Introduction
It is known that human-mediated transport of nonindigenous species (NIS) around the world has greatly increased in the twenty-first century and its continued growth will affect marine ecosystems more frequently and more severely in the future
[1]. NIS also impacts on economies (e.g., fishing, clog water intake pipes, agriculture, incrustation on piers and hulls of ships) and concomitantly creates serious issues for marine conservation science [
1,
2]. In marine ecosystems, the impact caused by NIS has been severe and biological invasion has been regarded as the second biggest cause of biodiversity change, after habitat destruction
[3]. Therefore, this phenomenon constitutes one of the four greatest threats (land-based pollution sources, overexploitation of living marine resources, destruction, and physical modification of the marine habitats) to the world’s oceans
[4].
Ships can transport marine organisms (i.e., bacteria and other microbes, small invertebrates and eggs, cysts and larvae of various species) colonizing external or internal submerged surfaces, such as hulls or in the water held in internal ballast water tanks or inlets, or in the ballast water itself. The problem is compounded by the fact that virtually all marine species have life cycles that include a planktonic phase. Therefore, ballast water used by NIS has perhaps become the best-known mechanism of how unintentional introductions occur in marine systems [
5,
6]. In response to the threats posed by invasive species, United Nations called on International Maritime Organization (IMO) and other international bodies to take action to address the transfer of harmful organisms by ships. IMO developed rules on ballast water discharge to prevent the spread of alien species, and has been active in ballast water issues since 1992. Today a very wide range of stakeholders, including shipping companies, ports, environmental groups, tourism bodies, public health organizations, seafood producers, etc., are working on various aspects internal (local or regional) and external (international) in order to find solutions for the problem brought by ballast water
[4].
The general obligations according to International Convention for the Control and Management of Ships’ Ballast Water and Sediments are to prevent, minimize, and ultimately eliminate the transfer of harmful aquatic organisms and pathogens through the control and management of ships’ ballast water and sediments, consistent with international law. Reballasting at sea, the exchange of coastal ballast water for mid-ocean ballast water, as recommended by the IMO guidelines, currently provides the best-available measure to reduce the risk of transfer of harmful aquatic organisms. However, it is subject to serious ship-safety limits. Even when it can be fully implemented, this technique is less than 100% effective in removing organisms from ballast water. The efficiency is extremely important and adding to this, other alternatives, effective ballast water management, and/or new treatment methods are being developed to replace or reinforce reballasting at sea
[4].
It is known that transport of NIS by ships is fast and dynamic and it operates over a larger geographical scale than natural colonization processes [
7,
8]. After transport of individuals from their native range, release and establishment to a novel location, NIS face a range of physical and chemical (e.g., temperature, salinity, and light) and biological factors, including negative biological interactions such as competition and predation, which will determine whether they will persist into the future in the receptor community
[9].
A very small fraction of transported and introduced species become invasive
[10] and the success of these organisms in new habitats will depend on suitable site and invader characteristics (physiology tolerances and reproduction modes), ecological interactions with native species and history (transport from donor to receptor region); these will determine when and where a NIS may bring about impact at the individual, population, and community levels
[6]. A well-known example is the green alga
Caulerpa taxifolia that invaded the Mediterranean Sea and brought biological and economic negative impacts to the new ecosystem.
Disturbance (natural or anthropogenic) and invasion can be strongly linked. A disturbance may create vacant spaces, release resources or alter species interactions, allowing invaders to establish in a new area [
6,
11]. This may give NIS a foothold from which they can create changes in the structure and function of ecosystems [
1,
12,
13]. Once established, many NIS can modify their own habitat as engineering species (i.e., species that create new microhabitats by causing physical state changes in biotic and abiotic materials that, directly or indirectly, modulate the availability of resources to other species) and may facilitate the invasion of other NIS, a process termed the “invasional meltdown scenario” [
14,
15]. A classic example is invasion by the zebra mussel (
Dreissena polymorpha) in the Laurentian Great Lakes which resulted in an increase in the diversity of macroinvertebrates, which used zebra mussel shells as refuge from predators
[16].
Spatial heterogeneity is known to be an important factor in invasion dynamics and NIS may produce impacts at the community level which range from the possibility that native species are driven to extinction to alterations in abundance patterns of organisms within the community [
17,
18]. Indeed, more species may be able to invade a fluctuating environment compared with a homogeneous one because fluctuating environments provide more niche opportunities for both NIS and natives
[17]. Therefore, after recent developments in the theory of invasion, some researchers now suggest that environmental heterogeneity plays an important role in biological invasion and the fate of the community
[18].
The relationship between community diversity and bioinvasion was a hypothesis first proposed by Elton
[19], when he predicted that less invaded communities were more diverse. It follows that more diverse communities show more complete or efficient use of resources and thus avoid leftover niche spaces that may be available for occupation by new species. However, some studies questioned this hypothesis, after comparing the number and abundance of NIS with the native diversity [
20–
23]. For instance, large scale studies reported positive correlations between the number of invasive and native species [
17,
21,
24–
27]. On the other hand, patterns analyzed at smaller scales found negative relationships between native diversity and invader success [
17,
26,
28–
34].
Many organisms produce organic compounds that are not directly involved in the normal growth, development, or reproduction (i.e., are not primary metabolites). These substances are known as secondary metabolites that can help organisms, for instance, to survive against predators and competitors, due to its negative effects over them. Terpenes, steroids, fatty acids, polyphenols, polyketids, and alkaloids are structural classes of secondary metabolites with ecological functions and are typically...
