Diversidad y control biológico de insectos - CyberTesis UACh ...

Diversity.
To determine whether treatment affected diversity, we calculated Shannon index as
= − H '
S
∑
i=
1
p p ln
i i
where pi = proportion of individuals represented by each taxon; i = i-th species and S =
observed number of species. As Shannon index is sensitive to changes in rare species, we
used it in conjunction to other community metrics.
Species richness and rarefaction statistics.
Large differences on abundance were observed between treatments. As treatments with
more individuals may have artificially inflated species richness, we employed rarefaction
statistics to compare species richness between sites while controlling for abundance
differences (Hurlbert 1971). Because of differences at the taxonomic resolution of groups,
we will use species richness and taxa richness interchangeably.
Species richness was expressed as the number of expected species (E(S)) within a sub-
sample of n specimens. The size of the sub-sample (n) used for comparing treatments was
equivalent to the least abundant treatment at every sampling date.
Evenness.
We applied Hurlbert’s probability of interspecific encounters (PIE; Hurlbert, 1971) as an
evenness measure, which establishes the probability of encounters between two individuals
of different species, assuming that every individual in the collection can encounter all the
other individuals. This index has a low sensitivity for rare species and gives more
importance to evenness of distribution of individuals between species (Barbieri et al.,
1999), thus it provides information complementary to the Shannon index. In addition, PIE
is unbiased by sample size and number of species in a sample, unlike most other evenness
indexes. We used the species-richness module of ECOSIM (1000 iterations) to test
differences in arthropod evenness (Gotelli and Colwell, 2001) as
PIE
⎡ N ⎤⎡
⎢ ⎥⎢1
−
⎣ N + 1⎦⎢⎣
= ∑ i
⎛
⎜
⎝
Ni
N
⎞
⎟
⎠
2
⎤
⎥
⎥⎦
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