A long-term study of a small rocky reef

1. A long-term study of a small rocky reef Bill Ballantine Leigh Marine Laboratory New Zealand

2. This study aims to determine the variations over TIME in a NATURAL marine benthic community i.e. where there is no exploitation, no serious disturbance and no driving force for change. To make this valid, practical and generally meaningful requires a large number of decisions, choices, stratifications, etc. including -

3. Theoretical points: 1. For spatial comparisons the observations need to be made at the same time. Similarly, comparisons over time need observations at the same place. The study needs a fixed site. 2. Comparisons in space need to be separated by sufficient distance to avoid auto-correlation (pseudo-replication). Similarly, replicates in time need sufficient separation in time. The study must extend over multiple generations.

4. Practical considerations: • No human interference : in a marine reserve • No major natural disturbances (e.g. erosion) • Easilyaccessible for frequent observations • Simple topography – uniform, gentle, bedrock slope • Reduced secondary factors (e.g. rock type) • Low diversity – only ~15 significant species • Short generation times – < 1 - 5 years • Comprehensive – all significant species monitored • Small enough to allow census of most species • Large enough to provide multiple patch dynamics

5. The locality: Goat Island Bay, Leigh Standard Reef

6. Standard Reef: total area 5m x 4m

7. Standard Reef: 1–20 @ 1m2, A-L @ 0.1 m2 Most data that will be shown comes from 1-10 m2

8. Square 2 : open rock and crevices

9. The Standard Reef biological community Trophic level Carnivorous whelk 3 Cellana radians Sypharochiton pelliserpentis Melagraphia aethiops Turbo smaragda Grazing molluscs 4 species Mussels 2 Barnacles Ralfsia Phytoplankton Benthic microflora 1 Good data: regular census Some data: infrequent sampling

10. Grazing molluscs: • RESIDENTS • A chiton • Sypharochiton pelliserpentis A patellid limpet • Cellana radians • VISITORS • A turbinid snail • Turbo smaragda • A trochid snail • Melagraphia aethiops

11. Sypharochiton pelliserpentis Homing to crevices (< 30 cm) Slow growing and long-lived (>3 years) Small changes (<10% per month and <50% per year) No seasonality Range of biomass over time ~ 4x

12. * * Total wet weight

13. Cellana radians Home-ranging (< 1 m) Fast growing and short-lived (< 2 years) Rapid changes (up to 50% per month) Strong seasonality (summer peaks) Range of biomass over time >20x

14. Cellana radians biomass (g) 1-10 m2 (all data) Standard Reef, Echinoderm Reef

15. Turbo smaragdus Wide-ranging (up and back to lower zone) Moderate growth rate and longevity Very rapid short term changes (>50% per month) and rapid changes per year >70%) Weak seasonality Range of biomass over time >20x

16. Turbo biomass (g) 1-10 m2 (all data) Standard Reef, Echinoderm Reef

17. Melagraphia aethiops Very wide-ranging (at this level) Moderate growth rate and longevity Long periods of low (< 30) or high (> 50) abundance No seasonality Range of biomass over time >20x

19. Conclusions for grazing molluscs: The variations of biomass over time are LARGE and important. 2. The variations are NOT PREDICTABLE (beyond very short time frames). 3. The variations are NOT RANDOM and the patterns are distinctive for each species. 4. The variations show no persistent patterns of competition. None of these conclusions were expected, and they do not match well with existing theory on food web models.

20. Comparison of 4 grazing molluscs Cellana Melagraphia

21. Sessile species: A small sheet-forming barnacle Chamaesipho columna A black encrusting alga Ralfsia (cf confusa) A small mussel Xenostrobus pulex Despite only 3 species, the patch dynamics are complex

22. Barnacles: Chamaesipho columna

24. Ralfsia covered barnacles

25. months)

27. Conclusions for sessile species are the same as for the grazing molluscs – (a) large variations over time (b) unpredictable (c) different patterns for each species (d) low correlations between species

28. Sessile species dynamics Barnacles settle only on bare rock Ralfsia only grows well on or between barnacles Mussels settle on Ralfsia, barnacles or themselves, but not on bare rock Settlement (all species) occurs as strong pulses, but is only weakly seasonal. Ralfsia grows over barnacles but does not harm them Mussels grow over and smother barnacles and Ralfsia Ralfsia dies back after ~ 12 months There is no equilibrium state.

29. Barnacles Variations with time Ralfsia 1999 Barnacles Ralfsia Bare Barnacles Ralfsia Mussels Barnacles Bare Mussels 2001 2003 Mussels

30. Correlation coefficients (r) for 1-10 m2 on Standard Reef 1997 onwards

31. Extra time A further 9 years of data is available but includes a 2 year gap. Conclusions from extra time confirm and reinforce previous conclusions especially: The range of variation (b) The specifically distinct patterns

33. * * Three lines of evidence indicate a similar event occurred in 1981

34. Turbo smaragdus biomass (g) 1-10 m2

36. Discussion: 1. All species in this natural and undisturbed community show variations over time which are: (a) Large and ‘ecologically important’ (b) Unpredictable (except for very short periods) (c) Non-random and distinctive in their patterns (d) Largely independent Given (a) and (d), it follows that the interactions between species are varying over time.

37. 2. There is little or no comparable data because (a) these are very difficult to obtain even if time is available and undisturbed sites exist (b) the topic does not seem interesting to most workers (c) career paths and grant agency policies tend to prevent their collection.

38. 3. Existing knowledge is mainly from studies that are: (a) short-term (b) detailed and precise (c) focused on active processes and limiting factors Such studies are necessary and important, but are effectively just short clips from a movie.

39. Cellana radians biomass 10m2 The conclusions from the 3 periods would be quite different.

40. 4. Existing theory on temporal variation in biological communities consists mainly of implicit and untestedassumptions that such variation is (a) small, except when disturbed from outside (b) and /or periodic (e.g. seasonal) (c) and /or random (d) and /or unimportant

41. 5. Existing models of biological community dynamics implicitly assume that (a) the community is maintained by active processes (b) these processes can be recognized and estimated (c) the estimates can be used to make a useful model (d) it is not necessary to include any temporal variation (other than that produced by external factors) e.g. Branch (2008) Trophic interactions in sub-tidal rocky reefs on the west coast of South Africa

43. 6. Such models are useful as descriptions, but they cannot be made predictive in any precise way because they are equilibrium models and are unable to cope with continuous or complex temporal changes.

44. Conclusions 1. In a simple undisturbed rocky shore community the main species showed large changes in abundance over time- scales that included multiple generations. 2. These changes were not predictable, except over very short time frames but were not random. Each species had a distinctive pattern but the details never repeated precisely.

45. 3. Despite all the changes and the absence of any equilibrium state, the community persisted through time as frequently recurring similar structures and patterns. 4. There is little comparable data (spatially-explicit, multi-generational), and no clear theory on what temporal changes should be expected in an undisturbed community.

46. 5.It is not known if the kind of changes found in this study would occur in other communities, but it seems likely. • It is well-known that even simple physical systems can show complex intrinsic dynamics, if the system is externally forced and governed by non-linear processes.

47. Biological communities are such systems and consequently are likely to show similar intrinsic dynamics. • Furthermore, although these intrinsic dynamics will be overlaid by ‘external’ disturbances (such as exploitation, severe storms, pollution, etc.) they are likely to continue to operate.

48. A useful analogy ? In the 1960s, weather forecasters were confident that with better data and analysis, their forecasts would improve indefinitely. Edward Lorentz proved that this is not true. “Complex systems” are completely deterministic and show recognizable types of order, but do not reach equilibrium and never repeat exactly the same state. Consequently, detailed predictions are not possible (except for short periods), no matter how much is known about the present situation or the governing processes. Biological communities are likely to be “complex systems” of this type.

49. If ecologists considered the component species of a community analogous to the weather at a locality and the entire community analogous to the climate, I believe considerable practical and theoretical advances could be made with existing data. Community predictability will become a matter of pattern and probability not precision. The climate of an area is composed entirely of ‘weather events’ none of which can be precisely predicted, but we know that the climate of an area has real and useful levels of predictability, indeed most of our activities depend on this (e.g. successful farming is possible).

50. Three problems with this study 1. No spatial replication 2. Very small area 3. Simple community (low biodiversity) I could only manage a single, small, simple area for a long period. Better data would not only require large amounts of work and finance, it would also require a long time. Consequently it seems sensible to extract as much information as possible from the present study.