Diversity and Distributions, (Diversity Distrib.) (2015) 21, 534–547

BIODIVERSITY Historical legacies accumulate to shape RESEARCH future biodiversity in an era of rapid global change Franz Essl1,2,3*, Stefan Dullinger3, Wolfgang Rabitsch2, Philip E. Hulme4, Petr Pysek5,6, John R. U. Wilson1,7 and David M. Richardson1

1Centre for Invasion Biology, Department of ABSTRACT Botany and Zoology, Stellenbosch University, Aim Biodiversity responses to changing environmental forcing on are Private Bag X1, Matieland 7602, South 2 often characterized by considerable time-lags (= relaxation times). Although Africa, Environment Agency Austria, Spittelauer L€ande 5, 1090 Vienna, Austria, changes to the occurrence and abundance of species likely have cascading 3Division of Conservation Biology, effects (e.g. on species of other trophic levels, genes, community structure and Vegetation and Landscape Ecology, Faculty ecosystem processes), current concepts addressing lagged biodiversity responses Centre of Biodiversity, University of Vienna, are limited to single drivers affecting a few biodiversity components (e.g. Rennweg 14, 1030 Vienna, Austria, 4The debt in terms of species numbers or population size). Little attention Bio-Protection Research Centre, Lincoln has been paid to the interacting and cumulative nature of time-lag phenomena.

A Journal of Conservation Biogeography University, PO Box 84, Canterbury, New Here, we synthesize current knowledge, mechanisms and implications of Zealand, 5Department of Invasion Ecology, delayed biodiversity responses and propose a ‘cumulative biodiversity lags- Institute of Botany, Academy of Sciences of framework’ which aims to integrate lagged responses of various components of the Czech Republic, 252 43, Pruhonice, biological organization. Czech Republic, 6Department of Ecology, Location Global. Faculty of Science, Charles University in Prague, Vinicna 7, 128 44 Praha 2, Czech Results Effects of change in environmental forcing are transmitted along a ser- 7 Republic, South African National ies of linked cause–effect relationships which act on different biodiversity com- Biodiversity Institute, Kirstenbosch Research ponents (e.g. individuals, populations, species, communities). We show that Centre, Claremont, 7735, South Africa lagged responses to environmental forcing are caused by different mechanisms (e.g. metapopulation dynamics, dispersal limitation, successional dynamics), which operate sequentially on these intermediary links. Lags manifest them- selves on the respective biodiversity component which changes over time; the full relaxation time of a focal system will therefore depend on the aggregate length of different lags. We elucidate key mechanisms and circumstances which are likely to cause cumulative lagged responses, and propose research avenues to improve understanding of cumulative biodiversity lags.

Main conclusions The failure to give adequate consideration to widespread cumulative time-lags often masks the full extent of biodiversity changes that have already been triggered. Effects that are particularly relevant for human livelihoods (e.g. changes in the provision of ecosystem services) may emerge with the most pronounced delay. Accordingly, the consideration of appropriate temporal scales should become a key topic in future work at the science–policy and Distributions *Correspondence: Franz Essl, Centre for interface. Invasion Biology, Department of Botany and Keywords Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa. Biological invasions, , framework, global change, invasion debt, E-mail: [email protected] management, thresholds, time-lags.

biodiversity (Rockstrom€ et al., 2009; Butchart et al., 2010; BIODIVERSITY DYNAMICS AND TIME-LAGS Devictor et al., 2012), and the many services derived from

Diversity Humans are modifying the biosphere at a global scale. ecosystems (Hooper et al., 2012; Hobbs et al., 2013). It is The concomitant changes increasingly impact negatively on well known that responses to changing environmental forcing

DOI: 10.1111/ddi.12312 534 http://wileyonlinelibrary.com/journal/ddi ª 2015 John Wiley & Sons Ltd Historical legacies shape biodiversity changes on species are often characterized by considerable time-lags population sizes and extent (Fig. 2a,b). If average effects on (= relaxation times) (e.g. Tilman et al., 1994; Kuussaari individual fitness are negative, populations will decline and et al., 2009; Essl et al., 2011; Devictor et al., 2012; Dullinger might eventually be driven to . Lagged popu- et al., 2012, 2013; Gilbert & Levine, 2013). Such responses lation-level responses will occur if (1) stochastic will likely also trigger effects on other biodiversity compo- of small populations are not immediate and (2) metapopula- nents (e.g. other trophic levels, genes, community structure tions survive long after connectivity has decreased (if coloni- and ecosystem processes). The possible additional delays zation–extinction dynamics are slow) (Hylander & Ehrlen, before these subsequent accumulating effects become appar- 2013). Progressive isolation and loss of populations may ent are much less well understood (e.g. Galloway et al., 2003; finally drive species towards regional extinction. Changing Galetti et al., 2013; Svenning & Sandel, 2013). Delays environmental forcing may also have positive impacts on between the start of the influence of drivers of events and individual fitness of other species whose populations will the unfolding of their full, complex effects can be consider- consequently increase, often leading to range expansions. able. This means that the full extent of biodiversity changes Again, population expansion into new geographic or ecologi- assessed at any given time might be substantially underesti- cal space will be associated with temporal delays (Jackson & mated. The true cause of changes, and ecosystem degrada- Sax, 2009; Svenning & Sandel, 2013). The direction of tion, may also be masked. Such cumulative effects are response of particular species will be shaped by the interac- increasing in importance in the current era of rapid and per- tion of their traits and niche requirements with the rate and vasive environmental change. As a corollary, if present-day magnitude of environmental change and the responses of biodiversity patterns are in disequilibrium with current con- populations of other species to the same forcing. ditions, interventions responding to apparent drivers of Rapid environmental change may, however, trigger further change will, in many cases, be insufficient (or indeed may be effects on biodiversity, with each subsequent change intro- entirely inappropriate) for stemming further biodiversity loss. ducing additional delays (Fig. 1b,c). First, direct impacts will Consequently, the capacity to understand and steer current trigger responses of dependant species (e.g. hosts, mutualists, biodiversity changes (e.g. by increasing resilience and mana- predators, parasites) (Fig. 2c,d). These will in turn affect ging risks, Seidl, 2014) may be severely compromised. other species (e.g. through competition) such that impacts Here, we synthesize current knowledge, mechanisms and on species accumulate at higher hierarchical levels of biodi- implications of what we call cumulative biodiversity lags and versity (i.e. communities and ecosystems, Svenning & Sandel, discuss the implications of the nature of delayed biodiversity 2013), thereby mediating interactions between species, flows responses. and subsequently stocks of energy and matter, and physical properties of ecosystems (Isbell et al., 2011; Hooper et al., 2012). Simultaneously, through altered selection, individual- CUMULATIVE BIODIVERSITY LAGS: level impacts trickle down to lower hierarchical levels of bio- INTRODUCING A FRAMEWORK THAT diversity (e.g. genetic diversity), affecting composition and ACCOUNTS FOR ACCUMULATING DELAYED spatial structure of the gene pool (Balint et al., 2011). The RESPONSES interaction of these biodiversity responses will affect ecosys- Effects of change in environmental forcing are transmitted tem functioning, and the provision of ecosystem services along a series of linked cause–effect relationships on different (Hooper et al., 2012), with feedbacks that will affect the fit- biodiversity components. We argue that lagged biodiversity ness of individuals. Finally, changes in different biodiversity responses are caused by different mechanisms (e.g. metapop- components may trigger societal responses and adaptations ulation dynamics, dispersal limitation, successional dynamics, (e.g. conservation or restoration measures), aimed at reduc- soil development; Hanski & Ovaskainen, 2002; Hylander & ing or preventing negative impacts. Ehrlen, 2013; Svenning & Sandel, 2013) which operate at Thus, the full relaxation time a focal system is committed these intermediary links. Each of these mechanisms takes its to consists of the cumulative, yet interconnected, relaxation own specific and varied time to unfold. Taken together, each times of the responses (Galloway et al., 2003) (Fig. 3). Attri- of them hence contributes to a cascade of lagged responses butes which modify the length of relaxation times are, for at the subsequent biodiversity components (Appendix 1). example, degree of specialization, longevity, dispersal capac- Changes in environmental forcing will initially affect indi- ity, and trophic position for species (Allendorf & Hard, viduals of a few particular species, namely those which are 2009; Kuussaari et al., 2009; Krauss et al., 2010; Cousins & most sensitive to the forcing (e.g. some plant species will Vanhoenacker, 2011; Ledger et al., 2013); generation time, respond directly to eutrophication, Fig. 1a). More specifi- population size and mating system for genetic diversity cally, the forcing will modify the fitness of the individuals (Balint et al., 2011); and functional traits of keystone species directly affected (i.e. their success in passing genes to the as well as the levels of redundancy or complementarity in next generation). These changes might be associated with species interactions and ecosystem function (Schweiger et al., lags, for instance due to survival of resistant life cycle stages 2010) for community properties. after habitat quality change (Hylander & Ehrlen, 2013) and For conceptualizing this phenomenon of multiple and will scale up to altered population dynamics thereby affecting accumulating relaxation times, we propose the term

Diversity and Distributions, 21, 534–547, ª 2015 John Wiley & Sons Ltd 535 F. Essl et al.

(a)

(b)

(c)

Figure 1 Conceptual model of the cumulative biodiversity lags-framework. Effects of changing environmental forcing are transmitted along cause–effect relationships (links are indicated by arrows) on biodiversity components. Responses at the level of individuals and populations are shown for a hypothetical species (in green) (a). Pressures impact first on fitness of individuals, scaling up to populations and then to the full range of the species, which may cause positive (range expansion, new populations in old range, in blue) and negative (range retraction, loss of populations in old range, in red) responses in different parts of its range across ecological gradients (e.g. climate). Impacts on species accumulate at higher (community) and, via altered selection, on lower (genetic) levels of biodiversity organization (b). Together, these changes will modify the provision of ecosystem services. Finally, changes in different biodiversity components may trigger societal responses aimed at reducing negative impacts or at increasing adaptive capacity. Lagged direct and indirect responses of different biodiversity components to pressures thus follow a hierarchy of biodiversity changes (c). Lagged responses [as shown in (a) and (b)] can occur at each of the intermediary links and will manifest themselves at the subsequent biodiversity component. Consequently, responses of biodiversity components at higher levels in the hierarchy of biodiversity responses will occur later, as they accumulate lagged responses of previous cause–effect reactions. Yellow font (a) indicates where pre-existing thematic concepts of delayed biodiversity response apply. For simplicity, feedbacks (e.g. changes to biodiversity that can result in changes to drivers) are not shown.

536 Diversity and Distributions, 21, 534–547, ª 2015 John Wiley & Sons Ltd iest n Distributions, changes. and little Diversity trigger relatively will be thresholds, will is of there equilibrium crossing thresholds, abundance concomitant such new the below a and and quality, feedbacks above in habitat amplifying whereas time or However, changes, relaxation quantity off. rapid Thus, habitat paid and parasites. in been for large and loss has prerequisite particularly predators a debt a of After extinction is immigration (e) the plants the levels. after of for trophic available, reached immigration necessary across becomes The turn increases patch times. in system habitat relaxation is the focal new different presence to the a show whose responds After will herbivores, species (d) levels species of partner time. trophic on immigration their relaxation or depend the when full their groups that paid the pay functional but be increase will different change only will change of environmental will debt environmental species the debt extinction the by This indirect by affected debt. Thus, affected directly extinction forcing. directly not indirect environmental system Species experience (c) focal reached. will time. the is debt relaxation in equilibrium extinction longer species new experiencing a the a gene and quality, until different equilibrium habitat off with abundance or debt sources higher quantity extinction different a habitat if from to in However, immigration leading through loss off. to increased, (e.g. a paid due be species After been admixing may focal has the fitness a deficit by changes), for immigration caused evolutionary available the (e.g. pools,?or? becomes after immigration patch reached if during lower habitat is However, increases to new equilibrium paid. diversity leading a abundance been genetic reduced, After new event has further (b) a forcing debt be time. a succession), extinction may relaxation Following restoration, the fitness longer high. habitat after erosion, a is reached genetic to patch is to and habitat equilibrium leads equilibrium a new connectivity abundance in a metapopulation species quality, and a or size of quantity population abundance habitat declining equilibrium, in At reduction (a) a system. causes study that a within richness species and 2 Figure ehnsso uuaielg netntosadimgaina h ee fmtpplto audneo n oa species) focal one of (abundance metapopulation of level the at immigration and extinctions in lags cumulative of Mechanisms c (d) (e) (b) (c) (a) 21 534–547, , Exncon ª 05Jh ie osLtd Sons & Wiley John 2015

Reacon at threshold Immigraon itrcllgce hp idvriychanges biodiversity shape legacies Historical

Species diversity Populaons 537 F. Essl et al.

Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 (a1) (b)

Critical load

(a2) (c)

Critical N leaching

(a3) (d) (e) (f)

Critical response Biological responses Chemical responses Nitrogen deposition

t1 t2 t3 t3 t3 t4 t5 t6 t6 t6

DDT RDT

Figure 3 Schematic representation of cumulative biodiversity lags caused by eutrophication. Nitrogen deposition increases (a1, b) until it reaches a critical threshold at which point the soil nitrogen content (a2, c) starts increasing rapidly. This causes a cascade of delayed effects on the biotic components (a3) – plants (d) show a physiological response to the changes in soil chemistry (black line in a3), the change in plant tissue chemistry and plant species composition affects the herbivores (e) (dark grey line in a3) and then affect higher trophic levels (f) (light grey line in a3). This can create a time-lag between each subsequent component losing its resilience, the Damage Delay Time (DDT, red shades); and similarly once nitrogen deposition is reduced there can be delayed recovery, that is the Recovery Delay Time (RDT, green shade). Note also that functional groups can be both directly affected and indirectly affected (in this case soil organisms in particular) complicating the measurement of DDT and RDT. Modified from Posch (2003). © F. Essl (1x), Wikimedia Commons (4x).

‘cumulative biodiversity lags’. We suggest linking this frame- to a quick recovery of declining native plant species (Shaw work to the DPSIR conceptual model (EEA, 1999) which dis- et al., 2011) and, similarly, indirect responses (e.g. changes tinguishes root causes or ultimate drivers (D) from in ecosystem structure, reduced erosion) may occur rapidly. immediate or proximate pressures (P) (e.g. changes in bio- Such rapid responses are most likely to occur when the physical processes) that affect different components of biodi- responding biodiversity components are still present in the versity (i.e. states, S), thereby creating impacts (I), which system, and when delaying indirect effects on the response of may trigger societal responses (R) (Fig. 1b). It thereby links the focal biodiversity component (e.g. top-down feedbacks, the biophysical to the socio-economic sphere. cf. Bergstrom et al., 2009) are of little importance. Although we argue that accumulating time-lags will prevail in many (if not most) cumulative biodiversity responses, we Integrating pre-existing concepts of biodiversity acknowledge that indirect effects of changing environmental changes forcing are not necessarily associated with increasing lag times (e.g. populations or species experiencing losses in The understanding of delayed biodiversity changes has increased abundance may lose dependent species in advance of full rapidly over the last two decades. However, currently available range losses). In addition, it has been shown that the concepts apply only to certain mechanisms (e.g. extinction debt) removal of introduced alien herbivores on islands may lead of delayed responses of one biodiversity component (e.g. spe-

538 Diversity and Distributions, 21, 534–547, ª 2015 John Wiley & Sons Ltd Historical legacies shape biodiversity changes cies) in isolation. Little attention has been paid to the interacting periods and at large spatial scales. Climate change provides a and cumulative nature of time-lag phenomena. valuable example. Increases in energy consumption bring The extinction debt-concept describes the likely relaxation about rising emissions of greenhouse gases which translate, time between a change in environmental forcing and the with considerable delays, into a new equilibrium of atmo- resultant extinction of species. When first proposed, this spheric greenhouse gas concentrations (IPCC, 2013). Major concept dealt with loss and fragmentation of habitats (Til- elements of the climate system respond to changing atmo- man et al., 1994), but subsequent work has expanded the spheric greenhouse gas concentrations with considerable pressures considered and has greatly improved the under- delays ranging from decades (e.g. atmospheric temperatures, standing of this phenomenon (e.g. Hanski & Ovaskainen, precipitation, ocean acidification) to centuries (e.g. polar gla- 2002; Allendorf & Hard, 2009; Kuussaari et al., 2009; Cous- ciers, sea level) (IPCC, 2013). Similarly, adjustments in spe- ins & Vanhoenacker, 2011; Dullinger et al., 2013; Gilbert & cies distributions and abundances track the lagged change in Levine, 2013). In a recent conceptual development, Hylander climatic space with further delay (Thomas et al., 2004; Loarie & Ehrlen (2013) showed that multiple processes operating et al., 2009). Even the current moderate pace of climate on individuals and metapopulations contribute to delayed change causes delayed range adjustments at large (Devictor extinctions. Proven relaxation times at local scales are often et al., 2012) and small (Bertrand et al., 2012; Dullinger et al., in the range of decades to more than a century (Allendorf & 2012) spatial scales. Rates of range shifts that will be required Hard, 2009; Kuussaari et al., 2009), especially for species in the future to track the velocity of predicted climate change near their and for specialized, long-lived are likely to be much higher (Loarie et al., 2009; but see taxa (Hanski & Ovaskainen, 2002). Chen et al., 2011; Hulme, 2014). Evidence suggests that Jackson & Sax (2009) introduced the term immigration many temperate plant species were not able to fully colonize credit for delayed species range gains and immigrations fol- suitable habitats during the millennia following the last glaci- lowing a forcing event, and Essl et al. (2011) used the term ation and population ranges are still expanding, that is the invasion debt for a related phenomenon, that is delayed system has not yet relaxed (e.g. Svenning & Skov, 2007). invasion of alien species into new territories. Studies of Ecosystem-level responses to changing climate will follow lagged responses related to alien species invasions have species range changes with further delay (Svenning & Sandel, shown that such phenomena play a key role at all invasion 2013), for instance the emergence of old-growth stands of stages (sensu Blackburn et al., 2011). For instance, time-lags boreal and temperate forests (including dead wood, hollow between alien plant introduction and first record of it escap- trees) needs several centuries after tree species have colonized ing from cultivation are on average almost 150 years for a region (Fig. 4). As effects on communities, ecosystem long-lived woody species in Central Europe (Kowarik, 1995) properties and services often depend less on the presence of while the delay between naturalization and population species than on their abundance (Hooper et al., 2012; Sven- expansion is often > 50 years (Aikio et al., 2010). Finally, ning & Sandel, 2013), the consequences of delayed species subsequent range filling may take many decades (Gasso responses will only become fully realized with cumulative, et al., 2010; Svenning & Sandel, 2013). Similarly, to describe yet interconnected lags (Strayer et al., 2006; Hulme et al., delayed species range and abundance shifts due to changing 2013). climate, Devictor et al. (2012) coined the term climatic debt. Changes in communities subjected to incremental environ- Interestingly, most studies only address lagged responses mental forcing unfold slowly, and initially, when the signal of biodiversity components on species. One of the few stud- of change is small, are inherently difficult to detect (Scheffer ies addressing genetic diversity is the work by Helm et al. et al., 2009; Hughes et al., 2013). For instance, the capacity (2009), who found evidence that changes in European land of species involved in specialized mutualistic or antagonistic use over the past century have affected the genetic diversity interactions to track forcing events is limited by the slowest of a grassland plant species (Briza media). The paucity of responding partner. Pathogens and parasites will only be able such studies addressing lagged responses in gene frequencies to colonize new space after host population density or size is probably because genetic change is much less immediately thresholds have been reached in the new range (Keesing apparent, although some direct effects have been clearly et al., 2010; Britton, 2013). Consequently, additional lags are demonstrated (Skelly et al., 2007; Richmond et al., 2009; to be expected for the full establishment of ecological inter- Pauls et al., 2013), for example the evolution of smaller body actions. For instance, Pysek et al. (2011) showed that alien size in response to size-selected harvesting (Allendorf & species in Central Europe co-opt pollinators of native flora Hard, 2009). Similarly, lagged responses of communities and and accumulate insect pollinators with increasing residence ecosystems are particularly difficult to measure and quantify. time. Similarly, European species introduced to North Amer- ica have accumulated pathogens over centuries (Mitchell et al., 2010). Moreover, most of the substantial and increas- The evidence of cumulative biodiversity lags ing impacts on resident communities caused by alien species Cumulative biodiversity lags unfold over long periods, invasions world-wide (Vila et al., 2011; Hulme et al., 2013) especially where changes in environmental forcing are are density-dependent, and many, if not most, alien species incremental (Hughes et al., 2013) or operate over long have not spread to their limits in their novel ranges (Gasso

Diversity and Distributions, 21, 534–547, ª 2015 John Wiley & Sons Ltd 539 F. Essl et al.

Equilibrium ImmigraƟon credit Abundance lag Ecosystem funcƟoning and structure lag

Lag Ɵmes – Several decades to several millenia Several decades to several centuries c. 100 to > 1000 years

Mechanisms – Dispersal limitaƟon, CompeƟƟon, metapopulaƟon Successional dynamics compeƟƟon dynamics (a) (b) (c) Soil formaƟon: c. 100 to >1000 years

Carbon sequestraƟon: Several 100 years Occupied Occupied Unsuitable Unsuitable Abundance

Newly occupied Newly VegetaƟon-dependent ecosystem structures: Newly suitable & - Old trees: several 100 years Years unoccupied - Coarse woody debris: several Climate change 100 years -CaviƟes for nesƟng sites: c. 100 to several 100 years

Figure 4 Schematic representation of cumulative biodiversity lags of vegetation dynamics under climate change in temperate forests (with a focus on leading range edge dynamics). Under stable climate, tree species distributions are in climatic equilibrium (a), that is all climatically suitable regions are occupied, and abundance within sites is in climatic equilibrium. Under climate change, climatically suitable regions are shifting (b). Tree species are colonizing climatically suitable regions with delay, with the large variation in lagged immigration depending mostly on the distance to source populations, species traits and interactions with the resident community. Once tree species have colonized a region, reorganizations in the abundance of tree species within the communities occur (c) (each line represents a tree species in eastern North America, Pacala et al., 1996). It may take several centuries before equilibrium is reached, as shown, for example by forest-gap models. Changes in the composition and abundance of tree species have cascading effects on ecosystem functioning (e.g. soil formation, carbon sequestration) and structure (e.g. vegetation-dependent habitat structures). Note that each biodiversity change and concomitant lag is highly dependent on the change at the preceding step in the hierarchy of change, for instance, soil formation (e.g. as a consequence of the immigration of broadleaved at the expense of conifer tree species) will only fully be triggered when tree species composition and abundance have reached equilibrium. For each stage of biodiversity change, the most important mechanisms causing lags and typical relaxation times are given. Items (a) and (b) were modified from Svenning & Sandel (2013); data on relaxation times are taken from the same source. et al., 2010), so the full scale of impacts is likely yet to be (Dunne & Williams, 2009; Potts et al., 2010). More rarely, realized (Byers & Goldwasser, 2001). The invasion of alien delayed co-extinctions may occur when the dependant spe- species may have positive feedbacks via interspecific facilita- cies has a persistent life cycle stage, as is often the case in tion on the abundance of species which had invaded the sys- island vascular plants that have lost their specialized pollina- tem earlier, a phenomenon termed invasional meltdown tors but which may still survive for decades as adult plants (Simberloff & Von Holle, 1999). There is evidence that such after reproduction has ceased (Koh et al., 2004). responses do occur with substantial delays of several decades (Grosholz, 2005). Regime shifts and nonlinear behaviour in complex However, indirect effects of changing environmental forc- ecological systems ing are not necessarily associated with increasing cumulative lag times. This is particularly the case when indirect Processes in complex systems under changing conditions responses are fully realized in advance of direct responses, often exhibit nonlinear behaviour, for example, due to and when the relaxation time of indirect responses is short amplifying interacting feedback loops and multiple causalities compared to that for direct responses. For instance, popula- (Stallins, 2006; Brook et al., 2008). In practical terms, this tions or species experiencing losses in abundance will lose means that a small perturbation could cause a large effect, a dependent species (e.g. co-extinction of specialist parasites, proportional effect or no effect at all (Scheffer & Carpenter, Koh et al., 2004; Dunne & Williams, 2009) and density- 2003). Such nonlinearity will mostly be restricted to specific dependent functional roles in ecosystems (Anderson et al., regions or rates of change that are close to thresholds or tip- 2011) in advance of full range losses. Theoretical consider- ping points (Hanski & Ovaskainen, 2002; Brook et al., 2013). ations and available evidence suggest that linked extinctions, Often, there is a significant time-lag between the dynamics which had been proposed as one of the main mechanisms of the drivers and the expression of impacts, causing great contributing to increased extinction rates (Diamond, 1984), difficulties in ecological management (Leadley et al., 2010). indeed play a prominent role (Dunn et al., 2009; Hylander & The importance of nonlinear behaviour should not be under- Ehrlen, 2013), particularly for specialists (Dunn et al., 2009; estimated, and it has substantial consequences for the detec- Potts et al., 2010) and species at higher trophic levels tion and correct assessment of changes that have already

540 Diversity and Distributions, 21, 534–547, ª 2015 John Wiley & Sons Ltd Historical legacies shape biodiversity changes been triggered (Scheffer & Carpenter, 2003; Brook et al., into conservation activities. For example, societal responses 2013; Hughes et al., 2013). Nonlinear behaviour in cause– to environmental degradation tend to be belated and insuffi- effect relationships characterized by substantial lag phases cient as interventions and environmental policies are mod- means a high probability of underestimating the scale and elled on changes in observed biodiversity states not on rate of biodiversity change, even when a tipping point of an pressures. As is the case with climate change, diverse human ecological system has already been crossed (Scheffer et al., activities have already committed biodiversity to substantial 2009). The longer the lag time, the greater is the probability changes, the dimensions and trajectories of which will only of underestimation and the more influential are the implica- be fully revealed in the future. This is particularly worrisome, tions. Amplifying feedbacks and the concomitant crossing of as current conservation efforts are already judged to fall far thresholds will trigger particularly large and rapid changes short of curbing biodiversity loss (Butchart et al., 2010; Tit- (Fig. 2e). For instance, bleaching of coral reefs (i.e. corals tensor et al., 2014), even without considering the effect of expelling their symbiotic zooxanthellae) is a symptom of lags. Increased efforts in creating biodiversity indicators that stress, and most large-scale events have been caused by tem- respond to pressures, rather than biodiversity states or porary rises in sea water temperatures over a specific thresh- human responses, are necessary to accurately monitor recent old (e.g. summer maxima increase of 1–2 °C for 3–4 weeks; and predict forthcoming biodiversity changes (Butchart Hoegh-Guldberg et al., 2007). As a consequence of climate et al., 2010). More generally, the consideration of appropri- change, this threshold is crossed more often. Below this tip- ate temporal scales should become a key topic in the work at ping point, however, there is relatively little negative impact the science–policy interface (e.g. IPBES, www.IPBES.net) on corals. If unfavourable conditions prevail for too long, (Perrings et al., 2011). corals will die and the ecosystem will switch to a different, There is already substantial empirical evidence that the total less complex and less species-rich stable state. Increasing relaxation time of a focal biodiversity component increases ocean acidification is expected to exacerbate future coral reef with the cumulative, yet interconnected relaxation lag times of endangerment as when aragonite saturation of sea water falls the responses. As a consequence, effects that are particularly below 3.3 Oaragonite, carbonate accretion of corals approaches relevant for human livelihoods (e.g. changes in ecosystem zero (Hoegh-Guldberg et al., 2007). Importantly, due to the structure, functioning and provision of services) (Fig. 1c) may inertia of the oceans, both sea temperature rise and aragonite emerge with the most pronounced delay. Models that try to saturation lag substantially behind increasing atmospheric assess possible future responses of species, communities and

CO2-concentrations, and the full effects of both pressures ecosystems to global change drivers need to become more clo- will realize only belatedly. sely integrated to improve the representation of cumulative lags in biodiversity dynamics and the mechanisms that may cause it. The trade-off between model complexity, uncertain- PRIORITIES FOR RESEARCH AND SOCIETAL ties and robustness will be a major limitation of such inte- RESPONSES TO CUMULATIVE BIODIVERSITY grated models and the accuracy of their predictions (Dukes LAGS et al., 2009; Svenning & Sandel, 2013). Thus, sensitivity analy- Our current understanding of the mechanisms causing ses and ensemble forecasts based on model families with dif- cumulative biodiversity lags is fragmentary. Empirical evi- ferent representations of ecological complexity and dence of the scale and relevance of lagged biodiversity stochasticity, which have become standard tools in climate responses is largely limited to losses and gains in abundance change science (Knutti & Sedlacek, 2013), should also be more and range of species, predominantly at fine spatial scales widely applied for predicting biodiversity change. (habitats to landscapes) (Jackson & Sax, 2009; Kuussaari We currently have only a limited causal understanding of et al., 2009) (Fig. 5). Most studies have examined the effect the mechanisms and the interactions of key factors modulat- of one pressure in isolation, and there is a strong bias ing lagged biodiversity responses. In particular, the interac- towards pressures which manifest themselves visually in land- tions of species traits and environmental context scapes (e.g. habitat loss, fragmentation) (Ewers & Didham, (biogeography, climate, ecosystems) in shaping cumulative 2006) and are thus easier to observe. Moreover, substantial lags need increased scientific attention. Furthermore, driving biases are evident in taxonomic coverage (in favour of plants, forces differ in their character and dynamics. For example, birds and some insect groups such as butterflies), geographic studies analysing whether continuous incremental change or distribution (temperate regions of the Northern Hemisphere) increased likelihood of pulsed events (fires, extreme climatic and environments (terrestrial ecosystems) (Kuussaari et al., events, Kreyling et al., 2011) have varying effects on lag times 2009; Svenning & Sandel, 2013). As most studies build on and how velocity of change affects relaxation times are correlative statistical approaches, a mechanistic understand- urgently needed. It is also largely unknown whether, and if ing of how observed patterns of cumulative biodiversity lags so to what extent, feedbacks potentially offset or amplify are created by ecological processes is limited. cumulative lagged biodiversity responses. Although the repre- The lack of appreciation of lagged biodiversity responses sentation of feedbacks is particularly challenging in predictive in scientific enquiry has prevented adequate integration of models, progress in process-based models increasingly allows the implications of widespread cumulative biodiversity lags for the incorporation of temporal dynamics of change.

Diversity and Distributions, 21, 534–547, ª 2015 John Wiley & Sons Ltd 541 F. Essl et al.

(a) (b)

(d)

(c)

Figure 5 Examples of lagged biodiversity responses due to changing pressures. (a) Proportions of nationally Red-listed species in Europe (e.g. Iberian Lynx, Lynx pardinus) show time-lags of up to at least 100 years to changing socio-economic drivers (Dullinger et al., 2013) (extinction debt). (b) Vascular plant species of the European Alps (e.g. Leontopodium alpinum) show lagged range shifts due to climate change, especially at the receding range margin (climatic debt) (Dullinger et al., 2012). (c) The European green crab (Carcinus maenas), introduced to California in 1993, preys voraciously on native clam species. The reduction in the native clam has, over time, led to a large increase in abundance of an introduced clam species that had been rare in the first half century following the introduction of the green crab (invasion debt) (Grosholz, 2005). (d) The invasion of cheatgrass (Bromus tectorum) increases fire frequency and alters nitrogen dynamics in arid grasslands in the western USA, but the full scale of these effects only unfolded with substantial time-lags (invasion debt) (Sperry et al., 2006). © Programa de Conservacion Ex-situ del Lince Iberico (www.lynxexsitu.es), Harald Pauli (2x), Wikimedia Commons.

Special emphasis needs to be given to identifying critical tip- databases such as GBIF and genetic databases such as Gen- ping points in biodiversity responses (Fig. 2e) (Brook et al., Bank), is particularly promising. Experiments exposed to 2013). Given the complexity and extensive data requirements cumulative changes in pressures, nested studies addressing of process-based models, their application will likely be lim- different scales and study systems where insights are com- ited in the near future and should be directed towards case bined from modelling, experiments and real-world data are studies which provide insights into above-mentioned key fea- needed to improve our understanding of patterns and tures of cumulative biodiversity lags. processes. Making use of already available long-term data on biodi- versity and environmental conditions (including changes in ENSURING A SAFE OPERATING SPACE IN AN magnitude of pressures), sampled with standardized proto- ERA OF RAPID GLOBAL CHANGE cols on many locations world-wide, for example those gath- ered in the global LTER-network (Redman et al., 2004) or Although indirect effects of changing environmental forcing stored in large repositories (herbaria, global biodiversity are not necessarily associated with increasing cumulative lag

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Specific rate or level of change in a focal system causing Biodiversity hierarchy strong qualitative or quantitative responses. Crossing such The three levels of biodiversity organization, i.e. genetic, spe- thresholds will cause a system to shift to novel stable system cies and ecosystem diversity. states rapidly.

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BIOSKETCH the impact of climate change on the distribution of biota and the resulting consequences for nature conservation.

Franz Essl is an ecologist at the University of Vienna and Author contributions: F.E. conceived the ideas and led the the Austrian Environment Agency and is a Research Associ- writing, all authors contributed substantially to the writing. ate at the DST-NRF Centre of Excellence for Invasion Biol- ogy (http://academic.sun.ac.za/cib/). He is interested in causes and patterns of biological invasions, in the processes Editor: Christoph Kueffer governing diversity patterns of species and habitats, and in

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