Fast versus slow: differing life history strategies of two New Zealand damselfly (Odonata: Zygoptera) species

Abstract

Life history strategies are important for all organisms and are studied in large part to understand how an individual, population or species reacts to/lives in/survives in its environment and how it/they adapt to changes within that environment. No single strategy is optimal because environmental, morphological and physiological constraints lead to trade-offs between different traits, and how a species responds to these constraints determines their life history. Two variations of life history strategies are slow and fast, which are characterised by differences in activity, development, metabolism, behaviour and their environment. These differences can result in species divergent preference for different habitats, influencing species distribution.

Around 90% of New Zealand’s insects are endemic and most are not well represented in the literature. Their life histories often lack synchronicity, seasonality, and winter diapause, when compared to their Northern Hemisphere counterparts, traits which are often associated with a mild, maritime climate. Odonata are model insects for life history studies but their diversity in New Zealand is low. Two species of damselfly (Odonata: Zygoptera) are present in the South Island: Austrolestes colensonis and Xanthocnemis zealandica. Both of these species were selected to study because they are easy to locate, abundant and widely distributed. They are well described, taxonomically distinct, and are easy to collect, and maintain in the lab. These two species also permit the study of two closely related, cohabitating, predator species that potentially differ in their life history strategies.

The primary objective of this thesis was to determine if a fast-slow life history strategy dichotomy exists between A. colensonis and X. zealandica by investigating aspects of their life history and distribution. A long term (two year) field study was used to investigate differences in life history characteristics between both species in six different populations over an altitudinal gradient. A survey of the lower half of the country was conducted to create a distribution map for each species and investigate any species-specific habitat preferences. Laboratory studies were conducted to investigate differences in metabolism, by measuring movement behaviour and ability to withstand starvation, as well as the likelihood of winter diapause occurring.

A. colensonis naiads were found to grow larger, move more, have a decreased starvation tolerance, and a preference for lower altitude. These characteristics are consistent with a life history at the fast end of the continuum. X. zealandica, on the other hand, display characteristics more in alignment with a slow life history, because naiads of this species are smaller, move less, and can survive longer periods of starvation. X. zealandica require permanent habitats; however, they can take advantage of low quality habitats, particularly those at high altitudes, which don’t suit A. colensonis. Therefore, a fast/slow life history strategy dichotomy is confirmed to be present and may influence how these species interact with the environment and each other. Additionally, like many other endemic New Zealand invertebrates, A. colensonis and X. zealandica undergo quiescence rather than diapause when overwintering. This allows these species to opportunistically take advantage of a mild changeable climate and hence the periods when suitable conditions for growth prevail.