Trends in Ecology & Evolution
OpinionCollapsing population cycles
Introduction
Multi-annual population cycles (i.e. the regular, high-amplitude density oscillations displayed by some animal populations) are among the most studied and discussed ecological phenomena. Prime examples of cyclic population dynamics include the three–four-year cycles in voles and lemmings, the four–ten-year cycles in ptarmigan and forest grouse, and the nine–eleven-year snowshoe hare and forest insect cycles. Ecologically, population cycles, where they occur, have profound influences on the functioning of ecosystems. Population cycles are also important because they provide unique insights into the mechanisms of population and community dynamics. Modern textbooks exploit the examples of cyclic populations to introduce students to state-of-the-art theory of population regulation and trophic interactions, to analytical tools for analyzing ecological dynamics and to discussion of the effects of pulsed disturbance and flows on ecosystem structure and function.
However, there is now reason to believe that some of the most well-known examples of cyclic dynamics have become lessons of history rather than analyses of contemporary ecology. During the past two decades, cycles in voles, forest grouse and forest insects have been fading out in Europe. The first indications of such changes were published during the mid-1990s 1, 2, 3, although their significance then was questioned [4]. Here, we provide an updated evaluation of the phenomenon of collapsing cycles, in the light of both the most recent empirical evidence and relevant insights derived from earlier research on population cycles. Although drifting in and out of cyclic dynamics might be expected to be within the range of normal nonlinear population dynamics, we argue that the recent events of collapsing cycles are more widespread and simultaneous than would be expected from an accumulation of independent events.
Section snippets
Modeling collapsing cycles
Mathematical and statistical modeling is a central issue in research on population cycles. The modeling literature is vast, but many useful syntheses have been made 4, 5, 6, 7. Models differ along a continuum from complex mechanistic models to rather simple phenomenological models. Whereas the mechanistic models are primarily used to deduce population dynamics from assumed or known biological mechanisms [5], the phenomenological models are mainly tools for inferring processes from patterns
Collapsing cycles in space
More than eighty year ago Charles Elton was the first to realize that population cycles of snowshoe hares and voles were regionalized in the sense that they were mainly northern phenomena. His conjecture has been verified analytically and extended taxonomically to many species of mammals, birds and insects [11]. Although ecologists initially emphasized a north–south dichotomy between cyclic and noncyclic dynamics, the focus later changed to explore clinal geographical patterns, whenever panels
The cases
Among all cases of cyclic population dynamics, the vole cycle in Fennoscandia is probably the most celebrated owing to the many long time series and the richly geographically patterned dynamics. The first recent incidence of a deviation from the ‘normal Fennoscandian vole cycle’ was reported from northern Finnish taiga two decades ago [26]. Soon similar events of missed cyclic peak years were reported to have taken place at approximately the same time (i.e. mid-1980s) over vast tracts of the
Patterns of transition in time
Whereas the shift from cyclic to noncyclic dynamics appeared to happen abruptly in some cases, it was preceded by a period of gradual amplitude dampening in others (Table 2). The most astonishing example of gradual amplitude dampening before loss of cyclicity is from Birger Hörnfeldt's spatially extensive monitoring program of boreal voles in northern Sweden (Figure 3). Statistical evidence for change in period duration (i.e. period lengthening) preceding the collapse of the cycle has so far
Collapsing cycles: a case for intrinsic systemic variability?
Except in the extreme case of the larch budmoth (Figure 2), all other cases of collapsing cycles have been demonstrated in time series shorter than 60 years, usually shorter than 30 years. Obviously, the duration of the time series constrains empirically based inferences about the causal mechanism. A challenge is thus to distinguish between ‘normal’ intrinsic variability and systemic changes due to some external force. The inherent variability in stochastic log–linear systems sometimes leads to
Collapsing cycles resulting from climatic forcing
The strongest causal inferences have been derived from analyses in which the collapse process has been related to concurrent climate change. A particularly illuminating case is the detailed analysis of the process of collapse of the field vole cycles in northern England [28], where the increasingly shorter winters since the early 1990s have been decisive (Figure 4). This study complements the earlier studies of geographic clines in vole dynamics along gradients of seasonality. Indeed, vole
Conclusions and perspectives
Paradigms on how population cycles vary in space have shifted as more and longer population time series have accumulated and more refined analytical tools have been used. Hence, to some extent, the current realization of temporally changing cycles might also be data and method driven. Indeed, drifting in and out of cyclic dynamics over long time scales can be expected to be within the range of the normal behavior of some populations and ecosystems. However, the many cases of collapsing cycles
Acknowledgements
We thank Birger Hörnfeldt and Xavier Lambin for comments on the manuscript.
References (52)
Response to Korpimaki et al: vole cycles and predation
Trends Ecol. Evol.
(2003)- et al.
Rodent dynamics as community processes
Trends Ecol. Evol.
(1988) Spatial population dynamics: analyzing patterns and processes of population synchrony
Trends Ecol. Evol.
(1999)Transients: the key to long-term ecological understanding?
Trends Ecol. Evol.
(2004)- et al.
Monitoring for conservation
Trends Ecol. Evol.
(2006) - et al.
Vole cycles, snow depth and fox predation
Oikos
(1994) - et al.
Predation on competing rodent species: A simple explanation of complex patterns
J. Anim. Ecol.
(1996) Spatial and temporal patterns of small-rodent population dynamics at a regional scale
Ecology
(1996)Population cycles in voles and lemmings: density dependence and phase dependence in a stochastic world
Oikos
(1999)Why do populations cycle? A synthesis of statistical and mechanistic modeling approaches
Ecology
(1999)
Noisy clockwork: time series analysis of population fluctuations in animals
Science
Complex Population Dynamics: A Theoretical/Empirical Synthesis
Analytical Population Dynamics
Experimentally-induced transitions in the dynamic behavior of insect populations
Nature
The macroecology of population dynamics: taxonomic and biogeographic patterns in population cycles
Ecol. Lett.
Specialist predators, generalist predators, and the microtine rodent cycle
J. Anim. Ecol.
Large-scale spatial dynamics of vole populations in Finland revealed by the breeding success of vole-eating avian predators
J. Anim. Ecol.
Geographic gradient in small rodent density-fluctuations – a statistical modeling approach
Proc. Biol. Sci.
The collapse of cycles in the dynamics of North American grouse populations
Ecol. Lett.
The population dynamics of the vole Clethrionomys rufocanus in Hokkaido, Japan
Res. Pop. Ecol.
Vole population cycles in northern and southern Europe: is there a need for different explanations for single pattern?
J. Anim. Ecol.
A new geographical gradient in vole population dynamics
Proc. Biol. Sci.
Is the heart of Fennoscandian rodent cycle still beating? A 14-year study of small mammals and Tengmalm's owls in northern Norway
Ecography
Why the larch bud-moth cycle collapsed in the sub-alpine larch-cembran pine forests in the year 1990 for the first time since 1850
Oecologia
Waves and synchrony in Epirrita autumnata/Operophtera brumata outbreaks. I. Lagged synchrony: regionally, locally and among species
J. Anim. Ecol.
Analysing noisy time-series: describing regional variation in the cyclic dynamics of red grouse
Proc. Biol. Sci.
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