if it should be a closed canopy forest lite data, Souza et al. (2003) mapped that wears away the capacity of an eco- but is actually open or savannah, then forest in the Amazon region of Brazil system to recover, such as through the the state has changed. These would be that had been excessively burned or gradual attrition of species over time. considered negative changes in state, as heavily logged and burned, and Strand For example, forest fragmentation is a they degrade the forest, from a biodiver- et al. (2007) reported on several cases process that opens up continuous forests sity perspective and from a production in which remote sensing was used for through multiple disturbances. A forest perspective, and would generally affect monitoring forests affected by invasive can readily tolerate some loss of spatial the level of goods and services available. tree species and insects from several continuity and still maintain its species Often, the degradation of forests results regions of the world. and functions, but studies suggest that from the use of poor harvesting tech- certain levels of fragmentation are actu- niques over a period of time. However, TIPPING POINTS ally tipping points, with a resulting loss forests can also become degraded for Forests may not always recover after of forest biodiversity and f unction and a many reasons not involving logging. For severe and protracted disturbances. reduced capacity to produce goods and example, forests may appear intact but Thresholds exist for populations of indi- services (e.g. Andrén, 1994; Arroyo- be missing most large animal species as vidual species and for individual pro- Rodríguez et al., 2007). a result of over-hunting (e.g. Redford, cesses within ecosystems, and ultimately Ecosystems can be used and harvested 1992). As a result, there could be long- for the ecosystems themselves. The point for services, but the derivation of those term consequences for forest health at which the ecosystem loses its capacity services cannot exceed sustainable levels, because of increased insect herbivory to recover, or at which its resilience and nor can goods be removed in a manner that resulting from a lack of control by preda- integrity are lost, is referred to as a tip- destroys ecosystem processes (Figure). tors, or reduced seed dispersal, functions ping point, or an ecological threshold. If Once a tipping point is reached, changes that the missing animals might have per- there is too much disturbance, a cascade to the ecosystem are large and nonlinear, formed. Another example of degradation of effects with marked changes to the often unpredictable, and usually dramatic might be the successful establishment of forest ecosystem will result, ultimately (e.g. Scheffer and Carpenter, 2003). For an invasive species that out-competed moving the forest to a new state.

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