Chapter 3: How human colonisation history has impacted the community structure of New
3.2 Materials and methods
3.2.1 Study area.
The North Island of New Zealand (113,729 km2) is part of the New Zealand biodiversity hotspot (Conservation International) and the 14th-largest island in the world. It is the most populated island of New Zealand, supporting around 75.5% of the country’s human population. The North Island is defined by two ecoregions, temperate broadleaf and mixed forest (e.g., Northland temperate forest, Northland temperate Kauri Forest), high in native species richness for both flora and fauna.
The Auckland region (4,894 km2) is one of the nine governmental regions of the North Island. This region has attracted large populations of settlers during both the human colonisation events, due to its landscape and its milder climate. Consequently, the region is currently the most populated of New Zealand (2013 Census; 1.415 million people, ~ 33.4% of NZ population; New Zealand Government Statistic 2016). The first human contact in the region is estimated at around 1300 AD (Davidson 1978a).
3.2.2 Ecosystem structure and dynamics of change
Anthropogenic effects on natural ecosystems were estimated using the degree of forest lost following each period of colonisation. The area of pre-human forest cover was obtained from Ewer et al. (2006). Forest cover in 1840 was estimated for the North Island and the Auckland region based on work by McGlone (1983). Current estimates of land cover for agriculture, forest and artificial cover (e.g., road, building) were obtained from the Statistics New Zealand website (New Zealand Government Statistics 2009).
3.2.3 Bird species
To estimate the response of native terrestrial birds to human settlement, I determined the presence or absence of avian species in the North Island of New Zealand and in the Auckland region based on historical records. Terrestrial bird species that were only present on offshore islands in each area were considered as absent. I
3.2 Materials and methods
3.2.1 Study area.
The North Island of New Zealand (113,729 km2) is part of the New Zealand biodiversity hotspot (Conservation International) and the 14th-largest island in the world. It is the most populated island of New Zealand, supporting around 75.5% of the country’s human population. The North Island is defined by two ecoregions, temperate broadleaf and mixed forest (e.g., Northland temperate forest, Northland temperate Kauri Forest), high in native species richness for both flora and fauna.
The Auckland region (4,894 km2) is one of the nine governmental regions of the North Island. This region has attracted large populations of settlers during both the human colonisation events, due to its landscape and its milder climate. Consequently, the region is currently the most populated of New Zealand (2013 Census; 1.415 million people, ~ 33.4% of NZ population; New Zealand Government Statistic 2016). The first human contact in the region is estimated at around 1300 AD (Davidson 1978a).
3.2.2 Ecosystem structure and dynamics of change
Anthropogenic effects on natural ecosystems were estimated using the degree of forest lost following each period of colonisation. The area of pre-human forest cover was obtained from Ewer et al. (2006). Forest cover in 1840 was estimated for the North Island and the Auckland region based on work by McGlone (1983). Current estimates of land cover for agriculture, forest and artificial cover (e.g., road, building) were obtained from the Statistics New Zealand website (New Zealand Government Statistics 2009).
3.2.3 Bird species
To estimate the response of native terrestrial birds to human settlement, I determined the presence or absence of avian species in the North Island of New Zealand and in the Auckland region based on historical records. Terrestrial bird species that were only present on offshore islands in each area were considered as absent. I
established a list of avian species for the three main time periods (pre-human, post Polynesian settlement and post European settlement), based on unpublished reports, publications and personal observations (Hutton 1870, Buller 1870, Anonymous 1940, Anonymous 1942 – 1944, Anonymous 1946, Lovegrove 1980, Robertson et al. 2007) (seeSuppl. 3 ). Native species from the Order Anseriformes were not included in the list due to their potential to migrate outside the study zone.
3.2.4 Extinction rate
Using the New Zealand avifauna records and the extinction records during each period of human colonisation (Holdaway et al. 2001, New Zealand online 2013), I calculated the extinction rate for each colonisation period using the number of species extinctions (E) per year per total number of species following Pimm et al. (2006). In order to allow comparison with other research (Pimm et al. 2006, Pimm et al. 2014), I adjusted it to per million species years (MSY), rather than using the absolute numbers.
3.2.5 Data analysis
I used the well-established mathematical model of the species-area relationship (SAR) equation (S = cAz), to calculate the extinction of species after deforestation (McWethy et al. 2010, Hanski 1998, May and Stumpf 2000, Brook et al. 2003). In this equation, S is the number of species, A is the habitat size (area) and c and z are two constants (Diamond 1972, Drakare et al. 2006). When the size of a habitat A is reduced to Anow, the number of species in the reduced habitat Snow can be predicted using the equation:
Snow
S =
Anow
A
2
Previous research has established that if z = 0.1, this offers a good estimation of the number of short- term species extinctions that will occur following habitat loss (Rybicki and Hanski 2013). Conversely, long- term extinction patterns are more likely to occur when z = ~0.25 (Rybicki and Hanski 2013). To estimate the
(1)
established a list of avian species for the three main time periods (pre-human, post Polynesian settlement and post European settlement), based on unpublished reports, publications and personal observations (Hutton 1870, Buller 1870, Anonymous 1940, Anonymous 1942 – 1944, Anonymous 1946, Lovegrove 1980, Robertson et al. 2007) (seeSuppl. 3 ). Native species from the Order Anseriformes were not included in the list due to their potential to migrate outside the study zone.
3.2.4 Extinction rate
Using the New Zealand avifauna records and the extinction records during each period of human colonisation (Holdaway et al. 2001, New Zealand online 2013), I calculated the extinction rate for each colonisation period using the number of species extinctions (E) per year per total number of species following Pimm et al. (2006). In order to allow comparison with other research (Pimm et al. 2006, Pimm et al. 2014), I adjusted it to per million species years (MSY), rather than using the absolute numbers.
3.2.5 Data analysis
I used the well-established mathematical model of the species-area relationship (SAR) equation (S = cAz), to calculate the extinction of species after deforestation (McWethy et al. 2010, Hanski 1998, May and Stumpf 2000, Brook et al. 2003). In this equation, S is the number of species, A is the habitat size (area) and c and z are two constants (Diamond 1972, Drakare et al. 2006). When the size of a habitat A is reduced to Anow, the number of species in the reduced habitat Snow can be predicted using the equation:
Snow
S =
Anow
A
2
Previous research has established that if z = 0.1, this offers a good estimation of the number of short- term species extinctions that will occur following habitat loss (Rybicki and Hanski 2013). Conversely, long- term extinction patterns are more likely to occur when z = ~0.25 (Rybicki and Hanski 2013). To estimate the
long term extinction, I used the estimated value z = 0.27 that has been established using a meta-analysis of SAR (Diamond 1972).
To estimate the amount of time elapsed between European settlement and the new equilibrium in native terrestrial avian species richness, I calculated the ‘relaxation index’ (I), using the ratio of the number of extinctions after a given time period (T) (Brooks et al. 1999):
I=
Snow
Soriginal
where Snow represents the number of species available at a given time, and Soriginal is the number of species present at the start of the period of change. At the start of the relaxation process (i.e., initial urbanisation development), when Snow = Soriginal , I will be equal to 1, and it will subsequently decline with time and
urbanisation progress. I calculated the half-life of the declining avifauna using the equation of Books et al. (1999) and assumed that the species decline, represented by I, follows a first-approximation exponential relationship (Diamond 1972).
I=exp
−k×T
where T is the time after the start of urbanisation and k is a constant. Half-life was calculated using I = 0.5 to represent the potential time taken for the extinction of half of the species from the community to occur.