Chris J. Winks

Are invasives bigger? A global study of seed size variation in two invasive shrubs.

We explored the spatial structure of seed size variation and tested whether seed size differed between native and exotic populations in two invasive species. Seed of Cytisus scoparius (Scotch broom) is significantly heavier in its exotic range, whereas seed of Ulex europaeus (European gorse) is no different between ranges. This result suggests that seed size in C. scoparius is either adaptively or phenotypically responsive to conditions in its exotic range or that plants with large seeds were preferentially introduced. We found that modern ornamental broom seed was no bigger than seed from natural or naturalized populations, suggesting that large seed size in the exotic range is not due to preferential introduction of ornamental varieties with large seeds. Most previous studies of trait differences between native and exotic ranges in invasive species have not taken variation throughout the ranges into account. This is the most comprehensive survey of seed size variation in any species, and the first time that variation in a trait of an invasive species has been studied from individual plant level up to global ranges. Demographic rates can be affected by seed attributes making this study an important first step in understanding how population processes may differ between native and exotic ranges.

Invasive interactions: can Argentine ants indirectly increase the reproductive output of a weed?

The direct and indirect interactions of invasive ants with plants, insect herbivores, and Hemiptera are complex. While ant and Hemiptera interactions with native plants have been well studied, the effects of invasive ant–scale insect mutualisms on the reproductive output of invasive weeds have not. The study system consisted of Argentine ants (Linepithema humile), boneseed (Chrysanthemoides monilifera monilifera), and sap-sucking scale insects (Hemiptera: Saissetia oleae and Parasaissetia nigra), all of which are invasive in New Zealand. We examined the direct and indirect effects of Argentine ants on scale insects and other invertebrates (especially herbivores) and on plant reproductive output. Argentine ants spent one-third of their time specifically associated with scale insects in tending behaviours. The invertebrate community was significantly different between uninfested and infested plants, with fewer predators and herbivores on ant-infested plants. Herbivore damage was significantly reduced on plants with Argentine ants, but sooty mould colonisation was greater where ants were present. Herbivore damage increased when ants were excluded from plants. Boneseed plants infested with Argentine ants produced significantly more fruits than plants without ants. The increase in reproductive output in the presence of ants may be due to increased pollination as the result of pollinators being forced to relocate frequently to avoid attack by ants, resulting in an increase in pollen transfer and higher fruit/seed set. The consequences of Argentine ant invasion can be varied; not only does their invasion have consequences for maintaining biodiversity, ant invasion may also affect weed and pest management strategies.

Growth and development of captive-reared Mercury Islands tusked weta, Motuweta isolata Johns (Orthoptera: Anostostomatidae)

The Mercury Islands tusked weta, Motuweta isolata (Orthoptera: Anostostomatidae), occurs naturally only on Middle Island, a 13 ha island in the Mercury Island group, New Zealand. Growth and development were studied to support a conservation programme to establish them on two other islands and thereby reduce chances of accidental extinction. The egg incubation period was determined from 784 eggs laid by eight weta (range 6-275 eggs per female) exposed to varying temperature similar to Middle Island. Most eggs (240) hatched after 12-39 weeks but 17 eggs laid in late winter hatched after 48-63 weeks. Overall, 87% hatched between October and December, and all hatched between August and February. Egg mortality was 67% (range 50-100% for individual females). Growth and postembryonic development was observed in the laboratory at 16-18°C, and a 14 : 8 h (light : dark) photoperiod. Males and females became adult at either the 10th or 11th instar. Lengths of the head, head with jaws, pronotum, protibia, tympanal membranes, mesotibia, metatibia, metafemur, cercus, ovipositor, tusks, hypandrium and stylus are given for each instar. Tusks appeared as slight bulges on the mandibles of males at the 7th instar, and subsequently enlarged allometrically at each ecdysis. The lifespan from hatching averaged 718 ± 51 d (±95% CI) and ranged from 595 to 902 d. No significant differences were detected between the lifespans of either males or females, or weta that became adult at the 10th or 11th instar. Adult status was confirmed for one 10th instar male when it copulated with 11th instar adults. Males made multiple unsuccessful attempts to copulate with two 10th instar females and 284 eggs were found in one 10th instar female after it died. Only one 11th instar female laid eggs (83) but none hatched in the laboratory.

Invertebrate fauna of boneseed, Chrysanthemoides monilifera ssp. monilifera (L.) T. Norl. (Asteraceae: Calenduleae), an invasive weed in New Zealand

Boneseed, Chrysanthemoides monilifera ssp. monilifera, is a serious invasive weed in New Zealand, particularly in coastal habitats throughout the North Island, northern parts of the South Island, and in the Port Hills near Christchurch. The invertebrate fauna associated with boneseed in New Zealand was surveyed in 1999-2000 as part of a biological control programme for the weed. Boneseed is attacked by a wide range of native and exotic insects but damage is usually not severe. Potential biological control agents that feed on foliage or seeds are unlikely to meet with significant competition from species already resident in this country. However, the combined effect of generalist predators, such as Argentine ants and spiders, and parasitoids could inhibit the effectiveness of some of the invertebrates that would be potential biological control agents. In New Zealand, most (if not all) herbivore niches on boneseed are under-utilised, and there is considerable scope for the introduction of specialised invertebrates that could reduce the vigour of this invasive weed.