The equilibrium theory of island biogeography was developed by

By Tom Hewitt

Abstract:

As Earth’s human population continues to grow along with our per capita consumption, the area of land available for nature conservation is being reduced. This leaves us to question how best to utilise the land available to maximise conservation benefits and preserve the greatest number of species. The equilibrium theory of island biogeography (ETIB) proposed by MacArthur and Wilson (1967) attempts to assist us in this endeavour by theorising which geometric features allow for the greatest overall species richness in a habitat. Namely that larger habitats and habitats located close together will hold the greatest number of species. Although (ETIB) has received varying degrees of empirical support since it was first proposed but has nevertheless been extremely influential in the field of conservation biology and still provides us with an important theoretical framework to guide future conservation.

Introduction:

The Equilibrium Theory of Island Biogeography (ETIB) first proposed by MacArthur and Wilson (1967) concerns the effect of island size and distance on the number of species residing on an island. Under this theory species richness is pushed towards equilibrium by the rate of species extinction and colonisation, as species richness increases the rate of colonisation declines and extinction increases until both are in balance shown in figure 2. It argues that larger islands can support a higher number of species than smaller islands, referred to as the species area relationship and that isolated islands receive fewer immigrants than islands closer together, referred to as the species distance relationship.MacArthur and Wilson saw islands as “natural laboratories” and believed that these principles could be adapted and provide insight on the formation of ecosystems. Based on these principles Diamond (1975) came up with a set of rules for the design of nature reserves shown in figure 1, he stated that reserves should be as large as possible (A), closer together (C), with many dispersal corridors (E) and roughly circular with the least amount of edge possible (F). These principles have been highly influential in conservation and utilised extensively in the design of nature reserves particularly in the 1980’s however the empirical nature of such principles has been questioned (Higgs,1981; Sarkar, 2012).

Figure 1 Key principles of MacArthur-Wilson, Islands hold species in equilibrium due to the balance of extinction and immigration (colonisation), Image adapted from Warren et al (2015)

Figure 2 Diamond’s (1975) principles for reserve design, Image adapted from Diamond (1975)

Criticisms:

There are many valid criticisms of the ETIB and to what extent it can be used to inform conservation. One of the most obvious problems is that nature reserves are rarely functionally equivalent to islands, species utilise land outside of reserves to varying degrees (Itescu, 2018), and conservation practice must incorporate management of the surrounding landscape when considering biodiversity. There is a varying amount of empirical support for the principles put forth in the ETIB and by Diamond (1975). Whilst the is strong empirical support for species area and species distance relationship, the assumption of an equilibrium state and Diamond’s suggestion that circular habitats hold more species than elongated ones are less well supported and appears to vary according to species (Fattorini, 2016).

Arguably treating conservation land as if they were islands has resulted in a fixation on reservation type ecology focussing on preserving large areas of habitat in what has sometimes been referred to as fortress conservation (Goldman, 2009). This has potentially played a part in the neglect of other more marginal habitat types and reconciliatory conservation in the wider landscape, leading to a self-fulfilling prophecy where nature reserves become increasingly biologically isolated from their surroundings which are increasingly neglected. Ecological theories such as the ETIB can be highly useful for helping us to understand ecological functioning, however they are rarely perfect, and we must be cautious when applying them to the real world.

Usefulness:

Despite its shortcomings the principles of ETIB can provide a useful framework for conservation management in some situations. In the urban environments area of vegetation are much more like islands in that they are surrounded by a “sea” of concrete which prevents the dispersal of many species, particularly ground dwelling invertebrates and natives (Fattorini, 2016). Island biogeographic principles can therefore inform the location of green spaces in urban environments, placing new green space next to existing green space for example elevates the potential species richness of both habitats. This can even be applied at micro scales with features such as green roofs, lawns, gardens, roadside verges and street trees providing habitat and increasing the species richness of the other nearby habitats (Fattorini et al, 2018).

In a New Zealand context, the ETIB could potentially have important implications in the creation of mainland islands free of mammalian predator species. New Zealand currently has several mainland islands which areas are where predators are eliminated or controlled to allow for the survival of indigenous species such as Kokako and Kiwi which otherwise cannot establish amongst the “sea” of predators (Russel et al, 2015). These areas of predator control can be highly isolated from one another and rarely do individuals disperse between these islands without the assistance of humans. With new techniques the eradication of mammalian predators is increasingly achievable across larger areas and with reduced labour cost. However, it is not possible to eradicate predators entirely and we must make decisions about which areas to focus our efforts to promote the greatest recovery of indigenous species. Island biogeographic principles should be used to guide the location of these new pest control projects, this means expanding existing projects to create larger continuous areas of habitat and locating new projects close to those which already exist. This will help facilitate the dispersal of desired colonists from nearby reserves. The appearance of Duncan the Kokako in the predator abundant  Auckland suburb of Glendowie in 2011, having travelled 31 km from his home in the Waitakere ranges (Department of Conservation, 2013), demonstrates the capacity of even poor dispersers to reach new habitat. Many new projects based on such principles are beginning to take shape such as the North-West Wild link north of Auckland which attempts to establish a network of pest free habitats to facilitate the recovery of native bird species.

Conclusion:

Inevitably the relevance of the ETIB to conservation depends on the specific nature of the habitat being managed, and it is at best a simplification of underlying mechanisms which determine species richness. Although not all the principles of the ETIB have proven to be as useful, the species area relationship and the species distance relationship were important insights and have been highly influential in modern conservation with implications for conservation work today. These principles can be particularly useful in the fields or urban ecology and invasive species management in countries such as New Zealand. ETIB should continue to be drawn upon to inform ecosystems management but in doing so we should recognise the limitations of such theories and ensure that our decision making is based on real world considerations and on understanding the unique elements of the ecosystems and species we manage.

Bibliography:

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Department of Conservation. (2013) The adventure of Duncan the Kokako. Retrieved from: https://blog.doc.govt.nz/2013/05/30/kokako-duncan/

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