Impacts on food web properties of island invertebrate communities vary between different human land uses

Human land use is of growing concern for island ecosystems. Besides direct
impacts on biodiversity, land uses can alter the functioning and structure
of ecosystems. Central to this are impacts on food webs. The release of
additional nutrients from human origin, habitat homogenization, or
environmental filtering due to human land use can change the diet of
individual consumer species (i.e., their trophic niches) and the
distribution and overlap of trophic niches within a food web. However, it
remains largely unclear whether the effects on food web properties vary
between the different and predominant human land uses present on islands.
Here, we investigated the impact of two dominant human land uses on small
oceanic islands (i.e., urban and tourism development) and tested if and
how different land uses on islands affect food web structure. To
disentangle human land uses, we investigated islands, which were either
privately owned by a tourist facility (i.e., exclusively tourism land use)
or experienced urban development from the local population (i.e., urban
land use), or remained uninhabited, serving as reference sites free of
direct land use. Using stable isotope analysis, we show that isotope
signature, trophic (isotopic) niches, and overall food web properties of
the investigated island invertebrate communities were significantly
changed under both land use regimes. While trophic diversity was reduced
and trophic niche widths increased under tourism land use, the
investigated food webs showed reduced trophic diversity at the food web
base and a more uneven trophic niche distribution under urban land use. In
summary, these findings show that different human land uses can have
contrasting impacts on oceanic island food webs. As oceanic islands
experience rapidly growing human land conversion, our results indicate
that they may also face increasing yet unpredictable long-term changes in
food web dynamics.