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Keywords:

  • breeding success;
  • habitat choice;
  • human disturbance;
  • wader

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • 1
    Increased human pressure on coastal habitats has contributed to the global population decline in waders. Although coastal development can be particularly rapid and poorly regulated in tropical countries, very little research has been conducted to assess the extent of these impacts in the tropics.
  • 2
    We examined the potential effects of human disturbance and tourism-related habitat changes on Malaysian plovers breeding on sandy tropical beaches.
  • 3
    In 2004 and 2005, we monitored 54 and 79 pairs of Malaysian plovers in the Gulf of Thailand, and used logistic habitat models to identify factors influencing habitat selection and breeding success. These models included variables affected by anthropogenic changes such as human disturbance and vegetation structure, as well as other natural factors such as prey availability and predator densities. We also assessed causes of nest failure and conducted 372 h of behavioural observations to identify mechanisms that relate important habitat variables to plover productivity.
  • 4
    Plovers selected wide beaches with low levels of human disturbance that had a low percentage cover of tall trees backing the beach. The likelihood of hatching clutches and fledging chicks was greater in territories with low levels of human disturbance, low conspecific density and high percentage cover of 0·5–5 m tall vegetation backing the beach.
  • 5
    Nest monitoring and behavioural observations suggested that heightened vulnerability to tidal inundation, trampling, heat stress, predators and territorial conflicts may have contributed to the results from the habitat models.
  • 6
    We conclude that tourism development on Thai beaches affects both habitat availability and productivity of Malaysian plovers by enhancing beach erosion rates, converting medium vegetation into tall monocultures and intensifying human disturbance. These direct effects of habitat loss may be exacerbated by density-dependent reductions in productivity.
  • 7
    Synthesis and application. This study demonstrates the value of combining three approaches: habitat modelling, nest monitoring and behavioural observations, for identifying impacts of anthropogenic changes on breeding birds and assigning ultimate causes. In understudied regions where there are pressing threats to wildlife, this approach may focus research efforts so that the necessary data can be obtained rapidly in order to assess and predict human impacts.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Sandy tropical beaches have tremendous economic value (Clark 1997) and are coveted areas for tourism development. Such development can lead to increased levels of disturbance and alterations in habitat structures that may threaten beach-nesting waders (International Wader Study Group 2003; Weston & Elgar 2005). Although the effects of tourism development on temperate beaches has been studied widely (Lord et al. 2001; Ruhlen et al. 2003), there have been few studies in the tropics even though nearly 40% of red-listed waders breed in these areas (Baillie, Hilton-Tayler & Stuart 2004). Factors such as weather and predation that may influence the vulnerability of waders to tourism development (Flemming et al. 1988; Clark & Nudds 1991) are likely to differ between temperate and tropical environments (Martin 1996).

In this study we focused on the Malaysian plover Charadrius peronii Schlegel, a sedentary plover that breeds on quiet, sandy beaches in south-east Asia. Malaysian plovers are red-listed, along with 11 other waders breeding in this region (Baillie, Hilton-Tayler & Stuart 2004). With some of the world's fastest growing economies and high human coastal densities, these waders are increasingly threatened by anthropogenic habitat change ( World Resources Institute 2004). However, no detailed or quantitative studies have been conducted on the breeding ecology or the potential impacts of development on the Malaysian plover or any of these other species.

The conflicts between tourism development and conservation are clearly evident in Thailand (Parr, Mahannop & Charoensiri 1993; Kontogeorgopoulos 1999). With a 3000-km coastline and strong government support for tourism, beaches are rapidly being converted into resorts, restaurants and seawalls to meet the demands of international tourism as well as expanding domestic tourism (Kontogeorgopoulos 1999).

Previous studies on temperate beaches have shown that human disturbance can lead to trampling of nests and chicks (Ruhlen et al. 2003), thermal stress (Weston & Elgar 2005), greater predation risk of chicks and clutches (Bolduc & Guillemette 2003) and reduced feeding times for chicks that may lead to starvation (Leseberg, Hockey & Loewenthal 2000).

Changes in habitat structure because of tourism development may also reduce habitat availability and breeding success. For example, changes in vegetation structure can influence both predator density and the vulnerability of waders to predators (Pampush & Anthony 1993; Manzer & Hannon 2005). In Thailand, one consequence of tourism-related development is the conversion of shrubby dune vegetation, mangroves and Acacia bushes to tall Casuarina trees (Bamroongrugsa, N. Wetlands International, unpublished data). These trees provide a shady environment with sparse underlying vegetation that is favoured by tourists. However, tall trees may provide perch sites and hence increase predation risk from avian predators (Wolff et al. 1999; Dekker & Ydenberg 2004). Moreover, the removal of ground cover and bushes along beaches may reduce fledgling success because these types of vegetation provide cover for chicks from visual predators (Clark & Nudds 1991; Pampush & Anthony 1993). Furthermore, a reduction in beach width as a result of development may contribute to increased vulnerability of nests to flooding (Koenen, Leslie & Gregory 1996) and predation (Liley 1999). Finally, in our study area there is substantial erosion partly as a result of the construction of seawalls and jetties at adjacent beaches (Brown & McLachlan 2002).

In this study, we assessed whether human disturbance and alterations in habitat structures influence habitat availability and fledgling success of Malaysian plovers using logistic habitat models. These models included variables affected by anthropogenic changes such as vegetation structure, beach width and human disturbance, as well as natural factors such as prey availability and predator densities. Although previous studies have demonstrated changes in breeding distributions and productivity as a result of tourism development (Beale & Monaghan 2004; Finney, Pearce-Higgins & Yalden 2005), most studies have examined anthropogenic changes in isolation from ecological factors and have not identified the mechanisms leading to changes in productivity. It is important to measure a series of ecological variables because they reflect natural constraints on productivity and may influence the fitness impacts of development. Understanding the mechanisms by which anthropogenic changes affect productivity may help quantify the impacts on wildlife (Gill & Sutherland 2000) and identify the best management practices to mitigate these effects (Beale & Monaghan 2004). In order to interpret the results of our habitat model more effectively, we also conducted detailed behavioural observations and ascertained causes of nest failure in order to identify potential predators and other sources of disturbance.

Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

study areas

All data were collected in four study areas in Prachuap Khiri Khan and Petchburi provinces in the Gulf of Thailand from 25 April to 25 July in 2004 and 2005. From south to north these study areas were: Bornok (BO; 99°53′ 12°00′), Khao Sam Roi Yod (KSRY; 99°56′ 12°05′), Pranburi River mouth (PR; 100°00′ 12°25′) and Laem Phak Bia (LPB; 100°05′ 13°03′). PR was only monitored in 2005. Land use ranged from pastures for cattle, coconut plantations and prawn aquaculture to roads, resorts and restaurants in more developed areas.

Between 2004 and 2005 there were substantial changes in land use. In 2005, four resorts were constructed along an 8-km beach in BO. Six resorts, a dirt road and a paved road were built on a previously undeveloped 5-km beach at PR in 2005. The most significant habitat change was at LPB. In 2004 there was little human disturbance at the north end of LPB because a 1-km long mangrove forest and river separated the 3-km long sandspit from any human structures. However, in 2005 the mangrove forest and sandy peninsula were converted into a seawall during the breeding season. With many construction workers and a new dirt road allowing easier access to the area for local fishers, disturbance levels increased dramatically in 2005.

monitoring of breeding pairs

The majority of plovers were caught and individually colour-banded either during the preceding winter, summer or on nests using noose mats or funnel traps (number of banded adults:chicks in 2004 and 2005, respectively, 118:88 and 75:103). Sixty-five, 70%, of the breeding adults were banded. Forty-eight birds caught during the winter of 2004 did not breed in our study sites.

We found nests by searching in areas where pairs were frequently observed or by watching birds return to nests. Eggs were floated regularly to detect embryo mortality and estimate lay date and hatch date (Westerkov 1950; Powell 2001; Yasué & Dearden 2006). Eggs were laid over a 2–4 day period and incubation began after the first egg was laid. We assumed a 30-day incubation period based on 21 nests for which we were able to measure incubation length accurately (30·4 ± 0·95 days). We checked the nests every 3–5 days to determine nest survival or cause of failure. If any clutches less than 25 days old disappeared between checks and there was no sign of inundation (destroyed nest structure, tidal debris at nest) or trampling (crushed nest structure and eggs, tracks of vehicles and footprints), we assumed that the nests had been predated. Local people did not collect eggs for consumption. During nest checks we resighted adults to ensure that they were still present and to discriminate between desertion and predation as causes of failure.

Close to the hatch date we visited nests every day and recorded the number of peeping or pipped eggs so that we could predict and visit the nest on hatch day. This allowed us to differentiate between nests that were predated before hatch and successful nests in which the chicks died soon after hatch. Detailed observations of adults and chicks helped us to determine whether nests had failed or succeeded. Adults with failed nests roosted or fed in pairs on the mudflat, whereas adults with young chicks conducted conspicuous distractive displays when we approached the nest site. We captured and banded chicks within 2 weeks of hatch and returned to the nesting territories weekly to assess chick survival up to 30 days. Family groups could be identified because at least one of the chicks or adults was colour-banded. Assessing fledgling success was not difficult because the plovers were restricted to beach habitats and did not move more than 200 m from the nest sites. In total, 86 and 126 nesting attempts of 54 and 79 pairs were monitored in 2004 and 2005.

habitat model

Measurement of habitat characteristics

Malaysian plovers defended a rectangular 100–300-m long, 5–40-m wide beach section and adjacent mudflats (20–400 m exposed at low tide). We characterized 200-m long breeding territories and random sites that included the mudflat, beach and vegetation backing the beach.

At each 200-m beach section, we measured the beach width at three transects 60 m apart. Percentage cover of ground cover (height < 0·5 m), medium vegetation (height 0·50–5 m) and tall vegetation (height > 5 m) was estimated visually along three 50-cm wide transects running perpendicular to the tide line and extending 20 m into the vegetation, from the beginning of the vegetation line on the upper shore of the beach (Daubenmire 1959). In 2005 we also used the same method to estimate the percentage ground cover on the beach because vegetation may reduce an incubating plover's predator-detection abilities (Wiebe & Martin 1998; Amat & Masero 2004).

In 2005, there were strong correlations between the different height categories of vegetation cover backing the beach (Pearson's r= 0·13–0·65). Thus we used factor analysis and reduced the three variables (short, medium and tall vegetation) into two orthogonal variables that accounted for 70% of the variance. Component 1 described territories with dense medium vegetation cover (correlations with independent variable and principal components r= 0·88) and low percentage ground cover (r = −0·29). The second component described areas with high percentage cover of large trees (r = 0·86) and low percentage ground cover (r = −0·49). These two components were used in the logistic habitat model.

Large (4–6-cm wide carapace) Ocypode (ghost) crabs shared both the mudflat and beach with Malaysian plovers. Previous studies have suggested that Ocypode crabs predate plover eggs and chicks (Watts & Bradshaw 1995; Gregory-Smith 1998). Thus at each sampling section we recorded the number of Ocypode burrows (diameter greater than 5 cm) in three randomly selected 12-m2 quadrats on the beach.

Small Scopimera (bubbler) crabs are a major prey item for Malaysian plovers (M. Yasué, unpublished data). At each sampling section, we measured the width of the mudflat where crab burrows were present. We paced the mudflat and visually estimated the proportion of the area with high, medium or low crab densities. These proportions were used to calculate weighted average crab densities for the section. At each of the three density categories we counted all burrows that had a 2–10 mm diameter in two 0·46-m2 randomly placed quadrats. We then multiplied the width of the mudflat with weighted average burrow densities to calculate a crab abundance estimate for the sampling section. Crab sampling was conducted between 08:00 and 13:00 at 0·7–1·0 m (based on published tide table values) on 15–25 July in 2004 and 2005. Although direct Scopimera counts would have yielded more accurate estimates of prey availability than burrow counts, our method was necessary because of the large number of sites that needed to be sampled over a short period. Moreover, burrows were a good indicator of crab availability because crabs created burrows at every low tide period to feed at the surface (Takahashi, Suzuki & Koga 2001).

We counted people and dogs on the mudflat, beach or up to 20 m in the vegetation behind the beach along 500–800-m long sample sections. These counts were standardized to an average number of people or dogs per kilometre at each site. Repeated counts were conducted at sampling sections on different times, days and month (n = 9–29 section−1). Census sections were larger than 200 m because people frequently walked or drove along the length of the beach and using 200-m would have yielded many zero counts, making comparisons between breeding territories difficult.

Human visitation rates are likely to increase dramatically in tourist areas during Thai holidays and in less tourist-orientated areas during short-term periods of high fish stocks. Maximum rather than average human visitation may be important for plover breeding success. Consequently we also qualitatively ranked the human disturbance level within a 500-m radius of each nest site based on the number of houses, restaurants, hotels, roads, shrimp ponds, trails or other human-made structures. Scrubland with no human use was ranked 0, and areas with interconnected resorts and paved roads were ranked 5.

We used a factor analysis to reduce these three correlated human disturbance variables (Pearson's r= 0·33–0·54) into one variable for each year that accounted for 69% and 61% of the variation (Fig. 1a,b). All three variables were positively correlated to the principle component (component matrix factor loadings = 0·69–0·85).

image

Figure 1. Scatter plots showing the relationship between instantaneous human (left) and dog (middle) counts and qualitative human disturbance ranking (right) to the human disturbance principal component for 2004 (top) and 2005 (bottom). Although human disturbance rankings were integers, we displaced the x-axis by 0·1 or 0·2 so that so it is easier to differentiate the site symbols on the graph.

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Habitat selection model

To identify factors influencing territory selection, we compared the habitat characteristics of beach breeding territories to control sections without nests in the four areas (2004 and 2005, breeding:control sections, respectively, n= 55:36 and 79:40). Control sites were selected based on proximity to nest sites and interspersed among nest sites as much as possible to avoid spatial autocorrelation (Legendre 1993). All beach sections without nests that were immediately adjacent to nesting territories were included as control sites. To increase sample size of control sites, other beach sections that were within 2 km of nest sites were also included in our study.

Control sites were visited at least once per month from January to July to ensure that there were no breeding plovers. We did not conduct more frequent checks at these sites because Malaysian plovers generally occupied territories at least 1 month prior to laying a clutch and usually did not change habitats after a failed nesting attempt (M. Yasué, unpublished data). The variables that were included in the habitat selection models were: beach width, the percentage cover of ground, medium and tall vegetation behind the beach, Ocypode burrow densities, Scopimera burrow abundance, the human disturbance factor, and site. As discussed above, in 2005 two principal components were used instead of direct vegetation cover behind the beach and percentage cover of beach vegetation was also included in the analysis. For all independent variables we checked the variable inflation factor (all < 2), tolerance (all > 0·5) and bivariate Pearson's correlation coefficients (all < 0·5) to ensure there was no multicollinearity in the data set.

Breeding success model

To minimize pseudoreplication, we calculated reproductive success per breeding territory for each pair of plovers rather than by each breeding attempt. Thus for pairs renesting during a season, the habitat characteristics for successive breeding attempts were averaged and the number of chicks hatched or fledged was summed and then divided by the number of breeding attempts per pair.

In the breeding success models, we identified habitat characteristics that influenced the number of chicks hatched per breeding attempt and the number of chicks fledged per breeding attempt (npairs for hatch success 2004, fledge success 2004, hatch success 2005, respectively, 49, 49, 71). We used these measures of breeding ‘efficiency’ rather than total eggs hatched or chicks fledged because multiple breeding attempts in a season could entail costs to adult survivorship (Ghalambor & Martin 2000).

We tested the same variables as in the habitat selection model and also included conspecific nest density because if there are density-dependent declines in breeding success, these indirect effects may lead to a greater combined impact on populations than could be attributed to the direct effects of habitat loss alone (Sutherland & Norris 2002). Also, a previous study on Kentish plovers suggested that more nests were predated in areas with higher nesting densities (Page et al. 1983). The nest density estimate was the number of conspecific nests within 200 m of the nest, based on Universal Transverse Mercator (UTM) coordinates obtained at each nest.

For the habitat selection, hatch and fledgling success analysis, we used binary logistic regression and selected the most important variables using log-ratios stepwise backwards elimination (removal if P > 0·10) in SPSS version 11·0, while controlling for site. The dependent variable for the habitat selection model was the presence or absence of nesting plovers. For breeding success, we recorded the number of chicks hatched and fledged per breeding attempt of a pair into high and low categories that had approximately equal sample sizes (high categories for hatch 2004, fledge 2004, hatch 2005 were eggs hatched clutch−1≥ 1·5, chicks fledged clutch−1≥ 0·67, eggs hatched clutch−1≥ 1, chicks fledged clutch−1≥ 0). Although scoring breeding success reduces resolution, this was necessary because data could not be transformed into a normal or a Poisson distribution using standard data transformation techniques. We conducted analyses for the 2 years separately because different independent variables were tested between these years (i.e. percentage beach ground cover in 2005 and factors instead of raw data for vegetation structure in 2005).

To check for the independence of successive nesting territories or control sites, we ordered the 200-m beach sections (south to north) within each site and used scatter plots, runs test and lag-1 autocorrelation functions to quantify the degree of serial autocorrelation in the residuals of our model (Pindyck & Rubinfeld 1998; Keitt et al. 2002; Diniz-Filho, Bini & Hawkins 2003). Data showed no sign of strong spatial autocorrelation (significance values for the runs test and autocorrelation functions were all > 0·05).

nest and brood disturbance observations

In 2004 and 2005, respectively, we conducted 70 h and 193 h of observation on incubating birds and 32 h and 77 h of observations on broods from either a hide or by sitting on the mudflat more than 150 m from the beach. Between the 2 years, the observations were conducted on 108 and 54 different nests and broods. All error bars represent standard errors. Incubation watches lasted 1–2 h (mean length 75·7 ± 2·3 min, n= 211) and brood watches lasted 45 min to 1 h (mean length 50 ± 1·3 min, n= 134). Behavioural observations were conducted at random times throughout the day between 06:30 and 17:30. During incubation watches we continuously observed the nests and recorded the total amount of time incubating birds were flushed off nests because of disturbances. Disturbance times during brood watches were measured from the time when the first adult began responding to a disturbance until the last adult stopped. When one of the adults led the chicks into the vegetation behind the beach during disturbances, we were only able to measure the end time of the disturbance based on the behaviour of the adult remaining on the beach. However, in most cases, soon after the beach adult stopped disturbance behaviour the adult that was in the vegetation returned to the beach. One of the adults was always visible during disturbance observations. Birds were considered disturbed if they were exhibiting anti-predatory behaviour, such as neck out-stretched vigilance posturing, flushing onto the mudflat, ‘rat-running’, chasing intruders, calling to distract predators, false brooding and crouching (Gochfeld 1984). If multiple nest or brood observations were conducted on the same pair, these values were averaged for subsequent analysis.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

productivity

Overall productivity at the three sites was higher in 2004 (1·08 ± 0·15) than in 2005 (0·53 ± 0·11 chicks fledged pair−1). Mayfield stage survival estimates for 2004 egg/chick and 2005 egg/chick (survival, nnests) were, respectively, 0·61 ± 0·06, 86/0·51 ± 0·7, 56 and 0·39 ± 0·05, 116/0·53 ± 0·06, 64 (Mayfield 1961).

Twenty nests were inundated by tides and 15 nests were trampled (two by cows, 11 by construction vehicles, two by motorbikes; Fig. 2). We attributed predation as the cause of failure for 26 nests. For most of these suspected predations, entire clutches disappeared between checks. We were unable to determine the predator type.

image

Figure 2. Causes of nest failure for 26 and 60 Malaysian plover nests in 2004 and 2005.

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In almost all cases of chick mortality we were unable to find carcasses or determine the cause of death. One chick in a heavily disturbed site was found trampled in a human footprint, and four other chicks were found within 1 m of the nest with no apparent physical damage. Adults remained in territories after chicks had disappeared and so we were confident that chicks had died rather than moved to different habitats.

habitat model

Factors influencing habitat selection

In both years, plovers selected wide beaches that had a low percentage cover of tall vegetation backing the beach and low levels of human disturbance (Table 1 and Fig. 3a–c). In 2005, birds also appeared to select beaches with a low percentage cover of beach ground cover (Table 1). In 2004 it appeared that plovers may have been selecting areas with a high percentage cover of medium vegetation height; however, this result was not statistically significant (P = 0·075).

Table 1.  Results of a binary logistic model predicting the presence of Malaysian plover nests within 200-m beach sections in the Gulf of Thailand (n = 2004 and 2005 breeding:control section, respectively, 55:36, 79:40)
VariablesHabitat selection
20042005
b Wald P b Wald P
Site  5·50·066 5·40·144
Human disturbance factor−2·88 8·00·005−0·7343·90·049
Beach width (m)0·6414·5< 0·00010·0674·80·029
Tall vegetation (% cover)−0·057 4·80·028−0·786·80·009
Medium vegetation (% cover)0·037 3·20·075   
Beach ground cover (%)NA  −6·0270·008
Model χ255·1  45·7  
d.f.6  7  
P < 0·0001  < 0·0001  
Area under Receiver Operating Characteristic (ROC) curve0·932  0·835  
image

Figure 3. (a–c) Variation in the proportion of beach sections with or without Malaysian plover nests in relation to (a) human disturbance, (b) beach width and (c) tall vegetation cover backing the beach (factor in 2005) in 2004 (left) and 2005 (right). For the highest independent variable category in each graph, we pooled all higher values so that there was sufficient sample size in each bar (n > 5). Binomial error bars are shown. Black dots represent a fitted logistic regression curve showing the predicted probabilities from the model. For this curve, the other significant independent variables were set to mean values. The bar graph and the logistic curve are plotted on the same scale.

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Factors influencing breeding success

In 2004, plovers were more likely to hatch at least 1·5 chicks clutch−1 in areas with low Ocypode burrow densities, low conspecific density and low levels of human disturbance (Table 2 and Fig. 4a,b). Ocypode burrow density and conspecific nest density both also affected the likelihood of hatching at least 1 chick clutch−1 in 2005.

Table 2.  Results of a binary logistic model predicting the number of Malaysian plover chicks hatched per clutch (npairs for 2004 and 2005, respectively, 49, 71)
VariablesChicks hatched clutch−1
2004: likelihood of ≥ 1·52005: likelihood of ≥ 1
b Wald P b Wald P
Site 2·510·284 3·30·347
Conspecific density (pairs within 200 m)−0·793·90·047−0·614·20·04
Ocypode burrow density 5·66·70·009−4·590·003
Human disturbance−1·715·50·021   
Model χ218·2  18·9  
d.f.5  5  
P 0·003  0·002  
Area under ROC curve0·795  0·79  
image

Figure 4. (a–b) The effects of (a) conspecific nesting density and (b) human disturbance on the proportion of Malaysian plovers that hatched at least 1·5 and 1·0 chicks clutch−1 in 2004 and 2005, respectively. Black dots represent a fitted logistic regression curve showing the predicted probabilities from the model. The bar graph and the logistic curve are plotted on the same scale.

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In 2004, the probability of fledging at least 0·67 chicks clutch−1 was greater in areas with a high percentage cover of medium vegetation (b = 0·084, Wald = 7·6, P= 0·006, model χ2 = 18·1, d.f. = 5, P= 0·003, AUC = 0·79; Fig. 5). The data suggested that conspecific density may influence fledge success rates; however, this was not statistically significant (b = −0·654, P= 0·095). In 2005, none of the habitat variables predicted the likelihood of fledging more than 0 chicks clutch−1 (model χ2 = 7·54, d.f. = 4, P= 0·11).

image

Figure 5. The effect of percentage cover of vegetation of medium height on the proportion of Malaysian plovers that fledged at least 0·67 chicks clutch−1 in 2005. Black dots represent a fitted logistic regression curve showing the predicted probabilities from the model. The bar graph and the logistic curve are plotted on the same scale.

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incubation and brood disturbance observations

Observations on the extent of anthropogenic disturbances, types of predators and anti-predator responses of Malaysian plovers may help to interpret the results of our binary logistic habitat models by identifying the underlying mechanisms that cause avoidance or reduced breeding success in certain types of habitats. Overall, during both incubation (211 observations on 108 pairs) and chick rearing (134 observations on 54 pairs), adults spent very little time overtly responding to anthropogenic and natural disturbances (incubation 3·2% of total time, chick-rearing 10·1%). The majority of anthropogenic disturbances were people, dogs, cattle and motor vehicles on the beach. Low anthropogenic disturbance rates were probably a result of very low human and dog densities at breeding areas (5·2 ± 0·7 people km−1 and 1·4 ± 0·3 dogs km−1 for nesting territories).

Natural disturbances

Natural causes of disturbance included potential predators such as ghost crabs (Ocypode spp. Ortmann, on 10 occasions), small Indian mongoose Herpestes javanicus Hodgson (three occasions), peregrine falcons Falco peregrinus Tunstall (three occasions), collared kingfishers Halcyon chloris Boddaert (most significant single source, 30% of all natural disturbance), attacks from neighbouring conspecifics as well as false alarms (Fig. 6). Compared with previous studies on Charadrius plovers (Page et al. 1983; Brook & Tanacredi 2002), we observed very few predators and no predation events in 372 h of observations. However, we did not systematically monitor the density of these potential sources of natural disturbance and it is possible that we missed cryptic predators such as snakes. Thus the low amount of time plovers spent responding to natural disturbances could be the result of low diurnal predator densities as well as plovers not overtly responding to less obvious predators.

image

Figure 6. The proportion of time Malaysian plovers spent responding to different types of disturbances during incubation and chick-rearing.

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Responses to disturbances

During false alarms, anthropogenic, conspecific and mammalian predator disturbances, adults quickly led chicks into the vegetation behind the beach. The chicks remained still in the vegetation, while one or both of the adults returned to the beach to monitor and distract the predators. Eventually, the chicks resumed feeding within the vegetation and then progressively moved to the beach and to the mudflats. These observations demonstrated how medium vegetation cover may improve breeding success for Malaysian plovers.

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Sandy tropical beaches are under tremendous development pressure. This study suggests that development on Thai beaches can reduce both habitat availability and breeding success for Malaysian plovers. Malaysian plovers share several habitat requirements with related temperate Charadrius plovers (Flemming et al. 1988; Powell 2001) and they are similarly vulnerable to human disturbance, changes in vegetation cover and reductions in beach width. Although numerous studies have examined these impacts on temperate environments, this is the first detailed study demonstrating the potential impacts of coastal development on a tropical Charadrius plover. Moreover, tropical species (and in particular species in south-east Asia) may be even more vulnerable to development because of weaker conservation regulations in many of these countries (Kontogeorgopoulos 1999).

human disturbance

In 2004, human disturbance affected both habitat availability and hatch success. This is one of the first studies demonstrating the effects of human disturbance on a breeding tropical wader. Clearly nest monitoring showed that trampling and construction affected hatch success (Liley 1999; Ruhlen et al. 2003). Trampling may be exacerbated by reduced beach width because plovers may be forced to nest on footpaths, cattle routes and motorcycle pathways. In LPB, where a seawall was being constructed, birds repeatedly nested on the dirt road for construction vehicles because the rocky boulders of the seawall replaced most of the sandy beach.

Previous studies have shown that human disturbance may affect hatch success by forcing adults off nests, thereby making eggs more vulnerable to opportunistic predators such as gulls (Bolduc & Guillemette 2003). Diurnal nest predation was probably not a significant causative factor, because we observed few opportunistic predators. However, it is possible that predation may be an important mechanism at night because there were many fishers on the beaches during this time. People dig for Ocypode crabs on beaches near Malaysian plover nests. Moreover, these crabs are more active at night, when they may pose a much greater threat (Watts & Bradshaw 1995).

Heat stress may be an important mechanism linking human disturbance levels and productivity. Malformations and embryo mortality begins to occur at 40·5 °C in some birds (Webb 1987) and a concurrent study at this field site showed that egg temperatures can rise above this temperature after only a few minutes of exposure to the sun (Yasué & Dearden 2006). Thus even at the very low levels of disturbance at our study site, embryo survival may be reduced (Lundy 1969). Although there have been few studies on human disturbance and heat stress in plovers, Weston & Elgar (2005) suggested that reduced brooding because of human disturbance may have resulted in lower fledgling success for the hooded plover Thinornis rubricollis in Australia.

Twenty-three per cent of failed nests were caused by desertion. This may be partly attributed to human disturbance. Adults may choose to abandon nests if they are constantly forced off nests on a hot day and embryo survival seems unlikely. Alternatively, as waders probably perceive people as predators (Frid & Dill 2002; Beale & Monaghan 2004), constant disruption by people may make adults perceive a greater personal risk to incubation and increase the likelihood of desertion (Ghalambor & Martin 2001).

Beale & Monaghan (2004) showed reductions in reproductive success in seabirds disturbed by people. As with our study, the mechanism causing reduced breeding success was difficult to identify because birds spent very little time overtly responding to human disturbance. Stress and physiological responses, such as increased heart, rate, which are not detectable by observations, may contribute to lower adult body condition, lower adult attendance and greater nest desertion, leading to reduced breeding success (Beale & Monaghan 2004).

Human disturbance had no effect in 2005. This may be attributed to the presence of one or two effective wide-ranging predators such as a mongoose on two very undisturbed beaches in KSRY in 2005. Thirty-five per cent and 40% of the nests monitored in 2004 and 2005 were on these beaches. In 2005, 16 of these nest were predated, whereas only five were predated in 2004. Also in 2005, our methods may have underestimated the amount of human disturbance perceived by birds at the construction sites. Although we counted more people on these beaches compared with 2004, birds may have perceived an even greater predation threat because of the presence of large, noisy machinery.

Fledging success was not affected by human disturbance. Although Malaysian plover chicks may be occasionally trampled by people and motor vehicles, in previous temperate research the main mechanism causing reduced chick survival in heavily disturbed areas is the reduction of chick feeding time (Flemming et al. 1988; Piatt et al. 1990; Lord, Waas & Innes 1997). Chick starvation risk may be less important for Malaysian plovers because of the relatively low human disturbance rates at our study site, the more consistent prey base in the tropics, and the lower feeding requirements because of the warmer weather (Martin 1996). Moreover there may be little remaining variability in human disturbance to detect differences in fledgling success because human disturbance already affected habitat selection and (in 2004) hatch success.

conspecific nest densities

In both years, breeding success was lower in areas of high conspecific nesting density. This may be because of greater predation risk (Page et al. 1983) or direct infanticide by neighbouring adults (Fraga & Amat 1996). Alternatively, in areas with high nesting densities, adults may spend more time defending territories, leaving the chicks more vulnerable to predation or heat stress. If habitats are lost as a result of tourism development, birds may be forced to nest in higher densities in remaining areas and suffer lower breeding success until population sizes are reduced to match the lower habitat availability (Goss-Custard et al. 1995). In LPB, approximately 80% of the habitat was lost because of the construction of the seawall. In 2005, the mean nearest nest distance for this site was only 30·4 ± 3·89 m (n = 6), compared with 604 ± 159 m for all other sites combined and 190·6 ± 25·6 m (n = 9) at the same site in 2004.

changes in habitat structure

Beaches backed by vegetation of medium (0·5–5 m) height (Acacia and mangroves) appeared to provide better nesting habitat than beaches backed by tall vegetation (Casuarina pine trees). Although there were very few raptors observed in the breeding season, migrating peregrine falcons and sparrowhawks (Accipiter spp.) were frequently observed during the non-breeding season (DeCandido et al. 2004) and may thus still influence plover habitat selection in the breeding season. During chick rearing, Acacia and mangrove bushes appeared to be an important hiding and foraging habitat for chicks (Clark & Nudds 1991). In areas with high disturbances, chicks fed mainly within the vegetation or along the beach close to the vegetation, instead of on the mudflats (Liley 1999; Weston & Elgar 2005). These bushes also provided shade for chicks when adults were not able to shade them during adult feeding bouts and disturbances (Visser & Ricklefs 1993; Weston & Elgar 2005).

significance and management implications

Our study suggests that there has been a rapid decline in available beach habitat for the Malaysian plover in Prachuap Khiri Khan and Petchburi provinces in the Gulf of Thailand. This is a concern because, as far as we know, this study was conducted on the largest plover population and least disturbed breeding habitat in Thailand. It is likely that the impacts of tourism are much greater for the small populations that may continue to exist in small, undisturbed pockets within more tourism-orientated beaches throughout Thailand. In a concurrent study we interviewed ornithologists and visited numerous beaches on both coasts of Thailand to census Malaysian plovers (M. Yasué, unpublished data). Almost all the beaches where plovers bred less than 10 years ago were now highly developed tourist areas with no plovers.

Well-managed protected areas that do not allow habitat conversions and limit the number of people on beaches, may help conserve plover populations by increasing the amount of available breeding habitat and potentially reducing nesting densities. The habitat model suggests that beach width, tree heights, Ocypode crab densities and human disturbance rates should be considered when selecting areas for protection. In terms of management of these protected areas, sign-posted fencing around beach areas where plovers breed (Lord et al. 2001) may reduce human access during periods when birds are most sensitive to disturbance, such as in the hottest months of April and May. Moreover, in key Malaysian plover breeding areas, tall trees could be removed or beach height could be artificially raised to create more suitable habitat (Ziewitz, Sidle & Dinan 1992; Round, Erwin & Porter 2004).

However, with continued rapid tourism growth, ineffective land-use planning and strong government support for tourism (Kontogeorgopoulos 1999; Kontogeorgopoulos 2004), these types of recommendations are unlikely to be implemented in Thailand. Part of the KSRY and PR sites are in protected areas, and at present there is little tourism development in there and relatively few changes occurred during 2004 and 2005. However, protected area status in Thailand does not necessarily preclude development (Dearden & Chettamart 1997). In 2004 and 2005, land speculators visited the most important waterbird breeding habitat within the protected area of KSRY. In many protected areas, managers may be more interested in generating revenue through tourism development than in meeting conservation goals. Moreover, on beaches within protected areas, cleaning vehicles regularly run along the beaches, sweeping up garbage and destroying potential nests of breeding waders (C. Chettamart, National Park Superintendent, unpublished data). In the few protected areas that actively discourage development there are still no active habitat restoration projects or exclosures (C. Chettamart, unpublished data).

conclusion

This study demonstrates the value of using multiple approaches, including habitat modelling, nest monitoring and behavioural observations, to evaluate and interpret the effects of habitat loss and human disturbance on a tropical wader population. By examining a series of habitat variables relating to predation risk and prey availability, we were able to identify important habitat characteristics for the Malaysian plover and assess how anthropogenic changes may influence habitat suitability. We were also able to evaluate possible direct links between human disturbance and changes in habitat structure and productivity, by conducting behavioural observations and assessing causes of nest failure.

Our work demonstrates the importance of conducting conservation-related studies on tropical beaches (Martin 1996; Brown & McLachlan 2002; Fish et al. 2005). Differences in climate and environment can affect both the natural constraints on breeding and vulnerabilities to different types of anthropogenic change. Many tropical countries have similar conflicts between tourism and conservation and there are several red-listed beach-nesting species such as the Chinese-crested terns Sterna bernsteini Schlegel and green turtle Chelonia mydas L. (Hall & Page 2000). In Thailand, little terns Sterna albifrons and red-wattled lapwing Vanellus indicus Boddaert breed alongside Malaysian plovers. Multispecies studies could also help to determine cumulative impacts of tourism development on wildlife (Root, Akcakaya & Ginzburg 2003).

In regions where there is a paucity of ecological research, such as south-east Asia, this type of behavioural approach may be an effective means to acquire data rapidly that has greater generality than site-specific monitoring studies and requires shorter study periods than demographic studies (Sutherland 1998). Although current policy barriers restrict the conservation value of this study, we hope that the methods used may have value for other regions with similar environments, threatened beach wildlife and a more favourable socio-political climate for wildlife conservation.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Thanks to Josh Malt, Alan Burger and anonymous referees for valuable suggestions and comments on a previous draft; Bruce Catton, Mark Flaherty, David Duffus, Dov Lank and Trisalyn Nelson for statistical advice; George Gale, Philip Round and Andrew Pierce in Bangkok; staff at Khao Sam Roi Yod National Park and the villagers of Bonok for equipment and logistical support; field assistants Allison Patterson and Prathew Tonghom. M. Yasué was supported by a PGS-D NSERC Canada and research was funded by P. Dearden's SSHRC, Canada research grant.

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  6. Discussion
  7. Acknowledgements
  8. References
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