Figure 1. Chart describing processes and predictions of the
hypotheses. Panel (a) describes the underlying processes that may give
rise to the patterns predicted by the outlined hypotheses. Underpinning
hypothesis 1 (either phenology or range shift) is the
assumption that species differ fundamentally in their abilities to
adjust either in situ or via dispersal. As both strategies are likely to
be adaptive, being able to use either will lead to an increased
probability of presence, which should be reflected in positive
population trends. Positive feedback loops through larger population
size further enhance the ability of both mechanisms to act. Underpinning
hypothesis 2 (both phenology and range shift), on the
other hand, is the assumption that adaptive in situ responses in
phenology increases the fitness of the individuals, leading to higher
rates of survival and/or more offspring. This in turn increases the
probability of presence (stronger population trends) and thus higher
colonization rates which leads to the species being able to expand into
habitats becoming suitable as climate changes (=shift in the northern
range boundary (NRB)). In this study, the hypothesized underlying
processes are investigated through proxies for range shift, phenology
shift, and probability of presences as depicted by derived estimates in
yellow versus blue font in the process charts: shift in NRB as a measure
of species range shift; change in adult flight period as a proxy for
phenology shift; and population trends as a proxy for probability of
presence across the distribution. Panel (b) describes the expected
patterns in the data, i.e. the combinations of responses, as regards NRB
and phenology shift estimates, that would support hypotheses 1
(either advanced phenology or northwards shifting NRB), 2
(both advanced phenology and northwards shifting NRB), and
0 (neither advanced phenology nor northwards shifting
NRB). Although these proxies do not allow us to infer evidence for the
underlying processes, they can inform us of the patterns across a wide
sample of species. By combining them with information on population
trends, we can infer how successful the strategies are on their own and
in combination for species experiencing climate change, and what may be
the consequences if species cannot utilize either of the highlighted
strategies.