**2.2. Freshwater and continental environments**

themselves from microbes, competitors, ultraviolet (UV) rays, and predators [1], reduce plant

Throughout 350 million years, plants and insects have been keeping a close relationship [4] resulting in an efficient defense system in plants that can recognize signals from herbivore and activate the plant immune response against them. To arrest herbivore attack, plants produce specialized metabolites with negative physiological effects against herbivores, such as

Metabolites that implicate in defense against herbivores can be modified by biotic and abiotic factors, such as humidity, altitudinal gradient, nutrient availability, herbivores diversity, etc., [6]. Then, we asked: at distinct environments, are plant defense mechanisms the same? If they

To answer, we select three very distinct ecosystems to compare plant defense traits: the first great difference is between aquatic and terrestrial environments, and the latter we divide in tempered and tropical forest. The objective is recognizing the ecological and evolutionary

In aquatic environments, there is a great diversity of photosynthetic organisms that interact and maintain complex ecological relationships with herbivores. In aquatic habitats, these interactions are considered very important since they affect the nutrient cycle and energy

Generally, when a plant is attacked, its defense mechanisms are activated through the production of diverse compounds generically termed plant secondary metabolites (PSM) [9]. It has been recognized that the secondary compounds may either serve as feeding deterrents or attractants in terrestrial plant-animal interactions or function as allelopathic chemicals or antibiotics; the same evolutionary pressures responsible for the many biologically active compounds found in terrestrial vegetation have been predicted to have parallels in marine [1, 10] and freshwater vegetation [11]. However, it is possible that the constraints in aquatic habitat

The primary producers most widely distributed in marine habitats are the seaweeds (red, brown, and green algae); these photosynthetic organisms have developed several defenses in response to herbivores, for example, by having a resistant or unpalatable physical structure or a morphology that makes the feed difficult for the herbivore or by having spatial and temporally diverse stages of life cycle and by the production of chemical defense against herbivores ranging from unpalatable to toxic. Marine algae are known to produce a wide range of secondary metabolites with various biological actions [1], many of them with medicine and

lead to some differences in the production and action of these natural compounds.

are different, are there some recognizable patterns at separate environments?

diversification of plant defense traits at distinct environments.

**2. Aquatic environments**

92 Pure and Applied Biogeography

flows of food chains [7, 8].

**2.1. Marine environments**

agriculture human uses [12].

tissue quality, and produce chemical and mechanical defenses [2, 3].

toxins, deterrent, dissuasive, and/or no nutrition [5].

In continental and freshwater environments, angiosperms are more abundant than macroalgae; therefore, they contribute significantly to primary productivity, and they maintain numerous interactions with aquatic consumers such as birds, mammals, fishes, crayfish, insects, and mollusks [22, 23]. For a long time, it was considered that the herbivory on freshwater macrophytes was infrequent and with minimal impact [24, 25]. Contrary to this point of view, a growing body of evidence suggests that the evolutionary and ecological importance of herbivory occurs in an aquatic context as in terrestrial habitats [9, 11]. Interactions between herbivores and aquatic plants have been reported in a wide range of habitat types, including freshwater lakes, rivers, estuaries, wetlands, and shallow seas [26, 27]. Accordingly, interactions between herbivores and aquatic plants have global distribution, and herbivores are present wherever submerged, floating, or emergent plants are present [27]. It is a fact that aquatic herbivores have a strong impact on aquatic plant biomass, productivity, and species composition [22, 28]; thus, like in terrestrial angiosperms, selection may favor aquatic plants that have chemical and other types of antiherbivore defenses [9].

Defense and resistance mechanisms against herbivores have been poorly understood in freshwater; even so we now know that freshwater plants are frequently chemically or structurally defended from consumers [29–31]. Structural defenses are more commonly found among upland plants than wetland plants [22]; in some cases, we can find thorns or tough leaves [32]. Chemical defenses are more widespread in macrophytes [23, 31] as well as in various algae, cyanobacteria [22].

Diverse groups of chemical compounds are known in aquatic plants, including alkaloids [33, 34], flavonoids, steroids, saponins, phenolics (including tannins), cyanogenic glycosides, glucosinolates [23, 29], quinines, and essential oils [32]. The different types of chemical defenses can vary between species, localities, time, and environmental conditions [31]. Many of them have not been identified; some studies have found multiple dissuasive components in the chemical extracts analyzed, but the low concentrations or their unstable state makes their identification difficult and therefore their correlation with the dynamics of the aquatic community [35].

In the aquatic environment, plant-herbivore interactions are different from terrestrial ecosystems because water provides different physicochemical conditions compared with air or soil, which should affect the herbivore access and the dispersal of released compounds [36].

#### *2.2.1. Macrophyte growth adaptations*

The growth forms of macrophytes are the most significant adaptation to freshwater environments and have important consequences for aquatic plant-herbivore interactions. The structure of the macrophytes and the presence of leaves and flowers above or below the water level determine the access and type of herbivores [36], so structures above the water surface can be consumed by terrestrial herbivores while the submerged parts by aquatic herbivores. Therefore the growth forms may have different mechanisms to prevent herbivory. Compared with terrestrial vegetation, freshwater aquatic plants produce less phenolic compounds, and a different phenolic amount in the aquatic growth forms has been observed. Lodge [22] indicated that the rank of mean phenolic content in wetland plants is tree > floating leaves plants > emergent > submersed > algae. Submerged macrophytes have much lower content than emergent or floating leaved macrophytes [37]. These differences are because emergent plants need more structural tissue, thicker cell walls, and a more complex cuticle to limit evapotranspiration and provide stability; therefore they present structural defenses, while submerged macrophytes are less structurally defended because they have little lignification, thin cuticles to facilitate gas exchange with water, and less exposure to ultraviolet light [36]. As a consequence, interactions with herbivores are modified, fully aquatic leaves of amphibious species and submersed plants exhibited higher grazing rates than aerial leaves, possibly due to a lower structural defense [38].

It is considered that in freshwater plants, constitutive chemical resistance against herbivores are frequent [31, 39, 40] presumably because of a high and lengthy exposure to mostly generalist herbivores [34]. Plants, which would be attacked by generalist herbivores, tend to be defended by a diverse collection of toxins in small concentrations, whereas plants attacked by specialist herbivores tend to employ higher levels of compounds, which reduce digestibility, as it happens in numerous terrestrial plants that are consumed by specialist herbivores.

Aquatic plant-herbivore interactions are highly variable across aquatic ecosystems [11], and we have little information about the presence, levels, types, and function of PSMs; thus we require further analysis in order to make suitable generalizations.
