Introduction
Over recent decades, global human transportation networks have led to
the establishment of once geographically restricted species in new
ecosystems (Hulme 2009; Hulme et al. 2008; Roderick & Navajas
2015; Sinclair et al. 2020). Introduced taxa may act as
“ecosystem engineers” by changing the structure of an ecosystem
through alteration of abiotic and biotic factors (Jones et al.1994); such non-native taxa may then be called invasive because of their
negative impact on ecosystem services and native species (Richardsonet al. 2011). Invasive plants are particularly pervasive and can
act as ecosystem engineers by changing habitat structure, soil chemistry
(Ehrenfeld 2003), nutrient cycling (Weidenhamer & Callaway 2010) and
microclimates (Ruckli et al. 2013). For example, the Hawaiian
invasion of the firetree Myrica faya increases nitrogen
availability in a generally nitrogen-limited system (Vitousek et
al. 1987). One consequence of ecosystem engineers is invasional
meltdown, in which modification driven by one invasive facilitates the
establishment of other non-native taxa (Green et al. 2011;
Simberloff & Von Holle 1999). Major compositional shifts in the
community often occur following introduction of multiple non-native
taxa, resulting in trophic cascades and shifts in biotic interactions
(Borges et al. 2006; Cucherousset et al. 2012; Wainrightet al. 2021). Through changed abiotic and biotic factors, invaded
ecosystems may then function as a habitat “sink” for native taxa in a
heterogenous landscape; in this case, endemic species would migrate from
less-disturbed, productive “source” habitat into invaded “sink”
habitat where reproductive output is diminished below population
sustainability (Pulliam 1988). By studying source-sink dynamics across
adjacent invaded and non-invaded habitat, we can start to quantify how
alien invasion impacts ecosystem structure and function at a landscape
scale.
To understand source-sink dynamics caused by invasion, it is important
to go beyond single taxa and study entire biological communities.
However, source-sink dynamics can produce misleading results when
quantifying communities using classic biodiversity metrics. This may
explain the contrasting results of studies examining how invasions
impact biotic communities. For
instance, the response of arthropod communities to plant invasion shows
mixed effects, with many studies detecting abundance and richness
decreases while others show increases (Litt et al. 2014). In a
source-sink context, species presence and abundance values in a habitat
would not indicate a productive system. Through changes in biotic
interactions associated with invasion (Bezemer et al. 2014) such
as increased predation risk, lack of the most nutritionally beneficial
prey items or a higher interaction with parasites (Mattos & Orrock
2010; Suarez et al. 2000), invaded habitat may serve an
ecological trap. Understanding the new network of biotic interactions
between native and non-native taxa following invasion will provide
insight into functional shifts that occur in ecosystems and indicate if
invasion can induce an ecological trap for native taxa.
In our study, we use a recent invasion of kāhili ginger (Hedychium
gardnerianum ) in Hawai‘i to measure how the establishment of an
invasive ecosystem engineer influences biotic interactions. Kāhili
ginger is one of the world’s worst invasive species and has invaded the
Azores, Madeira, Jamaica, Réunion, New Zealand and Hawai‘i, while also
expanding in South and Central America, Australia and Southern Africa
(Pereira et al. 2021). Native to the Himalayas, H.
gardnerianum was brought to Hawai‘i in 1940 and is found in wet forest
throughout the Hawaiian Islands, where it can form almost impenetrable
stands up to 3 m in height (Santos et al. 1992; Vorsino et
al. 2014). It is a very aggressive, shade-tolerant plant, and can
invade and establish in intact native rain forest habitat, displacing
native understory vegetation and altering composition of soil microbial
decomposers (Kao-Kniffin & Balser 2008; Minden et al. 2010).
Compositional differences between native forest and ginger-invaded sites
have been noted using invertebrate communities in New Zealand, though
the impact was context dependent (Yeates & Williams 2001). Changes in
the abundance of fungivores and decomposers is the most consistent
result of the invasion (Bassett 2014). The evidence of compositional
shifts caused by ginger most certainly causes differences in the
interactions between native and non-native taxa. Because of the global
prevalence of ginger, understanding such impacts on biotic interactions
are crucial to mitigate the costs associated with invasion.
Our current study uses the generalist endemic spider (Pagiopalus ,
Philodromidae) (Suman 1967) to assess whether modified sites could serve
as sink habitat. While detailed demographic studies are essential to
confidently identify source-sink dynamic, this is logistically
challenging in most systems. Instead, by assessing shifts in biotic
interactions, we can ask if major shifts in ecological function occur
for native taxa; because such changes will likely carry a cost, altered
biotic interactions may then indicate suboptimal habitat. We utilize
metabarcoding to compare dietary communities and parasite loads in
spiders across sites in ginger-invaded habitat and native-forest.
Through the diet of a generalist predator, we not only can assess biotic
interactions between predator and prey, but we also obtain a window into
the arthropod community composition. The study focuses on a site on the
island of Maui, specifically the mesic forest of Waikamoi on East Maui.
Here, a sharp boundary exists between the ginger invasion and the native
forest due to the efforts of the Nature Conservancy of Hawai‘i in
protecting their lands (The Nature Conservancy of Hawai‘i 2011). The
adjacency of native forest and ginger-invaded sites creates the
possibility of connectivity between metapopulations. This provides an
optimal system to assess differences in the quality of neighboring
habitat and ask if ginger habitat serves as a “sink” by changing the
biotic interactions of an endemic generalist predator found in both
habitats.
In pursuit of this question, we have four hypotheses. First, becausePagiopalus are generalist predators capable of eating a wide
diversity of prey times, we expect to find establishment of the spider
in ginger, reflected in similar abundances across sites. Second, the
altered environmental conditions in ginger will result in arthropod
communities differing from native forest sites; we expect, then, to
detect compositional differences in the diets between spiders collected
in ginger and native forest. Third, ginger sites will host more
non-native prey items, and this will be reflected in the diet ofPagiopalus collected in ginger habitat while the diets ofPagiopalus in native forests will instead consist of mostly
native prey. Lastly, given the hypothesis that the novel environments
created by ginger will serve as sink habitat by increasing diversity of
non-natives, we hypothesize a higher interaction with non-native
parasites indicative of poor habitat quality.
Methods