Stenothermal habitat specialists such as monophagous insects may be
threatened by climatechange induced range shifts, if resource
requirements along migration routes or future habitats are not met.
Little is known about altitudinal shifts of inconspicuous, less mobile
species relative to latitudinal range shifts in prominent, mobile
organisms such as butterflies. Here, we address three questions: What
are the resource requirements-habitat area, quality and
fragmentation-for two specialist herbivores? How do habitat
characteristics change with altitude? Do species resource requirements
suggest an increased vulnerability towards temporal or spatial
bottlenecks under climate change?
We mapped the spatial distribution of a riparian shrub, Veronica stricta
(Plantaginaceae), at local (300 m elevational gradient, n=252 patches)
and regional scales (600 m gradient; n=102) in the Tongariro National
Park, New Zealand. Patch occupancy and patch-level population dynamics
of Trioza obscura (Hemiptera: Triozidae) and an undescribed gall midge
(Diptera: Cecidomyiidae) were recorded in 2010 and 2011. Habitat
suitability and population dynamics were predicted with generalized
linear models, applying information theoretic model averaging.
Habitat area had the largest positive influence on insect presence and
survivorship, the latter benefitting from larger insect populations.
Habitat quality showed a species-dependent effect on occurrence (T.
obscura: negative effect of plant shade; cecidomyiid: positive effect of
host inflorescences) and survival (T. obscura: positive response to
leaf-nitrogen and host size; cecidomyiid: positive response to
inflorescence abundance). Only cecidomyiid colonization had a weak
negative response to habitat fragmentation. Altitude contributed
positively to patch suitability and survivorship of the cecidomyiid, but
decreased survivorship of T. obscura.
Altitude-related changes in the landscape matrix, such as the transition
from forested to subalpine vegetation, positively affected habitat area
and fragmentation, which in turn offset a steady decrease in host plant
size (i. e., patch quality). Our study revealed that habitat area,
quality and fragmentation do not always follow simple linear trends
along elevational gradients. We recommend that species distribution
models should consider these complex patterns when predicting range
shifts. We suggest that species vulnerability to climate change depends
on whether resource requirements allow for upward migration and suitable
habitats are available in future distribution ranges.
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