Introduction
Whilst droughts are natural phenomenon, their frequency and severity are both increasing due to climate change (Mukherjee et al. 2018), driven by reduced regional precipitation and increased global evaporation (Ault 2020, Dai 2011). Drought has devastating impacts on species and ecosystems, causing increase in species mortality and extinction (Choat et al. 2018, Harrison 2000), loss of biodiversity (Peterson et al. 2021, Tilman and Elhaddi 1992), and declines in ecosystem function and productivity (Atkinson et al. 2014, Ciais et al. 2005).
These pressures are occurring against a backdrop of increasing habitat fragmentation, driven by a range of factors including the extraction of resources, development of settlements, increasing transport links, and proliferation of farming globally. Consequently, many populations are increasingly isolated, impacting their risk of extinction (Crooks et al. 2017, Reed 2004). Corridors have been suggested as positive conservation actions which can be used to reverse the negative effect of fragmentation by promoting dispersal and maintaining gene flow, allowing species to move or adapt in the face of environmental change. Previous studies have shown that enhancing habitat connectivity by corridors can reduce the likelihood of extinction and enhance species diversity in patchy habitats (Chisholm et al. 2011, Damschen et al. 2019), potentially buffering species loss against increasingly inhospitable environments.
Extreme climatic events such as droughts have a devasting impact on the viability of natural populations. Previous studies showed that droughts reduced the survival rate of populations in habitat patches by inducing ecological traps, a phenomenon that occurs when species settle in maladaptive habitats with a poor habitat selection (Hale and Swearer 2016, Robertson and Hutto 2006). For example, Coho salmons (Oncorhynchus kisutch ) that inhabit intermittent streams used remaining pools as habitats when connectivity was lost, but during drought years a lower survival rate was found in some pools than others (Vander Vorste et al. 2020). Such phenomena have also been reported in other ecosystems (Robertson and Hutto 2006). In this case, dispersal might be key to maintain the dynamics of metapopulations and communities (Hale and Swearer 2016, Hale et al. 2015, Wolfe et al. 2023), as climatic extremes are likely to occur dynamically across time and space, and the quality of habitats can deteriorate or recover when the climate regime changes. Yet, there is relatively few data on the population changes with temporarily and spatially dynamic drought stressors.
Corridor quality is one of the important physical properties determines corridor effectiveness and dispersal success (Bennett et al. 1994, Habel et al. 2020, Li et al. 2021). Specifically, high quality corridors can promote dispersal, leading to greater movement rates to colonised patches compared to poor quality corridors (Li et al. 2021). Hence corridor quality may affect the stability and longevity of corridor-connected metapopulations, by facilitating or impeding movements between habitat patches. However, we currently lack empirical evidence on how corridor quality promotes population persistence, and how it interacts with the increasing severity and frequency of drought stressors.
While historical data has successfully investigated the population consequences of extreme drought events on habitat networks (e.g., Oliver et al. 2013), quantitively measuring the effect of increasing drought severity and frequency on population declines and – importantly – how this stress might interact with corridors is difficult. However, disentangling the effects of corridor quality on metapopulation persistence in such natural systems (in the absence of high replication and control treatments) is complex. Experimental microcosms provide one alternative method to achieve this, as they allow landscape style manipulations at an observable scale (Altermatt et al. 2015, Benton et al. 2007). Indeed, such systems have previously been used to study the effect of network modularity (e.g., Gilarranz et al. 2017), and the impact of habitat configuration on metacommunities (e.g., Chisholm et al. 2011, Wolfe et al. 2022).
Here we investigate the effect of habitat connectivity on the persistence of metapopulations under drought stressors. We manipulated both drought severity and the number of patches affected by drought in landscapes connected by either good or poor quality corridors. We use 3D printed “four-patch, four-corridor” microcosms containing the soil Collembola Folsomia candida as experimental metapopulations and use a fully factorial manipulation where we change the moisture of corridors (good vs poor quality corridors), amount of water added to the patches (severity of drought), and number of patches (from 1 to 4) affected by reduced water availability (number of patches affected by drought). Drought is a known abiotic stressor to F. candidapopulations, as they need a high moisture to survive and reproduce. By monitoring the changes in population abundance in microcosms for a relatively long-period (c.16 weeks, ~ 5 generations), we can measure the effects of corridor quality interacting with drought severity and number of drought patches on the time of extinction, the maximum rate of population decline, and the variability of abundance among patches.