Discussion
Our results show that, in our experimental microcosms, increased
connectivity can buffer a population from extinction caused by drought.
Specifically, habitats connected by good quality corridors, not only
enabled populations to persist longer when they are threatened by
drought severity, but slowed down the rate at which the number of
patches across landscapes affected by drought decreased the populations.
Surprisingly, connectivity differently affected the survival of
populations in each individual patch, as the variability of patch
abundance increased or slightly decrease then followed by a sudden
decrease in the habitats connected by poor quality corridors, whilst it
constantly decreased in the good quality corridors. In what follows, we
first discuss the limitations of our approach, and then discuss the
results in the context of the literature.
There are two main limitations to our approach. The first is that we did
not count individuals in the corridors, as corridors can, potentially at
least, provide extra habitats and serve as temporal refugia for species
when stressed (Eversham and Telfer 1994). The aim of the work was to
investigate how patchy populations response to environmental stressors,
thus the individuals dispersing in corridors are largely ignored. The
second is the potential confounding effect introduced the set up of the
experiment which may explain some of the observed patterns in the
abundance dynamics between treatments (Fig 2). This may be due to
systematic differences in the body size distributions of the populations
in different treatments, however the extinction dynamics largely conform
to the expected patterns.
Species are expected to seek new habitats when natal habitats
deteriorate. Field studies showed that Collembola have a high dispersal
ability for new habitats when experiencing dry conditions (e.g., Ferrin
et al. 2022). Consequently, increasing movement between habitats may
save populations from local extinction when patches experience
inhospitable environmental conditions. Our previous work showed that
Collembola were more likely to colonise new habitats connected by good
quality corridors, thereby increasing population growth rate in the
colonised patches (Li et al. 2021). In this regard, corridor quality
corridors may increase their probability of survival by facilitating
movements and avoiding dry habitats. Collectively, we demonstrated that
good quality corridors reduced the rate of population decline and
increased time before extinction caused by drought extremes,
highlighting the importance of connectivity in persisting populations in
the face of climate change. This not only agrees with previous studies
which show that increasing connectivity using corridors can increase the
persistence of metapopulations (Gonzalez et al. 1998, Swart and Lawes
1996), but proves their ability to slow down extinction when species are
exposed to disastrous environmental conditions. Our results suggest that
increasing spatial connectivity across a landscape can to some extent
mitigate the negative impact of climatic extremes on populations.
The most striking results were that the variability of patch abundance
was negatively related with increased corridor quality, suggesting that
connectivity can play an important role in shaping the viability of
patchy populations. Thus, when corridor quality was poor, Collembola
were constrained in patches as their dispersal ability was limited.
Intriguingly, this implies that when connectivity is good the increased
migration of individuals around the landscape could put increased
pressure on habitats to support large population sizes, increasing the
impacts of density dependence if such patterns persist in the long-term.
Conversely, individuals being stuck in a patch could cause uneven
survival rates among patches when drought extremes occur unpredictably.
In such settings, local extinctions were more likely to occur when some
patches were turned into ecological traps due to environmental change
(Hale and Swearer 2016), thereby increasing the variability of patch
abundance in habitats with poor habitat connectance. Similar findings
have been previously reported in freshwater systems where drought
disrupted the connectivity of habitat pools for some fish species (e.g.,
Vander Vorste et al. 2020). Furthermore, the changes in variability of
patch abundance over time indicates a contrasting pattern of stressed
populations towards extinction between habitats with good and poor
connectivity. Specifically, a constant decrease of the variability found
in well-connected populations indicates that the density of population
in each patch became uniform when experiencing stress, whilst a large
variability found in poor connected populations demonstrated an
increasing risk of local extinction because of loss of connectivity. Our
results suggest that the variability of patch abundance may be a good
predictor of population status and monitoring how it changes over time
might provide a useful guide to evaluate landscape connectivity for
species.
Drought is a climatic extreme which has serious consequences for the
persistence of metapopulations, seen here as an increasing drought
severity significantly reducing the time that population persisted and
increasing the rate of population decline, a phenomenon which is
consistent with the prediction that severer drought would cause higher
speed of extinction (Cady et al. 2019, Cayuela et al. 2016). Increasing
the severity of drought is more likely to cause a strong effect of
desiccation, causing a high rate of mortality when reaching species’
limits to drought. Collembola are well adapted in humid soil
environments and need to absorb water vapor to over their entire life
cycles (Bayley and Holmstrup 1999). Some essential life stages such as
reproduction cycles and egg incubation in Collembola are highly depended
on moisture levels (Holmstrup 2019, Waagner et al. 2011). Meanwhile,
young individuals of Collembola are more vulnerable to desiccation than
old (Hilligsoe and Holmstrup 2003), increasing drought severity are more
likely to reduce the fitness of young adults and cause a further loss of
fecundity. This may explain the extinction event occurred at the end of
our experiment, and the extinction happens faster with increased
severity. Indeed, extreme environmental stressors such as drought has
been shown shaping the population dynamics and persistence of many
species (e.g., Johansson et al. 2020).
The amount of the favourable habitats within a landscape has been shown
to be important for maintaining the persistence of populations (Dytham
1995, Meli et al. 2014). Surprisingly, we found that increasing the
number of patches affected by drought at different speeds only
negatively affect the maximum rate of population decline, rather than
the timing of extinctions, which may suggest a tipping point for
environmental conditions as a result of increased drought habitats
destabilising populations. The reasons underlying this pattern are
largely unexplored though. One possible explanation is that Collembola
were able to accumulate sugar and polyols when experiencing dry
conditions, temporarily prolonging their survival from dehydration
(Fountain and Hopkin 2005, Waagner et al. 2012). Hence, it is possible
that individuals living in a habitat patch under drought manipulation
(i.e., no water added for a week in our case) might not experience an
immediate local extinction due to the accumulation of polyols and
sugars, though drought was shown suppressing the viability of
populations based on our results (Fig. 2). Adjusting their body
conditions may allow them to temporarily survive over a short drought
period, however, it may also contribute to a fast decline when the
environmental conditions reach to tipping points (Dai et al. 2012). More
empirical evidence is needed to examine how increased environmental
stress, associated with tipping points, impacts the resilience of
spatially structured populations.
In summary, our analyses add to the growing literature describing how
increasing connectivity among habitats can buffer metapopulations
against population decline and extinction. However, we also show that
increased connectivity may increase the variability of populations
between patches in a landscape, as individuals can move more easily into
patches with more favourable conditions, potentially increasing the
effects of density dependence in these highly utilised patches. As
habitat fragmentation creates a mosaic of landscapes hosting a variety
of spatially structured populations, maintaining good spatial
connectivity allows species to move out the dangerous area, and/or
allows conservation actions to be taken to avoid mass extinctions when
facing extreme climatic events. Ultimately, our work highlights the
importance of habitat connectivity in maintaining population viability
in the face of climate change. Field evidence is needed to better
understand the role of habitat connectivity in persisting populations.