Cold tolerance strategies phylogeny geography benefits

The apparently clear distinction between freeze intolerance and freezing tolerance has prompted questions regarding the relative benefits of these strategies under different circumstances, the coexistence of species with different strategies in relatively similar microhabitats, and the reasons why some species adopt one strategy over another. Traditionally, it has been thought that the higher insect orders contain more freezing tolerant species than the other orders, but that there is otherwise no clear evidence of phylogenetic constraint (Block 1982; Duman et al. 1991). In addition, it has also been thought that freezing tolerance is a characteristic of Arctic species from particularly cold environments (Block 1995; Bale 1996), and Duman et al. (1991) have suggested that freezing tolerance is unlikely if insects are exposed to high subzero temperatures and periodic thaws occur during winter. Nonetheless, Duman et al. (1991) point out that if insects are routinely exposed to nucleators, such as external ice crystals in damp environments, or those associated with feeding, then freeze tolerance may be a strategy that ensures survival of subzero temperatures, particularly aseasonal freezing events. That the likelihood of inoculation and variability in temperature might be important in influencing the strategy adopted by a species has been suggested for alpine insects and for insects inhabiting more maritime environments in the Northern Hemisphere (Somme and Zachariassen 1981; van der Laak 1982).

Recently, these arguments have been explored in considerable detail by Sinclair et al. (2003c), based on the compilation of a large quantitative database on SCPs and LLTs by Addo-Bediako et al. (2000). It appears that freeze intolerance is the most common strategy among insects and is widespread within the Arthropoda, characterizing entire lineages within this phylum, including the springtails, mites, ticks, and spiders (Fig. 5.22). Most millipedes are freeze intolerant as are the few scorpions and terrestrial isopods that have been investigated.

Within the insects, in terms of the absolute numbers of species, freeze intolerance is dominant, although freezing tolerance has now been documented from many orders (Fig. 5.22). The majority of orders examined to date rely on either one or the other of the strategies, suggesting mutual exclusivity. However, six orders include both strategies, while two orders (Mecoptera, Siphonaptera) include only freeze-intolerant species, and two (Blattaria, Neuroptera) include only freezing tolerant species (Table 5.2). Within the insects there is a strong phylogenetic component to the SCP, and LLTs, with most variation being explained at the family and generic levels (Addo-Bediako et al. 2000). However, insufficient data are available to examine the extent to which this applies throughout the insects, or to attempt a similar sort of comparison at the level of each of these strategies. In sum, it appears that freeze intolerance is basal within the arthropods, and that freezing tolerance has evolved many times within different taxa.

The geographic distribution of the strategies was also examined by Sinclair et al. (2003c). They found quantitative support for the idea espoused by Klok and Chown (1997) that freezing tolerance is more common in Southern than in Northern Hemisphere species. However, this tolerance is of quite a different form to that found in many Northern Hemisphere insects. The large majority of Southern Hemisphere freezing tolerant species are moderately freezing tolerant, with mortality generally occurring less than 10°C below the freezing point. In contrast, many of the Northern Hemisphere species are strongly freezing tolerant, tolerating very low subzero temperatures (Section 5.3.1). Sinclair et al. (2003c) concluded from this that the specific nature of cold insect habitats in the Southern Hemisphere, which are characterized by oceanic influence and climate variability around 0°C must lead to strong selection in favour of freezing tolerance in this hemisphere. Thus, there are two main scenarios where it would prove advantageous for insects to be freezing tolerant. In the first, characteristic of cold continental habitats of the Northern Hemisphere, freezing tolerance allows insects to survive very low temperatures for long periods of time, and to avoid desiccation. These responses tend to be strongly seasonal, and insects in these habitats are only freezing tolerant for the overwintering period. In contrast, in mild and unpredictable environments, characteristic of habitats influenced by the Southern Ocean, and by strong El Nino Southern Oscillation events (see Sinclair 2001«), freezing tolerance allows insects which habitually have ice nucleators in their guts to survive summer cold snaps, and to take advantage

Palpigradi Pycnogonida Ricinulei Araneae ■ Amblypygi Uropygi Schizomida Scorpiones ■ Pseudoscorpiones Solifugae Opiliones Chilopoda ■ □ Diplopoda ■ Pauropoda Symphyla Collembola ■ Protura Diplura

Figure 5.22 Phylogenetic distribution of known cold tolerance strategies of larvae/nymphs and adults of (a) terrestrial arthropod taxa and (b) insect orders. Filled squares are freeze intolerant, open squares are freezing tolerant. Source: Redrawn from Sinclair et al. (2003c).

Figure 5.22 Phylogenetic distribution of known cold tolerance strategies of larvae/nymphs and adults of (a) terrestrial arthropod taxa and (b) insect orders. Filled squares are freeze intolerant, open squares are freezing tolerant. Source: Redrawn from Sinclair et al. (2003c).

Insects Cold

Archaeognatha Thysanura Ephemeroptera Odonata Plecoptera Embioptera Orthoptera Phasmatodea Mantophasmatodea Grylloblattodea Dermaptera Isoptera Mantodea Blattaria Zoraptera Thysanoptera Hemiptera Homoptera Psocoptera Phthiraptera Coleoptera Neuroptera Megaloptera Raphidioptera Hymenoptera Trichoptera Lepidoptera Strepsiptera Diptera Siphonaptera Mecoptera

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