Introduction

There has been considerable research on interactions between aquatic plants (hereafter macrophytes) and fish due to the importance of fisheries to society and structure and food macrophytes provide to aquatic habitats. A primary linkage between plants and fish are macroinvertebrates, many of which are associated with plants and a source of food for numerous fish species. Motivated by widespread biological invasions occurring in aquatic systems, researchers have begun studying ecological effects of invasive macrophytes on aquatic communities. They have found that invasive species (including macrophytes) are the second greatest threat to imperiled fish species in Canada and the United States after habitat loss and impact 63–70% of the listed species (Lassuy, 1995; Dextrase & Mandrak, 2006). Because native macrophytes typically have positive effects on fish and macroinvertebrate communities, this finding begs the question, “If invasive macrophytes are affecting fish and macroinvertebrate communities differently than native macrophytes, how are they doing it?” To address this question, we must first define accepted models of ecological dynamics in communities with native macrophytes.

In general, vegetated habitats have greater densities of fish than unvegetated habitats. Within vegetated habitats, structural variation influences fish community composition, density, behavior, and population dynamics (see review by Dibble et al., 1996). Fisheries researchers indicate that intermediate densities of macrophytes tend to support the greatest species richness of fish and the greatest growth and survival rates (e.g., Savino & Stein, 1989; Ferrer & Dibble, 2005; Strakosh et al., 2009). However, due to variability in size and feeding behavior among fish, response to vegetated habitats is species dependent. For example, certain fishes prefer areas of fairly dense macrophytes (e.g., Lepomis macrochirus Rafinesque) and others prefer areas with sparser vegetation (e.g., Micropterus salmoides Lacepede) (Johnson et al., 1988). Response of macroinvertebrates to macrophyte density is generally less varied than fish response and tends to increase linearly with macrophyte density (e.g., Cyr & Downing, 1988). Researchers have emphasized how macrophyte structure affects fishes and macroinvertebrates; however, macrophytes play other key roles such as providing food and influencing the aquatic environment physiologically (for a review on submerged macrophyte effects on ecosystem processes, see Carpenter & Lodge, 1986).

Based on the afore-mentioned relationships among native macrophytes, macrointerebrates, and fish, an increase in plant density within aquatic habitats due to an invasion would be expected to increase macroinvertebrate density, decrease fish foraging efficiency, and influence community composition. Due to the multitude of ways plants influence aquatic food webs, environmental conditions, etc., there are likely other ways that invasive species could influence fish and macroinvertebrate communities. Invasive species have specific traits that facilitate their invasion such as high growth rates, defense strategies (i.e., allelopathy), and adaptations to a broad range of environmental conditions (see review by Ren & Zhang, 2009). We posit that if invasive species affect fish and macroinvertebrate communities differently than natives, it is due to traits that enable them to invade because they are responsible for changes to aquatic environments. Furthermore, certain traits will have different effects on the aquatic community than others. If supported by evidence, knowing this information would allow us to better predict impacts of invasive macrophytes on fish and macroinvertebrate communities.

Our review has four primary aims: identify mechanisms by which invasive macrophytes affect fish and macroinvertebrate communities, summarize effects of invasive macrophytes on macroinvertebrates and fish, assess how invasive species traits are connected with how invasive macrophytes affect macroinvertebrates and fish, and specify areas for future research. Dibble et al. (1996) and Petr (2000) reviewed interactions of macrophytes and fish within inland systems extensively, and we will address numerous advances in knowledge since 1997, specifically focused on impacts of invasive macrophytes on fish and macroinvertebrate communities. Our use of the term invasion refers to the following definition, “a biological invasion consists of a species’ acquiring a competitive advantage following the disappearance of natural obstacles to its proliferation, which allows it to spread rapidly and to conquer novel areas within recipient ecosystems in which it becomes a dominant population” (sensu Valéry et al., 2008). An invasive macrophyte is thus the aquatic plant species (floating, submerged, or emergent) referred to in the preceding definition. The definition we used is not limited to non-native species; therefore, we made an explicit effort not to exclude native species from the invasive species included in this review. We excluded certain macrophyte–fish interactions from this publication to either avoid overlap with recent reviews on a topic (i.e., grass carp; Dibble & Kovalenko, 2009) or to limit the scope to freshwater systems and to exclude aquaculture.

Mechanisms driving invasive macrophytes impact on fish and macroinvertebrate communities

Invasive macrophytes affect aquatic communities through biomass production, photosynthesis, decomposition, and substrate stabilization (Fig. 1). Here, we detail various mechanisms driving invasive species effects on biotic and abiotic factors and resulting impacts on fish and macroinvertebrate communities (see Table 1 for a species-specific description of invasive macrophytes and their effects on fish, macroinvertebrates, and native vegetation).

Fig. 1
figure 1

Effects of invasive macrophytes on the ecosystem. Plant functions (white) and associated effects on abiotic (light gray) and biotic (dark gray) factors are shown. The plus (+) and minus (−) signs indicate the positive or negative response of the variable to an increase in the invasive macrophyte, respectively. In certain cases, the biotic response is unimodal instead of linear (as in the response of fish to habitat complexity), therefore both signs are used to depict the relationship

Full size image
Table 1 Invasive macrophytes and their interactions with macroinvertebrates and fish
Full size table

Effects of invasive macrophytes on plant biomass volume and habitat complexity

Invasive macrophytes can form dense monotypic stands, which can change macroinvertebrate and fish densities and community structure as well as interactions between macroinvertebrates and fish (Dibble et al., 1996). Many studies focused on aquatic community response to structural changes as a result of macrophyte invasion; therefore, we have organized this section to address specific responses of macroinvertebrates and fish.

Macroinvertebrate abundance/density

Macroinvertebrate density tends to increase only when both plant biomass and habitat complexity increases, as was the case with invasive species with highly dissected leaves or roots (e.g., Hydrilla verticillata (L. f.) Royle, Thorp et al., 1997; Myriophyllum spicatum L., Balci & Kennedy 2003; Trapa natans L., Strayer et al., 2003; Lagarosiphon major (Ridley) Moss, Kelly & Hawes, 2005; Cabomba caroliniana A. Gray, Hogsden et al., 2007). For instance, researchers in New Zealand found that beds of invasive L. major had 300% greater plant biomass and 113% greater macroinvertebrate densities compared with native vegetation (Kelly & Hawes, 2005). In contrast to the above-mentioned species, an invasion by Hymenachne amplexicaulis (Rudge) Nees resulted in 30 times greater plant biomass; however, because it replaced native submerged macrophytes with highly dissected leaves, habitat complexity was reduced (Houston & Duivenvoorden, 2002). Native vegetation had 2.7 times greater macroinvertebrate abundance than invaded areas (Houston & Duivenvoorden, 2002).

Despite an increase in habitat complexity due to invasion of certain macrophytes, investigators have found macroinvertebrate densities decreased (Cheruvelil et al., 2002; Hessen et al., 2004; Stiers et al., 2011) or were not different (Phillips, 2008; Theel et al., 2008) compared with native vegetation. For example, macroinvertebrate densities were negatively related to percent cover of three invasive macrophytes (Hydrocotyle ranunculoides L. f., Ludwigia grandiflora (Michx.) Greuter & Burdet, and Myriophyllum aquaticum (Vell.) Verdc.), probably due to anoxic conditions caused by dense mats that limited diffusion of oxygen and excess detritus (Stiers et al., 2011).

Macroinvertebrate assemblages

Changes in macrophyte composition and structural complexity can also alter macroinvertebrate assemblages (i.e., species diversity and composition). Species diversity of macroinvertebrates increased when habitat heterogeneity increased due to macrophyte invasion (e.g., Phragmites australis (Cav.) Trin. ex Steud., Holomuzki & Klarer, 2010; Eichhornia crassipes (Mart.) Solms, Masifwa et al., 2001; Brendonck et al., 2003; Kouame et al., 2010). For instance, in Lake Erie wetlands, presence of P. australis indirectly increased macroinvertebrate diversity by shading out Lemna spp. and reducing the prevalence of a dominant detrivore (Kulesza et al., 2009; Holomuzki & Klarer, 2010). Researchers found structural complexity of native and invasive species accounted for differences in macroinvertebrate species composition (e.g., H. amplexicaulis, Houston & Duivenvoorden, 2002; H. verticillata, Colon-Gaud et al., 2004; Mormul et al., 2010). In the Atchafalaya Basin, densities of Gastropods were four times greater and Hydrachnida were two times greater in native Ceratophyllum demersum L. than H. verticillata likely due to higher dissection of C. demersum leaves and related increase of surface area available to periphyton (Colon-Gaud et al., 2004).

Fish abundance

Effects of invasive macrophytes on fish abundance depend in part on quantity and quality of habitat prior to invasion. Systems with historically low densities of submerged vegetation tended had greater fish abundances after macrophyte invasion because habitat and food availability increased for fish (e.g., Kelly & Hawes, 2005; Barrientos & Allen, 2008; Bickel & Closs, 2008). In relatively vegetated areas, fish abundance decreased in invaded vegetation following a drop in water depth due to increased habitat complexity and decreased dissolved oxygen concentrations (Troutman et al., 2007). In contrast, other studies have shown that fish abundance did not differ between areas with or without invasive macrophytes (e.g., M. spicatum, Slade et al., 2005; H. verticillata, Hoyer et al., 2008; Phragmites austalis, Aday, 2007; Kulesza et al., 2009).

Fish foraging, growth, and diet

At a fine scale (<1 m2), foraging efficiency of fish decreases with the presence of an invasive macrophyte when habitat complexity surpasses a threshold limit (Valley & Bremigan, 2002a; Theel & Dibble, 2008). For example, largemouth bass had longer prey search times and fewer attacks in dense monocultures of artificial M. spicatum than less dense communities because prey fish were able to hide in dense canopies (Valley & Bremigan, 2002a). At the whole lake scale, increased habitat complexity can affect fish growth rates when shallow lakes become choked with invasive vegetation (Brown & Maceina, 2002; Cheruvelil et al., 2005; Sammons & Maceina, 2006). In Lake Seminole, Georgia, cover of H. verticillata was reduced from 76 to 22% and growth of largemouth bass increased; however, bass diets did not change to a great extent suggesting that growth responded primarily to increased predation efficiency (Brown & Maceina, 2002; Sammons & Maceina, 2006).

In some cases, fish diets changed when invasive macrophytes were present, either due to an increase in macroinvertebrate density (Njiru et al., 2004; Bickel & Closs, 2008) or differences in plant architecture (Dibble & Harrel, 1997). For instance, Njiru et al. (2004) speculate that the shift in Nile tilapia (Oreochromis niloticus L.) diet from a predominance of algae to insects is due to invasion of E. crassipes into the largely unvegetated Lake Victoria in Kenya. However, Dibble & Harrel (1997) found that prey of largemouth bass was predominately macroinvertebrates in enclosures of native Potamogeton nodosus Poir., but switched to preying on predominately fish in M. spicatum enclosures.

Fish habitat and assemblage

Non-native fish species can benefit from shelter and nesting habitat resulting from a macrophyte invasion (Houston & Duivenvoorden, 2002; Nico & Muench, 2004; Lapointe et al., 2010). For example, H. verticillata beds in a shallow Florida lake facilitated an invasive catfish, Hoplosternum littorale Hancock, by providing nest material and refuge from predators (Nico & Muench, 2004). Native fishes also benefit from habitat provided by invasive macrophytes, depending on environmental conditions (Collingsworth & Kohler, 2010). Following a water level drawdown, juvenile sunfish (Lepomis spp.) shifted habitat preference from native Potamogeton nodosus to M. spicatum, which colonized in deeper water (Collingsworth & Kohler, 2010).

There is some evidence that invasive macrophytes influence species composition and structure (age and size classes) of fish assemblages. For example, Houston & Duivenvoorden (2002) found that 79% of the fish in areas invaded by H. amplexicaulis were non-native compared with 3% non-native fish species in areas of native macrophytes. Weaver et al. (1997) found that age 0 bluegill were associated with areas of patchy M. spicatum, whereas yearling and adult bluegill were associated with areas of native macrophytes in Lake Mendota in the USA. Similarly, in Lake Chivero, Zimbabwe, smaller fish were found in the littoral zone dominated by E. crassipes in comparison to the pelagic zone (Brendonck et al., 2003). However, the above-mentioned studies did not show any evidence for an effect of invasive macrophytes on species diversity; and Slade et al. (2005) and Hoyer et al. (2008) found no differences in fish assemblages between invaded and native macrophyte communities.

Effects of invasive macrophytes on physico-chemical properties of the aquatic environment

Macrophytes alter physico-chemical properties of aquatic environments both structurally and functionally. Macroinvertebrates and fish are sensitive to changes in the water environment including dissolved oxygen levels, light penetration, water clarity, and allelopathic chemicals. The extent to which these parameters change influences the aquatic community.

Dissolved oxygen concentration

Invasive macrophytes limit concentrations of dissolved oxygen in water when they form dense floating mats that decrease atmospheric exchange with water (Masifwa et al., 2001; Strayer et al., 2003; Troutman et al., 2007; Villamagna & Murphy, 2010; Kornijow et al., 2010). Most aquatic species including macroinvertebrates and fish are sensitive to low dissolved oxygen concentrations; therefore, species assemblages would be expected to be different under floating mats of invasive macrophytes. Indeed, macroinvertebrate communities in ponds invaded by mat-forming macrophytes (H. ranunculoides, L. grandiflora, and M. aquaticum) were less diverse and were comprised primarily of species that tolerate low oxygen levels (e.g., Chironomidae and Naidadae) than non-invaded ponds with submerged vegetation (Stiers et al., 2011). Toft et al. (2003) found lower densities of epibenthic and benthic macroinvertebrates in floating mats of E. crassipes than native H. umbellata L. likely due to relatively lower oxygen levels beneath E. crassipes. However, researchers found greater densities of macroinvertebrates in roots of T. natans floating mats compared with native submergent species Vallisneria americana Michx., indicating that organisms can use oxygen exuded from T. natans roots and find refuge from underlying hypoxia (Strayer et al., 2003; Kornijow et al., 2010).

Light penetration

Floating mat species intercept light entering aquatic environments. Under reduced light conditions, submersed macrophyte and algae production is limited, thus changing aquatic community composition and food web structure with the introduction of a floating macrophyte (Troutman et al., 2007; Villamagna & Murphy, 2010). In a lake in Italy, researchers estimated that floating mats of T. natans reduced light transmission to 7% and supported two to eight times less algal biomass and two to ten times fewer macroinvertebrates (#g DW−1) than submerged species (Cattaneo et al., 1998).

Water clarity

Presence of rooted macrophytes, especially submerged macrophytes, is associated with greater water clarity through minimizing sediment resuspension and phytoplankton populations (see reviews by Barko & James, 1998; Madsen et al., 2001). Many fish species respond to an increase in turbidity by foraging less efficiently and selecting different prey (e.g., Hargeby et al., 2005; Shoup & Wahl, 2009). Researchers have not directly tested how invasive macrophytes affect water clarity; however, specific mechanisms could enable them to be more effective at reducing turbidity than native species (e.g., tolerance of extreme environmental conditions, Irfanullah & Moss 2004; high surface area to increase sedimentation, Rybicki & Landwehr 2007; allelopathy limiting phytoplankton growth, Hilt & Gross, 2008). For instance, Irfanullah & Moss (2004) reestablished non-native Elodea nuttallii (Planch.) H. St. John in a turbid, shallow lake in England to return the lake to a clear water state. The authors concluded that reestablishment was successful due to the species’ high tolerance of nitrogen concentrations and ability to grow under low light conditions.

Allelopathic chemicals

Certain invasive macrophytes exude allelopathic chemicals that affect epiphytic, herbivore, and fish growth and survival. Elodea canadensis Michx. and E. nuttallii were both shown to have allelopathic effects on cyanobacteria and lepidopteron larvae resulting in a competitive advantage over native species, which are depredated by herbivores (Erhard, 2005). M. spicatum can be lethal or sublethal to fish larvae (i.e., Neomysis integer Leach and Praunus flexuosus Müller, and Gasterosteus aculeatus L.) and has the potential to change fish distributions and occurrence of affected species in invaded habitats (Linden & Lehtiniemi, 2005). Kovalenko & Dibble (unpublished data) hypothesize that M. spicatum also changes the epiphytic community by exuding allelopathic compounds, resulting in macroinvertebrates avoiding M. spicatum as feeding habitat, and therefore, insectivorous fish as well.

Effects of invasive macrophytes on trophic dynamics

Trophic dynamics may also be affected by invasive macrophytes; however, these effects are likely species specific (Kornijow et al., 2005; Kovalenko & Dibble, 2011). Kovalenko & Dibble (2011) found that invertivorous and piscivorous fish in lakes dominated by M. spicatum had twice the isotopic niche (or trophic diversity) of fish in lakes dominated by native vegetation, indicating that primary consumers were feeding on a greater variety of carbon sources in invaded communities. As food resources decrease, there tends to be an increase in variety of resources that are incorporated into diets, which would show up as a greater range of 13C depletion all the way up the food chain. This could point to negative effects of M. spicatum on typical food resources for primary consumers, especially epiphyton (Kovalenko & Dibble, unpublished data). However, researchers found that E. canadensis did not affect Rutilus rutilus feeding preference for zooplankton even in dense vegetation, thus trophic dynamics were not significantly altered by the invasive plant (Kornijow et al., 2005).

Invasive plant traits and effects on the aquatic community

In 86% of studies we reviewed (43 out of 50 articles), researchers rejected the null hypothesis that aspects of aquatic communities did not differ based on presence or absence of invasive macrophytes. For instance, invasive macrophytes increased habitat complexity, hypoxia, allelopathic chemicals, facilitation of other exotic species, and inferior food quality leading to a decrease in abundance of native fish and macroinvertebrate species. However, mechanisms underlying invasive macrophyte impacts on fish and macroinvertebrate communities (i.e., biomass production, photosynthesis, decomposition, and substrate stabilization) were not fundamentally different than those of native macrophytes. In fact, in certain instances, there were increases or no changes in abundance, richness, etc., of native aquatic fish and macroinvertebrate species resulting from presence of an invasive macrophyte; even a species was shown to have the opposite effect in a different study system (e.g., H. verticillata, Sammons & Maceina, 2006; Barrientos & Allen, 2008; Hoyer et al., 2008; Theel et al., 2008). These results indicate that fish and macroinvertebrate communities respond to invasive species depending on both characteristics of the invader as well as the environment they are invading.

To explain what was fundamentally different in responses of fish and macroinvertebrate communities to invasive species as opposed to native species, we looked at specific characteristics that define invasions and invasive macrophytes. Shea & Chesson (2002) recommend using niche theory to understand biological invasions and identified three main factors that determine success of an invader: resources, natural enemies, and the physical environment. Putting invasive macrophytes into the “niche opportunity” framework developed by Shea & Chesson (2002), we see that resources (e.g., nutrients and light), competitors, and herbivores limit invasive macrophytes potential for expansion (Fig. 2). For example, Chase & Knight (2006) found that M. spicatum was only able to out-compete native vegetation under high nutrient conditions and absence of snails. However, generalist herbivores that consume competitors and carnivores that consume herbivores have indirect positive effects on invasive macrophyte success. For example, M. spicatum biomass was less in fish exclosures because sunfish reduced the abundance of herbivores, and therefore reduced herbivore consumption of M. spicatum (Ward & Newman, 2006). Thus, the extent of an invasion is a function of environmental/ecological factors and species’ ability to overcome limitations to growth.

Fig. 2
figure 2

Direct (solid line) and indirect effects (dashed) of the physical environment and biota on invasive macrophytes (modified from Shea & Chesson, 2002). Whether the interactions affect the invasive macrophyte positively or negatively are denoted with + and − signs, respectively

Full size image

Researchers have proposed a number of mechanisms invasive plants use to overcome these limitations (see review by Levine et al., 2003). For instance, some species have greater growth rates as a result of being released from natural enemies (i.e., enemy release hypothesis, Keane & Crawley, 2002). Another strategy is to use allelopathy to limit competitors [i.e., novel weapons (NWs) hypothesis, Callaway & Ridenour, 2004]. Other species have high resource-use efficiency and phenotypic plasticity to adapt to a range of environmental conditions (see review by Ren & Zhang 2009). It is important to note that traits associated with invasive species are only meaningful relative to traits expressed in the native community (Levine et al., 2003). Invasive macrophytes reviewed here utilized a variety of these mechanisms, and it is the expression of certain traits associated with successful invaders that resulted in greater impacts on the environment and biotic community.

In a recent review, Ren & Zhang (2009) categorized invasion mechanisms and traits of 133 species including 5 of 15 species detailed in this review. Each species was categorized based on literature that either explicitly stated invasive traits of a certain species or in which traits were readily apparent. Pertaining to invasive macrophytes included in our review, four species were categorized as having a NW (i.e., C. caroliniana, E. crassipes, E. canadensis, and E. nuttallii). One species was categorized as having phenotypic plasticity to adapt to a range of environmental conditions (PPET) (i.e., E. crassipes). One species was categorized as having phenotypic plasticity allowing for a greater range of resource allocation (PPRA) (i.e., P. australis). And one species was categorized as having a high growth or reproduction rate (High GRR) (i.e., E. crassipes). We further categorized an additional ten species included in this review based on the literature. M. spicatum and L. grandiflora have been documented as allelopathic (NW). T. natans, Egeria densa, Potamogeton crispus, H. ranunculoides, H. amplexicaulis, H. verticillata, L. grandiflora, and M. aquaticum have been shown to have a high growth or reproduction rate relative to native species (High GRR). Finally, H. ranunculoides, H. verticillata, H. amplexicaulis, L. major, L. grandiflora, and M. aquaticum have been shown to have phenotypic plasticity allowing them to have a greater range of environmental tolerance (PPET).

Many of these invasive traits (e.g., high growth rate, phenotypic plasticity, and NWs) can be directly related to how invasive species affected aquatic community dynamics (see Table 1 for a species-specific description of invasive macrophytes and their effects on fish, macroinvertebrates, and native vegetation). Researchers indicated changes in habitat structure and environmental conditions in part explained invasive species effects on fish foraging, growth, and abundance as well as macroinvertebrate richness and abundance. How invasive macrophyte species change habitat structure and environmental conditions can be attributed to a high growth rate (increase in density/habitat complexity and formation of thick mats that create hypoxic zones) and greater colonization of areas due to an increased ability to tolerate extreme environmental conditions (increase in total vegetated area). However, effects on fish and macroinvertebrate communities depended on characteristics of the invaded environment. For instance, researchers found that aquatic communities invaded by H. verticillata did not differ from communities without the invasive macrophyte if they were either located in an area with a small littoral zone or a nutrient-limited area (Barrientos & Allen, 2008; Hoyer et al., 2008).

Other studies indicated that NWs (i.e., allelopathic chemicals) determined impacts on fish and macroinvertebrate communities, particularly on reducing abundance and survival of macroinvertebrates that feed on these allelopathic species (e.g., Elodea spp., Erhard et al., 2007). In terms of fish, invasive macrophytes exhibiting a NW trait can have lethal and sublethal effects on certain fish through direct effects of toxicity and a potential reduction in food items due to effects on macroinvertebrates (Linden & Lehtiniemi, 2005; Erhard, 2005).

Conclusion

In this review, we sought to define specific ways in which invasive macrophytes affect fish and macroinvertebrate communities to answer the question: “If invasive macrophytes are affecting fish and macroinvertebrate communities differently than native macrophytes, how are they doing it?” We reviewed literature spanning continents, aquatic ecosystem types, and a wide variety of invasive plant species, and we defined specific mechanisms by which invasive species affect fish and macroinvertebrate communities (Fig. 1). Considering the accepted model of how plant structure influences fish and macroinvertebrate communities, we would expect that an increase of plant density would have positive effects on macroinvertebrate abundance and negative effects on fish foraging, growth, and populations. We found that positive effects of invasive macrophytes on fish and macroinvertebrate communities were associated with characteristics held in common with native macrophytes such as photosynthesis, increasing habitat complexity, and stabilizing substrate. Thus, removal of all or most plants in aquatic systems tends to have a negative effect on fish and macroinvertebrate communities (e.g., Mangas-Ramírez & Elías-Gutiérrez, 2004; Parsons et al., 2009), whereas selective removal of invasive species and/or immediate restoration of native vegetation tends to maintain the system’s diversity and density (e.g., Rybicki & Landwehr, 2007; Kovalenko et al., 2010). Negative effects, on the other hand, were associated with traits that invasive species use to invade. We found that three traits are largely responsible for negative effects on fish and macroinvertebrate communities: increased growth rate, allelopathic chemical production, and phenotypic plasticity that allow for greater adaptation to environmental conditions and resource utilization than native species.

It is apparent from our review that invasive traits are related to specific effects on fish and macroinvertebrate communities. While researchers have proposed using invasive traits to predict future invaders (Ren & Zhang, 2009), to our knowledge, there is currently no framework to predict effects on communities using these traits. Work on this topic would integrate data on environmental conditions with plant-specific information to model changes in structural and environmental aspects of aquatic systems as well as aquatic community response (Strayer, 2010). Furthermore, ecological engineering, keystone, and foundational effects of invasive macrophytes should be assessed in addition to functional traits, because impacts on ecosystems and communities likely result from a combination of these characteristics (Ehrenfeld, 2010).

Future research needs

After reviewing the literature, we identified areas we feel would further improve our knowledge of how invasive macrophytes influence fish and macroinvertebrate communities. Currently researchers have focused on responses of sport fishes to invasive species; therefore, a diversification of research effort to assess impacts on phytophylic fishes (especially rare and threatened species) is necessary to assess what different responses fish species have to invasions. For instance, Valley et al. (2010) observed that fishes with declining populations (i.e., killifish, darters, and rare minnows) were positively associated with plant biovolume of a native macroalgae (Chara sp.). In the event of an invasion, Chara would likely be replaced by invasive macrophytes, but fish response to this change would be difficult to predict. Furthermore, most studies focused on only one component of aquatic communities; however, interactions among community components are likely and research into impacts of invasive macrophytes on trophic relationships among plankton, macroinvertebrates, and fishes in early life and adult stages would give insight into these dynamics.

Due to incongruences of fish and macroinvertebrate responses at different spatial scales, an assessment of appropriate scales and methods to approach specific questions (multi-scale manipulative experimentation in ponds, mesocosms, field or laboratory) is necessary to clarify whole-system responses to macrophyte invasions. At a fine scale, investigation into whether habitats for parasites and pathogens are plant specific are necessary to determine mechanisms of invasion meltdown whereby introduction of one species leads to facilitation of another, and so on (Simberloff & Von Holle, 1999). At coarser scales, for example, research on how littoral–pelagic interactions are affected when an invasive species colonizes the littoral zone will increase understanding of different factors that determine responses at the system level.

Research is also necessary to develop management applications using information about how invasive macrophytes influence aquatic communities. For instance, applications using invasive traits to predict effects of an invasive species on a system would be a key tool for managers interested in prioritizing management efforts. Also, managers impact aquatic communities through aquatic plant management (e.g., herbicide, mechanical shedding, shading, etc.). While we did not address management effects in our review, an evaluation of these effects and costs should also be conducted in addition to invasive macrophyte effects when developing a comprehensive management plan.