Antropónimos

This experiment set o ut to measu re and compare the rate and fidelity of returning fauna after severe d istu rbance to the asse mblages appropriate to two different habitats, one native, the other domi nated by the i nvader Calluna vulgaris. Measures of the attributes of the assemblages before and after the disturbance allowed j udgments to be made as to which assemblage was most complex, and so which should exhibit the best resilience, persistence and consistency, and hence stability.

Complexity

Complexity ( the number of i nteractions - connections - i n an assemblage) i ncreases with increasing d iversity, which increases with successional age (Margalef 1 963, 1 968, Flaherty 1 969, Odum 1 969, H u rd and Wolf 1 974, Frank and McNaughton 1 99 1 ). Thus complexity increase should lead to an increase i n stability, but not all researchers agree - at least not as a general statement (Goodman 1 974, Orians 1 974a) .

Complexity is formed from a combination o f measures of species richness, abundance, evenness, and connectence. It is n ot often straight-forward. The measures themselves are multi­ compartmented and do not always indicate the same trends. I n this experiment species richness and even ness within guilds favoure d Calluna, yet total abundance favou red tussock, as did evenness of abundances. Connectence measured better in Ca/luna. It was not obvious which of these measu res of complexity to emphasise. I propose that the n u mber of taxa present , the d istribution of taxa in g ui lds, and the connectence, are more crucial than evenness of abundances and the total abundance, especially when total abundances are not very d ifferent. The meas u re of connectence already incorporates the abundance features. If the taxa that are the most prominent prey are in h igh

abundance, the system's overall functioning (i.e the dependent t rophic structure) is n ot compromised by rarification of other prey taxa (a 'bottom-up' stability). The presence or absence of a taxon wil l have g reater effect on the system than the evenness of abundances.

Evidence of systems' complexity and stability

The Monte Carlo simulations showed that the differences between the two h abitats were n ot due to chance, each habitat having its own assemblage, with res pect to taxon number and abundances, rei nforcing the long-term field invertebrate work of C hapter 3.

The b reakdown of the assemblage differences by species richness, abundances, evenness, and connectence describes where and how those d ifferences occu r. The species rich ness results clearly demonstrate that the Gal/una habitat was richer in the number of taxa, primarily through ',ts more extensive spider fau na. Contrary to C hapter 3's summarised data, equal numbers of herb ivores were sampled here, but this is probably due to the season (Spring) in which this experiment was done.

Separation of the taxa into guilds showed the strengths of each assemblage, the tussock

having well developed ground fauna. Scavengers (especial Formicidae) and other Hymenoptera are very important elements of the tussock assemblage. The p redominance of Formicidae in the tussock, not seen in the Gal/una, may be a result of the plant architectural differences between the two habitats (see Chapter 2) and implies that ground scavenger functioning is much more developed i n tussock grasslands. Predation, on the other hand, through spiders, is a component of the system that is much better developed in the Gal/una shrub land, and may reflect the differential mobility capabilities of the two guilds.

The g rouping of taxa i nto guilds allowed a different resolution of comparison when examining the abundance structure of each habitat. Total abundance values showed tussock to hold g reater numbers of individuals but it became apparent that this was due to the predominance of only one taxon (i.e. Formicidae). Non-spider predators (beetles and flies) were similar in number, even though the enlarged spider fauna in Gal/una must have been capturing a large proportion of this resource. This could mean that the replacement rate (through immigration and/or reproduction) of prey in Gal/una is much higher than in tussock. Abundances of the other guilds (herbivores, detritivores and

Hymenoptera (parasites)) s howed similar totals.

By observing the evenness of taxa in guilds and the evenness of individual abundances, a feel for the 'balance' in assemblage composition can be gleaned. Diversity measures are contentious (Chapter 3). M any of the measures commonly employed give conflicting results (Hairston et al. 1 968). It was just so in this experiment; the Shannon index suggested that the diversity was highest in Gal/una,

even without the spider fauna. The separation into measures of taxa evenness in guilds and evenness of abundances adds better resolution. The Simpson index applied to guilds and abundances, clearly showed that, while Gal/una held better evenness of taxa over the guilds, the abundances of taxa were better spread in tussock. The histograms (Fig. 5.9) strongly support this allocation. Though both

habitats showed the majority of taxa were present in low numbers, there was a very distinct 'J' curve 'tail' of abundances in tussock. Gal/una was bimodal in distribution of abundances (cf. Chapter 3), suggesting that a few g roups, orb spinning spiders, Chironomidae, Sciaridae, and Thysanoptera, were more able to exist in larger numbers, perhaps excluding others from maintaining higher abundances. The picture painted, then, of evenness of abundances appears to suggest that the tussock assemblage is more evenly distributed, but (and more importantly) that the Gal/una assemblage has a more even d istribution of taxa in its guilds.

Connectence in invertebrate field studies is not an easy parameter to measure. It usually requires in-depth information on each taxon present; its diet, habits, predators, mobility and strength of interactions. I n New Zealand many taxa are not even taxonomically resolved, let alone their d iets, habits etc. known. Thus attempts at this measure must be held, often , as rudimentary. Yet my i ndex and webs based on field observations and elucidation of diets from related overseas fauna, s upply useable

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