Exploring the concept of niche convergence in a land without rodents: The case of weta as small mammals

Abstract

The distinctiveness of New Zealand's large endemic orthopterans and lack of small mammals in our forest ecosystems led to the description of weta as ecologically equivalent to rodents in other countries. We review the use of this metaphor and the characteristics, such as diet and reproductive behaviour, given to support it. We note, however, that species are rarely specified when comparisons are made, thereby neglecting the ecological diversity of both weta and rodents. We suggest that if these taxa are to be compared, the details of their ecology are important and the scale of their influence in an ecosystem must be taken into account. We consider in particular the relevance of the 'invertebrate mouse' cliché in understanding evolutionary ecology in New Zealand and find it misleading. We show that reproductive potential and scale of change in population size differ greatly between mice and tree weta. We find that endothermic mice (Mus musculus) have a metabolic rate almost 20 times faster than ectothermic tree weta (Hemideina sp.), an intrinsic rate of increase some 275 times higher, and consume a high quality diet dominated by seeds and invertebrates and devoid of leaves, in contrast to tree weta diets. Comparative quantitative analyses of the influence of different animals on ecosystem services, biomass, nutrient cycling and energy turnover of forests in New Zealand and elsewhere will contribute to interpretation of the evolutionary history of the New Zealand biota.

Full text

302 New Zealand Journal of Ecology, Vol. 35, No. 3, 2011
FORUM ARTICLE
Exploring the concept of niche convergence in a land without rodents:
the case of weta as small mammals
Melissa J. Grifn, Steve A. Trewick*, Priscilla M. Wehi and Mary Morgan-Richards
Ecology Group, Institute of Natural Resources, Massey University, Private Bag 11-222, Palmerston North, New Zealand
*Author for correspondence: s.trewick@massey.ac.nz
Published on-line: 21 March 2011
Abstract: The distinctiveness of New Zealand’s large endemic orthopterans and lack of small mammals in
our forest ecosystems led to the description of weta as ecologically equivalent to rodents in other countries.
We review the use of this metaphor and the characteristics, such as diet and reproductive behaviour, given to
support it. We note, however, that species are rarely specied when comparisons are made, thereby neglecting
the ecological diversity of both weta and rodents. We suggest that if these taxa are to be compared, the details
of their ecology are important and the scale of their inuence in an ecosystem must be taken into account. We
consider in particular the relevance of the ‘invertebrate mouse’ cliché in understanding evolutionary ecology
in New Zealand and nd it misleading. We show that reproductive potential and scale of change in population
size differ greatly between mice and tree weta. We nd that endothermic mice (Mus musculus) have a metabolic
rate almost 20 times faster than ectothermic tree weta (Hemideina sp.), an intrinsic rate of increase some 275
times higher, and consume a high quality diet dominated by seeds and invertebrates and devoid of leaves, in
contrast to tree weta diets. Comparative quantitative analyses of the inuence of different animals on ecosystem
services, biomass, nutrient cycling and energy turnover of forests in New Zealand and elsewhere will contribute
to interpretation of the evolutionary history of the New Zealand biota.
Keywords: Anostostomatidae; ecosystem function; invertebrate mice; New Zealand; niche; omnivory
Introduction
The fauna of New Zealand is widely recognised as distinctive
and unusual in terms of composition and ecology (Daugherty
et al. 1993; Trewick & Morgan-Richards 2009; Wallis
& Trewick 2009 and refs therein). An example of this
distinctiveness is the prominent place in New Zealand’s
culture of certain Orthoptera and in particular members of
the family Anostostomatidae, known as weta1 (Johns 1997;
Trewick & Morgan-Richards 2009). Four groups of weta are
present in the New Zealand anostostomatid fauna; all are
ightless and nocturnal, and comprise 11 species of giant
weta (Deinacrida), seven tree weta species (Hemideina), three
tusked weta species (Anisoura, Motuweta) and approximately
40 species of ground weta (Hemiandrus). The prominence of
these large crickets in New Zealand contrasts with a natural
absence of small mammals, which in other parts of the world
are a major component of terrestrial ecosystems. This perhaps
explains why a comparison of these very different animals
has been made. In this paper we review the uses of the weta/
rodent or weta/small mammal comparison and consider its
relevance and value in understanding the evolutionary ecology
of New Zealand.
Immediately prior to the arrival of humans (c. 13th century
AD; Wilmshurst & Higham 2004) New Zealand had few
native mammals. Although at least one, probably ightless,
small mammal was present in the mid-Miocene (Worthy et al.
2006), abundant Holocene and late Pleistocene bone deposits
collected in caves, swamps and dunes provide strong evidence
that terrestrial mammals were absent in more recent times
(Worthy & Holdaway 2002). Thus, the recent evolutionary
history of New Zealand’s biota has proceeded without a
well-developed mammalian fauna. Inferred outcomes of this
situation include a relatively high diversity of ightless birds
and the occupation by non-mammalian taxa of ecological
niche space utilised by mammals elsewhere in the world
(Diamond 1990). For example, some small birds, such as the
now extinct Stephens Island wren, Traversia lyalli, are said
to have had a niche parallel to that of small mammals such
as mice and rats (Diamond 1990). At the other size extreme,
New Zealand moa (Dinornithiformes) have been described
as occupying the niche of ungulates (Ramsay 1978), and the
unique characteristics of the kiwi (Apteryx spp.) have seen it
given the status of honorary mammal (Calder 1978). There are,
however, few data that explicitly explore or test the accuracy
or relevance of such descriptions.
Probably the most frequently cited example of putative
niche convergence involves the comparison of weta with
rats (Ramsay 1978; Southern 1979; Daugherty et al. 1993),
mice (Fleming 1973, 1977; King 1974, 1991; Ramsay 1978;
Southern 1979; Daugherty et al. 1993), rodents (Ramsay
1978; Stevens 1980; Daugherty et al. 1993; Guignion 2005),
and small mammals generally (Fleming 1973; Southern 1979;
Duthie et al. 2006; King et al. 2011) including voles (Fleming
1977; King 1974). In the more recent literature, most authors
simply acknowledge these comparisons without necessarily
1 The Maori name weta is applied to Orthoptera of two species-rich cricket families, Anostostomatidae and Rhaphidophoridae, but the latter are usually
called ‘cave weta’. Cave weta have not been compared to small mammals.
New Zealand Journal of Ecology (2011) 35(3): 302-307 © New Zealand Ecological Society.
Available on-line at: http://www.newzealandecology.org/nzje/

303
Grifn et al.: ‘Invertebrate mice’
supporting such a treatment (e.g. Trewick & Morgan-Richards
2005; Watts et al. 2008a, b; Gibbs 2010), whereas at least
one author marginalised the proposition, stating that when
Pacic rats (Rattus exulans) invaded New Zealand, the ‘rodent
niche was empty’ (Gibbs 2009). We believe that comparisons
between weta and various small mammals might be useful if
they lead to a better understanding of niche convergence, or
the ecology and potential ecosystem services of weta within
the New Zealand forest ecosystem, including diet and seed
dispersal studies, as seeds are an important part of rodent diet
(Trewick & Morgan-Richards 2004).
Basis for the metaphor
Reference to weta as equivalent to some form of small mammal
is rst attributed to H.N. Southern in 1964 (Fleming 1973;
Ramsay 1978). Since then various characteristics of weta have
been used as putative evidence for their rodent-like nature.
These include nocturnal foraging (Fleming 1977; Ramsay
1978; Stevens 1980; Daugherty et al. 1993; McIntyre 2001),
occupation of diverse habitats (Fleming 1977), retreat to
daytime roosts (Fleming 1977; Stevens 1980; Daugherty et al.
1993), frass similar in size to rodent droppings (Fleming 1977;
Ramsay 1978; Southern 1979; Stevens 1980; Daugherty et al.
1993), combined biomass (Ramsay 1978; Daugherty et al.
1993), polygamous reproduction (Ramsay 1978; Daugherty
et al. 1993), omnivory (Ramsay 1978; McIntyre 2001),
relatively large individual size (McIntyre 2001), nocturnal
terrestrial activity (McIntyre 2001) and seed dispersal (Duthie
et al. 2006). Each of these features characterise only partially
or inconsistently the subjects being compared and are founded
on few if any data.
Validity of the metaphor: weta as small
mammals
An initial and important difculty with the comparison of weta
with small mammals is that it is vague. Reference to small
mammals (e.g. Duthie et al. 2006) is misleading because there
are many species in this diffuse group (Pough et al. 2005). Even
reference to rodents potentially encompasses an ecologically
diverse range of species. Among approximately 5000 species of
mammals (Delany 1974; Stoddart 1979) there are some 1814
rodents, ranging in size from 4 g to 50 kg (Ellenbroek 1980;
Pough et al. 2005), and most are described as small mammals.
Similarly, the species of weta used in the metaphor vary; tree
(Hemideina sp.) and giant weta (Deinacrida sp.) are often
cited in comparisons (e.g. Southern 1979; Daugherty et al.
1993; Morgan-Richards 1997), but generalisation as ‘weta’
is also commonplace (e.g. Fleming 1973; King 1991; Burns
2006). Failure to qualify which species are being discussed
means that interesting diversity in weta habits is not addressed.
For instance diet, which is a commonly used parameter in
ecological niche construction, cannot be addressed using
this loose terminology. Ground weta (Hemiandrus sp.) and
tusked weta (Anisoura, Motuweta) are primarily predators or
scavengers of animal foods (Cary 1983; Winks et al. 2002),
whereas tree and giant weta are unusual among their family
in eating leaves (Green 2005; Trewick & Morgan-Richards
2005; Wilson & Jamieson 2005; Wehi & Hicks 2010). The
diets of small mammals are similarly diverse; 7 g common
shrews (Sorex araenus) consume terrestrial arthropods and
earthworms (Malmquist 1985), whereas 15 g house mice (Mus
domesticus) are omnivores with a large proportion of their diet
consisting of seeds when available (Badan 1986; Tann et al.
1991). The various species of weta range in size from less than
one gram to more than 50 g. Small mammals that also span this
range include the common shrew, Sorex araneus, and pygmy
shrew, S. minutus (7 g and 4 g respectively; Dickman 1988),
Mus musculus (15 g; Hamilton & Bronson 1985) and the bank
vole, Myodes glareolus (23 g; Verhagen et al. 1986).The largest
weta are about one-third the size of the smallest rat (Rattus
exulans) in New Zealand (130 g; McCallum 1986).
Does the phrase ‘invertebrate mice’ help our science or
our understanding?
Although the comparison of weta to a particular mammal is
not consistent, the phrase ‘invertebrate mice’ has become a
popular cliché (e.g. King 1974; Fleming 1977; Ramsay 1978)
and many authors have applied it even when making non-
mouse comparisons (e.g. King 1974; Fleming 1977, referred
to voles). Despite this inconsistency, we shall focus here on
mice (Mus) and tree weta (Hemideina). Tree weta species
have allopatric or parapatric distributions (their ranges rarely
overlap) and this suggests the biology of each species is very
similar and thus subject to competitive exclusion where they
meet (Trewick & Morgan-Richards 1995, 2004, 2005). In
contrast, the co-occurrence of tree weta and introduced mice
in New Zealand suggests they are not in competition for
resources. Major differences between tree weta and mice limit
the usefulness of this comparison and may also allow them to
exist in sympatry. The fecundity, energetics, and abundance of
tree weta and mice are very different and putative similarity
of some other traits provides only partial insight because the
scale of inuence within an ecosystem may be very different
(Table 1). For instance, while mice and tree weta have some
similarity in their predation of seeds (e.g. in New Zealand,
tree weta Hemideina thoracica and mice Mus spp. both eat
seeds of kauri and rimu; Mirams 1957; Beveridge 1964; Badan
1986; Ruscoe et al. 2004), the quantitative effect that each has
upon the tree species is likely to differ.
Table 1. Summary of the differences between tree weta
(Hemideina spp.) and the mouse (Mus spp.). (Data from:
aMiller 1999; bRowe 2009; cRuscoe et al. 2004; dTownsend
et al. 1997, Moller 1985; eWyman 2009).
____________________________________________________________________________
Mouse Tree weta
____________________________________________________________________________
Weight 15 g 8 g
Rate of population r-selected k-selected
growth
Metabolic rate
(ml g–1 h–1 O2) 2.27a 0.114b
Density (per hectare) 8–28c 180–5000d
Diet (per night/per 8000–28 000 20–500 g
hectare) seedsc leavese
34–120 g
arthropodsa
____________________________________________________________________________

304 New Zealand Journal of Ecology, Vol. 35, No. 3, 2011
Fecundity
Although body mass of Orthoptera and rodents converge at
about the size of small mice, the attributes of large insects and
small mammals are nevertheless very different. Tree weta and
mice are at different ends of their respective distributions, and
this highlights a difference that is expressed in their reproductive
capacity. Small Orthoptera (such as Gryllidae crickets) often
exhibit a ‘boom or bust lifestyle’ like many small mammals;
whereas larger Orthoptera, such as weta, tend to have slower
growth and lower replacement rates (Whitman 2008).
The potential reproductive rate of mice far exceeds that
of weta. Wild female mice (Mus musculus) become sexually
mature at about 60–70 days old (Bronson 1984), have a 3-week
gestation period, a litter size averaging six offspring and the
ability to become pregnant soon after giving birth (Pelikan
1981). The reproductive characteristics of mice compared
with other mammals can be characterised as r-selected,
with a high intrinsic rate of increase (MacArthur & Wilson
1967). In contrast, tree and giant weta take about one year to
reach sexual maturity, and once adult, probably experience
just one breeding season and are thus, compared with many
smaller Orthoptera, K-selected (MacArthur & Wilson 1967).
Although details of weta reproduction are scarce, for two
tree weta species held in captivity (Hemideina thoracica,
H. crassidens), between 34 and 120 eggs per female were laid
over approximately 6 weeks and hatching rates were between
zero and 70% (Morgan-Richards, unpubl. data). Two wild-
caught adult female tusked weta (Motuweta isolata) laid 153
eggs in captivity before death, although only 21 juveniles were
recovered and 15 reared to adults (Stringer 1998). Using the
tusked weta example, assuming 76 eggs per female and 100%
fertility and survival, and an equal sex ratio, we calculated
that a pair of tusked weta could in theory increase to 109 000
in 3 years. During the same time period a pair of mice (Mus
musculus) could generate a population of over 30 million.
The mouse intrinsic rate of increase is thus around 275 times
greater than the weta.
The reproductive capacity of mice (and many other
rodents) is highly responsive to short-term changes in resource
availability (also a characteristic of r-selected species;
MacArthur & Wilson 1967). As mice are not limited to seasonal
breeding they can respond to food abundance at any time of
the year (Brockie 1992). For example, in New Zealand, seed
masting of Nothofagus beech stimulates a rapid increase of
mice (King 1983; Choquenot & Ruscoe 2000; Ruscoe et al.
2005). There is no evidence that any weta do or could respond
to such resource uctuations in this way.
Metabolism
One reason that mice have such a high growth rate and
responsive reproductive rate is that they are endothermic.
Mammals expend a large proportion of the energy they consume
maintaining their high body temperature (Bennett & Ruben
1979; Pough et al. 2005). Small mammals, such as mice, are at
the physiological limits for vertebrate endotherms (cf. reptiles
and amphibians) because their relatively large surface area
to volume ratio results in inefciency, compared with larger
endotherms, and requires consumption of disproportionately
large amounts of food to maintain their metabolic rate (Pough
1980; Pough et al. 2005). While some large mammals are leaf
eaters, leaves do not appear to be sufciently high quality food
sources for small mammals. In this respect elephants could
be considered a more appropriate mammalian equivalent of
giant weta (Deinacrida spp.), in contrast to mice that consume
a range of energy-rich foods, including insects and seeds.
Ectotherms use solar energy as a heat source, so most of the
energy they ingest goes to growth and reproduction (Pough
1980; Pough et al. 2005). This greater energy conversion
efciency means that insects such as tree weta and giant weta
can exist in thermodynamically demanding temperate and
alpine environments even when consuming mainly leaves
(e.g. Trewick & Morgan-Richards 1995; Sinclair et al. 1999;
Joyce et al. 2004). However, the ability to survive in these
circumstances is accompanied by relatively slow growth
of individuals and populations. Even if tree weta and mice
consumed the same types of food (which does not appear
to be the case), their respective effects on the environment
would be different.
The different energy requirements of these animals are
reected in their metabolic rates. Wild house mice have an
average resting metabolic rate of 2.27 ml g–1 h–1 O2, and to
sustain this, mice in New Zealand need to consume about 91
kJ day–1 (Miller 1999). On a diet of seeds alone, this equates
to 3 g dry weight per day of rimu (Dacrydium capressinum)
seeds (30 kJ g–1 rimu seed; Ruscoe et al. 2004) or approximately
970 seeds (Ruscoe et al. 2004). An invertebrate-only diet
requires 4.3 g day–1 (dry mass; Miller 1999). One investigation
of mice in New Zealand found that they ate a mixed diet of
adult arthropods, larva of Lepidoptera and seeds in exotic pine
(Pinus radiata) plantations and native kauri (Agathis australis)
forests (Badan 1986). Diets of mice and ship rats on Rangitoto
Island were shown to consist primarily of invertebrates, with
tree weta (Hemideina thoracica) being the most common
species consumed (Miller & Miller 1995).
Equivalent information on weta is scarce. However,
comparable data for Romalea guttata, a large, herbivorous,
North American grasshopper, where females weigh up to 6 g
(similar to the weight of adult tree weta and Mus musculus),
are available (Hadley & Quinlan 1993). In this case, resting
metabolic rate measured at 25°C was found to be 0.125
ml g–1 h–1 O2 (Hadley & Quinlan 1993). Recent investigation
into the metabolism of tree weta (Hemideina crassidens, H.
thoracica) yielded a similar estimate of 0.114 ml g–1 h–1 O2
at 16°C (Rowe 2009). This is about 0.05 the rate in mice.
Individual tree weta (H. crassidens, H. thoracica) in captivity
consume approximately 30 times less food than a mouse
(average of 0.1 g (wet weight) of leaf material per weta per
night at 14°C; Wyman 2009).
Environmental impacts
There are clearly major reproductive and thermodynamic
differences between mice and tree weta and we expect these to
be reected in the impact of these animals on the ecosystem they
occupy. However, there are currently few data for comparison
even in terms of population densities of mice and weta. The data
that do exist indicate considerable variation in space and time.
For example, mouse densities range from 8 to 28 per hectare
in Waitutu Forest, South Island, New Zealand (Ruscoe et al.
2004). In contrast, estimates of tree weta densities range from
180 weta ha–1 on Banks Peninsula, South Island (Townsend
et al. 1997) to over 5000 weta ha–1 on an island lacking native
and introduced predators (Moller 1985). Thus we infer that
each night mice may be consuming 8000–28 000 tree-seeds
or 60–220 g (wet weight, assuming 80% water) of arthropods
per hectare, while at the same time tree weta may be eating
between 20 and 500 g of leaf material.
Although tree and giant weta appear to disperse and

305
Grifn et al.: ‘Invertebrate mice’
also predate seeds like some mammals (Duthie et al. 2006;
Wyman et al. 2010), their inuence depends on the number
of seeds consumed and destroyed and the distance travelled.
In tree weta the number of seeds eaten is probably low, the
proportion destroyed high and the distance travelled minimal
(Wyman et al. 2010), so their importance as seed dispersers
is likely to be low compared with other animals such as mice
and native birds. However, data for comparison of the actual
amount consumed by various animals or even home range
size are few. For example, estimates of mouse home ranges
of between 250 and 470 m2 (Maly et al. 1985; Mikesic &
Drickamer 1992) are not directly equivalent to data showing
nightly movements of tree weta of <12 m (Kelly 2006).
Conclusion
The usefulness of the comparison between the ecological niches
of weta and mice (and other small mammals) is constrained
by unspecic terminology and the supercial nature of
initial comparisons, which obscure much of the ecological
and evolutionary distinctiveness of weta. The very different
metabolic and reproductive rates and diets of these animals
(e.g. mice vs tree weta) likely mean they have signicantly
different impacts on ecosystems (summarised in Table 1).
Persistence of the invertebrate-mouse cliché, despite a lack
of supporting evidence for similarity, can best be attributed to
lack of knowledge of weta. Thus studies of weta reproductive
strategies and mate choice, population size and dynamics,
fecundity, dietary repertoire, nutrient optimisation, and resource
partitioning among weta taxa deserve close attention. Suitable
data on these would also enable comparisons with taxa related
to weta that co-occur with native mammals in other parts of
the world (e.g. Australia).
Quantication of the effects that different weta species
have on seed predation and dispersal, pollination, predation,
and nutrient/energy cycling is critical and would enable
comparison with other animals in New Zealand ecosystems.
The co-occurrence of weta and introduced rodent species in
New Zealand today provides the experimental framework for
comparative analyses of the ecological niches occupied by
weta species and the energetics of New Zealand ecosystems.
This will in turn contribute to better interpretation of the
evolutionary history of the New Zealand biota and provide
an empirical basis for testing what are, in many cases to date,
ad hoc interpretations.
Acknowledgements
We thank Niki Minards for her helpful comments on the
manuscript and George Gibbs for getting us started. Two
anonymous referees helped improve the manuscript.PMW
is funded by New Zealand Foundation for Research Science
and Technology postdoctoral fellowship MAUX0905. MJG
is supported by a grant from the Entomological Society of
New Zealand.
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Received 24 September 2010; accepted 16 December 2010