Sabine S. Nooten

Atmospheric change causes declines in woodland arthropods and impacts specific trophic groups

Arthropod assemblages form a fundamental part of terrestrial ecosystems, underpinning ecosystem processes and services. Yet, little is known about how invertebrate communities, as a whole, respond to climatic and atmospheric changes, including predicted increases in carbon dioxide concentrations (CO2). To date, woodland Free Air CO2 Enrichment (FACE) studies have focused entirely on northern hemisphere managed plantations. We manipulated atmospheric CO2 in a mature, native Eucalyptus woodland (0.15 ha, >32 000 m3) in Australia, using the Eucalyptus FACE (‘EucFACE’) facility. We used three complementary sampling methods (vacuum sampling, pitfall and sticky trapping) to record invertebrate abundances under ambient and elevated levels of CO2 (400 versus 550 ppm). Based on the collection of over 83 000 invertebrates, we found significant declines in the overall abundance of ground‐dwelling (14.7%) and aerial (12.9%) arthropods under elevated CO2, with significant decreases in herbivore, omnivore, scavenger and parasitoid functional groups. Even though several groups showed varying declines in abundance, elevated CO2 did not measurably affect community composition. The results of the present study indicate that atmospheric CO2 levels predicted within the next 35 years may cause declines in arthropod abundances in Eucalyptus woodland. Declines found in several functional groups suggest that elevated atmospheric CO2 has the potential to affect ecosystem processes, possibly including nutrient cycling by herbivores and omnivores, as well as biocontrol by parasitoids.

The power of the transplant: direct assessment of climate change impacts

Understanding the factors that limit species distributions has become increasingly important in the face of rapid climate change. Many approaches have been used to predict responses of species and communities to new environmental challenges, including species distribution modelling, glasshouse and growth cabinet experiments, and small-scale field manipulations, all of which have both advantages and limitations. Here, we review the use of a powerful, direct method to predict how species and communities will respond to the changing climate: the field transplant experiment. We discuss how transplant experiments can elucidate the factors that limit species distributions; disentangle the role of genetic change vs. phenotypic plasticity in species’ responses; and improve understanding of the role of species interactions in driving community change. Several generalisations about potential species’ responses to climate change are emerging from these studies, including the critical role of specific life stages in response to warming trends, the role of natural enemies and new hosts in limiting or promoting adaptive capacity, and the role of niche saturation in conferring community stability at a functional guild level. Transplant experiments have also confirmed likely mechanisms of recent range shifts and highlighted the potential for some modelling exercises to overestimate future range changes. With the prospect that accelerating warming over the next few decades will increase extinction rates and accelerate ecosystem degradation, we urge researchers to utilise this powerful but underused method more widely.

What shapes plant and animal diversity on urban golf courses

Recent concern over increasing loss of biodiversity has prompted considerable interest in the role of urban green spaces as reservoirs of local biodiversity. This study assessed the diversity of three indicator taxa - plants, ants and birds - on golf courses spanning a wide range of environmental variation in terms of climate, elevation, course age, size and connectivity to native woodland. Species richness and community composition was further compared between contrasting on-course habitat types that reflect different management intensities. We identified a set of taxon-specific environmental correlates indicating an intricate interplay of landscape- and local-scale variables that affect local species diversity. Our results show that floristic diversity is positively associated with the amount of rainfall, whereas ant and bird diversity are related to local-scale factors, particularly the number of trees and the size of water features on a site. The amount of on-course native habitat was a strong predictor of plant and ant diversity and was also associated with the number of unique species at the site level; this reinforces the value of remnant habitat patches as local biodiversity reservoirs that represent mini hot-spots in an otherwise species-poor urban landscape. Community composition for all three taxa differed markedly between non-playing and playing areas, with boundary and remnant habitats generally having more diverse, species-rich communities. Our results suggest that local floral and faunal biodiversity on urban golf courses can be enhanced by creating woody non-playing areas and, especially, by preserving, restoring or expanding remnant habitats.

Elevated atmospheric carbon dioxide concentrations promote ant tending of aphids

Animal mutualisms, which involve beneficial interactions between individuals of different species, are common in nature. Insect-insect mutualism, for example, is widely regarded as a keystone ecological interaction. Some mutualisms are anticipated to be modified by climate change, but the focus has largely been on plant-microbe and plant-animal mutualisms rather than those between animals. Ant-aphid mutualisms, whereby ants tend aphids to harvest their honeydew excretions and, in return, provide protection for the aphids, are widespread. The mutualism is heavily influenced by the quality and quantity of honeydew produced by aphids, which is directly affected by host plant quality. As predicted increases in concentrations of atmospheric carbon dioxide (eCO2 ) are widely reported to affect plant nutritional chemistry, this may also alter honeydew quality and hence the nature of ant-aphid mutualisms. Using glasshouse chambers and field-based open-top chambers, we determined the effect of eCO2 on the growth and nutritional quality (foliar amino acids) of lucerne (Medicago sativa). We determined how cowpea aphid (Aphis craccivora) populations and honeydew production were impacted when feeding on such plants and how this affected the tending behaviour of ants (Iridomyrmex sp.). eCO2 stimulated plant growth but decreased concentrations of foliar amino acids by 29% and 14% on aphid-infested plants and aphid-free plants, respectively. Despite the deterioration in host plant quality under eCO2 , aphids maintained performance and populations were unchanged by eCO2 . Aphids induced higher concentrations of amino acids (glutamine, asparagine, glutamic acid and aspartic acid) important for endosymbiont-mediated synthesis of essential amino acids. Aphids feeding under eCO2 also produced over three times more honeydew than aphids feeding under ambient CO2 , suggesting they were imbibing more phloem sap at eCO2 . The frequency of ant tending of aphids more than doubled in response to eCO2 . To our knowledge, this is the first study to demonstrate the effects of atmospheric change on an ant-aphid mutualism. In particular, these results highlight how impending changes to concentrations of atmospheric CO2 may alter mutualistic behaviour between animals. These could include positive impacts, as reported here, shifts from mutualism to antagonism, partner switches and mutualism abandonment.

Roles of family and architecture in driving insect community structure: a comparison of nine Australian plant species

While there has been longstanding interest in the factors that shape the composition and structure of insect assemblages, many fundamental questions remain. We compared species composition, and the distribution of feeding guilds within assemblages of two large insect orders (Coleoptera and Hemiptera) collected from nine plant species within three of the largest Australian plant families. We investigated the relative role of host plant family and architectural traits in explaining the characteristics of the insect assemblage. Assemblage composition varied significantly among plant species within families, as there was little commonality in Coleoptera and Hemiptera morphospecies among plants within each family. When the feeding guild structure of the Coleoptera and Hemiptera assemblages was considered as a whole, there was no consistency within host plant families and among plants with similar architectural traits (leaf size). In contrast, when the guild structure of only the phytophagous members of these two orders was considered, assemblage structure was found to be consistent among plant species with a similar leaf size.