Biotic interactions shaping species assemblage and distribution of nettle-feeding butterfly communities Hélène Audusseau*, Sylvain Londe, Nils Ryrholm, Reto Schmucki Background Biotic interactions structure communities and shape species co-occurrence patterns.
Because these processes operate in complex networks, biotic interactions are difficult to study and it is unclear how they affect species response to environmental changes. I. Observations and case study(a)
II.(b)Niche partitioning
Over the last two decades, Araschnia levana has substantially expanded its distribution range. In Sweden, the newly established nettle-feeding butterfly is likely to interact with resident species feeding on the same host-plant (Urtica dioica).
We characterized the realized niche of A. urticae and A. io in two regions, where A. levana has and has not established, and over two time periods during its establishment. We then quantified the overlap in realized niches among species and over time. Where the three species overlap, we found
Water
Urban land
PC1
Phosphorus flow Nitrogen flow
N Polygonia c-album
Aglais urticae
PC2
Mean slope
Non-intensive agriculture
Forest
Open land
Arable land
Principal axes of environmental variations of the habitat available to Mean aspect the species
Differences in density of occurrence for each time period between species A. Urticae – A. levana
axis1 = 23.7 % axis2 = 19.3 % axis3 = 15.7 %
(c)
PC2
A. io – A. levana
Water
Urban land
Non-intensive agriculture
Nitrogen flow
Aglais io
DK
Open land
PC1
Phosphorus flow
Forest
Vanessa atalanta
We are studying how the establishment of Arashnia levana influences indirect species interactions and shapes nettlefeeding butterfly communities. 0
50
100
150
Mean aspect
axis1 = 24.8 % axis2 = 16.8 % axis3 = 12.9 %
Analyses based on a public database of species records - Sweden
We collected butterfly larvae (2017-2018), fortnightly throughout the reproductive period of the species and 19 sites distributed along a latitudinal gradient where A. levana is recorded to be present ( ) and absent ( ). b
a
Niche partitioning
200 km
III. Field study
aa
Niche partitioning
Mean slope Arable land
Period 2
Araschnia levana
Period 1
S
Audusseau et al. (2017) J. of Biogeography 44, 28-38
Throughout the expansion of Arashnia levana, we detected directional changes in the distribution of A. urticae and A. io. We suggested that these changes are likely to be triggered by modifications in parasitoïd pressure in relation to the arrival of A. levana, increasing apparent competition where the species co-occur.
Stockholm area b
b
c
b
b
May
June
July
c
IV. Theoretical tests of apparent competition
b
c
We are developing spatial explicit simulations to test hypotheses related to apparent competition in host-parasitoïd systems where environmental structure, parasitoïd’s virulence, butterfly dispersal and environmental niche can be controlled to assess their effect on species distribution and niche partitioning. Apparent competition in a structured environment
August
a) Environment
For each species, we collected
Parasitoïd pupa and adults
• Up to 4 nests of 5 L2 • Up to 3 nests of 20 L4 • Up to 20 individuals of L5
a) Brachonid wasp b) Ichneumonid wasp c) Tachinid fly
• We found phenological change in parasitoïd load and all four butterfly species share common parasitoids. • Butterfly population dynamics seem to be influenced by parasitoïds and regulation processes are likely to be cumulative and shared across species.
c) With parasitoï ds
Resources a) Environmental suitability is optimal in the upper right corner. b) Both species have a similar realized niche optimum in the upper right corner. c) The realized niche of the host most sensitive to the parasitoïd shifts to the lower right corner. Hosts
*
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b) Without parasitoïds
Temperature
South Sweden
c
Virulence
Sub-optimal temperature
0.3
5 °C
0.7
12 °C
Our simulations suggest that apparent competition can induce divergence in the realized niches of butterfly species having overlapping fundamental niches.
