KA PUNAEWELE MEA AI O NA LOKO IA: THE USE OF d15 NITROGEN AND d13 CARBON ISOTOPES AS AN EXTENSION OF ONE’S KILO TO UNDERSTAND TROPHIC INTERACTIONS WITHIN A LOKO IA

The east side of Hawaiʻi Island, with its abundant rain and fertile soils, is susceptible to invasion by non-native flora and faunal species.  The traditional functioning of the loko iʻa (traditional Hawaiian fishpond systems) of Waiāhole and Kapalaho are challenged by an abundance of non-native invasive species including California grass (Brachiaria mutica), Mexican mollies (Poecilia mexicana), Marquesan mullet (Moolgarda engeli), and tilapia (Sarotherodon melanotheron). Not only are these invasive species physically reshaping the environment thro ugh reduction in open water and  increased sedimentation, but they may also be changing the water chemistry and the structure of native food webs within these loko iʻa .  

Figure 1. Native (closed) and non-native (open) species at the loko iʻa in functional groupings of primary producer (PP), primary consumer (C1), and secondary consumer (C2) based on δ¹⁵ N signatures.

In an attempt to quantify the energy dynamics at the loko iʻa , Kamehameha Schools Kumuola Marine Science Education Center (Kumuola ) partnered with Ke Ana Laʻahana Public Charter School (KALPCS) to collect and process samples of aquatic and semi-aquatic  native and non-native  organisms for δ ¹⁵  nitrogen and δ ¹³  carbon stable isotopic analysis to further understand present-day energy sources and trophic niches at the loko iʻa of Waiāhole and Kapalaho .

The generalized functional groupings of primary producer (PP), primary consumer (C1) and secondary consumer (C2) were assigned based upon δ¹⁵N clustering for all resident loko iʻa organisms analyzed (Fig. 1) .  The Mexican molly, kanda and tilapia, the three significant non-native invasive fish species at the loko, group with the native primary consumers and detritivores and have δ¹³C signatures that overlap with important native food fish and crustaceans. Secondary consumers in the studied loko iʻa were all native, clustered tightly in δ¹⁵N isotope signatures, and had broadly overlapping δ¹³C signatures.  Observational data by KALPCS students supports their grouping as the top-level consumers in these loko iʻa.

The invasion and establishment of non-native species to the Hawaiian Islands is a reality that we will continue to face going forward; understanding the nature of the resultant impact allows us to manage and combat the invasions to maintain the function ality of our culturally important places.