Procesamiento computacional del sistema

Any study of population or community ecology depends on an ability to accurately census the population of interest, and to record changes in population density or dynamics. For many small mammal communities, especially in complex

environments where direct enumeration is not possible, this necessitates the use of relative density indices. In studies of rodents and predators in New Zealand, this has been attempted using trap capture rates, and more recently, through the use of

footprint tracking tunnels (King and Edgar 1 977) to measure changes in activity and abundance. However, relative indices are subject to a number of biases that may affect their reliability. Therefore, before using relative density indices to investigate changes in rodent populations following predator reduction, I tested the reliability and

�

J

Lake Waikaremoana

D Beech Tfiwa�podocarp III Rodent monitoring sites

Highway JS

Figure 1.1 Map of the study area at Lake Waikaremoana, Te Urewera National Park, North Island, New Zealand. Predators were trapped on Puketukutuku peninsula (750 ha)

from May 1995 to March 1998. T l , T2 and T3 refer to rodent-monitoring areas on the treatment peninsula, while NTl and NT2 refer to non-treatment areas on Whareama peninsula (450 ha), and NT3 to a non-treatment area in Maraunui bay, where no predator removal was conducted. Scale on the small figure represents 100 km.

Chapter 1. General introduction _{1 3}

Plate 1.4 View of Wairau arm of Lake Waikaremoana, New Zealand. Puketukutuku, the treatment peninsula is in the middle-distance, while W hareama peni nsula, the non-treatment area, is on the right, with the Panekiri B luffs behind.

repeatability o f density estimates obtained from density indices commonly used i n New Zealand (Chapter 2 ; Calibration o f tracking tunnels, snap traps and Fenn traps : D o they tell the same story?).

The ability to persist in a wide range of habitats, and respond to an unpredictable resource such as the irregular energy input from synchronous beech seeding, suggests that ship rats have a fairly catholic diet that can respond to changes in food availability and quality. An omnivorous diet has been reported elsewhere in the ship rat, both in New Zealand (Best 1 969; Daniel 1 973; Innes 1 979; Gales 1 982) and internationally (Clark 1 980; 1 982). An investigation of such diet flexibility is an important step in understanding the factors that govern small mammal populations, especially in understanding how large changes in food availability during a beech masting affect ship rat and mouse populations. Therefore, in Chapter 3 (Diet and diet selection in ship rats in mixed forest at Lake Waikaremoana, New Zealand), I present data on the food consumed by ship rats caught in the study area over an 1 8 month period, and compare this to estimates of food availability in the environment. Chapter 3 also presents data on infection rates of stomach parasites in rats caught during the study, to investigate if parasite loads might influence population density or dynamics.

In Chapter 4 (Habitat use of house mice and ship rats in a mixed forest mosaic at Lake Waikaremoana, New Zealand), I investigate the distribution of house mice and ship rats among two broad forest types at Lake Waikaremoana. I test whether mice and rats are habitat specialists (confined to a small subset of micro habitats) or habitat generalists (associated with broad-scale environmental gradients).

Given the primary importance of changes in food availability in driving the irregular eruptions of rodents in New Zealand forests (King 1 98 3 ; Murphy 1 992; Fitzgerald et al. 1 996; Chapter 3, this study), the question remains, what role do predators play, if any, in the rodent population dynamics? In Chapter 5 CA computer simulation of rodent and predator population dynamics in an eruptive system) I present a synthesis of the current understanding of the small mammal assemblage in New Zealand forests, through the construction of a computer model of house mouse, ship rat and stoat popUlation dynamics. The model was constructed using current knowledge on rodent and predator biology and ecology in New Zealand and elsewhere, and was used to

Chapter 1. General introduction _{1 5}

generate a number o f predictions regarding the role o f predators i n a primarily food­ driven system. It serves to highlight what we do not currently know, as much as what

we do know, about the biology and ecology of rodents and their predators in New Zealand.

In Chapter 6 (The role of predators in ship rat and house mouse population eruptions: Drivers or passengers?) I test a number of predictions regarding the role of predators in a population eruption generated from the literature and the computer model developed in Chapter 5. Large-scale predator reduction was carried out in the study area, and the role of predation in prey population dynamics was tested by comparing the responses of ship rat and house mouse populations in areas with and without predator reduction.

Chapter 7 presents a synthesis of the factors influencing the eruptive population dynamics of rodents in New Zealand mixed forests. Specifically, I discuss the relative importance of food availability, habitat heterogeneity and predation pressure in