Effects of native forest and pastoral land use on the population dynamics and trophic role of the New Zealand freshwater crayfish Paranephrops planifrons (Parastacidae)

Deforestation and conversion to pasture of over 50% of land area has affected the water quality, physical habitat, and fish and benthic invertebrate communities of New Zealand streams, but the impact of land use change on crayfish populations is unknown. The population dynamics, growth, and diet of crayfish in Waikato pastoral streams were compared to crayfish inhabiting native forest streams to determine whether conversion to pasture has adversely affected crayfish populations and whether the change in land use has affected their role in the stream community. Crayfish densities were greater in native forest streams due primarily to greater juvenile recruitment. Crayfish populations in pastoral streams exhibited much greater growth than in native forest streams, but their longevity was shorter. Reproductive maturity occurred after 1 year in pasture streams, as opposed to 2 years in the native forest streams, and consequently juvenile crayfish endured only one winter period, where greatest mortality occurred, before reproducing. Temperature was the primary factor influencing the faster egg development and greater growth rates of juvenile P. planifrons in pasture sites, although food source may also have been a significant factor. Crayfish in pasture streams consumed more invertebrates than in native forest streams where leaf detritus dominated stomach contents. In laboratory growth experiments, invertebrate food sources increased the moult increment of juvenile crayfish and decreased the length of the intermoult period, whereas conditioned leaf litter did not promote growth and was not assimilated into body tissue. Warmer water temperatures decreased the length of the intermoult period, but had no effect on the size of moult increment. Crayfish annual production estimates did not differ between land use, as a consequence of higher crayfish densities in native forest streams and faster growth of crayfish in pasture streams, suggesting that the overall success of the crayfish populations in pasture streams was similar to native forest streams. However, the removal of riparian trees (particularly the loss of their root systems) seems to have undermined the stability of habitat available to crayfish. Habitats of crayfish in native forest streams were primarily tree roots, undercut banks, and accumulations of woody debris. Deforestation has resulted in the loss of many of these habitats, and cobble substrates, macrophytes and terrestrial vegetation growing within the stream, now provide important habitats for crayfish in pasture streams. Crayfish in pasture streams were not as resistant to a major flood disturbance as those in native forest streams, suggesting that the habitats in these pasture streams did not provide stable refugia. Furthermore, the rate of recovery of a crayfish population in one pasture stream following the flood was very slow indicating that P. planifrons does not exhibit resilience traits and is dependent on stable refugia to persist during floods. Crayfish were identified as top invertebrate predators in these hill-country streams despite omnivorous feeding habits that allowed crayfish to exploit a variety of food resources. Crayfish in native forest streams had greater volumes of leaf detritus in their gut contents than those in pasture streams. However, stable carbon and nitrogen isotope analyses and bioenergetic studies of the trophic basis of crayfish production identified aquatic invertebrates as the major energy source for crayfish growth in both native forest and pasture streams. Snails, mayflies and chironomids were the most important taxa to crayfish diet particularly in native forest streams, but by acting largely as a predator of a wide range of invertebrate taxa in pasture streams, crayfish were able to take advantage of both allochthonous and autochthonous energy pathways. As omnivores, crayfish may have dual functional roles as both predators and processors of organic matter, and through production of FPOM they may make leaf matter available to a wider range of invertebrates (e.g. collectors) in stream ecosystems. Juvenile crayfish had higher rates of assimilation for leaf detritus and higher ingestion rates of invertebrates and leaf detritus than adult crayfish, and most of the annual production of crayfish populations in both native forest and pasture streams was from juveniles. Age affected the role of crayfish in native forest streams where adult crayfish processed more leaf litter into FPOM, and juveniles were more carnivorous. However in pasture streams, the roles of juvenile and adult crayfish were similar, as large crayfish were able to utilise the abundant invertebrate resources. The role of crayfish as predators or processors of leaf detritus may change according to their growth requirements, but more importantly, according to environmental influences on local food resources. Flexible feeding habits have allowed P. planifrons to maintain their populations in pasture streams, despite considerable land use disturbance