Dynamics of a declining amphibian metapopulation: Survival, dispersal and the impact of climate

Abstract

Climate can interact with population dynamics in complex ways. In this study we describe how climatic factors influenced the dynamics of an amphibian metapopulation over 12 years through interactions with survival, recruitment and dispersal. Low annual survival of great crested newts (Triturus cristatus) was related to mild winters and heavy rainfall, which impacted the metapopulation at the regional level. Consequently, survival varied between years but not between subpopulations. Despite this regional effect, the four subpopulations were largely asynchronous in their dynamics. Three out of the four subpopulations suffered reproductive failure in most years, and recruitment to the metapopulation relied on one source. Variation in recruitment and juvenile dispersal was therefore probably driving asynchrony in population dynamics. At least one subpopulation went extinct over the 12 year period. These trends are consistent with simulations of the system, which predicted that two subpopulations had an extinction risk of >50% if adult survival fell below 30% in combination with low juvenile survival. Intermittent recruitment may therefore only result in population persistence if compensated for by relatively high adult survival. Mild winters may consequently reduce the viability of amphibian metapopulations. In the face of climate change, conservation actions may be needed at the local scale to compensate for reduced adult survival. These would need to include management to enhance recruitment, connectivity and dispersal.

Introduction

Climatic factors may influence population dynamics deterministically or stochastically, and can operate at both local and regional levels. Identifying the role of climatic factors in population declines and extinctions therefore poses significant challenges (e.g. McCarty, 2001, Thomas et al., 2004). For example, short-term fluctuations in rainfall can influence the hydroperiod of a temporary pond in an unpredictable way, and this may impact the survival of aquatic organisms in a stochastic fashion. On the other hand, global warming may result in the pond drying up altogether, and in the longer-term this can result in deterministic extinctions (McMenamin et al., 2008). Equally, local rainfall can interact with hydrology in complex ways so that water levels of adjacent ponds vary asynchronously and unpredictably. Alternatively, a reduced water table at the regional scale can result in synchronous reductions in water level across all water bodies. As the persistence of a metapopulation is ultimately determined by birth rates and death rates, the impact of climate on survival within different subpopulations is fundamental to understanding the dynamics and conservation of the system.

Because many species breed in discrete, patchily distributed water bodies, metapopulation ideas have frequently been applied to the conservation of amphibians (e.g. Gill, 1978, Sjögren, 1991, Sjögren-Gulve, 1994, Sinsch, 1992, Vos and Chardon, 1998, Hels, 2002, Hels and Nachman, 2002, Griffiths, 2004). However, if population sizes are controlled by regional factors (e.g. a regional drought), then all local populations may be impacted in the same way and behave like a single population rather than like a metapopulation that depends on asynchronous dynamics (Hanski, 1999, Liebhold et al., 2004). Consequently, identifying whether a patchily distributed population displays the stochastic extinction–recolonization dynamics required by a metapopulation (sensu stricto) remains problematical (Harrison, 1991, Harrison, 1994, McCullough, 1996, Baguette, 2004). Managing amphibian populations on the basis that they conform to a metapopulation structure may therefore be an oversimplification (Marsh and Trenham, 2000, Smith and Green, 2005). Because of the lack of clarity in knowing whether amphibian populations meet the assumptions of metapopulation models, here we use the term ‘metapopulation’ in the broadest sense to refer to a subdivided population that has a spatial structure (e.g. Harrison, 1994, Wiens, 1996, Marsh and Trenham, 2000).

In this study, we combine capture-mark-recapture data collected over a 12 year period with population viability analysis to explore the impact of variation in survival on the metapopulation dynamics of the great crested newt (Triturus cristatus). Declines of great crested newts in Europe have led to provisions under EU legislation that require member states to implement protection measures for this species. Although the great crested newt frequently exists in subdivided populations (e.g. Arntzen and Teunis, 1993, Miaud et al., 1993, Kupfer and Kneitz, 2000, Joly et al., 2001, Griffiths, 2004, Jehle et al., 2005, Meyer and Grosse, 2007), there have been no studies of long-term population dynamics within such subdivided systems. In particular, we test the hypotheses that: (1) annual survival varies between subpopulations; (2) annual survival is related to climatic factors and possibly climate change; and (3) subpopulations display asynchronous dynamics.