Shifting ranges of two tree weta species (Hemideinaspp.): competitive exclusion and changing climate

Abstract

Aim Species' responses to climate change are likely to depend on their ability to overcome abiotic constraints as well as on the suite of species with which they interact. Responses to past climate change leave genetic signatures of range expansions and shifts, allowing inferences to be made about species' distributions in the past, which can improve our ability to predict the future. We tested a hypothesis of ongoing range shifting associated with climate change and involving interactions of two species inferred to exclude each other via competition.Location New Zealand.Methods The distributions of two tree weta species (Hemideina crassidens and H. thoracica) were mapped using locality records. We inferred the likely modern distribution of each species in the absence of congeneric competitors with the software Maxent. Range interaction between the two species on an elevational gradient was quantified by transect sampling. Patterns of genetic diversity were investigated using mitochondrial DNA, and hypotheses of range shifts were tested with population genetic metrics. ResultsThe realized ranges of H. thoracica and H. crassidens were narrower than their potential ranges, probably due to competitive interactions. Upper and lower elevational limits on Mount Taranaki over 15 years revealed expansion up the mountain for H. thoracica and a matching contraction of the low elevation limits of the range of H. crassidens. The observed nucleotide diversity in H. thoracica was consistent with a species that persisted in northern areas during Pleistocene glacial periods, from where it expanded at warmer times. In contrast, a two-tailed distribution of nucleotide diversity in H. crassidens was as expected for a species that expanded northwards during glacials and southwards during interglacials.Main conclusions Range shifts resulting from climate change involve complex species interactions. Competition among related species is an important factor limiting realized ranges. In New Zealand, H. thoracica is likely to continue to displace H. crassidens as human-induced global warming proceeds.

Viability and fertility of hybrid New Zealand tree wētā Hemideina spp. (Orthoptera: Anostostomatidae)

“…Female infertility would prevent mtDNA passing the species boundary (introgressing), and this may explain why no evidence of mtDNA introgression has been seen in previous studies (Bulgarella et al 2014, Mckean et al 2016, despite evidence of a low level nuclear DNA and possible phenotypic introgression (Mckean et al 2016). The low number of F 1 hybrids seen in the wild suggests that these wētā are forming a bimodal hybrid zone (Mckean et al 2016), and with reduction in fertility of at least a 50% (due to female infertility), production of hybrids is probably costly.…”

“…The other three putative hybrid individuals were assumed to be F 1 hybrids as their phenotypes were completely consistent with the F 1 hybrids that had been genetically identified. No cryptic hybrids were identified in previous studies of tree wētā from this population (Trewick and Morgan-Richards 1995, Morgan-Richards and Gibbs 2001, Bulgarella et al 2014, Mckean et al 2016. Live wētā were held in individual containers at a constant temperature of 14°C.…”

“…To detect signs of impaired growth (hybrid inviability) the hind tibia length of both dead and living adult hybrids was measured with electronic calipers and compared via ANOVA to wild adult females of both parent species measured in a previous study (15 H. thoracica and 19 H. crassidens;Mckean et al 2016), and to a separate sample of adult males from both species (25 H. thoracica and 22 H. crassidens), that were sampled from the same locations as the hybrids. Hind tibia length is a reliable proxy for body size in tree wētā , Bulgarella et al 2014.) The sex of hybrids and instar at maturity for male wētā were both recorded.…”

“…Tree wētā (Orthoptera: Anostostomatidae: Hemideina) are a genus of seven nocturnal arboreal insects, with high morphological and ecological similarity (Field and Bigelow 2001, Dewhurst 2012, Bulgarella et al 2014. They are hypothesized to have speciated in allopatry during the Pliocene or earlier (Trewick and MorganRichards 2005), and they now have broadly parapatric distributions Morgan-Richards 1995, Bulgarella et al 2014).…”

“…Introgression is the signal of past hybridization, and an ability to successfully hybridize might be of fundamental importance to the future of a species, while climates and environments continue to change (Grant and Grant 1993, Allendorf et al 2001, Becker et al 2013, Taylor et al 2015, Sivyer et al 2018). There is evidence that H. crassidens formerly occupied much of central North Island that is now the range of H. thoracica (Bulgarella et al 2014). Isolated populations of H. crassidens remain in regions of high elevation, which suggests they are adapted to colder environments.…”