Abstract
AbstractInvasive species fundamentally change the bio‐physical and ecological characteristics of the ecosystems they invade. Rapidly expanding invasive species may facilitate the spread of other invasive species, and successive invasion events may lead to novel species interactions that may push the system beyond its equilibrium state and change successional pathways. Knowing the direction of the invasion front may be useful to predict impacts of invasive species. Water primrose (Ludwigia spp.), one of the invasive floating macrophytes in the Sacramento‐San Joaquin River Delta (hereafter, Delta), has increased in cover rapidly over the past three decades likely outcompeting native and non‐native species, changing their functional relationships, with cascading effects in the macrophyte communities of the aquatic ecosystem. In this study, we analyze the directionality of water primrose invasion and assess which spaces it occupies, whether it has overcrowded or outcompeted other vegetation communities, and its implications for succession in the Delta. We used imaging spectroscopy data acquired in June of 2004, June of 2008, November of 2014, and October of 2016 for the 2500 km2 of the Delta to map the communities of submerged macrophytes, floating macrophytes, and emergent marsh. We found that water primrose cover increased fourfold in the Delta over the past 13 yr, changing significantly in the central Delta and Liberty Island region from 122 ha in 2004 to 471 ha in 2016. Water primrose expanded first by spreading over open water and submerged macrophytes and, when that habitat was exhausted, primrose invasion switched direction and encroached into emergent marsh. This bilateral expansion to both open water and the marsh is likely to change rates of succession and affect the restoration of the native Delta marshes. Understanding the mechanisms behind the expansion dynamics of this invasive will allow managers to counter its impact on newly established vulnerable marshes.
Highlights
Invasive aquatic species have a tremendous impact on fragile wetland ecosystems by modifying ecosystem processes such as nutrient availability, nutrient cycling, soil chemistry, water tables, hydrology, food webs, and habitat (Gordon 1998, Scheffer et al 2003, Dukes and Mooney 2004, O’Farrell et al 2009, Pejchar and Mooney 2009)
Until 2014, the majority of this expansion was over open water and into areas freed of water hyacinth, but in the last two years, water primrose started to encroach into areas previously dominated by emergent marsh
Our previous study showed that water hyacinth spread over submerged mats as often as it spread over open water habitat (Khanna et al 2012), while our current results show water primrose spreading into open water far more effectively than over the submerged mats
Summary
Invasive aquatic species (hereafter “invasives”) have a tremendous impact on fragile wetland ecosystems by modifying ecosystem processes such as nutrient availability, nutrient cycling, soil chemistry, water tables, hydrology, food webs, and habitat (Gordon 1998, Scheffer et al 2003, Dukes and Mooney 2004, O’Farrell et al 2009, Pejchar and Mooney 2009). Invasives may significantly impact invertebrate distribution, diversity, and abundance (Meerhoff et al 2003, Toft et al 2003, Stiers et al 2011); induce anoxic conditions detrimental to fish and other aquatic life (Penfound and Earle 1948, Dandelot et al 2005, 2008, Nehring and Kolthoff 2011); and act as barriers for fish movement (Penfound and Earle 1948, Stiers et al 2011, Thouvenot et al 2013a) They reduce open water habitat for water birds and other wildlife (Thouvenot et al 2013a). Removal of invasives may facilitate the establishment or expansion of either native or non-native species into the niche that was created and modified by the invasive (Klo€tzli and Grootjans 2001, Lugo 2004, Khanna et al 2012). Gaertner et al (2014) highlighted the risk of regime shifts in ecosystems invaded by aggressive non-native species due to feedback mechanisms and ecosystem engineering characteristics of such invasives (Crooks 2002)
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