Australasian Plant Conservation
Originally published in Australasian Plant Conservation 22(2) September - November 2013, p 2-4
Plants and their role in coextinction
Environmental Decisions, School of Plant Biology, University of Western Australia
Does David Coates’ lacebug (Ceratocader coatsi), known from a single specimen on a mountain, require active management? We know very little about most of the herbivorous insect species in Australia, the majority do not even have names yet. Photo: M.L. Moir
Mature grasstrees (Xanthorrhoea sp.) are important hosts to many invertebrates such as bugs, beetles, spiders and pseudoscorpions (shown from top is the herbivorous planthopper Cyptobarsac rubriops, a predatory pseudoscorpion species, the herbivorous bug Baclozygum brachypterum and the blood-sucking bug Neoclerada westraliensis). Often these particular invertebrate species do not occur on other plants as they are adapted to the microhabitat or the food source provided by grasstrees (this grasstree has been shaved to demonstrate the dense microhabitat). Photos: M.L. Moir
Plants act as hosts to many different types of organisms, from fungi and mites to butterflies and birds. For many invertebrates, interactions with plants are important in directly providing food through plant parts (e.g. for herbivorous insects, pollinators, seed predators), indirectly through the animals that visit plants (e.g. for flower spiders, sundew mirid bugs that feed on invertebrates captured by the plant), or shelter (domatia mites, galling insects, leaf-curling spiders). When these interactions become species-specific, then the loss of the plant species could result in the extinction (also termed coextinction) of those invertebrates that rely on that particular plant for survival. The number of cascading extinctions through this mechanism is predicted to be huge.
In this article, I focus on herbivorous insects. Insect herbivores are more diverse than plants; in England for example, they represent 26% of terrestrial diversity, whereas plants contribute 22% (Strong et al. 1984). The majority of plant species host at least one species of monophagous insect (only able to feed on a single plant species). The extinction of one plant, therefore, could translate into loss of multiple insect species. Herbivorous insects are predominantly lost before the extinction of their host plant. For example, two monophagous mealybugs, Clavicoccus erinaceus and Phyllococcus oahuensis, have gone extinct in Hawaii despite their host plants maintaining populations, although one plant is now critically endangered. The insect herbivore is extinguished before the host plant because there is a critical point at which the host population becomes too small to sustain a viable population of insect herbivore. Consider the conservation of any dependent insect species is, therefore, required promptly when a decline in a plant species is first recognized.
Globally, butterflies garner by far the most support in terms of active conservation for herbivorous insects. Within Australia, butterflies also receive the most attention but, very occasionally, other charismatic species manage to steal the spotlight (and conservation dollars), such as the giant Lord Howe stick insect. But what of the vast majority of species, particularly the micro-insects that are generally less than 5 mm in length? Unfortunately, for most groups we don’t know what species are out there and therefore struggle to identify which may be threatened; many are undescribed, or known from very few individuals. Consider, for example, the lacebug recently named in honour of the Australian Network for Plant Conservation president David Coates; Ceratocader coatsi. It is known only from a single specimen collected on conservation listed Gastrolobium subcordatum (WA Government: Priority 4), on a mountain in south west Australia. Does it require conserving? Too few surveys have been conducted that specifically target plant-dwelling invertebrates to be able to confidently make a call. Despite the difficulties in identifying invertebrates in need of conservation, we should persist as their extinction could facilitate cascading extinctions, which potentially include losing the plant species as well.
Conservation management options
When we do identify invertebrate species in need of conserving, then precautionary action beyond monitoring needs to be taken to assist with their survival. It is insufficient to assume that the invertebrate will survive if we conserve the plant. The survival of the plant is, of course, of foremost importance to herbivorous insects. However, if only the welfare of the plant is considered, then the fate of the insect (and perhaps a multitude of other associated organisms) may be doomed. Ex-situ conservation techniques such as translocations, seed banks, cultivation in botanical parks and even assisted migration do not automatically facilitate the survival of insects, which may not be able to reach these new areas (Moir et al. 2012a,b). Restoring threatened communities can increase the potential habitat for both the insects and the plant host (e.g., Hogbin 2012). Having said this, in situ conservation is not the only way forward. Joint translocation can capitalize on the expenditure already invested in the plant conservation program, and with a little extra investment can assist in the survival of multiple species.
Trials of such joint translocations are occurring in the southwest of Australia in partnership with Western Australian Department of Parks and Wildlife (DPaW), formerly Department of Environment and Conservation. The trials include introducing the mealybug (Pseudoccoccus markharveyi) onto its critically endangered host plant Banksia montana. In addition, translocated individuals of the critically endangered Banksia brownii are receiving the equally critically endangered plantlouse (Trioza barrettae) (so named after the hard-working DPaW flora officer in this floral biodiversity hotspot, Sarah Barrett). An interesting dilemma has arisen with such joint translocations; when should we introduce the host and insect to the new site to maximize establishment and growth for both populations? Currently PhD student Michaela Plein is working on this question from a modelling perspective.
Another management option is the reintroduction of insect species onto naturally occurring populations of host plant that are currently uninhabited by the insect. One of the main dilemmas with this action is whether the herbivore will displace other species at the new site. This option does have multiple benefits, including:
- translocation can be immediate as plants are of a suitable age and structure
- not requiring botanists/land managers to translocate plants (an issue for host plants that do not have “critically endangered” status)
- possible reintroduction into the insect’s historic range and therefore potentially more successful due to similar environmental factors
- mitigates threats to the species as a whole by developing new populations away from the source population.
The critically endangered plantlouse (Acizzia veski) (IUCN 2013) is currently undergoing such reintroduction trials on populations of its conservation listed host plant Acacia veronica (WA Government: Priority 4) in the southwest.
General applications for restoration
The majority of current restoration projects focus on returning the plants, but not on active reinstatement of animal species. Some insects may take many years to recolonize restored habitats, if at all. Certain insects with poor powers of dispersal but with close host-specific ties with particular plants could be actively introduced into restored areas by entomologists and land managers. This would increase the similarity of fauna between restored and undisturbed sites, and provide the ecosystem functions offered by these specialist insects (e.g. pollination, herbivory and recycling) (Moir et al. 2010).
The road ahead for land managers and conservationists may look daunting, particularly given the overwhelming number of insect species potentially at risk, and the dire predictions of the impact that climate change will have on native plant species. However, considering plants as a part of a network of interacting species is the beginning toward the conservation of approximately 26% of global terrestrial biodiversity - herbivorous insects.
Grants from the National Climate Change Adaptation Research Facility (TB11-06) and the Australian & Pacific Science Foundation (APSF 13/7) are supporting insect translocation trials.
Hogbin, T. (2012). Better bush for your buck: targeting restoration efforts and exploring restoration methods in the lower Hunter Valley, NSW Australasian Plant Conservation 21(2), 5-6.
Moir, M.L., Brennan, K.E.C., Majer, J.D., Koch, J.M. and Fletcher, M.J. (2010). Plant species redundancy and the restoration of fauna habitat: Lessons from plant-dwelling bugs. Restoration Ecology 18, 136-147.
Moir, M.L., Vesk, P.A., Brennan, K.E.C., Hughes, L., Keith, D.A., McCarthy, M.A., Coates, D.J. and Barrett, S. (2012). A preliminary assessment of changes in plant-dwelling insects when threatened plants are translocated. Journal of Insect Conservation 16, 367-377.
Moir, M.L., Vesk, P.A., Brennan, K.E.C., Poulin, R., Hughes, L., Keith, D.A., McCarthy, M.A. and Coates, D.J. (2012). Considering extinction of dependent species during translocation, ex situ conservation and assisted migration of threatened hosts. Conservation Biology 26, 199-207.
Strong, D.R., Lawton, J.H. and Southwood, T.R.E. (1984). Insects on Plants. Blackwell Scientific Publications, Oxford.