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Critical Thresholds Associated with Habitat Loss: a Review of the Concepts, Evidence, and Applications

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Critical Thresholds Associated with Habitat Loss: a Review of the Concepts, Evidence, and Applications Biol. Rev. (2010), 85, pp. 35–53. 35 doi:10.1111/j.1469-185X.2009.00093.x Critical thresholds associated with habitat loss: a review of the concepts, evidence, and applications Trisha L. Swift* and Susan J. Hannon Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 2E9 Canada (Received 6 July 2008; revised 30 June 2009; accepted 9 July 2009) ABSTRACT A major conservation concern is whether population size and other ecological variables change linearly with habitat loss, or whether they suddenly decline more rapidly below a ‘‘critical threshold’’ level of habitat. The most commonly discussed explanation for critical threshold responses to habitat loss focus on habitat configuration. As habitat loss progresses, the remaining habitat is increasingly fragmented or the fragments are increasingly isolated, which may compound the effects of habitat loss. In this review we also explore other possible explanations for apparently nonlinear relationships between habitat loss and ecological responses, including Allee effects and time lags, and point out that some ecological variables will inherently respond nonlinearly to habitat loss even in the absence of compounding factors. In the literature, both linear and nonlinear ecological responses to habitat loss are evident among simulation and empirical studies, although the presence and value of critical thresholds is influenced by characteristics of the species (e.g. dispersal, reproduction, area/edge sensitivity) and landscape (e.g. fragmentation, matrix quality, rate of change). With enough empirical support, such trends could be useful for making important predictions about species’ responses to habitat loss, to guide future research on the underlying causes of critical thresholds, and to make better informed management decisions. Some have seen critical thresholds as a means of identifying conservation targets for habitat retention. We argue that in many cases this may be misguided, and that the meaning (and utility) of a critical threshold must be interpreted carefully and in relation to the response variable and management goal. Despite recent interest in critical threshold responses to habitat loss, most studies have not used any formal statistical methods to identify their presence or value. Methods that have been used include model comparisons using Akaike information criterion (AIC) or t-tests, and significance testing for changes in slope or for polynomial effects. The judicious use of statistics to help determine the shape of ecological relationships would permit greater objectivity and more comparability among studies. Key words: critical thresholds, habitat loss, fragmentation, configuration, time lag, Allee effects, landscape, simulation, conservation, statistics CONTENTS I. Introduction ......................................................................................... 36 (1) Purpose and structure of review .................................................................. 36 II. Possible explanations for critical thresholds, and conservation implications .......................... 37 (1) Configuration effects at low habitat cover ........................................................ 37 (2) Allee effects at low habitat cover ................................................................. 38 (3) Time lags ........................................................................................ 39 (4) Habitat loss ...................................................................................... 39 III. Evidence for critical thresholds ...................................................................... 39 ∗ Address for correspondence: PO Box 668, Athabasca, Alberta, Canada T9S 2A6 E-mail: [email protected] Biological Reviews 85 (2010) 35–53 © 2009 The Authors. Journal compilation © 2009 Cambridge Philosophical Society 36 Trisha L. Swift and Susan J. Hannon (1) Simulation studies of critical thresholds .......................................................... 40 (a) Effects of simulated landscape characteristics on critical thresholds .......................... 40 (b) Effects of species characteristics on critical thresholds ........................................ 44 (c) Evidence for causes of critical thresholds in simulations ...................................... 45 (2) Small-landscape studies of critical thresholds .................................................... 45 (a) Evidence for causes of small-landscape thresholds ........................................... 45 (3) Large-landscape studies of critical thresholds .................................................... 45 (a) A comparison of landscape-level study designs ............................................... 45 (i) Fragmentation effects at low habitat proportion ......................................... 46 (ii) Allee effects .............................................................................. 46 (iii) Time lags ................................................................................ 46 (iv) Habitat loss effects alone, for certain response variables ................................. 46 (b) Evidence for presence and values of large-landscape critical thresholds ...................... 46 (c) Evidence for causes of large-landscape thresholds ............................................ 47 IV. General comparisons of simulation, small-landscape, and large-landscape study results ............. 48 V. Utility of critical thresholds in conservation .......................................................... 48 VI. Statistical considerations ............................................................................. 49 VII. Conclusions .......................................................................................... 50 VIII. Acknowledgements .................................................................................. 50 IX. References ........................................................................................... 50 I. INTRODUCTION also of conservation interest. For example, all else being equal an increase in the critical threshold level for a species’ abun- Numerous studies have documented the detrimental effects dance would be considered undesirable, since the steeper of habitat loss on various ecological responses (e.g. bird decline would begin ‘‘sooner’’ in relation to habitat loss. body condition: Burton et al., 2006; amphibian population declines: Cushman, 2006; plant reproduction: Aguilar et al., 2006). Recently, there has been growing interest in the (1) Purpose and structure of review shapes of these relationships. Do ecological responses change The purpose of this review is to present theoretical and linearly with habitat loss, or are there ‘‘critical threshold’’ empirical evidence for critical thresholds in species’ responses levels of habitat? A critical threshold is ‘‘an abrupt, nonlinear to habitat proportion in the landscape. We will discuss change that occurs in some parameter across a small range (a) possible explanations for critical thresholds, (b)evidence of habitat loss’’ (With & King, 1999b). The response variable for their occurrence and value in simulated and real land- undergoing this abrupt change may be individual behaviour, scapes, (c) the effect of species and landscape characteristics the abundance of a species, or community composition, on the existence and value of critical threshold levels, and among others. The key point is that the magnitude or slope (d) potential uses and misuses for critical threshold informa- of the relationship between the response and the amount tion in landscape management. The review will conclude of habitat in the landscape changes once that amount of with a summary of major trends and recommendations for habitat falls below a critical threshold level. For example, the future research. abundance of a species in a landscape may decrease more Included are studies for which the authors explicitly steeply with habitat loss once the amount of remaining habi- addressed the presence or absence of ‘‘critical thresholds’’, tat falls below some proportion of the total landscape area. ‘‘thresholds’’, ‘‘sudden changes’’, or ‘‘nonlinear’’ relation- For simplicity, this proportion will henceforth be referred to ships, as well as a few which only presented data suggesting as the ‘‘critical threshold level’’ or the ‘‘threshold level’’, and such relationships (e.g. data plots). Different authors seem to ‘‘habitat proportion’’ will mean the amount of habitat as a estimate exact critical threshold levels in different ways, or not fraction of total landscape area. at all. Therefore, when there was no formal statistical assess- The existence of critical thresholds in habitat proportion ment of critical threshold levels, we re-estimated them as is of conservation concern, because small additional losses either the point at which the slope changed (for sharp thresh- of habitat below the critical threshold level may lead to olds), the midpoint of the curve around which the slope abrupt population declines or other important ecological changed (for more gradual thresholds), or the range over changes. Unanticipated, such changes may preclude timely which the response value changed markedly (for categorical conservation measures. Such thresholds also indicate that data), rounded to the nearest 5% (Fig. 1). This was done to some other factor (such as fragmentation, see below) may make the results of different studies more comparable, not become substantive only below certain proportions of habi-
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