Chapter 6 General discussion
6.4 Avenues for future research
6.4.1
Study species – some important unknowns
The climatic limits of A. arboreum, A. haworthii and C. orbiculata on Banks Peninsula have been established, but some unanswered questions remain that are relevant to their potential distributions. Quantifying seed bank longevity, seed dispersal, individual life span, and fecundity beyond current range limits for the three species would allow for temporally explicit projections of spread. More interestingly, the strength of biotic interactions and their effects on population dynamics remain unknown. While insect pollination occurs in all three species, Aeonium species are also pollinated abiotically, and all are also capable of autogamy (Alamo et al. 1996; Zietsman 1998). There are anecdotal tales of C. orbiculata being pollinated by New Zealand birds (Di Carter, pers. comm. 2010), which is plausible given that sunbirds are pollinators in its native range. If autogamy lowers seed output and progeny fitness in Aeonium species as it does in C. orbiculata (Zietsman 1998), pollinator interactions could strongly influence rates of spread for all three species.
Similarly, natural enemies and the effect of competition on the three species are unknown, though I hypothesized that interspecific competition is an important factor in habitat selection, given the low photosynthetic capacity of CAM plants. In the Canary Islands, Aeonium species are found only in specific habitats and regions, and Lems (1960) posits interspecific competition and adaptive radiation as underlying mechanisms. As a consequence of interspecific competition, I expect land use to be an important factor for all three species, and hypothesize that grazing facilitates C. orbiculata invasion by reducing competition and increasing vegetative spread. This hypothesis could be explicitly tested by grazing exclusion field trials [e.g. similar to studies of other succulents by Moolman and Cowling (1994) and Dean et al. (2015)] and would establish whether C. orbiculata can persist amongst vegetation without herbivores.
All three species have unanswered questions related to their taxonomy that merit further investigation. One of these is the effect of hybridization in Aeonium species, which hybridize readily (Jorgensen & Olesen 2001). Supposedly, natural hybrids are rare in the native ranges (Lems 1960) but on Banks Peninsula, they are common to the point that identification to species level can be difficult (Bill Sykes, pers. comm., 2010). Natural hybrids might affect the genetic diversity of Aeonium populations and their climatic tolerances, thereby shaping range limits, as such studying the genetics of natural populations and testing climatic limitation in hybrids could be beneficial. Meanwhile, the taxonomy of C. orbiculata is highly contested (Van Jaarsveld & Koutnik 2004), with debates over classifying the five varietals as distinct species that are unlikely to be resolved soon (Ernst Van Jaarsveld, pers. comm., 2014). In this thesis, I offered a hypothesis that the less-hardy regional populations of South African C. orbiculata such as those in coastal areas (Van Coller & Stock 1994) may be ancestral to those in New Zealand. Genetic or historical studies in New Zealand and South Africa could pinpoint the South African origins of New Zealand introductions. Such studies could improve projections of the
species’ potential distributions by establishing whether the extremely frost-tolerant genotypes of C. orbiculata are present in New Zealand. Finally, and perhaps most interestingly, the apparent climatic niche shift in all three species in New Zealand relative to their global ranges merits further investigation. While New Zealand is undoubtedly novel climate for all three species, it is not feasible to distinguish realized from fundamental niche shifts from correlative approaches alone (Guisan et al. 2014). There are numerous reports of niche shifts during invasions (Broennimann et al. 2007; Mata et al. 2009; Gallagher et al. 2010), but the underlying mechanisms have rarely been investigated empirically (Petitpierre et al. 2012). Alien Crassulaceae in New Zealand could provide an ideal case study due to the dramatic climatic differences between ranges. Fully reciprocal transplant experiments between New Zealand and the native ranges would evidence whether the phenomenon can be considered a fundamental niche shift or not, while phylogenetic analyses could investigate underlying mechanisms, e.g. whether the shifts arise from evolutionary changes or are in fact a result of underlying phenotypic plasticity or exaptation. These species present an opportunity to improve our understanding of niche shifts and their mechanisms.
6.4.2
Limitations, and lessons learned
The majority of limitations have been discussed in corresponding chapters, but it is important to emphasize that the final refined projections for Banks Peninsula still over-predict suitable climate (over-prediction rate was 27%, 24% and 5% for A. arboreum, A. haworthii and C. orbiculata, respectively). Final projections also do not account for habitat suitability, thus they are valuable starting points but not predictions of spread. As discussed earlier, fine-scale iSDMs trained on the fundamental presence and absence points could improve projections for Banks Peninsula significantly. In addition, there is room for improvement in the IPMs. Some parameters were omitted (e.g. seed bank), population growth was predicted only in the steady state, and fecundity parameters were extrapolated from surveys. The IPMs could also be improved by running multiple simulations to estimate stable values of lambda that allow colonization. However, this would require temporally explicit projections that incorporate stochasticity, and an estimate of seed rain to un-colonized sites. Some of the limitations of this thesis have highlighted issues to bear in mind in future work, the most important being choice of study species. While A. arboreum, A. haworthii and C. orbiculata are biologically interesting and of immediate concern to local practitioners, they are far from ideal species for a modelling exercise. In hindsight, other characteristics would have been desirable, for example, the species should not have highly restricted microhabitat requirements that are unlikely to be represented at a 1 km grid resolution, and should ideally not have a disjunct distribution in climate space or non-analog climate between the native and invaded ranges. A well-studied species would ensure a higher quality of distribution data in the native range, as well as sound a priori reasoning for choosing climatic variables and understanding the role of non-climatic effects. An annual or fast- growing species would maximise changes in demography or vital rates over a two-year field season.
Despite all of this, it was exciting to work with species that have scarcely been studied, and I am grateful for the opportunity to contribute to our understanding of the Crassulaceae.
6.5
Conclusions
To summarize, my four thesis objectives were answered as follows;
1. The current distributions of A. arboreum, A. haworthii and C.orbiculata in New Zealand are climatically distinct from their distributions elsewhere, and this constitutes a shift in at least the realized niches into cooler and wetter climatic conditions. I provided SDM projections of the species’ potential distributions at a national scale, although it was necessary to use simple models.
2. Performance was strongly climate-limited along the gradient present on Banks Peninsula, and declined with increasing elevation and precipitation. Beyond the species’ current distributions, at higher elevations, performance declined. The decline in performance was sharpest for Aeonium species, but none of the three species have yet reached climatic equilibrium.
3. Fecundity varied significantly within and among wild populations on Banks Peninsula, but very little of this variation was directly linked to climate. Plant size was the key predictor of fecundity, and across their current distributions in the region, climate does not appear to limit reproduction.
4. Testing SDMs against predicted population growth revealed that the SDMs over-predicted suitable climate, both at range edges and within the “climate envelope”. It appeared unlikely that the over-prediction was solely because of the SDM algorithm. Scale, occurrence data and climate data are also likely sources of error, factors that are relevant to all SDM studies. Despite over-prediction, SDM and higher predicted population growth were ranked similarly, so projections were refined to improve estimates of range limits on Banks Peninsula.