INTERSPECIFIC ABUNDANCE-OCCUPANCY RELATIONSHIP REMAINING STABLE ACROSS ISLANDS FOR EPIGAEIC ARTHROPODS: IS THE COMMUNITY AT EQUILIBRIUM?

RIGAL, F.1, TRIANTIS, K.A.1,2, WHITTAKER, R.J.2, BORGES, P.A.V.1 1 Azorean Biodiversity Group, (CITA-A), Universidade dos Açores, Departamento de Ciências Agrárias, Rua Capitão João d`Ávila, São Pedro, 9700-042 Angra do Heroísmo, Terceira, Portugal.

2 Biodiversity Research Group, Oxford University, Centre for the Environment, South Parks Road, Oxford, OX1 3QY, United Kingdom.

The interspecific abundance–occupancy relationship (IAOR) is one of the most common pattern in ecology. Strong negative departure from the IAOR highlight spatial aggregation (species restricted to few sites but displaying high abundance), while positive departure highlight widespread but sparse distribution (Fig.1). Here, IAOR deviation was analysed as well as the abundance and the occupancy which are the main characteristics of the endemic (E), native (N) and introduced (I) soil arthorpod communities, of native fragmented forests (different sizes) from six Azorean islands.

Thus, potential differences between these three communities will be assessed in order the highlight groups of species (i) risking extinction or (ii) those that dominate the overall community of soil arthropods.

METHODS We compared abundances of E, N, I species for each island using a GLM. We controlled the heteroscedasticity structure by using categories as a fixed variance weight. Abundances were log-transformed to normalise the residuals. Occupancy was compared using a binomial GLM. To examine the departure (i.e. residuals values) of the three categories (E/N/I) from the overall community abundance-occupancy relationship, we applied a model averaging procedure (Burnham & Anderson 2002) based on six models (Table 1) to define a consensus IAOR for each island.

STUDY SITE AND DATA The Azores is a young oceanic archipelago, comprising nine islands and some small islets aligned about 615 km on a WNW ESE axis near the Mid Altantic Ridge (37-40° N, 25-31° W)

Randomly placed transects of 30 pitfall traps were placed in native forest isolates on six of the islands in summer of 1999, 2000 and 2001 (Fig. 2). Araneae, Opiliones, Pseudoscorpiones and Insects were identified at a species level and classified into three colonization categories: native but no endemic, endemic and introduced (see Borges et al. 2005).

Table 1. The six models used in this study. p is occupancy and μ abundance.

0.03  

RESULTS

azorean biodiversity

group

Figure 3. The IAORs for soil arthropods. Solids lines are the fits resulting of the model averaging and dashed lines the non parametr ic boots t rap conf idence in ter va l . Key: endemic ,single island e n d e m i c , n a t ive a n d introduced . The goodness-of-fit for the averaged fit was evaluated by R square using the following formula:

where yi is the observed values, is the fitted values and is the mean of the observed values. Red bold numbers are the p values of the Kruskal-Wallis one-way ANOVA by ranks between the three categories for the positive (above) and negative (below) departure. df=2 for all the tests.

R2=0.89 R2=0.83 R2=0.71 R2=0.87 R2=0.82 R2=0.89

Category  effect  (E/N/I)  

Model  log(Abundance)   Model  occupancy  

Island   F   p  value     Dev.  Res.   p  value  Flores   0.41   0.66   0.81   0.66  

Faial   0.10   0.10   0.20   0.90  

Pico   0.27   0.75   0.44   0.79  

Models

Reference Nachman (1981) Hanski and Gyllenberg (1997)

He and Gaston (2003) He and Geston (2003) Leitner and Rosenzweig (1997)

Freckleton et al. (2005)

0.03  

0.42  

0.18  

0.50  

0.30  

0.06  

0.52  

0.38  

0.50  

0.11  

0.80  

0.35  

Category  effect  (E/N/I)  

Model  log(Abundance)   Model  occupancy  

Island   F   p    value   Dev.  Res.   p  value  São  Jorge   0.13   0.87   0.40   0.81  

Terceira   1.67   0.19   0.32   0.84  

São  Miguel   0.19   0.82   0.18   0.91  

Table 2. ANOVA results for GLM for log(abundance) using distributional categories as a fixed variance weight and analysis of deviance using chi-squared test for binomial GLM for occupancy. For all tests, d.f. =2.

OUTCOMES 1)  We find no clear differences for the departure from the IAOR between endemic, native and exotic species suggesting that the three

groups are randomly distributed on both sides of the overall IAOR (Fig 3) whatever the island. 2)  The three different groups of species have no significantly different mean abundance and occupancy at the island scale whatever the

island.

This implies that the fundamental community dynamics of three groups are broadly similar (i) whatever other biological differences they may display, (ii) regardless of the surface of remaining native forest and (iii) the fragmentation of each island of the archipelago.

F=2  (L,  M)  T=12  S=1571  

F=2  (M,  S)  T=8  S=226   F=3  (L,  M,  M)  

T=16  S=952  

F=2  (M,  M)  T=8  S=293  

F=5  (L,  L,  M,  M,  S)  T=28  S=2345  

F=3  (M,  S,  S)  T=12  S=331  

Azores  archipelagos  (Portugal)  

Flores  

Corvo  

Faial  

Pico  

São  Jorge  

Graciosa  

Terceira  

NW

SW

W

NE  

SE  

Figure 2. The Azorean islands. Islands with underlined bold letters are those selected in the present study. F=number of native vegetation fragment and size class of each with L=large, M=medium and S=small, T= number of transect (for Large fragment, T=8, for medium and small, T=4) and S=total surface of native vegetation (hectares)).

AtlanRc  ocean  São  Miguel  

Santa  Maria  

negative departure

positive departure

Abundance

Occ

upan

cy

Figure 1. Schematic IAOR and departure measures.

References: Borges, P.A.V., C. Aguiar, J. Amaral, I.R. Amorim, G. André, A. Arraial, A. Baz et al. 2005. Ranking protected areas in the Azores using standardized sampling of soil epigean arthropods. Biodiversity and Conservation 14: 2029-2060. Burnham, K.P and Anderson, D.R. 2002. Model selection and multimodel inference: a practical information-theoretic approach. (Springer, New-York). 2nd Ed. Freckleton, R.P., Gill, J.A., Noble, D. and Watkinson, A.R. 2005. Large-scale population dynamics, abundance-occupancy relationship and scaling from local to regional population size. Journal of Animal Ecology. 74:353-364. Hanski, I. and Gyllenberg, M. 1997. Uniting two general patterns in the distribution of species. Science 284: 397-400 He, F. and Gaston, K.J. 2003. Occupancy, spatial variance and the abundance of species. American Naturalist 162: 366-375 Leitner, W.A. and Rosenzweig, M.L. 1997. Nested species-area curves and stochastic sampling: a new theory. Oikos 79:503-512 Nachman, G. 1981. A mathematical model of the functional relationship between density and spatial distribution of a population. Journal of Animal ecology 50: 453-460

A nonlinear least squares method was used to fit each of the six models and we applied a non parametric bootstrap method to determine the confidence intervals (CI) (2.5%-97.5%).We then performed Kruskal-Wallis one-way analysis of variance by ranks to test the difference of positive and negative departures for the three categories (E/N/I).

In the study, we have considered only native vegetation. However, introduced species displaying low occupancy and low abundance in native forest could present an adverse pattern in exotic vegetation and land uses surrounding native forest emphasizing the occurrence of a threatening source near forest edges. Further investigations has to be carry out to address this issue.

0.1 0.5 2.0 5.0 20.0

0.1

0.2

0.5

1.0

Flores

Abundance

Occupancy

0.2 0.5 2.0 5.0 20.0

0.2

0.4

0.6

0.8

1.0

Faial

Abundance

Occupancy

0.05 0.20 1.00 5.00 20.00

0.1

0.2

0.5

1.0

Pico

Abundance

Occupancy

0.2 0.5 2.0 5.0 20.0

0.2

0.4

0.6

0.8

1.0

San Jorge

Abundance

Occupancy

0.05 0.20 1.00 5.00 20.00

0.05

0.10

0.20

0.50

1.00

Terceira

Abundance

Occupancy

0.1 0.5 2.0 5.0 20.0

0.1

0.2

0.5

1.0

Sao Miguel

Abundance

Occupancy

São  Miguel  Terceira  São  Jorge  Pico  Faial  Flores  

PERSPECTIVES

INTRODUCTION