[Taxacom] NZ biogeographer's exam Q3

John Grehan calabar.john at gmail.com
Sun Jan 29 10:05:14 CST 2017

Recent or not, the interesting point about Cook Strait is that it is not
the Strait (the sea) that is biogeographically significant, but Cook Strait
as a biogeographic boundary which includes the adjacent terrestrial
landscapes. As Geoff points out, the present Strait itself is not all that
old. It might be an ecological barrier in the present (in the sense that
any distributional limit is an ecological barrier by definition). The
reality of Cook Strait as a biogeographic boundary is illustrated below
(and note the reference to predating the Strait in its modern form). Note
that the role of Cook Strait is the same for understanding the range limits
of particular taxa either side of the Strait as it is for those that are
endemic to either side of the Strait. Also note further examples of the
ongoing pervasive tendency for molecular clock studies to misrepresent
minimal estimates as actual or maximal.

“At a local scale, there are many species breaks at Cook Strait, for
example, in the cicada genus *Maoricicada*. *M. campbelli *was discussed
earlier with reference to the Moonlight and Waihemo tectonic zones.  *M.
myersi* is endemic north of Cook Strait, while its sister species *M.
lindsayi* occurs south of the strait (Marshall et al., 2009).

Of the eight main cades in the cicada *Kikihia*, three are restricted to
north of the strait (*K. scutellaris, K. cauta*, and clade 4), and three (*K.
horologium*, clade 2, and clade 5) are only found south of the strait
Marshall et al., 2008). Clade 1 and clade 3 (the *K. subalpina* complex)
both occur in North and South Islands, but each includes a genetic break at
the strait.

Phylogenetic breaks at Cook Strait in skinks (Greaves et al., 2008) and in
galaxiid fishes (Waters et al., 2006) have been dated as Pliocene (minimum
age), and so these predate the strait in its modern form. In contrast,
Marshall et al. (2009) suggested that the Cook Strait breaks in *Kikihia*
subalpine clades date only to the mid- to late Pleistocene, around the time
that the strait appeared. Their study relied on a “standard” evolutionary
rate used for insects (2.3% divergence/Myr), but the calibration that this
is based on has been criticized (Heads, 2012a).
In an earlier study, Marshall et al. (2008) dated the first split in*
Kikihia* at 6.6 Ma, with most species originating in the Pleistocene. They
calibrated the phylogeny by assuming the endemic *K convicta* on Norfolk
Island was no older than t he island, and that *K. subalpina* in the North
Island was no older than the subalpine habitats there (these appeared with
uplift and volcanisms at 1.2 Ma). Both these dates are likely to be
underestimates of clade age (see Chapter 5).”

On Sat, Jan 28, 2017 at 11:02 PM, Geoff Read <gread at actrix.gen.nz> wrote:

> If Cook Strait only last opened 15 000-16 000 years ago (I saw this as the
> date used in another paper on marine mussels) it's not going to be very
> significant as an ancient barrier for land biota.  I imagine it has come
> and gone over time. Sure is a barrier since though.
> On the article on estuarine Austrovenus stutchburyi - interesting but they
> missed out sampling a very large population in Porirua Harbour bordering
> Cook Strait (only sampled Wellington Harbour). They also threw in the
> possibility of human-mediated translocation messing up the picture (these
> are edible cockles), which is a new one to me. There's no supporting
> citation for the idea.
> Geoff
> On Sun, January 29, 2017 12:37 pm, John Grehan wrote:
> > NZ biogeographer's exam Q3
> >
> > Why is it not necessary to assume that a Cook Strait boundary in a
> > terrestrial group is the result of the sea barrier?
> >
> > “It is natural to assume that a Cook Strait boundary in a terrestrial
> > group
> > is the result of the sea barrier. Yet many southern groups reach their
> > northern limit at the *northern* side of the strait (e.g., the cicada
> > *Amphipsalta
> > strepitans*; Marshell et. al., 2012). In a similar way, many northern
> > groups have their southern limits at the southern side of the strait. For
> > example,* Hebe parviflora* (PLantaginaceae) is widespread in the eastern
> > North Island and has its southern limit along the northeastern shores of
> > the South Island (Marlborough Sounds, Cape Campbell) (Bayly and Kellow,
> > 2006). These distributions indicate that it is the Cook Strait region –
> > not
> > the strait itself – that marks the phylogenetic break.
> >
> > In a similar pattern to that of these last, terrestrial groups, Ross et
> a.
> > (2012) reported a Cook Strait break in the estuarine bivalve,
> *Austrovenus
> > stuchburyi*, but noted that the break does not coincide exactly with the
> > modern strait. Instead, northwest Nelson specimens belong to the North
> > Island clade, and Wellington specimens are in the South Island clades.
> > Again, the pattern suggests that the modern topography of the Cook Strait
> > region is not relevant to the biogeographic break."
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> >
> > Nurturing Nuance while Assaulting Ambiguity for 30 Years, 1987-2017.
> >
> --
> Geoffrey B. Read, Ph.D.
> Wellington, NEW ZEALAND
> gread at actrix.gen.nz

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