Transcript Homogenization of soil seed bank communities associated
The Invasive Species Ireland Forum 2009
Long-term implications of plant invasions: the significance of the soil seed bank
Margherita Gioria & Bruce Osborne
School of Biology and Environmental Science University College Dublin
Predicting invasions by IAS
A number of generalizations have been proposed to predict the factors that are responsible for successful invasions Only a few have provided consistent results ◦ ◦ ◦ Stochastic factors ◦ Disturbance Propagule pressure Residence time Changes in land use Impacts at community level – standing vegetation The impact of IAS on the soil seed bank (SSB) has been largely neglected
Soil seed banks
Determining plant community dynamics Source of diversity and genetic variability Survival of a species at a locality Mitigating the effects of unfavourable seasons Colonization of new habitats Dispersal in space and in time - ‘memory’ Thompson et al. (1997) classified SSBs : ◦ Transient - Short-term persistent - Long-term persistent
SSBs and IAS
Alterations in the seed bank of resident species The formation of a large SSB seed input, germination, viability species recruitment from the seed bank additional effects on the vegetation Understanding the potential long-term implications of plant invasions changes in SSB must be examined
Mechanisms
Species displacement from the vegetation Reduce seed input Formation of a large above and below-ground biomass Limitation mechanisms Changes in conditions for germination Reproductive strategies of resident species Saturation Gioria 2007
Objectives
3 large herbaceous plant invaders:
Fallopia japonica var. japonica FJ Gunnera tinctoria GT Heracleum mantegazzianum HM
Seed bank of GT and HM Effects on the structure (diversity, composition, and abundance) of resident SSB communities Comparative assessment of the effects of these invaders
Characteristics
Large stature, biomass, and litter
Reproduction: FJ: exclusively by vegetative means GT: sexual and asexual HM: exclusively by seeds Reproductive potential: GT: 700,000 seeds per plant (Osborne et al. 1991) HM: 10,000-20,000 fruits per plant (Pyšek et al. 2007) Residence time: FJ: 3-5 years GT: 30-50 years HM: 30-40 years
Methods
Multi-site comparative approach 3 sites for each invader Comparable invaded and uninvaded areas 4 – 4m 2 plots 5 soil cores 3 depths (0-5, 5-10, 10-15 cm) May and October Seedling emergence approach (Thomspon & Grime 1979) Unheated greenhouses 240 samples per site
Seed bank of GT and HM
Gunnera tinctoria
• 32,120 ± 31,837 SD seedlings m −2 • 28,308 ± 16,176 SD seedlings m −2 in May in October • 20% seedlings (5-10 cm) • 10% seedlings (10-15 cm) • Asynchronous
Persistent seed bank
(sensu Thompson et al. 1997)
Heracleum mantegazzianum
• 9,762 ± 390 SD seedlings m −2 • 0-5 cm • Synchronous germination • Requirement for chilling period in October
Transient seed bank
Impacts of GT
May October
Fig. 1.
nMDS configurations representing SSB communities invaded by GT at three sites
Impacts of HM
May October
Fig. 2.
nMDS configurations representing SSB communities invaded by HM at three sites
Impacts of FJ
May October
Fig. 3.
nMDS configurations representing SSB communities invaded by FJ at three sites
Dominance GT
May October
Fig. 4
. Dominance-diversity curves based on SSB data collected in May and October at three sites
Dominance HM
May October
Fig. 5
. Dominance-diversity curves based on SSB data collected in May and October at three sites
Dominance FJ
May October
Fig. 6
. Dominance-diversity curves based on SSB data collected in May and October at three sites
SSB invaded by GT
Stellaria uliginosa Spergula arvensis Ranunculus acris Juncus bufonius Urtica dioica Juncus effusus 0 Cardamine pratensis Stellaria uliginosa Spergula arvensis Urtica dioica Ranunculus acris Juncus bufonius Juncus effusus 0 May 10 20 30 40
% Contribution to similarities
50 October 10 20 30
% contribution to similarities
40 60 50
Figure 7.
Similarity percentages analysis showing the species that most contributed to similarities between invaded seed bank communities at sites invaded by GT (Bray-Curtis, 4rt root tramsformed data)
SSB invaded by HM
Juncus effusus May Urtica dioica 0 20 40 60
% Contribution to similarities
80 100 October Urtica dioica
Figure 8.
Similarity percentages analysis showing the species that most contributed to similarities between invaded seed bank communities at sites invaded by HM (Bray-Curtis, 4rt root tramsformed data) 0 20 40 60 80
% Contribution to similarities
100
SSB invaded by FJ
Cirsium arvense Juncus bufonius Ranunculus acris May Urtica dioica Ranunculus repens Ranunculus acris Epilobium hirsutum Juncus bufonius Juncus effusus Urtica dioica 0 0 10 20 30 40 50 60
% Contribution to similarities
70 80 90 10 20 30 40 50
% Contribution to similarities
October 60 70
Figure 8.
Similarity percentages analysis showing the species that most contributed to similarities between invaded seed bank communities at sites invaded by FJ (Bray-Curtis, 4rt root tramsformed data)
Effect of invasive species identity
Table 1.
Results of PERMANOVA analyses testing the effect of ‘invasive species identity’ (Sp) on soil seed banks Source of variation df SS MS
F P
SS MS
F P
Sp D S(Sp) Sp x D P(S(Sp)) S(Sp) x D P(S(Sp)) x D Residual Total 2 2 6 4 27 12 54 432 539 67.71
20.20
136.68
9.76
20.98
31.98
27.95
290.66
605.91
May
33.86
10.10
22.78
2.44
0.78
2.66
0.52
0.67
1.49
3.79
29.32
0.92
1.15
5.15
0.77
0.176
0.003
0.001
0.592
0.024
0.001
1
October
75.50
16.98
108.27
12.99
35.38
37.75
8.49
18.05
3.25
1.31
2.09
4.50
13.77
1.72
1.77
0.062
0.002
0.001
0.024
0.001
22.66
38.53
319.45
629.76 1.89
2.65
0.71
0.97
0.74 0.001
0.723
May
Invaded seed bank
Fig. 9.
nMDS plots displaying multivariate patterns in invaded seed bank communities for the three invaders, at each study site and within each plot October
Conclusion 1
Major effects on the seed bank of invaded areas Invaded SSB less diverse, abundant More persistent component Dominated by seeds of agricultural weeds and Juncus species GT formed a large persistent seed bank - 30,000 seedlings m 2 ◦ Eradication non realistic HM formed a transient bank - 10,000 seedlings m 2 October ◦ Eradication feasible FJ did not set any viable seed
Conclusions 2
No effect of Species similar SSBs Independent of the reproductive strategy of the invader Independent of the initial SSB diversity FJ: despite not setting any viable seed on invaded communities significant effects In a short period of time (3-5 years), compared to 40-50 years for GT and HM higher invasive potential
Implications
Alterations of SSBs could be an important determinant of the invasive success of large invasive plants Long-term implications Improving our understanding of such effects Important for the development of control and conservation programmes Disturbance in an attempt to eradicate invasive species Promote the germination of seeds of undesirable species Need for seeds of desirable species