Extrinsic and intrinsic forcing of abrupt ecological change: case studies from the late Quaternary

Abstract

Summary1. Abrupt changes and regime shifts are common phenomena in terrestrial ecological records spanning centuries to millennia, thus offering a rich opportunity to study the patterns and drivers of abrupt ecological change. 2. Because Quaternary climate changes also often were abrupt, a critical research question is to distinguish between extrinsic versus intrinsic abrupt ecological changes, i.e. those externally driven by abruptly changing climates, versus those resulting from thresholds, tipping points, and other nonlinear responses of ecological systems to progressive climate change. Extrinsic and intrinsic abrupt ecological changes can be distinguished in part by compiling and analysing regional networks of palaeoecological records. 3. Abrupt ecological changes driven by spatially coherent and abrupt climate changes should manifest as approximately synchronous ecological responses, both among different taxa at a site and among sites. However, the magnitude and direction of response may vary among sites and taxa. Ecological responses to the rapid climatic changes accompanying the last deglaciation offer good model systems for studying extrinsic abrupt change. 4. When abrupt ecological changes are intrinsically driven, the timing and rate of ecological response to climate change will be strongly governed by local biotic and abiotic processes and by stochastic processes such as disturbance events or localized climatic extremes. Consequently, at a regional scale, one should observe 'temporal mosaics' of abrupt ecological change, in which the timing and rate of ecological change will vary among species within sites and among sites. These temporal mosaics are analogous to the spatial mosaics observed in ecological systems prone to threshold switches between alternate stable states. The early Holocene aridification of the North American mid-continent and the middle-Holocene aridification of North Africa may be good examples of temporal mosaics. 5. Synthesis. Past instances of extrinsic and intrinsic abrupt change are of direct relevance to global-change ecologists. The former allow study of the capacity of ecological systems to quickly adjust to abrupt climate changes, while the latter offer opportunities to understand the ecological processes causing abrupt local responses to regional climate change, to test tools for predicting critical thresholds, and to develop climate-adaptation strategies.

Community ecology in a changing environment: Perspectives from the Quaternary

“…Specific critical transitions can be targeted for intensive study, based on various criteria (e.g., knowledge of natural history of taxa, existence of independent paleoclimatic records, precision of paleoecological records, availability of multiple paleoecological sites for replication or pattern analysis, potential significance of observed patterns). The rapid increase or decline of a dominant species or the disappearance of a community, for example, is of obvious interest in conservation context and may be driven by rapid environmental change, cross-scale interactions, or both (23,84,92). Intensive, integrated study of a carefully selected array of paleoecological case studies would test fundamental theory and indicate the extent to which community turnover follows general rules.…”

Post-disturbance vegetation dynamics during the Late Pleistocene and the Holocene: An example from NW Iberia

“…Many of them have been reliably recorded by 53 environmental archives in the form of long-term records, which contain key information that 54 offers a unique opportunity to study the patterns of ecological change ; 55 Williams et al, 2011). Among these records those related to vegetation dynamics are the most 56 investigated.…”

What do we mean by regime shift? Distinguishing between extrinsic and intrinsic forcing in paleoecological data

“…In contrast, intrinsically mediated responses result from a combination of site-specific abiotic factors (e.g. soil characteristics, groundwater regime and physiography), or from local-scale biotic processes (competition, facilitation and disturbance) (Williams et al 2011). Under these conditions, abrupt ecological changes can occur following only a gradual external forcing (e.g.…”

“…One of the key insights presented in the framework presented by Williams et al (2011) is that abrupt ecological responses can occur as a result from both linear and non-linear dynamics. this is important to recognize, because not all abrupt changes observed in the paleoecological record might represent intrinsic regime shifts.…”

“…A theoretical framework for understanding abrupt ecosystem change was proposed by Williams et al (2011), contrasting extrinsically forced against intrinsically mediated ecosystem responses to an environmental forcing (Fig. 1).…”